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=Part I. Microscopic Anatomy=
<center><big>'''Part I. Microscopic Anatomy'''</big></center>


* [[Book - A text-book of histology arranged upon an embryological basis (1913) 1-1|1.1. Cytology]]
=I. Cytology=
* [[Book - A text-book of histology arranged upon an embryological basis (1913) 1-2|1.2. General Histology]]
* [[Book - A text-book of histology arranged upon an embryological basis (1913) 1-3|1.3. Special Histology]]


==The Cell==


I. Cytology.  
Since 1838 it has been known that all plants and animals are composed of small structural elements called cells (Latin, cellula; Greek, *vros). The lowest forms of animals and of plants are alike in being single cells throughout life. The more complex organisms are groups of cells, which have been derived by process of repeated division from a single cell, the fertilized ovum. Thus the human body, which begins as one cell, becomes in the adult an aggregation of cells variously modified and adapted to perform special functions. Since the liver is a mass of essentially similar cells, the problems of its functional activity are the problems of the functions of a single one of its cells. The diseases of the liver are the result of changes occurring in these cells, which must be restored to a normal condition to effect a cure. As this is equally true of other organs, it is evident that cytology, the science of cells, is a basis for both physiology and pathology.


The Cell.  
A cell may be defined as a structural element of limited dimensions, which under certain conditions can react to external stimuli and perform the functions of assimilation, growth, and reproduction. Because of these possibilites a cell may be considered an elementary organism. It is described as a mass of protoplasm containing a nucleus. A third element, the centrosome, is found in the cells of animals, but it is doubtful whether it exists in the cells of the higher plants. It becomes prominent when a cell is about to divide. Some authorities regard the centrosome as a temporary structure, which forms shortly before division begins and disappears after it is completed. Others consider it as a permanent and essential part of a cell, which accordingly consists of protoplasm, nucleus, and centrosome.


Since 1838 it has been known that all plants and animals are composed
of small structural elements called cells (Latin, cellula; Greek, *vros).
The lowest forms of animals and of plants are alike in being single cells
throughout life. The more complex organisms are groups of cells, which
have been derived by process of repeated division from a single cell, the
fertilized ovum. Thus the human body, which begins as one cell, becomes
in the adult an aggregation of cells variously modified and adapted to
perform special functions. Since the liver is a mass of essentially similar
cells, the problems of its functional activity are the problems of the functions of a single one of its cells. The diseases of the liver are the result of
changes occurring in these cells, which must be restored to a normal condition to effect a cure. As this is equally true of other organs, it is evident
that cytology, the science of cells, is a basis for both physiology and
pathology.


A cell may be defined as a structural element of limited dimensions,
===Protoplasm===
which under certain conditions can react to external stimuli and perform
the functions of assimilation, growth, and reproduction. Because of
these possibilites a cell may be considered an elementary organism. It is
described as a mass of protoplasm containing a nucleus. A third element,
the centrosome, is found in the cells of animals, but it is doubtful whether
it exists in the cells of the higher plants. It becomes prominent when a
cell is about to divide. Some authorities regard the centrosome as a temporary structure, which forms shortly before division begins and disappears
after it is completed. Others consider it as a permanent and essential
part of a cell, which accordingly consists of protoplasm, nucleus, and
centrosome.


Protoplasm is the living substance of which cells are composed. More specifically the term is applied to this living substance exclusive of the nucleus, or to the corresponding dead material, provided that death has not changed its physical properties. It has been proposed to substitute the name cytoplasm for protoplasm in the restricted and earlier sense of the term, to call the nuclear substance karyoplasm, and to consider both cytoplasm and karyoplasm as varieties of protoplasm. Although these names are often employed, the cell substance apart from the nucleus is ordinarily called protoplasm.


Protoplasm is a heterogeneous mixture of substances forming a soft viscid mass of slightly alkaline or neutral reaction. ("The terms may be used interchangeably for an alkalinity which is so slight" Henderson.) It is ordinarily more than three-fourths water, and the remainder consists of salts and organic substances, some in solution and some in a colloidal state. The organic bodies are classed as proteins, glycogen or some allied carbohydrates, and lipoid (fat-like) bodies. Protoplasm may exist in a numberless variety of forms.


HISTOLOGY






PROTOPLASM.  
FIG. r. DIAGRAM OF A CELL.


Protoplasm is the living substance of which cells are composed. More
In the four quadrants different types of protoplasmic structure are represented namely, homogeneous, granular, foam-like, and fibrillar.
specifically the term is applied to this living substance exclusive of the
nucleus, or to the corresponding dead material, provided that death has
not changed its physical properties. It has been proposed to substitute
the name cytoplasm for protoplasm in the restricted and earlier sense of
the term, to call the nuclear substance karyoplasm, and to consider both
cytoplasm and karyoplasm as varieties of protoplasm. Although these
names are often employed, the cell substance apart from the nucleus is
ordinarily called protoplasm.  


Protoplasm is a heterogeneous mixture of substances forming a soft
viscid mass of slightly alkaline or neutral reaction. ("The terms may be used interchangeably for an alkalinity which is so slight" Henderson.)
It is ordinarily more than three-fourths water, and the remainder consists of salts and organic substances, some in solution and some in a colloidal
state. The organic bodies are classed as proteins, glycogen or some allied
carbohydrates, and lipoid (fat-like) bodies. Protoplasm may exist in a
numberless variety of forms.




On microscopic examination, even with lenses of the highest power, the protoplasm of certain living cells appears homogeneous and structureless. But most of the cells which the histologist examines are not living.




FIG. r. DIAGRAM OF A CELL.  
They have been killed by various reagents, selected as causing the most rapid fixation possible. The protoplasm of such cells usually exhibits granules, fibrils, or networks with closed or open meshes. Whether these structures are wholly due to precipitation and coagulation is difficult to determine, but indications that they preexist have been observed in certain living cells. In any case, the various forms of coagulation occur with such constancy that their study is of the utmost importance to the histologist.


In the four quadrants different types of protoplasmic structure are represented namely, homogeneous, granular, foam-like, and fibrillar.  
Even the ground substance of protoplasm, in which the fibrils or granules are imbedded, is not necessarily homogeneous. According to Biitschli's interpretation it has the structure oj foam or of an emulsion that is, it consists of minute droplets of one substance completely surrounded by walls of another substance. In these walls, granules and filaments may be lodged, as seen at the margins of the upper right quadrant of Fig. i. The complex chemical activities of a cell are said to be manifestly impossible in any homogeneous mass; but in such a heterogeneous medium as an emulsion, they are conceivable (Alsberg). In other words, the vital qualities of protoplasm may not depend so much on hypothetical complex and unstable living molecules, as upon the interaction of various substances, made possible by their arrangement in droplets and investing films.


The various structures commonly observed in protoplasm may be grouped as follows:


i. Granules. Ultra-microscopic granules doubtless exist in protoplasm, since the smallest of those observed approach the limit of visibility. The minute granules, if abundant, give the Nissl's bodies.


On microscopic examination, even with lenses of the highest power,  
protoplasm a dark color. Often they are absent from the peripheral layer of protoplasm, or exoplasm, which is then clear, somewhat firmer, and chemically different from the inner endoplasm (Fig. i). In addition to minute granules such as may be found in most preserved protoplasm, certain cells contain larger granules, which are important secretory products elaborated by the cell. In active gland cells these granules are well defined and abundant, and they diminish as the cell becomes exhausted.  
the protoplasm of certain living cells appears homogeneous and structureless. But most of the cells which the histologist examines are not living.  




FIG . 2 . CLUMPS OF~GRANULES (NissL's BODIES) IN A NERVE


PROTOPLASM 3


They have been killed by various reagents, selected as causing the most
rapid fixation possible. The protoplasm of such cells usually exhibits
granules, fibrils, or networks with closed or open meshes. Whether these
structures are wholly due to precipitation and coagulation is difficult to
determine, but indications that they preexist have been observed in certain living cells. In any case, the various forms of coagulation occur
with such constancy that their study is of the utmost importance to the
histologist.


Even the ground substance of protoplasm, in which the fibrils or
Various forms of white blood corpuscles may be distinguished by the size and staining reaction of the granules imbedded in their protoplasm. In certain nerve cells (Fig. 2) granules occur in large groups, known as Nissl's bodies. As Crile has shown, these become disorganized as a result of surgical shock or muscular fatigue. It is evident, therefore, that the careful observation of protoplasmic granules is of very great importance.
granules are imbedded, is not necessarily homogeneous. According to
Biitschli's interpretation it has the structure oj foam or of an emulsion
that is, it consists of minute droplets of one substance completely surrounded by walls of another substance. In these walls, granules and filaments may be lodged, as seen at the margins of the upper right quadrant
of Fig. i. The complex chemical activities of a cell are said to be manifestly impossible in any homogeneous mass; but in such a heterogeneous
medium as an emulsion, they are conceivable (Alsberg). In other words,  
the vital qualities of protoplasm may not depend so much on hypothetical complex and unstable living molecules, as upon the interaction of various substances, made possible by their arrangement in droplets and investing films.  


The various structures commonly observed in protoplasm may be
grouped as follows:


i. Granules. Ultra-microscopic granules doubtless exist in protoplasm, since the smallest of those observed approach the limit of visibility.
The minute granules, if abundant, give the


Xissl's bodies.


protoplasm a dark color. Often they are absent
from the peripheral layer of protoplasm, or
exoplasm, which is then clear, somewhat firmer,
and chemically different from the inner endoplasm (Fig. i). In addition to minute granules
such as may be found in most preserved protoplasm, certain cells contain larger granules,
which are important secretory products elaborated by the cell. In active gland cells these
granules are well defined and abundant, and F JG . 2 . CLUMPS OF~GRANULES


.ij..., ., IT i i j (NissL's BODIES) IN A NERVE  
FIG. 3. 'FIBRILS IN A NERVE CELL.


they diminish as the cell becomes exhausted. CELL.


Various forms of white blood corpuscles may


be distinguished by the size and staining reaction of the granules imbedded in their protoplasm. In certain nerve cells (Fig. 2) granules
FIG. 4. VACUOLES IN A YOUNG FAT CELL.
occur in large groups, known as Nissl's bodies. As Crile has shown,
these become disorganized as a result of surgical shock or muscular






FIG. 5. CANALS (TROPHOSPONG IUM) IN A NERVE CELL.


HISTOLOGY




 
2. Fibrils. Protoplasm may be permeated with a delicate meshwork of fibrils, which collectively constitute the spongioplasm, or filar mass. This is imbedded in the clear hyaloplasm, or interfilar mass (Fig. i). In certain cells there are filaments, known as mitochondria, which are formed by the coalescence of rows of granules. The relation between these structures and the reagents used is discussed by Kingsbury (Anat. Rec., 1912, vol. 6, pp. 39-52). The spongioplasm may form an irregular network, or its constituent fibrils may be parallel, passing from one end of the cell to the other. In oblique and transverse sections of such cells, the filaments are cut across, so that they appear as short rods, or even as granules. Fibrils may be extremely slender, as in the case of those which radiate through the protoplasm at the time when the cell divides; or they may be quite coarse, like the permanent fibrils characteristic of certain muscle and nerve cells
fatigue. It is evident, therefore, that the careful observation of protoplasmic granules is of very great importance.
 
 
 
 
 
FIG. 3. 'FIBRILS IN A NERVE CELL.
 
 
 
FIG. 4. VACUOLES IN A YOUNG
FAT CELL.
 
 
 
Nucleus.
 
 
 
Nucleolus.
f
 
 
 
 
Capsule. Canaliculi.
FIG. 5. CANALS (TROPHOSPONG
IUM) IN A NERVE CELL.
 
 
 
2. Fibrils. Protoplasm may be permeated  
with a delicate meshwork of fibrils, which collectively constitute the spongioplasm, or filar mass.  
This is imbedded in the clear hyaloplasm, or  
interfilar mass (Fig. i). In certain cells there are  
filaments, known as mitochondria, which are  
formed by the coalescence of rows of granules.  
The relation between these structures and the  
reagents used is discussed by Kingsbury (Anat.  
Rec., 1912, vol. 6, pp. 39-52). The spongioplasm  
may form an irregular network, or its constituent  
fibrils may be parallel, passing from one end of  
the cell to the other. In oblique and transverse  
sections of such cells, the filaments are cut across,  
so that they appear as short rods, or even as  
granules. Fibrils may be extremely slender, as in  
the case of those which radiate through the protoplasm at the time when the cell divides; or they  
may be quite coarse, like the permanent fibrils  
characteristic of certain muscle and nerve cells  


Reticular apparatus.  
Reticular apparatus.  
FIG.. 6. RETICULAR NETWORK (Fig. 3).
3. Vacuoles. Protoplasm often contains large
or small drops of clear fluid, fat, or some other
substance less highly organized than the surrounding material (Fig. 4). In preserved cells the
spaces which were occupied by these droplets appear clear and empty, and are known as vacuoles.
tiey vary greatly in size, and one or several of
them may be found in a single cell.
4. Canals. The protoplasm of certain cells is said to contain fine tubes
or clefts which communicate with lymphatic spaces outside of the cell
PROTOPLASM 5
(Fig. 5). Prolongations from the surrounding capsule-cells have been
described as entering these canals and as performing, together with the
lymph, a nutritive function. Hence the network of canals has been called
trophospongium. But it has not been shown conclusively that these
canals open to the exterior of the cell. They may be similar to the closed
networks or "reticular apparatus" lying wholly within the protoplasm,
shown in Fig. 6. Such networks have been described in nerve cells, cartilage cells and gland cells. The network is said to be of a thick fluid consistency. In certain gland cells there are canals within the protoplasm,
which convey the secretion to the free surface of the cell. These may be
simple, branched, or arranged in a network. Like the other forms of
intracellular canals, they can be studied only in special preparations.
5. Inclusions. Various foreign bodies, such as other cells or bacteria,
which may have been ingested by the protoplasm, are grouped as inclusions. This term is applied also to crystalloid substances formed within
the protoplasm (Fig. 7), and to coarse masses of pigment granules which
appear extraneous.
NUCLEUS.
The nucleus (Latin, nucleus, "the kernel of a nut"; Greek, Kdpvov, "a
nut") is typically a well-defined round body, situated near the center of
the cell, appearing denser or more coarsely granular than the surrounding
protoplasm (Fig. i). There are characteristic variations in the shapes of
nuclei, in their position within the cells and in their structure.
Ordinarily the karyoplasm, or nuclear substance, is sharply marked off
from the cytoplasm by the nuclear membrane. Sometimes, in preserved
tissues, the cytoplasm has shrunken away from the nuclear membrane,
so as to leave a narrow space partially encircling it; and in certain living
cells, the nucleus migrates through cytoplasm, as if it were an independent
body. But there are phases of cell-development in which the nuclear
membrane disappears and no line can be drawn between karyoplasm and
cytoplasm. At all times they have a common structural basis. The
ground substance of the nucleus, corresponding with the hyaloplasm, is
the nuclear sap; and it contains, for spongioplasm, a meshwork of delicate
linin fibrils. These help to form the nuclear membrane, in which they
terminate. The nuclear membrane, nuclear sap, and linin reticulum do
not stain deeply, and are therefore grouped together as the achromatic
constituents of the nucleus.
The principal chromatic constituent of the nucleus is known as chromatin. It stains deeply, since it contains a large amount of nucleic acid,
which has a marked affinity for basic stains. Chromatin occurs in the
form of granules, which are bound together in strands or masses by the
6 HISTOLOGY
linin fibers (Fig. i). The masses, known as chromatin knots, occur especially at the points of intersection in the linin meshwork. Sometimes they
are attached to the nuclear membrane, or so distributed over its surface
that it appears to consist of chromatin. It forms morphologically the
most important part of the nucleus.
Certain nuclei contain one or more round bodies, which belong with
the chromatic elements because of their deep staining, but which are
chemically different from chromatin. These bodice, known as nucleoli,
are stained with acid or neutral dyes. They are said to be composed of
paranuclein, whereas chromatin is composed of nuclein. In distilled
water the structures formed of nuclein disappear, but those consisting of
paranuclein remain. The nuclei of nerve cells contain typical nucleoli
(Figs. 3 and 5). Sometimes a nucleolus, lodged in the nuclear reticulum,
is more or less covered with chromatin (Fig. 9, A), but the term should
not be applied to irregular knots of chromatin, even when most of the
chromatic material within a nucleus is gathered into one or two such
bodies. These are the so-called false nucleoli (pseudonucleoli).
Every nucleus, therefore, consists of ground substance or nuclear sap,
a network of linin, and granules and masses of chromatin. Usually it is
surrounded by a membrane, and sometimes it contains a nucleolus. Most
cells contain a single nucleus; but occasionally a single cell contains two
nuclei, as is frequent in the liver, or even several nuclei, as in certain cells
associated with bone. Non-nucleated bodies, like the mammalian red
blood corpuscles, and the dead outer cells of the skin, have lost their
nuclei in the course of development.
Functionally the nucleus is regarded as a center for chemical activities
necessary for the life of the cell. It is believed to produce substances
which pass out into the cytoplasm, where they may be further elaborated.
Evidences of nuclear extrusions into the cytoplasm have been frequently
recorded. But the interactions between nucleus and cytoplasm, of such
nature that they cannot be observed under the microscope, are presumably of far greater biological importance.
CENTROSOME.
The centrosome is typically a minute granule in the center of a small
sphere of differentiated protoplasm. Often the term is applied to this
entire structure, but it refers particularly to the central granule; the
enveloping sphere is known as the attraction sphere, and it is composed
of archoplasm^ When a cell is about to divide, delicate fibrils, either rearranged from the protoplasmic reticulum or formed anew, radiate from
the archoplasm toward the periphery of the cell. The central granule
becomes subdivided into two, which then move apart. In resting cells,
CENTROSOME 7
or those which are not undergoing division, the centrosome may already
have divided into a double body or diplosome preparatory to the next
division of the cell (Fig. i).
Centrosomes have been detected in many forms of resting cells, and
it is assumed by some authorities that the centrosome is an invariable
constituent of the cells of the higher vertebrates. According to this
opinion the centrosome may become inconspicuous but it never loses
its identity. Often they are found very close to the nuclear membrane,
which may be indented to accommodate them; and rarely, as in certain
cancer cells and in one form of the worm Ascaris, they have been reported
as within the nucleus. They may occur near
the free surface of certain cells, usually in the
form of diplosomes, as shown in cell a, Fig. 8.
Just above the diplosome, such cells may send
out contractile projections of protoplasm
(pseudopodia), with the activity of which the!
diplosome may be in some way associated.j
Pseudopodia, with an underlying diplosome,
have been observed in the columnar cells of
the human large intestine. In cell b of Fig. 8
there are four diplosomes, one of which lies
beneath the protoplasmic projections. It is
believed that the diplosomes may multiply
by fission, and that thus they may give rise to the numerous motile
hairs, or cilia, which project from certain cells. Of these they form the
basal bodies (Fig. 8, c). In many gland cells the centrosome lies in the
midst of the protoplasm where the secretion accumulates. The discharge
of the secretion is accomplished by the contraction of the protoplasmic
strands in which the centrosome is lodged. In all these relations the
centrosome appears to be a center for motor activities, and it is described
as the kinetic or dynamic center of the cell.
FIG. 8. CELLS OF THE EFFERENT
DUCTS OF THE TESTIS OF A
MOUSE. (After Puchs.)
To show diplosomes, and (in c)
cilia with basal bodies.
CELL WALL.
The protoplasm at the surface of certain cells floating in the blood or
lymph forms a thin pellicle, apparently as a result of protoplasmic concentration, or other reaction to the surrounding medium. Cells which
line the greater part of the digestive tube, and have only one surface
directed toward the intestinal contents, are provided with a thick wall on
the exposed surface. Such a wall is called a cuticular border, or cuticula.
On the other sides of these cells, the membrane is much thinner, and on
the basal surface it is sometimes lacking. In such cases the protoplasm
appears to be continuous with that of the underlying cells. In other cases
8 HISTOLOGY
the entire cell is devoid of any membrane. The cell membrane, therefore,
is not an essential part of a cell; if present it ranges from a thin pellicle,
on the border line of visibility, to a well-defined wall, which may be formed
as a secretion of the underlying protoplasm. If the several surfaces
of the cell are in relation to different environments, there is often a corresponding difference in the structure of their walls.
In examining a group of cells, it will be important to determine whether
they are merely in contact, or actually continuous. Sometimes cells
are so completely fused that their nuclei are irregularly distributed through
a single mass of protoplasm. Such a formation is a syncytium in which
the position of the nuclei is the only means of estimating the territory of a
single cell. A syncytium may arise from the fusion of cells, or, as in striated muscle fibers, it may be due to the multiplication of nuclei in an undivided mass of protoplasm. Instead of being completely fused, cells are
often joined to one another by protoplasmic processes of varying length
and width, thus forming cellular networks. Fibrils within such a syncytium may pass continuously from the protoplasm of one cell into that of
another.
Although cell membranes are often inconspicuous in animal cells, they cannot be
overlooked in plants. Thus cork is a mass of dead cells from which nuclei and protoplasm have disappeared, leaving only the cell walls. In describing cork, Robert
Hooke introduced the name "cell," in 1664. He wrote: "I took a good clear piece of
Cork and with a Pen-knife sharpen'd as keen as a Razor, I cut a piece of it off, and
thereby left the surface of it exceeding smooth, then examining it very diligently with
a Microscope, me thought I could perceive it to appear a little porous. . . . These
pores, or cells, were not very deep, but consisted of a great many little Boxes ."
In this way one of the briefest and most important of scientific terms was introduced.
FORM AND SIZE OF CELLS.
Cells are regarded as primarily spherical in form. Spherical cells
are comparatively numerous in the embryo, and in the adult the resting
white blood corpuscles, which float freely in the body fluids, assume this
shape. Such cells are circular in cross section. When spherical cells
are subjected to the pressure of similar neighboring cells, they become
polyhedral and usually appear six-sided in cross section. Such cells, as
a whole, may be cuboidal, columnar, or flat. Certain cells become fusiform (spindle-shaped) or are further elongated so as to form fibers; others
send out radiating processes and are called stellate. Thus the form of
cells is extremely varied. The shape of the nucleus tends to correspond
with that of its cell. It is usually an elliptical body in elongated cells,
and spherical in round or cuboidal cells. In stellate cells it is either
spherical or somewhat elongated. Crescentic nuclei, and others more
SIZE OF CELLS 9
deeply and irregularly lobed, are found in some of the white blood corpuscles and in giant cells.
The size of cells ranges from that of the yolks of birds' eggs which are
single cells, at least shortly before being laid down to microscopic structures four thousandths of a millimeter in diameter. The thousandth of a
millimeter is the unit employed in microscopic measurements. It is called
a micron, and its symbol is the Greek letter /*. The small cells referred to
are therefore four microns (4 ju) in diameter. The size of any structure
in a section of human tissue may be roughly estimated by comparing its
dimensions with the diameter of a red blood corpuscle found in the same
section. These red corpuscles are quite uniformly 7.5 p. in diameter.
CYTOMORPHOSIS.
Cytomorphosis is a comprehensive term for the structural modifications which cells, or successive generations of cells, undergo from their
origin to their final dissolution. 1 In the course of their transformation,
cells divide repeatedly, but the new cells begin development where the
parent cells left off. Cell division, therefore, is an unimportant incident
in cytomorphosis.
Cytomorphosis is a continuous advance in which four successive stages
are recognized first, the stage in which the cells are undifferentiated ;_
second, the stage of specialization or differentiation: third, the stage of
degeneration; and fourth, the stage in which the cells die and are removed.
These may be considered in turn.
Undifferentiated cells, as can be seen in sections of young embryos,
are characterized by large nuclei and little protoplasm. They multiply
rapidly, but the rate of division declines with the gradual increase of the
protoplasm and the consequent functional differentiation of the cell. In
the adult, relatively undifferentiated cells are found in many situations,
as, for example, in the deepest layer of the epidermis. As the cells at the
surface die and are cast off, new ones come up from below to take their
places. But since the basal cells can produce only epidermal cells, they
are themselves partly differentiated. From this point of view the fertilized ovum, which can produce all kinds of cells, must be regarded, in spite
of its size and great mass of yolk-laden protoplasm, as the least differentiated cell.
Differentiated cells may preserve a round or cuboidal form, but usually
they are elongated, flattened, or stellate. The cytoplasm usually contains coarse granules, fibrils, masses of secretion or other special forma
1 The term cytomorphosis was introduced by C. S. Minot in 1901 in a lecture entitled "The
Embryological Basis of Pathology" (Science, 1901, vol. 13, p. 494). Cytomorphosis is further
discussed by Professor Minot in "The Problem of Age, Growth, and Death," published by G. P.
Putnam's Sons, 1908.
IO
HISTOLOGY
tions. As a result of their own protoplasmic activity, the cells of many
tissues become surrounded by intercellular substances, which may far
exceed in bulk the cells which produced them. Intercellular substances
may be solid or fluid. When present in small amount they form thin
layers of cement substance between closely adjacent cells; in large amount
these substances constitute a ground work in which the cells are imbedded,
as, for example, in cartilage and bone.
Although the differentiation of cells is chiefly cytoplasmic, there is
some evidence of corresponding nuclear changes. Thus while the muscle
cells of the salamander are elaborating complex fibrils, the nuclei
become modified as shown in Fig.
9. The significance of the nuclear
changes is unknown.
Degeneration is the manifestation of the approaching death of
the cell. In nerve cells this process
normally takes place very slowly.
These cells remain active throughout life, and if destroyed, they
can never be replaced. In many
glands, in the blood and in the
skin, however, the cells are constantly dying and new ones are
being differentiated. In a few
organs the cells perish, but no new
ones form, so that the organ to
which they belong atrophies.
Thus a large part of the mesonephros (Wolffian body) disappears during embryonic life; the
thymus becomes vestigial in the
adult; and the ovary in later years loses its chief function through the
degeneration of its cells.
The optical effects of degeneration cannot at present be properly classified. In a characteristic form, known as "cloudy swelling," the cell enlarges, becoming pale and opaque. In another form the cell shrinks and
stains deeply, becoming either irregularly granular or homogeneous and
hyaline. The nucleus may disappear as if in solution (karyolysis, chromatolysis) ; or it may become densely shrunken or pycnotic, and finally break
into fragments and be scattered through the protoplasm (karyorhexis).
If the process of degeneration is slow, the cell may divide by amitosis.
It may be able to receive nutriment which it cannot assimilate, and thus
FIG. 9. NUCLEI OF STRIATED MUSCLE FIBERS FROM
YOUNG SALAMANDERS (NECTURUS). (Eycleshymer.)
A, From a 7 mm. embryo; B, from one of 26 mm.; ch,
chroma tin knot; g. s, ground substance; 1, linin
fibril; n, nucleolus; n.m, nuclear membrane.
CYTOMORPHOSIS 1 1
its protoplasm may be infiltrated with fat and appear vacuolated. It
may form abnormal intercellular substances, for example, amyloid; or
the existing intercellular substances may become changed to mucoid
masses, or have lime salts deposited in them. Thus an impairment or
perversion of function is often associated with optical changes in the cell
substance.
The removal of dead cells is accomplished in several ways. Those
near the external or internal surfaces of the body are usually shed or desquamated, and such cells may be found in the saliva and urine. Those
which are within the body may be dissolved by chemical action or devoured by phagocytes.
Every specimen of human tissue exhibits some phase of cytomorphosis.
In some sections a series of cells may be observed from those but slightly
differentiated, to those which are dead and in process of removal. Because
of the similarity and possible identity of this normal "physiological"
regression, with that found in diseased tissues, such specimens should be
studied with particular care.
VITAL PHENOMENA.
The vital properties of cells are fully treated in text-books of physiology. They include the phenomena of irritability, metabolism, contractility, conductivity, and reproduction. Under irritability may be
grouped the response of cells to stimuli of various sorts, such as heat, light,
electricity, chemical reagents, the
*' V, 1 2 2,
nervous impulse, or mechanical inter
tt**^ f^'f-t *-!.$ gL A cm
ference. Metabolism, in a wide sense, ,yp> i^L %M.
^ ^Vj" ' ^"* WT&f~
includes the ingestion and assimilation of food, the elaboration and
secretion of desirable products, together with the elimination of waste 3 "*
.!. t, FIG. 10. LEUCOCYTES OF A PROG. * au.
products. Contractility may be Changes in form observed during ten minutes;
r ,v i r 4.v,_ o, at the beginning of the observation;
manifest m the locomotion Of the J.a half minute later, etc.
entire cell, in the vibratile action of
slender hair-like processes, the cilia, or in contraction of the cell body.
Conductivity is the power of conveying impulses from one part of the
cell to another. Reproduction is seen in the process of cell division.
Many phases of these activities are observed in microscopic sections and
as such they will be referred to in later chapters. A few which are of
general occurrence will be described presently.
AMCEBOID MOTION.
The unicellular animal, Amoeba, exhibits a type of motility known
as amoeboid, which has been observed in many sorts of cells in the verte
12 HISTOLOGY
brate body. In marked cases, as in certain white blood corpuscles (the
leucocytes), the cell protoplasm sends out fine or coarse processes which
divide or fuse with one another, causing the cell to assume a great variety
of forms. The processes may be retracted, or they may become attached
somewhere and draw the remainder of the cell body after them, the result
of which is locomotion or the so-called wandering of the cell. Such wandering cells play an important part in the economy of the animal body.
Their processes can flow around granules or cells and thus enclose them
in protoplasm. Some of these ingested bodies may be assimilated by
the cell as a result of complex chemical and osmotic reactions. Cells
which feed on foreign particles and can alter or digest them are known as
phagocytes. Amoeboid movements take place very slowly. In preparations from warm-blooded animals they may be accelerated by gently
heating the object.
Another form of motion is that which occurs within the protoplasm
of fresh cells, whether living or dead, and consists in a rapid oscillation
of minute granules, due to diffusion currents. Although these movements
were first observed within protoplasm, it was soon shown that they occurred when various inert particles were suspended in a liquid. Robert
Brown described the motion in 1828, in an essay entitled "On the General
Existence of Active Molecules in Organic and Inorganic Bodies," and
the phenomenon is called the molecular or Brownian movement. It
may often be seen in salivary corpuscles.
FORMATION AND REPRODUCTION OF CELLS.
In the past, two sorts of cell formation have been recognized, namely
the spontaneous generation of cells, and the origin of cells through the
division of pre-existing cells. According to the theory of spontaneous
generation it was once thought that animals as highly organized as intestinal worms came into existence from the fermentation of the intestinal
contents. After this had been disproved, it was still thought that unicellular animals arose spontaneously and that cells might be formed
directly from a suitable fluid, the cytoblastema. Something of the sort
may have occurred when life began, and it is the expectation of certain
investigators that conditions may yet be produced which shall lead to
the formation of organic bodies capable of growth and reproduction. At
present, however, only one source of cells is recognized the division of
existing cells. "Omnis cellula e cellula." A nucleus likewise can arise
only by the division of an existing nucleus; it cannot be formed from nonnucleated protoplasm.
DIRECT DIVISION 13
AMITOSIS.
The simplest form of cell division is one which rarely occurs. Ordinarily the division of the cell is accompanied with the production of protoplasmic filaments, and the process is therefore called mitosis (Greek, /uVos,
a thread). But in direct division or amitosis these filaments are not
developed. The nucleus merely becomes increasingly constricted at
the middle until divided in two; or it may be bisected by a deep cleft or
fissure. Preceding the division of the nucleus, the nucleolus, if present,
may subdivide and supply each half of the nucleus with a nucleolus (Fig.
n). Cells which divide by this method are usually degenerating, and
X /
Beginning Completed Beginning Completed
Division of the nucleolus. Division of the nucleus.
FIG. ii. AMITOSIS ix EPITHELIAL CELLS FROM THE BLADDER OF A MOUSE. Xs6o.
Such preparations as that shown in the figure are made by pressing the lining of a freshly obtained
piece of the bladder against a clean cover-glass. Certain of the superficial cells adhere to it, and they are
then fixed and stained.
the process may terminate with the multiplication of nuclei. If carried
to completion, the protoplasm also divides, and a cell membrane develops
between the daughter nuclei. The role of the centrosome in amitosis
has not been determined. Maximow finds it in a passive condition
between the two halves of the nucleus, or beside the stalk connecting
these halves if the division is not complete (Anat. Anz., 1908, vol. 33, p.
89). He states that certain mesenchymal cells which divide by amitosis
in the rabbit embryo are not degenerating, but may later divide by mitosis, and thus he confirms Patterson's similar conclusion in regard to certain cells in the pigeon's egg. These instances are regarded as exceptional.
In the human body the detachment of a portion of the lobate nucleus
of certain leucocytes has been described as amitotic division, but the
superficial cells of the bladder furnish more typical examples. E. F.
Clark has found many cells dividing by amitosis hi the degenerating parts
of a human cancer. The occurrence of two nuclei within one cell by no
means indicates this form of division. Associated with such cells, others
containing nuclei of the dumb-bell shape, or those partially bisected
by clefts must be found, in order to prove that amitotic division is taking
place.
14 HISTOLOGY
MITOSIS.
Mitosis, also called indirect division and karyokinesis, is the ordinary
mode of cell division. Although it is a continuous process, it has been
conveniently divided into four successive phases the prophase, metaphase,
anaphase, and telo phase. During the prophase the chromatic material
of the nucleus prepares for division and collects in the center of the cell.
It is divided in halves in the metaphase, and the two halves move apart
during the anaphase. The chromatic material becomes reconstructed
into resting nuclei during the telophase. The various patterns which the
chromatic material and protoplasmic fibrils present during these phases
are known as mitotic figures.
Mitotic figures are found hi all rapidly growing tissues, but especially
favorable for preliminary study are the large cells in the root tips of plants.
In longitudinal sections of root tips, the cells are cut at right angles to
the plane of cell division, which is desirable; and often in a single section
5 mm. long, all the fundamental stages may be quickly located. The
following general description of mitosis is based upon such easily obtained preparations, and the plant selected is the spiderwort (Tradescantia virginiana). 1 They may be satisfactorily stained with saffranin,
or with iron haematoxylin and a counter stain such as orange G. There
are many descriptions of mitosis in root tips, among them the following:
Rosen, (Hyacinthus oriental/is) Beitr. zur Biol. der Pflanzen, 1895,
vol. 7, pp. 225-312; Nemec, (Allium cepa} Sitz.-ber. kon. Ges. der Wiss.
Prag, 1897, No. 33, pp. 25-26, and Jahrb. fur wiss. Bot., 1899, vol. 33, pp.
313-336; Schaffner, (Allium cepa) Bot. Gaz., 1898, vol. 26, pp. 225-238;
Hof, (Ephedra major) Bot. Centralbl., 1898, vol. 76, pp. 63-69, 113-118,
166-171, 221-226; Gregoire and Wygaerts, (Trillium grandiflorum) La
Cellule, 1904, vol. 21, pp. 1-76; Farmer and Shove, (Tradescentia mrginiana) Quart. Journ. Micr. Sci., 1905, vol. 48, pp. 559-569; Richards,
(Podophyllum peltatum] Kansas Univ. Sci. Bull., 1909, vol. 5, p. 87-93.
The cells to be described are found in the interior of the root tip, just
back of the protecting cap of cells which covers its extremity. They are
oblong in shape and their long axis corresponds with that of the root.
The walls are very distinct, and the cells consist of granular vacuolated
protoplasm, which in preserved specimens is generally irregularly shrunken.
The resting cells (Fig. 12, A) contain large round nuclei in which the
chromatin is in the form of fine granules evenly distributed throughout
the nucleus. A nucleus usually contains from two to five round nucleoli,
each of which, when in focus, is seen to be surrounded by a clear zone.
The nuclear membrane is distinct.
1 Good specimens may be obtained from any rapidly growing root tip. Those starting from
hyacinth bulbs placed in water are very favorable. Onion root tips have been extensively
used, and also those of bean and corn seedlings. The pointed ends are snipped off and dropped
into Flemming's stronger solution
INDIRECT DIVISION 15
Prophase. The first indication of approaching division is a change
in the chromatin, which becomes gathered into fewer and coarser granules
and takes a deeper stain. Portions of the linin network break down, so
that the chromatin granules come to be arranged in long convoluted
threads. Such threads are developing in the cell, Fig. 12, B, but are
more perfectly formed in C. It is possible that at a certain stage the
nucleus contains only a single continuous thread, but this condition cannot be demonstrated in Tradescantia. The stage of nuclear division in
which the chromatic material appears to be arranged in a coiled thread
or skein is called a spireme. The "close spireme" (B) is succeeded by
the "loose spireme" (C). Successive stages in the development of the
spireme in animal cells are seen in Fig. 20, D, E, and F.
As the spireme develops, the nuclear membrane becomes less distinct,
and the clear zones disappear from around the nucleoli. The nucleoli
become apparently less regular in outline, and forms which suggest that
two of them have fused (Fig. 12, B) are perhaps more frequently seen
than in resting cells. Usually it is stated that the nucleoli break up into
smaller bodies toward the time of their dissolution, and that some of
these escape into the cytoplasm after the disappearance of the nuclear
membrane. Farmer and Shove believe that the nucleoli contribute to
the chromatin; Richards regards them as a store of food material for the
rest of the cell; and others believe that they form the achromatic
"spindle" which will be described presently. Their function in animal
cells is equally uncertain.
In the stage shown in Fig. 12, D, which may be regarded as the end
of the prophase, the nuclear membrane and the nucleoli have disappeared,
and the spireme thread has become divided into a number of segments
or chromosomes. These are straight or curved rods of different lengths.
Sometimes they appear as bent V-shaped bodies, but these often represent
two chromosomes with their ends together. J-shaped forms, with one long
and one short arm, have been described in various plants. The chromosomes become so arranged that one end of the rods, or the apices of the V's,
are situated in the equatorial plane, which extends transversely across the
middle of the cell. Often it is temporarily tilted (as in D and E) as if the
mitotic apparatus had shifted to a position in which it obtained more
space. It may do this mechanically if the contents of the cell are under
pressure. When the chromosomes are gathered at or in the equatorial
plane, they constitute collectively the equatorial plate. Because of their
stellate arrangement at this stage, which is best seen in transverse sections
of the cell, this mitotic figure is known as the aster.
The manner in which the chromosomes are formed from the spireme
thread is difficult to determine. According to Gregoire and Wygaerts,
the linin and chromatin, which have often been regarded as closely related
i6
HISTOLOGY
substances, are identical, and linin is merely a name for slender filaments
of chromatin. Accordingly the chromatin simply draws together to
FIG. 12. MITOTIC CELL DIVISION IN THE ROOT TIP OF Tradescanliavirginiana. Xias diam.
A, resting cell; B, C, D, prophase; E, metaphase; F, anaphase; G, H, I, telophase.
form chromosomes, and the beaded appearance of the spireme thread is
due to alternate enlargements and constrictions of one substance. Others
consider that a different substance connects the granules of chromatin
INDIRECT DIVISION
with one another; and Rosen states that each chromatin granule is completely imbedded in a broad strand of linin. Davis similarly interprets
the spireme shown in Fig. 20, F. Whatever the actual structure may be,
the chromatin granules in the spireme thread early divide in two, so that
the thread appears double. When the thread shortens and condenses to
form the chromosomes, the rows of granules may coalesce so as to produce a rod already divided lengthwise, although its halves are in close
apposition. Occasionally the ends of the chromosomes are seen to be
slightly separated.
Metaphase. In the metaphase (Fig. 12, E), the two longitudinal
halves of each chromosome are being drawn apart toward the opposite
poles of the cell. If the chromosome is V-shaped, the separation of the
two halves begins at the apex of the V.
At this stage an achromatic figure, known as the spindle, is evident
in plant cells, but it is more sharply defined in those of animals. As seen
in the diagram (Fig. 13), it consists of fibrils which pass from the equatoria
Polar radiation. Nuclear spindle.
FIG, 13. EARLY METAPHASE.
FIG. 14. LATE METAPHASE.
plate toward either pole, where, in animal cells, there is a well-defined
granule, the centrosome. Around each centrosome there are radiating
protoplasmic fibrils, forming the polar radiation (Figs. 13 and 14). The
polar radiation is also called an aster, and the two asters connected by
the spindle are known as the amphiaster. Some of the spindle fibers
are attached to the chromosomes and appear to pull their halves apart;
others pass from pole to pole without connecting with the chromosomes.
In animal cells the spindle arises as the two centrosomes, lying beside
the nucleus, move apart (Fig. 20, A). As they pass to the opposite poles
of the nucleus, the spindle forms between them, either from the nuclear
reticulum, or the cytoplasmic reticulum, or hi part from both. These
conditions appear to vary in different animals.
In the cells of root tips, a condensation of protoplasm forms a cap at
the poles of the nucleus at the time when the nuclear membrane and
nucleoli are disappearing. From the "polar cap," spindle fibers develop
l8 HISTOLOGY
which invade the nucleus, and also radiations which have been traced
even to the cell walls. But as Rosen states, sun-like figures, such as certain botanists have pictured, do not occur. Schaffner has described a
distinct centrosome or central granule in the root tip of the onion, but
Richards finds that in Podophyllum there is no such structure, and the
weight of evidence appears to be against the existence of a definite centrosome in the higher plants.
Anaphase. In the anaphase the halves of each chromosome move to
the opposite poles (Fig. 12, F). The figure thus produced is known as a
double star or diaster. Since each chromosome has divided into two,
the original number of chromosomes is preserved, and an equal number
of rods will be found in either star. They cannot all be brought into
focus together, and because of overlapping, they are hard to count.
Sometimes one chromosome, longer than the others, remains for a time
as a continuous bar from one aster to the other. Between the asters
there are always straight spindle fibers, but they vary in distinctness.
(The anaphase in an animal cell is well shown in Fig. 21, D.)
Telophase. After the chromosomes have reached the opposite poles,
they form two dense masses. They are generally said to unite end to
end, thus forming a spireme thread. But in the root tips of Trillium,
Gregoire and Wygaerts state that they come into contact with one another
laterally; and as they separate, transverse connections are retained,
which, with the vacuolization of the chromosomes, restore the nuclear
reticulum. This may not be the correct interpretation, but immediately
after the anaphase the chromosomes form a very compact mass, easily
overstained so that it appears solid. Subsequently the mass enlarges
(Fig. 12, H), and the chromosomes become coarsely granular, taking the
form of wide bands. Nucleoli reappear, and according to Richards,
"it is a general rule that they arise on the side of the nucleus nearest the
new cell wall." This accords with Nemec's statement that they form
from the outer fibers of the spindle. Nemec and Rosen agree that they
first appear outside of the nucleus, which they enter before the nuclear
membrane develops. These are details which require confirmation.
The new cell wall arises in plants as a series of thickenings of the
interzonal spindle fibers, which at this stage form a barrel-shaped bundle
(Fig. 12, G). The thickenings coalesce to form a membrane which does
not at first reach the sides of the cell. While this wall is developing the
nuclei are in a condition resembling the spireme stage of the prophase.
The entire mitotic figure is therefore called the double spireme or dispireme. The cell wall is soon completed and the nuclei return to the
resting condition (Fig. 12, I).
The time required for mitotic cell division varies from half an hour
(in man) to five hours (in amphibia). After death, if the tissues are not
INDIRECT DIVISION 19
hardened by cold or reagents, it is thought that mitoses go on to completion. Forty-eight hours may elapse before they entirely disappear from
the human body.
Pluri-polar mitosis. Under abnormal conditions, as in the cancer
cells shown in Fig. 15, spindles may develop simultaneously in connection
with three or four centrosomes. Similar pluri-polar spindles have been
produced experimentally, by treating cells with various poisonous solutions. An unequal distribution of chromatin may occur under such
conditions, and this may happen also with bipolar spindles, as shown in
Fig. 15, a.
Number and individuality of the chromosomes. It is now generally
believed that every species of plant or animal has a fixed and characteristic number of chromosomes, which regularly recurs in the division of all
FIG. 15. MITOSES IN HUMAN CANCER CELLS. (From Wilson, after- Galeotti.)
a, Asymmetrical mitosis with unequal distribution of chromatin; b, tripolar mitosis; c, quadripolar" mitosis.
its cells, with the exception of the germ cells, in which the number is
reduced. In certain species, however, the two sexes regularly differ from
one another in the number of their chromosomes, and one sex may contain an odd number. Usually the number of chromosomes is believed to
be even.
There is considerable difficulty in counting the chromosomes. Generally it is possible that some have been cut away in the process of sectioning, so that, if the number is believed to be invariable, the highest number
found in any cell is assumed to occur regularly. Another source of error
lies in the fact that a bent chromosome may be counted as two, or rods
with their ends overlapping may appear as one. Farmer and Shove
have ventured to state that the number in Tradescantia "varies from
about twenty-six to thirty- three." Nemec found that twelve chromosomes occur regularly in young tissues of the onion, but that in older
tissues the number diminishes even to four. Sixteen have been recorded
in the onion by other botanists. Podophyllum is said to have sixteen
(Mottier), but Richards records counts of fourteen. In man the number
has been placed at 16 and 32, but it is now believed to be 24. Gutherz,
with particularly favorable material, emphasizes the difficulty of counting
20 HISTOLOGY
the chromosomes in man. He found only two cells in which a count
could be made, in neither case with absolute certainty. But he agrees
with Duesberg that the reduced number is twelve, according to which
the whole number should be twenty-four. Recently, however, Wieman
has found cells in the brain of a 9-mm. human embryo which contained 33
chromosomes. Some cells in the nasal epithelium and mesenchyma of
this specimen contained 34, and others 38. Thus Wieman concludes
that the number in man is certainly greater than 24 and is perhaps variable (Amer. Journ. Anat., 1913, vol. 14, pp. 416-471).
In the grasshoppers, which are among the most favorable objects for
the study of mitosis, not only is the number of chromosomes for a given
species believed to be constant, but each cell appears to contain a definite
series of chromosomes, the members of which vary somewhat in shape
and size. Recent studies of such cells favor Rabl's hypothesis of the
individuality oj the chromosomes, according to which the chromosomes
persist in the resting nucleus, although disguised by their lateral branches
and diffuse granular form. If this hypothesis is correct, when a nucleus
prepares for division the same chromosomes which entered it will reappear.
Sometimes in the prophase the bands of chromatin are arranged hi a
polar field such as is seen in the telophase (Fig. 12, H). This arrangement has been observed by Farmer and Shove in the prophase of Tradescantia, and by others in various plants and animals. It is regarded as
evidence that the chromosomes are "independent and continuously
perpetuated organs of the cell." Nevertheless it is generally true that
in resting nuclei no trace of individual chromosomes can be made out.
The great importance of accurate knowledge of the chromosomes is shown
by the following considerations.
As a result of mitotic cell division, it is evident that every new cell
regularly receives one-half of each chromosome found in the parent cell, and
thus the number of chromosomes remains constant. But in the germ
cells the number is invariably reduced, and hi some animals it becomes
exactly one-half of the number found elsewhere in the body. In such a
case, when the male sexual cell, or spermatozoon, unites with the female
sexual cell, or mature ovum, in the process of fertilization, the original
number is restored. Each parent thus contributes one-half of the chromosomes found in the cell which gives rise to a new individual; and since
each of these divides with every subsequent cell division, it is evident
that one-half of the chromatin in every cell of the adult body is of maternal origin and one-half of paternal origin. The process by which the
sexual cells acquire the reduced number of chromosomes and become
ready for fertilization is known as maturation. The production of the
sexual cells in the male is called spermatogenesis and in the female
ob'genesis.
SPERMATOGENESIS
21
FIG. 16. DORSAL (a) AND
LATERAL (b) VIEWS
OF THE ABDOMEN OF
GRASSHOPPERS. (After
Hyatt and Scudder.)
SPERMATOGENESIS .
In its essential features, the process of spermatogenesis in insects
corresponds with that in mammals, and very favorable material can be
obtained in abundance from grasshoppers of various
genera.
The males may be distinguished from the females by the
shape of the abdomen. In males it is more rounded (Fig. 16)
with various appendages directed dorsally. The abdomen
of the female is pointed, terminating in the ovipositor, the
parts of which as seen from the side may be together, or
widely separated dorso-ventrally. The genital glands can be
readily removed by dissecting as follows: Male grasshoppers,
which have been chloroformed, are opened by a mid-ventral
incision. The abdominal walls are pinned out on a wax plate
under normal salt solution (0.6 per cent.). The intestinal
tube, which is usually black or green, is taken out with forceps,
and the yellow or orange testes are seen close together at the
upper end of the abdomen, attached to the back. Each testis
consists of a number of separate cylindrical lobes, and it should
be worked loose from the surrounding tissue with forceps in
such a way that these lobes remain together. The tissue
may be preserved in Flemming's strong solution or in Hermann's fluid, and stained with iron haematoxylin.
Among the many publications upon spermatogenesis
in the grasshoppers, the following may be cited: McClung,
C. E., The accessory chromosome sex determinant? Biol.
Bull., 1902, vol. 3, pp. 43-84; Sutton, W. S., On the morphology of the chromosome group in Brachystola magna, Biol.
Bull., 1902, vol. 4, pp. 24-39; McClung, C. E., The chromosome complex of orthopteran spermatocytes, Biol. Bull.,
1905, vol. 9, pp. 304-340; Robertson, W. R. B., The chromosome'complex of Syrbula admirabilis, Kansas Univ. Sci. Bull.,
1908, vol. 4, pp. 273-305; Davis, H. S., Spermatogenesis in
Acrididae and Locustidae, Bull. Mus. Comp. Zool., 1908, vol.
53> PP- 57-IS7; Wilson, E. B., The sex chromosomes, Arch,
fur mikr. Anat., 1911, vol. 77, pp. 249-371.
f As seen in sections, each lobe of the testis of the
grasshopper contains a considerable number of closed
sacs or cysts, which are filled with sexual cells; and
all the cells within a cyst are in approximately the
same stage of development. The cysts are shown
in Fig. 17, which represents a longitudinal section
of a single lobe. Usually the testis is sectioned as a
whole, and the specimen consists of a group of lobes
cut transversely or obliquely. Cross sections from the apical portion,
furthest from the outlet, will contain younger stages than the sections
d
FIG. 17. LOBE OF THE
TESTIS OF A GRASSHOPPER. Xso. (After
Davis.)
a, apical cell.
b, spermatogonia.
c, spermatocytes.
d, spermatocytes dividing.
e, spermatids.
f, spermatozoa.
22
HISTOLOGY
Spermatogonia
lower down in the lobe, since the cysts form at the apex and gradually
move downward. At the apex, according to Davis, there is an apical
cell which is surrounded by young sexual cells known as spermatogonia
(Fig. 17, a). The spermatogonia move away from the apical cell, and
each becomes enclosed in a cyst-wall derived from the surrounding
tissue. Within the cysts thus formed, the spermatogonia multiply, and
the cysts in the upper part of the lobe are filled with spermatogonia
(Fig. 17, b). After repeated divisions the spermatogonia pass through a
period of growth, accompanied by a rearrangement of their nuclear contents. The large cells with characteristic nuclei which are thus produced,
are known as primary spermatocytes. They fill the cysts further down
in the lobe (Fig. 17, c). Each primary spermatocyte divides into two
secondary spermatocytes, and
each of these divides into
two spermatids, after which
no further cell division is
possible until fertilization
takes place. But each spermatid becomes transformed
from a round cell into a linear
body with a whip-like tail,
and is then capable of independent motion. Since in
this form these cells were
once thought to be parasitic
animals living in the spermatic fluid, they received the
name spermatozoa, which they
still retain. 1 Cysts containing spermatozoa occur near the outlet of the lobe, or if the grasshoppers
are collected late in the season, they may be found throughout most of the
testis. Specimens from young grasshoppers, in which the spermatocyte
divisions are abundant, are more desirable, even though no spermatozoa
have become fully developed.
The succession of cell divisions described in the preceding paragraph
is shown in tabular form in Fig. 18. Except for the number of chromosomes within the various cells, this diagram is quite as applicable to man
as to the grasshopper. In this figure, however, only two spermatogonial
divisions have been included. The number of times which the spermatogonia may divide before becoming spermatocytes is considerable and
1 It has been proposed to substitute the term spermium for spermatozoon; and consequently
spermiocyte, spermid, etc., for spermatocyte and spermatid. The secondary spermatocytes are
sometimes called praespermatids or praespermids; but these changes in names are of questionable
value
Secondary Spermatocytes
Spermatozoa
FIG. 1 8. DIAGRAM OF THE CELL DIVISIONS IN SPERMATOGENESIS. The figures indicate the number of chromosomes found in the cells of certain grasshoppers.
SPERMATOGENESIS 23
presumably indefinite. As seen in sections, the spermatogonia, spermatocytes, and spermatids may be described as follows, using for illustrations Davis's figures of a common grasshopper Dissosteira Carolina.
Spermatogonia. The nucleus of each spermatogonium contains the full
number of chromosomes, which in most of the grasshoppers (Acrididee)
is 23. With every spermatogonial division, each chromosome is split
lengthwise. In this and other respects the mitotic figures are quite like
those occurring elsewhere in the body. They are shown in Fig. 20, A,
B, and C. When the twenty- three chromosomes have formed the equatorial plate, it is sometimes possible to see all of them in a single transverse section of the cell (Fig. 19, A). It then appears, as found by Montgomery (1901) in certain Hemiptera, and a year later by Button in grasshoppers, that the chromosomes vary in size, but the "gradations in volume
are not between individual chromosomes but between pairs, the two
members of which are of approximately equal size." In Fig. 19, A, twelve
forms of chromosomes have been identified by Davis; and all of these are
paired except the one numbered 4. The members of a pair are often, but
by no means invariably, side by side. In some cases, owing to foreshortening, their resemblance in size is not apparent in the drawing. The behavior of the odd or accessory chromosome is of special interest, since according to McClung's hypothesis, now well established, this accessory chromosome is the bearer of those qualities which determine sex.
Primary spermatocytes. After the last spermatogonial division, the
cells begin their "growth period." At this time the chromatin tends to
collect on one side of the nucleus, in a condition known as synapsis (or
more recently as synizesis). This distribution of the chromatin has been
frequently observed, but it has not been shown to be of special significance. In the primary spermatocytes drawn in Fig. 20, D, E, and F, the
chromatin is evenly distributed. All of the chromosomes, except the
accessory chromosome, have become filamentous, but the accessory chromosome remains as a compact, darkly staining body close to the nuclear
membrane. It resembles a nucleolus, for which in fact it has been mistaken. True nucleoli may occur in these cells, together with the accessory chromosome, but they stain differently.
As the primary spermatocytes prepare for the next division, the spireme becomes resolved into eleven loops, each of which represents the two
members of a pair of chromosomes joined end to end. The granules imbedded in the linin thread divide as usual, so that each loop contains a
double row of granules (Fig. 20, F). These loops contract to form eleven
chromosomes, which, because of their four parts, are known as tetrads.
The structure of the tetrads is shown in Fig. 19, B-G. The filaments seen
in the upper row of drawings contract into corresponding solid forms of
24 HISTOLOGY
chromosomes seen in the lower row, in which the place of attachment to
the spindle fibers has been indicated.
Each tetrad represents two chromosomes joined end to end and split
lengthwise. The simplest forms are shown in Fig. 19, B and C, which
illustrate respectively two ways in which the tetrad may later divide.
The two component chromosomes may simply be pulled apart, as indicated in Fig, 19, B, in which the spindle fibers are attached to the ends of
the rod. If this takes place, each secondary spermatocyte will receive
one member of every pair of chromosomes which occurred in the spermatogonium, but no part of the other member. Such a division, which
eliminates one-half of the chromosomes from the daughter cell, is known
as a reductional division. The other form of chromosome division is
known as equational. When it takes place, every chromosome divides
lengthwise, and the daughter cells receive one-half of every chromosome in
the parent cell. This occurs in ordinary cell division, and also in the division of the tetrads provided that the spindle fibers are attached to the
place where the two component chromosomes come together (Fig. 19, C).
FIG. 19. A, POLAR VIEW OF THE METAPHASE OF A SPERMATOGENIAL DIVISION IN Dissosteira Carolina.
X 1450 (After Davis.) The pairs of chromosomes have been numbered. B-G, various forms of tetrads,
rom primary spermatocytes. (After Davis and Robertson.)
As a stage in the separation of the two halves of a rod-shaped tetrad, crossshaped forms are produced (Fig. 19, D). If the separation is almost
complete, such shapes are seen as in Fig. 19, E. The arms of the tetrad
which are not attached to the spindle fibers may bend toward one another
and unite, so as to form rings (F), or they may twist about like a figure
8, as shown in G. If the spindle fibers are attached to the points xx in the
upper figure in G, the division will be equational; if as shown in the lower
figure it will be reductional.
Usually it is considered that the division of the tetrads into double
bodies or dyads, is equational, and that the division of the dyads, which
takes place when the secondary spermatocytes divide, is reductional.
According to Davis, however, the first division of the tetrads is reductional
and the second division is equational. In either case the end-result is the
same. Each spermatid will contain one of the four parts of each tetrad,
and thus one member of every pair of chromosomes will be eliminated from
any given spermatid.
SPERMATOGENESIS
Since in the testis tetrads occur only in the primary spermatocytes,
the cells shown in Fig. 20, G-J, are easily identified. These are success
F
FIG. 30. SPERMATOGENESIS IN Dissosleira Carolina A-FXI4SO; G-LX966. (Davis.)
A, B, C, prophase. metaphase. and telophase of a spennatogonial division. D-L, successive stages in the
.division of a primary spermatocyte into secondary spermatocytes.
ive stages in the division of the primary spermatocyte. In G the accessory
chromosome is seen as a rod-shaped body above and to the right; in H it is
below and to the right. In J it is obliquely placed just above the equatorial
26 HISTOLOGY
plate and in K it is passing to the upper pole of the spindle. In the spermatogonial divisions the accessory chromosome always divides with the
others; but in the division of the primary spermatocyte it passes undivided into one of the daughter cells. Thus one secondary spermatocyte
will receive eleven chromosomes (dyads) and the other will receive twelve
(eleven dyads and the accessory chromosome) . In the late anaphase shown
in Fig. 20, L, the accessory chromosome cannot be recognized.
Secondary spermatocytes. The secondary spermatocytes pass rapidly
from the condition shown in Fig. 20, L, to that of Fig. 21, A. A nuclear
membrane has developed, and the dyads have become somewhat filamentous. Without passing through a complete resting stage they proceed to
divide as shown in Fig. 21, B-F. The dyads separate into their component halves. In those secondary spermatocytes which received the
accessory chromosome, that body will be seen dividing with the dyads, and
each spermatid will receive one-half of it. It has been questioned whether
the division of the accessory chromosome is longitudinal and therefore equational, or transverse and reductional. Many cytologists
consider that if a chromosome splits lengthwise, all of its parts will be
represented in the resulting halves, but if it divides transversely, essential
elements will be lost. This conception lends importance to the question
of transverse or longitudinal division of the accessory chromosome. By
the division of this chromosome it comes about that one-half of the spermatids contain twelve chromosomes, and one-half contain eleven, as indicated in the diagram, Fig. 18. The spermatids shown in Fig. 21, F, contain the accessory chromosome.
Spermatids and Spermatozoa. In forming spermatozoa, the spermatids
become elongated, passing from the condition shown in Fig. 21, F, to that
of Fig. 21, G. The chromatin within the nucleus is distributed in fine
granules throughout the linin reticulum. Close to the nuclear membrane a small dark body has appeared, from which a slender filament has
grown out. This body is usually described as the centrosome. A condensation within the cytoplasm, seen also in F, is known as the paranucleus.
It is of uncertain origin, but may proceed from the cytoplasmic structure
called mitochondrium. The paranucleus forms a sheath about the axial
filament.
Successively later stages are shown in Fig. 21, H, I, and J. The chromatin within the nucleus becomes homogeneous and very dense; at the
same time the nucleus elongates and forms the head of the spermatozoon.
This is enveloped by the cell membrane, but there is no appreciable layer
of protoplasm around it. The centrosome elongates and forms the middle
piece of the spermatozoon; and the axial filament, with a covering derived
from the paranucleus and cytoplasm, constitutes the tail. Only a portion
of the tail is included in the figure. The human spermatozoon likewise
SPERMATOGENESIS
2 7
consists of a head, which is essentially the nucleus, a middle piece containing
the centrosome, and a tail; but the form of the head is very different from
FIG. 21. SPERMATOGENESIS IN Dissosteira Carolina. Xi4So. (Davis.)
A-F, successive stages in the division of a secondary spermatocyte into spermatids. G-J, successive stages
in the transformation of spermatids into spermatozoa.
that in the grasshopper. It will be described in a later chapter
Although the spermatozoa of the grasshopper appear alike, it has been
shown that one-half of them contain eleven chromosomes, and one-half
28 HISTOLOGY
contain twelve. The mature ova all contain twelve chromosomes. If a
spermatozoon with eleven chromosomes unites with an ovum with twelve,
a male animal will be produced, in every cell of which there will be twentythree chromosomes. But if the spermatozoon contains twelve chromosomes, a female animal is formed, containing twenty-four chromosomes
in every cell. Thus sex appears to be determined by the presence or
absence of a chromosome within the spermatozoon.
In some cases, as in several Hemiptera described by Wilson, the accessory chromosome is paired, but its mate is of small size. Thus the spermatozoa all have the same number of chromosomes; but half of them contain
the large member of the pair and will produce females, and the other half
contain the small member and will produce males. The mature ova all
contain the large member. In tfyese insects, therefore, both sexes contain the same number of chromosomes, but the cells of the male contain
a small chromosome, whereas the corresponding one in the female is
large. From these observations it is reasonable to conclude that sex may
be determined by a difference in the nature of certain chromosomes in
those animals in which there are no appreciable differences in size or
number.
In man, a difference in the number of chromosomes in the sexes has
been reported, but the observations have not been confirmed. It is supposed that the spermatogonia contain twenty-four chromosomes, but it
has not been shown that they exist as pairs. The spermatocytes, spermatids and spermatozoa apparently contain twelve. As the principal constituents of the spermatozoon, the chromosomes are believed to be the
essential agents in the transmission of all qualities inherited from the male
parent, and certain of them may determine sex.
OtfGENESIS.
Mature ova result from a succession of cell divisions closely comparable
with those which produce spermatozoa. The primitive female sexual
cells correspond with the spermatogonia, and are called oogonia. They
are provided with the full number of chromosomes, and divide an indefinite number of times. After a period of growth they become primary
oocytes, in which the number of chromosomes is reduced one-half. The
primary oocytes divide to form secondary oocytes; and these again divide
to produce the mature ova, which are incapable of further division
unless fertilization takes place. (The term ovum is ordinarily loosely
applied, so that it includes not only the mature cells, but also oocytes,
and the clusters of cells resulting from the division of the fertilized ovum.)
Although the mature ovum and the spermatozoon are closely similar
OOGENESIS
2 9
in their nuclear constitution, they differ radically as to form, size, and cytoplasmic structure. The ova are very large cells, stored with nutriment
for the embryo which each one may later produce. In the higher vertebrates they are formed in relatively small numbers. According to Hensen's estimate, about two hundred, ready for fertilization, are produced
by the human female in a life-time. But the male, according to Lode,
produces 340 billion spermatozoa, or, as stated by Waldeyer, nearly 850
million per ovum. A large number must be produced, since many will
fail to traverse the uterus and tube so as to find the ovum at the time of
its discharge from the ovary. The ova of lower vertebrates, which
are fertilized and develop outside of the body, are discharged in great
numbers; in certain fishes from three to four million are produced
annually.
The multiplication of oogonia in the human ovary takes place before
birth, and about fifty thousand are produced. At birth, or shortly thereafter, all the oogonia have become primary ob'cytes (Keibel). At first the
oocytes are small, but they enlarge at varying rates, and the largest are
indistinguishable from mature ova except by their nuclear contents. Since
some grow more rapidly than others, the ovary in childhood contains
primary oocytes of many sizes. Each oocyte becomes enclosed in a cyst
or follicle. The way in which these follicles develop, and the. manner in
which the oocyte escapes into the uterine tube by the rupture of these
follicles, will be described in connection with the ovary. Between the cells
of the follicle and the oocyte, there is a broad, radially striated membrane,
known as the zona pellucida or zona radiata (Fig. 2 2) . This zona has sometimes been regarded as a cell membrane, but the oocyte divides within it as
if enclosed in a capsule. It does not invest the daughter cells like a membrane. The radial striations have been interpreted as slender canals
containing processes of the f ollicular cells, and the zona has been considered
as a product of these cells. In certain cases a perivitelline space has been
described as encircling the oocyte and thus separating it from the zona,
but this space has been considered as artificial, or as a refractive line
wrongly interpreted as a space.
The cytoplasm of the oocyte becomes charged with yolk granules or
spherules. They constitute the deuteroplasm (or deutoplasm), but this
term is equally applicable to fat droplets and other secondary products of
the protoplasm. In the human oocyte the granules are centrally placed
(Fig. 22), and they are so transparent, when fresh, as to cause only a slight
opacity. In the eggs of many animals the yolk is more highly developed,
and it may be evenly distributed or gathered at one pole. Within the
cytoplasm of the developing oocyte, a large dark body of radiate structure
is sometimes conspicuous. It is inappropriately known as the yolk nucleus,
and is probably a derivative of the centrosome and surrounding archo
30 HISTOLOGY
plasm. Other "vitelline bodies," of uncertain origin and significance,
have been described. Some have been considered as nuclear extrusions.
The nucleus of the oocyte is very large and vesicular. The chromatin
occurs chiefly along the nuclear membrane and about the nucleolus. The
nucleous is also very large, and Nagel stated that in the fresh condition it
exhibits amoeboid movements, but this observation has not been verified.
The nuclei of the oocytes ordinarily show no signs of mitosis, and they
may remain in the resting condition for thirty years or more and then divide. Many of them, however, will degenerate without division.
zp
PIG. 22. THE OVUM AS DISCHARGED FROM A VESICULAR FOLLICLE OF AN EXCISED OVARY OF A WOMAN
THIRTY YEARS OF AGE. Examined fresh in the liquor folliculi. (Nagel.)
c. i., Corona radiata composed of cells of the follicle; n., nucleus; p., granular protoplasm; p. s., perivitelline space; y., yolk; z. p., zona pellucida. (From McMurrich's "Embryology.")
The cell divisions which give rise to the secondary oocyte and the mature ovum respectively, have never been observed in man. Some of the
cells within the ovary may be secondary oocytes and the cell shown in Fig.
22 may be of this sort, or possibly a mature ovum, but this cannot be determined. From what is known of other mammals, however, it may confidently be assumed that the cell divisions take place as shown in the diagram, Fig. 23.
When the primary oocyte divides, the chromosomes, reduced in number,
also divide and are equally distributed to the daughter cells; but the great
mass of cytoplasm remains with one of these cells, namely, the secondary
OOGENESIS 31
oocyte. The other cell, which is relatively very small, is known as the
first polar body, or polar cell. It has the same nuclear contents as the secondary oocyte, and may divide into two other polar bodies, equivalent to
mature ova. More often it degenerates without division. When the
secondary oocyte divides, it likewise produces one large cell, the mature
ovum, and one small cell, the second polar body. The latter is said to be capable of fertilization, but to what extent it may develop is unknown.
Functionally the production of polar bodies serves to prevent the subdivision and distribution of the nutritive material elaborated within the
primary oocyte. One mature ovum with abundant yolk is provided at
the expense of three ova (polar bodies) which degenerate.
Although the maturation of the ovum has not been observed in man,
nor even the presence of definite polar bodies, the entire process has been
Oogonia
Polar Bodies
Secondary Oocyte
Mature Ovum
FIG. 23. DIAGRAM OF THE CELL DIVISIONS IN OOGENESIS. (Compare with Fig. 18.)
carefully studied in other mammals, notably in the mouse. 1 It has been
shown that the maturation of the ovum of the mouse takes place rapidly,
both of the oocyte divisions being accomplished within from four to fifteen
hours. The first polar body usually forms before the oocyte is discharged
from the ovarian follicle in other words, before ovulation takes place.
The second polar body is usually formed in the uterine tube, after the spermatozoon has entered the oocyte. Long and Mark have found that the
chromosomes of the primary oocyte are tetrads, or bodies showing transverse and longitudinal divisions; and that those of the secondary oocyte are
dyads. They believe that the first division is transverse or reductional,
and that the second is equational.
1 Among the most important papers are: Sobotta, J., Die Befruchtung und Furchung des
Eies der Maus. Arch. mikr. Anat., 1895, vol. 45, pp. 15-91.
Long, J. A., and Mark, E. L. The maturation of the egg of the mouse. Carnegie Inst.
Publ. No. 142, 1911, pp. 1-72.
32 HISTOLOGY
The difficulty of counting chromosomes is apparent from the varying
numbers which have been reported in the mouse After reduction the
number has been placed at 8, 12, 16, 18 and 20 by different observers.
The polar bodies in the mouse are relatively large. In the upper part
of Fig. 24, A, a polar body is about to be formed, and it is completely cut
off from the oocyte in Fig. 24, C. In D and G, two polar bodies are shown.
FERTILIZATION.
In the mouse, from six to ten hours after coitus, spermatozoa have made
their way to the distal end of the uterine tube, where fertilization takes
place. According to Long and Mark, the maturation of ova usually occurs
at some time during the period from "13! to 28^ hours " after the mouse has
given birth to a litter; and during the process of their maturation, the
oocytes are discharged from the ovary and enter the distal end of the tube.
Here, if fertilization takes place, a single spermatozoon penetrates the zona
pellucida. In a section obtained by Sobotta, the entrance of the spermatozoon has been partially accomplished (Fig. 24, B). Its tail lies outside
of the zona, and appears to have become thickened. In another specimen
Sobotta found the head, middle piece and a part of the tail within the
cytoplasm of the oocyte. The tail had broken as it crossed the zona, and
the portion remaining outside had drawn together and was disintegrating.
In some animals it is said that the entire spermatozoon enters the ovum,
but in others only the head and middle piece. In any case the tail appears
to be a propelling apparatus which becomes functionless after the head
and middle piece have passed through the zona. It has entirely disappeared in the stage shown in Fig. 24, A, in which the head of the spermatozoon is seen within the oocyte on the right side of the figure. Meanwhile
the oocyte is becoming a mature ovum by undergoing divisions and producing the second polar body; and the anaphase of this division is shown in
Fig. 24, A. Sobotta stated that no centrosomes occur in connection with
the spindles of the maturation divisions, and Long and Mark have likewise failed to find any "typical centrosomes."
In Fig. 24, C, the second polar body has become a separate cell. The
chromosomes of the ovum, which is now mature, have formed a compact
mass. They next become resolved into a chromatic reticulum, and a
resting nucleus is produced, provided with a nuclear wall and distinct nucleoli (Fig. 24, D and E). This nucleus, which becomes large and moves
toward the center of the cell, is known as the female pronucleus. Meanwhile the head of the spermatozoon has enlarged and formed the male
pronucleus, as shown in Fig. 24, C, D and E.
The two pronuclei, which are very similar, develop rapidly, "probably
within a few minutes after the entrance of the spermatozoon." Simultaneously they prepare for division, and the chromatic reticulum of each
FERTILIZATION
33
becomes resolved into the reduced number of chromosomes which it received during maturation (Fig. 24, F). A centrosome with astral radiations is now seen between the two groups. In Fig. 24, G, it has divided
in two, and the spindle has developed. There has been much discussion
as to the origin of these centrosomes. Since in this case they arise by the
division of a single body, the possibility that one comes from the sper
H ' J
FIG. 34. MATURATION AND FERTILIZATION OF THE OVUM OF THB MOUSE .A,C-J,Xsoo; 8X750.
(After Sobotta.)
A-C, entrance of the spermatozoon and formation of the second polar body. _ D-E, development
of the pronuclei. F-J, successive stages in the first division of the fertilized ovum.
matozoon and one from the ovum has been eliminated. Moreover in the
mouse they cannot be derived from the surviving centrosome of the last
maturation division of the ovum, for that division takes place without
centrosomes. Therefore the centrosome must either be brought in by the
spermatozoon as a constituent of its middle piece, or it must be a new formation. Sobotta accepted the former alternative, and he observed a centro
3
34 HISTOLOGY
some in connection with the head of the spermatozoon in certain stages
(Fig. 24, C) but not in all. It is probable, according to Conklin, that
"the source of the cleavage centrosomes may differ in different animals,
or even in the same animal under different conditions."
Later stages in the division or "cleavage" of the fertilized ovum into
two cells are shown in Fig. 24, H-J. The two groups of chromosomes
come together upon the spindle so that the full number, characteristic
of the species, is restored. Each chromosome then divides lengthwise,
and thus each daughter cell receives one-half of its chromosomes from
the male parent and one-half from the female parent. This is strikingly
evident when the eggs of the fish Fundulus, which have long rod-shaped
chromosomes, are fertilized with the sperm of Menidia, which has shorter
rods. Moenkhaus, who performed this experiment (Amer. Journ
Anat., 1904, vol. 3, pp. 29-64), states that the two kinds of chromosomes
remain grouped and bilaterally distributed on the spindles during the first
and second divisions of the fertilized ovum, but that later they become
gradually mingled.
Important information in regard to the nature of fertilization has
been obtained by experiments upon unfertilized eggs. Changes in the
concentration or composition of the sea water in which the eggs of marine
animals have been placed, mechanical agitation, or, in the case of frogs'
eggs, puncturing the outer layer with a needle, have led to repeated cell
divisions. In this way embryos or larvae have been produced from
unfertilized eggs, and, in a few instances, adult animals. Loeb, who
has been a foremost investigator in this field, concludes that the spermatozoon causes the development of the egg by carrying a substance into
it which liquefies the cortical layer of the egg, and thereby causes the
formation of a membrane. "This membrane formation, or rather the
modification of the surface of the egg which underlies the membrane
formation, starts the development." At the same time there is an acceleration of the oxidations in the egg. "What remains unknown at present
is the way in which the destruction of the cortical layer of the egg accelerates the oxidations."
For the physicist and chemist, the production of mitotic figures
and the process of fertilization, have been subjects of great interest, and
discussions of their significance will be found in various text-books of
physiology and biological chemistry. For further morphological details
the student is referred to "The Cell in Development and Inheritance,"
by E. B. Wilson (2nd ed., New York, 1900) and to the chapters on "Die
Geschlechtszellen" and "Eireife, Befruchtung und Furchungsprozess,"
by W. Waldeyer and R. Hertwig respectively, in vol. i of Hertwig's
"Handbuch der vergl. u. exp. Entwickelungslehre der Wirbeltiere,"
(Jena, 1906).
II. GENERAL HISTOLOGY.
HISTOGENESIS.
SEGMENTATION AND THE FORMATION OF THE GERM LAYERS.
The body is composed of groups of similarly differentiated cells, similar
therefore in form and function. Such groups are known as tissues. Histology (Greek, IO-TO'S, "a textile fabric") is the science of tissues, and histogenesis deals with their origin. There are as many tissues in the body as
there are "sorts of substance;" thus the liver consists essentially of hepatic
tissue, and the bones of osseous tissue. All of these, however, are modifications of a small number of fundamental tissues, the development of which
may now be considered.
It has already been stated that a new individual begins existence as a
single cell, the fertilized ovum. This cell then divides by mitosis into a
pair of cells, Fig. 25, A; and these again divide, making a group of four,
Fig. 25, B. By repeated mitosis a mass of cells is produced, which because
of its resemblance to a mulberry, is called a morula (Fig. 25, C). Development to this point is known as the segmentation of the ovum.
A section through the morula of the rabbit is shown in Fig. 25, D. An
outer layer of cells surrounds the inner cell mass. Soon a cup-shaped
cleft, crescentic in vertical section, forms between the outer and inner
cells as shown in E, and this cleft enlarges until the entire structure becomes
a thin-walled vesicle, within and attached to one pole of which is the inner
cell mass (Fig. 25, F). Cells from this mass gradually spread beneath
the outer layer until they form a complete lining for the vesicle. The
inner layer is called entoderm, and the outer layer ectoderm.
Before the entoderm has encircled the vesicle, a third layer has appeared
between the other two. This middle layer is the mesoderm (Fig. 25,
G). It arises from the place where the ectoderm and entoderm blend
with one another. The layers may be separated and floated apart
except at this spot where they are "tied together." This place is therefore called the primitive knot. The mesoderm also spreads laterally from
a longitudinal thickening of the ectoderm, which extends backward from
the primitive knot and marks out the future longitudinal axis of the
embryo. This thickening is the primitive streak. Arising from the primitive knot and primitive streak, the mesoderm spreads out rapidly between
the ectoderm and entoderm, and very soon it splits into two layers (Fig.
25, H). One of them (the somatic layer) is closely applied to the ecto
35
36 HISTOLOGY
derm, and the other (the splanchnic layer) to the entoderm. Between
them is a cavity, known as the body cavity or coslom, which in the adult
becomes subdivided into the peritoneal, pleural, and pericardial cavities.
The ectoderm and the somatic mesoderm together form the body wall or
somatopleure; the entoderm and the splanchnic mesoderm together form
the intestinal wall or splanchnopleure.
Reviewing the preceding paragraphs it is seen that the fertilized ovum,
through segmentation, forms a morula, which later becomes a vesicle
composed of three germ layers, the outer or ectoderm, inner or entoderm,
and middle or mesoderm. By the folding of these layers the body as a
whole acquires its form; and by their growth and differentiation all the
organs and tissues are produced, together with the fetal membranes which
surround the embryo. Omitting for the present all reference to the membranes, the fundamental changes which the germ layers undergo may be
briefly considered, as follows:
Ectoderm. A portion of the ectoderm forms a layer of cells covering
the body of the embryo. In the adult this becomes the outer layer of
the skin, or the epidermis, and from it, hairs, nails and the mammary,
sebaceous and sweat glands develop. It is reflected under the eyelids
and over the front of the eye, and forms the lachrymal glands. It etxends
into the external auditory opening and there forms the ceruminous glands;
and into the nasal, oral, anal and urogenital apertures. Within the
mouth it forms the salivary glands, the enamel of the teeth, and the cells
associated with the sense of taste. Thus it extends far back toward
the pharynx, and dorsally, in its deepest part, it produces the anterior
lobe of the hypophysis, which will be described in a later chapter. In
the nose it also extends far inward, so that it lines the accessory cavities
which push out from the nasal cavity into certain bones of the head, and
it forms the olfactory cells. An inpocketing of the ectoderm produces
the lining of the deep portion of the ear, including the auditory cells, and,
as will be seen, the ectoderm gives rise to the lens and retina of the eye.
Thus the ectoderm not only forms the outer covering of the body, with
extensions into the several apertures, but it produces various sensory
cells which are stimulated from external sources.
The second great derivative of the ectoderm is the nervous system.
It arises in young embryos as the medullary groove. This is a longitudinal
groove or furrow, situated in front of the primitive knot and appearing
in cross section as a median dorsal depression (Fig. 25, G and H). Later
the groove becomes a tube by the coalescence of its dorsal edges, which
are about to unite in Fig. 25, H. The tube then becomes completely
separated from the epidermal layer of ectoderm, as in Fig. 29.
The closure of the medullary groove to form a tube begins near the
anterior end of the embryo and proceeds backward. Thus for a time the
GERM LAYERS
37
tube opens to the exterior both anteriorly, at the anterior neuropore, and
posteriorly, at the posterior neuropore. Eventually the neuropores become
closed over, and the tube is then whoUy detached from the epidermal
layer. The form of the tube is shown in Fig. 27, which represents a dissected reconstruction of a chick embryo. In this dissection the epidermal
layer, which covers the upper or dorsal surface of the embryo, has been
almost all removed. A portion of it which forms a fold under the head
and around the anterior neuropore has been left in place, and also a portion around the rhomboidal sinus, which may be regarded as an expanded
posterior neuropore. By removing the epidermal layer, the medullary
tube has been exposed. Anteriorly it shows a succession of expansions
B
D
E
FIG. 25. SEGMENTATION OF THE OVUM AND FORMATION OF THE GERM LAYERS IN THE RABBIT. (A-E
after van Beneden; F-H, after Duval.)
A-C represent surface views of the two-cell stage, four-cell stage and morula respectively. D-H are vertical sections. In D and the inner cell mass is heavily shaded. Ect., ectoderm. Ent., entoderm
Mes., mesoderm.
which are to form the brain, and also a pair of lateral outpocketings, or
optic vesicles, each of which will become the retina of an eye. Posteriorly
the tube is slender, and this part becomes the spinal cord. The brain
and spinal cord, which are derived directly from the medullary tube,
constitute the central nervous system. The peripheral nervous system
consists of bundles of nerve fibers which ramify throughout the body,
together with masses of nerve cells associated with these fibers. The
nerve cells are detached ectodermal cells, arising chiefly from the dorsal
part of the medullary groove, and the fibers are protoplasmic outgrowths
of these detached cells and of others which remain in the wall of themedul
3o HISTOLOGY
lary tube. Thus the entire nervous system, central and peripheral, is
ectodermal in origin.
Entoderm. Before considering the chief derivatives of the entoderm,
the notochord (or chorda dorsalis) may be briefly described. In the
lowest vertebrates it is an important supporting structure, and is regarded
as "the primitive forerunner of the vertebral column." It arises in
young mammalian embryos as a median longitudinal band of cells in
the entodermal layer, immediately below the floor of the medullary groove.
In the diagram, Fig. 25, H, it is shown as an elevation; in Fig. 29, it appears as a group of cells completely detached from the underlying entoderm. It then forms a longitudinal rod extending forward from the primitive knot to the under side of the brain, as seen in the longitudinal
section of the chick embryo, Fig. 28. Later it becomes surrounded by
mesodermal cells, which develop into the bodies (or centra) of the vertebrae together with the intervertebral ligaments between them. These are
HNch.
C
Nch.
Nch.
FIG. 26. THE NOTOCHORD.
A, in a sheep embryo of 14.6 mm. (after Minot); B, in a cod fish; C, in man (after Dwight).
shown in Fig. 26, A, as alternating light and dark areas respectively. The
notochord in passing through them shows "segmental flexures" (Minot).
In the vertebral column of a fish (Fig. 26, B) the central notochordal
rod has expanded between the bodies of the vertebrae so as to form large
lenticular masses of gelatinous pulp. These retain a very slender connection with one another. In the human adult, the notochord is represented by the series of detached expansions, or nuclei pulposi, one of which
occurs in each intervertebral ligament (Fig. 26, C). These nuclei are
composed of a peculiar tissue, the development of which has been described by L. W. Williams (Amer. Journ. Anat., 1908, vol. 8, pp. 251284). The notochord is very rarely the source of tumors. Occasionally,
owing to its connection with the entoderm, which is retained longest
anteriorly, it gives rise to a pharyngeal recess (Huber, Anat. Record,
1912, vol. 6, pp. 373-404).
GERM LAYERS 39
In young mammalian embryos the entire entoderm, with the notochordal cells included in its dorsal part, forms the lining of a spherical
sac, known as the yolk-sac (Fig. 25, H). In birds the mass of yolk,
which may be regarded as lodged in the thickened ventral wall of the
yolk-sac, is so extensive that the cavity of the sac is merely a flattened
dorsal cleft. The yolk-sac gives rise to the entire intestinal tube, together
with all its outgrowths. They are therefore lined with entoderm, and
they develop as follows.
At first, in the chick embryo (Figs. 27 and 28) a flattened finger-like
extension of the yolk-sac projects forward into the head, under the notochord. This outpocketing is the fore-gut, which gives rise to the pharynx,
oesophagus, stomach, and anterior part of the small intestine. Near its
anterior extremity it comes in contact with the entoderm and fuses with
it, thus forming the oral membrane. By the rupture of this membrane,
an opening from the exterior into the pharynx is produced.
Similarly the hind-gut develops as a pocket from the posterior part
of the yolk-sac. It gives rise to the lower portion of the small intestine
and the entire large intestine, and fuses with the ectoderm, forming the
cloacal membrane. In later stages the ventral part of the posterior
end of the hind-gut becomes cut off from the dorsal part; the ventral
subdivision forms the bladder, and the dorsal subdivision becomes the
lowest part of the rectum. At the same time the cloacal membrane is
correspondingly subdivided into the urogenital membrane which closes
the outlet of the bladder, and the anal membrane which closes the rectum.
Later these membranes rupture, and the line of separation between ectoderm and entoderm is then difficult to determine. The entoderm apparently lines the entire urethra in the female, but only the upper or prostatic portion in the male; the remainder is lined with ectoderm.
In addition to forming the lining of the pharynx and entire digestive
tube, together with the bladder and its outlet, the entoderm lines the
following important organs, which arise as outgrowths of the pharynx and
digestive tube: the auditory tube, extending from the pharynx to the ear;
the thyreoid gland and certain constituents of the thymus; the entire
respiratory tract, including the larynx, trachea and lungs; the liver; and
the pancreas.
Mesoderm. The mesoderm has already been described as forming
splanchnic and somatic layers. These are indicated in the diagram Fig.
25, H, but are more accurately shown in Fig. 29, which corresponds to
the upper part of Fig. 25, H, under higher magnification. Where the
somatic and splanchnic layers come together they are greatly thickened,
and the thickened part becomes cut into block-like masses by a series
of transverse clefts. The masses are called mesodermic somites, and a
pair of them occurs in each transverse segment of the body. They in
HISTOLOGY
crease in number as new ones become cut off from the unsegmented
mesoderm in the posterior part of the embryo. At first each somite may
Med. groove
Prim, knot
Fig. 27. Fig. 28.
PIGS. 27 AND 28. RECONSTRUCTIONS OF A CHICK EMBRYO. INCUBATED APPROXIMATELY 30 HOURS. X3O.
FlG. 27 represents a dorsal view. The ectoderm has been removed except around the rhomboidal sinus
and under the head. On the left side, all the mesoderm except the blood vessels has also been removed; a. portion including nine somites remains on the right side. The lowest layer beneath the
vessels, is the entoderm. Fig. 28 is a median sagittal section, except that the entire heart has been
included. Ant. neur., anterior neuropore; Med. groove, medullary groove; Med. tube, medullary
tube; Mes. som., mesodermic somite; Opt. ves., optic vesicle; Oral mem., oral membrane; Peric. cav.,
pericardial cavity; Pr. knot, primitive knot; Pr. str., primitive streak; R. sinus, rhomboidal sinus;
Vit. v., vitelline vein; W. duct, Wolffian duct.
contain a cavity, which is an extension of the ccelom, but the cavity is
soon obliterated by a plug of cells. In dorsal view some of the somites
are shown on the right side of Fig. 27; the rest have been cut away.
GERM LAYERS
In later stages each somite gives rise to a stream of cells which spread
around the medullary tube, nojochord and aorta After these cells have
been given off, the somite appears as a plate-like structure (Fig. 30),
known as the dermo-myotome. The principal derivative of the dermomyotome is the voluntary musculature of the body. In producing the
various voluntary or skeletal muscles, certain cells of the dermo-myotome become transformed into muscle fibers. These are at first arranged
in segmental masses, but the masses become subdivided into groups
representing the individual muscles. The groups become separated
from one another and shift to their final positions. Subsequently they
Med. tube
/
t-Int.
FIG. 29.
FIG. 30.
FIG. 29. TRANSVERSE SECTION OF A RABBIT EMBRYO MEASURING 4.4 MM. (pj DAYS). X6o.
FIG. 30. TRANSVERSE SECTION OF A RABBIT EMBRYO MEASURING 5 MM. (n DAYS). X4O.
Ect., ectoderm; Ent., entpderm; Int., intestine; Med. tube, medullary tube; Msnch., mesenchyma;
Msth., mesodennal epithelium; Nch., notochord; Som., somatopleurej Som. mes., somatic mesoderm; Spl., splanchnopleure; Spl. mes., splanchnic mesoderm; W. d., Wolffian duct.
acquire their connections with the bones, which develop later than the
muscles. The remainder of the dermo-myotome breaks up into cells
which are contributed to the deep portion of_the skin.
Connecting the somites with the lateral somatic and splanchnic
layers of the mesoderm, there is a narrow neck of cells (as seen in cross
section, Fig. 29) which is known as the intermediate cell mass, or
nephrotome. The nephrotomes at first are not segmentally divided, but
form the floor of a longitudinal groove in the mesoderm, lateral to the
somites (Fig. 27). The nephrotomes give rise dorsally to a longitudinal
cord of cells, which later becomes a tube, and is known as the Wolffian
duct (Figs. 27, 29, and 30). It lies in the groove above the nephrotomes.
This duct grows posteriorly and acquires an opening into the entodermal
bladder. The nephrotomes then become separated from the somites
and from the lateral layers of the mesoderm, and their cells become arranged so as to form coiled tubes, which empty into the Wolffian duct.
In this way the mesoderm gives rise to the Tenal_system, which consists
essentially of coiled mesodennal tubes, receiving urinary products from
42 HISTOLOGY
the blood and conveying them through the Wolffian duct to the bladder.
Later, parts of the urinary system lose their primary function and become
the ducts of the genital system.
The lateral somatic and splanchnic layers of the mesoderm produce
the lining of the pleural, pericardial, and peritoneal subdivisions of the
ccelom, as already stated: They give rise also to an important tissue
known as mesenchyma. With the production of mesenchyma the tissues
Epi.
i.s.
B.V
FIG. 31. SECTION FROM THE HEAD OF A RABBIT EMBRYO OF ioj DAYS, 4.4 MM., TO SHOW MESENCHYMA.
Epi. and M. T., Ectodermal epithelium of the epidermis and medullary tube, respectively. N., nucleus,
P., protoplasm, and I. S., intercellular substance of a mesenchymal cell. Two of these cells show
mitotic figures. B. V., Blood vessel, lined with endothelium. One of the blood vessels contains an
embryonic red blood corpuscle.
of the embryo may be divided into two sorts, namely, epithelium which
covers an external or an internal surface of the body, and mesenchyma
which fills the space between two layers of epithelium. These relations
are clearly shown in the cross section of the abdomen (Fig. 30). The
body wall consists of a layer of ectodermal epithelium externally, and of
mesodermal epithelium internally, with a thick layer of mesenchyma
between the two. Similarly the intestinal wall consists of mesodermal
epithelium toward the ccelom, and entodermal epithelium toward the
intestine, with mesenchyma between them. Epithelium is thus produced by all the germ layers, but mesenchyma is almost exclusively the
product of the mesoderm. It is formed not only from the lateral splanchnic
GERM LAYERS
43
and somatic layers of the mesoderm, but also from the somites. The
tissue which has been described as spreading from the somites around
the medullary tube, notochord and blood vessels, and into the deep
portion of the skin, is mesenchyma. It also surrounds the tubules derived
from the nephrotome.
Under higher magnification, as in Fig. 31, it is seen that epithelium
is a layer of closely compacted cells, but that mesenchyma is a protoplasmic network, the meshes of which are filled with a fluid intercellular
substance. If this substance is abundant, the nuclei of the mesenchyma
are widely separated, as in the figure; but if it is scanty they are quite
close together. Mesenchyma gives rise to a great variety of tissues,
including involuntary muscle, adipose tissue, cartilage, and bone. Both
the cells and the intercellular substance may become variously modified.
The most widespread derivative of mesenchyma is connective tissue,
which invests the nerves, vessels, muscles and epithelial structures, binding them together in organs, and filling the interstices of the body.
FIG. 32. WALL OF THE YOLK-SAC FROM A CHICK OF THE SECOND DAY OF INCUBATION. (Minot.)
Mes., Splanchnic mesoderm; Ent., entoderm,. four distinct cells of which are shown at c; V, V, blood
vessels containing a few young blood cells.
The origin of the blood and blood vessels remains to be considered.
In very early stages the vessels appear as cellular strands, some of which
contain a lumen, situated between the mesoderm and entoderm. Associated with these strands, but further out on the yolk-sac, there are
clusters or "islands" of blood cells, surrounded by a thin layer of flattened
cells known as endothelium. The entire system soon forms a network
of distinct vessels situated in the splanchnopleure (Figs. 29 and 32).
The formation of this primary vascular network in rabbit embryos has
been described by Bremer (Amer. Journ. of Anat., 1912, vol. 13, pp.
111-128). Generally the vessels and the corpuscles within them are!
considered to be mesodermal, but some authorities have regarded them as]
entodermal, and others have proposed to describe them as forming a
separate germ layer or "angioblast" (more appropriately angioderm) .
In the chick embryo shown in Figs. 27 and 28, the network of vessels
44 HISTOLOGY
in the splanchnopleure has formed a complete circulatory system. By
a process of folding, portions of the net have been brought together
under the fore-gut, where the vessels from the two sides have fused and
formed a single median tube, the heart. The two large trunks, derived
from the network, which convey the blood from the yolk-sac to the heart
are known asjntelline veins^ The heart divides anteriorly into two
vessels (the aorta) which pass from the under side of the fore-gut to the
upper side, and then extend posteriorly. They finally connect by
branches with the network over the yolk, from which they have been
derived. Through this system, nutriment taken from the yolk is brought
to the heart by the vitelline veins, and distributed throughout the body
by the aortae.
In mammals also, a complete system of vessels is established early
in development, and it is believed that all later vessels arise as branches
of this primary endothelial network. If this opinion is correct, none of
the later vessels are formed by the coalescence of mesenchymal spaces,
or by transformation of mesenchymal cells into endothelial cells, but
only as outgrowths of pre-existing endothelium. There is, however,
a very close connection between the endothelium and the surrounding
mesenchyma, as shown in Fig. 31.
The histogenesis of the blood is likewise very difficult to follow. The
simplest interpretation is one which has not been disproven, namely, that
all forms of blood corpuscles are descendants of the cells found in the
blood islands of the yolk-sac. According to this hypothesis these cells
multiply in certain places to which they have been carried by the circulating
blood, for example in the liver in later embryonic life and in the bone
marrow of the adult; and they differentiate into the red and white corpuscles of various kinds. The difficulties which this hypothesis encounters
will be discussed in later chapters.
THE FUNDAMENTAL TISSUES.
From the foregoing outline of embryological development, it is clear
that all the organs of the body are derived from a relatively small number
of fundamental tissues. After the fertilized egg has segmented, it gives
rise to layers of cells, of which the ectoderm and entoderm are epithelial
from the beginning. The mesoderm very early divides into two tissues
epithelium, which lines the body cavity, and mesenchyma, which forms
the internal substance of the body wall and intestinal wall. Thus epithelium and mesenchyma may be regarded as the primary tissues of the body.
The groups of blood corpuscles, which are probably derived from the
mesenchyma, and the endothelium which surrounds them, also arise
very early, and these may be set apart as vascular tissue.
GERM LAYERS 45
The nervous system develops from epithelium, but its cells, singly
or in groups, become imbedded in strands and masses of nerve fibers which
these same cells send out as processes. Thus little remains in the adult
to suggest that the brain or peripheral nerves come from a layer of cells
covering a surface, and the nervous system is therefore described as consisting of nervous tissue.
The voluntary muscles are formed from cells derived from the epithelium of the mesodermic somites, but they develop as the somite breaks
up and its epithelial character is lost. The involuntary muscles are produced by a transformation of mesenchymal cells into elongated muscle
cells. For physiological reasons these two kinds of muscle, which are
of diverse origin and structure, are classed together as muscular tissue.
The relation of the germ layers to the five fundamental tissues which
have now been recognized, is shown in the following summary.
ORIGIN OF THE TISSUES FROM THE GERM LAYERS.
The ectoderm produces:
1. EPITHELIUM of the following organs: the skin (epidermis) including the cutaneous glands, hair and nails; the cornea and the lens; the external and internal ear; the
nasal and oral cavities, including the salivary glands, the enamel of the teeth and anterior lobe of the hypophysis^ the anus; the cavernous and membranous parts of the male
urethra; together with that epithelium of the chorion which is toward the uterus and
of the amnion which is toward the embryo.
2. NERVOUS TISSUE forming the entire nervous system, central, peripheral and
sympathetic.
3. MUSCULAR TISSUE, rarely, as of the sweat glands, and iris.
The mesoderm produces:
1. EPITHELIUM of the following four sorts: (i) epithelium of the urogenital organs
(except most of the bladder and the urethra) and the epithelioid cords of cells in the
suprarenal gland; (2)epithelium of the pericardium, pleurae, and peritoneum and the
continuation of this layer over the contiguous surfaces of amnion and chorion; (3) epithelium lining the blood vessels and lymphatic vessels; and (4) epithelium lining the
joint cavities and bursae.
2. MUSCULAR TISSUE, striated (voluntary), cardiac, and smooth (involuntary).
3. MESENCHYMA, an embryonic tissue, which forms in the adult, connective and
adipose tissue, bone (including the teeth except their enamel), cartilage, tendon, and
various special cells.
4. VASCULAR TISSUE, the cells of the blood and lymph, consequently the essential
elements of the lymph glands, red bone marrow and spleen.
The entoderm produces:
1. EPITHELIUM of the following organs: the pharynx, including the auditory
tube and middle ear, thyreoid and thymus glands; the respiratory tract, including
larynx, trachea, and lungs; the digestive tract, including the oesophagus, stomach,
small and large intestine, rectum, liver, pancreas, and the yolk-sac; and part of the
urinary organs, namely most of the bladder, the female urethra, and prostatic part
of the male urethra (including the prostate).
2. NOTOCHORDAL TISSUE, which occurs in the nuclei pulposi.
46 HISTOLOGY
In the following pages the fundamental tissues will be considered in
turn. In connection with them, certain organs will be examined. An
1 organ is a more or less independent portion of the body, having a connective tissue framework, and a special blood, lymph, and nerve supply,
in addition to its characteristic essential cells. The essential cellular
substance of an organ, in distinction from the accessory tissues, is often
called its parenchyma; the accessory supporting tissues constitute the
stroma (Gr. arrpupa, bed), in which the parenchyma is imbedded.
Such structures as the pancreas and liver are obviously organs. An
individual muscle or a particular bone, which has a connective tissue
covering or framework, and a supply of vessels and nerves, besides its
essential substance, may also be regarded as an organ. The organs which
are of widespread occurrence, such as the bones, muscles, tendons and
large vessels, will be described with the tissues. The more complex
organs are reserved for a later section, entitled "Special Histology."
EPITHELIUM.
The Dutch anatomist, Frederik Ruysch, recognized that the covering
of the margin of the lips is not identical with the epidermis. "Therefore," he wrote, "I shall call that covering the epithelis, or papillary integument of the lips" (Thesaurus anat. Ill, 1703, No. 23, p. 26). It is
an unfortunate name (CTTI, upon O^Xrj, Latin papilla, the nipple) since it
does not refer to the layer upon the nipple, but to that which covers a
great number of nipple-like elevations of the underlying tissue. Such
elevations or papilla are indeed abundant in the lips, but they occur also
under the epidermis. Ruysch substituted epithelia for epithelis in other
sections of his work, and Haller, writing some years later, used the neuter
epithelium, so that epithelia thus became a plural.
As the term epithelium is now used, it includes the epidermis, and the
lining of the various internal tubes and cavities. It has been defined as
a layer of closely compacted cells, covering an external or internal surface
of the body, having one of its surfaces therefore free, and the other resting on underlying tissue. But the term is also correctly applied to solid
outgrowths from such layers, either in the form of cords or masses of
cells. Usually these outgrowths subsequently acquire a Cavity, or lumen,
around which the cells become arranged in a layer.
The epithelia which cover the skin and line the digestive tube and
urogenital organs are thick, as compared with those which line the bodycavity, the vessels, and the synovial cavities. For these thin layers His
(1865) introduced the term endothelium. He wrote as follows:
We are accustomed to designate the layers of cells which cover the serous and
vascular cavities as epithelia. Jut all the layers of cells which line the cavities within
EPITHELIUM 47
the middle germ layer have so much in common, and from the time of their first appearance differ so materially from those derived from the two peripheral germ layers,
that it would be well to distinguish them by a special term either to contrast them, as
false epithelia, with the true, or to name them endothelia, thus expressing their relation
to the inner surfaces of the body.
The name endothelium, etymologically absurd, has become generally
accepted for the lining of the blood vessels and lymphatic vessels. For
the other forms of epithelium which it was intended to include, special
names have been proposed.
Minot (1890) introduced mesothelium to designate the layer of mesodermal cells which bounds the body cavity. Thus mesothelium does not
include the endothelium of the vessels, or the lining of the synovial cavities; but it does include the cells of the nephrotome, through which the
body cavity may extend, and also the epithelium which bounds the somites
in early stages. Professor Minot applies the term also to the thick epithelium of the renal organs, which is derived from the cells of the nephrotome.
As seen in Fig. 33, the epithelium lining the vessels closely resembles
that which lines the body cavities, and to a certain extent this justifies
the use of the term endothelium for
both layers as proposed by His. But
it has been shown embryologically
that the vessels and body cavity are
of different origin, and are distinct
even in the earliest stages. Moreover the linings of the synovial cavities, tendon sheaths, and the chambers
of the eye form a third separate group.
They arise relatively late in develop- A> Surface view of a ^3i uin from the mesen .
ment by the confluence of intercel- * e r r t y B> surface view of endothelium from an
lular spaces in the mesenchyma, and
they are therefore bounded by flattened mesenchymal cells.
In accordance with these embryological facts, the following use of
terms is here proposed:
Endothelium should be restricted to the lining of the blood vessels
and lymphatic vessels.
Mesothelium, except in young embryos, should be restricted to the
lining of the body cavity and its subdivisions.
Mesenchymal epithelium (or false epithelium) should be applied to
the lining of joint cavities and bursae.
All of these forms of epithelium are primarily thin and are derived
from the mesoderm. The lining of the body cavity is, however, thickened
in places. Thick epithelium may be ectodermal, entodermal or mesodermal in origin.
4 8
HISTOLOGY
Epithelia differ from one another, not only in origin, but also in the
shape of their cells, the number of layers of which they are composed, and
the differentiation of their cells. These features should be examined in
every specimen studied, and something under each heading should be
recorded in any complete description of an epithelium.
SHAPES OF EPITHELIAL CELLS AND THE NUMBER OF LAYERS.
An epithelium which consists of but one layer of cells is called a simple
epithelium, and its cells may be fiat, cuboidal or columnar. These terms
refer to the appearance of the cells when cut in a plane perpendicular to
the free surface. If in such a section the outlines of the cells are approximately square, as along the upper surface in Fig. 34, the epithelium is
cuboidal; if they are stretched out in a thin layer so that they appear
linear, as along the lower surface in Fig. 34, the epithelium is flat. Endothelium is an extremely flat epithelium, in which the cells are so thin that
the nuclei cause local bulgings of the cell membrane. If the epithelial
cells are laterally compressed, so that tall forms result as in Fig. 35, B,
the epithelium is columnar. Such epithelium is less accurately called
Cuboidal epithelium.
"' Connective tissue.
Flat epithelium.
FIG. 34. PORTION OF THE MEMBRANES SURROUNDING A PIG EMBRYO MEASURING 6o_MM. (Allantios
above, and amnion below.)
cylindrical, and both cuboidal and flat epithelia are sometimes referred
to as pavement epithelium. Intermediate forms, which are described
as low columnar or low cuboidal, frequently occur. The cells of certain
epithelia change their shape temporarily, as in the bladder during distention, in the oesophagus during deglutition, and, to some extent, in the
arteries with every pulsation. During post-mortem contraction the
arterial endothelium is considerably thickened. Moreover during embryonic development, epithelial cells may change from one form to another.
On surface view the epithelial cells of all types are polygonal and usually
six-sided (Figs. 33 and 35, A). Geometrically a circle would come in
contact with six surrounding circles of equal diameter, and a cell is usually
in contact with six surrounding cells. The cells, however, vary in diameter, and are often surrounded by five or seven cells and occasionally
by four or eight.
An epithelium which consists of several superimposed layers is known
as stratified epithelium (Fig. 37). In such cases the basal cells are usually
EPITHELIUM
49
columnar and closely crowded. They multiply by mitosis and give rise
to cells which are pushed toward the free surface. After leaving the basal
layer they enlarge and become polygonal in outline. Toward the free
surface they become gradually flattened, and may be more or less cornified
or transformed into horny material. These scale-like cells are called
FIG. 35. SIMPLE COLUMNAR EPITHELIUM
FROM A HUMAN INTESTINAL VILLUS.
A, Surface view; B, vertical section. The
prominent cell outlines in A are due to
terminal bars, shown in section in B.
Cut., cuticular border.
FIG. 36. DETACHED SQUAMOUS CELLS FROM THE
MOUTH.
FIG. 37. STRATIFIED EPITHELIUM FROM THE (ESOPHAGUS
OF A CHILD.
squamous cells and they readily become detached (Fig. 36). Stratified I
epithelium is found in the vagina, oesophagus, pharynx and oral cavity ;f
and in its most complex form, with many layers, some of which are peculiarly modified, it constitutes the epidermis.
Columnar cells""
Fusiform cells'
..... Basal cells
'' -Conn, tisues.
FIG. 38.
FIG. 39
FlGS. 38 AND 39. PSEUDO-STRATIFIED CILIATED EPITHELIUM FROM THE HUMAN RESPIRATORY TRACT.
Fig. 39 is a diagram of the condition shown in Fig. 38. X72O.
In certain organs and especially in embryos, simpler forms of stratified
epithelium occur, which are described as four-layered, or two-layered as
the case may be. The superficial cells may be flat, cuboidal, or columnar.
A characteristic epithelium with dome-shaped outer cells and tall basal
cells, found in the bladder and ureter, is known as "transitional epithe
50 HISTOLOGY
Hum" as if it were intermediate between the simple and stratified forms.
When the bladder contracts the cells are heaped up in several layers, but
when distended the number may be reduced even to two.
If the cell walls are indistinct and the sections are thick or oblique,
the number of layers in an epithelium may be very difficult to determine.
Thus in a simple epithelium the nuclei may be at different levels (Fig.
35, B), and if the section is not vertical it will show several layers, approaching the condition of the tangential section, Fig. 35, A. Fig. 38
represents a vertical section of an epithelium with nuclei at three levels,
and in two forms (the basal nuclei being round and the others elongated) ;
but yet, as interpreted in Fig. 39, it is not stratified. It is of the form
known as pseudo-stratified, in which all the cells reach the underlying
connective tissue, but only a limited number extend to the free surface.
Pseudo-stratified epithelium occurs in the upper part of the respiratory
tract, including the trachea and larger bronchi, and in the epididymis.
PERIPHERAL DIFFERENTIATION OF EPITHELIAL CELLS.
Free surface. The free surface of epithelial cells is often provided
with a thickened top-plate or cuticula. Under high magnification the
cuticular border of the columnar cells in the intestine is seen to be vertically
striated (Fig. 35, B), and these striations have been interpreted as minute
canals through which protoplasmic processes may be sent out beyond
the free surface. In some cases, however, the striated cuticula appears
to consist merely of short, parallel protoplasmic rods. In certain cells
of the kidney, the rods may become somewhat divergent, giving rise to
what is known as the "brush border." Longer processes, which are
vibratile but not retractile, are called cilia (the Latin term for eyelashes). They project from the free surface of certain epithelial cells
in the trachea and bronchi (Figs. 38 and 39), in the uterus and uterine
tube, in the efferent ducts of the testis, and in the nasal part of the pharynx
together with the auditory tube and naso-lachrymal duct which open
into it. In the living condition the motion of cilia may be observed in
various unicellular animals. It may be studied advantageously in
fragments from the margin of the gills of a clam, or in epithelium from
the roof of the mouth of a frog. The cilia are numerous, and in the snail
Heidenhain counted no arising from a single cell. They do not act
together, but rapidly succeeding waves, due to the bending of the cilia,
pass over the entire surface. By bending sharply downward, each
cilium creates a forward current in the overlying fluid, and passes the
particles above it to the cilium in front. No sooner does a cilium begin
to bend than the next in front takes up the movement and thus the ciliary
waves are propagated. In some animals, however, the wave proceeds
EPITHELIUM 51
in a direction opposite to that of the effective stroke. The cilia in man
produce currents toward the outlets of the body. In the uterine tube
the stroke is toward the uterus, presumably favoring the passage of the
ova, but the spermatozoa ascend this tube against the current.
The structure of cilia, because of their small size, is difficult to determine, but in many cases a differentiation between the exoplasm and
endoplasm has been observed. The simplest cilia, as shown in the
diagram (Fig. 40, a), are essentially permanent pseudopodia, with contractile sheaths and fluid contents. They may develop very rapidly
in the protozoa. Thus Prowazek has seen processes grow out in eight
minutes, which were then vibrating 19 times in 20 seconds. Schafer
PIG. 40.
a, b, c, Diagrams to illustrate the structure
of cilia. (After Williams.)
FIG. 41.
Diagram of a ciliated cell (after Prenant) ,
showing yibratile cilia; b, cells of the
human epididymis (after Fuchs), showing
non-motile cilia.
considers that cilia are primarily pseudopodia, and that their motion
is caused by the alternating ingress and egress of fluid to and from the
central part, due to variations in the surface tension.
Many cilia, however, appear to contain more or less solid axial rods,
which generally proceed from round basal bodies resembling centrosomes.
That these bodies arise from the centrosome has recently been denied.
Sometimes the bodies are double, and extensions from them downward
into the cytoplasm may occasionally be observed (Fig. 41, a). These
roots approach one another beside the nucleus, and it has been discussed
whether or not they unite. The roots, and portions of the cytoplasmic
reticulum at right angles to the shafts of the cilia, have been thought
to act as levers. Others conjecture that the central shaft is a supporting
structure, perhaps elastic, which is surrounded by a contractile sheath.
The contractile elements may extend the whole length of the cilium or be '
confined to its base, as indicated in the diagram (Fig. 40, b and c). If
HISTOLOGY
the sheath were equally developed about the entire circumference of the
axis, the cilia should be able to strike in any direction. Usually the
effective stroke is in one direction only, but in some cases it may be
reversed. In reversible cilia, such as occur on the labia of the sea anemone,
the effective stroke is either toward the mouth or away from it, according
to the chemical composition of the substances in contact with the
cilia (Parker, Amer. Journ. of Physiol., 1905, vol. 3, pp. 1-16).
In such a case the contractile material is supposed to be gathered in two
bands, on opposite sides of each cilium. In the irreversible cilia, such as
are found elsewhere in the sea anemone and in man, the contractile
material, according to Parker, must be gathered especially on one side
of the supporting axis.
The whip-like processes, or flagella, which form the tails of spermatozoa, may be compared with single cilia. Each springs from a body
resembling a centrosome, and consists of an axial filament with a surrounding sheath, but whether the filament or the sheath contains the
contractile substance is still uncertain.
Non-motile projections, somewhat resembling cilia, are found in the
cells of the epididymis (Fig. 41, b). They have no basal bodies, and
lack the distinctness of true cilia. Generally they appear in conical
clumps, which have been compared to the hairs of a wet paint brush.
They may be concerned with the discharge of secretion. Other nonmotile processes of epithelial cells are
the tapering projections of the sensory cells, apparently designed to receive stimuli. The lining of the
central cavity of the spinal cord and
ventricles of the brain is also provided with short projections, which
may be degenerating cilia. It is
questionable whether these are
motile.
Lateral surface. The lateral surfaces of epithelial cells may be in
close contact with one another,
sometimes without intervening cell
walls; or they may be separated by
a thin layer of intercellular substance, which is generally fluid. Immediately beneath the cuticular border of the cells lining the intestine,
the intercellular substance takes the form of a more solid bar encircling each cell and binding it to those which surround it. The
arrangement of these terminal bars is shown in the diagram, Fig. 42,
and in the section Fig. 35, b. If the section passes down through the
IntercellU'
lar substance.
PIG. 42. DIAGRAM OF THE NETWORK OF TERMINAL BARS.
The two cells on the left are divided lengthwise
into halves; the two on the right are drawn
as complete cylinders or prisms.
EPITHELIUM
53
middle of the cell, as on the left of Fig. 35, b, the bars are cut across and
appear as points; but if either the proximal or distal side of the cell is
included in the section, they appear as lines, as on the right of the figure.
Terminal bars have been found in many epithelia, especially in mucous
membranes and glands. They occur in the epididymis (Fig. 41, b) where
they appear as thickenings of the cell wall. According to Stohr they are
found in the stratified epithelium of the tongue and bladder.
The intercellular substance in endothelium and mesothelium is ordinarily inconspicuous, but it may be demonstrated by treating the tissue
with a solution of silver nitrate. The resulting precipitate occurs chiefly
in the intercellular "cement substance," which then appears as a wavy
black line bounding each cell (Fig. 33). It is of importance since various
forms of blood corpuscles make their way through it from the vessels into
the surrounding tissue.
In the lower layers of the epidermis and the thick oral epithelium,
the intercellular substance is clearly seen, and here it is bridged by spiny
processes from the adjacent cells. These intercellular bridges occur in endothelium and many
forms of epithelium, but they are most readily
observed in the deep layers of the thick stratified
epithelia (Fig. 43). Within the bridges, fibrils pass
from cell to cell. In the intercellular spaces between
the spiny processes, nutrient fluid makes its way to
the outer layers. Whatever nutriment they receive
must be derived from the intercellular fluid or DE S . F THE EPI *
through the bodies of the underlying cells, since
neither blood vessels nor lymphatic vessels penetrate the epithelium.
This is probably true of all epithelia in man, but in the bladder and renal
pelvis the blood vessels approach very close and may appear to enter,
and in the amphibia, according to Maurer, capillaries may be observed
well within the oral epithelium. Nerve fibers extend among the basal
cells of the epidermis and other epithelia, and ramify in contact with
these cells, but special methods are required to demonstrate them.
Basal surface. The basal cells of an epithelium sometimes seem to
send out processes which blend with the underlying connective tissue.
Usually, however, the lower surface is well defined, and the epithelium
is bound down by intercellular cement substance. Often, especially in
glands, the epithelium rests upon a thin, well-defined basement membrane
or membrana propria. This membrane is usually homogeneous and contains very few nuclei. Sometimes it is composed of elastic tissue. Certain
basement membranes have been considered as derivatives of the epithelium, but generally they are clearly of mesenchymal origin.
54 HISTOLOGY
PROCESSES OF SECRETION IN EPITHELIAL CELLS.
Many epithelial cells elaborate and discharge substances which do
not become parts of the tissue. Such cells are called gland cells, and their
products are either utilized by the body (secretions) or eliminated as waste
products (excretions). The process of elaboration and discharge of the
secretion or excretion may often be recognized by changes in the form
and contents of the cell. A gland cell which is full of secretion, or discharging it, is called "active," and one in which the secretion is not
apparent, though it may be in process of formation, is called "resting."
The appearances during secretion differ in two types of gland cells the
serous, which produce watery secretions, like saliva; and the mucous,
which form thick secretions, like those of the nose and throat. These
will be considered in turn.
Serous gland cells, when empty, are small and darkly staining. As
Granule.
.-,, ..^-^~- l r, , -...* * ^ ' New granule.
Basal filaments.
Nucleus.
^W^_
Large nucleolus.
A B
FIG. 44. Two SEROUS GLAND-CELLS FROM THE SUBMAXILLARY GLAND OF A GUINEA-PIG. X 1260.
In cell B the granules have passed into the unstainable state; new stainable granules are beginning to
develop in the protoplasm.
the formation of secretion begins, the cells, if prepared with special
methods, exhibit granules which stain intensely. These granules have
become cut off from the basal filaments or mitochondria (Fig. 44, A).
They enlarge, lose their staining capacity, and are transformed into drops
of secretion. The entire cell becomes larger and clearer than before, and
the alveolar structure of its protoplasm is well marked (Fig. 44, B).
Finally the droplets become confluent and are discharged from the free
surface of the cell. A portion of the mitochondria remains behind as
the source of further secretion. In many gland cells the cytoplasmic
differentiation is accompanied by changes in the nucleus. In the empty
cell the nucleus has distinct nucleoli and a fine chromatic reticulum, but
in cells full of secretion the nucleoli have enlarged or disappeared and the
chromatin is in the form of coarse masses. Particles pass from the nucleus
into the cytoplasm, and these have been said to give rise to secretory
granules.
In mucous cells the process of secretion also begins with granule formation, but the mucigen granules gather near the free surface of the cell
GLANDS
55
FIG. 45. EPITHELIAL CELLS SECRETING Mucus.
From a section of the mucous membrane of the human stomach
Xs6o. p, Protoplasm; s, secretion; a, three cells, two empty,
the third showing the beginning of mucoid metamorphosis; e, the
cell on the right is discharging its contents; the granular protoplasm has increased and the nucleus has become round again.
where they become changed into clear droplets of mucus. A discoid
mass of secretion is thus produced which is quite sharply marked off from
the underlying cytoplasm
(Fig. 45, a and b). As
the cytoplasm becomes
increasingly transformed
into secretion, the elongated nucleus becomes at
first round, and then flattened. It is forced to
the base of the cell where
it is lodged in a small amount of unchanged cytoplasm (Fig. 45, b-d).
The secretion is then gradually discharged through the distended topplate, which is often ruptured in sections, and the nucleus again becomes
round and moves toward the center of the cell. Most gland cells
are not destroyed by the act of
secretion, but may repeat the
process several times. An exception occurs in the case of the
sebaceous glands, in which the
cells disintegrate and are cast off
with their products. In the
mucous cells of the intestine,
secretion is formed below and
discharged from the free surface
at the same time. The cells, as
seen in Fig. 46, arise near the
bottom of tubular depressions
lined with simple columnar epithelium. By the formation of
new cells below them they are
pushed toward the outlet of the
tube. Thus the youngest cells
are at the bottom of the pit and
the oldest are at the top. For a
time the secretion develops faster
than it is discharged, and the
cells enlarge as seen in the middle
part of the gland; later, as elimination exceeds production, they
become narrow, and their final
stages, as compressed cells with
Gland lumen.
FIG. 46. INTESTINAL GLAND FROM A SECTION OF THE
HUMAN LARGE INTESTINE. X 165.
The secretion formed in the goblet-cells is here colored blue; usually it is pale as in Fig. 45. In zone I
the goblet-cells show the beginning of secretion; that
expulsion has begun is evident from the presence of
drops of secretion in the lumen of the gland. 2,
Goblet-cells with much secretion. _ 3, Goblet-cells
containing less secretion. 4, Dying goblet-cells,
some of which still contain remnants of secretion.
50 HISTOLOGY
a remnant of secretion, are found near the orifice of the gland. Cells
such as have been described, which appear like cups filled with mucus, are
known as goblet cells.
In certain stratified or pseudo-stratified epithelia, the formation of
mucus has been seen to take place in some of the deeper cells, but the discharge of the secretion can occur only when these cells have reached the
free surface.
THE NATURE AND CLASSIFICATION OF GLANDS.
The simplest form of gland is merely a single secreting cell situated
apart by itself in an epithelium. Such unicellular glands are abundant
in invertebrates and are represented in man by scattered goblet cells.
In the higher animals the secreting cells usually occur in groups, and they
are generally found in tubular or saccular outpocketings of the epithelium.
Excretory duct.
Secretory duct.
Intercalated duct
End pieces.
FIG. 47. DIAGRAM OF VARIOUS FORMS OF GLANDS.
The arrangement of ducts in D is that of the human submaxillary gland.
An unbranched tubular gland is shown in vertical section in Fig. 46, and
in the diagram, Fig. 47, A. The secreting cells may be distributed throughout the tube, or they may be limited to the lower part. In such cases
the upper part forms the duct of the gland. Sweat glands are unbranched
tubes, with a coiled secreting portion in the deeper part of the skin,
and a relatively long duct which conveys the secretion to the surface.
Many glands are branched, as in Fig. 47, B. The main stem becomes
the duct, and the characteristic secretion is formed in saccular or tubular
" end pieces."
GLANDS 57
Such glands as have been described, either branched or unbranched,
occur in great numbers as constituent parts of some organ, and they are
classed as simple glands. The sebaceous and sweat glands of the skin,
intestinal glands, and uterine glands are examples of this class. Many
glands are much larger than these, owing to the fact that the epithelial
outgrowth has branched repeatedly. It becomes invested with a connective tissue capsule, which sends partitions, or septa, among the ramifications of the epithelial tube, thus dividing the gland into lobes and lobules. A lobule usually contains a terminal branch of the duct together
with the cluster of end pieces which empty into it. The large glands not
only have a connective tissue framework, but also a special supply of
nerves, blood vessels and lymphatic vessels. Thus they form independent
organs, and they are classed as compound glands. They include the liver,
which discharges its secretion through a single duct; the pancreas, which
is formed by the fusion of two glands and therefore has primarily two
ducts; and many smaller organs, like the prostate, which is a compact group of glands each
of which has a separate duct.
All the glands thus far considered are alike
in being outpocketings of epithelium. Most of
them develop as masses or cords of epithelial
cells which later acquire a central cavity or
lumen. The secreting cells may discharge their
products from their free surfaces directly into
the lumen; or the secretion may enter minute
canals, either within the cells (intracellular), or FlG< 48 ._Di7^A^F A SIMPLE ALbetween the cells (intercellular). Intercellular
secretory canals (also called capillaries) are
found in the serous glands of the tongue and
in the serous portions of the salivary glands; they occur also in the
liver, the gastric and pyloric glands, sweat glands, lachrymal gland and
bulbo-urethral gland. Various forms are shown in the right half of the
diagram Fig. 48. They occur where two or more cells come together
and consequently they are in relation with two or more terminal bars.
In longitudinal sections the bars may be seen to extend downward along
the canals. Through such intercellular canals the basal cells in a glandular epithelium may discharge their secretion into the central cavity, as
shown in Fig. 48. Intracellular secretory canals, shown in the left half
of Fig. 48, are less definite in outline, and are never in relation with terminal bars. They may be transient vacuoles opening at the surface. Sometimes they anastomose and form a network of canals within the cell. They
have been observed, together with intercellular canals, in the sweat
58 HISTOLOGY
glands, the liver, and the gastric glands. There are apparently no secretory canals in any mucous gland, and they have not been found in the
duodenal, intestinal, uterine and thyreoid glands, the kidney or the
hypophysis.
The ducts have a clear-cut lumen and are typically lined with a very
regular epithelium, showing distinct cell boundaries. The cells usually
do not contain the rods, granules or vacuoles characteristic of secreting
protoplasm, and the nuclei are not crowded to the base of the cells.
In some cases, however, the ducts contain mucous cells, and in the salivary glands a specialized portion of the ducts is believed to discharge
salts into the secretion as it passes through them. In such a gland
(Fig. 47, D) the duct, as it leaves the end pieces, consists of simple flat
epithelium. This intercalated duct gives place to the secretory duct which
is lined with columnar epithelium, having basal rows of granules. The
outer excretory portion consists of simple or stratified non-glandular
epithelium.
The end pieces of the glands, as already noted, vary in shape from
saccular to tubular. Usually a minute dissection or a reconstruction is
necessary to determine what the shape may be. A round termination
is called an acinus (Latin, a grape or berry) or an alveolus (Latin, a trough
or tray). These terms are often used interchangeably. The elongated
forms are called tubules.
During the development of the thyreoid gland the duct becomes
obliterated, so that the secretion within the end pieces cannot escape.
The end pieces become closed epithelial sacs, known as follicles (Latin,
folliculus, a leather bag, shell, or husk). In addition to the material
enclosed within the follicles, the thyreoid gland secretes substances which
are taken up by the surrounding blood vessels and lymphatic vessels.
Secretions of this sort are called internal secretions.
The epithelioid glands are masses or cords of cells which produce
internal secretions only. They are never provided with a duct or lumen,
although in some cases their cells arise from the wall of an epithelial tube.
They are closely related to the glands with obliterated ducts.
Finally there are glands which produce cells and are therefore called
cytogenic glands. These include the ovary and testis, which are epithelial
structures consisting of follicles and tubules respectively. They produce
the ova and spermatozoa. The other cytogenic glands are non-epithelial
bodies which produce various forms of blood corpuscles. They will be
considered in a later chapter.
The classification of glands, as presented in the preceding paragraphs,
is summarized in the following table:
GLANDS
59
I. Epithelial glands, with persistent ducts, producing external secretions.
1. Unicellular glands.
2. Simple glands.
a. Ectodermal, e.g., sweat and sebaceous glands.
b. Mesodermal, e.g., uterine glands.
c. Entodermal, e.g., gastric and intestinal glands.
3. Compound glands.
a. Ectodermal, e.g., mammary and lachrymal glands.
b. Mesodermal, e.g., epididymis and kidney.
c. Entodermal, e.g., pancreas and liver.
II. Epithelial glands, with obliterated ducts, producing internal secretions.
a. Ectodermal, anterior lobe of the hypophysis (the duct of the
posterior lobe is partially obliterated).
b. Entodermal, thyreoid gland.
III. Epithelioid glands, never having duct or lumen, producing internal
secretions. .
a. Ectodermal (through their relation to the sympathetic nerves),
chromaffin bodies; and medulla of the suprarenal gland.
b. Mesodermal, cortex of suprarenal gland; interstitial cells of
the testis; corpus luteum.
c. Entodermal, islands of the pancreas; epithelioid bodies in
relation with the thyreoid gland; thymus (?)
IV. Cytogenic glands, producing cells.
a. Mesodermal, epithelial the ovary and testis.
b. Mesodermal, mesenchymal the lymph glands, haemolymph
glands, spleen, red bone marrow, and many smaller lymphoid
structures.
THE MESENCHYMAL TISSUES.
Mesenchyma (/wos middle, trxyf^, an infusion) is a term introduced
by O. Hertwig, in 1883, for the tissue produced by cells which have
wandered out from the epithelial germ layers into the spaces between them.
It is found only in young embryos. In the adult it is represented by a
large group of derivatives, including connective tissue, adipose tissue,
cartilage, bone, smooth muscle fibers, tendons, fasciae, and various
special forms of cells. Mesenchyma arises chiefly from different parts
of the mesoderm, as already described (p. 42), but in the head of the
chick embryo a portion of it comes from the ectoderm, and in the wall of
the intestinal tube, according to Hertwig, the entoderm contributes to
its formation. Together with the blood islands it constitutes the entire
non-epithelial tissue of the embryo in early stages. It consists of a net
60 HISTOLOGY
work of branching cells, in the meshes of which there is a homogeneous,
fluid, intercellular substance. The intercellular portion of the tissue
becomes highly developed and variously modified.
Although typical epithelium and mesenchyma are radically different,
as shown in Fig. 31, p. 42, there are conditions in which they are comparable. Thus dense mesenchyma, in which the cells are closely packed
and have very little intercellular substance, resembles epithelium, and it
may give rise to groups or cords of epithelioid cells. Moreover epithelium may resemble mesenchyma by forming a vacuolated syncytium, or
as seen in Fig. 49, a branching protoplasmic network. In epithelium the
intercellular spaces arise as vacuoles in the exoplasm, and the intercellular substance of mesenchyma may also be considered as occupying
coalescent vacuoles.
Intercellular spaces.
Nuclei
Intercellular bridges.
PIG. 49. FLAT EPITHELIAL CELLS FROM THE BRANCHIAL PLATE OF A LARVAL SALAMANDER. X 300
The tissue of the adult which most closely resembles mesenchyma is
known as reticular tissue. It cannot, however, be regarded as an immature connective tissue, or a persistence of the primitive mesenchyma,
since it arises rather late in embryonic development (e.g., in the lymph
glands which first appear hi human embryos measuring about 45 mm.,
and in the oesophagus of embryos of 30 mm.). It is therefore considered
to be a special form of connective tissue.
RETICULAR TISSUE.
Reticular tissue forms the framework of lymph glands, red bone
marrow and the spleen; it occurs as a layer immediately beneath the
epithelium of the digestive tract, and has been reported in many other
organs. It consists of a network of cells in relation with an abundant
RETICULAR TISSUE
6l
fluid intercellular substance (Fig. 50). The protoplasmic processes of
the primitive mesenchyma have become transformed into flattened strands
or slender fibers, which are clear and homogeneous, and anastomose ?
freely. The cells associated with these fibers contain pale," flattened,
oval nuclei, with few chromatin granules. In ordinary sections reticular
tissue will be most readily recognized by the cells lodged in the fluid intercellular substance. These cells, which are chiefly lymphocytes, having
round nuclei and a narrow rim of protoplasm, are often so abundant that
the tissue appears as a dense cellular mass in which the framework of
reticular tissue is almost completely hidden. Upon careful examination,
however, some of its nuclei and fibers can always be detected.
FIG. 50. RETICULAR TISSUE SEEN IN A FROZEN SECTION OF A DOG'S SPLEEN WHCIH HAD BEEN INJECTED
WITH SILVER NITRATE. Xaso. (Mall.)
A, artery with its ampullae (a) : V, vein. .
In order to study reticular tissue advantageously, the lymphocytes
and other forms of free cells should be disengaged from its meshes. This
may be accomplished by shaking or brushing the sections; or by artificially digesting the specimen (which if properly done will destroy the
cells, including those of the reticular tissue, but will leave the network
of fibers); or by the following ingenious method devised by Mall. A
piece of fresh spleen is distended by injecting gelatin into its substance;
it is then frozen and sectioned. The sections are put in warm water, which
dissolves out the gelatin, carrying the loose cells with it, and leaves areas of
clear reticular tissue. Professor Mall has also shown how to wash out the
62 HISTOLOGY
pulpy contents of the entire spleen, so as to leave the framework of connective and recticular tissue, which may be inflated and dried (Zeitscbr.
f. Morph., 1900, vol. 2, pp. 1-42). Such preparations give an idea of
the intricacy of the reticular meshwork that can be obtained in no other
way, and yet the finer ramifications have been destroyed by this process.
There has been considerable discussion as to whether the fibers of reticular tissue are
chemically different from those of ordinary connective tissue. They differ from the
elastic elements of connective tissue, since reticular fibers are dissolved by both acids
and alkalis which leave the elastic fibers intact; and they are not destroyed by pancreatic
digestion which causes the elastic fibers to disintegrate. But the differentiation of the
reticular fibers from the "white fibers" of connective tissue has not been successfully
accomplished. Mall has shown, however, that tendon, consisting largely of white fibers,
is dissolved more readily by boiling in p.c. solutions of potassium hydrate or hydrochloric acid, respectively, than sections of lymph glands; and the name reticulin has
been introduced for a constituent of the reticular fibers which does not yield gelatin on
boiling. Reticulin is not generally recognized as an independent substance, and
reticular tissue often appears to blend with white fibrous connective tissue.
The recognition of reticular tissue depends, therefore, on its form rather than on
its chemical constitution.
Mucous TISSUE.
The substance of the umbilical cord is composed of mucous tissue.
At birth it is a peculiar gelatinous mass of pearly luster, which has long
been known anatomically as Wharton's jelly. During its development
from mesenchyma, a large amount of mucus becomes deposited in its
intercellular spaces. This mucus, like that produced in the goblet cells
and that found in the cornea and vitreous body of the eye, is a translucent substance which contains mucin. Chemically there are many
(varieties of mucins. They are compound protein bodies containing a
'carbohydrate complex in their molecules, and are therefore known as
glycoproteins. True mucins are formed in abundance in goblet cells and
in mucous tissue; to a less extent they occur in all embryonic connective
tissue. Related substances, called mucoids, have been obtained from
tendon, cartilage and bone.
In the umbilical cord the mucus may be regarded as a secretion which
is produced without the formation of special granules or vacuoles, and is
discharged equally from all surfaces of the cells. It is a homogeneous
ground-substance, in which extremely delicate fibrils are imbedded. These
are gathered in wavy bundles (Fig. 51, a). Fibrils of the same sort,
generally arranged in denser bundles, are found in ordinary connective
tissue, and constitute the white fibers. Chemically they are said to consist of collagen, an albuminoid body which on boiling yields gelatin, the
source of glue. The origin of the collagenous fibers has been the subject
of repeated investigation. Henle (1841) considered that they arose in
MUCOUS TISSUE 63
the intercellular substance, apart from the cells, and Merkel defends this
idea in the following passage, here somewhat abbreviated (Anat. Hefte,
Abt. i, 1909, vol. 38, pp. 323-392):
The mesenchymal syncytium secretes an amorphous gelatinous substance, which
may be scanty (as in reticular tissue) or abundant (as in the umbilical cord) . The fibers
arise exclusively in this gelatinous substance; the cells take no direct part in the formation of the fibers but serve only for the production of the jelly. At their first appearance
the fibers are not collagen, and generally they are not yet smooth and glistening like
true connective tissue fibers. Instead they are granular, and not infrequently varicose.
Later, though often very soon, they acquire the characteristic appearance of fully
a
FIG. 51. Mucous TISSUE OF THE HUMAN UMBILICAL CORD, STAINED WITH PHOSPHO-TUNGSTIC ACID
H*MATOXYLIN. (Mallory.)
a, White fiber, b, fibroglia.
formed connective tissue fibers. They may arise as a very delicate network, which,
through the breaking down of the least utilized threads, becomes transformed into
smooth and unbranched fibers. But in places where from the first there is a decided
stretching, as in tendon, parallel unbranched fibers are formed directly. Professor
Heiderich has shown me preparations of a mucin, in which, by the addition of acid,
structures were formed which were strikingly similar to developing connective tissue
without any stretching, nets with round meshes; but with the slightest traction, long
fibers isolated from one another. Thus connective tissue fibers are merely the effects
of mechanical conditions upon the gelatinous intercellular substance.
A very different idea of the origin of the white fibers is that of Flemming, recently further elaborated by Meves (Arch. f. mikr. Anat., 1910,
vol. 75, pp. 149-208), according to whom the fibers arise within the cytoplasm. By special methods Meves has demonstrated coarse filaments,
which he names chondrioconta, within the protoplasm of both epithelium
and mesenchyma. These granule-rods or chondrioconta (probably
comparable with the mitochondria of gland cells) are regarded as a part
of the fundamental protoplasmic network or spongioplasm. If they
are short they are called chondriosomes. Meves describes the development of white fibers as follows:
64 HISTOLOGY
Connective tissue fibrils are produced from the chondrioconta which come to lie at
the surface of the cell. They then change their chemical constitution and are no longer
stained by iron haematoxylin or fuchsin. At this stage those which are in a row unite
end to end. Thus in the formation of a fibril numerous cells take part, each producing
a section. The fibrils again change their chemical constitution and become intensely
stained by the collagen stains. Finally they become free from the cells and lie in the
intercellular spaces. From the time of their first formation they have a wavy course,
which may become more marked later. This clearly means that the connective tissue
fibers have grown in length more than the surrounding elements. They increase also
in diameter through independent growth, and for a time new fibers are produced by the
cells I differ with Flemming since I consider that connective tissue fibers
are not formed within the cell body but are produced at the cell surface (by transformia
tion of the chondrioconta) ; I agree with him in deriving them from the cytoplasmcfilaments.
The umbilical cord "has long been regarded as a particularly favorable
object for the study of white fibers, but the way in which they arise remains
undetermined. In addition to these white fibers, the umbilical cord contains stiff fibers of a different nature, found at the periphery of the cells.
They are similar to the fibers of a tissue which forms the framework for
the branching nerve cells, thus binding them together, and accordingly
named neuroglia (vevpov, nerve, y\ta, glue). Fibers similar to those
of the neuroglia, found at the periphery of muscle cells, are called border
fibrils or myoglia. In 1903 Mallory described similar border fibrils in
connective tissue and named them fibroglia. They are seen at the periphery of the cells in the umbilical cord (Fig. 51, b). Mallory describes
them as follows (Journ. Med. Res., 1905, vol. 13, pp. 113-136):
Neuroglia, myoglia and fibroglia fibrils morphologically and in certain staining
reactions more or less closely resemble one another. They touch or form part of the
periphery of the cell protoplasm, but continue away from the cell in two directions, *..,
they do not begin or end in the cell which produces them. How far the fibroglia are
accompanied by protoplasmic processes cannot be determined. The number of these
fibrils to a cell is not constant, but it is usually in the neighborhood of a dozen.
Professor Mallory has found no transitions between the fibroglia and
the white fibers. Meves likewise considers them as entirely distinct,
and states that the production of white fibers by the cells of the umbilical
cord terminates by the fifth month. The fibroglia are present at birth,
and probably no tissue is more favorable for their study than the umbilical
cord at term.
In addition to the mucous matrix, the white fibers, and the fibroglia,
mucous tissue contains cells and intercellular spaces. The cells, at first
stellate with many anastomoses, become elongated and more or less disconnected from one another. Three of their nuclei are shown in Fig. 51,
but their cytoplasm forms a thin layer, the limits of which can scarcely
be determined. The intercellular spaces contain a fluid through which
CONNECTIVE TISSUE
cells may migrate. There are no capillaries, lymphatic vessels, or nerves
within the mucous tissue of the umbilical cord, and no elastic fibers. The
three large blood vessels which pass through the cord, and the tissue in
their walls, will be considered later.
CONNECTIVE TISSUE.
Connective tissue occurs in various forms. Dense connective tissue
is a tough fibrous substance, such as that part of the skin from which leather
is made; and loose connective tissue, or areolar tissue, is a spongy cobweb
of delicate filaments, such as occurs between the muscles. Both forms
when fresh are very white, and they are composed of similar fibers. A
small mass of fresh connective tissue, subcutaneous or inter-muscular,
may be spread out with needles upon a slide, thus forming a thin film.
After adding a drop of water and applying a cover glass, it will present
V
FIG. 52. SUBCUTANEOUS TISSUE FROM A CAT.
The fiber a has been treated with dilute acetic acid; the other fibers have been teased apart and examined,
unstained, in water, a, c, White fibers; b, fat cell; d, connective tissue cell; e, elastic fibers.
such an appearance as shown in Fig. 52. The bulk of the tissue is seen
to consist of white or collagenous fibers felted together (Fig. 52, c). They
are the same in origin and structure as those already described in the
mucous tissue of the umbilical cord, but in ordinary connective tissue
their fibrils are gathered into denser bundles. Each bundle or fiber is
composed of exceedingly minute fibrils, bound together by a small amount
of cement substance. The addition of picric acid causes the fibers to
separate into their constituent elements. Often a bundle of fibrils turns
aside from the main trunk, so that the fiber branches, but the fibrils
themselves are unbranched.
Upon the addition of dilute acetic acid the white fibers swell and disintegrate, some of them passing through the condition shown in Fig. 52, a.
Such fibers show a succession of constrictions at places where they are
encircled by rings or spiral bands of a refractive substance not affected
5
66
HISTOLOGY
by the acid. These rings have been observed by Ranvier in living connective tissue fibers, and it is therefore improbable that they are remnants
of a sheath which surrounded the entire fiber, as some have thought.
They are probably formed of elastic substance.
In addition to the white or collagenous fibers, connective tissue contains fibers of a second sort, known as elastic fibers.' They are absent from
corneal tissue, the mucous tissue of the umbilical cord and generally,
though not always, from reticular tissue. Since they develop later than
the white fibers, they are not found in young connective tissue; but otherwise they are present, though varying
greatly in abundance, in all forms of connective tissue. They are not destroyed^
*
by dilute acids or alkalies, and are described as composed of elastin, an albuminoid body which does not vieldjrelatin on
boiling. Unlike the white fibers they are
not composed of smaller elements or fibrils,
but each fiber is a structureless homogeneous thread. In favorable cases, however,
an enveloping sheath may be seen. In
tissue which has not been torn apart the
elastic fibers form a net (Fig. 53, A). The
fibers meet and fuse with one another; and
across the angles thus formed, one or two
delicate strands are commonly to be found. When the tissue is pulled
apart so that the net is broken, the fibers kink and recoil like tense wires
(Fig. 52, e).
The origin of the elastic fibers has not been determined. They have
been said to arise within the cells by the fusion of granules of elastin.
Mall's idea of their exoplasmic origin is illustrated by their relation to the
cells in Fig. 53, B. Others consider that they are formed from the intercellular substance.
Although elastic fibers are clearly seen in fresh connective tissue,
they are often invisible in specimens stained with haematoxylin and eosin.
In order to determine their presence, sections may be stained with resorcin-fuchsin, which leaves the white fibers nearly colorless, but makes
the elastic fibers dark purple; or other special stains may be used. In
some situations, however, the elastic tissue is highly developed and may
be seen with any stain. This is true of the fenestrated membranes found
in many blood vessels. A fenestrated membrane is a network of elastic
fibers in which the fibers are so broad that they appear to form a perforated plate (Fig. 54, A). The greatest development of elastic tissue probably occurs in the ligament of the neck in grazing animals, which consists
\
FIG. S3.
A, Elastic fibers of the subcutaneous areolar tissue of a rabbit. (After Schafer.)
B, Cells in relation with elastic fibers,
after treatment with acetic acid. Subcutaneous tissue of a pig embryo.
(After Mall.)
CONNECTIVE TISSUE
6 7
of very coarse elastic fibers with very little white fiber. It is therefore commonly used for the histological and chemical study of elastic tissue (Fig.
54, B and C). In man the stylohyoid ligament and the ligamenta flava
are of this class, and they exhibit the yellowish color which is characteristic of elastic tissue. Elastic fibers are found also in the ground substance
of certain cartilages, which will be described later.
Connective Tissue Cells. In addition to white collagenous fibers and
yellow elastic fibers, connective tissue contains cells and intercellular
spaces. The cells which produce fibers are known as fibroblasts (/JAao-ros,
a bud, is used in many terms to indicate a formative cell, with a prefix
which usually designates the structure which it produces). Actively
FIG. 54. ELASTIC FIBERS.
A, Network of thick elastic fibers below, passing into a fenestrated membrane above. From the human
endocardium. B, Thick elastic fibers (f) from the ligamentum nuchae of the ox; b, white fibers.^C,
Cross section of the ligamentum nuchae, lettered as in B.
growing fibroblasts, both in the embryo and in the adult, exhibit fibroglia
fibrils at their borders, but in mature connective tissue these fibrils are
seldom found. The cells of fully formed connective tissue are generally
flattened or lamellar, consisting of a thin pale layer of almost homogeneous
protoplasm, which is sometimes vacuolated. Such cells when seen on
edge are spindle-shaped. They may be spread out in flat layers, retaining
the protoplasmic connections characteristic of mesenchyma, as seen in the
mesentery (Fig. 55, c). In dense connective tissue the cells also exhibit
broad thin protoplasmic processes (Fig. 56, c), but they have become more
or less detached from one another. The cells are bent to conform with
the adjacent fibers, to which they are closely applied, and along which,
in living tissue, they have been observed to migrate. The nuclei of these
cells are elliptical on surface view, and rod-shaped when seen on edge.
They contain fine chromatin granules, and sometimes a small but distinct
nucleolus. Occasionally the nuclei are indented on one side. The centro
68
HISTOLOGY
some, in a clear area of protoplasm, has been found close beside the nucleus. In ordinary specimens, stained with haematoxylin and eosin, the
centrosome is not seen, and the entire cytoplasm is quite inconspicuous;
I- b.v.
FIG. 55. CONNECTIVE TISSUE CELLS (c) AND A MAST CELL (m )FROM THE MESENTERY OF A RAT. X 1000;
b.v. a small blood vessel lined with endothelial cells. The specimen was fixed in alcohol and stained with
Unna's methylene blue.
but the nuclei stand out prominently along the edges of the fibers (Fig.
56, x).
Cells in connective tissue which differ from the fibroblasts by having
abundant protoplasm in the form of large round cell bodies, were named
Pic. 56. CONNECTIVE TISSUE CELLS (c) A LYMPHOCYTE 0) AND PLASMA CELLS (p) FROM A LACTATING
HUMAN BREAST. X 1000.
A vacuolated plasma cell is shown at v, and a connective tissue cell on edge is seen at x.
plasma cells by Waldeyer (Arch. f. mikr. Anat., 1875, vol. n, pp. 176194). He stated that they develop from connective tissue cells, and are
always arranged about the blood vessels. Two years later, in the same
CELLS IN CONNECTIVE TISSUE 69
journal, Ehrlich published the first of his far-reaching investigations on
the effects of various anilin dyes upon protoplasm. He showed that the
plasma cells found near the vessels in the mesentery of the rat, when
stained with basic dyes, exhibit very coarsely granular protoplasm (Fig.
55, m). Further studies led him to separate these granular cells from the
other forms of plasma cells. He was inclined to believe that they arose
from over-nourished connective tissue cells, and accordingly named them
mast cells (Mastzellen), referring to the mast or acorns on which animals
are fattened (Arch. f. Physiol., 1879, pp. 166-171). In another communication in the same volume (pp. 571-579), he introduced a further
subdivision of cells which may be alike in form but which react differently to the anilin dyes. In contrast with the basic granules of the mast
cells, which are not stained with the acid dye eosin, he found other granules
which stain deeply with eosin but do not respond to the basic dyes.
These granules are now generally known as eosinorjhjHCi and the cells
which contain them are called eosinophiles. Mast cell granules are often
referred to as basophilic, but since some confusion results from calling
the entire cells basophiles, they are still known as mast cells. Cells of
both classes are found in the circulating blood, and will be described with
the blood corpuscles; both kinds are found also in the intercellular spaces
of connective tissue. It is known that various forms of blood corpuscles
develop in the reticular tissue of lymph glands and bone marrow, from
which they enter the blood vessels; and it is also very evident that cells
leave the vessels and enter the intercellular spaces of connective tissue.
There has been endless discussion as to whether the eosinophiles of connective tissue and blood are the same sort of cell; and also whether the
"mast leucocytes" in the vessels and the mast cells in the surrounding
tissue are identical. Maximow states that there is no genetic relation
between mast cells and mast leucocytes in the adult, but "whether in
embryonic life they are likewise independent is still undecided." As to
the eosinophiles, he says: Those found in the connective tissue are
generally eosinophilic corpuscles which have emigrated from the vessels.
"Any proof of a local origin in connective tissue is lacking." But Weidenreich considers that eosinophilic granules are derived from broken-down red
corpuscles, which are taken up by white blood corpuscles and by connective tissue cells, both of which become thereby eosinophilic.
In ordinary sections of connective tissue, stained with haematoxylin
and eosin, eosinophiles are seldom overlooked, because of the brilliant
color of their granules. Mast cells, however, should be sought for in
tissue preserved either in formalin or alcohol, and stained with Unna's
polychrome methylene blue or some other basic dye. The preparation
shown in Fig. 55 is a portion of the mesentery preserved by being tied
across the end of a short glass tube and immersed in alcohol. The tissue
70 HISTOLOGY
was then stained with methylene blue, and mounted without being sectioned. Most of it is colored pale blue, but the granules of the mast
cells are deep purple. Such granules, which assume a color different from
that of the stain employed, 'are called by Ehrlich metachromatic. The
granules of mast cells are so coarse that in favorable places, when examined
with an immersion lens, they can readily be counted. They spread over
and obscure the nucleus, which appears as a pale central area.
Mast cells and eosinophiles were removed by Ehrlich from the miscellaneous group of plasma cells described by Waldeyer. Another type of
cell was discovered in syphilitic connective tissue by Cajal, and independently described in tuberculous tissue by Unna (Monatsch. f. prakt.
Dermatol., 1891, vol. 12, pp. 296-317). He states that these cells (to
which the name plasma cells has come to be restricted) arise from normal
connective tissue cells by the increase and rounding off of the cell body.
As described by Unna, the granulation of the protoplasm is so fine that
even with the highest magnification the individual granules cannot be distinctly recognized as such.
Typical plasma cells are shown in Fig. 56, p. They usually have
very round nuclei with characteristic coarse masses of deeply staining
chromatin. These masses may appear as wedge-shaped bodies with their
broad ends against the nuclear membrane so that they resemble the
spokes of a wheel ("Radkern"); or the chromatin blocks may suggest
the squares of a checker-board. The nucleus occupies an eccentric position in the mass of dense and deeply staining protoplasm. Specific granulation, such as occurs in mast cells and eosinophiles, is absent. In certain
plasma cells, vacuoles are seen (Fig. 56, v) which contain a "homogeneous,
semifluid, colloid-like substance which has a strong affinity for acid dyes."
If the affinity for such dyes has become well marked, these vacuoles
form conspicuous structures, known as Russel's bodies. Usually they
are regarded as degenerative products, but some investigators consider
them as secretions.
Associated with plasma cells, lymphocytes are often found (Fig. 56, 1).
These cells constitute an important class of white blood corpuscles or
leucocytes. They differ from plasma cells in having only a small rim
of pale protoplasm about the nucleus, but the nuclei of these two sorts of
cells are very similar. Although Ehrlich (1904) agreed with Unna that
only one source for the plasma cells had been established, "namely, an
origin from hypertrophied connective tissue cells," many authorities
now believe that they develop from lymphoid cells or lymphocytes.
Councilman expresses this opinion as follows (Journ. Exp. Med., 1898,
vol. 3, pp. 393-420):
As to their origin I hold the same opinion as Marschalko, that they are derived from
lymphocytes. In the kidney they enter into the interstitial tissue by emigration from
CELLS IN CONNECTIVE TISSUE 71
the blood vessels. They may emigrate from the vessels as plasma cells, or they may
be formed from emigrated lymphoid cells. They have been seen in the act of emigration and the shapes of many of the cells in the interstitial tissue can leave no doubt as
to their amoeboid character. We are led to the belief that the plasma cells have their
origin in the lymphoid cells from the similarity of their nuclei to those of lymphoid cells
and from the presence of transitional forms.
Downey (Folia haemat., 1911, vol. n, pp. 275-314) supplies a useful
review of the literature of plasma cells, and expresses his opinion that
they arise from several sources.
Plasma cells are found in connection with chronic inflammation of
many sorts. They occur normally in abundance in the mucous membrane
of the digestive tube from the stomach to the rectum, and they may be
seen in bone marrow and in the lymphoid organs. Occasional plasma
cells may be expected in subcutaneous tissue and in the breast.
Reviewing the preceding paragraphs it is seen that connective tissue
contains nbroblasts_or connective tissue cells, and that mast cells, eosinophilic cells, plasma cells and lymphocytes may be lodged in the intercellular spaces. Except the plasma cells, which probably develop from lymphocytes, these are all comparable with forms of blood corpuscles normally
found within the vessels. The source of these corpuscles will be further
considered with the blood, together with other forms which sometimes
leave the vessels but which are never regarded as constituents of connective tissue.
In the connective tissue of amphibia and mammals, Ranvier described certain
slender branched cells which he named clasmatocytes (Arch. d'Anat. micr., 1900, vol. 3,
pp. 122-139). This term refers to the detachment of portions of their processes, which
Ranvier believed took place normally as a method of discharging a secretion. The
breaking down was observed chiefly in amphibian cells which are now considered to be
mast cells. Like other mast cells they are prone to distintegrate. The cells in mammals, to which Ranvier referred, are regarded by Maximow as derived from wandering
lymphocytes. He believes that these may send out several processes, or become
spindle-shaped, thus producing "clasmatocytes," but since this name is inappropriate
he calls them resting wandering-cells. He finds that they contain a limited number of
vacuoles and coarse granules, but the granules are said to differ from those of mast cells
(Arch. f. mikr. Anat., 1906, vol. 67, pp. 680-757). The significance of these cells is
uncertain.
Connective tissue contains two additional types of cells, which are so
distinct that they may be regarded as separate tissues. These are the
pigment cells and the fat cells; the latter will be described as adipose
tissue.
Pigment cells. The color of the various tissues is due to pigments,
which may be involution, like the haemoglobin in red blood corpuscles and
the lipochromes in fat; or they may occur as granules imbedded in the
protoplasm. The granules, which are yellow, brown, or black, often
j 2 HISTOLOGY
retain their natural color in stained specimens. They are said to consist
of "melanin," which represents an ill-defined group of substances, some of
which are haemoglobin derivatives. In the lung, inhaled soot is taken
into the protoplasm of certain cells which thus become pigmented with
extraneous material. Pigment granules are widely distributed, and
may be found in the liver, spleen, heart, brain, and other organs.
In certain situations, pigment is extensively developed in branched
connective tissue cells such as are shown in Fig. 57, A. In man these
are of limited occurrence, being found near the eye, and in the pia
mater, especially under the medulla oblongata and upper portion of the
spinal cord. Weidenreich con
-*&! -^T^^ siders that this represents the
remains of a general pigmented
sheath for the entire nervous
system. In lower vertebrates
branching pigment cells are
often abundant in the subcutaneous tissue, and changes
FIG. 57. A, Two pigment cells from the deep, peripheral __1__ ,.,,-1, oc nrrnr in frr>a<s
part of the cornea of the rabbit. B, Pigmented m COlOr, SUCn aS OCCUr in irOgS,
epithelium from the conjunctiva of the guinea-pig. j , 4.-L- r ^-f ^^OI'^T-, /-.
The pigment is chiefly in the basal layer. are due tO the extension OT
retraction of these processes.
Such pigmented connective tissue cells are called chromatophores or
chromatocytes. But in the human skin the pigment granules are in the
epidermis, chiefly in the basal layers. In the stratified epithelium of the
conjunctiva of the eye, toward the cornea, numerous pigment granules
are found in the basal layers, and scattered groups occur also in the outer
layers, as shown in Fig. 57, B. Pigment in this situation occurs frequently in the Caucasian race, and regularly in the other human races.
Simple epithelium may be densely pigmented, as in the external epithelium of the retina. Thus it is seen that pigment cells are by no means
limited to connective tissue.
ADIPOSE TISSUE.
If in a freshly killed animal a loop of intestine is drawn out of the
abdominal cavity, the blood vessels ramifying in its mesentery will be
seen to be imbedded in a band of fat, which branches when the vessels
branch, and diminishes in width toward the intestine as the vessels
become small. The close relation between the distribution of fat and the
course of the vessels is notable also in sections. Fat cells occur in groups
or lobules around the vessels, and are found, with few exceptions, wherever
there is loose connective tissue. They may also occur singly, as in some
parts of the denser connective tissue of the breast.
ADIPOSE TISSUE
73
When examined fresh, each fat cell appears as a large round oil-drop,
which is more or less compressed into a polyhedral shape by the surrounding cells. It is highly refractive, having a border which becomes
alternately bright and dark on changing the focus. The liquid fat or
oil which fills the cell, leaving only an imperceptible film of protoplasm
around it, may escape by the rupture of the membrane, thus forming
smaller drops. In the specimen shown in Fig. 52 the fat was seen coming
out from the upper surface of one of the cells, and the droplets thus
emerging ran together forming larger ones. As fat cells develop,
a coalescence of small drops occurs in the protoplasm.
The earliest formation of adipose tissue is said to occur in human
embryos of the fourth month. It may be studied advantageously in
the subcutaneous tissue of embryos of the fifth month (Fig. 58). In
such specimens there are areas of loose and very vascular mesenchyma,
found at the level of the roots of the hairs, in which certain cells exhibit
vacuoles. These cells are at first quite like the surrounding fibroblasts,
FIG. 58. SUBCUTANEOUS FAT CELLS FROM A HUMAN
EMBRYO OF THE FIFTH MONTH X 520.
n Nucleus; f.v., fat vacuole; p. r., protoplasmic rim.
FIG. S9- FAT CELLS FROM NEAR THE KIDNEY OF
A NEW-BORN CHILD. X 520.
being fusiform or stellate. Their protoplasm contains several small
vacuoles, some of which unite to form one large drop, and the nucleus
together with the greater part of the protoplasm, is pushed to one side
(Fig. 58, n). Sections of such cells have the form of "signet rings."
Frequently small vacuoles are seen in the accumulation of protoplasm
beside the nucleus. With further development the fat droplet becomes
so large that the protoplasmic rim appears as a mere line or membrane,
just within which is the greatly flattened nucleus. During the formation
of the fat cells, the branching processes become very short, but it is
doubtful whether they are altogether lost.
For some years after birth fat cells containing several vacuoles are
found in certain situations, as around the kidney (Fig. 59) and in the
outer layer of the cesophagus. Usually these are regarded as immature
forms.
Adipose tissue of the adult, when well preserved, presents cells of
74
HISTOLOGY
rounded form as shown in Fig. 60; often, however, their thin walls are
bent or collapsed. If the sections are thick, a network of a different
pattern, representing another layer of cells, will come into view on changing the focus. The nuclei of the fat cells are pale oval bodies, with finely
granular chromatin (Fig. 60, n), often containing one or two small vacuoles.
The protoplasm around the nucleus forms such a thin layer that it is
scarcely appreciable on surface view. Both nucleus and protoplasm
are much darker when seen on edge, since a thicker layer of substance is
thus presented. When sectioned in this position the nuclei within the
cells must be carefully distinguished from those of the connective tissue
just outside. Many of the fat cells will show no nuclei, since the entire
cell is usually not included within the limits of one section.
In extreme emaciation, the fat cells become small and the protoplasmic
rim thickens, so that the cells again assume the signet-ring form. A
FIG. 60. NORMAL ADIPOSE TISSUE FROM AN ADULT.
X 400.
Connective tissue is seen at the left of the figure and
(as at c. t.) between the fat cells; n, nucleus
of a fat cell.
FIG. 6r. FAT CELLS FROM THE OMENTUM IN A
CASE OF EXTREME EMACIATION. X 520.
b. v. f blood vessel; f. c., fat cell.
delicate reticulum appears between the shrunken, cells as shown in Fig.
61. Some of the fibers proceed directly from the fat cells, indicating
that the processes have never wholly disappeared. Others come from
the fibrofrlasts which from the first are scattered among the fat cells.
The great difference between the appearance of fresh fat cells and
those seen in sections is due to the fact that fat is dissolved by the reagents
ordinarily used in preserving the tissue. Thus the sections usually
show empty vacuoles and no fat whatever. Occasionally, as a result
of cooling, the fat has formed insoluble crystals in the shape of radiating
needles, and these, or an amorphous precipitate which takes a bluish
stain with haematoxylin, may be seen within the cells. Although fat
is the commonest substance to be found within the vacuoles in human
ADIPOSE TISSUE 75
tissues, it is not the only material which may have filled them, and
therefore to demonstrate the presence of fat, special methods must be
employed. Fresh tissue may be preserved in osmic acid, which blackens
not only fat but some related substances; or frozen sections of tissue may
be stained with Sudan III or Scharlach R, which color fat droplets red
and demonstrate them even when minute. These stains may also be
used after preservation of the tissue in formalin. It may be noted that
Sudan III has been fed to animals, thus imparting a pink color to the
living adipose tissue. If the animal is lactating, the fat globules in the
milk also become pink.
Fat vacuoles occur in many sorts of cells which do not belong to adipose tissue, such as the cells of the liver, cartilage, and striated muscle.
These cells are not called fat cells, even if their protoplasm contains many
vacuoles, and they do not resemble the cells of adipose tissue.
Since fat cells occur in lobular masses in definite places, as under the skin, around
the kidney, in the bone marrow, etc., and since they supply the body with nutriment,
it has been proposed to regard them as constituting glandular organs. They receive
fat from the adjacent vessels and store it, or quite possibly they absorb carbohydrates
and convert them into fats. The formation of fat has been said to begin in or near
the nucleus with the production of granules, but the part which the nucleus plays
is uncertain. The small vacuoles often seen within it apparently arise after the cell
is full of fat. Mast cells have often been found associated with fat cells and it has
been supposed that they contained secretory granules which were concerned with
fat production. Like an internal secretion, fat is taken from the cells into the vessels
and distributed over the body.
TENDON.
Tendons consist essentially of very dense connective tissue. They
are composed almost wholly of parallel white or collagenous fibrils, com
FIG. 62. LONGITUDINAL SECTION OF THE FIG. 63. TENDON CELLS FROM THE TAIL
TENDON OF THE FLEXOR LONGUS DIGITORUM. OF A RAT. STAINED WITH METHYLENE BLUE.
X 160. INTRA VITAM. (Huber.)
pactly bound together in bundles. The cementing matrix contains tendomucoid. Closely applied to the bundles are the tendon cells which produced them. In ordinary longitudinal sections of tendon, the protoplasm
of the cells is indistinct or imperceptible, but the nuclei appear in rows
as seen in Fig. 62. In special preparations, particularly in those of the
7 6
HISTOLOGY
P .b
delicate tendons found in the tail of a rat or mouse (Fig. 63), it is seen that
the cytoplasm of tendon cells forms a plate-like layer which is folded
about the fiber bundles, tending to encircle them. Moreover the cells
are provided with lamellar or wing-like projections, which extend out between adjacent fiber
bundles. Apparently there are protoplasmic
connections, end to end, between the cells, which
thus form longitudinal rows or chains; and in
cross sections of the tendon some of the wing-like
projections anastomose as seen in Fig. 64. Thus,
as in connective tissue, the original syncytial arrangement of the mesenchyma is partially, preserved.
The primary tendon bundles, which consist
chiefly of white fibers and tendon cells, contain
also a small amount of elastic tissue in the form of
fine, wide-meshed networks. The elastic fibers
are said to occur especially near the cells and their
processes. The primary bundles are generally
grouped in secondary bundles or fasciculi, which
are bounded by partitions or septa of looser connective tissue (Fig. 65). Within the septa there are nerves and blood
vessels, in relatively small number. Lymphatic vessels are said to be
FIG. 64. FROM THE CALCANEAN TENDON (TENDO
ACHILLIS) OF A RABBIT.
(After Prenant.)
p. b., Primary bundle bounded
by a cytoplasmic sheath.
sh., which extends from a
tendon cell, t.c. p., process
extending into a primary
bundle. The entire figure
is a portion of a secondary
bundle.
.!
: i
Septum. Blood vessel. Fasciculus. Fibrous sheath.
FIG. 65. FROM A CROSS SECTION OF A TENDON FROM AN ADULT MAN. X 40.
confined to the sheath of connective tissue which surrounds the entire
tendon, with which the septa are continuous (Fig. 65).
The fibrous sheath or vagina fibrosa, which surrounds the tendon,
TENDON
77
may contain a cavity filled with fluid. It is then called a mucous sheath
or vagina mucosa. The cavity arises as a cleft in the embryonic connective
tissue and its walls are formed of mesenchymal epithelium. The cells
have become flattened and the fibers felted together to bound the space.
It contains a fluid like that of the joint cavities, being chiefly water and
a mucoid substance which renders it viscid, together with protein material
and salts. The function of the mucous sheath is to facilitate the movements of the tendon. By its formation the tendon is freed from the local
connection with surrounding tissue, and the sheath generally occurs where
such connection would especially interfere with motion. The mucous
burs<z are similar structures in relation with muscles or bones. The joint
cavities, to be described later, belong in the same class, having a similar
origin and function.
Aponeuroses, fasciae and ligaments are connective tissue formations,
resembling tendon in possessing a more or less regular arrangement of
cells and fibers. Elastic elements may be abundant.
Mes.
Pre.Cart.
Cart.
CARTILAGE.
Cartilage is a mesenchymal derivative, the development of which it
is difficult to follow, since at certain stages its nuclei are so crowded that
they obscure the transformation of the intercellular substance. Two
interpretations of its development are illustrated in Fig.
66, A and B. As represented
in A, the mesenchymal cells
multiply and come together so
that the intercellular spaces
are obliterated. Thus precartilage is formed, consisting
of large closely adjacent cells,
separated from one another by
thin walls which stain red
with eosin. This type of precartilage has been frequently
described in the lower vertebrates. It becomes cartilage by the thickening and chemical transformation of its exoplasmic walls. They form an intercellular ground substance or matrix, which stains blue with haematoxylin. According to
Professor Mall the same result is produced in another way, as shown in
Fig. 66, B. The mesenchymal cells in becoming precartilage produce a
fibrillated exoplasm. The nuclei with the surrounding endoplasm then
become "extruded from the syncytium" and lie in the intercellular spaces.
FIG. 66. DIAGRAMS OF THE DEVELOPMENT OF CARTILAGE
FROM MESENCHYMA.
A, Based upon Studnifika's studies of fish; B, upon Mall's
study of mammals. Mes.. Mesenchyma; Pre. Cart.
precartilage; Cart., cartilage.
78 HISTOLOGY
At the same time the fibrillated exoplasm becomes transformed into the
homogeneous matrix of the cartilage, which stains blue with haematoxylin.
Whether or not the cells are extruded may be questioned, but the relation of the fibrous to the homogeneous matrix, which is shown in the
figure, may readily be observed around the vertebrae in pig embryos.
After the cartilage has formed, the cells occupy cavities, or lacuna, in
the matrix. It is probable that in the living condition the cartilage cells
completely fill their lacunae, but in preserved specimens they are often
irregularly shrunken. Usually the protoplasm of each cell is of a spongy
vacuolated texture, which is in part due to fat droplets and in part to
glycogen; in ordinary sections, both of these substances have disappeared,
leaving empty spaces.
Glycogen is a carbohydrate which resembles starch and is therefore sometimes
called "animal starch." It is soluble in water, and soon after death it becomes
converted into glucose. For both of these reasons it disappears from ordinary sections. Fresh tissues, preserved in strong alcohol and stained with tincture of iodine,
exhibit glycogen as brownish-red granules which may be aggregated in masses of
considerable size. Glycogen is found not only in cartilage cells but also in striated
muscle and in the cells of the liver. In the embryo it has a wider distribution. At
certain stages of development, according to Gage, it occurs in the cells of the nervous
system and is abundant in the epidermis, the digestive tube, and the ccelomic epithelium. Its production, like that of fat, varies with nutritive conditions, and it accumulates in well-nourished individuals.
The cartilage cells are said to be enclosed in capsules, which are often
transparent and inconspicuous linings of the lacunas. Sometimes they
appear as rather broad bands which are concentrically striated, indicating that they were deposited in successive layers. The layers of newly
formed matrix, which bound the lacunae, usually stain very dark blue
with haematoxylin. The deep color is probably due to chondromucoid.
Peripherally the color blends with that of the older matrix, which takes
a pale blue stain. Like the intercellular substance of connective tissue
the matrix of cartilage may contain white and elastic fibers, but in its
commonest form it appears homogeneous and hyaline. Chemically it is
a mixture of collagen, chondromucoid, chondroitin sulphuric acid in combination, and albuminoid substances (albumoid). The old term "chondrin" really means little else than the matrix of cartilage, which on superficial examination is found to be a dense body. Within it, however, the
cells produce new ground substance and push themselves apart from one
another by interstitial growth. The cells in the interior of the cartilage
are often much larger than those at the periphery, and the increase in
the size of their lacunae is probably accomplished by the resorption of the
adjacent matrix. The cells divide by mitosis, and after division two of
them are found in a single capsule. They then move apart, and a partition, at first very slender, is formed between them. They may remain
CARTILAGE 79
grouped as a pair, forming a bisected elliptical figure, or they may divide
again, producing either a row of cells or a cluster of three or four (Fig.
66). Since the cells change their positions with difficulty in the dense
matrix, they are regularly found in very characteristic groups. It has
been asserted that certain cartilage cells undergo mucoid degeneration
and become lost in the matrix. In old cartilage dark spots, staining intensely with haematoxylin, are suggestive of such a process. Such cells
must be carefully distinguished from tangential sections of the deeply
staining pericapsular matrix.
Cartilage grows not only by the interstitial increase of the cells and
matrix in its interior, but more especially by appositional growth, through
A B
FIG. 67. THE THREE TYPES OF CARTILAGE: A, HYALINE; B, ELASTIC; C, FIBROUS. (Radasch).
a, b, Outer and inner layers of perichondrium; c, young cartilage cells; d, older cartilage cells; e, f, capsule
surrounded by deeply staining matrix; g, lacuna.
the formation of new cartilage over its external surface. Around every
cartilage in the adult, there is a connective tissue envelope, the perichon-\
drium, containing undifferentiated cells which multiply and become
transformed into cartilage cells (Fig. 67, A). These are added at the
surface, undergoing in a thin layer such changes as are shown in Fig. 66.
The young generations of cells are therefore at the periphery of the
cartilage, and the oldest cells, or the groups which they have produced,
are in the center. Between them an interesting series of cytomorphic
changes may be observed. Since the perichondrium is the formative
layer, a more or less perfect regeneration of cartilage may occur after
surgical operations if the perichondrium is left in place, but not otherwise.
The perichondrium contains vessels and nerves, none of which pene
80 HISTOLOGY
trate the matrix of the cartilage. In some cases, however, vascular connective tissue occupies an excavation in its peripheral portion. Whatever nutriment the cells in the interior of the cartilage receive is obtained
by diffusion through the matrix. It has been asserted that this diffusion
takes place through a system of canals penetrating the matrix, and passing from one lacuna to another as in bone. But in mammalian cartilage the only canals which have been recorded are presumably the
result of shrinkage, such as may be produced by treating the specimen
with absolute alcohol or ether.
The three principal forms of cartilage hyaline, elastic, and fibrocartilage and the exceptional "vesicular supporting tissue" may be
further described as follows:
Hyaline cartilage, the commonest type, is characterized by its clear,
pale bluish or pearly translucent matrix, which is ordinarily free from
fibrils. The nasal cartilages, most of the laryngeal cartilages, and the
tracheal and bronchial rings are of this variety, together with the xiphoid
and costal cartilages, and the articular cartilages which cover the joint
surfaces of the bones. In embryos the greater portion of the skeleton
is at first formed of hyaline cartilage. Although the matrix usually appears homogeneous, it may be resolved into bundles of parallel fibers by
artificial digestion, and its behavior toward polarized light indicates an
underlying fibrillar structure. Sometimes, as a degenerative process, a
network of fibers may appear in the matrix, staining red with eosin, and
resembling the elastic fibers shown in Fig. 68, 3. Such a condition has
been observed in the trachea. In degenerating portions of the laryngeal
and costal cartilages, fibers having a luster like asbestos (or the mineral
amianthus) are sometimes seen; according to Prenant these "amianthoid
fibers" are neither white nor elastic. In old age a deposit of calcareous
granules often occurs in the matrix of hyaline cartilage, and in some of
the laryngeal cartilages this change may begin by the twentieth year.
With the increase and coalescence of the granules, the cartilage becomes
calcified, and blood vessels may enter it; but it does not form true bone.
As with other calcified structures, such as tendon, treatment with acids
shows that the underlying tissue has retained its characteristic features,
and remains quite different from bone.
Elastic cartilage contains, in its matrix, granules, fibers or networks
of elastic substance (Figs. 67, B, and 68) ; consequently its color is yellowish.
It is found in the external ear, the auditory (Eustachian) tube, the epiglottis, and in certain small cartilages of the larynx, namely the corniculate and cuneiform cartilages and the vocal processes of the arytsenoid
cartilages. It develops from hyaline cartilage, which it closely resembles.
Within its matrix, granules of elastic material are deposited, which
later coalesce to form fibers. Some authorities have stated that they
CARTILAGE
8l
arise from the cells, but according to Schafer "their formation apart
from the cells can be easily verified in the arytsenoid cartilage of the calf."
The elastic nature of fibers within the cartilage matrix can be demonstrated by special stains, such as resorcin-fuchsin; they stain like the elastic
fibers of connective tissue.
1. 2. 3.
FIG. 68. ELASTIC CARTILAGE. X 240.
i, Portion of a section of the vocal process of an arytaenoid cartilage of a woman thirty years old; the elastic
substance is in the form of granules. 2_and 3, Portions of sections of the epiglottis of a woman sixty
years old; a fine network of elastic fibers in 2, a denser network in 3. z, Cartilage-cell, nucleus invisible;
k, transparent capsule.
Fibro-cartilage cannot be regarded, like elastic cartilage, as a late modification of hyaline cartilage. In its early development, as seen in the
intervertebral disc of an embryo, its matrix is primarily fibrous. It is
composed of anastomosing bundles of fibers which blend with the hyaline
matrix of the adjacent vertebral cartilage as shown in Fig. 66, B. Instead
of becoming transformed into hyaline
cartilage, however, it develops into a
cartilaginous modification of dense connective tissue. It is found typically developed in the intervertebral and interpubic fibro-cartilages. According to Stohr
it forms the articular cartilage lining the
sterno-clavicular, acromio-clavicular, and
mandibular joints, together with the joints
of the costal cartilages, and it covers the
head of the ulna. Usually it is said to
form the rims deepening the sockets of the
shoulder and hip joints, together with the
interarticular discs of the mandibular,
sterno-clavicular and knee joints but these, according to Stohr, consist of
dense connective tissue without the characteristic cartilaginous matrix. A
portion of their cells are round, however. Even when typically developed,
fibro-cartilage consists chiefly of interwoven bundles of white fibers
With haematoxylin and eosin this ground substance is diffusely stained,
since the fibers, colored by the eosin, are imbedded in a chondro-mucoid
6
FIG. 69. FROM A HORIZONTAL SECTION
OF THE INTERVERTEBRAL Disc OF MAN.
g, Fibrillar connective tissue; z, cartilagecell (nucleus invisible); k, capsule
surrounded by calcareous granules.
X 240.
82
HISTOLOGY
matrix which stains with haematoxylin. The cells are not flattened as in
connective tissue. They are lodged in well-rounded lacunae (Fig. 69),
bounded by capsules and zones of blue-staining matrix; and they are frequently arranged in pairs or small groups such as occur in other forms
of cartilage. Their protoplasm is extensively vacuolated and is sometimes shrunken.
"Vesicular supporting tissue" is a form of precartilage which consists
of large vesicular cells in close contact, bound together by firm walls; it
is a "cartilage without a matrix." In many invertebrates it is an important tissue, but in adult mammals it is of limited occurrence. In man
such a tissue is said to be present on the inner surface of the tendon of
insertion of the M. quadriceps femoris, and in the sesamoid cartilage in
the tendon of the M. peronaeus longus. This form of cartilage resembles
the notochordal tissue at a certain stage of development, and it is called
"chord oid tissue" by Schaffer.
NOTOCHORDAL TISSUE.
Although the notochord is of entodermal origin (cf. p. 38), it gives
rise to a tissue which has often been called cartilage. Notochordal tissue
^ > A ^
MHMBMCv p *
FIG. 70. A PORTION OF A NUCLEUS PULPOSUS FROM A HUMAN EMBRYO OF THE SIXTH MONTH. X 225.
The notochordal syncytium is seen in the center of a mucoid matrix. The vertebrae are toward the right
and left, beyond the limits of the figure.
differs, however, from any of the types thus far considered. The principal
stages in its development in the pig have been described by Williams
(Amer. Journ. Anat., 1908, vol. 8, pp. 251-284), whose account may be
summarized as follows:
NOTOCHORD 83
In an embryo measuring 5.5 mm. the notochord is a rod of cells surrounded by a
thin notochordal sheath. A cross section contains about eight wedge-shaped cells.
In an embryo measuring 9 mm. it is larger, and a cross section shows about fifteen
cells at the periphery, and three or four at the center. In an embryo of 1 1 mm. the
cells have lost all definite arrangement and are more or less vacuolated. The vacuoles
increase in size and number, and are found to contain mucin or a gelatinous mucinlike substance. In an embryo measuring 17 mm. the cell walls, which up to this
time have remained intact, are breaking down (or being absorbed) and the mucin
escapes from the vacuoles. The cells are united by strands of cytoplasm and the
notochordal tissue now resembles mesenchyma. The syncytial network continues
to enlarge, both by growth, and by the formation of a greater number of vacuoles.
In a much older embryo (250 mm.) the formerly continuous peripheral sheet of
syncytial tissue is broken in many places by large masses of mucin. In the center
of this accumulation, the slender syncytial network seems suspended (cf. Fig. 70).
In the adult the syncytium has become divided into groups of vacuolated cells
imbedded in a gelatinous matrix. Thus it acquires a resemblance to cartilage in
several particulars, but it should be regarded as a distinct tissue.
The human notochord undergoes a development similar to that of
the pig. After it has ceased to be an epithelioid rod of cells, its most
characteristic condition is that shown in Fig. 70, which includes a portion
of the nucleus pulposus from an embryo of the fifth month. The notochordal tissue forms a vacuolated syncytium suspended in the gelatinous
matrix, which, at the periphery of the nucleus pulposus, is bounded by a
structureless membrane. Very rarely the notochord is the source of
tumors which are composed of tissue similar to that normally found
within the nucleus pulposus.
BONE.
Bone develops relatively late in embryonic life, after the muscles,
nerves, vessels, and many of the organs have been formed. The skeleton
at that time consists of hyaline cartilages, which are later replaced by
the corresponding bones of the adult. According to Kolliker, Robert
Nesbitt was the first to point out that the bones are not indurated or
transmuted cartilages, but are new formations, produced around the
cartilages which are later destroyed. Moreover, in his "Human Osteogeny
Explained in Two Lectures" (London, 1736), Nesbitt showed that
certain bones develop directly from connective tissue without having
been preformed in cartilage. These are now called membrane bones in
distinction from cartilage bones. The membrane bones are the bones
of the face and the flat bones of the skull. They include the interparietal
or upper part of the occipital, the squamous and tympanic parts of the
temporal, the medial pterygoid plate of the sphenoid, the parietal, frontal,
nasal, lachrymal, zygomatic (malar) and palate bones, together with the
vomer, maxilla and almost the entire mandible. Nesbitt correctly
8 4
HISTOLOGY
concluded that there is but one method of bone formation, whether or
not it takes place in relation with cartilage, but he was unaware of the
existence of cells, and believed that bones were produced from an ossifying
juice derived from the blood.
Development of bone. Bone formation begins with the production
of a layer or spicule of matrix which stains red with eosin. As to the
origin of this matrix there is the same difference of opinion which obtains
in regard to other intercellular products. It has been asserted that it
proceeds from osteogenic fibers, which are modified white fibers of the
connective tissue. Frequently a spicule of matrix is seen to fray out
into the connective tissue, as shown in the lower part of Fig. 71. Between
the osteogenic fibers, calcareous granules may then be deposited until
Osteoblasts. Calcifying connective tissue bundles. Bone matrix. Bone cells.
FIG. 71 FROM A SECTION OF THE MANDIBLE OF A HUMAN EMBRYO OF FOUR MONTHS. X 240.
the fibers are lost in a homogeneous calcified matrix. According to
this opinion the matrix is essentially an intercellular formation. Others
consider that the matrix is produced by a transformation of the exoplasm
of bone-forming cells, or osteoblasts.
Osteoblasts are derived from mesenchymal or young connective
tissue cells through an increase in their protoplasm and a shortening of
their processes. They are found in contact with the surface of spicules
of bone, arranged in an epithelioid layer (Fig. 72). There is great variation in their shape. Often they are pyramidal, but they may rest upon
the bone either by a broad base or a pointed extremity. Their round
nuclei may be in the part of the protoplasm next to the bone, or away
from it as far as possible. Active osteoblasts tend to be cuboidal or
columnar, but as bone production ceases they may become quite flat.
They form bone only along that surface which is applied to the matrix.
As the strand of bone grows broader through their activity, it encloses
here and there an osteoblast, which thus becomes a bone cell (Fig. 72).
Apparently bone cells do not divide, and if they produce matrix, thus
BONE 85
becoming more widely separated from each other, it is only to a slight
extent and in young bones; they are therefore quite inactive. Each bone
cell occupies a space in the matrix, called as in cartilage, a lacuna, but
unlike the lacunae of cartilage those in bone are connected by numerous
delicate canals, the canaliculi. In ordinary specimens the canaliculi are
visible only as they enter the lacunas, which are thus made to appear stellate. The matrix around the lacunae resists strong hydrochloric acid
which destroys the ordinary matrix, and so may be isolated in the form
of "bone corpuscles." The "corpuscles" correspond with the capsules
of cartilage, which may be isolated in the same way. The bone cells
nearly fill the lacunae and send out very slender processes into the canaliculi. These may anastomose with the processes of neighboring cells, as
Osteoblast becoming a bone cell. Bone cell. Osteoblast.
Uncalcified
matrix.
Calcined
matrix.
FIG. 72. PART OF A CROSS SECTION OF THE SHAFT OF THE HUMERUS, FROM A HUMAN EMBRYO OF THE
FOURTH MONTH. X 675.
can be seen in the embryo, but it is doubtful if this condition is retained
in the adult. The processes, moreover, are so fine that ordinarily they are
invisible.
The spicules of bone, containing bone cells and beset with osteoblasts,
increase in size and unite with one another, so as to form a spongy network enclosing areas of vascular connective tissue. These areas are
not entirely surrounded by bone, but retain connections with the exterior,
through which the vessels may enter and leave. It is evident that if the
spicules continued to thicken, while new ones were added at the periphery,
the bone would soon become quite solid and heavy. This is prevented
by the destruction or resorption of certain spicules, which begins at a
very early stage. It may be studied advantageously in the developing
mandible of a pig embryo, 10 cm. in length. At this stage the teeth
are growing rapidly, and around each tooth the spicules of bone are
being destroyed so as to produce a larger socket; at the same time the
jaw is increasing in thickness by the formation of new bone over its outer
86
HISTOLOGY
^
surface. Toward the area of resorption the osteoblasts become flatter
and less numerous, finally disappearing.
In sections of bone, the places where resorption is going on may be
recognized by the presence of large multinucleate cells, which Kolliker
in 1873 name d "bone destroyers" or ostoclasts (preferably spelled osteoclasts). They are shapeless masses of protoplasm without any limiting
membrane, containing usually from one to twenty nuclei (Fig. 73). In
the largest of them, Kolliker counted from fifty to sixty nuclei. He
Osteoblasts.
Haversian canals in the
process of formation.
Blood vessels.
Perichondrial bone.
Finished Haversian
canal.
-- Empty lacunae.
Osteoclast.
Endochondrial borderline.
' Endochondrial bone.
FIG. 73. PORTION OF A CROSS SECTION OF A TUBULAR BONE OF A NEWBORN KITTEN.
believed that they arose from osteoblasts through repeated nuclear division. Apparently they are not due to a fusion of cells; and they have
nothing in common, except their large size, with the giant cells of the bone
marrow, which will be described in connection with the blood. Osteoclasts are found along the surface of the bone, sometimes forming rounded
elevations or caps at the extremities of spicules, and sometimes imbedded
in shallow excavations known as Ho-wship's lacuna. There seems to be
no satisfactory evidence that the osteoclasts are the active cause of bone
destruction. On the contrary they appear to be degenerating cells, produced by those conditions which lead to the dissolution of bone.
BONE 87
The processes of bone formation and resorption just described take
place both in membrane and in cartilage bones. As the membrane bones
enlarge, the central portion, through resorption, becomes loose spongy bone
(substantia spongiosa), which is enclosed on all sides by an outer layer of
compact bone (substantia compacta). In the flat bones of the skull the
compact substance forms the outer and inner " tables, " which have the
spongy "diploe" between them. The cartilage bones likewise consist
of spongy and compact portions.
Hyaline
cartilage.
Primary
marrow
space.
Perichondrial
bone.
PlG. 74- A DORSO-PALMAR LONGITUDINAL SECTION OF A PHALANX OF THE LITTLE FlNGER, FROM A HUMAN
EMBRYO OF THE SIXTH MONTH. X 60.
Replacement of the skeletal cartilages. The changes within the skeletal
cartilages during the formation of bone may be studied advantageously
in longitudinal sections of any developing "long bone," or in transverse
sections of the vertebrae from pig embryos measuring about 10 cm. The
vertebiae exhibit several processes which will be cut lengthwise in transverse sections. Fig. 74 represents a longitudinal section of a phalanx
88
HISTOLOGY
around which ossification has begun. On either side of the shaft of
hyaline cartilage, the matrix of which stains blue with haematoxylin,
there is a strip of bone, the matrix of which is stained red with eosin.
These strips are sections of a band of bone which completely encircles
the middle part of the cartilage. It has been formed by osteoblasts which
developed in the perichondrium. The portion of the cartilage which is
surrounded by bone has begun to degenerate. Its capsules have been
Enlarged
cartilage
cells.
Endochondrial
bone.
Perichondrial
bone
Periosteum.
Perichondrial
bone.
FIG. 75. A DORSO-PALMAR LONGITUDINAL SECTION OF A MIDDLE-FINGER PHALANX,
FROM A HUMAN EMBRYO OF THE FOURTH MONTH. X 60.
resorbed, and the enlarged lacunae are beginning to coalesce. The matrix
of the cartilage in this region takes a deeper stain, and calcareous granules
are being deposited within it.
On the left of Fig. 74, a bud of perichondrial tissue is seen entering
the shaft of the cartilage, and similar buds may invade it from other
sides. Within the cartilage the ingrowing perichondrial tissue forms the
primary marrow, which is a very vascular connective tissue. As it advances, the walls of the lacunae are resorbed, setting free the cartilage
cells. Formerly it was thought that these cartilages cells became osteoblasts, but they are now considered to be dying cells, without further
function.
BONE
8 9
Meanwhile the cartilage continues to grow, especially in length.
This is brought about by successive transverse divisions of the cells of the
shaft, so that they become arranged in more or less definite longitudinal
rows (Fig. 75). The thin transverse walls of the lacunae in these rows are
Hyaline cartilage (cells in
groups).
Hyaline'cartilage (cells
enlarged).
Periosteum
Endochondrialbone
Osteoblasts. Osteob'lasts. Blood Osteoclasts.
vessels. Marrow
cells.
PIG. 76. FROM A LONGITUDINAL SECTION OF THE PHALANX OF THE FIRST FINGER OF A HUMAN EMBRYO OP
THE FOURTH MONTH. X 220.
dissolved more readily than the thicker longitudinal walls, and the deepblue ragged spicules of calcified matrix which are thus produced, are therefore generally elongated. Osteoblasts, derived from the primary marrow,
arrange themselves on these spicules, and form bone in the same manner
9
HISTOLOGY
as elsewhere. Thus the spicules of calcified matrix, staining blue, become
encased in the matrix of bone which stains red (Figs. 75 and 76).
From what has been said, it is clear that bone is formed both around
the cartilage (perichondrial bone) and within the cartilage (endochondrial bone). In long bones and flat bones, ossification is at first perichondrial and later endochondrial; in short bones it is endochondrial until
the cartilage has been entirely replaced. Thus the part taken by endochondrial and perichondrial ossification varies greatly in different bones.
As the bone grows, the older parts which have formed in relation with
Periosteum.
Haversian
Endochondrial Perichondrial
bor
Haversian canal.
Calcified matrix
between endochondrial and
perichondrial
bone.
Blood vessel.
Marrow.
Remains of calcined matrix of
cartilage.
FIG. 77. FROM A CROSS SECTION OF THE SHAFT OF THE HUMERUS, FROM A HUMAN EMBRYO OF THE FOURTH
MONTH. X 80.
the cartilage are resorbed. In the shaft of the humerus from a human
embryo of the fourth month (Fig. 77), only a thin and interrupted layer
of calcified cartilage remains to mark the boundary between perichondrial
and endochondrial bone, and in the adult all traces of this layer have disappeared. This is true of most bones, but in the auditory ossicles calcified cartilage is found throughout life.
The final stages in the replacement of the cartilages by bone take
place long after birth, when the bones have increased greatly in diameter
and length. The growth in diameter is accomplished by the deposition
of new layers externally, and the enlargement of the marrow cavity.
BOXE
---diaph..
.
through resorption, internally. This explains why a band of metal placed
around the bone of a young animal is later found within the marrow.
The internal resorption takes place in such a way that a meshwork of
spicules and plates, denser toward the periphery, remains within the shaft,
and the marrow occupies its interstices. To a limited extent new bone is
formed in the interior of the shaft by osteoblasts in its lining membrane,
called the endosteum. The deposition of new layers externally is produced
by osteoblasts in the periosteum, which is a specialized connective tissue
layer surrounding the bone. It replaces, and apparently is derived from,
the perichondrium of the original cartilage. The extent to which new
bone is formed, and its distribution, may
be determined by feeding madder to growing animals. This dye, as has long been
known, imparts a red color to the matrix
of bone deposited while it forms a part of
the diet. By this means Kolliker determined that the deposition of periosteal
bone is not uniform. In a given bone,
there will be unstained areas, where no
new bone is being formed, or where an external resorption is taking place. In this
way the bones acquire their characteristic
modelling.
Growth in length occurs chiefly through
the activity of the uncalcified cartilage.
In a long bone, ossification first produces a
band of bone encircling the cartilage, and then a hollow shaft of bone
with a rounded mass of cartilage at either end (Fig. 78, A, B). The cells in
these masses continue to divide, prolonging the longitudinal rows of cells
such as are seen in Fig. 75. As ossification takes place at one end of these
rows, new cells are formed at the other, and thus the length of the shaft or
diaphysis increases. Certain bones have been found to grow more at one
end than at the other. After a time osteogenic tissue invades the cartilages
at the extremities of the bone, extending into them from the marrow
cavity of the shaft. It forms a small bone within each, and these are
known as epiphyses (Fig. 78, D). Between the epiphysis and the diaphysis there remains a layer of cartilage, called the epiphyseal synchondrosis,
which allows further growth in length. The cells which it produces are
added chiefly to the shaft. The relation of the epiphyses to the growth
of bone was demonstrated by early experiments, in which metal pegs
were placed in the bones of young animals. Pegs in the shaft scarcely
separate from one another during growth, but a peg in the epiphysis
moves away from one in the diaphysis. The epiphyses are formed at
art.
D
FIG. 78. PLAN OF OSSIFICATION IN A
LONG BONE, BASED UPON THE TIBIA.
Cartilage is drawn in black, and bone is
stippled. art., Articular cartilage;
ep., epiphysis; diaph., diaphysis.
92 HISTOLOGY
r various times after birth, or, in the tibia, shortly before birth; they unite
I with the diaphyses usually between the eighteenth and twenty-second
I years, when the bones have acquired their full length. At that time nothing is left of the original cartilage except the layer of articular cartilage
which covers the joint surfaces. Details in regard to the time when
ossification begins in the various bones, the number of centers involved
(for many bones have more than the three which have here been described), and the time when these join the main bone, will be found in textbooks of anatomy, and, together with many references to important
studies of bone development, in Bidder's " Osteobiologie " (Arch. f. mikr.
Anat., 1906, vol. 68, pp. 137-213).
Structure of Bone in the Adult. The properties of adult bone are essentially those of its matrix, which consists of organic and inorganic constituents intimately blended, and perhaps chemically combined. Of the inorganic matter, over 80% is calcium phosphate, Ca 3 (PO^; the remainder
includes chlorides, carbonates, fluorides and sulphates of calcium, sodium,
potassium and magnesium. In order to cut sections of bone, this inorganic matter must be removed, and decalcification is usually accomplished by placing the specimen, after it has been preserved, in dilute nitric
acid (3-5%) for several days or weeks. The matrix then has the consistency of cartilage. Its organic portion, which remains, is composed
chiefly of collagen, together with osseo-mucoid. The collagen occurs in
very fine white fibrils which are gathered in bundles, arranged in thin
layers or lamella. Within these layers the fibers occur in parallel sets
which tend to cross one another at right angles, thus producing a lattice
work. These "decussating fibers" are seen only in special preparations
in which a lamella has been peeled off, so that it can be examined in surface view. The calcareous matter is said to be deposited in the cement
substance between the fibers, and not within them. Coarser uncalcified
fibers are found in embryonic bone and in certain situations in adult bone
for example, at the sutures and the places where tendons are inserted.
They also extend into the bone from the periosteum (Fig. 79), constituting the "perforating fibers" (Sharpey's fibers). The perforating fibers of
the bones of the skull are entirely collagenous. These bones in the adult,
together with the entire skeleton at birth, contain no elastic fibers; but in
other bones of the adult elastic fibers accompany the perforating fibers
(Schulz, Anat. Hefte, Abt. i, 1896, vol. 6, pp. 117-153).
The periosteum consists of two layers. It has an outer layer of dense
connective tissue, rich in blood vessels and containing also lymphatic
vessels and nerves. It blends with the surrounding looser connective
tissue and in places with fasciae and tendons. The inner layer has few
vessels but contains an abundance of elastic fibers. They are chiefly
parallel with the long axis of the bone, but in the periosteum of the bones
BONE 93
of the roof of the skull they form an interlacing network (Schulz). Perforating fibers, such as were described in the preceding paragraph, may
arise from this layer; and others, both white and elastic, derived from tendons, may pass through it into the bone. In this way the tendons acquire
a very firm insertion. The cells of the inner layer of the periosteum are
spindle-shaped or flattened connective tissue cells, together with the more
cuboidal osteoblasts which rest against the bone. In young bones these
are so numerous as to form a third layer of the periosteum. In the
adult they are few in number, but are capable of proliferation, and together with those in the endosteum. they are the source of new bone after
injury. The periosteum, in bodies which have been kept a week at 15
C., is said to be capable of producing bone when transplanted to another
body; and after operations in which a shaft of bone has been shelled
out from its periosteum, a new shaft may be formed.
Beneath the periosteum, as seen in the cross section of the shaft of a
long bone (Fig. 80) , there are layers or lamellae of bone which are parallel
Suture. Perforating fibers. Periosteal lamellae.
/ I
Blood vessel. Volkraann's canal. Haversian canal.
FIG. 79. SECTION ACROSS A SUTURE IN THE SKULL OF AN ADULT.
Prepared by Bielschowsky's method. X 80.
with the surface. These are the "outer ground lamellae" or periosteal
lamella. They are traversed by Sharpey's perforating fibers and by small
blood vessels lodged in the so-called Volkmann's canals. The bone
cells occupy lacunae, situated between the lamellae, and in Fig. 80 they
are seen as small spots. In the lowest part of the figure, a portion of
the marrow has been included. The marrow is surrounded by the
endosteum, external to which are the "inner ground lamellae" or endosteal
lamella. These are parallel with the inner surface of the bone.
Between the periosteal and the endosteal lamellae there is a dense
mass of matrix unlike anything found in embryonic bone. Scattered
through it, numerous blood vessels are seen in cross section. Each
vessel is surrounded by concentric lamella which present a very charac
94
HISTOLOGY
teristic figure. Such vessels are said to occupy Haversian canals (named
for the English anatomist, Clopton Havers). Volkmann's canals contain
vessels, but they are not surrounded by concentric lamellae. An Haversian canal often contains two vessels, an artery and a vein, together
with a small amount of connective tissue and occasional fat cells; flattened
osteoblasts may rest against the surrounding bone, and send processes
into it. The concentric lamellae enclosing an Haversian canal constitute
Resorption line.
Volkmann's canals.
V
Periosteum.
ifif^" Periosteal lamellae.
xT..- Perforating fibers.
' :: :&*;- ^ ^
_~ Haversian lamellae.
Haversian canal.
-;^
- Interstitial lamellae.
. Endosteal lamellae.
U Marrow.
FIG. 80. PART OF A CROSS SECTION OF A DECALCIFIED PHALANX FROM AN ADULT.
an Haversian system. Interstitial lamella, irregularly arranged, fill the
intervals between the Haversian systems.
The way in which the compact bone of the adult is formed from the
trabecular network of the embryo is indicated in the diagram, Fig. 81
(cf. also Fig. 73). After an area of vascular tissue has been surrounded
by bone, the osteoblasts form lamellae, gradually closing in from all
sides until only a slender canal remains. Successive stages are shown
in Fig. 81, B. V., H. C 1 , and H. C 2 , respectively. The deposition of the
concentric lamellae is not continuous. It, is interrupted by periods of
BONE
95
B.V:
FIG. 81. DIAGRAM OF THE DEVELOPMENT OF BONE.
(In part, after Duval.)
f., Fibrous layer of periosteum; o., osteogenic layer of periosteum; os., osteoblast; b.c., bone cell; B. V., blood vessel;
H. C 1 ., beginning Haversian canal; H. C 2 ., complete
Haversian canal; i. 1., interstitial lamellae, c. 1., concentric lamellae; Sh., Sharpey's perforating fibers.
resorption, after which the deposition of bone is resumed. Resorption
lines are frequently seen in the Haversian systems (Fig. 80).
Longitudinal sections of
decalcified bone show the way
in which the Haversian canals
connect with one another
(Fig. 82). The lamellae are
not so strikingly subdivided
into the groups seen in cross
sections, since both the concentric lamellae and the
ground lamellae are longitudinal layers. The lacunae of
the Haversian systems, however, are flattened, parallel
with the course of the Haversian canals, whereas those of
the interstitial lamellae are
more rounded or stellate.
The Haversian lacunas have been described as shaped like melon seeds.
Certain features of bone which can scarcely be seen in decalcified
specimens are rendered
conspicuous in layers of
dried bone, ground upon
an emery wheel until
thin enough to be translucent. The Haversian
canals and lacunae with
the canaliculi projecting
from them, are then
empty, except for air
and particles of bone
dust. The specimens
are mounted in thick
balsam, which spreads
over the bone without
filling the lacunae and
canaliculi. When seen
under the microscope
these structures appear
black (Fig. 83), the air
within them being highly refractive. In such preparations the way in
which the canaliculi pass from one lacuna to another, their connections
Periosteum.
Fat .drops.
FIG. 82. FROM A LONGITUDINAL SECTION OF A HUMAN META
CARPAL. X 3O.
Fat drops are seen in the Haversian canals. At z Haversian canals
open on the outer, and at xx on the inner surface of the bone.
9 6
HISTOLOGY
\
with the Haversian canal, and their manner of branching may be readily
observed. Although these canals are all present in the decalcified bone,
they are usually inconspicuous and often invisible. It has been impossible to determine absolutely whether the bone-cells anastomose with one
another through these canals, but it is considered probable that their
processes do not extend very far into them.
Vessels and Nerves in Bone. The blood vessels of the marrow, bone
and periosteum freely connect with one another. Small branches from
the arteries and veins of the periosteum enter the bone everywhere,
through the Volkmann's and Haversian canals, and anastomose
with the vessels in the marrow.
The marrow receives its blood
from the nutrient artery, which
gives off branches on its way
through the compact bone and
forms a rich vascular network in
the marrow. Of the larger veins
which drain this network, one
passes out beside the nutrient
artery and others connect freely
with the vessels in the compact
bone. Lymphatic vessels are found
only in the outer layer of the
periosteum. Numerous medullated and non-medullated nerves
are present in the periosteum,
where some of them end in lamellar
corpuscles. Others enter the Haversian canals and marrow, chiefly to
innervate the vessels. The nerves will be described in a later chapter.
FIG. 83. CROSS SECTION OF COMPACT BONE, FROM
THE SHAFT OF THE HUMERUS, SHOWING THREE
HAVERSIAN SYSTEMS AND PART OF A FOURTH.
(Sharpey.)
THE JOINTS.
Bones may be joined in two ways, either by a synarthrosis which
allows little or no motion between them, or by a diarthrosis which permits
them to move freely upon one another.
In a synarthrosis the mesenchymal tissue between the adjacent bones
may form dense connective tissue, such as passes from one bone to another across the sutures of the skull (Fig. 79) ; or it may form cartilage,
in which case the joint is known as a synchondrosis. The cartilage may
be hyaline, as in the epiphyseal synchondroses, but often it is fibrous, as
in the intervertebral synchondroses.
JOINTS
97
In a diarthrosis the connective tissue between the bones remains comparatively loose in texture, and a cleft forms within it, containing tissue
fluid. This is the joint cavity (Fig. 84). It is bounded in part by flattened connective tissue cells, which spread out and form an imperfect
epithelium (Fig. 85). This is not a continuous layer of cells, since in
many places the fibrous tissue comes to the surface. The connective
tissue layer blends with the perichondrium, which in turn passes into
cartilage, and a portion of the cartilage, uncovered by perichondrium,
helps to bound the joint cavity.
FIG. 84. PHALANGEAL JOINT
FROM A HUMAN EMBRYO OF
THB FOURTH MONTH.
car., Cartilage; j. c., joint cavity;
8. f., stratum fibrosum; s. s.,
stratum synoviale.
FIG. 85. AN ENLARGED DRAWING OF THE LEFT PART OF THE JOINT
SHOWN IN FIG. 84.
b. v., Blood vessel; car., cartilage; j. c., joint cavity; mes. epi.
mesenchymal epithelium.
The articular cartilages are sometimes fibrous (as noted on p. 81) but
usually they are hyaline. They vary in thickness from 0.2 mm. to 5 mm.,
being thinner at the periphery. The cells near the free surface are flattened. In the middle strata they are rounded and are often arranged in
groups; in the deepest layers they tend to be in rows perpendicular to
the surface. The matrix becomes calcified as the cartilage connects
with the bone, and a line of demarcation separates the calcified from the
uncalcified portion (Fig. 86). In the uncalcified cartilage, cells with
processes extending into the adjacent matrix have been described, and
the deeper layers of flattened cells may exhibit lobed nuclei.
The joint capsule consists of an outer layer of dense connective tissue,
the stratum fibrosum; and an inner loose layer of which the mesenchymal
epithelium is a part, the stratum synoviale (Fig. 84). The fibrous layer
is specially thickened in various places to form the ligaments of the joint.
7
9 8
HISTOLOGY
It may cover the end of the bone, coming between it and the joint cavity;
thus the distal articular surface of the radius is covered with dense fibrous
tissue. In other joints, as in the shoulder and hip, such tissue forms a
rim, deepening the socket of the joint. These rims are called labra
glenoidalia. Large folds or plates of dense fibrous tissue may project
into the joint, covered by the synovial layer, thus forming the menisci
of the knee joint, and the articular discs such as are interposed inthesternoclavicular and mandibular joints. Nerves and vessels are absent from
the articular cartilages of the adult, and also from the labra and articular
discs.
Hyaline
cartilage.
FIG. 86. VERTICAL SECTION THROUGH THE
HEAD OF A METACARPAL OF AN ADULT MAN.
X 50.
FIG. 87. SYNOVIAL VILLI WITH
BLOOD VESSELS FROM A HUMAN
KNEE JOINT. X so.
The epithelium has fallen from the
apex of the left yillus, exposing
the connective tissue.
The synovial layer consists of loose connective tissue, generally with
abundant elastic elements. In many places it contains considerable
quantities of fat. It has nerves which may terminate in lamellar corpuscles, numerous blood vessels, and lymphatic vessels which may extend
close to the epithelium. The "epithelium" is a smooth glossy layer of
connective tissue with parallel fibers and small round or stellate cells
containing large nuclei. The cells are sometimes infrequent, as in places
where there is unusual pressure. Elsewhere they may be spread in a
single thin layer, or heaped together, making an epithelium of three or
four strata. The synovial membrane may be thrown into coarse folds
(plied) or into slender almost microscopic projections (villi}. The latter
impart a velvety appearance to the membrane on which they occur.
JOINTS
99
On microscopic examination the synovial villi are seen to vary greatly
in shape. They are covered by a simple or double layer of synovial
epithelium, and usually, but not invariably, they contain vessels. The
synovia (synovial fluid) consists chiefly of water (94%), the remainder
including salts, albumin, mucoid substances, fat droplets and fragments
of cells shed from the membrane.
Enamel.
Dentine.
Crown
TEETH.
A tooth consists of three parts, crown, neck, and root or roots. The
crown is that portion which projects above the gums; the root is the part
inserted into the alveolus or socket in
the bone of the jaw; and the neck,
which is covered by the gums, is the
connecting portion between the root
and crown. A tooth contains a dental
cavity filled with pulp. The cavity is
prolonged through the canal of the
root to the apex of the root, where it
opens to the exterior of the tooth at
the Joramen apicis dentis. The foramen is shown, but is not labelled, in
Fig. 88. The solid portion of the
tooth consists of three calcified substances, the dentine or ivory (substantia eburnea], the enamel (substantia adamantina), and the centent
(substantia ossea) . Of these the dentine is the most abundant. It forms
a broad layer around the dental cavity and root canal, and is interrupted
only at the foramen. Nowhere does
the dentine reach the outer surface
of the tooth. In the root it is covered
by the cement layer, which increases
in thickness from the neck toward
the apex; and in the crown it is enclosed by the broad layer of enamel. The enamel, however, becomes
thin toward the neck, where it meets and is sometimes overlapped by the
cement. The pulp, dentine, and cement are of mesenchymal origin, the
dentine and cement being varieties of bone. The enamel is an ectodermal
formation, but so intimately associated with the others that it may be
described with them.
Root.
Cement.
FIG. 88. LONGITUDINAL GROUND SECTION OF A
HUMAN INCISOR TOOTH. X 4.
IOO
HISTOLOGY
The Development of the Teeth. The first indication of tooth development in human embryos is a thickening of the oral epithelium, which
has been observed in specimens measuring 11-12 mm. At this stage
the oral plate, which marks the boundary between ectoderm and entoderm, has wholly disappeared, but it is evident that the thickening takes
place in ectodermal territory. The tongue is well developed, but the
upper and lower lips are not as yet separated by depressions from the
structures within the mouth. Soon after the thickening has appeared, it
grows upward in the upper jaw, and downward in the lower jaw, into the
adjacent mesenchyma, thus forming an epithelial plate which follows the
circumference of either jaw. It undergoes the same sort of transformation
in both the maxilla and mandible, and the following description of the
conditions in the mandible is therefore applicable to both. As the plate
descends into the mesenchyma, it divides into a labial lamina in front,
a b c d
FIG. 89. SAGITTAL SECTION THROUGH THE TONGUE AND LOWER JAW OF A HUMAN EMBRYO OF 22 MM
X 20.
a, Labial lamina; b, dental lamina; c, Meckel's cartilage; d, tongue.
which brings about the separation of the lip from the gum, and a dental
lamina behind, which is concerned with the production of the teeth (Fig.
89). At first the dental lamina is inclined decidedly inward or toward
the tongue, as seen in the figure, but later it descends from the oral epithelium almost vertically. Taken as a whole it is a crescentic plate of cells
following the line of the gums, along which the teeth will later appear.
The further development of the dental lamina is shown diagrammatically in Fig. 90, A-D, each drawing representing a part of the oral epithelium above and dental lamina below, freed from the surrounding mesenchyma. The labial side is toward the left and the lingual side toward
the right. Almost as soon as the dental lamina has formed, it produces a
series of inverted cup-shaped enlargements along its labial surface (Fig.
90, B), and these become the enamel organs. There is a separate enamel
organ for each of the ten deciduous teeth in either jaw, and they are all
present in embryos of two and one-half months (40 mm.). They not
TEETH
IOI
only produce the enamel but extend over the roots, so that they are described as forming moulds for the teeth which develop within their concavities. The tissue enclosed by the enamel organ is a dense mesenchyma, constituting the dental papilla. It becomes the pulp of the tooth,
and produces, at its periphery, the layer of dentine. As the tooth develops, the connection between its enamel organ and the dental lamina
Oral epithelium.
Enamel
organs.
Dental
groove
Dental lamina.
Papillae.
Enamel organs. Necks of enamel organs.
ABC D
FIG. 90. DIAGRAMS SHOWING THE EARLY DEVELOPMENT OF THREE TEETH.
(One of the teeth is shown in verticle section.)
becomes reduced to a flattened strand or neck of epithelial tissue, which
subsequently disintegrates.
In order to produce enamel organs for the three permanent molars,
which develop behind the temporary teeth on either side of the jaws, the
dental lamina grows backward, free from the oral epithelium. This
backward extension becomes thickened and then inpocketed by a papilla,
thus forming the enamel organ for. the first
permanent molar in embryos of 17 weeks (180
mm.) . It grows further back, and gives rise to
the enamel organ for the second molar at about
six months after birth, and for the third or late
molar (wisdom tooth) at five years. In rare
cases, several of which have been reported,
there is a fourth molar behind the wisdom tooth,
and it is assumed that in these cases the dental
lamina continued its backward growth beyond
the normal limits (Wilson, Journ. Anat. and
Physiol., 1905, vol. 39, pp. 110-134).
The permanent front teeth develop from enamel organs on the labial
side of the deep portion of the dental lamina (Fig. 91). Owing to the
obliquity of the lamina the permanent teeth are on the lingual side of the
deciduous teeth. The enamel organs for the incisors develop slightly in
advance of those for the canines, but all of these are indicated in an embryo
of 24 weeks (30 cm.) described by Rose. He found the enamel organs
for the first premolars in an embryo of 29 weeks (36 cm.) and for the second
QE.
D.R.
E.O.
FIG. 91. TEETH FROM A HUMAN
EMBRYO OF 30 CM. (Modified
from R6se.)
E. and E. O., Enamel organs of
a deciduous and of a permanent tooth respectively; D.
R., dental lamina; O. E., oral
epithelium; P., papilla.
102 HISTOLOGY
premolars at 33 weeks (40 cm.). Each front tooth develops in the alveolus occupied by the corresponding deciduous tooth, but later a bony
septum forms between the two teeth and subdivides the alveolus. When
the deciduous teeth are shed, the partitions are resorbed, together with
the dentine and cement of the roots of the deciduous teeth. This resorption is accompanied, as in bone, with the production of osteoclasts.
The portion of the dental lamina which is not utilized in producing
enamel organs becomes perforated and forms irregular outgrowths (Fig.
91). This disintegration begins in the front of the mouth and spreads
laterally. Epithelial remnants from the lamina have been found in the
gums at birth and have been mistaken for glands. Like other epithelial
remains they occasionally develop abnormally, forming cysts and other
tumors. The deepest part of the lamina, below the enamel organs of
the permanent teeth, is considered by Rose to be a possible source of a
third set, and he states that a case has been reported to him in which
such a set, consisting of thirty-two teeth, developed on the lingual side
of the permanent teeth. The models which Rose prepared, showing
the enamel organs in various stages of development, form the basis of
present accounts of tooth development. They are described and well
illustrated in the Arch. f. mikr. Anat., 1891, vol. 38, pp. 447-491.
ENAMEL ORGAN AND ENAMEL.
The basal cells of the oral epithelium may be followed as a distinct
layer over the dental lamina and enamel organ, as shown in Fig. 92.
This suggests that the enamel organ should be regarded as an infolding
of the oral epithelium, and the occurrence of a transient dental groove
immediately above the lamina (Fig. 90, C) favors this interpretation. The
basal surface of the epithelium of the enamel organ is therefore directed
toward the surrounding mesenchyma, and the superficial cells are found
in the interior of the organ. At first these internal cells are in close
contact, like those of ordinary epithelium, but later, through an accumulation of gelatinous intercellular substance, they constitute a protoplasmic
reticulum which resembles mesenchyma, and is known as the enamel
pulp (Fig. 93). No vessels or nerves penetrate this pulp. On the side
away from the dental papilla the enamel pulp is bounded by the outer
enamel cells. At first these are typical cuboidal epithelial cells, but later
they become flattened and transformed into a feltwork of pulp fibers.
Toward the dental papilla the enamel pulp is bounded by inner enamel
cells, which develop differently over the upper and lower parts of the
tooth respectively. Over the lower portion of the dental papilla they
remain as cuboidal or low columnar cells. Here, through a thinning of
the pulp, they are brought into contact with the outer enamel cells, and
TEETH
103
the two layers together form the epithelial sheath of the root (Fig. 102).
Over the upper part of the dental papilla, the inner enamel cells elongate
and become enamel-producing cells or ameloblasts (Fig. 93).
The ameloblasts produce enamel along their basal surfaces, which
are toward the dental papilla, but they become so transformed that
their basal surfaces appear like free surfaces, and the entire cells seem
inverted. In columnar epithelial cells the nuclei are generally basal,
and the secretion gathers near the free surface, but in the ameloblasts
these conditions are reversed. The nuclei are toward the enamel pulp,
If';
Thickened ' W!-^ff&$i&
oral
ithelium. . '.
Outer'enamel cells
Enamel pulp
Inner enamel cells
Free edge of the
dental lamina .
Papilla.
FIG. 92. FROM A CROSS SECTION OF THE UPPER JAW OF A HUMAN EMBRYO OF FIVE MONTHS. X 43.
and the latter forms a dense layer over the ameloblasts, suggesting a
basement membrane (Fig. 93). According to Cohn (Verh. phys.-med.
Ges. Wiirzburg, 1897, vol. 31, No. 4) both ends of the ameloblasts are
encircled by terminal bars. These bars may be regarded as modifications of the thin film of cement substance found between the ameloblasts.
Near the center of each cell, and therefore on the basal side of the nucleus,
Cohn has described typical centrosomes or diplosomes.
Toward the dental papilla the protoplasm of the ameloblasts contains
granules or droplets which blacken with osmic acid and presumably
indicate secretory activity. The basal surface of each ameloblast presents
IO4
HISTOLOGY
a cuticular border and gives rise to a tapering projection known as Tomes's
process. Tomes's processes extend into the developing enamel, but
they may readily be seen in specimens in which the layer of ameloblasts
has shrunken away from the enamel, as in Fig. 93. Around these processes minute globules are deposited, which resemble the granules within
the cells, since they blacken with osmic acid. They are described as
composed of a horny substance similar to that found in the epidermis.
This material may become fibrillar, and Tomes's processes also readily
break up into fibrils. There is therefore an uncalcified fibrillar layer of
Cuticular Tomes's Enamel
Dorder. processes, cement. Calcified, . . uncalcified dentine.
Enamel pulp.
Outer enamel cells.
Odontoblasts. Pulp.
Inner enamel cells
(ameloblasts).
FIG. 93. PORTION OF A LONGITUDINAL SECTION OF AN INCISOR TOOTH FROM A NEWBORN KITTEN. X 300.
In this section the Tomes's processes have shrunken away from the enamel cement.
Rectangle enclosing the portion
of the tooth shown highly magnified in the adjoining part of the
figure.
enamel next to the ameloblasts. Further from the ameloblasts the
enamel is calcified and consists of rods known as enamel prisms (sometimes
called enamel fibers) which are bound together by calcified matrix or
enamel cement. The way in which the prisms develop has not been fully
determined. They have been regarded as the calcified ends of the
ameloblasts and also as intercellular deposits.
The formation of enamel begins at the top of the crown of each tooth
and spreads downward over its sides. If the tooth has several cusps, a
cap of enamel forms over each, and these caps later coalesce. The enamel
increases in thickness by the elongation of the prisms, which extend
across it from the inner to the outer surface.
TEETH 105
When the tooth comes out through the gum, or erupts, the enamel is
covered with a "persistent capsular investment" described by Nasmyth
(1849) an d called "Nasmyth's membrane" (cuticula dentis). Huxley
studied this structure as it covers the teeth in an embryo of the seventh
month (Trans. Micr. Soc. London, 1853, v l- I > PP- I 49~ I ^4)- He found
that the inner enamel cells could be easily removed, leaving the surface
of the enamel covered with a finely wrinkled or reticulated structureless
membrane. Upon adding strong acetic acid the membrane became
voluminous and transparent, and was thrown into coarse folds. The
ends of the enamel prisms could be seen through it. This dental cuticula
is now generally considered to be composed of the last-formed uncalcified
ends of the enamel prisms, which are composed of horny material. After
the eruption of the tooth it is gradually worn away, remaining longest in
the depressions of the enamel.
The fully developed enamel is the hardest substance in the body.
Several analyses have shown that it contains less than 5% of organic
matter. No cells or protoplasmic structures are found within it, but it
exhibits various markings, shown in Fig. 94. The outer surface of the
enamel of the permanent teeth, especially on the sides of the crown and
on young teeth, presents a succession of circular ridges and depressions,
which may be seen with a hand lens. These were discovered by Leeuwenhoek (1687), whose figure of them is reproduced in Fig. 94, A. He considered that they marked the intervals during the eruption of the tooth,
and wrote, "For example, let us assume that the tooth has fifty circles
or ridges; if this is so, the tooth has been pushed through the gum during
fifty successive days or months." This explanation is not supported by
any evidence.
The enamel, as seen in ground sections passing lengthwise through
the tooth, shows numerous brownish bands which are broadest and most
distinct toward the free surface (Fig. 94, B). These are the contour lines
or lines of Retzius, first described in Miiller's Archiv, 1837 (pp. 486-566).
The coarsest of them may be seen with the naked eye, but upon magnification these are resolved into a number of finer lines, and many new
lines appear. Their direction is shown in the figure; they arch over the
apex of the crown, and on its sides tend to be parallel with the long axis
of the tooth. Thus they cross the enamel prisms, and are not the lines
along which the enamel most readily fractures. Apparently they indicate the shape of the entire enamel at successive stages in its development,
and for this reason they are called contour lines. When Leeuwenhoek's
ridges are present, the lines of Retzius end in the furrows between them.
It was once supposed that their brown color was due to pigment, and it is
well known that the enamel of certain teeth in rodents is deeply pigmented
and brown. But when the lines are highly magnified, no pigment granules
io6
HISTOLOGY
are found. It then appears that the lines are due to imperfect calcification
of the enamel cement, which is often vacuolated where a line crosses it.
Another set of lines crosses the enamel radially, taking the shortest
course from the dentine to the free surface. These radial lines are due
to the arrangement of the enamel prisms, and fractures of the enamel
tend to follow them. As seen in reflected light, under low magnification,
they appear as alternating light and dark bands, often called Schreger's
lines. The prisms in crossing the enamel are bent in such a way that they
are cut in alternating zones of cross and longitudinal sections, respectively
(Fig. 94, C). These zones vary in shape and sometimes the prisms in
cross section form an island surrounded by longitudinal sections. Since
an entire prism cannot be isolated or included within the limits of a single
section, the course which they take is difficult to determine. There is no
F
FIG. 94. THE'MARKINGS OF THE ENAMEL IN ADULT TEETH.
A, Leeuwenhoek's figure showing ridges encircling the enamel. B, Longitudinal ground section of a canine
tooth; c, cement; c. 1., contour lines (lines of Retzius); d. c., dentinal canals; i. s., interglobular spaces.
C, Longitudinal section of the enamel of an incisor tooth, the dentinal surface being toward the left.
The enamel shows zones of transverse and longitudinal sections of enamel prisms. D, Fragment of
enamel showing prisms in longitudinal view, slightly affected by hydrochloric acid. X 350 (Koelliker).
E, Cross section of the decalcified enamel of a canine tooth from a child of three years. X 350 (Koelliker). F, Cross section of enamel prisms of a permanent molar from a child of about eight years.
(Smreker.)
evidence that they branch, and the greater surface which they cover at
the periphery of the enamel, as compared with the dentinal surface, has
been explained by an increase in the diameter of the prisms as they pass
outward. Such an enlargement is not well marked, however, and is partly
offset by an outward thinning of the interprismatic cement. Apparently
there is an increase in the number of ameloblasts as the tooth becomes
larger, and there may be some late-formed enamel prisms which do not
reach the dentinal surface. The plan according to which the prisms bend
is discussed in Koelliker's Gewebelehre (6th ed.) but it has never been
fully explained.
The individual enamel prisms, when seen lengthwise, exhibit transverse markings. These may be made out in ground sections, but they
become more evident after the prisms have been treated with acid (Figs.
94, D and 99). They have been regarded as artificial products, but probably they indicate successive stages in the elongation of the prism. Fre
TEETH 107
quently the prisms, when isolated, appear beaded, with transverse bands
at the places of constriction.
When seen in cross section the prisms have highly refractive outlines,
from 3-6 fi in diameter. They were formerly described as polygonal
and primarily hexagonal (Fig. 94, E) but Smreker finds that they are
crescentic, as shown in Fig. 94, F (Arch. f. mikr. Anat., 1905, vol. 66,
pp. 312-331). The convex side of the crescent, along which the interprismatic cement is most abundant, is always toward the dentine. The
hollow of the crescent receives an adjacent prism which appears to have
been pressed into it. Isolated prisms of this sort are therefore hollowed
out on one side, and it is possible that they connect with one another by
flanges or bridges (von Ebner, Arch. f. mikr. Anat., 1905. vol. 67, pp. 18-81).
DENTAL PAPILLA, DENTINE, AND PULP.
The dental papilla has already been described as a mass of dense
mesenchyma, enclosed and probably moulded by the enamel organ. At
the end of the fourth month, shortly before the formation of enamel has
begun, the outermost cells of the papilla become
elongated and arranged in an epithelioid layer.
Since they produce the dentine, which is the
principal part of the tooth, these cells are known
as odontoblasts . At first they rest against the
inner enamel cells. Later a thin layer of predentine extends like a membrane between the
ameloblasts and odontoblasts; it is seen as a
white line in Fig. 92. As the layer of predentine
widens and becomes calcified, the odontoblasts
remain on its inner surface, which is toward the
pulp. Five of them are shown in Fig. 95,
together with their branching processes, one of
FIG. 95- FIVE ODONTOBLASTS.
which proceeds from the cuticular border of each FROM WHICH TOMES'S FIBERS
EXTEND UPWARD INTO THE
cell and occupies a canal in the dentine. These DENTINE, FROM A TOOTH OF A
NEWBORN CAT. (Prenant.)
dental or dentinal canals (canaliculi dentales) are
readily observed in adult teeth. Their existence, and the fact that they
open into the pulp cavity, were recorded by Leeuwenhoek in 1687.
"The presence of fibrils of soft tissue within the dentinal tubes" was
established by Tomes in 1856 (Phil. Trans., pp. 515-522). He found
that if a section of a fresh tooth is placed in dilute hydrochloric acid and
then torn across the tubes, fibrils will be seen projecting from the broken
edges; and that if the pulp is pulled away from the dentine, fibrils can be
drawn out from the tubes. By the latter method the cells shown in Fig.
96 were obtained. The fibers within the dentinal canaliculi are called
dentinal, dental or Tomes' s fibers.
io8
HISTOLOGY
Recently von Korff, with special methods, has demonstrated another
sort of fibers which lie between the odontoblasts and pass from the pulp
into the predentine (Fig. 97, A). The fibers are apparently collagenous
FIG. 96.
Six odontoblasts with dentinal (or Tomes's)
fibers, f. p., pulp processes. From the pulp
at birth. X 240.
A B
FIG. 97. THE DEVELOPMENT OF DENTINE
IN PIG EMBRYOS. (After v. Korff.)
d., Calcified dentine; e. c., inner enamel
cells; f., fibrous ground substance of
dentine; od., odontoblasts; p., mesen
chymal cells.
and among them, immediately beneath the layer of enamel cells, calcareous granules begin to be deposited (Fig. 97, B). These granules become abundant, and fill the ground substance of the dentine. Von Korff
concludes that it is not the odontoblasts but the fibrils |
of the pulp which give rise to the dentine, and similarly'
he finds that in bone the osteogenic fibers develop from
the surrounding mesenchyma rather than from osteoblasts (Arch. f. mikr. Anat., 1907, vol. 69, pp. 515-543).
Studnicka agrees with von Korff that "the odontoblasts
are really gland cells, which are only secondarily concerned in the formation of dentine and do not produce
ground substance; their processes (the Tomes's fibers)
serve to convey certain nutrient material to the parts
far removed from the inner surface, and thus nourish
the dentine." (Anat. Anz., 1909, vol. 34, pp. 481-502.)
Von Ebner, however, maintains that von Korff's fibers
are produced by the odontoblasts as part of the process
of dentine formation.
Other very fine collagenous fibrils in the dentmal
matrix are arranged like the decussating fibers in the
lamellae of bone. They cross one another as they run
longitudinally in the successively deposited layers of
dentine. These layers are sometimes marked out by distinct contour
lines, the direction of which is shown in Fig. 98. They indicate the shape
of the entire dentine at various stages in its development, and show that
FIG. 98. DIAGRAM OF
THE ARRANGEMENT
OF THE DENTINAL
LAMELLA AND
CONTOUR LINES IN
AN INCISOR. (Koelliker.)
TEETH
IOQ
the root of the tooth forms after the crown is essentially complete. The
innermost layers are formed last. In addition to the contour lines, dentine seen in reflected light shows the radial Schreger's lines, which follow
the course of the dentinal canals but are said to be due to the fibrillar
structure of the matrix between them.
Dentine when fully developed is not so hard as enamel and contains
a much larger amount of organic matter (approximately 25%). When
the inorganic substances are removed from enamel, the remaining tissue
scarcely holds together, but dentine and bone, when so treated, leave a
gelatinous matrix which preserves the form of the original object. The
dentinal canaliculi pass radially through the dentine, often following a
somewhat S-shaped course as shown in Fig. 94, B. In addition to these
primary curves, they may show spiral twists and secondary curves. As
they cross the dentine, they divide dichotomously a few times and give
off many slender lateral branches, some of which anastomose with those
from adjacent canaliculi (Fig. 99). They finally become very slender
Enamel prisms.
Dentine. Enamel.
FIG. 99- FROM A LONGITUDINAL SECTION OF THE LATERAL
PART OF THE CROWN OF A HUMAN MOLAR TOOTH.
X 240.
i, Dentinal canaliculi, some extending into the enamel; 2,
globules of calcified dentine projecting into the interglobular spaces, 3.
Cement.
FIG. 100. FROM A LONGITUDINAL SECTION OF THE ROOT OF A HUMAN
MOLAR TOOTH. X 240.
I, Dentinal canaliculi interrupted by a
stratum with many small interglobular
spaces, 2. 3, bone lacunas and canaliculi.
and generally end blindly, but some terminal loops have been described.
Each canal is surrounded by a resistant uncalcified layer known as Neumann's sheath. This sheath may be isolated with acids, and thus it is
comparable with the "corpuscles" of bone and the capsules of cartilage.
It is difficult to determine whether the processes from the odontoblasts
extend the whole length of the canaliculi, but they are believed to do so.
Tomes observed that the peripheral portion of the dentine is more sensitive than the deeper part, and considered that the fine ramifications of
the odontoblasts respond like nerve fibers to stimulation. Nerves
have been traced to the odontoblast layer at the base of the dentine, but
it is doubtful whether they extend into the dentinal canals as some have
reported.
110 HISTOLOGY
The contact between the dentine and enamel is usually quite smooth.
Each enamel prism rests in a shallow socket on the dentinal surface,
and in places the dentinal canals extend into basal clefts in the enamel
cement. A short distance beneath the enamel the dentine exhibits a
layer of spaces, which in ground sections are filled with air and appear
black (Fig. 94, B, i.s.). They occur along the contour lines, and
are due to imperfect calcification of the cement in that region of the
matrix which was the first to form. Each space is bounded by spherules of
calcified matrix which project into it from all sides, and the cavities are
therefore known as inter globular spaces (Fig. 99). Toward the root of the
tooth they are smaller and more numerous than in the crown. They are
said to be particularly abundant in poorly developed teeth.
The pulp consists of a fine network of reticular tissue together with the
peripheral layer of odontoblasts already described. The odontoblasts
persist throughout life, and may continue to produce dentine so that the
root canals may become nearly or quite obliterated. They are also active
in repairing injuries. Some of the late-formed dentine contains blood
vessels and resembles bone, so that it has been called osteo-dentine. The
odontoblasts connect with one another and with the rest of the pulp by
protoplasmic processes. The pulp tissue is free from elastic fibers and
from bundles of white fibers. It is very vascular. The small arteries
entering the apical foramina send capillaries close to the odontoblasts, but
normally they do not enter the dentine. Lymphatic vessels, according to
Schweitzer, are found by injection to begin as a tuft of branches in the
pulp of the crown; they empty into one or a few very wide vessels passing
through the root (Arch. f. mikr. Anat., 1907, vol. 69, pp. 807-908). The
nerves of the pulp are the medullated dental branches of the alveolar nerves,
which enter through the apical foramina, lose their sheaths and form a loose
plexus beneath the odontoblasts, between which they terminate in free
endings.
DENTAL SAC, CEMENT, AND PERIODONTAL TISSUE.
Each embryonic tooth, consisting of its enamel organ and papilla, is
completely surrounded by mesenchyma, which extends from the oral
epithelium to the bony trabeculae of the developing jaw (Fig. 101). This
mesenchyma gives rise to the dental sacs enclosing the teeth; each sac
consists of a dense outer layer and a loose inner layer of young connective
tissue (Fig. 102). Toward the base of the dental papilla the tissue of the
sac is separated from the dentine by the epithelial sheath, which is a part
of the enamel organ. After the crown of the tooth is well developed, the
epithelial sheath disintegrates or becomes penetrated by cells of the dental
sac, which are then transformed into osteoblasts and deposit bone directly
TEETH HI
upon the outer surface of the dentine. This bone is a part, of the tooth and
is known as the substantia ossea or cement. It is thinnest at the neck of
the tooth, and increases in thickness downward toward the apex of the root,
over which it forms a considerable cap (Fig. 88). The deeper part of the
root develops after the eruption of the crown.
The cement contains typical bone cells, enclosed in large lacunae which
connect with one another through canaliculi (Fig. 100). The dentinal
surface sometimes appears resorbed and the dental canaliculi then end
abruptly; occasionally they appear to anastomose with those of the cement.
Cross section of the
orbicularis oris muscle.
Labial gland.
Dental lamina.
Enamel organ.
Enamel.
FlG. 101. VERTICAL SECTION THROUGH THE LIP AND JAW OF A HUMAN EMBRYO OF Six AND A HALF MONTHS. X 9
The lamellae of the cement, which are seldom well marked, are concentrically placed around the root. In young teeth Haversian canals are absent,
but in old teeth they occur in the outer layers near the apex of the root.
Connective tissue fibers, comparable with Sharpey's fibers in bone, pass
radially through the cement. They cross the dental sac and enter the
bone of the alveolus, thus binding the tooth to its socket,
As the tooth enlarges and fills the socket, the dental sac becomes reduced to a thin layer consisting of the alveolar periosteum externally and
the dental periosteum internally, with vascular connective tissue between.
Frequently these are described as a single layer. It may contain fragments
of the epithelial sheath. It has few elastic fibers, but is well supplied with
112
HISTOLOGY
vessels and nerves which are branches of those about to enter the apical
foramen. Around the neck of the tooth, dense connective tissue forms the
circular ligament (Lig. circulare dentis).
The gum (gingiva) is the part of the lining of the mouth which surrounds the tooth. It is covered by the stratified oral epithelium, in which
Dental sac.
Outer layer. Inner layer.
Outer enamel cells.
Enamel pulp.
Inner enamel cells.
Enamel.
Epithelial
sheath.
Dentine
Odontoblasts
Dental papilla
(future pulp)
Blood vessel.
Bony trabecula of the lower jaw.
FIG. 102. LONGITUDINAL SECTION OF A DECIDUOUS TOOTH OF A NEWBORN Doc. X 42.
The white spaces between the inner enamel cells and the enamel are artificial, and due to shrinkage.
intercellular bridges are well developed, and this epithelium rests on tall
connective tissue elevations or papillae. There are no glands in the gums.
When the tooth erupts it makes a hole through the epithelium, but the
margins of the aperture become inverted. Thus the epithelium extends
TEETH 113
close to the tooth and turns down as a sheath surrounding the neck. At
the level of the upper part of the cement it ends abruptly. The connective
tissue of the gums blends below with the circular ligaments. It contains
few elastic fibers, but is very vascular and is often infiltrated with lymphocytes. Its lymphatic vessels drain outward, along the margin of the cheek
and gums, and inward, over the floor or roof of the mouth, as shown by
Schweitzer.
MUSCULAR TISSUE.
Contractility is a fundamental property of protoplasm. In muscle cells
it attains its highest development. Muscle cells are elongated structures,
known as muscle fibers, which contain numerous longitudinal fibrils within
their protoplasm. By the shortening of these fibrillated cells, muscular
action results. The muscle fibrils, or myofibrils, may be free from transverse markings, as in smooth muscle; or they may exhibit a succession of
dark and light transverse bands, as in striated muscle. Smooth muscle
fibers enter into the formation of the viscera, and their action, almost
without exception, is involuntary. Striated muscle, in so far as it constitutes the entire system of skeletal muscles, is voluntary, or under the control
of the will, but the striated fibers of the diaphragm and upper part of the
oesophagus are apparently involuntary. The special form of striated muscle, known as cardiac muscle, which makes the bulk of the heart and extends
some distance in the wall of the pulmonary veins, is involuntary. The
three principal forms of muscle, smooth, skeletal, and cardiac, are mesodermal in origin. Within the basement membrane of the sweat glands
there are elongated ectodermal cells which have been described as smooth
muscle fibers, but their contractile nature is still questioned. It is well|
established, however, that the muscles of the iris, which control the size of/
the pupil, are derived from ectodermal cells which bud off from those f orm-P
ing the optic cup. Ectodermal muscles in man are limited to these
examples.
SMOOTH MUSCLE.
Smooth muscle fibers are derived from mesenchymal or young connective tissue cells. Usually they are produced in layers which surround
some tubular organ, such as a blood vessel, duct, or a part of the^ digestive
tube. The fibers in these layers are generally parallel, and are usually
either circular or longitudinal in relation to the organ which they envelop.
Occasionally they are oblique, or irregularly interwoven. Fibers which
encircle an organ are called circular or transverse fibers; they may be cut
across or split lengthwise according to the plane in which the organ is
sectioned. The same is true of the longitudinal fibers, which run lengthwise of the organ.
8
114 HISTOLOGY
The formation of smooth muscle may be studied advantageously in
the oesophagus of pig embryos, and its development in this position
has been carefully described by Miss McGill (Internat. Monatschr. f.
Anat. u. Physiol., 1907, vol. 24, pp. 209-245) A part of a longitudinal
section of the oesophagus of a 27-mm. pig is shown in Fig. 103. In such
a section the developing longitudinal smooth muscle fibers or myoblasts
are cut lengthwise (s.l.) ; and the circular fibers, which form a layer internal
to the longitudinal fibers, are cut across (s.c.). The loose mesenchymal
network, from which these fibers arise, is continuous with them above
and below. A third thin layer of muscle fibers is forming at m.m., and
at the top of the figure, the entodermal epithelium which lines the
oesophagus has been included, together with the basement membrane
beneath it.
In becoming smooth muscle cells, the mesenchymal cells change from
a stellate to a spindle-shaped form and come closer together, but they do
not lose their protoplasmic connections with one another. In the outer
part of their protoplasm coarse border fibrils or myoglia fibrils are produced,
which are similar to the fibroglia fibrils of connective tissue (p. 64).
According to Meves, the fibroglia and myoglia are identical. The latter
are at the periphery of the muscle cells and pass from one cell to another
for long distances. These fibrils may be strikingly demonstrated in the
oesophagus of a 24-mm. pig, stained with phospho-tungstic acid haematoxylin.
The coarse fibers shown by Miss McGill in both the circular and longitudinal muscle layers in Fig. 103 are "often found lying in part near the
surface of the cell, resembling the border-fibrils of Heidenhain." She
states that they are produced by a coalescence of granules within the
protoplasm, forming at first spindle-shaped bodies which later join end
to end, making varicose fibers. Subsequently they become smooth.
They may split into fine fibrils, and usually they decrease in number as
the embryo grows older. "In places they may be entirely absent in the
adult tissue; rarely they are abundant."
In addition to the peripheral myoglia fibrils, the protoplasm of smooth
muscle cells contains fine longitudinal fibrils, which have been described as the active agents in muscular contraction. Thus Miss McGill
finds that in the contracted portions of the muscle fibers the myofibrillae
show "a distinct increase in caliber." She states that the fine myofibrils
do not arise until the pig embryo reaches a length of about 30 mm. They
are apparently homogeneous from the beginning, and are distributed uniformly throughout the protoplasm. Some of them are shown in the muscle
layer m.m. in Fig. 103. Ordinarily these fibrils are indistinguishable in the
close-grained, deeply staining protoplasm which characterizes the muscle
cells.
SMOOTH MUSCLE
Along the sides of the muscle fibers there are at first protoplasmic
processes which bind them together. Later these seem to be replaced
by white fibers, like those of ordinary connective tissue. They form a
network investing the muscle cells, as shown in Fig. 104. This intermuscular reticulum, produced directly from the muscle fibers, is unusually
well shown in the walls of the blood vessels in the umbilical cord. To
some extent, according to Miss McGill, it is produced from special mesen
ept.
FIG. 103. FROM A LONGITUDINAL SECTION OF
THE OESOPHAGUS OF A 2?-MM. PlG EMBRYO.
X 700. (After McGill.)
b. m., Basement membrane; epi., epithelium;
mes., mesenchyma; m. m., muscularis mucosae;
n., nerve cells; s. c., circular smooth muscle
cut across; s. 1., longitudinal smooth muscle
cut lengthwise.
FIG. 104. FIBROUS TISSUE IN
RELATION WITH SMOOTH
MUSCLE FIBERS, FROM THE
BLADDER OF A PIKE. (After
Prenant.)
c., Connective tissue network;
n., p., f., nucleus, granular
protoplasm, and fibrillar protoplasm of a muscle cell.
chymal cells within the muscle layer, which develop into connective tissue
cells. In many layers of smooth muscle, however, connective tissue cells
are difficult to demonstrate. Finally it should be noted that elastic
fibers are found between the muscle cells. They vary greatly in number,
being especially abundant in the walls of arteries.
From what has been said, it is ev'dent that smooth muscle retains its
original syncytial nature, and that to some extent it resembles connective
tissue. It consists of elongated contractile cells which are joined together,
especially toward their extremities, by myoglia fibrils, and which are
bound together laterally by a white fibrous network containing inter
'll6 HISTOLOGY
spersed elastic fibers. These features, which are essential for understanding the action of smooth muscle, are usually difficult to observe in the compact tissue of the adult.
Smooth muscle fibers in the adult are fusiform, cylindrical or slightly
flattened cells, varying in length from about 0.02 mm. in some blood
vessels to approximately 0.5 mm. in the pregnant uterus. In the intestine
they are said to measure about 0.2 mm. Their diameter, through the
widest part, is from 4-7 /*,. Owing to the length of these fibers and the
fact that they are not perfectly straight, they are seldom wholly included
in a single section. Moreover they are usually so closely packed that their
outlines are hard to follow. They may be isolated, however, by treating
fresh tissue with a 35% aqueous solution of potassium hydrate, or 20%
nitric acid. The fibers when shaken apart appear as in Fig. 105. Owing
FIG. 105. SMOOTH MUSCLE FIBERS FROM THE SMALL INTESTINE OF
A FROG. X 240.
to the readiness with which they may be disassociated, the existence of
connections between them has sometimes been overlooked or underestimated; but it is evident that independent cells, by shortening cannot
cause the contraction of a tube. Branching fibers have been isolated
from the aorta, and are said to occur also in the ductus deferens and
bladder.
The fibers when sectioned longitudinally (see Fig. 17 7, p. 186) somewhat resemble connective tissue, from which they may be distinguished
by the staining and texture of their protoplasm and the position of their
nuclei, which are within the fibers. With haematoxylin and eosin the muscle substance takes a deeper and more purple stain than the connective
tissue fibers, and it is not so refractive. In doubtful cases Mallory's connective tissue stain may be used, which colors the muscle substance red and
the white fibrous tissue blue.
The nuclei of smooth muscle fibers are elliptical or rod-like bodies, containing a characteristic chromatic reticulum and sometimes several nucleoli (Fig. 9, A, p. 10). When the muscle fiber contracts, the nucleus
shortens and broadens, but according to measurements made by Miss
McGill (Anat. Rec., 1909, vol. 3, pp. 633-635) there is no change in its
volume. She finds, however, that the chromatin tends to collect at the
poles of the contracted nucleus, and states that "the nucleus appears to
take an active part in the process of contraction." Frequently spirally
twisted or bent nuclei are found in layers of contracted muscle (Fig. 106)
and they have been regarded as occupying contracted fibers. It is
SMOOTH MUSCLE 1 17
probable, however, that the spiral nuclei occur in relaxed fibers, which
have been crumpled together by the contraction of adjacent fibers.
Along one side of the nucleus the centrosome may be found, occupying
a shallow indentation of the nuclear membrane.
At the poles of the nuclei there is often an accumulation of granular
protoplasm (Fig. 104, p. 115) which is sometimes pigmented. The fibrils
diverge to pass by the nucleus, and the granular protoplasm occupies the
conical non-fibrillated space which is thus produced.
The surface of the smooth muscle fibers is covered by ^^^szi^^
a delicate membrane, which is sometimes thrown into
transverse wrinkles by the contraction of the fiber.
Possibly the fibrils terminate in it. They do not appear p
to become more compact as they extend into the tapering ^FIBERS FROM
ends of the fibers and presumably they do not all extend DOG?' RTERY F A
the whole length of the cell.
In transverse sections the fibers present rounded or polygonal outlines
(Fig. 107). They vary in size, since some are sectioned through the
tapering extremity and others through the thick central part which
contains the nucleus. In the figure the substance between the fibers appears solid, and it has sometimes been described as cement, or as a membrane rather than as a reticulum.
The relation of the myoglia, reticulum and muscle fibers to the process
of contraction has never been adequately explained. In the intestine, with
the normal accumulation of food, the diameter of the tube becomes four
times as great as in the contracted state,
and the muscle layer becomes reduced
to somewhat less than one-fourth of its
A ' ^ original thickness. The muscle cells
appear to slip by one another and to
-T^T -\f <B f rm a l aver on ly a f ew fibers thick.
After a certain amount of distention
c
FIG 107. CROSS SECTION OF SMOOTH MUSCLE the tube will expand no further, and
FIBERS FROM THE STOMACH. X 560. 111
a, Connective tissue septum; b, section of a added F^SSUrC CaUSCS it tO TUptUrC.
iblr ttrou g h% t he e nu u cil e u u s? : ' sei *" f a Presumably the elastic and white fibers
aid in restoring the normal caliber.
With extreme contraction, however, the white and elastic fibers no longer
aid the muscles, but become crumpled into coarse folds, as seen frequently in contracted arteries. As to the muscle fibers themselves,
Meigs concludes that during contraction fluid passes from them into the
intercellular spaces, so that the fibers shrink in size and become darker; he
states that they decrease greatly in length but remain of about the
same diameter, while the spaces between the fibers become larger
(Amer. Journ. Physiol., 1908, vol. 22, pp. 477-499). According to Miss
Il8 HISTOLOGY
McGill, the deeply staining nodular thickenings of muscle fibers indicate
a normal form of contraction in which the fiber does not contract as a
whole, but a wave of contraction passes over it. In these contraction
nodes the diameter of the fiber becomes increased (Amer. Journ. Anat.,
1909, vol. 9, pp. 493-545). The enlargement of such muscular tubes as
the vessels and intestine appears to be passive and due respectively to
the pressure of the blood or food within. After extreme contraction the
elastic tissue probably serves to dilate the tube to a certain size.
Smooth muscle is nourished by capillary blood vessels which tend to
follow the course of the fibers, and it is innervated by slender branches of
the sympathetic nervous system.
SKELETAL MUSCLE.
The skeletal muscles develop primarily from the mesodermic somites,
which have been briefly described in a previous section (p. 39). The transformation of a portion of each of these blocks of tissue into layers and masses
of skeletal muscle fibers has recently been reviewed by Williams, from whose
work Fig. 108 has been taken (Amer. Journ. Anat, 1910, vol. n, pp. 55100). In Fig. 108, A, the core of the somite has fused with the ventral
and medial walls of the mass, and the tissue thus formed is streaming
over the aorta and toward the notochord. This tissue, the sclerotome,
becomes mesenchyma and gives rise to smooth muscle and various other
mesenchymal derivatives. In the part of the somite left in place, near
the groove x, the striated muscle fibers begin to develop. The cells here
elongate at right angles with the plane of the figure, and 'thus lengthwise
of the embryo. In an older stage (Fig. 108, B) these myoblasts have
multiplied and have begun to form a plate of muscle tissue, the myotome,
which extends ventrally as shown in C and D. The dorso-lateral wall
of the somite has meanwhile become a plate of tissue, the dermatome, which
with the myotome associated with it, is often called the dermo-myotome.
The dermatome according to Bardeen produces only striated muscle
fibers; Williams finds that it forms only dermal connective tissue, and
others consider that it gives rise both to muscle and connective tissue.
The myotome is "entirely transformed into muscle fibers." The way
in which the myotomes extend ventrally and break up into the ventrolateral trunk and neck musculature, and the longitudinal fusion and
splitting of the dorsal part of the myotomes to produce the deep back
muscles of the trunk and neck, have been described by Warren Lewis
(Keibel and Mall, Human Embryology, 1910). The skeletal muscles
of the limbs have usually been described as arising from cells which have
migrated into the limbs from the ventral part of the myotomes. If this
takes place the cells which migrate become indistinguishable from mesen
SKELETAL MUSCLE
IIQ
chymal cells, but Bardeen and Warren Lewis consider that " the myo tomes
play no part whatever in the origin of the musculature of the limbs."
Moreover, Lewis states that " the idea that myotomes play a role in the
origin of the muscles of the head must be abandoned." A radical differ
C
D
FIG. 108. TRANSVERSE SECTIONS THROUGH THE MIDDLE OF CERTAIN SOMITES IN SUCCESSIVELY OLDER
CHICK EMBRYOS. A, B, AND C., THROUGH ONE OF THB SECOND PAIR OF SOMITES IN EMBRYOS OF
NINE, FIFTEEN, AND TWENTY-FIVE SEGMENTS RESPECTIVELY: D, THROUGH ONE OF THE FORTY-FOURTH
PAIR IN AN EMBRYO OF FIFTY-TWO SEGMENTS. X 230. (Williams.)
ao., Aorta; d, dermatome; m, myotome; m. t., medullary tube; n, notochord; s, sclerotome; z, angle at which
the myotome develops.
ence in the source of smooth and striated fibers has therefore not been
demonstrated, but the two forms of muscle develop very differently.
The myoblasts which produce striated muscle are found in the midst
I2O
HISTOLOGY
of a mesenchymal or connective tissue network, thus differing from the
myoblasts of smooth muscle. The latter unite with one another through
protoplasmic or fibrous processes; the striated fibers are bound together
by connective tissue sheaths. In producing striated fibers, the myoblasts become greatly elongated cylindrical structures, with rounded or
blunt ends. Although according to Schafer they generally do not exceed
36mm. in length, they sometimes measure from 53-123 mm. (Stohr);
their diameter is o.oi-o.i mm. During the growth of the myoblast,
mi to tic nuclear division takes place repeatedly, producing multi-nucleate
cells; and in the adult fibers, a further multiplication of nuclei through
amitosis has been reported. Each developing myoblast thus acquires
a row of centrally placed nuclei, imbedded in granular protoplasm. In
the outer part of the myoblasts coarse myofibrils develop, which, as seen
in cross section, form an encircling ring about the nuclei and axial core
of protoplasm (Fig. 109). The entire myoblast is surrounded by a membrane, to the formation of which the adjacent mesenchyma contributes.
Bundles of fibrils
s (Cohnheim's areas)
FIG. 109. CROSS SECTION OF MYOBLASTS
AND MESENCHYMAL CELLS FROM AN
iS-MM. PIG.
mes., Mesenchymal cell; f., myofibril; n.
nucleus of a myoblast; s., sarcolemma.
Connective tissue
FIG. no. CROSS SECTION OF FOUR MUSCLE FIBERS OF THE
HUMAN VOCAL MUSCLE. X 590.
The group of cells shown in Fig. 109 corresponds with a portion of the
myotome in Fig. 108, D. It is sectioned in the same plane, but represents
a later stage. In the adult, such an area of tissue as shown in Fig. 109
becomes a group of fibers as in Fig. no. The myoblasts have greatly
enlarged, and their protoplasm is filled with myofibrils which are often
arranged in "fields," known as Cohnheim's areas. These fields are cross
sections of longitudinal bundles of fibrils known as muscle columns, which
Schafer later named sarcostyles (i.e., muscle columns). The term sarcostyle is, however, often loosely applied to the separate myofibrils. It has
been supposed that the fibrils in a column arise by the longitudinal splitting of a primitive myofibril, but in sections it often appears that the areas
or columns are due to shrinkage. As the fibrils multiply, the nuclei, each
SKELETAL MUSCLE
121
surrounded by a small amount of granular protoplasm, migrate to the
periphery of the fiber and rest just beneath the connective tissue investment. Occasionally a nucleus is found which has not reached the surface.
Toward the end of the muscle fiber, the nuclei are numerous, and may
retain their central position. The growth of the fiber in length is supposed
to occur at the extremities.
The central position of the nuclei in myoblasts in pig embryos was clearly described by Schwann, in the second part of his treatise which established the cellular
structure of animals (1839). He believed, however, that the myoblasts were formed
by the coalescence of primary round cells arranged in a row. The gradual and nearly
complete transformation of the protoplasm into longitudinal fibrils was correctly
observed. Schwann found that the secondary cells, or mature fibers, were completely
enclosed in structureless membranes, which were clearly seen in shrunken fibers (Cf.
Fig. in).
Every striated muscle fiber is completely invested by a membrane
named the sarcolemma (o-ap, flesh; Xe/x/xa, husk or shell). This term
B S5-5
Sarcoplasm.
FIG. in. STRIATED
MUSCLE FIBER OF
FROG, TEASED APART
i N WATER. BEING
TORN AT x, AND
SHOWING THE SARCOLEMMA AT S AND S 1 .
Light band. Dark band.
FIG. iia. -LONGITUDINAL SECTIONS OF STRIATED MUSCLE.
A., Sketch to show the relation between the cells and fibers
according to Baldwin, a., Fibrous membrane; b., nucleus
of a muscle cell in vertical section; c., sarcolemma; d.,
myonbrils artificially separated.
B., Part of a fiber from a straight muscle of the eye of a calf.
X 1000. The nucleus is seen in surface view; the sarcoplasm contains chondrioconta.
was introduced by Bowman (Phil. Trans., 1840) who described the membrane as "a tubular sheath of the most exquisite delicacy, investing every
fasciculus ( or fiber) from end to end, and isolating its fibrillae from all the
surrounding structures." He confirms Schwann's statement that it is
not a fibrous structure derived from the surrounding connective tissue, and
he states that the nuclei of the muscle come to lie "at or near the surface
but within the sarcolemma." He adds, however, that he has seen similar
cells in the sarcolemma itself. Since Bowman's time there has been
prolonged discussion as to the nature of this membrane. The outer portion, which may occasionally contain nuclei, appears to be of connective
tissue origin, and is comparable with a basement membrane. The inner
122
HISTOLOGY
part, or true sarcolemma, is a structureless membrane closely applied to
the surrounding connective tissue. It appears to be much more definite
than any membrane which invests smooth muscle fibers, to which the term
sarcolemma has been extended by Heidenhain and others. The sarcolemma of striated muscle, however, is not yet thoroughly understood.
Although the muscle cells are generally said to be within it, Baldwin finds
that they are outside of the sarcolemma, between it and the fibrous basement membrane (Fig. 112, A). Accordingly he agrees with Apathy in
regarding the myofibrils as comparable with connective tissue fibers. The
possibility that the myofibrils are intercellular will be discussed under
cardiac muscle.
The appearances of skeletal muscle which have caused it to be called
striated are found only in longitudinal sections, including those which are
obliquely longitudinal. It is then seen that the myofibrils, which run
lengthwise, are composed of alternating light and dark portions, and that
they are so arranged that the dark parts of one fibril are beside the dark
parts of the adjacent fibrils. As a result of the close crowding of the fibrils,
alternating light and dark transverse bands appear to pass from one side
of the fiber to the other, and these are the striations. They are shown in
Fig. 1 1 2, A and B (at the right of A, the fibrils are represented as artificially
frayed apart).
Bowman (1840) stated that "a decisive characteristic of voluntary muscle consists
in the existence of alternate light and dark lines, taking a direction across the fasciculi."
He added that Leeuwenhoek had described
the striae repeatedly, believing in the earlier
years of his inquiry that they were circular
bands around the fibrils, but later regarding
them as of spiral arrangement, comparable
with an elastic coil of wire, and in some way
capable of retraction. Bowman recognized
that they were caused by the " coaptation of
the markings of neighboring fibrillae." He
found that the muscle fibers can readily be split
into longitudinal fibrillae with transverse markings, but that "in other cases their natural
cleavage is into discs, and in all instances these discs exist quite as unequivocally as
the fibrillae themselves." The discs are produced when the ends of a muscle fiber
are pulled apart (Fig. 113). Bowman regarded each disc as a plate of agglutinated
segments, receiving a single segment from every fibrilla which crossed it. These segments he named sarcous elements; they are united endwise to form the myofibrils and
crosswise to form the discs. Usually the longitudinal cohesion is much greater than
the lateral, and in the wing muscles of insects, according to Schafer, the fiber "never,
under any circumstances, cleaves across into discs."
The finer structure of the fibrils is illustrated in the diagram, Fig. 114,
which represents a part of seven myofibrils, including three dark bands
FIG, 113. A HUMAN STRIATED MUSCLB FIBER SEPARATING INTO Discs. (After
Bowman.)
SKELETAL MUSCLE
123
and portions of four light bands. Under polarized light the dark bands
are doubly refractive or anisotropic, and the light ones are singly refractive
or isotropic. Following Rollett's suggestion, the striations are often
designated by letters. The dark band is called Q (an abbreviation for
Querscheibe, or transverse band) and the light band is called J (applied by
Rollett to a subdivision of the isotropic layer) . The light band is bisected
by the ground membrane, or Krause's membrane, which appears as a very
FIG. 114. DIAGRAM OF MUSCLE STRIATIONS. (After Heidenhain.)
The fibrils consist of alternating dark bands, Q, and light bands, J. J. is traversed by the ground membrane Z, and Q by the median membrane M. In the right of the three muscle segments shown in
the Sgure, the bands, N, have been drawn.
slender dark line, Z (Zwischenscheibe, or intermediate disc) . The lines
Z are believed to represent continuous membranes which divide the muscle
fiber into compartments called muscle segments, or sarcomeres. At the
sides of the fiber, Krause's membranes join the sarcolemma, which bulges
between them when the fibers are contracted (Fig. 112, A). Between
Z and Q, in the highly developed striated muscles of insects, a band N
has been described (Nebenscheibe, or accessory band). The dark band
FIG. 115. FIBRILS OF THE WING-MUSCLES OF A WASP; THE UPPER ONE CONTRACTED; THE MIDDLE ONE
STRETCHED; AND THE LOWEST ONE UNCONTRACTED. X 2000. (Schafer.)
Q becomes gradually lighter toward its central part (thus forming h or
Qti), and in its central part it is sometimes seen to be crossed by Hensen's
median membrane, M (Mittelscheibe). The latter is believed to be similar
to Krause's membrane, but more delicate. Like the other bands it may
appear dark or light according to the focus. In the muscle fibrils shown
in Fig. 115, the bands Q, J, and Z may be readily identified; M appears
as a rather broad white line which may include Qh.
124 HISTOLOGY
Between the myofibrils and completely surrounding them is the sar coplasm, which is a fluid substance containing interstitial granules, fat droplets, and glycogen. It differs from the protoplasm of the muscle cells
which is found about the nuclei, and which is cut off from the sarcoplasm,
according to Baldwin, by the sarcolemma. The granules have been carefully studied by Bullard (Amer. Journ. Anat., 1912, vol. 14, pp. 1-46) who
discusses their staining reactions and probable composition. The significance of the interstitial granules could not be determined. The fat
droplets are regarded as reserved food material, and they vary in abundance according to the quantity of fat in the food. Schafer has found no
evidence that the isolated sarcoplasm of insect muscles is contractile, but
he readily observed the contractility of isolated myofibrils. Moreover the
activity of certain muscles in living embryos begins soon after the fibrils
are differentiated.
In the process of contraction, according to Schafer, the hyaline substance of the myofibril passes from the light segment / into the dark segment Q, so that each sarcomere becomes short and broad. He refers to
the photograph of the lowest fibril in Fig. 115 as showing that the dark
substance is porous (note the end of the fiber toward the right). The
sarcolemma bulges between the successive Krause's membranes, which are
brought closer together (Fig. 112, A), and the length of each sarcomere is
greatly reduced. The dark band Q may become light through the accumulation of hyaline substance within it, and the shortened and condensed J
may become quite dark, causing a reversal of the original color relations.
The sarcoplasm is said to be forced from between the dilated myofibrils in
Q, into /. Others consider that contraction is due to a passage of fluid
from the sarcoplasm into the myofibrils, and that the beaded form which
the myofibrils often present, results from an intake of fluid through the
ultra-microscopic membranes which are supposed to surround them. The
latter interpretation is defended by Meigs (Zeitschr. f . allg. Physiol., 1908,
vol. 8, pp. 81-120), and vigorously attacked by Schafer (Quart. Journ.
Exp. Physiol., 1910, vol. 3, pp. 63-74). The older theories of contraction
and the numerous papers on the finer structure of striated muscle are admirably reviewed by Heidenhain (Anat. Hefte, Abth. 2, 1899, PPi-iu).
Adult muscle is composed of such fibers as have been described in the
preceding paragraphs. They are arranged in compact bundles, shown in
cross section in Fig. 116. Around all the larger muscles there is a connective tissue sheath, or external perimysium, which extends into the muscle in
the form of septa, thus subdividing it into bundles or fasciculi. These
septa constitute the internal perimysium, and the connective tissue extends from them around the individual muscle fibers, blending with the
sarcolemma. In the connective tissue of the diaphragm, elastic fibers are
SKELETAL MUSCLE
125
abundant; but the muscles of the extremities are poor in elastic tissue,
containing only fine, chiefly longitudinal fibers, found especially in the
perimysium externum.
Cross sections of striated muscle fibers are readily recognized. They
have rounded-polygonal outlines formed by the sarcolemma and fibrous
membrane, within which are the myofibrils, often shrunken from the
membrane. The fibrils stain intensely with eosin. They appear as coarse
granules, but their rod-like form becomes evident as they are followed up
and down by changing the focus. The shifting picture thus presented is
quite characteristic. Some fibers stain more darkly than others, owing to
the varying abundance of sarcoplasmic granules.
External perimysium.
Muscle bundles.
Internal perimysium.
Cross section of artery.
Muscle spindle.
Cross section of nerve
FIG. 116. FROM A CROSS SECTION OF THE OMOHVOID MUSCLE OF MAN. X 60.
In many animals, as in the rabbit, two sorts of striated muscles may be recognized
red muscle (e.g., the M. semitendinosus and M. soleus); and pale or white muscle
(e.g., the M. adductor magnus). Correspondingly there are two sorts of fibers.
First, there are dark fibers with abundant sarcoplasm, well defined longitudinal
striation, and poorly developed transverse markings, having in general a small
diameter; these occur in red muscles. Secondly, there are pale fibers, with less sarcoplasm and better defined transverse striations, having a greater diameter. These
are the more highly differentiated fibers. Although in some animals these two sorts
of fibers are found in separate muscles, in others, as in man, they are mingled in
single muscles. In general the most constantly active muscles (cardiac, ocular,
masticatory and respiratory) contain the most fibers with abundant sarcoplasm.
The muscles having many fibers with scanty sarcoplasm contract more quickly but
are exhausted sooner.
126 HISTOLOGY
The size of the muscle fibers is subject to considerable variation. They
are said to enlarge at a uniform rate throughout the body until birth, when
their diameter is about twice as great as in embryos of four months. After
birth the fibers of certain muscles become much coarser than those in
others. Thus the gluteal muscles have large fibers (av. diam. 87.5/0
and the ocular muscles have small ones (av. diam. 17.5 /A), as determined by
Halban (Anat. Hefte, Abth. i, 1894, vol. 3, pp. 267-308). He finds that
the diameter of the adult fibers in general is about five times greater than
at birth. As a result of exercise the diameter of muscle fibers in rats may
show an average increase of 25% according to Morpurgo (Arch. f. path.
Anat., 1897, vol. 150, pp. 522-554). He states that the enlargement of the
muscle takes place without an increase in the number of its fibers, but merely
through the thickening of existing elements. The fibers which grow most
are those which originally were thinnest, and which act as a reserve
material with great capacity for growth. The enlargement of fully formed
fibers apparently takes place through an increase in the sarcoplasm, with
Transition zone.
Nucleus tendon. Q Z
FIG. 117. BRANCHED STRI- FIG. 118. LONGITUDINAL SECTION OF A PART OF A MUSCLE
ATED MUSCLE FIBER FROM FIBER FROM A HUMAN INTERNAL INTERCOSTAL MUSCLE. SHOW
THE TONGUE OF A FROG. ING ITS TRANSITION TO TENDON. X 750.
out multiplication or thickening of the fibrils. After injury striated muscle gives slight evidence of regeneration, but it has been thought that
latent myoblasts may become active. A proliferation of nuclei toward
the injured ends of the muscle fibers has been recorded, but repair is chiefly
through the production of connective tissue.
Longitudinal sections of skeletal muscles may be easily recognized
by the presence of unbranched striated fibers, bounded by well-defined
membranes, associated with which are the flattened peripheral nuclei. The
striations Q and / are visible under low magnification. In a few situations, striated muscle fibers branch (Fig. 117). Branching has been reported toward the place where the muscle fibers of the tongue are inserted
into the mucous membrane, and where the facial muscles end in the subcutaneous tissue. The way in which the fibers connect with tendon has
SKELETAL MUSCLE
127
Muscle fiber. Connective tissue.
Connective tiss
been studied with conflicting results. Schultze finds that at the end of
the muscle fiber the myofibrils are no longer differentiated into light and
dark bands, but pass directly into the tendon fibrils, with which they are
continuous (Fig. 118). "Muscle fibrils and tendon fibrils are parts of a
single structure." (Arch. f. mikr. Anat, 1912, vol. 79, pp. 307-331). But
Baldwin finds that the ends of the muscle fibers are primarily conical and
are covered with sarcolemma; and the tendon fibrils connect with the
sarcolemma at the apices of the cones. The processes of sarcolemma are
thus primarily "dovetailed" into the tendon. Secondarily .the cones
may blend to form a thickened flat layer to which perichondrial or periosteal fibers are attached. In no case is the sarcolemma penetrated by
muscle fibrils or tendon fibrils, and therefore there is no continuity between
them (Morph. Jahr., 1912, vol. 45, pp. 249-266). Thus Baldwin defends
the generally accepted opinion.
Muscles are abundantly supplied with vessels and nerves, which are
imbedded in the perimysium. The lymphatic vessels end in the septa
without extending among the individual muscle fibers; but the blood
vessels, through capillary
branches, continue further and
run between adjacent fibers,
thus forming a plexus with
elongated rectangular meshes.
The nerves are chiefly motor,
and a branch ends in contact
with every muscle fiber, to
which it transmits the impulse
for contraction. Muscles also
contain sensory nerves, having
"free endings" and probably
terminating also around the muscle spindles. The spindles are slender
bundles of poorly developed fibers, generally situated near the septa
formed by the internal perimysium, as seen in Figs. 116 and 119. All
the muscle spindles are formed during embryonic life, and their abundance and distribution in the various muscles in embryos have been
studied by Gregor (Arch. f. Anat. u. Entw., 1904, pp. 112-194). They
have not been found in all muscles, and in certain muscles they are
regularly more numerous than in others. Thus they have been reported
as absent from the muscles of the eye, face, pharynx, small muscles of
the larynx, the Mm. ischiocavernosus and bulbocavernosus, and certain
others, including a large part of the diaphragm. They are numerous in
the distal muscles of the limbs, and in certain muscles of the neck. The
finer structure of the nerve terminations, both motor and sensory, will be
considered with the nervous system.
Cross section Muscle fibers Nucleus Nucleus of the
of nerve. of the of the sarcolemma.
spindle, perimysium.
FIG. 119. THE MUSCLB SPINDLE SHOWN IN FIG. 116.
X 240.
128
HISTOLOGY
CARDIAC MUSCLE.
A portion of the mesenchymal syncytium from which cardiac muscle
develops is shown in Fig. 1 20. Its nuclei are found in the axial part of the
protoplasmic strands, at varying intervals from one another. Peripherally
a few myofibrils have developed from the chondrioconta, or protoplasmic
granules, and these fibrils extend for considerable distances through the
syncytium regardless of cell areas. They multiply rapidly, and form a
peripheral layer of fibrils surrounding the central nuclei and axial protoplasm. Thus as seen in cross section, the strands of cardiac syncytium
and the myoblasts of skeletal muscle
resemble one another. The fibrils
exhibit alternating dark and light
bands which are arranged as in
skeletal muscle, and ground membranes (Z) develop across the fibers,
bisecting the light bands (/). Thej
striations, however, are not as regular
and as highly developed as in 1
skeletal muscle. At the periphery
of the fibers there is a sarcolemma,
which is thinner than that of skeletal
muscle, and was formerly overlooked.
In early stages the muscle fibers in
many places rest close against the
endothelium of blood vessels; later
they are surrounded by more or less
connective tissue.
In the adult the cardiac muscle fibers anastomose freely, thus retaining
their original syncytial arrangement (Fig. 121). They do not, however,
form an irregular network, but are arranged in layers, in which the fibers
tend to be parallel. Thus they are cut longitudinally in Fig. 121 and
transversely in Fig. 172 (p. 179). The nuclei retain their central position.
They are elliptical bodies with a conical mass of protoplasm at either pole.
This protoplasm, as in smooth muscle, occupies the interval left between
the fibrils as they diverge to pass by the nucleus. It is granular, and
frequently contains brown pigment.
According to Apathy (Biol. Centralbl., 1888, vol. 7) "the contractile substance
is a product of the muscle cell and the muscle cell is represented by the nucleus and
surrounding area of protoplasm." "The myofibrils of the contractile substance are
the histogenetic homologues of connective tissue fibrils, however much they may
differ from them chemically or functionally." Baldwin has recently advanced a
similar interpretation. He finds that the sarcoplasm between the fibrils differs from
the protoplasm around the nucleus. Moreover he states that the perinuclear proto
FIG. 120. CARDIAC MUSCLE FROM A DUCK
EMBRYO OF THREE DAYS. (M. Heidenhain, from
McMurrich's "Embryology.")
CARDIAC MUSCLE
129
Nucleus. Sarcoplasm. Fibrils. Lateral branch.
plasm, in both skeletal and cardiac muscle, is separated by the sarcolemma from the
myofibrils and sarcoplasm (Fig. 112, A). In regard to smooth muscle, however,
Baldwin merely notes that it should be reviewed in the light of these facts. The
existence of a membrane around the nu
icleus and granular protoplasm at its peles
['would place smooth muscle in the same
J category, and make the fibrils extracellular.
With muscle, therefore, as with connective tissue, the distinction between intracellular and extracellular appears to be
arbitrary and conventional. It is interesting to note that the extrusion of the nuclei
from the precartilage matrix to its surface,
as described by Mall, may be comparable
with the passage of the nuclei from the center to the surface of skeletal muscle fibers.
Baldwin's papers are found in the Zeitschr.
f. allg. Physiol., 1912, vol. 14, pp. 130-160,
and, as regards cardiac muscle, in the
Anat. Anz., 1912, vol. 42, pp. 177-181.
A feature of cardiac muscle which
is unlike anything observed in smooth
or skeletal fibers is the presence of intercalated discs. These are transverse
lines across the fibers, which were
formerly interpreted as cell boundaries, and some authorities still regard
them as such. In the guinea-pig
Jordan and Steele find that they first appear during the week before birth.
Thus they are late in development, and they are relatively less abundant
and simpler in the young than in adults (Amer.
Journ. Anat, 1912, Vol. 13, pp. 151-17.3). If
the cardiac syncytium ultimately became resolved into cells, it would resemble certain other
syncytia in this respect; and cardiac muscle can
be broken up into cell-like blocks, apparently
along these discs. However, the discs occur at
variable distances from one another, and very
frequently they mark off non-nucleated portions of the syncytium. As many as four of
them may extend partly across a single nucleus,
as shown by Jordan and Steele, indicating that
they are peripheral modifications of the
myofibrils, and cannot be regarded as cell walls. Heidenhain (Anat.
Anz., 1901, vol. 20, pp. 33-78) pictures them as always connected on one
9
x Conn, tissue. Capillaries.
FIG. 121. LONGITUDINAL SECTION OF A PAPIL
Y MUSCLE FROM THE HUMAN HEART. X 24<J
The transverse lines (x) are partly light (where the
fiber has broken) and partly dark (intercalated
iscds.)
FIG. 122. INTERCALATED Disc (d)
FROM HUMAN CARDIAC MUSCLE,
STAINED WITH THIAZIN RED AND
TOLUIDIN BLUE. Z, Krause'a
membrane. (Heidenhain.)
130 HISTOLOGY
side with a ground membrane Z (Fig. 122), and states that they are somewhat narrower than a sarcomere (i.e., the distance between two successive
ground membranes) . He regards them as the places where new sarcomeres
form, thus providing for the growth of the heart. Jordan and Steele,
among others, consider that they are places where individual fibrils are
contracted, and the fact that they are shorter than adjacent sarcomeres
favors this interpretation. The discs may extend straight across the fiber,
but frequently they are broken into "steps" as shown in the figure.
There are, therefore, three peculiarities of cardiac muscle through which
it differs conspicuously from skeletal muscle, namely, its anastomosing
fibers, central nuclei, and intercalated discs.
NERVOUS TISSUE.
General features. In nervous tissue the protoplasmic functions of irritability and conductivity attain their highest development. Irritability
is that property which enables the cell to react to various stimuli, such as
pressure or light; and through conductivity the effects of stimulation are
transmitted to distant parts of the cell, or to adjacent cells. In all animals
the cells of the outer or ectodermal layer are those most exposed to stimulation, and the ectoderm accordingly gives rise to the entire nervous system.
In some animals all the ectodermal cells have been described as equally
responsive to stimulation, and the name "sensory layer" has been applied
to the ectoderm as a whole. Usually, however, the sensory cells become
specialized in definite and limited areas of the ectoderm. M. Schultze
(1862) showed that the sensory cells of the nose and eye are epithelial
elements, the bases of which are prolonged into filaments which serve as
nerves to convey sensation. He taught that the specific functions of the
sense organs depend on their respective epithelial cells, which accordingly
may be designated as olfactory, gustatory, auditory or visual cells.
Not only does the ectoderm produce sensory neuro-epithelial cells, the
nucleated bodies of which remain in the epithelium, but it gives rise to more
deeply placed nerve cells, which connect with the epithelial cells and place
them in communication with the muscles. In simple forms of animals this
connection is very direct, and the response of the muscle to epithelial
stimulation is quite automatic. In the higher animals there are both
direct and indirect paths from the sensory endings to the muscles, and muscular action may be inhibited or initiated by certain of the centrally
placed nerve cells.
The centrally placed cells in vertebrates constitute the spinal cord and
brain, which together form the central nervous system. The bundles of
fibers which convey impulses to and from the central nervous system, together with the cells associated with them, constitute the peripheral nervous
system.
NERVOUS TISSUE 131
In the olfactory epithelium of vertebrates there are neuro-epithelial
cells which send fibers directly into the central nervous system, but in
other cases the nucleated bodies of the sensory cells are not found in the
epithelium. They occur in circumscribed masses or ganglia, from which
fibers extend both into the central nervous system, and outward to various
sensory structures, where they terminate in contact with cells which stimulate them. Thus the stimulus which gives rise to a tactile sensation is
received by the terminal ramifications of a nerve fiber in the skin. The
stimulus is conveyed along this fiber (Fig. 123, a), through the spinal
ganglion (b), into the spinal cord, where it produces several branches (at c).
One of these branches passes to a motor cell, d, to which, through contact, it
FIG. 123. DIAGRAM OF THE SPINAL CORD SHOWING A SENSORY FIBER, a; A MOTOR FIBER, e; AND THE
FIBERS WHICH CONNECT THEM WITH EACH OTHER AND WITH THE BRAIN.
transmits its stimulus. The motor cell sends a fiber outward (e) to terminate in contact with a striated muscle, which is thereby stimulated so that
it contracts. This direct path from the sensory ending to the muscle, provides for reflex or unconscious action, such as is taken when the hand is
suddenly withdrawn from a painful contact. In such a case a considerable
group of muscles may contract together, since the sensory fiber sends
branches up and down the cord (/"), and these in turn give off collateral
branches which pass to motor cells at different levels.
The cell which conveys the tactile sensation from the skin to the spinal
cord gives rise to branches which terminate in contact with other cells in
the spinal cord, as shown in Fig. 123, g. From these cells processes cross
to the opposite side of the cord and pass up to the brain (ti), where they
connect with nerve cells through which the sensations become conscious.
These brain cells presumably become permanently modified by the sensations which they receive, so that they store experiences. As a result of the
sensation transmitted from the skin, certain cells in the brain may send
stimuli downward to the motor cells of the cord, which then cause the
132
HISTOLOGY
muscles to act voluntarily. The descending fiber crosses to the opposite
side during its descent, and occupies the position in the cord shown in Fig.
123, i. A branch is shown passing to the motor cell, d.
From this sketch of the constitution of the nervous system, it is seen
that it consists essentially of cells, made up of cell bodies and of fibers; the
fibers are prolongations of the cell bodies. The cells are sensory, or afferent,
conveying impulses toward the central nervous system; and motor, or
efferent, conveying impulses away from the central system. Within the
cord these cells connect with others, forming ascending and descending
tracts, or bundles of fibers passing toward the brain and away from it,
respectively. Fibers which serve to connect different levels of the cord
with one another are known as association fibers; those which connect
the opposite sides are commissural fibers.
Certain features in the development of the nervous system in lower animals, of
interest in connection with the mammalian nervous system, are shown diagrammatically in Fig. 124. In sponges, according to Parker, there is no nervous tissue of
any sort, but beneath the thin epithelium he finds elongated contractile cells which
"resemble primitive smooth muscle fibers" (Fig. 124, A). They have been regarded
as modified epithelial cells. Parker finds that they are stimulated directly, as a result
FIG. 124. A, DIAGRAM OF THE MUSCULAR MECHANISM IN A SPONGE. (Parker.) B, DIAGRAM OF THE
NBURO-MUSCULAR MECHANISM IN A MEDUSA. (Parker, after Hertwig.) C, DIAGRAM OF THE
VENTRAL NERVOUS CHAIN (c) AND ADJACENT STRUCTURES IN AN EARTHWORM. (Parker, after
Retzius.)
Longitudinal muscle; b, motor fiber; d, sensory fiberj e, epithelium on the under surface of the
body, containing neuro-epithelial cells.
of changes in the sea- water, so that they slowly contract and close the orifices around
which they are situated. Since the sponges are lower than any animals which are
known to have nerve cells, Parker concludes that muscular tissue arose independently
of nervous tissue, and is the more primitive (Journ. Exp. Zool., 1910, vol. 8, pp. 1-41).
In the medusae, neuro-epithelial cells, nerve cells, and both smooth and striated
muscle fibers are present. According to Oskar and Richard Hertwig, the muscle
cells are derived from the deep part of the ectodermal epithelium, and from the
first they are connected with nerve cells or neuro-epithelial cells (Fig. 124, B). In
other words, in the medusas muscle and nerve develop in primary communication
with one another (Das Nervensystem der Medusen, Leipzig, 1878).
In the earthworm (Fig. 124, C) neuro-epithelial cells in the ventral body wall
send fibers to a cord of nervous tissue which constitutes a central nervous system.
From cells in this cord, processes extend to the muscles, as shown in the diagram.
Thus the neuro-epithelial cell does not stimulate the muscle directly; it conveys an
impulse to the motor cell which in turn acts upon the muscle. In addition to the
NERVOUS TISSUE
133
cells shown in the diagram the cord contains ramifying association and commissural
cells. Thus stimulation at one point on the surface of the animal may cause
coordinated muscular contractions in different parts of the body. As Retzius has
pointed out, if the neuro-epithelial cells should withdraw into the interior of the
animal, leaving their branching process in the epidermis, the conditions in vertebrates would be closely paralleled.
The development and structure of the central nervous system and the
sense organs will be considered in a later chapter. The following account
deals first with the development of the spinal nerves, the spinal sympathetic
system, and the cerebral nerves; and secondly, with the adult structure
of these parts, including the ganglia, nerve trunks, and nerve endings.
DEVELOPMENT OF THE SPINAL NERVES.
The formation of the medullary groove (or neural groove) as a longitudinal trough in the ectoderm, and its conversion into the medullary tube
V
FIG. 125. THE DEVELOPMENT OF THE NERVOUS SYSTEM AS SEEN IN CROSS SECTIONS OF RABBIT EMBRYOS:
A, 7$ DAYS; B, 8J DAYS; C, 9 DAYS; D, ioj DAYS; E, IA DAYS.
c. c. f Central cavity; d. r., dorsal root; d. ra., dorsal ramus; ep., ependymal layer; g. c., ganglion cells;
g. 1., gray layer; m. g., medullary groove; m. t., medullary tube; o. b., oval bundle; s. g., sympathetic
ganglion; sp. g., spinal ganglion; s. ra., sympathetic ramus; v. r., ventral root; v. ra., ventral ramus;
w. 1., white layer.
by the coalescence of its dorsal edges, have been described in a previous
section (p. 37). The anterior part of the tube expands to form the brain;
the posterior part becomes the relatively slender spinal cord.
At about the time when the medullary tube separates from the epidermal ectoderm, some cells become detached from the medial dorsal
portion of the tube and pass down on either side of it, as shown in Fig. 125,
C and D. These cells constitute the neural crest. They multiply by
mitosis and accumulate in paired masses, corresponding in number with
134 HISTOLOGY
the segments of the body. Thus they form the spinal ganglia. A typical
cell of a spinal ganglion is at first round, but later becomes bipolar by sending out two processes, one toward the periphery and the other toward the
medullary tube. These processes grow out from opposite ends of the cell
(Fig. 126). With further growth the nucleated cell body passes to one
side of the prolongations, with which it remains connected by a slender
stalk. Such T-shaped cells are characteristic of the spinal ganglia. The
fibers which grow toward the medullary tube enter its outer part and then
bifurcate, sending one branch toward the brain and the other down the
cord. These longitudinal fibers form distinct oval bundles just within
Bipolarcells. T-cell.
Since these bundles receive accessions of
fibers from every spinal ganglion, they enlarge as they approach the brain. The
fibers of the oval bundle branch freely at
their terminations, and along their course
they give off collateral branches, which enter
FIG. 126. SPINAL GANGLION CELLS. . . .
The bipolar forms are from a chick the deep substance of the cord. 1 he periph
eral fibers from the spinal ganglia grow out
ward through the mesenchyma, and terminate in sense organs or sensory endings, which will be described presently. The fibers of the spinal
ganglia are essentially sensory or afferent, conveying impulses from the
periphery toward the cord, and up the cord toward the higher nervous
centers.
The efferent or motor fibers arise chiefly from cells, the bodies of which
remain within the central nervous system. Each of these nerve-forming
cells, or neuroblasts, sends out one long process called a neuraxon (or axone).
The neuraxons of the motor cells leave the spinal cord, near its ventral
surface, in bundles which unite to form the ventral roots. The ventral
roots correspond in number with the dorsal roots, which are the bundles
of sensory fibers passing into the cord from each spinal ganglion. Peripherally the ventral root joins the bundle of fibers growing outward from
the spinal ganglion, and the two together form a spinal nerve. Every
spinal nerve consequently has a dorsal (sensory) root, and a ventral
(motor) root. The fibers from the two roots travel in the same connective
tissue sheath, but otherwise they remain entirely distinct.
The fundamental facts which have just been reviewed eluded anatomists for
centuries. The nerves, extending from the brain and cord to all the important organs,
were regarded as tubes, conveying a vital fluid necessary for organic activity; when
this supply was cut off, the organs ceased to perform their functions. Thus if nerves
to the skin were destroyed, the skin became insensible; or if those to muscles were
cut, the muscles could not contract. The possible existence of sensory and motor
nerves with different functions was debated and generally rejected, until Charles Bell
proved conclusively that "nerves entirely different in function extend through the
NERVOUS TISSUE 135
frame; those of sensation; those of voluntary motion; .... these nerves are sometimes separate, sometimes bound together; but they do not, in any case, interfere with
or partake of each other's influence." This brilliant discovery was verified by physiological experiments to determine "whether the phenomena exhibited on injuring
the separate roots of the spinal nerves corresponded with what was suggested by their
anatomy." Bell found that such was the fact. (An Exposition of the Natural
System of the Nerves of the Human Body, with a republication of papers delivered
to the Royal Society, London, 1824.)
It was at first supposed that the nerves grew out from the cord and brain and
acquired connections with their end-organs; but the apparent difficulty which the
fibers would have in reaching them, and the fact that the connections must be established before the nervous system can be functional, have led to the idea that the
nervous and muscular systems are connected at all stages of their development. In
tadpoles, however, Harrison has shown that such connection is not an indispensable
requisite for the normal development of the muscles, since they are formed in a normal
manner after the medullary tube and neural crest have been removed from the entire
posterior portion of the body. He finds further that nerves grow out into the adjacent
A B
FIG. 127. THE GROWTH OF NERVES IN TISSUE CULTURES. (Harrison.)
A, Two views of the same nerve fiber taken twenty-five minutes apart, during which time the fiber has
grown 2On', B, Two views of another fiber, at lower magnification, taken fifty minutes apart.
tissues from transplanted portions of the medullary tube. Therefore he concludes
that the nerves normally grow out to their end-organs and unite with them, but that
this takes place very early in development, when the paths are quite direct. Subsequent growth of the body causes the muscles to shift about and become widely separated from the central nervous system, so that the nerves become greatly elongated
and follow irregular courses (Amer. Journ. Anat., 1904, vol. 3, pp. 197-220; 1906,
vol. 5, pp. 121-131).
The participation of the mesoderm in the formation of nerve fibers has repeatedly
been asserted, and some authorities now consider that the long fibers passing from the
spinal cord to distant muscles are formed from chains of cells, either mesodermal
or ectodermal. Certain of Harrison's experiments were designed to show whether
the nerve fibers are formed by peripheral cells or grow out from the central nervous
system. In tissue, cultures, made by placing fragments of the medullary tube of tadpoles in lymph, at a stage when the tube consists entirely of round cells, he observed
the actual growth of the fibers. Examined after a day or two of cultivation, in a
considerable number of cases, they were seen extending out into the lymph clot (Fig.
127). Harrison concludes that the nerve fibers begin as an outflow of hyaline proto
136
HISTOLOGY
plasm from the nerve cells. The protoplasm is actively amoeboid, and, as a result of
this activity, it extends farther and farther from its cells of origin, retaining its pseudopodia at its distal end. Similarly enlarged "cones of growth," provided with spiny
processes, have been observed in preserved tissue by Cajal; and His, from embryological studies, had long maintained that the nerve fibers grow out from neuroblasts
in the central nervous system and spinal ganglia. Harrison concludes that his experiments "place the outgrowth theory of His upon the firmest possible basis" (Anat.
Rec., 1908, vol. 2, pp. 385-410).
Dorsal and Ventral Rami. Every spinal nerve, near the junction of
its ganglionic and motor roots, divides into a dorsal and a ventral branch I
FIG. 128. THE SYMPATHETIC SYSTEM IN A I6-MM. HUMAN EMBRYO. (After Streeter.)
nerve; St., stomach.
or ramus (Fig. 125, E). Each ramus receives both sensory and motor
fibers, and is therefore a mixed nerve. The dorsal rami are distributed to
the muscles and skin of the back; their terminal cutaneous branches enter
the skin along a line extending from the neck down the trunk of the body,
NERVOUS TISSUE 137
as may readily be shown in dissections of the adult. In embryos of 10-12
mm. these rami are present as short branches, which can be followed to the
muscular condensations derived from the myotomes, but apparently at
that stage they do not enter the skin. The ventral rami are longer. Most
of them anastomose with the ventral rami of adjacent nerves, thus giving
rise to the cervical, brachial and lumbo-sacral plexuses. They are distributed to the muscles and skin of the ventral body wall.
DEVELOPMENT OF THE SPINAL SYMPATHETIC SYSTEM.
In mammalian embryos measuring 10-12 mm., each of the thoracic
spinal nerves exhibits a branch directed toward the aorta, and ending in a
rounded mass of ganglion cells. This is the sympathetic or visceral ramus,
terminating in a sympathetic ganglion (Fig. 125, E). It is generally
believed that the cells in the sympathetic ganglia migrate outward from
those in the spinal ganglia, but an origin from cells of the medullary tube
which wander out along the ventral roots has also been asserted. Although the cells of the sympathetic ganglia were formerly considered to be
mesodermal (even after it had been shown that those of the spinal ganglia
were ectodermal), it is now generally admitted that the entire sympathetic
system is ectodermal. However, in the cervical region the spinal nerves
at first do not have sympathetic rami, and the sympathetic ganglia consequently appear isolated in the mesenchyma. Their cells may have migrated in detached groups. Instead of eight ganglia on either side of the
neck, corresponding in number with the spinal nerves, there are but three,
known as the superior, middle and inferior cervical ganglia, respectively,
and of these the middle ganglion may be merged with the superior. They
are elongated structures, especially the superior ganglion, and presumably
represent a fusion of segmental ganglia.
Each sympathetic ganglion in the thorax of the adult is connected
with its spinal nerve by two rami communicantes, known as the white and
gray rami, respectively. The white rami consist chiefly of fibers passing
outward from the spinal nerve, and they are probably a persistence of the
sympathetic rami of the embryo. The gray rami contain fibers passing
from the sympathetic ganglia back to the spinal nerves, and apparently
arise later. They are found not only in the thorax and abdomen, but
also in the neck where, as usually described, they place the superior cervical ganglion in connection with the first four cervical nerves, the middle
cervical ganglion in connection with the fifth and sixth, and the inferior
in connection with the sixth, seventh and eighth. The succession of
sympathetic ganglia on either side of the body, extending from the neck
to the pelvis, become connected with one another through bundles of
138
HISTOLOGY
longitudinal nerve fibers, and thus they form the ganglionated trunk of
the sympathetic nerve (Fig. 128).
From the ganglia of the trunk, bundles of nerve fibers grow out ventrally to supply the blood vessels and viscera. It is characteristic of these
branches that they unite with one another freely, forming net-like sympathetic plexuses, within which there are many scattered nerve cells.
When the nerve cells in these ganglionated plexuses are particularly abundant, the structure is called a ganglion, though generally retaining a
plexiform character.
The principal branches of the cervical sympathetic trunk are the superior, middle, and inferior cardiac nerves, which grow out from the corresponding cervical ganglia. They extend to the heart (Fig. 128) and form
the cardiac plexus, associated with which is the cardiac ganglion, situated
under the arch of the aorta. These nerves, which are joined by branches
from the vagus, innervate the heart. The cervical sympathetic trunks
also send out nerves which form plexuses around the aorta and the pulmonary, subclavian and carotid arteries together with their branches. These innervate
the smooth muscles in the walls of the vessels.
Some of the fibers accompany the thyreoid
arteries into the thyreoid gland and others are
distributed to the pharynx and larynx.
The upper thoracic ganglia supply nerves
to the aortic plexus and pulmonary plexus, and
the latter enters the lungs. Large bundles of
fibers proceeding from the "fifth or sixth to the
ninth or tenth " thoracic ganglia of the sympathetic trunk, unite to form the greater splanchnic
nerves, one on either side of the body, and
branches from the remaining thoracic ganglia
.__._. _ , form the lesser splanchnic nerves. These
coe. g., cceliac ganglion; my.pl. -i ,. ., ..i i ? i
myentenc plexus; sbm. pi., sub- splanciimc nerves pass into the abdominal cavity
mucous plexus. . . . .
and join one another, forming a large ganglionated plexus on the sides and front of the aorta (Fig. 128). The sympathetic trunks in the abdomen also send branches to join this plexus. The
great plexiform ganglion found around the cceliac artery, as it leaves
the aorta, is called the cceliac ganglion (or plexus). A similar plexus
surrounds the superior mesenteric artery. From these plexuses, as shown
in ^the diagram (Fig. 129), sympathetic nerves extend through the mesentery, and they form a microscopic ganglionated plexus surrounding the
intestinal tube, lodged between the longitudinal and circular layers of
smooth muscle. This is the myenteric plexus (plexus myentericus). It
innervates the muscle and sends branches into the tissue beneath the mu
PIG I29
NERVOUS TISSUE 139
cous membrane, where they form another plexus (the plexus submucosus). In this way the sympathetic system supplies the intestine.
It sends its fibers into other organs, following the arteries, thus forming
the hepatic, splenic, suprarenal and renal plexuses. In the pelvis the
sympathetic rami form the hypogastric plexus, with branches distributed
to the rectum, bladder and urogenital organs, and finally it accompanies
the arteries down the legs, innervating the muscles in the walls of the
vessels.
In 1664, Willis published a remarkably clear account of the nerve "commonly
called intercostal because it rests against the roots of the ribs." This nerve, which is
the ganglionated trunk of the sympathetic system, had generally been supposed to
descend from the cerebral nerves. Willis described its connections with these nerves
and, through each intercostal space, with the spinal cord. He noted the cardiac
branches, and stated that the great mesenteric plexus, placed in the midst of the others,
like a sun, sent its nerve fibers like rays in all directions (hence it came to be called the
"solar plexus"). Willis found that this nerve sent branches to all the abdominal
organs below the stomach. He considered that its function was to place the heart
and viscera in connection with the brain so that they should act in harmony
(Anatome cerebri, Amstelodami, 1664). Because of their frequent communications
with other nerves, Winslow (1732) called the ganglionated trunks the Nervi sympathetic? maximi.
Bichat (Anatomic Gen6rale, 1802, translated by Hayward 1822) subdivided the
nervous system into two parts "essentially distinct from each other, the one having
the brain and its dependencies for its principal center, and the other having the ganglions." The latter is "almost everywhere distributed to the organs of digestion,
circulation, respiration, and secretion." "Each ganglion is a distinct center, independent of the others in its action, furnishing or receiving particular nerves as the
brain furnishes or receives its own. . . . The continuous thread that is observed
from the neck to the pelvis is nothing but a series of communications These
communications are often interrupted, without any inconvenience in the organs to
which the great sympathetic goes." That the sympathetic system acts independently
of the central nervous system, at least to a great extent, is its most prominent physiological characteristic.
Thus the sympathetic system merits to some extent the terms organic, visceral,
or vegetative system, which have been applied to it. Burdach (1819) stated that it
might be called the "automatic system," and the term "autonomic system" has more
recently been used, but Burdach preferred sympathetic system, which has been internationally adopted by anatomists.
DEVELOPMENT OF THE CEREBRAL NERVES.
The nerves which are connected with the brain, supplying the skin and
muscles of the head together with certain viscera, are built upon the
same plan as the spinal nerves, of which they may be regarded as a continuation. They consist of dorsal sensory roots, and ventral motor roots
which, however, do not unite to form single nerves. Certain cerebral
nerves are wholly sensory and others consist merely of a ventral root, and
140
HISTOLOGY
are therefore entirely motor. Still others have no ventral roots, but receive motor fibers through lateral roots. The fibers in the lateral roots are
like motor fibers of the ventral roots in that they arise from cells within
the central nervous system, but their processes emerge from the lateral
wall of the brain instead of the ventral wall. They come out immediately
below the entering sensory fibers of the dorsal roots.
Beginning at the anterior end of the brain and proceeding toward
the spinal cord, the cerebral nerves occur in the following order: olfactory,
optic, oculomotor, trochlear, trigeminal, abducent, facial, acoustic, glossopharyngeal, vagus, accessory and hypoglossal.
FIG. 130. THE CEREBRAL NERVES OF A IZ-MM. PIG.
Olfactory (not shown). Optic (fibers in the stalk of the eye, the lens of which is marked L). Oculomotor
(Oc.). Trochlear (Tr.). Trigeminal, semilunar ganglion (s.-l.); ophthalmic (oph.), maxillary
(va..) and mandibular (md.) branches. Abducent (Ab.). Facial, geniculate ganglion (g.); large
superficial petrosal (1. s. p.). chorda tympani (ch. ty.), and facial (fa.) branches. Acoustic (A.), supplying the otocyst (Ot.). Glossopharyngeal, superior (s.) and petrosal (p.) ganglia; tympanic (t.), lingual
(1. r.) and pharyngeal (ph. r.) branches. Vagus, jugular (j.) and nodose (n.) ganglia; auricular (au.) and laryngeal branches, rec. being the recurrent nerve; the main stem proceeds to the abdomen. Accessory, internal ramus joining the vagus, and external ramus (ex.). Hypoglossal (Hy.).
Froriep's rudimentary hypoglossal ganglion (F.) sometimes sends fibers to the hypoglossal nerve, c.i,
c.2, c.3, cervical nerves.
It is desirable to use the names of these nerves rather than the numbers
often applied to them. The names are descriptive, but the numbers are
arbitrary and were very variously employed in the older anatomical works.
Unlike the spinal nerves, the cerebral nerves are not a series of similar
structures. Moreover the recent demonstration of the Nervus terminalis
in mammals indicates that the numbering may need further revision.
In embryos measuring about 10 mm., the cerebral nerves are all present
and show their primary branches. Except the olfactory nerve, they are
NERVOUS TISSUE 141
included in Fig. 130, in which parts derived from dorsal roots are unshaded;
those from lateral roots are black; and those from ventral roots are crosshatched. They may be briefly described as follows.
The olfactory nerve, on either side of the head, consists of about twenty separate
bundles of processes from the neuro-epithelial cells in the nasal mucous membrane.
These bundles of neuro-epithelial fibers pass directly into the olfactory bulbs, which are
portions of the brain. The wmero-nasal nerve is a bundle much longer than the others,
which arises from a tubular epithelial pocket in the mucous membrane of the nasal septum. This pocket is a rudimentary organ of considerable interest, known as the vomeronasal (or Jacobson's) organ. Associated with the vomero-nasal nerve, but said to be
distinct from it, there is a small ganglionated nerve which sends its fibers into the
brain caudal to the olfactory lobe. Distally it is "distributed chiefly to the vomeronasal organ." This is the Nervus terminalis, discovered in fishes by Pinkus in 1894,
and recently found in human and pig embryos and in adult dogs and cats (Johnston,
Journ. Comp. Neur., 1913, vol. 23, pp. 97-120; and McCotter, ibid., pp. 145-152).
The optic nerve is a round cord of fibers extending from ganglion cells in the retina
to the brain. It is quite unlike any portion of a spinal nerve, and will be described
in connection with the eye.
The oculomotor nerve has only a ventral root, and consequently it is entirely motor.
It is distributed to four of the six muscles which move the eye-ball (namely, the inferior
oblique and the superior, medial and inferior rectus muscles) and to the muscle which
raises the upper eye-lid (M. levator palpebra superioris).
The trochlear nerve arises from cells in the ventral part of the medullary tube, but
its fibers, instead of passing directly outward, grow to the dorsal surface of the tube and
cross to the opposite side before they emerge. Although the trochlear nerve must
be regarded as a ventral root, its fibers leave the brain more dorsally than those of
any other nerve. They come out at the notch or isthmus between the mid-brain and
the hind-brain, and all of them pass to the superior oblique muscle of the eye-ball.
This muscle, which runs through a fibrous ring or pulley (trochlea) attached to the
frontal bone, turns the eye outward and downward.
The trigeminal nerve consists of dorsal and lateral roots. Its sensory cells form the
semilunar ganglion, which gives rise to three large nerves, the ophthalmic, maxillary
and mandibular (hence the name trigeminal). In general terms, the ophthalmic is
the sensory nerve of the forehead and largely of the scalp; the maxillary is the sensory
nerve of the front of the face and the upper teeth; and the mandibular distributes
sensory fibers to the front of the tongue, the lower teeth, and the skin over the lower
jaw. Unlike the ophthalmic and maxillary nerves, the mandibular is a mixed nerve,
receiving all the motor fibers of the trigeminal. These motor fibers are distributed
chiefly to the muscles of mastication, through the masticator nerve.
The abducent nerve is wholly a ventral root, and its fibers all pass to the lateral rectus muscle, which abducts the eye-ball (i.e., turns it outward).
The facial nerve is largely a lateral root, and is the motor nerve of the facial muscles.
It has, however, a dorsal root (the so-called Nervus intermedius) and a ganglion
known as the ganglion geniculi, or geniculate ganglion, since it occurs at a bend in
the nerve. The facial nerve has three fundamental branches, all of which contain
both sensory and motor fibers; these are the large superficial petrosal nerve, the chorda
tympani, and the facial nerve (the name of the entire nerve being applied to one of
its parts).
The acoustic nerve, which is wholly associated with the internal ear, is entirely sen
142 HISTOLOGY
sory. Its large ganglion becomes subdivided into the vestibular ganglion, with fibers
to the semicircular ducts or "organ of equilibration," and the spiral ganglion, which
sends fibers to the auditory cells of the cochlea.
The glosso-pharyngeal nerve is chiefly sensory, but it has a small lateral motor
root. It has two ganglia, one above the other, the superior ganglion (ganglion superius)
and the petrosal ganglion (ganglion petrosum), respectively. The principal branches
are the sensory tympanic nerve, which supplies the mucous membrane of the middle
ear; the sensory lingual branch, which passes to the back of the tongue and ends in
contact with cells of the taste buds, being the nerve of taste; and the mixed pharyngeal
branch which is distributed to the pharynx. It supplies the stylo-pharyngeal muscle.
The vagus nerve, which is sensory, is joined by the accessory nerve, which is motor,
so that the vagus is regarded as a mixed nerve. It has two ganglia, the jugular ganglion
(ganglion jugulare} above, and the nodose ganglion (ganglion nodosum) below.
Its principal branches are the sensory auricular branch, which is distributed to the
skin of the external ear; the mixed superior laryngeal nerve, distributed to certain
laryngeal muscles and to the mucous membrane of the larynx down to the vocal
folds; the recurrent nerve, which terminates as the superior laryngeal in the vocal
muscles and mucous membrane of the lower part of the larynx; cardiac branches,
which anastomose with the cardiac sympathetic plexus; and finally, from the main
trunk of the nerve as it passes through the thorax into the abdomen, branches to
the oesophagus, trachea, lungs, stomach, small intestine, liver, spleen and kidneys.
Many of these branches anastomose with the sympathetic system. The wide
range of this nerve is indicated by the term vagus.
The accessory nerve is wholly motor, and consists of lateral roots which arise from
the hind-brain, and also from the spinal cord as far down as the sixth cervical ganglion.
Beginning as a small bundle of fibers underneath the dorsal roots on the side of the
spinal cord, it increases in size as it passes upward toward the brain, receiving accessions of fibers in its course. It arches toward the vagus and descends in contact
with it, finally dividing into external and internal branches. The external ramus
supplies the sterno-mastoid muscle and a part of the trapezius; the internal ramus
joins the vagus.
The hypoglossal nerve is made up entirely of ventral roots, and is the motor nerve
for the lingual muscles.
In the head the sympathetic system is intimately associated with the
cerebral nerves, along the main branches of which the ganglion cells migrate. They accumulate in four ganglia, all of which are associated with
the trigeminal nerve. These are the ciliary, spheno-palatine, otic and
sub maxillary ganglia (Fig. 128).
The ciliary ganglion receives its cells from the ophthalmic nerve and in part from
the oculomotor nerve, with both of which it remains permanently connected. The
sympathetic plexus which ascends around the internal carotid artery also sends fibers
to it. Branches from the ciliary ganglion are distributed to the front of the eye,
especially to the ciliary muscles and the dilator of the iris.
The spheno-palatine ganglion derives most of its cells from the maxillary nerve, but
it is in communication also with the large superficial petrosal nerve and the sympathetic plexus around the internal carotid artery. Some of its fibers reach the orbit, but
most of them are distributed to the mucous membrane of the nose and palate.
The otic and submaxillary ganglia both receive cells from the mandibular nerve,
NERVOUS TISSUE 143
and both are in connection with the sympathetic plexus around neighboring arteries.
The otic ganglion receives fibers from a prolongation of the tympanic nerve, and it
sends branches to the parotid gland. The submaxillary ganglion is joined by the
chorda tympani and sends branches to the submaxillary and sublingual glands.
The lower ganglia of the glossopharyngeal and vagus nerves the
petrosal and nodose ganglia differ from the other ganglia in the head by
being temporarily connected with rudimentary ectodermal sense organs.
Their contact with the ectoderm is transient, however, and their cells
are considered to have come down from the superior and jugular ganglia,
respectively. They are thus strikingly analogous to the ganglia of the
sympathetic trunk, and it may be considered that instead of being connected with their nerves by rami, they have remained in the main stems.
Moreover the vagus nerves produce myenteric and submucous plexuses
in the oesophagus and stomach, which are quite like those of the sympathetic system in the intestine, but the fibers pass from the nodose ganglion
to these plexuses without the interposition of a ganglion comparable with
the cceliac ganglion. In addition to sympathetic fibers, the vagus contains many direct fibers, which probably come especially from the jugular
ganglion. At present, however, both the upper and lower ganglia are
described as similar in structure and as resembling the spinal ganglia.
The opinion here advanced, that the nodose and petrosal ganglia are
sympathetic, must therefore be regarded as tentative.
STRUCTURE OF NERVOUS TISSUE.
Owing to the extent of the ramifying processes characteristic of nerve
cells, it is rare that an entire cell, even a small one, is included within a
single section. A motor cell, such as sends its fibers from the cord to
distant muscles, has never been seen as a complete, isolated structure.
From what is known of its several parts, however, a diagram of such a
cell may be put together, as shown in Fig. 131. At the top of the figure
is the nucleated cell body, which in different nerve cells varies in diameter
from 4-150 /*. Frequently this nucleated portion is referred to as the nerve
cell in distinction from the processes which grow out from it. The processes include the relatively short and irregularly ramifying dendrites,
which convey impulses toward the cell body, and a single fiber, the
neuraxon, chemically and physically different from the others, which
conveys impulses away from the cell body. If the various processes radiate
from the cell body in several directions, as in Fig. 131, the cell is described
as multipolar; if the neuraxon is at one end of the cell and a single dendrite
at the other, the cell is bipolar (Fig. 126); sometimes the nerve cell has
only one process and is unipolar, as in the mature cells of the spinal ganglion which have a T-shaped process, and in other cells in which dendrites
144
HISTOLOGY
Dendrites.
Cell body.
Collaterals.
Medullary sheath.
Neurolemma.
have not developed. The dendrites have the granular structure of the
protoplasm from which they grow out, and were therefore originally
named "protoplasmic processes." The neuraxon, although receiving delicate fibrils from the
protoplasm, as shown by special methods, seems
quite distinct from the cell body. At its origin
it often appears as a clear slender cone, free from
granules, implanted directly upon the cell body,
or upon the root of one of the larger dendrites.
It tapers as it passes outward, and its fibrils
come close together so that they appear to unite.
Beyond the apex of the cone, which is a place
where the neuraxon is easily broken, the fiber
enlarges and its constituent neurofibrils spread
apart so that they are more readily distinguishable. They are imbedded in a fluid interfibrillar
substance. The neuraxon may send out collateral branches, which are usually at right angles
with the main fiber.
As the neuraxon passes out from a motor cell
it is at first free from any surrounding sheath
(Fig. 131, a). In the outer layer of the spinal
cord it becomes coated with a layer of the refractive fatty substance known as myelin. This
is formed in the cord or medulla spinalis, and
fibers which have this sheath are said to be
medullated fibers (Fig. 131, b). The cells of the
neuroglia network, through which the nerve
passes while within the cord, may take part in
forming the myelin, but they do not produce a
membrane around each nerve, and they are not
shown in the diagram. On leaving the cord, the
neuraxon is still surrounded by the myelin sheath,
but the latter is invested by a membrane called
the neurolemma or sheath of Schwann (Fig. 131,
c). At quite regular intervals along the course
of the fiber, the myelin sheath is constricted or
interrupted, forming the nodes of Ranmer.
These are 0.08-1.00 mm. apart, being closer
together in growing fibers, and in the distal part
of adult fibers Midway between two nodes
there is a nucleus, which may be found at any point in the circumference of the fiber, just within the neurolemma; it occupies a depres
Node of Ranvier.
FIG. 131. DIAGRAM OF A NERVE
CELL.
NERVOUS TISSUE 145
sion in the myelin. Toward its distal end the fiber usually branches,
and the branches are given off at the nodes. The myelin then becomes
thin, so that the fiber is surrounded merely by neurolemma (Fig. 131, d),
and finally this ends. The naked axis cylinder then breaks up in its
terminal arborization, forming the motor organs attached to striated
muscle fibers. In comparison with the size of its cell body, the neuraxon
shown in the diagram is too short; in extreme cases, as in the neuraxons
extending from the spinal cord to muscles in the foot, it may be actually
more than a meter long, or several thousand times the diameter of the cell
body from which it comes.
The medullated nerve fibers were the first parts of the nerve to be studied microscopically, and were referred to as "cylinders;" the central fiber was called the axis
cylinder. Remak (Obs. anat. et micr. de syst. nerv. structura, Berlin, 1838) was the
first to describe non-medullated nerves, which are still known as "Remak's fibers,"
but their nervous nature was not readily admitted. Moreover. Remak recognized
that nerve fibers proceed from cells. Deiters (Untersuchungen iiber Gehirn und
Riickenmark, Braunschweig, 1865) supplemented these observations by showing
that all "ganglion cells" (referring to nerve cells within the spinal cord and brain)
are centers for two systems of true nerve fibers, (i) the generally broader and always
single and undivided axis cylinder process; and (2) the protoplasmic processes with
their extensive system of minute branches. He discussed whether the nerve cells
anastomose with one another, and concluded that all such anastomoses which had
been reported were due to deceptive appearances. Thus the nerve cells were
believed to communicate by contact and not by continuity.
The confused mass of interwoven fibers which sections of nervous tissue ordinarily
present, is, therefore, not a general syncytium from which sensory and motor fibers run
out, but an orderly arrangement of branching cells. Striking proof of this was afforded
in Golgi's description of the olfactory bulb (1875). In the plate which accompanied
his publication, the cells in the different layers, and their various processes, were
drawn in black with absolute assurance; similar figures of "Golgi preparations"
are now seen in all treatises on the anatomy of the nervous system (Fig. 132). Golgi
found that if fresh tissue is placed in a solution of potassium bichromate and osmic
acid, and is later transferred to a solution of silver nitrate, a heavy black deposit
occurs in certain nerve cells, extending throughout their minutest ramifications,
whereas adjacent cells are wholly unaffected. The process must be carried out
with great care, and even then it is capricious; but this method has afforded fundamental information in regard to the forms of individual nerve cells.
In order to emphasize that the nervous system is built up of separate
cells, the term neurone has been widely used to designate a complete nerve
cell, with all its branches. Fig. 131, therefore, represents a neurone, together with certain sheath cells.
Recently, however, there has been a tendency to regard such a neurone as a
syncytium, and in the latest editions of his "Lehrbuch," Stohr adopts this interpretation. He states that in so far as the neurone includes peripheral nerve fibers, it is
a biological or syncytial unit, but not a single cell. It is considered to be a "biological
unit" since it is well known that the cell body of the nerve cell is the nutritive or
10
146
HISTOLOGY
controlling center for the entire fiber; and any part of the fiber which is cut off from
the cell body undergoes degeneration. Stohr considers that Schwann (1839) had
the correct conception when he regarded the nerve fiber as "a secondary cell, developed by the coalescence of primary cells."
Opposed to the syncytial interpretation of a peripheral fiber are the experiments
of Harrison, some of which have already been cited. He has shown that in the
tadpole the sheath cells, or neurolemma cells, which are believed by some to produce
the segments of the fiber which they surround, all migrate from the brain along the
dorsal root. If the dorsal part of the cord is removed from tadpoles, the ventral
roots are deprived of their sheath cells, but the fibers of the ventral roots grow out to
their terminations nevertheless. If the ventral part of the cord is cut from beneath
the dorsal part, the dorsal roots develop and have with them the sheath cells which
Neuraxon with
branching collaterals.
FIG. 132. Two NERVE CELLS FROM THE CENTRAL NERVOUS SYSTEM. GOLGI PREPARATIONS. X 200.
A, Cell of Deiter's type, having a neuraxon ending at a considerable distance from the cell body; B, cell of
Golgi's type having a neuraxon with many branches ending near the cell body.
normally would enclose the fibers of the ventral root. These sheath cells do not
produce nerve fibers. Therefore Harrison concludes that the peripheral fibers are
not syncytial.
Recently W. H. and M. R. Lewis have caused sympathetic fibers to grow from
pieces of the intestine of chick embryos placed in various saline solutions. These
fibers show amoeboid endings. They branch freely and anastomose, but like
the nerve fibers from the central nervous system "they are outgrowths from nerve
cells and are not formed from pre-existing protoplasmic networks" (Anat. Rec., 1912,
vol. 6, pp. 7-31)
Another form of syncytium would result if neurofibrils passed across the places
of contact between the neurones. According to Apathy, who has studied the neurofibrils of invertebrates with special methods and faultless technique, the neurofibrils
pass freely from cell to cell (Mitth. Zool. Station, Naples, 1897, vol. 12, pp. 495-748).
It is possible that this takes place in the vertebrate nervous system also. Anastomoses
NERVOUS TISSUE
147
have been found between ganglion cells in the retina by Dogiel, and slender nerve
fibers appear to anastomose in tissue cultures; but the staining of individual cells
by the Golgi method, and the way in which degeneration may be limited to cell
territories, are regarded as strong evidence against the existence of a general syncytium.
STRUCTURE OF GANGLIA.
Although a ganglion is characterized by the accumulation of the bodies
of nerve cells, it is traversed by many fibers, as seen in the section of a
spinal ganglion (Fig. 133). Under higher magnification the cell bodies
appear as in Fig. 134. The nuclei are large vesicular structures, round
or oval in outline, containing a characteristic prominent nucleolus. They
are surrounded by abundant, darkly staining, finely granular proto
Blood vessel.
Fat.
Ganglion cells.
Nerve fibers.
Perineurium.
root
of a spinal nerve.
Center
of the
spinal
ganglion.
FIG. 133. LONGITUDINAL SECTION THROUGH A SPINAL GANGLION OF A CAT. X 18.
plasm, which exhibits its fibrillar structure only with special methods.
Frequently the protoplasm contains pigment granules. The "reticular
apparatus" is said to be present always, and slender intracellular canals
(trophospongium) have been described (Figs. 5 and 6, p. 4). Finemeshed reticular networks have been found covering the exterior of the
nerve cells, and they have been ascribed both to the terminal ramification
of nerve fibers and to branches of the supporting tissue. A ganglion cell
is often surrounded by flat or stellate cells arranged in concentric layers
so as to form a sheath. Within the sheath there is a homogeneous membrane or capsule, on the inner side of which are cells arranged in a single
layer, corresponding to the cells within the neurolemma of peripheral
148
HISTOLOGY
nerves. Connective tissue, containing small blood vessels, passes between the ensheathed cells of the ganglion.
In the embryo the cells of the spinal ganglia are bipolar, but generally
they become unipolar, with T-shaped processes, as already described.
In the ganglia of the acoustic nerve, however, the bipolar form is said to
be retained, and these cells are not surrounded by capsule or "mantle"
cells. In other ganglia of the cerebral nerves, and in spinal ganglia, the
cells are arranged as shown in the diagram, Fig. 135. Their branches
can be studied only in special preparations, made usually by
Ehrlich's methylene blue method, or CajaPs silver nitrate method.
Cross section of a medullated nerve fiber.
w^wiij^
' '<s*&*>7 h& '^ JjjP
S$! ,. Nucleus of the
capsule
u Protoplasm.
jp Nucleus.
Nucleolus.
Nerve
cell.
Pigment.
--"-''
Longitudinal view
of medullated
nerve fibers. Surface view of
nucleated sheath.
FIG. 134. FROM A CROSS SECTION OF A HUMAN SEMILUNAR GANGLION. X 240.
At x the beginning ofl a protoplasmic process has been included in the section; elsewhere the processes
cannot be seen.
The most characteristic cells (Fig. 135, 3) have large round bodies
and a single spirally coiled process, which arises from a conical
projection of the protoplasm. The process often winds about the
cell body. Soon after passing through the capsule it acquires a sheath
of myelin, and is covered with neurolemma. It may give off collaterals before it divides into its two main branches, which correspond with dendrite
and neuraxon respectively. Sometimes the process divides into three
branches (Fig. 135, 2); the branching takes place at a node of Ranvier.
Certain of the large cells, as found constantly in the human jugular ganglion, lack the coiled windings, so that the process passes directly through
the capsule and divides at once into its two branches.
Frequently the ganglion cells are provided with short processes which
end in rounded enlargements, either within the capsule (Fig. 135, 5) or
outside of it (Fig. 135, 6). Collateral branches may end in this way.
NERVOUS TISSUE
149
These "end discs" were first observed by Huber in frogs (Anat. Anz.
1896, vol. 12, pp. 417-425). They are found not only in spinal ganglia
but also in the central nervous system and in sympathetic ganglia; and
Dorsa root.
Motor cell of
the spinal cord.
Ventral
root.
Spinal
ganglion.
End disc
Capsule.
FIG. 135. DIAGRAM OF THE NERVOUS ELEMENTS IN A HUMAN SPINAL GANGLION.
after the distal part of a nerve has been cut away, the axis cylinders of the
proximal part send out many such buds, which grow into the myelin
toward the place of injury. In all cases they are regarded as abortive
HISTOLOGY
branches. They are said to occur normally only in adults, and especially
in old age, being very numerous in the nodose ganglion of the vagus
nerve.
Another feature which, in man, has been found almost exclusively
in the nodose ganglion of adults, is the occurrence of "fenestrated cells."
These are ganglion cells with peripheral vacuoles, which may break down
so that the cell appears multipolar (Fig. 135, 7). Sometimes they are so
arranged that the cell process seems to grow out by several roots (Fig.
135,8). Although the fenestrated cells increase in number with advancing
age, they are not considered pathological, since they occur in young dogs
and other animals.
Less conspicuous than the large cells with medullated fibers, but more
numerous, are small pyriform cells with non-medullated fibers (Fig. 135,
4). Ranson, from his own and previous observations, concludes that in
the cat and rat, in which the cells have been carefully counted, about
Nerve cell.
Nerve cell.
Sheath.
Sheath.
FIG. 136. CELLS OF THB HUMAN SYMPATHETIC GANGLIA. (Prepared by L. R. Muller.)
A, From the ciliary ganglion; B, from the superior cervical ganglion.
X 465.
two-thirds of the spinal ganglion cells may be classified as small, and are
associated with non-medullated fibers (Amer. Journ. of Anat., 1911, vol.
12, pp. 67-87).
The spinal ganglion cells are sometimes surrounded by fine networks
of non-medullated fibers, which are probably the terminal branches of
medullated fibers derived from cells in the sympathetic ganglia (Fig. 135,
i). Branches of the sympathetic fibers are also distributed to the blood
vessels in the ganglion. Whether any fibers pass through the spinal
ganglion without connecting with its nerve cells is still uncertain; they have
not been demonstrated in mammals.
Sympathetic Ganglia. The sympathetic ganglia consist of multipolar
cells which are smaller than those of spinal ganglia (Fig. 136). Their
round or oval nuclei, often eccentric, have prominent nucleoli and a loose
chromatin network, as in other nerve cells; some of them contain two
NERVOUS TISSUE
nuclei. The protoplasm is often pigmented. Around the cell bodies,
nuclei of the sheath cells may be abundant. Three types of sympathetic
ganglion cells are shown in Fig. 137. The motor cells, terminating in
contact with smooth muscle fibers, are by far the most abundant (Fig.
137, i). Their neuraxons are non-medullated fibers, which are provided
with very slender collaterals. The cell body is stellate and its branching
dendrites appear spiny. The second type (Fig. 137, 2) is possibly sensory,
Motor fiber from a spinal nerve.
V
Pericellular plexus
Capsule
Neuraxons
Sympathetic
nerve.
Sensory fiber from a
spinal nerve.
Sympathetic (?)
nerve fiber.
Section of the
pericapsular
plexus.
Surface view
of. pericapsular
plexus.
Stellate cell.
Xeuraxon.
muscle fibers
FIG. 137. DIAGRAM OF THE ELEMENTS OF Two SYMPATHETIC GANGLIA.
but the terminations of its fibers are not known. Its dendrites are long
and slender and may extend from one ganglion to another. Some of them
are accompanied by the neuraxon, which may acquire a medullary sheath,
often at a considerable distance from the cell body. Cells of the third
type (Fig. 137, 3) resemble those of the second type. They have long
branching dendrites which pass between the adjacent cells to the periphery
of the ganglion, where they form a plexus. Their non-medullated neurax
152 HISTOLOGY
ones pass out of the ganglion, but their terminations are unknown. Small
stellate cells, one of which is shown in the figure, presumably belong with
the supporting tissue.
Fibers from the spinal nerves may pass through the sympathetic
ganglia, or terminate within them. Thus spinal motor fibers, after
losing their myelin sheaths, form pericellular plexuses about the sympathetic motor cells, and their collaterals end in the same way. They
are apparently indistinguishable from the sympathetic fibers which pass
from one ganglion to another and terminate in pericellular networks.
Medullated sensory fibers, some of which arise from lamellar corpuscles,
extend through the sympathetic nerves to enter the spinal ganglia.
Chromaffin organs, or paraganglia, are masses or cords of cells which
originate in close association with sympathetic ganglia. Although they
have often been classed with nervous tissue, they are to be regarded as
glands which produce an internal secretion. This secretion acts upon the
smooth musculature in the walls of the blood vessels and causes it to
maintain a proper state of contraction, or tonus.
When fresh, chromaffin tissue is darkly colored. If preserved in fluids
containing chromic acid or salts of chromium, the cells which contain
secretion acquire a yellowish-brown stain. The term chromaffin refers
to this specific affinity for chromium, and does not mean that the cells
stain deeply.
Groups of chromaffin cells are found in connection with the ganglionated
trunk of the sympathetic system. In the new-born child these "chromaffin bodies" may reach a length of 1-1.5 mm - (Zuckerkandl) and several
of them may be associated with a single ganglion. They are always found
in the plexus at the bifurcation of the carotid artery, where they enter
into the formation of the carotid gland (glomus caroticum). They occur
in vary ing number sin the cceliac, renal and hypogastric plexuses, and extend
along the vessels so that chromafnn cells are found in relation with the
kidneys, ureters, prostate, epididymis and ovary. The largest bodies
(the organs of Zuckerkandl) are found on either side of the inferior mesenteric artery, and may connect with one another by a bridge across the
front of the aorta. At birth "the average length of the right one is n.6
mm., and of the left, 8.8 mm." Usually there are two chromaffin bodies
on either side in the hypogastric plexus, but the total number of bodies
connected with the abdominal plexuses varies greatly, "from 7 to 26, or
even more; in one case nearly 70" (Zuckerkandl). Although they undergo
regressive changes after birth, they do not dissappear.
The medulla of the suprarenal glands consists of chromafim tissue,
which has very important functions throughout life; it will be described
in connection with the suprarenal glands.
NERVOUS TISSUE
153
STRUCTURE OF NERVES.
Nerves are bundles of nerve fibers passing between the central nervous
system and the various parts of the body; they are so widely distributed
that they may be found in sections of most of the organs and tissues.
When examined fresh, in reflected light, nerves are seen to be of two sorts,
formerly known as white and gray nerves, respectively. Similarly, sections of the brain and spinal cord are formed of white substance and gray
substance. The obvious distinction in color is due to the presence or
absence of microscopic sheaths of myelin around the individual fibers.
Nerves which contain a large proportion of myelinated or medullated
fibers are white; and those which have few are gray. All nerve fibers
when first formed are non-medullated, and most of the sympathetic nerves
remain in this condition.
Non-medullated nerves can readily be found between the circular and
longitudinal layers of smooth muscle in any part of the digestive tube.
They are circumscribed bundles of fine fibers running through the coarser
connective tissue (Fig. 138). Many of them contain nerve cells, unmis
FIG. 138. A SYMPATHETIC NERVE FROM THE MYENTERIC PLEXUS OF A CAT. X 77S
a., Nucleus of a supporting cell; b., nerve cell; c., non-medullated nerve fibers. Above the nerve are circular smooth muscle fibers in longitudinal section; below it are longitudinal fibers in cross section.
takably characterized by large, round or oval, vesicular nuclei, having a
prominent nucleolus. Around the nucleus is dense protoplasm, starting
out in branching processes, all but the roots of which are cut away in
sectioning. Other cells are found, having relatively small nuclei and very
indefinite or wholly imperceptible protoplasmic bodies. These are supporting cells; they produce a syncytial framework in which the nerve cells
and their very delicate ramifications are imbedded. The framework
tends to form septa, subdividing the nerve into smaller bundles.
Some non-medullated fibers, but by no means all, are closely invested
by sheath cells. According to Schafer, the nuclei of these cells appear to
be interpolated in the substance of the fiber, and it is impossible to demon
154
HISTOLOGY
strata a distinct sheath (Fig. 139). Similarly Bardeen has stated that it
is "mainly a matter of judgment to decide whether the fibrils are surrounded by or imbedded within the sheath cells." They correspond with
the neurolemma cells of medullated nerves.
Medullated Nerves. The larger sympathetic nerves contain a considerable number of medullated fibers, and the splanchnic nerves are described
as white. In the trunks of the spinal nerves, however, the medullated
fibers attain their maximum development. Examined with low magnifi
FIG. 139. NON-MEDULLATED NERVE FIBERS. X 4o. (After Schafer.)
cation, such a nerve is seen to consist of round cords imbedded in loose
connective tissue (Fig. 140). This loose tissue, which surrounds the
entire nerve and its several cords, is the epineurium; its connective tissue
bundles are chiefly longitudinal, and are associated with abundant elastic
tissue and frequent fat cells; it contains the blood vessels which supply
the nerve. Each cord is surrounded by a dense lamellar layer of connective tissue, which contains flattened cells in contact with one another so
that they form more or less continuous membranes. This layer is the
Fat cells.
Artery.
Bundles of nerve fibers
Epineurium.
Perineurium.
Endoneurium.
FIG. 140. MKDULLATBD NERVE. PART OF A CROSS SECTION OF THE HUMAN MEDIAN NERVE. X ao.
perineurium. It is continuous with the outer membranes covering the
cord, and contains cleft-like spaces which are said to communicate with
the subdural and subarachnoid spaces, but which do not connect with
lymphatic vessels in the epineurium. Prolongations of the perineurium
extend as septa into the larger nerve bundles and constitute the endoneurium, which may penetrate between the individual nerve fibers, forming
the so-called "sheaths of Henle." Their nuclei are always outside the
neurolemma.
NERVOUS TISSUE
155
The individual nerve fibers vary in diameter, and the larger ones are
probably those which have a longer course. It is impossible to distinguish histologically between sensory and motor fibers. The sheath of
Endoneurium.
Fiber sheath.
Medullary
sheath.
FIG. 141. MEDULLATED NERVE. PART OF A CROSS SECTION OF THE HUMAN MEDIAN NERVE. X 220
myelin which surrounds the fiber varies greatly in thickness, as seen
in the cross section, Fig. 141. In ordinary preparations it forms light
zones around the dark fibers, suggesting the relation between protoplasm
and nucleus; but the rod-like nature of the central fibers is evident on
changing the focus. The myelin is surrounded by the membranous neurolemma,
within which the single internodal nucleus
is occasionally included in a given section.
Portions of isolated fibers, viewed longitudinally, are shown in Fig. 142.
Myelin is a mixture of complex fats and lipoid
substances, some of which are combined with
sugar. Like fat, it is dissolved by ether and
blackens with osmic acid. In preserved specimens the emulsion breaks down, giving rise to
various forms of shrinkage. A network which
appears after fibers have been treated with
alcohol and ether is said to be composed of
neurokeratin, a substance insoluble in these
reagents, which does not blacken with osmic
acid. The size of the meshes varies (Fig. 143,
A, B). In preparations blackened with osmic
acid, the myelin is often traversed by oblique
clefts, the incisures of Lantermann (Fig. 143, D).
The arrangement of these characteristic clefts
may be pictured by imagining a succession of stemless funnels strung along the
axis cylinder, not all of which are pointed the same way. The incisures are doubtless artificial, and their number is increased by pulling the nerve fibers apart; they
appear to be empty or crossed by strands of myelin, but in the preparation shown in
A Nu.
FIG. 142. Two NERVB FIBERS WITH
SHRUNKEN Axis CYLINDERS FROM
THB SCIATIC NERVE OF A RABBIT.
X 350.
A, Axis cylinder; M, medullary sheath
(myelin); N, neurolemma; Nu, nucleus of the neurolemma.
156
HISTOLOGY
Fig. 143, C, the neurokeratin framework is so arranged as to correspond with these
intervals. In transverse sections, incisures are included in Fig. 143, E and I; the
concentric, vacuolated and radial appearances of the myelin are represented in F-H.
The nodes of Ranvier, shown in the diagram, Fig. 131, are conspicuous in isolated
nerve fibers stained with osmic acid. Various interpretations of their structure are
represented in Fig. 144. According to the first (Fig. 144, A) the myelin occurs
like fat, within distinct cells wrapped around the nerve fibers; the node is the interval
between successive cells. The nucleus, which is flattened by the myelin against
the outer cell wall, mid- way between the nodes, is not shown. Corresponding with
the neurolemma on the outside, there is an "axolemma" next the axis cylinder;
neurolemma and axolemma come together at the node. If the nerve fibers are treated
with silver nitrate, a black precipitate is produced at the nodes, as if an intercellular
substance were present; the blackening may extend up the axis cylinder producing
cross-shaped figures (Fig. 144, B).
my
FIG. 143. MEDULLATED NERVE FIBERS.
A-D, Longitudinal sections; E-I, cross sections.
(A-B, after Gedoelst; C, E, F, after Hardesty; D
and I, osmic acid preparations, after Prenant
and Scymonowicz; G, alcoholic preservation,
after Koelliker H, picnc acid preservation, after
Schafer.) a. c., Axis cylinder; in., incisure; my.,
myelin; nu., nucleus of the neurolemma.
no D
FIG. 144. NODES.
A, Diagram of the intracellular explanation
of myelin; B, the cross obtained with
silver nitrate; C, the biconical enlargement (after Gedoelst); D, intercellular
myelin (after Hardesty) ; a. c., axis cylinder; ax., axolemma; my., myelin; ne.,
neurolemma; no., node.
As the axis cylinder traverses the node, its fibrils may spread apart, forming a
"biconical enlargement." The fibrils in the midst of the enlargement have been
described as thickened (Fig. 144, C). The same figure shows no axolemma and
suggests that the neurolemma passes across the node without interruption. This
is clearly shown in D, where the myelin layer also, though constricted, is not completely divided. The myelin has accordingly been regarded as an exoplasmic part
of the axis cylinder, and chemically it is said to be related to the interfibrillar substance
or neuroplasm. Bardeen (Amer. Journ. Anat., 1903, vol. 2, pp. 231-257) considers
that the myelin is derived from the intercellular substance between the fiber and the
sheath, and is "due to influences exerted by the axis cylinder fibrils." That the
axis cylinder plays the chief part in its production is indicated by the fact that the
myelin breaks down when the fiber degenerates, and that it forms around fibers in the
central nervous system where there are no continuous sheaths.
The production of myelin is said to begin at about the fourth month, at the central
ends of the nerves. It begins at different times in different tracts and systems, and
NERVOUS TISSUE
157
the medullary sheaths of the spinal nerves are not all formed until two or three years
after birth. They continue to increase in thickness into adult life.
NERVE ENDINGS.
SENSORY ENDINGS. The outward growth of nerve fibers from cells
in the ganglia of the spinal and cerebral nerves has already been described.
Near their terminations these fibers branch repeatedly at the nodes, lose
their myelin sheaths, and form terminal arborizations in contact with epithelial, connective tissue, or
muscle cells. These are the sensory endings, and
apart from those connected with the eye, ear, and
other organs of special sense, they may be described
as follows.
Free Endings. Sensory fibers to the epidermis
and to the corneal and oral epithelia penetrate the
basal layer, passing between the cells as unsheathed
fibers, and ramify among the cells in the outer layers
(Fig. 145). The extremities of the fibers, which may be pointed or
club-shaped, are in contact with the epithelial cells, but do not enter
them. In the process of branching the neurofibrils become distributed
FIG. 145. FREE NERVE
ENDING, IN EPITHELIUM. GOLGI PREPA R A T i o N . (After
Retzius.)
Intraepithelial
fiber.
Papilla
Capillaries.
Tactile cells.
meniscus.
Corium.
FIG. 146.
Tactile cells Nerve fiber,
in the corium.
FIG. 147.
FIGS. 146 AND 147. FROM VERTICAL SECTIONS THROUGH THE SKIN OF THE GREAT TOE FROM A MAN OF
TWENTY-FIVE YEARS. X 360.
in smaller and smaller bundles, which often anastomose, forming plexuses;
but whether the interlacing constituent fibrils unite with one another so
as to form a net has been questioned. At the ends of the branches, each
158
HISTOLOGY
fibril has become separate from the others; frequently it shows varicose
enlargements.
Free sensory endings occur not only in stratified epithelia, but also in
Medullated Muscle
nerves. fibers.
Medullated nerve fiber.
Terminal ramification. Tendon bundle.
FIG. 149. TENDON SPINDLE OF AN ADULT CAT. X 80.
t jL ^ ^
~ Medullated nerve fiber.
~f;
~^~~" Axis cylinder.
~ Nucleus of a tendon cell.
FIG. 150. THE LEFT PORTION OF FIG. 149. X 345.
muscle, tendon and connective tissue. In simple epithelia the free endings may be sensory, but in glandular
epithelia they are often efferent fibers, inciting the cells
to glandular activity. The ultimate branches of the
nerves are so delicate that they cannot be seen in
ordinary preparations; they have been demonstrated
chiefly by the methylene blue method, applied to very
fresh or living tissue.
In the epidermis, as a modification of the free endADULTCAT.XI35- j n g Sj fib ers are found terminating in disc-shaped networks (tactile menisci) at the base of modified cells (Fig. 147). These
tactile cells may occasionally be seen in ordinary preparations.
FIG. 148. MUSCLE
SPINDLE OF AN
NERVOUS TISSUE
159
The stellate "Langerhans cells" shown in Figs. 146 and 147 are usually
regarded as wandering cells lodged in intercellular spaces, but Stohr states
that intergrading forms connect them with the epithelial cells; and they
may act as sensory cells.
Muscle Spindles. As seen in ordinary preparations muscle spindles
are shown in Fig. 119 (p. 127). They are
slender groups of 3-20 muscle fibers, 1-4 mm.
long and 0.08-0.2 mm. wide, around which
nerve fibers terminate as shown in Fig. 148.
The spindles are surrounded by a thick connective tissue sheath or capsule, continuous with
the perimysium, and said to be divided into an
-jt inner and an outer layer by a space filled with
fluid. The muscle fibers of the spindle are
poorly developed. They are distinctly striated
toward their tapering and very slender ends,
but in their middle portions, sarcoplasm and
nuclei are abundant and the striations ill defined. Three or four nerves terminate in each
spindle. Their connective tissue sheaths blend
with the perimysial capsule, and they branch
Tactile cells, f"
Nerve fibrils.
Connective tissue sheath.
FIG. 151. TERMINAL CYLINDER.
(After Ruffini, from Ferguson's
Histology.)
gH, Medullary sheath; il, terminal
ramifications of the axis cylinder;
L, connective tissue.
FIG 152. TACTILE CORPUSCLE FROM A SECTION OF THE SKIN
OF A HUMAN FINGER. X 560.
(Prepared by van der Velde, after the Bielschowsky method.)
and lose their myelin as they pass through it to the muscle cells.
They may encircle the muscle fibers of the spindle, forming spirals or
rings (as in the upper part of Fig. 148), or they may form a panicle of
branches with enlarged club-shaped ends. Since they do not degenerate
after the motor roots have been cut, they are supposed to be sensory
fibers, but their function has not been established. Other sensory fibers
to muscle have free endings, as shown in Fig. 157.
i6o
HISTOLOGY
Tendon Spindles. Tendons possess free sensory endings, together
with the tendon spindles. These are small portions of the tendon, 1-3
mm. long and 0.17-0.25 mm. wide, enclosed in sheaths of connective
tissue. They stain more deeply than the surrounding tendon.
The few nerve fibers which terminate in a tendon spindle lose their
sheaths and branch freely, ending in club-shaped enlargements (Figs 149
and 150). They are found in all tendons and serve to transmit the sensation of tension, being active in connection with coordinated movements.
In connective tissue the sensory nerves may have free endings. In
addition to these the subcutaneous tissue near the coils of the sweat
glands, and in the corium of the fingers
and toes, sometimes contains terminal
cylinders (of Ruffini) which resemble
tendon spindles in the way that their
nerves ramify (Fig. 151). These cylinders lack the distinct capsules which
characterize the nerve corpuscles.
PIG. 153- GENITAL CORPUSCLE FROM THE
HUMAN GLANS PENIS. METHYLENE
BLUE STAIN. (After Dogiel, from Bohm
and von Davidoff.)
FIG. 154. BULBOUS CORPUSCLE FROM THE HUMAN CONJUNCTIVA. METHYLENE BLUE STAIN. (After Dogiel,
from B6hm and von Davidoff.)
Terminal corpuscles are nerve endings consisting of a coarse nerve
fiber, or knot of small branches, surrounded by a semifluid intercellular
substance (which is granular in preserved tissue), and enclosed in a connective tissue capsule. The terminal ramifications of the nerve show
irregular swellings or varicosities, and apparently they unite so as to make
a network. Often more than one fiber enters a corpuscle, and it has been
suggested that they include afferent and efferent fibers. Generally the
connective tissue sheaths of the entering fibers blend with the capsule,
and the myelin sheaths are lost just within it. Terminal corpuscles have
been grouped as tactile, genital, bulbous, articular, cylindrical, and lamellar.
Tactile corpuscles (or Meissner's corpuscles) are elliptical structures,
40-100 n long and 30-60 n broad (Fig. 152). They are characterized
by transverse markings, due to the corresponding elongation of the capsule
cells and the tactile cells within. From one to five medullated fibers
enter the lower end of a tactile corpuscle, losing their sheaths soon after
entering. They pursue a spiral course through the corpuscle, giving off
NERVOUS TISSUE
161
branches which end in enlarged terminal networks between and upon the
tactile cells. These corpuscles are found in some of the papillae, or connective tissue elevations just beneath the epidermis, being especially
numerous in those of the soles and palms (23 in i sq. mm.) and in the
finger tips; they occur also "in the nipple, border of the eyelids, lips,
glans penis and clitoris."
Genital corpuscles are large, round
or oval bodies 60-400 n long (Fig. 153)
which may receive as many as ten
nerve fibers. These ramify and send
branches to neighboring corpuscles,
and also to the epidermis. The genital corpuscles are deeply placed be- FlG neath the epithelium of the glans
penis, clitoris, and adjoining structures.
Bulbous corpuscles (of Krause) are smaller than the genital corpuscles,
having a diameter of 20-100 n (Fig. 154). They are most numerous
(1-4 in a sq. mm.) in the superficial connective tissue of the glans penis
and clitoris. Similar structures, either round or oval, are found in the
conjunctiva and "edge of the cornea, in the lips and lining of the oral
cavity, and probably in other parts of the corium." They have thinner
capsules and receive fewer nerves than
the genital corpuscles, which they resemble. The articular corpuscles, found
near the joints, belong in the same
category.
Cylindrical corpuscles (cylindrical
end bulbs of Krause) contain a single
axial nerve fiber with few or no branches,
terminating in a knob-like or rounded
extremity (Fig. 155). The fiber is surrounded by a semi-fluid substance,
sometimes described as an inner bulb,
and this is enclosed in a few concentric
layers of cells which are continuous
with the sheath of the nerve. Cylindrical corpuscles are found in the
mucous membrane of the mouth and in the connective tissue of muscles
and tendons.
Lamellar corpuscles (or Pacinian corpuscles) are macroscopic elliptical
structures 0.5-4.5 mm. long and 1-2 mm. wide (Fig. 156). They were
first observed in dissections, as minute vesicular bodies attached to the
terminal branches of nerves. Microscopically they are striking objects ;
Axis cylinder.
Inner core.
FIG. 156. SMALL LAMELLAR CORPUSCLE
FROM THE MESENTERY OF A CAT. X so.
The nuclei of the capsule cells appear as
thickenings. The myelin of the nerve
fiber may be traced to the inner core.
162
HISTOLOGY
suggesting an encysted foreign body. The axial core of the corpuscles
is surrounded by concentric layers, sometimes as many as fifty, which
represent a perineurium distended with fluid. A single large nerve fiber
enters one end of the corpuscle and loses its myelin as it traverses the
lamellae. It extends through the semifluid core without obvious branches,
sometimes being flattened and band-like; it may fork at its further end
or form a coil of branches, and it has been observed to pass out and enter
another such corpuscle. Usually the corpuscles are sectioned obliquely
or transversely so that the concentric layers completely encircle the inner
core.
Special methods have shown that the axial fiber may possess many
short lateral branches ending in knobs, and that one or more delicate
fibers may enter (or leave) the corpuscles in addition to the large one just
Sensory nerve fibers.
Muscle fibers.
Motor plate.
Medullated nerve fibers.
Nerve fiber bundle.
FIG. 157. MOTOR NERVE ENDINGS OF INTERCOSTAL MUSCLE FIBERS OF A RABBIT X 150.
described; they form a net surrounding the axial fiber. A small artery
may pass into the corpuscle beside the nerve and supply the lamellae
with capillaries. Lamellar corpuscles are abundant in the subcutaneous
tissue of the hand and foot and occur in other parts of the skin, in the
nipple, and in the territory of the pudendal nerve; they are found near
the joints (particularly on the flexor side) and in the periosteum and perimysium, in the connective tissue around large blood vessels and nerves,
and in the tendon sheaths; also in the serous membranes, particularly
in the mesenteries. According to Schumacher (Arch. f. mikr. Anat,
1911, vol. 77, pp. 157-191) the lamellar corpuscles become inflated when
the blood-pressure is increased, and "their structure and distribution,
together with the results of experiments, indicate that they are regulators
of the blood pressure."
NERVOUS TISSUE 163
MOTOR ENDINGS. The motor nerve endings are the terminations of
efferent nerves, in contact with smooth, cardiac or striated muscle fibers.
The nerves to the smooth muscles are a part of the sympathetic system.
They are non-medullated fibers which branch repeatedly, forming plexuses.
From the plexuses very slender varicose fibers proceed to the muscle cells,
in contact with the surface of which they end in one or two terminal or
lateral nodular thickenings. Probably each muscle cell receives a nerve
termination. Except that the nerve endings in heart muscle are a little
larger, often provided with a small
cluster of terminal nodules, they are
like those of smooth muscle.
Striated muscles are innervated
by the neuraxons of the ventral roots, B
which grow out from cell bodies re- L FlG IS8 ._ MOTOR PLATES
Within the Central System. A > Surface view, from a guinea-pig; B, vertical
* section, from a hedgehog. (After B6hm and
form n1pTH<;f>t; nf mprliillatfvl von Davidoff.) g., Granular substance of the
lorm piexubt motor p]ate . m ^ striat?d muscle . n . t nerve
fibers in the perimysium, from which j| r r ; * r - terminal ramifications f the ne
branching medullated fibers extend
into the fasciculi (Fig. 157). Each muscle fiber receives one of these
branches, or sometimes two placed near together. They are usually
implanted near the middle of the muscle fiber. The connective tissue
sheath of the nerve blends with the perimysium, and the neurolemma
is said to be continuous with the sarcolemma. On the inner side of
the sarcolemma the myelin sheath ends abruptly, and the nerve fiber
ramifies in a granular mass considered to be modified sarcoplasm, which
may contain muscle nuclei. This entire structure appears as a distinct
elevated area, estimated to average from 40 to 60 n in diameter; it has
been named the motor plate. A surface view and a section of a motor
plate are shown in Fig. 158.
VASCULAR TISSUE.
Vascular tissue includes the blood vessels, the heart, and the lymphatic
vessels, together with the blood and the lymph.
BLOOD VESSELS.
GENERAL FEATURES. The existence of blood vessels was well known
to the ancient anatomists, and a distinction was sometimes made between
pulsating and non-pulsating vessels. They were all included by Aristotle
under the term <\ty (vein). He described the two great vessels at the
back of the thorax, one of which is the vena cava; the other, as he states,
"by some is termed the aorta, from the fact that even in dead bodies
164 HISTOLOGY
part of it is observed to be full of air." He added that " these blood vessels
have their origins in the heart, for in whatever direction they happen to
run, they traverse the other viscera without in any way losing their distinctive characteristics as blood vessels; whereas the heart is, as it were,
a part of them" (Historia Animalium, Book 3, trans, by Thompson).
Subsequently the term artery was applied to the aorta and its branches,
which were found partly empty of blood after death, and were believed
to convey air; the windpipe was called the arteria as per a.
Vesalius described an artery as "a vessel similar to a vein, membranous, round,
and hollow like a pipe, by means of which vital spirit and warm blood, rushing impetuously, are distributed throughout the entire body; by the aid of these, and thus
through the motion of the artery itself (which is by dilatation and contraction) the
vital spirit and the natural warmth of the several parts are renewed" (De corporis
humani fabrica, 1543, 4th ed., 1604). Vesalius described the arteries and veins as
composed of coats (tunica) in which he found loose tissue and layers of fibers
circular, oblique, and longitudinal.
The valves of the veins, consisting of thin membranes projecting into their lumens,
were first described and clearly figured by Fabricius, under whom Harvey studied
at Padua (De venarum ostiolis, 1603). Fabricius observed that the ostiola are
found chiefly in the veins of the limbs and are "open toward the roots of the veins but
closed below." He considered that "to a certain extent they hold back the blood,
lest like a stream, it should all flow together either at the feet, or in the hands and
fingers." He stated that the veins can be easily dilated and distended, since they are
composed of a simple and thin membranous substance; and concluded that the veins
have valves to prevent over-distention, but the arteries, because of the thickness
and strength of their walls, do not require them.
In demonstrating the circulation of the blood (in 1628) Harvey contributed
little to the knowledge of the structure of the vessels. He could not find the microscopic connections between the arteries and veins, but they were discovered not many
years later by Malpighi (De pulmonibus, Ep. II, 1661). In the membranous lungs
of frogs and turtles, Malpighi found a rete or network of vessels connecting the artery
and vein, so that the blood was not poured out into spaces, but was driven through
tubules. He concluded that if in one case the ends of the vessels are brought together
in a rete, similar conditions exist elsewhere, and he observed the circulation taking
place in the diaphanous anastomosing vessels of the distended bladder of frogs.
Leeuwenhoek (1698) clearly figured the minute vessels which pass from the arteries
to the veins in the caudal fin of eels, and noted that the line of separation between the
artery and vein is arbitrary.
The vessels which connect the arteries with the veins, because of their
hair-like minuteness, were later called capillaries. Physiologically they
form the most important part of the vascular system, and anatomically
they are the most fundamental. They consist merely of endothelial
tubes. All larger vessels, not only the arteries and veins, but also the
heart, are derived from endothelial tubes and retain their endothelial lining.
The endothelium, however, becomes surrounded by layers of smooth
muscle fibers and connective tissue, which form the substance of the
BLOOD VESSELS
vessel walls. The arteries in general have thicker and more elastic walls
than the veins, and tend to remain open after death; the thinner walls
of the veins are prone to collapse.
DEVELOPMENT. In an early stage the blood vessels of the embryo
form a network in the splanchnopleure. In mammals, as in the chick
(Figs. 27 and 28, p. 40), the portion of the net nearest the median line
forms, on either side of the body, a longitudinal vessel, the dorsal aorta.
The part of the net folded under the pharynx constitutes successively
(beginning posteriorly) the vitelline veins, the heart, and the ventral aorta,
and the latter are continuous in front of the pharynx with the dorsal aortae.
The heart first appears as two dilated vessels, one on either side, which
are parts of the general network. They are brought together in the median
line under the pharynx and fuse. At
first the heart pulsates irregularly,
but with the establishment of the
circulation, its beats become rhythmical. The blood flows from the
general network through the veins to
the heart, and thence through the
arteries back to the net. All the
future vessels of the body are believed to be offshoots from the endothelial tubes just described. They
grow out, as shown in Fig. 159,
through the mesenchyma with which
they often appear to be inseparably connected. The sprouts are at
first solid, but soon become hollow except at the growing tips. They
may encounter similar offshoots from the same or other vessels and
fuse with them. Through the anastomosis of such sprouts new capillary
nets are produced.
The formation of a definite system of arteries and veins out of a general
network may be partly explained on mechanical principles. The vascular
outgrowths must take certain courses marked out by the epithelial structures. Thus in early stages they may grow between the somites, but not
into them, producing a series of segmental vessels; they pass around the
front of the fore-gut and up and down between its lateral outpocketings,
so that the regular system of aortic arches appears to depend upon these
epithelial obstructions; and they are guided along the under surface of the
developing brain in a very characteristic manner. Epithelial obstructions
therefore determine the position of the capillary plexuses. In each plexus
the favorable channels enlarge and become the main arteries and veins,
sending forth new branches and acquiring thick walls; whereas the
vessels in which the current is slow remain small or disappear.
FIG. 159
Blood vessels from a rabbit embryo of 13 days,
developing as endothelial sprouts (en) from
pre-existing vessels (b.v.); b.c., blood corpuscle within a vessel.
1 66 HISTOLOGY
These factors are further considered by Thoma (Histomechanik des
Gefasssystems, 1893).
The way in which main trunks develop from indifferent networks has been described
by Evans on the basis of extraordinarily perfect injections; thin fluid introduced into
the vessels of a living chick embryo is distributed throughout the vascular system
by the action of the heart (Anat. Rec., 1909, vol, 3, pp. 498-518). Obviously however if vessels are arising as mesenchymal spaces which subsequently become joined to
the vascular system, they would not be revealed by this method. The existence of
detached spaces in rabbit embryos has been denied by Bremer, after making very
careful graphic reconstructions of all the vessels in the anterior end of the specimens
studied. He finds that a network consisting largely of solid strands precedes the
network of open tubes (Amer. Journ. Anat., 1912, vol. 13, pp. 111-128). Schafer,
however, describes the formation of vessels by the vacuolization of connective
tissue cells, which then become connected with processes from pre-existing capillaries,
and so added to the endothelium. He states that "a more or less extensive capillary
network is often formed long before the connection with the rest of the vascular
system is established" (Text-book of Micr. Anat., 1912). His observations were
made upon subcutaneous tissue of the new-born rat. Similar appearances in the
subcutaneous tissue of human embryos may be interpreted quite differently, and
before it can be accepted that the cells containing red corpuscles are detached from
the vascular system, careful reconstructions are required.
The formation of anomalous vessels readily takes place by the persistence and
enlargement of channels usually unfavorable. This is discussed by S. R. Williams in
explaining the condition observed in an adult salamander, in which one of the long and
slender lungs received its artery at the anterior end and the other at the posterior
end (Anat. Rec., 1909, vol. 3, pp. 409-414). Innumerable forms of human vascular
anomalies may thus be explained embryologically; some of them represent persistent
vessels which are normally important at a certain stage of development, and others
represent connections which are as abnormal in the embryo as in the adult (cf. Lewis,
Amer. Journ. Anat., 1909, vol. 9, pp. 33-42).
A very characteristic form of circulation occurs in certain organs, in
which the endothelium of the vessel walls is closely applied to the epithelium of the secreting tubules, or other parenchymal structure (Fig. 160).
The walls of the vessels are as thin as those of capillaries, but their diameter is much greater, so that they have been described as lacunar vessels or "sinusoids," the term sinus being generally applied to the large
thin-walled veins in the dura mater about the brain (Minot, Proc. Boston
Soc. Nat. Hist., 1900, vol. 29, p. 185-215). Apparently the close apposition of the endothelium, on all sides, to the cells of the parenchyma
is the most essential characteristic of these vessels and must be of considerable physiological significance. There are few or no connective
tissue cells between the thin lining of the vessel and the epithelial tissue
which it nourishes. Capillaries, on the contrary, are imbedded in connective tissue, even though occasionally they approach close to an epithelium, sometimes appearing to enter it. In the lungs the capillaries are
compressed between epithelial plates, but they do not resemble the vessels shown in Fig. 160.
BLOOD VESSELS
167
Where sinusoids are most highly developed, as in the liver and Wolffian body of
embryos, they possess another very significant characteristic. They are not connections between an artery and a vein, like the capillaries, but are subdivisions of
veins. Thus in the liver, as shown in the diagram, Fig. 161, the portal vein enters the
organ and is subdivided by cords of hepatic cells into sinusoids, such as are shown in
section in Fig. 160. These reunite to empty into the vena cava inferior. The sinusoids
X300
FIG. 160. SINUSOIDS (Si) IN THE LIVER OF A CHICK EMBRYO OF ELEVEN DAYS. (Minot.)
h.c., Cords and tubules of hepatic cells.
of the liver have therefore been described as formed by the intercrescence of vascular
endothelium and hepatic parenchyma. This arrangement of veins constitutes the
hepatic portal circulation, taking its name from the entering vessel. The same type of
venous circulation occurs in the Wolffian bodies, where it constitutes the "renal portal
circulation," although it has no connection with the portal vein. It is probable that
this form of circulation, which is generally lacunar or sinusoidal, represents a primitive
VC.I.
Int. V Ar.
FIG. 161. DIAGRAM SHOWING ON THE LEFT THE LIVER AND ITS SINUSOIDS; ON THE RIGHT THE PANCREAS
AND ITS CAPILLARIES.
The connective tissue is represented by dots. Ar., Artery; Int., intestine; V., vein; V. C. I., vena cava
inferior; V. P., portal vein.
type of vascularization, since a single vessel passing by or through an organ provides it
with both afferent and efferent vessels. The arterio-venous circulation requires the
presence of two vessels with currents flowing in opposite directions. There are indications that various organs in the human embryo have a transient "portal circulation"
before the arteries connect with the network and become the main afferent channels.
CAPILLARIES. The capillaries are endothelial tubes of varying diameter, the smallest being so narrow that the blood corpuscles must pass
1 68 HISTOLOGY
through them in single file. Their walls are composed of elongated, very
flat cells, with irregularly wavy polygonal outlines which are clearly
demonstrated in silver nitrate preparations (Fig. 162). Between the cells,
the red and white corpuscles frequently make their way out of the vessel.
There are no pre-formed openings for this purpose, and the endothelial
cells come together after the corpuscles have passed out. Certain endothelial cells are phagocytic, devouring objects which float in the blood;
some of them may become detached and enter the circulation. Moreover endothelial cells are contractile, and may be stimulated to activity
by the sympathetic fibers in the delicate
perivascular plexus which is shown in
methylene blue preparations. Some of the
fibers end in contact with the cells and presumably control the caliber of the vessel;
other fibers may be afferent and receive a
stimulus when the vessel expands and
stretches the plexus. The bulging of endo
PREPARATION. (A fter Koelliker) *,-, , . .1 i e ir
thelial nuclei into the lumen of vessels, frequently seen in preserved specimens, is probably due to post-mortem
contraction; in life the lining is presumably smooth.
Although capillaries vary in diameter (4.5-12 /i), those in a given
territory are quite uniform, both as to caliber of individual vessels and
the size and pattern of the meshes in the network. The closest meshes
and largest capillaries occur in secretory organs and in the lungs, which
are therefore abundantly supplied with blood. The muscles are well
supplied by slender capillaries in a rectangular meshwork. Serous membranes and dense connective tissue have a scanty blood supply, from narrow capillaries in a coarse net.
ARTERIES. The walls of the arteries are composed of three layers
the tunica iniima, tunica media, and tunica externa, respectively. The
intima includes the endothelium and generally an underlying elastic
membrane, separated from the endothelium by a small amount of fibrous
tissue. The media is primarily a layer of circular smooth muscle fibers;
and the externa (formerly called the tunica adventitia) consists chiefly
of connective tissue. The thickness of all these layers is greatest toward
the heart. They become thinner at the places where the arteries branch,
and in the pre-capillary vessels nothing remains but the endothelium.
The small terminal arteries are called arterioles. They are endothelial
tubes encircled by scattered smooth muscle fibers. In Fig. 163, C, the
oval nuclei of the endothelium are seen to be elongated parallel with the
course of the vessel. As is usually the case, the walls of the endothelial
cells are not visible. The rod-shaped nuclei of the muscle fibers are at
right angles with the axis of the vessel. In the somewhat larger artery,
ARTERIES
169
B, the muscle fibers form a single but continuous layer, the media, outside of which the connective tissue is compressed to make the externa.
Its fibers tend to be parallel with the vessel. The walls of such an artery
are so thick that it is possible to focus on the layers separately; thus in A,
the endothelium, which with a delicate elastic membrane beneath it constitutes the intima, is not seen, being out of focus. The nuclei of the
. . ..
"H^ft
v
FIG. 163. FRAGMENTS OF HUMAN ARTERIOLES. X 240.
i, Nuclei of endothelial cells; m, nuclei of circular muscle fibers; a, nuclei of connective tissue.
In A, since the endothelium is out of focus, its nuclei are not seen.
media and externa are evident. A cross section of such a vessel is seen
in Fig. 177.
The larger arteries are lined with endothelium similar to that of the
capillaries, as shown in silver nitrate preparations (Fig. 164). This endothelium rests on a layer of connective tissue containing flattened cells
and a network of fine elastic fibers. The meshes of the fibrous and elastic
Endothelial cell.
\
Indentations made by smooth muscle fiber.
FIG 164. ENDOTHELIUM OF A MESENTERIC ARTERY OF A RABBIT. SURFACE VIEW. X 250.
tissue are elongated lengthwise of the vessel, and on surface view they
present a longitudinally striped appearance, f In addition to this subendothelial tissue and the endothelium, the intima includes the inner elastic
membrane (Fig. 165). This is usually a conspicuous layer thrown into
wavy folds by the post-mortem contraction of the vessel. It is easily
seen with ordinary stains, appearing as a refractive layer, and is deeply
170
HISTOLOGY
colored by resorcin-fuchsin and other elastic tissue stains (upper segment
in Fig. 165). In smaller arteries the endothelium appears to rest directly
upon the elastic network which replaces this membrane; and in such
large ones as the external iliacs, the principal branches of the abdominal
aorta, and the uterine arteries in young persons, the subendothelial tissue
is said to be lacking. The inner elastic membrane is not a continuous
sheet of tissue, since it is perforated by elongated apertures; it forms a
&% ;--3BWi- ! ' /: }!: :J&
This portion is
shown enlarged
on the left.
f Endothelium.
Intima \ Inner elastic
f membrane.
Media
Externa
Fie. 165. A SECTION THROUGH A HUMAN ULNAR ARTERY AND VEIN, SHOWING THE WALL OF THE ARTERY
ON THB LEFT AND OF THE VEIN ON THE RIGHT. THE UPPER PART OF THE FIGURE (a-d) is FROM A
SECTION OF THE SAME VESSELS STAINED WITH RESORCIN-FUCHSIN, AN ELASTIC TISSUE STAIN. X 550.
a, Circular, and b, radial elastic fibers of the media of the artery; c, external elastic membrane; d, elastic
fibers in the media of the vein; e, circular, and g, longitudinal muscle fibers of the media; f, endothelium.
fenestrated membrane and the development of such membranes from elastic
networks has already been described (cf. Fig. 54, p. 67). The membrane
is particularly thick in the larger arteries of the brain, and it is sometimes
double.
The media, which consists of but a single layer of circular muscle
fibers in the pre-capillary vessels, becomes many-layered in larger arteries.
Generally the fibers are all circular or perhaps oblique, but in the loose
musculature of the umbilical arteries, longitudinal fibers are numerous.
Longitudinal fibers are said to occur in certain other vessels near theintima,
being especially well developed in the subclavian artery. The post-mortem contraction of the circular fibers, which throws the intima into folds,
ARTERIES 171
causes a spiral crumpling of certain muscle nuclei, the significance of
which has already been discussed (Fig. 106, p. 117). Between the muscle
fibers there are circular elastic fibers, or plates in the larger vessels, which
are thrown into wavy folds. Radial fibers, which connect these in a
general network, are slender and require special staining. White fibers
are present, apparently formed in considerable part by the muscle fibers
which they bind together. The proportion between the muscular and
elastic tissue in the media varies in different arteries. In the smaller
vessels, the muscular tissue predominates, and this is true also of the
cceliac, femoral and radial arteries. But in the common iliac, axillary
and carotid arteries the elastic tissue prevails, and in this respect they
resemble the largest arteries the aorta and pulmonary artery.
The externa is a connective tissue layer which sometimes contains
scattered bundles of longitudinal muscle fibers. It has many longitudinal
elastic fibers, which are particularly numerous toward the media, where
they are often grouped as the external elastic membrane (Fig. 165). This
is not a fenestrated membrane, but is merely a dense zone of longitudinal
fibers. It is said to be well developed in the carotid, brachial, femoral,
cceliac and mesenteric arteries, and to be absent from the basilar and
other cerebral arteries.
Nerves and vessels ramify in the externa. The walls of the larger
arteries are supplied with small blood vessels, the vasa vasorum, derived
from adjacent arteries. These are distributed chiefly to the externa;
they may penetrate the outer part of the media but do not reach the intima. Lymphatic vessels form perivascular plexuses, and send branches
into the externa. The nerves are medullated and non-medullated. They
include vasomotor fibers which innervate the smooth muscle cells, and
sensory or afferent nerves which have terminal arborizations in the intima
and in the externa. Other nerve fibers end in lamellar corpuscles in the
externa of the aorta and other large vessels.
Ganglia are not seen in the walls of the vessels, and the sympathetic
fibers to the muscles therefore travel considerable distances to their terminations. In this respect the nerves to the smooth muscles of the vessels differ from those to the musculature of the digestive tube.
In the largest arteries (the aorta and pulmonary arteries) the intima
is very broad (Fig. 166), and it increases in thickness with age. Its endothelial cells are less elongated than those of smaller arteries. They
rest on a fibrous subendothelial tissue, containing flattened stellate or
rounded cells, and networks of elastic tissue/- The elastic fibers are thicker
toward the media, finally producing a fenestrated membrane which corresponds with the inner elastic membrane of smaller vessels, but which is
scarcely thicker than adjacent elastic lamellae. The broad media consists
of elastic membranes and muscle fibers, but the elastic tissue greatly
172
HISTOLOGY
b
predominates. On section the wall of the fresh aorta consequently
appears yellow, and not reddish like the more muscular walls of smaller
arteries. The elastic tissue is arranged in a succession of circular fenestrated membranes connected with one another by oblique fibers. Between them are the muscle cells. According to Koelliker, in the inner
layers of the media, the muscle cells form an anastomosing syncytium
of short, broad and flattened elements, somewhat resembling cardiac muscle (Fig. 167), but in the
outer layers the fibers are of the
ordinary type. The externa contains no outer elastic layer and is
relatively thin; its inner elastic
portion may have been taken over
into the media.
VEINS. Since the artery to
any structure and the returning
vein are often side by side, they
are frequently included in a single
section and may readily be compared. In embryos the veins are
of much larger diameter than the
corresponding arteries, and they
have thinner walls. Although
Nerve.
Vasa vasorum.
FIG. 1 66. FROM A TRANSVERSE SECTION OF THE
HUMAN THORACIC AORTA, STAINED WITH RESORCIN-FUCHSIN. X 80.
a, Endothelium; b, subendothelial fibrous tissue;
c, d, elastic membranes of the media.
FIG. 167. BRANCHED SMOOTH MUSCLE
CELLS FROM THE THORACIC AORTA OF
A CHILD AT BIRTH (a) AND AT FOUR
YEARS (b). (After Koelliker.)
the difference in diameter is less marked in the adult, it generally remains
a distinctive feature (Fig. 177, p. 186), and the difference in the thickness
of the walls becomes accentuated (Fig. 165). In comparing the diameters
of the ulnar vein and artery in Fig. 165, it should be remembered that the
ulnar artery is usually accompanied by two returning veins, only one of
which is shown in the figure. Because of their thinner walls, which contain relatively little elastic tissue, the veins are generally partly collapsed;
the lumen is therefore irregular, whereas that of the arteries tends to be
VEINS
173
This portion is enlarged below
Endothelium.
\
round (Fig. 165). Small veins full of blood may be round, however,
and the arteries are sometimes irregularly contracted.
The walls of the veins, like those of arteries, are composed of three
layers, the intima, media, and externa. The intima includes the primary
endothelium, which is composed
of polygonal cells, generally shorter
and broader than those of arteries.
The endothelium rests on a thin
layer of subendothelial fibrous
tissue. The inner elastic membrane of arteries is represented in
the smaller veins by a thin homogeneous membrane, but in larger
veins it is replaced by a network
of elastic fibers (Fig. 165). In
addition to these structures the
intima of certain veins contains
scattered oblique and longitudinal
muscle fibers; they are said to
occur in the iliac, femoral, saphenous and intestinal veins, the intramuscular part of the uterine veins,
and especially in the dorsal vein of
the penis near the suspensory ligament.
The media shows great variations. It is generally a thin layer
consisting of circular muscle fibers,
elastic networks and relatively
abundant connective tissue, and is
best developed in the veins of the
lower extremity (especially the
popliteal). In those of the upper
extremity it is not so well marked,
and it is still thinner in the larger
veins of the abdominal cavity; it FJG . I68 ._ r
is reduced to fibrous tissue and is
essentially absent from the vena
cava superior, the veins of the
retina, of the pia and dura mater, and of the bones.
The externa is the most highly developed layer of the veins. It consists of interwoven bundles of connective tissue, elastic fibers, and longitudinal bundles of smooth muscles which are more abundant than in the
A CROSS SECTION OF A HUMAN SUPRARENAL VEIN, STAINED WITH H^KMATOXYLIN. X 240.
, Circular muscle fibers of the media; b, connective tissue; c, d, longitudinal muscle fibers of the externa;
e, connective tissue; f, small vein; g, fat cell.
174 HISTOLOGY
arteries. In certain veins (e.g., the main trunk of the portal, the renal
and suprarenal veins) the longitudinal muscle forms an almost complete
layer of considerable thickness (Fig. 168).
The valves of veins are paired folds of the intima, each shaped like
half of a cup attached to the wall of the vein so that its convex surface
is toward the lumen. In longitudinal section they appear like the valves
of the lymphatic vessel shown in Fig. 179. The valves are generally
found distal to the point where a branch empties into the vein, and they
prevent its blood from flowing away from the heart. They are most
numerous in the veins of the extremities, but appear also in the intercostal, azygos, spermatic, and certain other veins; none are found in the
vertical trunks of the superior and inferior venae cavae. They counteract the effects of gravity upon the blood, and it has been suggested that
their arrangement in man corresponds rather to a quadrupedal attitude
than to an upright position. The endothelial cells on the surface of the
valve toward the lumen of the vein are elongated parallel with the current,
and beneath them there is a thick network of elastic tissue. On the side
of the valve toward the wall of the vein, the long axis of the cells is transverse, and there the cells rest upon fibrous connective tissue.
THE HEART.
DEVELOPMENT. The heart has already been described as a median
longitudinal vessel situated beneath the pharynx, formed posteriorly
by the union of the vitelline veins, and terminating anteriorly in the
two ventral aortae (Figs. 27 and 28, p. 40). This endothelial tube is
surrounded by the mesothelium of the body cavity, except along its
dorsal border, where it is attached, as it were by a short mesentery, to
the under side of the fore-gut. If the embryo is placed in an upright
position, corresponding with that of the adult, the relations of the heart
to the body cavity will be as shown in the diagram, Fig. 169, A. The
posterior part of the body cavity, which becomes the peritoneal cavity,
extends forward on either side and comes together across the median
line beneath the heart, thus forming the pericardial cavity. As the heart
develops it becomes bent upon itself as shown in Fig. 169, B; and below
it, a shelf of tissue forms across the body, representing the future diaphragm. Dorsal to the diaphragm, the pericardial cavity still communicates with the peritoneal cavity, on either side of the body. In the
region of this communication the lungs later develop, and partitions
separate the part of the body cavity around them, namely the pleural
cavity, from the pericardial and peritoneal cavities respectively. These
partitions are the pleuro-pericardial membrane and the membranous
part of the diaphragm (Fig. 169, C). Meanwhile the mesentery of
HEART
175
the heart has become thin and has ruptured in the hollow of the Ushaped bend, forming the sinus transversus pericardii, which persists
throughout life as a small but very definite structure.
While the heart is still a simple tube consisting of endothelium internally and mesothelium externally, with a space between them bridged
by protoplasmic strands, it beats regularly, although possessing neither
nerves nor muscles. Without causing any interruption of the circulation
the simple tube becomes divided into four chambers, namely the right
and left atria (or auricles 1 ) and the right and left ventricles. The process
of subdivision may be outlined as follows:
When the tube becomes bent into a U, the venous end of the heart
is carried anteriorly, dorsal to the aortic end, as shown in Fig. 1 70, A-C.
A B c
FIG. 169. DIAGRAMS ILLUSTRATING THB FORMATION OF THE PERICARDIAL CAVITY.
A.., Aortic end of heart; B. W., body wall; D., diaphragm; Ht., heart; Li., liver; Lu., lung; P. C., pericardia!
cavity: Per., peritoneal cavity; PI., pleural cavity; S.p-p., pleuro-pericardial septum, S. tr. p., sinus
transversus pericardii; V., venous end of the heart.
At the same time the ventral or aortic limb of the U is carried to the right
of the median plane (C). The dorsal limb is divided into two parts by an
encircling transverse constriction, the coronary sulcus (s.c.}. Its thickwalled portion, ventral to the sulcus, forms the ventricles; the thin-walled
dorsal portion becomes the atria. In the human embryo of three weeks
(C) the atria are represented by a single cavity subdivided into right and
left parts only by an external depression in the median plane. The right
portion receives all the veins which enter the heart (the vitelline veins and
their tributaries) and is much larger than the left portion. The cavities of
the atria not only freely communicate with each other but they have a
common outlet into the undivided ventricle. From the ventricle the
blood flows out of the heart through the aortic limb. In a complex manner,
described in text-books of embryology, a median septum develops,
dividing the heart into right and left halves.
In the heart of a i2-mm. pig embryo this septum has already formed
1 According to the anatomical nomenclature adopted at Basle, the term auricle (diminutive
of auris, ear) is restricted to what was formerly called the auricular appendix, and the term
atrium (chamber) is used for the cavity as a whole.
176 HISTOLOGY
(Fig. 170, D) and has been exposed by cutting away most of the left
atrium and left ventricle. The septum between the atria becomes perforated as it develops, so that in embryonic life the atria always communicate. The perforation in the septum is the foramen ovale.
Encircling the orifice which connects each atrium with the corresponding ventricle, the is a ring of mesenchyma which in the adult becomes
dense fibrous tissue the annulus fibrosus. Extending from this ring
into the left ventricle there are two flaps of tissue partly detached from
the ventricular walls. They constitute the bicuspid valve (or mitral valve).
Toward the apex of the heart each flap passes into strands of tissue attached to the walls of the ventricle. These strands become the chorda ten
D E F
FIG. 170. EMBRYONIC HEARTS. "*
A and B, From rabbits nine days after coitus; C, from a human embryo of three (?) weeks; D and E, from a
12-mm. pig (D sectioned on the left of the median septum, and E on the right of it); F, from a 13.6mm. human embryo, sectioned like E. The hearts are all in corresponding positions with the left
side toward the observer, the anterior end toward the top of the page, the dorsal side to the right,
ao., Aorta; c. s., coronary sinus; f. o., foramen ovale; i. f., interventricular foramen; 1. a., left atrium;
p. a., pulmonary artery; p. v., pulmonary vein; r. a., right atrium; s., septum membranaceum separating
the root of the aorta from tne right ventricle; s. c., coronary sulcus; v., ventricle; v. b., bicuspid
valve; v. t., tricuspid valve; v. v., vitelline vein; v. v. s., valves of the venous sinus.
dinecB of the adult, and the muscular elevations into which they are inserted
are the papillary muscles (musculi papillares). The differentiation of
these structures has not taken place in the stage shown in Fig. 1 70.
In the i2-mm. pig (Fig. 170, D) the median septum which has grown
up from the apex of the heart, so as to separate the right and left ventricles
from each other, is not complete. The ventricles still communicate
through the interventricular foramen, and through this aperture the blood
passes from the left side of the heart to enter the root of the aorta. The
root of the aorta is shown in E, a section of the same heart made on the
right of the median septum. The pulmonary artery and the part of the
aorta near the heart develop first as a single vessel; they become separated
from one another by the formation of a partition. As long as the dividing
HEART 177
wall is incomplete, the blood from either ventricle may pass out through
either artery as shown in E. In the more advanced human embryo, F,
the partition between the aorta and pulmonary artery has extended so
that it joins the interventricular septum, and causes the interventricular
foramen to open into the root of the aorta only (s). This portion of the
interventricular wall which is the last to form, is translucent in the adult,
and is known as the septum membranaceum.
As previously noted all the veins come together to enter the right
atrium. The original vitelline veins are no longer directly connected
with the heart, and their persistent cardiac outlet becomes the terminal
part of several large branches. These are the superior vena cava from
the head and arms, the inferior vena cava from the trunk and legs (receiving as branches the hepatic vein draining the portal system from the intestine, and the umbilical vein from the placenta); and the coronary sinus
which, as it passes across the heart in the coronary sulcus, receives
branches from the wall of the heart. All these veins come together in a
cavity, ill defined in mammals, known as the sinus venosus, and this sinus
empties into the right atrium through an orifice guarded by a valve with
right and left flaps. With further growth the sinus venosus becomes a
part of the atrium, and the superior and inferior venae cavse and coronary
sinus open separately, guarded by imperfect valves derived from the
valves of the sinus venosus. The left flap of this valve is said to assist
in closing the foramen ovale; the right flap becomes subdivided into the
rudimentary valve of the vena cava inferior (Eustachian valve) and the
valve of the coronary sinus (Thebesian valve). The degeneration of the
valve of the venous sinus seems to take place after the bicuspid and
tricuspid valves have become well formed, and have rendered.it superfluous. In early stages it must be regarded as the principal valve of the
heart. The tricuspid valve, between the right atrium and right ventricle,
develops from the cardiac walls in the same way as the bicuspid valve.
Their formation is discussed by Mall (Amer. Journ. Anat., 1912, vol. 13,
pp. 249-298).
In the embryonic heart, the left atrium receives most of its blood
through the foramen ovale, but the pulmonary veins early grow out from
it as a small vessel (Fig. 170, D) which sends four branches to the lungs.
These are given off near the heart, and with the enlargement of the atrium
they come to open into it separately. After birth they are the only
supply of the left atrium, and they convey the same quantity of blood as
the veins which enter the right atrium.
LAYERS OF THE HEART. Early in the development of the heart a
third layer, consisting of mesenchyma, forms between the endothelium
and mesothelium. It gives rise to the cardiac musculature, and toward
the primary layers it produces connective tissue! The wall of the heart
i 7 8
HISTOLOGY
in the adult is divided into three layers, the endocardium, myocardium
and epicardium respectively. The endocardium consists of the endothelium, which is continuous with that of the blood vessels, and of subendothelial fibrous tissue. According to Mall, this tissue is derived from the
endothelium. The myocardium is the muscle layer, which is thin in the
atria, but very thick in the ventricles; in the left ventricle it is much
thicker than in the right. The epicardium consists of the pericardial epithelium together with underlying connective tissue. This layer is also
called the visceral pericardium, and with the parietal pericardium it bounds
the pericardial cavity, forming a closed sac containing the pericardial
fluid. The general relations of
these layers in an embryonic
heart are shown in Fig. 171.
The epicardium is a smooth
layer. The musculature of the
ventricles is arranged in trabeculae covered with endothelium,
between which there are blood
spaces classed as sinusoids. In
the adult the musculature is
more compact, but internally it
is indented by many clefts and
irregular spaces, extending
among the trabecula carnecs and
the conical papillary muscles.
Endocardium. The endocardium consists of endothelium
which is a single layer of flat,
irregularly polygonal cells, and
of the underlying connective
tissue which contains smooth
muscle and many elastic fibers
(Fig. 172). Elastic fibers are more highly developed in the atria than in
the ventricles; they occur either as networks of thick fibers or fuse to
form fenestrated membranes. Smooth muscle fibers are more numerous
where the wall of the heart is smooth; they are most abundant in
front of the root of the aorta.
The atrio- ventricular valves are essentially folds of endocardium
containing dense fibro-elastic tissue continuous with the similar tissue
in the annuli fibrosi. The valves contain muscle fibers toward these
rings, and elastic fibers which are prolonged into the chorda tendinea.
Blood vessels are found only in the basal portion of the valves, where the
muscle fibers occur. The semilunar valves of the pulmonary artery and
FIG. 171 . SECTION OF THE HEART SHOWN IN FIG. 170. F.
ca., Capillaries; en., endothelium; 1. a., left atrium; 1. v.,
left ventricle; mes., mesothelium (of the epicardium,
or visceral pericardium) ; p. c., pericardial cavity; p. p.,
parietal pericardium; r. a., right atrium; r. v., right
ventricle; si., sinusoids; v.b., bicuspid valve; y. t.,
tricuspid valve; v. v. s., valves of the venous sinus.
HEART 179
aorta contain neither muscle fibers nor vessels. Their elastic fibers are
found chiefly on the ventricular sides of the valve, and in the noduli
(which are thickenings in the middle of the circumference of each segment,
to perfect their approximation when closed).
Myocardium. The myocardium consists of muscle fibers arranged I
in layers or sheets, which are wound about the ventricles in complex spirals, I
making a vortex at the apex of each ventricle. If the heart is boiled in
dilute acid these layers may be unwound, and the heart has frequently
been investigated in this way, most recently by Mall (Amer. Journ.
Anat., 1911, vol. n, pp. 211-266). The layers are composed of cardiac
muscle, which is a syncytium of striated fibers with central nuclei and
Myocardium.
Endothelium
Nuclei of connective tissue
Detached
endothelial cell
Endocardium
Small artery.
; Fibers of the atrioventricular system.
Connective tissue.
Nucleus of a connective tissue cell.
Nucleus of
cardiac muscle.
' Sarcoplasm.
WJ \
\
kj5\-r- -/ Capillaries.
FIG. 172. FROM A CROSS SECTION OF THE PECTINATE MUSCLES OF A HUMAN HEART (RIGHT ATRIUM)
X 240.
The muscle fibrils in transverse sections appear as points; at i they are radially arranged.
intercalated discs, as already described (p. 129). Cardiac muscle is shown
in longitudinal section in Fig. 121 (p. 129), and in transverse section in
Fig. 172. Between the muscle fibers there are capillary branches of the
coronary vessels which ramify in the epicardium. The capillaries come
into close relation with the muscle fibers and some of them extend into
the endocardium. Certain vessels, especially in the right atrium, empty
into the cavity of the heart as small veins known as the vena minima
(or veins of Thebesius). Minute veins in the papillary muscles have
been described as opening into the ventricle at both ends.
i8o
HISTOLOGY
In the heart of adult frogs, the system of intermuscular clefts or lacunar vessels
is the only blood supply of the ventricular musculature; the coronary vessels are
limited to the epicardium. In turtles the coronary vessels supply an outer layer of
the ventricular muscles, but the greater part is still nourished by the central lacunae
or sinusoids. This sinusoidal circulation, which is characteristic of the adult heart
in lower vertebrates, occurs also in mammalian embryos, but it becomes vestigial in
adult mammals.
A structure which has recently received much attention because of its
functional importance is a small band of muscle fibers, associated with
nerves, which passes from the septum between the atria into the septum
between the ventricles. This atrio-ventricular bundle or "bundle of His"
(discovered independently in
1893 by Kent and His, Jr.)
represents the only connection between the musculature of the atria and ventricles; it passes through the
fibrous tissue where the
annuli fibrosi come together.
The position of the bundle
is shown in Fig. 173, after
Curran (Anat. Rec., 1909,
vol. 3, pp. 618-632). Curran
finds more extensive branches
in the atria than others have
shown. They come from
both sides of the heart into
Rd
VC.L
FIG. 173. THE ATRIO-VENTRICULAR BUNDLE (F. a. v.), AND
THE POSITION OF THE "SlNO-ATRIAL NODE" (x) IN A
HUMAN HEART. (After Curran and Aschoff.)
Ao., Aorta; A. p., pulmonary artery; F. o., fossa ovalis; S. c.,
coronary sinus; R. d., right branch of the atrio-ventricular
bundle; and R. s., its left branch; V. c. i. f vena cava inferior; V. c. s., vena cava superior.
the inter-atrial septum, and converge from the fossa ovalis, the roots
of the tricuspid valve and the orifice of the coronary sinus to form
the atrio-ventricular node. This is "a small mass of interwoven fibers
in the central fibrous body of the heart, " and the main bundle, 2-3
mm. wide, passes from it into the inter-ventricular septum. It passes
under the pars membranacea, and divides into two branches which are
distributed to the right and left ventricles, respectively. Their extensive
ramifications have been modelled by Miss DeWitt. She describes the
models, and briefly summarizes previous investigations of the bundle, in
the Anatomical Record (1909, vol. 3, pp. 475-497); the subject is more
fully considered by Aschoff (Verh. d. deutsch. path. Gesellsch., 1910,
PP- 3-35)
The atrio-ventricular bundle is composed of muscle fibers which are
pale macroscopically. They are larger than those of ordinary cardiac
muscle, but contain fewer fibrils, peripherally placed and surrounded
by abundant scaroplasm (Fig. 172). In the ventricle they are specially
HEART l8l
rich in glycogen. In the node, however, according to Miss DeWitt, the
fibers, though varying greatly in size, are much smaller than those found
elsewhere in the heart. Several of them unite at a point, producing stellate groups, and the entire node is an intricate network.
The fibers of the atrio-ventricular bundle resemble those described by Purkinje
in the sheep, horse, cow and pig, but which he could not find in the rabbit, dog and man
(Arch. f. Anat., Physiol. u. wiss. Med., 1845, pp. 281-295). I n the walls of the ventricle, immediately beneath the endocardium, he observed "first with the naked eye, a
network of gray, flat gelatinous threads, which in part were prolonged into the papillary
muscles, and in part passed like bridges across the separate folds and clefts." Under
the microscope, they appeared very granular, but he decided that they were probably
muscular. Purkinje's fibers are regarded as imperfectly developed muscle fibers.
In the human heart they are not as distinct from the other cardiac muscle fibers as in
the sheep. It is possible that they are directly continuous with the cardiac syncytium, although, as noted by Miss DeWitt, if the transition is gradual it will be very
difficult to observe in sections.
At the junction of the superior vena cava and the atrium, Keith and
Flack have described a peculiar musculature imbedded in densely packed
connective tissue, composed of striated, fusiform fibers, plexiform in arrangement, with .well-marked elongated nuclei, "in fact, of closely similar
structure to the node" (Journ. Anat. and Physiol., 1907, vol. 41, pp.
172-189). These fibers are said to be in close relation with the vagus and
sympathetic nerves; they have a special arterial supply. According to
Keith and Flack they are situated at the junction of the sinus venosus
and the atrium, and they form the sino-atrial node (sino-auricular node).
The sino-atrial node is found immediately beneath the epicardium in the
position shown in Fig. 173. In it the impulse for the heart beat is believed to originate, and to be transmitted to the atrio-ventricular node;
the latter correlates the contraction of the atrium with that of the
ventricle.
Epicardium. The epicardium is a connective tissue layer, covered
with simple flat mesothelium and containing elastic fibers and many fat
cells. The latter are distributed along the course of the blood vessels.
Vessels and Nerves. The branches of the coronary vessels pass from
the epicardium into the myocardium, forming capillaries in intimate
relation with the muscle fibers. The heart is thus supplied with aerated
blood from the root of the aorta, as well as by the blood within its own
cavities; on the left side this is aerated, but not on the right.
The lymphatic vessels, draining toward the base of the heart, are
very abundant, and true lymphatic vessels are found in all layers of the
heart. The tissue spaces in the myocardium are also extensive.
The nerves to the heart have already been described as forming the
cardiac plexus. This plexus receives branches from the vagus, and from
the sympathetic cardiac nerves proceeding from the cervical sympathetic
182 HISTOLOGY
ganglia. It sends its fibers toward the heart, where they follow the
coronary vessels in their ramifications. The cardiac ganglion is associated with the superficial part of the cardiac plexus, and is under
the arch of the aorta. Other small ganglia occur on the posterior wall
of the atria, and scattered ganglion cells are found along the atrio-ventricular bundle. They have been reported along the nerves elsewhere in
the heart. The ganglion cells are probably in connection with efferent
fibers from the central nervous system, which include two sorts fibers
from the ventral ramus of the accessory nerve, which pass out with the
branches of the vagus and inhibit cardiac action; and fibers from the
spinal nerves, by way of the inferior cervical ganglion, which accelerate it.
Histologically nerve endings have been seen both within and around the
capsules of cardiac ganglion cells. It is said that the medullated nerve
fibers from the central system end within the capsules; and that nonmedullated branches from adjacent sympathetic ganglia end outside of
them. Motor endings in contact with cardiac muscle have also been
found. Sensory endings have been described both in the epicardium
and endocardium. They consist of terminal ramifications forming "endplates." Some of these fibers presumably connect with sympathetic
cells near at hand; others are terminations of afferent medullated fibers
which are said to pass to the medulla, along the vagus trunk, as the
"depressor nerve."
LYMPHATIC VESSELS.
GENERAL FEATURES. The lymphatic vessels are far less conspicuous
than the blood vessels, but they are no less important and are widely
distributed throughout the body. Those which occur in the mesentery
and are filled with a milky fluid after intestinal digestion has been going
on, are the most conspicuous. These "arteries containing milk" were
observed by Erasistratus, an anatomist of Alexandria who died in 280
B. C., but the observation was discredited by Galen. When Aselli in
1622 found the white vessels in a living dog which he had opened, and had
shown by cutting into them that they were not nerves, it was essentially
a new and great discovery. Aselli observed that the vessels were filled
only after digestion, at other times being scarcely visible. He traced
them to a mass of lymph glands which he mistook for the pancreas, and
believed that they passed on into the liver (De lactibus sive lacteis
venis, 1627). Years before the physiological observations of Aselli,
Eustachius (who died in 1574) had described the main trunk of the
lymphatic system in his treatise on the azygos vein (De vena sine
pan, Syngramma XIII, Opusc. anat., 1707). He states that from the
posterior side of the root of the left jugular vein (Fig. 174) "a certain
LYMPHATIC VESSELS
183
large branch is given off, which has a semicircular valve at its origin, and
moreover is white and full of aqueous humor."
" Not far from its source, it splits into two parts which come together a little further
on. Giving off no branches, and lying against the left side of the vertebrae, having
penetrated the diaphragm, it is borne along to the middle of the loins. There, having
become larger and folded around the great artery, it has an obscure ending, not clearly
made out by me up to the present time."
The vessel so well described by Eustachius is now known as the
thoracic duct. It has the structure of a vein, and empties its contents into
the blood at the junction of the left internal jugular and left subclavian
veins. It receives branches from the left side of
the head and the left arm, as well as from the
trunk of the body. There is a corresponding vessel
on the right side, known as the right lymphatic duct.
It drains the right side of the head, the right arm,
and adjacent territory, emptying at the junction of
the right internal jugular and subclavian veins.
Having no connection with the abdominal lymphatic
vessels, however, it is much smaller than the thoracic
duct on the left.
The connection between the lacteal vessels in
the mesentery, seen by Aselli, and the thoracic duct
observed by Eustachius, was demonstrated physiologically by Pecquet (Experimenta nova anatomica,
1651). He found a whitish fluid coming from the
vena cava superior of a dog from which the heart
had been excised, and observed that its flow was
increased by pressure on the mesenteries. Moreover
he described the receptaculum chyli, or enlargement
of the thoracic duct dorsal to the aorta, which receives the chylous fluid.
This is now called the cisterna chyli. The distribution, of the lymphatic
vessels, which are ramifications of these main trunks, was followed out
by skillful injections, and the results of such studies were presented in
great folios by Mascagni (1787) and Sappey (1874). Considered as a
whole the lymphatic system may be compared with a venous system
which has no corresponding arteries; it is composed entirely of afferent
vessels.
Recent anatomical studies of these vessels have been concerned with
their origin, and their relation in the adult to the surrounding connective
tissue. The vessels have long been known as absorbents, and it was
thought that they opened freely at their distal ends into the connective
tissue spaces; through these openings they were supposed to suck in the
tissue fluid which had escaped from the vessels, and the chylous fluid,
FIG. 174. THE AZYGOS
VEIN (a) AND THORACIC DUCT (b) AS
FIGURED BY EUSTACHIUS.
1 84
HISTOLOGY
charged with nutriment, which had entered the intestinal tissues, and to
convey this material back to the blood vessels. Thus the lymphatic
vessels were described as tissue spaces, which had elongated and coalesced
so as to form tubes bounded by flattened connective tissue cells, and
these vessels were thought subsequently to acquire openings into the
veins. Opposed to this conception is the idea of Ranvier that the
lymphatic vessels are primarily connected with the veins. They grow
out from the veins as endothelial sprouts, which form a closed system of
endothelial tubes, anastomosing freely with one another, but never with
the blood vessels. Thus they are connected with the veins by main trunks
comparable with the ducts of glands (Arch. d'Anat. micr., 1897, vol. i,
pp. 69-81). Fluids may pass through the thin endothelium almost as
readily as through open orifices, so that functionally the distinction does
not appear to be fundamental.
Ranvier's interpretation has been defended by MacCallum, on the basis of histological studies (Arch. f. Anat. u. Physiol., Anat. Abth., 1902, pp. 273-291), and by
Miss Sabin, from the injection of the lymphatic vessels in embryos (Amer. Journ.
Anat., 1902, vol. i, pp. 367-389). The most convincing evidence in its favor has been
supplied by Clark's observations on the growth of the lymphatic vessels in the tails of
tadpoles. The tadpoles were anaesthetized with chloretone. The membranous part
x" ~^C n
f ^ Jro
May 16.
11.30 A. M.
May 19,
II A. M.
May 19,
1 1 p. M.
FIG. 175- SUCCESSIVE STAGES IN THE GROWTH OF A LYMPHATIC VESSEL (lym.) IN THE TAIL OF A TADPOLE
(Rana paluslris). Xi35- (Clark.) b. v., Blood vessel; n., nucleus of the lymphatic vessel.
of the tail was then examined with immersion lenses, and certain of the lymphatic
vessels were drawn. The animals were restored to normal condition and were re-examined at intervals of twelve hours. The growth of a given lymphatic vessel was thus
demonstrated, as shown in Fig. 175. Its elongation and enlargement were seen to be
independent of the surrounding connective tissue, through which it made its way.
In some cases a blood corpuscle had escaped into the intercellular spaces. Toward
such a corpuscle the lymphatic vessel grew, and having reached it, the corpuscle was
taken in by the endothelial cells and transferred to the lumen of the vessel, through
which it was seen to travel toward the central vessels. As indicated in Fig. 175, the
nuclei of the living endothelium could be observed, and the multiplication of the endo
LYMPHATIC VESSELS
thelial cells during the growth of the lymphatic vessel was demonstrated (Anat. Rec.,
1909, vol. 3, pp. 183-198).
DEVELOPMENT. The development of the mammalian lymphatic system begins with the formation of a pair of very large sacs lined with
endothelium, situated at
the junction of the jugular
and subclavian veins (Fig.
176). These jugular lymph
sacs were first described
by Miss Sabin (loc. cit.}\
they appear in human embryos measuring about 10
mm. and are formed by
the union of several outgrowths from the veins.
In slightly older embryos,
another lymph sac is produced at the root of the
mesentery, below the place
where the renal veins
enter the vena cava inferior (Lewis, Amer. Journ.
Anat., 1920, vol. i, pp.
220-244). The opinion
that this sac is a derivative
of the adjacent veins has
been confirmed by certain
later embryological studies,
and by finding permanent
communications between
the lymphatic and the
venOUS SVStem at the level F IG - I7< *- LYMPHATIC VESSELS AND VEINS IN A RABBIT OF FOURJ TEEN DAYS, EIGHTEEN HOURS; 14.5 MM. X 11.5.
of the renal Veins in adult The lymphatics are heavily shaded, x being a vessel along the left
South American monkeys
(Silvester, Amer. Journ.
Anat., 1912, vol. 12, pp.
446-460). At other places,
which must be regarded as
secondary centers, lymphatic vessels appear to be derived from the veins
and to become detached from them. These vessels are seen in the
mesenchyma as isolated spaces, usually along the course of the veins,
at no great distance from the jugular and mesenteric lymphatics. Subsequently they become connected with one another by endothelial out
vagus nerve and y along the aorta. The large jugular lymph
sac is in contact with the internal jugular vein. In J.; it passes
to the junction of the external jugular (Ex. J.) and subclavian
veins, the latter being formed by the union of the primitive
ulnar, Pr. Ul., and external mammary veins, Ex. M. The
mesenteric sac is in front of the vena cava inferior (V. C. I.)
and below the renal anastomosis (R. A.). Other veins include
Az., azygos; V., vitelline; G., gastric; S. M., superior mesenteric; etc. The figures indicate the position of the corresponding cervical nerves.
i86
HISTOLOGY
growths, such as extend from the lymphatic vessels into the peripheral
tissues as described by Clark. The mesenteric sac thus becomes connected
with the left jugular sac (symmetrical connections with both jugular sacs
occur in some animals) and the connecting vessels constitute the thoracic
duct. The cisterna chyli is a secondary enlargement dorsal to the aorta.
In the adult the sacs are replaced by plexuses of smaller vessels.
The origin of the detached or apparently detached lymphatic spaces in embryos,
which precede the formation of the well-defined vessels, has been studied with great
diligence by Huntington (The Anatomy and Development of the Lymphatic System,
Mem. Wistar Institute, 1911) and McClure (Anat. Rec., 1912, vol. 6, pp. 233-248),
to whose many contributions references will be found in the papers cited. They consider that the lymphatic spaces arise in large part as mesenchymal spaces, but the
possibility suggested by Bremer's recent work on the blood vessels, that uninjectable
endothelial strands of great delicacy may pass to these cavities, has not been set aside,
and further work upon this subject is being conducted under Professor McClure's
direction. The reasons which led the writer to consider the origin of the lymphatic
vessels from mesenchymal spaces as improbable, were stated as follows (Amer. Journ.
Anat, 1905, vol. 5, pp. 95-120).
a bed
FIG. 177- BLOOD VESSELS AND LYMPHATIC VESSELS BETWEEN THE CIRCULAR AND LONGITUDINAL LAYERS
OF SMOOTH MUSCLE FIBERS IN THE SMALL INTESTINE OF A CAT. X 775.
a, d., Lymphatic vessels; b, vein; c, artery.
" i. The lymphatic spaces do not resemble mesenchyma even when it is oedematous,
but on the contrary, are scarcely distinguishable from blood vessels (Langer)."
" 2. After being formed, the lymphatics increase like blood vessels, by means of
blind endothelial sprouts, and not by connecting with intercellular spaces (Langer,
Ranvier, MacCallum, Sabin)." The subsequent work of Clark is here conclusive.
"3. In early embryos detached blood vessels may be seen without proving that
blood vessels are mesenchymal spaces. These detached vessels are not far from the
main trunks, from which they may have arisen by slender endothelial strands, yet
LYMPHATIC VESSELS
I8 7
FIG. 178. SILVER NITRATE PREPARATION OF A LYMPHATIC
VESSEL FROM A RABBIT'S MESENTERY, SHOWING THE
BOUNDARIES OF THE ENDOTHELIAL CELLS, AND A BULGING JUST BEYOND A VALVE.
often the connecting strands cannot be demonstrated." (Subsequently, Bremer
demonstrated such strands in great abundance.)
"4. The endothelium of the embryonic lymphatics is sometimes seen to be continuous with that of the veins" i.e., in certain places, as in connection with the jugular
sac, the origin of the lymphatic vessels from the venous endothelium can be clearly
seen; this fact is conclusively demonstrated by Huntington and McClure, who use
the term " veno-lymphatic " for transitional vessels (Amer. Journ. Anat., 1910, vol.
10, pp. 177-311).
LYMPHATIC VESSELS IN THE ADULT. In sections of the intestine from
an animal in which intestinal digestion was in progress, lymphatic vessels
may readily be found between
the muscle layers (Fig. 177).
Their walls are decidedly
thinner than those of blood
vessels of the same caliber,
and their contents are typically a granular or fibrinous
coagulum free from red corpuscles, but containing an
occasional lymphocyte. It
must be remembered, however, that blood vessels seen in sections are
not infrequently empty, and that blood corpuscles may be taken into
the lymphatic vessels. Having learned to recognize the lymphatics in
such favorable situations as the intermuscular tissue, one may readily
identify them in the connective tissue layer internal to the circular
muscle of the intestine, and in the connective tissue around the bronchioles in the lung; in the
embryonic lung they are
very conspicuous. They
may then be sought for in
various organs, but a sharp
distinction must be drawn
between the endotheliumlined lymphatic vessels and
the interfibrillar tissue spaces.
When prepared with silver
nitrate, the outlines of the endo thelial cells are seen to resemble those of blood vessels (Fig. 178), and in
the larger lymphatic vessels the endothelium with the underlying connective tissue forms a tunica intima. These lymphatics (0.2-0.8 mm. in
diameter), are often composed of three coats, though loose in texture. The
media contains circular smooth muscle fibers and a small amount of elastic
tissue; and the externa is composed of longitudinal connective tissue and
scattered bundles of longitudinal muscle. Thus they resemble the veins
FIG. 179.
Lymphatic vessel from a section of a human bronchus,
showing a valve, v. ; distal to the branch, br. Bundles
of smooth muscle fibers are seen at m. f.
1 88 HISTOLOGY
more closely than the arteries. Valves are very numerous in lymphatic^
vessels^ They are shown in section in Fig. 179. In the small vessels the
valves are described as folds of endothelium, such as would be produced
if the distal part of the vessel were pushed forward into the proximal part.
The vessels are often distended on the proximal side of the valve, producing bulbous enlargements, as shown in Fig. 178. Owing to the presence
of these valves, compression of tissue containing lymphatic vessels, or
the contraction of the muscles of the media, causes an onward flow of the
lymph. The nerves to lymphatic vessels are like those of the blood
vessels. Lymphatics are provided with vasa vasorum. As shown by
Evans (Amer. Journ. Anat., 1907, vol. 7, pp. 195-208) very small lymphatic vessels are accompanied by blood capillaries, and the larger lymphatics
are surrounded by a wide-meshed capillary network resting on the outer
side of the lymphatic media. (In the same volume of the Journal, pp.
389-407, Miller describes the network of blood capillaries around the
lymphatic vessels of the pleura.)
BLOOD.
Blood consists of round cells entirely separate from one another, floating in an intercellular fluid, the plasma. The plasma also contains as a
regular and apparently important functional constituent, the blood plates
(or platelets), together with smaller granular bodies. Blood cells or corpuscles are of two sorts, (i) red corpuscles or erythrocytes, which become
charged with the chemical compound, hcemoglobin, and which lose their
nuclei as they become mature; and (2) white corpuscles or leucocytes, which
are of several kinds, all of them retaining their nuclei and containing no
haemoglobin. The redness of blood is not due to the plasma, but is an
optical effect produced by superimposed layers of the haemoglobin-filled
red corpuscles. Thin films of blood, like the individual red corpuscles
seen fresh under the microscope, are yellowish green. Blood has a characteristic odor which has been ascribed to volatile fatty acids; it has an
oily feeling associated with its viscosity, an alkaline reaction, and a specific
gravity said to average in the adult from 1.050 to 1.060.
RED CORPUSCLES. Development. The first cells in the embryonic
blood are apparently all of one sort, derived from the blood islands. They
are large, round cells with a delicate membrane and a pale granular protoplasmic reticulum; their relatively large nuclei contain a fine chromatin
network with several coarse chromatin masses. Haemoglobin later develops in their protoplasm, giving it a refractive homogeneous appearance.
Stained with orange G or eosin it is clear and brightly colored, generally
quite unlike any other portion of the specimen. Often the haemoglobin
has been more or less dissolved from the corpuscles, which then appear
granular or reticular.
BLOOD
189
The developing red blood corpuscles are known as erythroblasts, especially in their younger stages when the nuclei are reticular. In later
stages the nuclei become densely shrunken or pycnotic, and stain intensely
with haematoxylin. The entire cells become smaller, and are then called
normoblasts. The transition from an erythroblast to a normoblast is
shown in Fig. 180, a; during this process the cells divide repeatedly by
mitosis.
It will be noticed that the terms applied to developing corpuscles are compounded
of words which describe the formative cells, instead of indicating what they produce.
Thus erythroblast signifies a red formative cell. Normoblast (Lat. norma, model or
type, and Gr. ^Xcwrtfy) is an objectionable term to designate a nucleated red corpuscle
of the usual size and form, in contrast with the large megaloblasts which occur in certain
diseases of the blood. Megaloblasts have reticular nuclei and presumably represent
a younger stage than the normoblasts. A reform in the nomenclature of blood cells
based upon morphological principles, is advocated by Minot (Human Embryology,
ed. by Keibel and Mall, 1912, vol. 2), and when agreement shall have been reached
regarding the relationships of the cells, it will be possible to adopt a reasonable
terminology.
In becoming mature red corpuscles the normoblasts lose their nuclei.
Before they disappear, the pycnotic nuclei often assume mulberry, dumbbell, trefoil or other irregular shapes.
According to older observations they then
fragment, and are dissolved within the
normoblasts; but it is now generally believed that they are extruded from the
cells, either in one mass (Fig. 180, b) or
in detached portions, and that the extruded nuclei are devoured by phagocytes.
The loss of the nuclei begins in human
embryos of the second month. In embryos
of the seventh month, nucleated corpuscles
in the circulating blood have become infrequent, and after birth it is
rare to find one, except under pathological conditions.
In withdrawing from the circulating blood the nucleated red corpuscles
do not disappear from the body. Since 1868 it has been known that the red
marrow, found within certain bones in the adult, contains an abundance
of erythroblasts, which multiply by mitosis. They are the source of the
new corpuscles constantly entering the circulation. In certain diseases
of the blood, imperfectly developed normoblasts also leave the marrow,
and circulate as in the embryo. Before the marrow assumes the bloodforming function, the liver is the chief haematopoietic organ. Beginning
in embryos of about 7.5 mm., and continuing until birth, erythroblasts
are found between the hepatic cells and the endothelial cells of the sinusoids, and in certain stages they occur in vast numbers. Toward birth,
FIG. 180. THE DEVELOPMENT OF RBD
CORPUSCLES IN CAT EMBRYOS.
(Howell.)
a, Successive stages in the development
of a normoblast; b, the extrusion of
the nucleus.
i go
HISTOLOGY
however, the erythroblasts in the liver are no longer abundant, and in a
few weeks after birth they are said to disappear entirely. Red blood
corpuscles are formed also in the embryonic spleen, though to a less
extent than in the liver, and in some mammals the spleen normally
contains erythroblasts in the adult.
In regard to the source of the
erythroblasts in the spleen, liver
and red marrow, two opinions are
held. It is well known that erythroblasts and fully-formed red
corpuscles may wander out of the
vessels into connective tissue.
Accordingly it is often stated that
the circulating erythroblasts, which
at first multiply in the blood vessels, later withdraw to the reticular
tissue of the liver, spleen, and
marrow and there proliferate. Others consider that the erythroblasts
are formed in situ in these various places from the endothelial or reticular
tissue cells.
Mature Red Corpuscles. In the lower vertebrates, !the mature red
Leucocyte in
motion; at rest.
Side view of
red corpuscles.
FIG. 181. BLOOD CORPUSCLES FROM A FROG.
4, 5, and 6, Surface views of red corpuscles; 6, after
treatment with water. X 600.
FIG. 182. RED CORPUSCLES FORMING ROULEAUX. FIBRIN IN FILAMENTS RADIATES FROM THE BLOOD
PLATES. (From Da Costa's Clinical Hamatology.)
corpuscles or erythrocytes are oval nucleated bodies, more or less biconvex, thus differing radically from those of adult mammals. They are
very large in the amphibia (Fig. 181). When a drop of freshly drawn
mammalian blood is spread in a thin film on a glass slide, beneath a cover
BLOOD 191
glass, it is seen to consist chiefly of biconcave discs, and of those in the form
of shallow saucers (Fig. 182). They have a remarkable tendency to pile
up in rouleaux, like rolls of coins. It is said that discs of cork weighted
so that they will float beneath the surface of water, will come together in
a similar way if their surfaces have been coated with an oily substance.
If the blood coagulates, filaments of fibrin will be seen in the plasma,
as shown in the figure. In fresh specimens there is no fibrin, and within
the blood vessels it does not form under normal conditions. Moreover
when they are within the endothelial tubes, red corpuscles do not come
together in rouleaux. It is evident that the thin film of blood, though
very fresh, is examined under extremely artificial conditions; and from
such preparations, conclusions as to the normal shape of the corpuscles
should not be hastily drawn. Within the blood vessels the red corpuscles are typically cup-shaped.
Rindfleisch (Arch. f. mikr. Anat., 1880, vol. 17, pp. 21-42) found that the corpuscles
in guinea-pig embryos, after losing their nuclei by extrusion, are at first bell-shaped;
but he considered that afterward they become biconcave discs from impact with others
in the circulating blood. Commenting upon this statement, Howell (Journ. of Morph.,
1890, vol. 4, pp. 57-116) writes:
"I feel convinced that the bell shape which Rindfleisch ascribes to the corpuscles
which have just lost their nuclei is a mistake. The red corpuscles even of the circulation, as is well known, frequently take this shape when treated with reagents of any
kind, or even when examined without the addition of any liquid. It seems very
natural to suppose that the biconcavity of the mammalian corpuscle is directly caused
by the loss of the nucleus from its interior. Certainly as long as the corpuscles retain
their nuclei, they are more or less spherical, and after they lose their nuclei they become
biconcave."
In the year preceding Howell's publication, Dekhuyzen discussed cup-shaped corpuscles (Becherformige rote Blutkorperchen, Anat. Anz., 1899, vol. 15, pp. 206-212)
which he found as a transient stage in mammals, and which his assistant saw in blood
drawn from his finger. Dujardin (Manuel de 1'observateur au microscope, 1842)
found many corpuscles shaped "like cups, or cupules (acorn cups) with thick borders"
in blood altered by the action of phosphate of soda. The first reference to such forms
is by Leeuwenhoek (1717) who put a drop of blood in a concoction of pareira brava,
and found that most of the globules which make the blood red, have "a certain bend
or sinus receding within, as if we had a vesicle full of water and by pressure of the finger
should hollow out the middle of the vesicle as a pit or depression." Von Ebner, in
Koelliker's Handbuch (1902), writes of bell or cap-shaped corpuscles produced in
warmed blood by the thickening of the border on one surface of the disc. Weidenreich
in 1902 (Arch. f. mikr. Anat.. vol. 61, pp. 459-507) after thorough study of blood
variously preserved, and also examined while circulating in the mesentery of a rabbit,
concluded that "the red corpuscles of mammals have the form of bells (Glocken)."
Weidenreich's conclusion has not been fully accepted by Jolly, David, Jordan, and
Schafer. Schafer (in Quain's Anat., vol. 2, 1912) states that " this opinion, although
shared by F. T. Lewis, Radasch and a few other histologists, cannot be accepted, for,
of examining the circulating blood in the mesentery and other transparent parts
in mammals, it is easy to observe that, with few exceptions, the erythrocytes are
biconcave; this shape must therefore be regarded as the normal one."
I9 2 HISTOLOGY
That the shape of corpuscles in the circulating blood is not easy to observe, is shown
by the fact that scientists have described it in very different ways. The circulating
corpuscles may be seen by spreading the mesentery of an anaesthetized guinea-pig
across the condenser of a microscope, having it' preferably in a warm room, and then
placing a cover glass directly over the vessels; they are examined with an immersion
lens. Sketches made during such observations are reproduced in Fig. 183. The
upper drawing shows a vessel stretched out abnormally, and the corpuscles are correspondingly elongated; the one at the left shows the hollow of the cup toward the observer, the others are seen in lateral view. Presumably normal conditions are shown in the
lower sketch, which includes two flat corpuscles,
one of which is almost biconcave, but this form
is exceptional. The corpuscles are very flexible,
bending around any obstruction, and when free,
again assuming their original form. They roll
about as they flow through the vessels, and when,
FIG. 183. RED CORPUSCLES, SKETCHED as the blood stagnates, the current in the vessels
WHILE CIRCULATING IN THE VES- . . ji.-L.-r J . ! ,
SELS OF THE OMENTUM OF A GUINEA- is sometimes reversed, their form does not change.
In 1903, following Weidenreich's publication, the
writer demonstrated the circulating corpuscles to Professor Minot, who describes the
cup-shape as the normal form in Keibel and Mall's "Embryology"; and in 1909 they
were shown to Dr. Williams who was convinced that they are cup-shaped.
A very important result of recent studies (which Schafer does not mention) is
the recognition that in well preserved tissues of all sorts, and with all fixatives such
as are relied upon to reveal the structure of other tissues, the mammalian erythrocytes are typically cup-shaped. Other forms are exceptional. In many specimens
the corpuscles and other tissues are irregularly shrunken, but where the tissues in
general are excellently preserved, the corpuscles appear as cups. The biconcave
discs are flattened cups.
In examining films of fresh blood, the biconcave discs will be seen to
change their appearance as the objective is lowered. When sharply in
focus the thin central portion appears light (Fig. 184, A); but in high
focus the center is dark, perhaps owing to the dispersal of light by the
lenticular corpuscles. The biconcave shape is apparent when the corpuscle
is seen on edge (Fig. 184, B). The cup-shaped forms are shown in Fig.
184, D; and E represents one of the innumerable shapes due to shrinkage.
The cups may be irregularly infolded, presenting shapes which can be
imitated by indenting a soft hat. If the corpuscles are placed in water
or a dilute solution, their haemoglobin passes out and water enters, so
that they are reduced to transparent membranes or shadows (Fig. 184,
F). Such forms are often seen in clinical examinations of urine. In
dense solutions, and in fresh preparations as the plasma becomes thicker
from evaporation, water leaves the corpuscles. They then shrink,
producing spiny or nodular round masses of haemoglobin, known as
crenated corpuscles (Fig. 184, G). A 0.6 per cent, aqueous solution of
common salt is said to cause the least distortion from swelling or shrinkage.
In life the corpuscles doubtless change their shape, responding to the
BLOOD 193
variations in their haemoglobin content and in, the surrounding plasma.
Occasionally they are spherical (according to Schultze, and others), and
deviations from the primary cup-shaped form are to be expected. In
these changes the corpuscles act like membranes filled with fluid. In
the mature corpuscles, however, the outer layer is thick, blending with
the contents within; and since no sharp bounding line can be seen histologically, the corpuscles have been described as lacking membranes.
The plastic nature of the membrane is shown by heating the blood film.
The corpuscles then become globular and send out slender varicose
processes, or round knobs attached by pedicles (Fig. 184, H). These
small spheres become detached in great numbers.
The dimensions of red corpuscles are quite constant. Those in human
blood average 7.5 n in diameter and ordinarily vary from 7.2 to 7.8 /*.
They sometimes surpass these limits. In biconcave form they are about
1.6 n thick. The cups average 7 ju in diameter and are 4 n in depth.
FIG. 184. RED CORPUSCLES IN VARIOUS CONDITIONS.
Spherical corpuscles are said to be 5 n in diameter. The blood of mammals
other than man also contains cups which become discs. The latter are
oval in the camel group but round in all others. Their average diameters
are less than in man (7.3 ju in the dog, 7.48 n in the guinea-pig), but the
species of animal cannot satisfactorily be determined from the diameter
of the corpuscles. In a given section, as already noted, the red corpuscles
furnish a useful gauge for estimating the size of other structures.
The number of red corpuscles in a cubic millimeter of human blood
averages five million for men, and four million five hundred thousand for
women. By diluting a small measured quantity of blood and spreading
it over a specially ruled slide, the corpuscles may be counted, and the
number per cubic millimeter calculated. A diminished number is of
clinical importance.
Histologically the red corpuscles usually appear as homogeneous
bodies, but with special methods a granular network has been found
within them, which has been interpreted as a reaction of the haemoglobin
to reagents, and also as a persistence of the protoplasmic reticulum of
the erythroblasts. It occurs especially in newly formed corpuscles
(seen in cases of anaemia). Instead of a net, there may be rings or round
13
IQ4 HISTOLOGY
bodies, some of which have been considered to be nuclear remains. A
few coarse granules of uncertain significance are sometimes conspicuous.
The fatty exoplasmic layer which invests the corpuscle and serves as a
membrane is not sharply marked out in stained specimens; it appears to
blend with the contents of the corpuscle. Although the corpuscles
may pass out of the vessels by " diapedesis," they are not actively
motile, and their margins never present pseudopodia. The characteristics
of haemoglobin may be described as follows:
Haemoglobin is an exceedingly complex chemical substance which combines readily
with oxygen to form oxyhamoglobin. To the latter the bright color of arterial blood is
due. Venous blood becomes similarly red on exposure to air. Through the oxyhaemoglobin, oxygen is transferred from the lungs to the tissues. Haemoglobin may be dissolved from the corpuscles by mixing blood with ether, and upon evaporation it crystallizes in rhombic shapes which vary with different animals. Those from the dog are
shown in Fig. 185, 4; in man they are also chiefly prismatic. Haemoglobin is readily
4
FIG. 1 8s- 1 Haemin crystals and 3, haematoidin crystals from human blood; a, crystals of common salt
(X 560); 4, haemoglobin crystals from a dog (X 100).
decomposed into a variety of substances, some of which retain the iron which is a part
of the haemoglobin molecule, others lose it. Hcsmatoidin, considered identical with a
pigment (bilirubin) of the bile, is an iron-free substance occurring either as yellow or
brown granules, or as rhombic crystals. The crystals (Fig. 185, 3) may be found in old
blood extravasations within the body, as in the corpus luteum of the ovary. Hcemosiderin, which contains iron, appears as yellowish or brown granules sometimes extremely fine, either within or between cells. The iron may be recognized by the ferrocyanide test which makes these minute granules bright blue. If dry blood from a stain
is placed on a slide with a crystal of common salt the size of a pin-head, and both are
dissolved in a large drop of glacial acetic acid which is then heated to the boiling point,
a product of haemoglobin is formed, called hamin. It crystallizes in rhombic plates or
prisms of mahogany brown color (Fig. 185, i). Such crystals would show that a suspected stain was a blood stain, but they afford no indication of the species of animal
from which it was derived.
The duration of the life of mature red corpuscles is unknown, but is
supposed to be brief. They may be devoured intact by phagocytes, but
generally they first break into numerous small granules. These may be
ingested by certain leucocytes, or by the peculiar endothelial cells of the
liver. Their products are thought to be eliminated in part as bile pigment. The destruction of red corpuscles occurs especially in the spleen
and haemolymph glands; to a less extent in the lymph glands and red bone
marrow. Pigmented cells in some of these structures derive their pig
BLOOD 195
ment from destroyed corpuscles. Sometimes a 'stippling' or granule
formation occurs within the corpuscle, which has been ascribed to degeneration of the haemoglobin. The dissolution of red corpuscles is known
as hamolysis and follows the injection of certain poisonous substances
into the blood. It occurs in various diseases. The study of the effects
of mixing the blood of one species of animal with that of another, has provided a very perfect means of distinguishing the species from which a
blood stain of unknown origin may have been derived.
WHITE CORPUSCLES. The white corpuscles or leucocytes are thoset
blood cells which retain their nuclei and do not contain haemoglobin.!
The youngest stages of erythroblasts, according to this definition, are
leucocytes, and like other leucocytes they are derived from the mesoderm.
In 1890 Howell wrote, "Before 1869 it was quite generally believed that
the red corpuscles are formed from the white corpuscles in fact, some
of the most recent investigations favor this view, although the evidence
is so overwhelmingly against it." It is still advocated by foremost investigators of the blood, and is referred to as the " monophyletic theory."
Those who believe in diverse origins of red and white corpuscles, and of
the various forms of white corpuscles, support the "polyphyletic theory."
Maximow (Arch. f. mikr. Anat., 1909, vol. 73, pp. 444-561) states that "the first
leucocytes, the lymphocytes, arise at the same time and from the same source as the
primitive erythroblasts; the latter represent a specially differentiated form of cell, but
the lymphocytes always remain undifferentiated. Therefore, like the primitive blood
cells from which they directly proceed, they are undifferentiated rounded amoeboid
mesenchymal cells." Weidenreich (Anat. Rec., 1910, vol. 4, pp. 317-340) concludes
that "the old, original view of the unified genetic character of all blood cells proves to
be correct," and he regards the lymphocyte as the primitive or young form of white
corpuscles. (For many other references, see Minot, in Keibel and Mall's Human
Embryology, vol. 2.)
Against the monophyletic interpretation, it has been asserted that the lymphocytes
of the adult are a different form of cell from the primitive blood cells, and that they are
not found in embryos until the time when lymph glands develop. These arise rather
late in rabbits of 25 mm. and in human embryos of 40 mm. (Lewis, Anat. Rec., 1909,
vol. 3, pp. 341-353). According to the polyphyletic view, the lymphocytes are first
formed from the reticular tissue in these glands and from similar tissue elsewhere. If
this is true, it becomes unnecessary to regard the lymph glands as organs for producing
young cells, and the bone marrow as an organ for producing old cells. The relation of
these organs to blood formation will be considered in a later chapter.
The number of white corpuscles in a cubic millimeter of human blood
is about eight thousand. If it exceeds ten thousand the condition is called
leucocytosis and becomes of clinical importance. There exists, therefore,
normally but one leucocyte for five or six hundred red corpuscles. In
the circulating blood the two sorts are said not to be evenly mixed; the
leucocytes are more numerous in the slower peripheral part of the blood
stream, near the endothelium. The leucocytes may be divided into three
HISTOLOGY
ig6
classes according to their nuclear characteristics, namely, into lymphocytes,
large mononuclear leucocytes, and polymorphonuclear leucocytes.
Lymphocytes have already been briefly described with the constituents
of connective tissue (Fig. 56, p. 68). Ordinarily they are small cells,
about the size of red corpuscles, 4-7.5 M in diameter. Large ones may be
double this diameter. Their protoplasm forms a narrow rim, sometimes
almost imperceptible, about the dense round nucleus (Fig. 186, A). The
chromatin is arranged in a network associated with coarse chromatic
masses such as cause a characteristic checkered appearance. Some of the
masses rest against the nuclear
membrane. Lymphocytes are
capable of amoeboid motion but
^r not to the extent of the poly
FIG. 1 86. LEUCOCYTES AS SEEN IN A SECTION OF Hu mOrphonUClear type. They
MAN TISSUE PRESERVED WITH ZENKER'S FLUID. err t H
A, Lymphocyte; B, large mononuclear leucocyte; C, form trom 22 tO 25% OI all
three polymorphonuclear neutrophiles. ,
leucocytes.
Large mononuclear leucocytes, sometimes 20 M in diameter, form only
from i to 3% of the leucocytes. They possess round, oval, slightly
indented, or crescentic nuclei, which are vesicular and usually eccentric
in position. Their chromatin occurs in a few large granules; as a whole
the nucleus is clear and pale (Fig. 186, B). The protoplasm, which is
abundant, usually lacks coarse granules or other distinctive features.
Sometimes it contains a few deeply staining granules as shown in one of
the cells in Fig. 187, II. The large mononuclear leucocytes are notably
phagocytic. In certain respects they are intermediate between lymphocytes and polymorphonuclear cells, and they were formerly known as
"transitional cells." Apparently, however, they are derived directly
from the modified endothelial cells lining the sinuses of the lymph glands,
and they have sometimes been regarded as the youngest of the forms of
cells shown in Fig. 186.
Polymorphonuclear leucocytes are cells somewhat larger than red corpuscles, being from 7.5 to 10 /* in diameter. They are characterized
by having nuclei with irregular constrictions leading to an endless variety
of shapes (Fig. 186, C). The nodular subdivisions may be connected by
broad bands or by slender filaments. It is said that in degenerating cells
the nucleus becomes divided into several separate masses. Such forms
can properly be called "polynuclear," an abbreviated term which is a
misnomer as applied to the ordinary cells; "mononuclear" as designating
the preceding types is also unfortunate since it implies that others have
several nuclei. The irregular shape of the polymorphous nuclei has been
ascribed to degenerative changes, comparable to those seen in the erythroblast nuclei. Within the concavity of the nucleus the centrosome may
be found, surrounded by a light area; usually it occurs as a diplosome.
BLOOD 197
(In the forms of corpuscles with round nuclei eccentrically placed, the
centrosome is on the side where the protoplasm is most abundant.)
The polymorphonuclear leucocytes are actively amoeboid, and particles
readily pass through their superficial layer, but like other forms of leucocytes they are covered with a very delicate cell membrane.
Max Schultze in the first paper published in the Archiv fur mikroskopische Anatomic (1865, vol. i, pp. 1-42) described an apparatus for the examination of microscopic
specimens at the body temperature, which he used in studying human blood. He observed the active creeping movements of the leucocytes, closely similar to those of the
most delicate amoebae, and watched them take up particles of carmine and other dyes
placed in a drop of fresh blood. "The act of ingestion," as he describes it, "is accompanied by no striking maneuver." He adds that he has never seen special processes
sent out to overcome foreign bodies, but that the creeping corpuscle, during its uniform
advance, passes over them and presses them into its substance. He diluted the blood
with two-thirds of its volume of fresh cow's milk, and observed that the leucocytes
moved with the same rapidity as before, and ingested the oil globules which are much
larger than the pulverized dye-stuff.
A fundamental characteristic of polymorphonuclear leucocytes is the j
development of distinct granules in their protoplasm. They can be seen
in fresh unstained specimens, in which it is evident that some of the cells
contain coarse granules, and others fine granules. The lymphocytes
and the large mononuclear leucocytes contain neither sort, and are therefore described as non-granular. In order to study the granules a drop
of blood is spread thinly over a cover glass and dried, afterward being
stained with a "blood stain," which is a carefully prepared mixture of
acid and basic dyes. The details of nuclear structure are not preserved by
this method, but the granules are clearly differentiated (Fig. 187). With
several of the blood stains the fine granules are colored purple or lilac;
and the coarse granules are found to be of two sorts, one kind staining
red with eosin, and the other blue with the basic dye. Only one sort of
granule occurs in a single cell.
Leucocytes containing coarse blue granules, which often obscure the
nucleus, are called mast cells. In order to distinguish between them and
the mast cells of connective tissue, which contain similar granules (see
Fig- 55> P- 68) those in the blood are often called mast leucocytes. They
form only 0.5% of the leucocytes, and in sections special methods are
required to demonstrate them. These cells have recently been interpreted as degenerating forms, but their significance has not been
fully established.
Leucocytes with coarse granules which stain red with eosin, an acid
stain, are called eosinophiles (sometimes oxyphiles, or acidophiles).
They constitute from 2 to 4% of the leucocytes in the blood. Eosinophilic cells, apparently distinct from those of the blood, occur also in
connective tissue, and since their granules are preserved by ordinary
198
HISTOLOGY
methods, and eosin is a dye used in routine examinations, these cells are
often seen. According to Weidenreich the eosinophilic granules are minute fragments of red corpuscles, or products of their degeneration, which
have been ingested. Badertscher (Amer. Journ. Anat., 1913, vol. 15,
pp. 69-86) finds that eosinophiles are very numerous in the vicinity of
the degenerating muscle fibers in salamanders, during the time when
their gills atrophy. He agrees with Weidenreich that the eosinophilic
granules are not products of protoplasmic activity but are derived from
JI.
FIG. 187. THE BLOOD CORPUSCLES. (WRIGHT'S STAIN.) (E. F. Faber, from Da Costa's
Clinical Hsematology.)
I, Red corpuscles, n, Lymphocytes and large mononuclear leucocytes, m, Neutrophiles.
IV, Eosinophiles. V, Myelocytes (not found in normal blood). VI, Mast cells.
material outside of the cells; and he likewise finds that they are taken up
by lymphocytes which thus become eosinophiles. Badertscher's work is
of interest in connection with cases of trichiniasis in man, in which the
number of eosinophiles in the blood becomes greatly increased, and at the
same time there is extensive degeneration of the muscles, caused by the
parasites. There is, therefore, reason to believe that esinophilic granules
are haemoglobin derivatives, but, as stated by Minot, "renewed investigation of the eosinophiles in man is very desirable."
BLOOD 199
The third type of granular cell, unlike the eosinophiles and mast cells,
contains fine granules, and these stain purple or lilac by taking both acid
and basic stains simultaneously. They are called neutrophiles, and form
between 70 and 72% of the leucocytes in the blood. They are actively
amoeboid and are the principal wandering cells of the body, leaving the
blood vessels more readily than other forms. In suppurative processes
they accumulate around the centers of infection, and they are of very
great clinical importance.
SUMMARY OF THE FORMS OF LEUCOCYTES.
Lymphocytes, 22 to 25% of the leucocytes, are small (about the. size
of a red corpuscle) or large (perhaps twice the diameter of a red corpuscle),
non-granular, with round checkered nuclei.
Large mononuclear leucocytes, i to 3%, may be two or three times the
diameter of red corpuscles. They are non-granular, or with few granules,
and have pale vesicular nuclei, round or crescentic.
Polymorphonuclear leucocytes, larger than red corpuscles, are granular, with nuclei variously constricted or bent. They include
Mast cells, 0.5%, with very coarse basophilic granules obscuring
the nucleus.
Eosinophiles, 2 to 4%, with coarse eosinophilic granules.
Neutrophiles, 70 to 72%, with fine neutrophilic granules.
Blood plates (Fig. 188) are small granular bodies (Kb'rnchenplaques)
which were recognized as a normal constituent of the blood by Schultze
in 1865. Previous references to them occur, and Zimmermann described them as "elementary corpuscles," believing that they gave rise to red corpuscles (Arch. f.
path. Anat., 1860, vol. 18, pp. 221-242). They are 2-4 n
in diameter, and between 245,000 and 778,000 have been
estimated to occur in a cubic millimeter of human blood.
They are readily reduced to granular debris in ordinary
PUSCLE.
sections, but when well preserved and properly stained,
they are found to consist of a central granular core and a hyaline outer
layer. Often they appear stellate, and on a warm stage they exhibit
amoeboid movements. They are concerned in the clotting of the blood,
or thrombus formation, and during coagulation threads of fibrin extend
out from them as seen in Fig. 182. It is possible, however, that they are
only passively involved in this process. In the amphibia certain small
spindle-shaped cells appear to be similarly related to fibrin-formation,
and they are called thrombocytes; the same term is sometimes applied to
the blood plates. In blood clots several days old, blood plates are still
found, indicating that they have more than a transient existence.
200 HISTOLOGY
The source of the blood plates has been known to American histologists
for several years, since they have had the opportunity of examining preparations made by J. H. Wright and described by him in 1906. The
specimen shown in Fig. 189 is one of several which were entrusted to the
writer for demonstration at the meeting of the American Association of
Anatomists in 1906; figures of them are reproduced in color in the
Journal of Morphology (1910, vol. 21, pp. 265-278). Fig. 189 represents
a giant cell of the bone marrow, sending out two processes or pseudopodia into a blood vessel; the endothelium is interrupted at their place
of entrance. By the special stain which Dr. Wright perfected, the central
and large part of the cytoplasm of the giant cells is seen to consist of red
or violet granules, identical in form and color with the granules in the
center of the blood plates. Moreover the giant cells are shown to have
a clear blue exoplasmic layer, which sends out slender processes, and
this exoplasm also is identical in structure with that of the blood plates.
Some of the blood plates are free in the vessels; others in rows or clumps
are still connected with the giant cells. Fig. 189 shows a few detached
FIG. 189. GIANT CELL FROM THE BONE MARROW OF A KITTEN. SHOWING PSEUDOPODIA EXTENDING INTO
A BLOOD VESSEL (V). AND GIVING RISE TO BLOOD PLATES (bp). (J. H. Wright.)
plates, and one which is budding off from a pseudopodium, but the colorcontrasts which make these preparations convincing are scarcely indicated.
Through Wright's investigations it has been made clear that blood plates
are detached portions of the cytoplasm of the giant cells in the bone
marrow, and of similar giant cells in the spleen; their granular center is
endoplasm, and their hyaline border is exoplasm.
According to Schafer (1912) Wright's "suggestion" seems improbable; and the
blood plates may be looked upon as minute cells. Others also have regarded the granular endoplasm as a nuclear structure. The blood plates are still described by many
writers as fragments of disintegrating white corpuscles, or fragmenting nuclei of red
corpuscles; and Stohr records that their origin is obscure.
BLOOD 2OI
Plasma is the fluid intercellular substance of the blood. It contains
various granules, some of which are small fat drops received from the
thoracic duct. Others occurring in variable quantity are refractive particles, not fatty, either round or elongated; they are known as haematoconia (or hsemoconia). In ordinary sections the plasma appears as a
granular coagulum. In the process of clotting, fibrin forms from the
plasma, and with the entangled corpuscles, it constitutes the blood-clot;
the fluid which remains is the serum. The process of fibrin formation
is of considerable histological interest, owing to a possible analogy with
fibril formation in connective tissue.
LYMPH.
The contents of the lymphatic vessels is called lymph. This fluid
is not identical with plasma, or with tissue fluid, yet all three are similar.
Nutrient material passes from the plasma into the tissue fluid and
thence to epithelial cells; and in return the products of epithelial cells
enter the tissue fluid from which they may be taken over either into
the plasma or lymph, first passing through the endothelial walls of the
vessels. Thus in the intestine much of the absorbed fat is transferred
across the tissue spaces to the lymphatic vessels (lacteals) within which
it forms a milky emulsion known as chyle. This form of lymph mingles
with other varieties coming from the various parts of the body, and
together they are poured into the plasma at the jugulo-subclavian junction.
Histologically lymph appears as a fine coagulum, containing lymphocytes
and large mononuclear phagocytic cells. The cells are not abundant.
Occasionally other forms of blood corpuscles are found in lymphatic
vessels, but the lymphocytes greatly predominate.
III. SPECIAL HISTOLOGY.
BLOOD FORMING AND BLOOD DESTROYING ORGANS.
BONE MARROW.
Bone marrow is the soft tissue found within the central cavities of
bones. Its source in the embryo is the vascular mesenchyma invading a
cartilage which is being replaced by bone. Early in its development it
contains osteoblasts and osteoclasts, and these cells may be found in adult
marrow, where it is in contact with the bone. The greater part of the
mesenchyma becomes reticular tissue with fat cells intermingled. The
meshes of the reticular tissue are occupied by an extraordinary variety of
cells, most of which are called myelocytes (marrow cells). In ordinary
sections the tissue of the marrow appears
to be riddled with large round holes.
Under high magnification the holes are
seen to be fat cells, the nuclei of which
are here and there included in the section (Fig. 190.) The reticular tissue
framework of the marrow consists of flattened cells, generally seen cut across; their
nuclei then appear slender and elongated.
The abundant meshwork of fibrils associated with these cells is not apparent in
FIG. 190. HUMAN BONE MARROW. ordinary sections. In the meshes are
e., Eosinophilic myelocyte; e-b., erythro- .
wast; e-c., erythrocyte; f. c., part of found giant cells; premyelocytes; myelocytes
the protoplasmic rim of a fat cell; g. c., J J J
'
r.. reticular tissueceii.
Wm ' Ch arC UtrOphtiic, bdSOpktiic Or eOSlHO
phUi C ; erythrocytes ; lymphocytes; and mature corpuscles both red and white.
The giant cells of the marrow have a single polymorphous nucleus.
They have therefore been named "megakaryocytes," in distinction from
the multinucleate osteoclasts or "polykaryocytes." The nucleus is so
large that it may be cut into several slices, and by combining these it has
been found that the entire nucleus is a hollow sphere with perforated walls;
the nuclei, however, are very irregular, and some may be of other forms.
With Wright's stain the protoplasm clearly shows an outer hyaline exoplasm and an inner granular endoplasm. It has been said that the latter
is divisible into two concentric zones, which differ from the protoplasm
within the nuclear sphere. In ordinary preparations these details are
202
BOXE MARROW
203
not evident (Fig. 191). A large number of centrosome granules (over
one hundred) have been found, and pluripolar mitoses have been observed.
A phagocytic function has been ascribed to these giant cells, but it has
also been denied. Their origin is unknown, but is said to be from the
leucocyte series of cells. Their important function of producing blood
plates has already been described (p. 200).
Premyelocytes are cells with large round vesicular nuclei containing
one or two coarse chromatin masses, and surrounded by basic protoplasm
free from specific granules (Figs. 190 and 191). These cells are parents of
Neutrophile. Lymphocytes. Giant cell.
Premyelocytes.
Mast cell. Erythroblast. Border of a fat cell.
FIG. 191. ELEMENTS OF HUMAN BONE MARROW.
A, From the femur at 10 years; B, from a cervical vertebra at 19 years; C, from the femur at 77 years;
D, from a rib at 59 years.
myelocytes, and are sometimes called "myeloblasts" a poor term, since
they do not produce marrow. Stohr refers to those in Fig. 191 as "plasma
cells"; others describe them as primitive wandering cells. Apparently
they are set free from the reticular tissue and they may produce not only
myelocytes but also erythroblasts.
Myelocytes are cells larger than polymorphonuclear leucocytes, having
round or crescentic nuclei and protoplasm containing a varying quantity
of specific granules, either neutrophilic, basophilic, or eosinophilic.
The young cells have round nuclei and few granules. The oldest become
the granular leucocytes ready to enter the blood vessels. Several generations, derived by mitosis, intervene between the young myelocytes and the
mature leucocytes. Most of the myelocytes are finely granular and neu
204 HISTOLOGY
trophih'c. Some are coarsely granular and eosinophilic; others contain
the basophilic mast cell granules, but these are not well preserved in ordinary specimens. In certain diseases myelocytes enter the circulating
blood, and they appear in smears as shown in Fig. 187, p. 198.
Erythroblasts are generally found in clusters, some being young with
vesicular nuclei, others being normoblasts with dense irregular nuclei, such
as have already been described. Rarely a nucleus may be found which
apparently is partly extruded. Cup-shaped corpuscles are seen in the
tissue meshes.
Lymphocytes are not a conspicuous element of the marrow, yet they
are present and sometimes in disease become abundant.
The relations of the blood vessels to the reticular tissue are of great
interest. It has been thought that the endothelium blends with the reticulum so that no sharp distinction can be made between the two. It
seems more probable that the endothelium is merely more permeable
than usual, by a freer separation of its cells. The same problem is presented by the blood vessels and reticular tissue of the lymph glands and
spleen.
The functions of the marrow are the production and dissolution of
bone, the storing of fat, the formation of granular leucocytes (neutrophiles,
eosinophiles, and mast cells), of red corpuscles, and to a less extent of
lymphocytes; to these some would add the destruction of red corpuscles,
as indicated by ingested fragments and intercellular granules.
LYMPH NODULES AND LYMPH GLANDS.
The lymph glands arise as nodules of dense tissue in close relation
with an artery, a vein and a lymphatic vessel, as seen in the photographs,
Figs. 192 and 193. The first distinct lymph glands in the body are a pair
in the axillary region, a pair in the iliac region, and a pair or two in the
maxillary region. They are found in rabbit embryos of about 30 mm.,
and in human embryos of about 40 mm. These first glands are soon followed by others in their vicinity, producing axillary, inguinal and cervical
groups, respectively; and scattered glands more peripherally situated
along the vessels develop later. At the same time, the tissue around the
jugular and mesenteric lymph sacs becomes transformed into dense lymphoid tissue, which is resolved into the chains of deep lymphatic glands.
These acquire a structure similar to that of the superficial glands. There
is no satisfactory evidence that the dense lymphoid tissue of which the
glands are composed is produced by the emigration of cells from either the
arteries, veins or lymphatics associated with them.
In further development the lymph glands become organized as shown
in the diagrams, Figs. 194 and 195. The left half of each diagram repre
LYMNPH GLADS
20 5
sents a younger stage than the right half. These instructive figures were
prepared by Stohr on the basis of Kling's studies (Arch. f. mikr. Anat.,
1904, vol. 63, pp. 575-610). In the youngest stage (Fig. 194) it is seen
that the blood vessels enter and leave the gland on one side, at a place
called the hilus (Lat. hilum, a small thing, applied to the eye of a bean,
and to similar hollows in bean-shaped organs). The lymphatic vessel,
as a plexiform peripheral sinus, encircles the entire structure. After
the gland has enlarged, lymphatic vessels extend into the mass of lymphoid
tissue, as shown on the right of Fig. 194, and eventually they pass clear
FIG. 192. THB FIRST AXILLARY LYMPH GLAND OF FIG. 193. ONE OF THE EARLIEST CERVICAL LYMPH
THE RABBIT. FROM AN EMBRYO OF TWENTY GLANDS. FROM A HUMAN EMBRYO OF 42
DAYS. 29 MM. X 60. MM. X 60.
a, Artery; g, lymph gland; I, lymphatic vessel; v, vein.
through it in a system of anastomosing sinuses. The lymph then flows
into the gland from the periphery, and out at the hilus; both the afferent
and efferent vessels are shown in Fig. 195. Finally a connective tissue
capsule develops around the larger glands, and in some of them it extends
into the interior, producing a system of supporting trabecula, either round
or lamellar. These may unite with one another as shown on the right of
Fig. 195. When present within the gland they are always found in the
central axes of the lymph sinuses.
By the production of the internal lymph sinuses, the substance of the
gland is subdivided into rounded nodules and elongated cords of lymphoid
tissue. The nodules are found at the periphery of the gland and collectively they form its cortex; the cords constitute the medulla. Several
other organs, e.g., the kidney and suprarenal glands, are divided into an
outer cortex (bark) and an inner medulla (pith). In the center of each
cortical nodule there is often a light spot, seen with low power, which
constitutes the germinal center. These general features of a lymph
gland are shown in Fig. 196. It is evident that certain of the secondary
206
HISTOLOGY
nodules in the cortex are imperfectly separated from one another, and that
they are continuous below with the anastomosing medullary cords.
The lymph glands of the adult (lymp ho glandules, also called lymph nodes)
are round or reniform structures varying in length from a few millimeters
Afferent lymphatic vessels.
Peripheral
lymph sinus.
Capsule.
Lymphoid tissue
Lymph sinus.
/ Blood vessels.
Lymphatic vessel. Lymphatic vessel.
FIG. 194.
Afferent lymphatic vessels.
Lymph sinus.
Capsule.
Trabecula.
Efferent lymphatic vessels.
FIG. 195. DIAGRAMS REPRESENTING FOUR STAGES IN THE DEVELOPMENT OF LYMPH GLANDS.
to a few centimeters. The largest of them show trabeculae and are subdivided into cortex and medulla as above described; the small ones remain
permanently in the various developmental stages shown in Figs. 194 and
195. The smallest structures consist of but a single nodule, with or without a germinal center; it contains a simple capillary network in its interior,
LYMPH GLANDS
207
and a lymphatic plexus over its surface. Such solitary nodules occur in
the mucous membranes of various organs. By contact with one another
laterally they constitute the noduli aggregati, or "Peyer's patches" of
the small intestine, which are macroscopic structures 1-5 cm. long.
Lymphoid nodules irregularly massed about epithelial pits become the
essential tissue of the tonsils. Wherever it occurs, lymphoid tissue has
essentially the same structure as that observed in the lymph glands.
Capsule.'
Trabeculae.
FIG. 196. LONGITUDINAL SECTION OF A HUMAN CERVICAL LYMPH GLAND. Xia.
Lymphoid tissue (formerly called adenoid tissue) consists of a framework of reticular tissue (see Fig. 50, p. 61, and the accompanying description), together with detached cells, chiefly lymphocytes, which fill
its meshes. Eosinophiles and the various forms of blood corpuscles
brought in by the blood vessels, are present in small numbers. The lymphocytes are like those of the blood, and the lymph glands are centers for
their production. Stained with haematoxylin, lympboid tissue, because
208
HISTOLOGY
Lymph
sinuses.
of the preponderance of nuclear material, is very dark, and its appearance
even under low magnification is quite characteristic; it is shown in the
medullary cords in Fig. 197, which illustrates also its relation to the
lymph sinuses.
The lymph sinuses are not well-defined endothelial tubes, but appear
rather as washed-out portions of the reticular tissue. If the endothelial
tubes which line the lymphatic vessels enter the lymph gland to form the
sinuses, it must be considered that their cells separate and that strands
of reticular tissue pass across them. Some authorities consider that the
endothelial tissue blends freely with the reticular tissue, so that any
distinction is here arbitrary. The reticular tissue cells, or endothelial
cells, lining the sinuses are highly phagocytic, and ingested fragments
may be seen within them in sections. Certain of these cells become detached, and there is reason to
believe that they are the source
of the large mononuclear leucocytes. Lymphocytes from the
adjacent cords and nodules
also enter the lymph as it
passes through the sinuses,
and thus they are added to
the circulation. Within the
cords and nodules they are
enclosed in a closer meshed
reticulum than that of the
sinuses, which may prevent
them from escaping too freely.
The germinal centers contain cells with larger and paler
nuclei than those of lymphocytes. These central cells
resemble premyelocytes, and
they are supposed to give rise to lymphocytes. Mitotic figures are abundant. The germinal centers, however, are not found in certain nodules,
and they are absent from the medullary cords. This has been explained
as due to the slower and more scattered multiplication of cells in those
places, but the germinal centers are absent also from the early stages of
embryonic glands. Presumably they are not adequately explained by
stating that they are centers for lymphocyte production.
The capsules of the lymph glands consist of fibrous connective tissue,
containing elastic elements which increase in abundance with age.
Smooth muscle fibers are present as scattered cells or as slender bundles.
The trabeculae, which are extensions of the capsule, are composed of the
FIG. 197. FROM THE MEDULLA OF A LYMPH GLAND OF AN
Ox. X 240.
LYMPH GLANDS
same tissues. They are completely surrounded by the lymph sinuses as
shown in Fig. 197. The flat cells over their surfaces may be regarded as
endothelial cells.
The blood vessels of a lymph gland enter chiefly at the hilus, but in
the larger glands some of them come in from the periphery and run in
the trabeculae; others however pass out through the trabeculae into the
capsule. The principal artery enters at the hilus and divides at once
into several branches, which travel in the trabeculae for a short distance,
and then pass over into the medullary cords. They extend through the
axes of the cords into the 'nodules, giving off small branches which form
a venous network at the periphery of these structures. The veins which
drain this network soon cross the sinuses and enter the trabeculae, in
which they travel toward the hilus alongside the arteries (Calvert, Anat.
Anz., 1897, vol. 13, pp. 174-180). A central artery surrounded by lymphoid tissue and drained by peripheral veins is found not only in lymph
glands, but also in the spleen.
Nerves to the lymph glands are not abundant. They consist of medullated and non-medullated fibers, which form plexuses about the blood
vessels, and supply the muscle cells in the capsule and trabeculae. They
have not been found in the nodules and cords.
The function of the lymph glands is not only to produce lymphocytes
which enter the lymphatic vessels and are conveyed through the thoracic
duct into the blood, but also to "filter the lymph." If certain poisonous
substances, inert particles, or bacteria are brought to the gland in the
lymph, they may be removed by the phagocytic endothelial or reticular
tissue cells. The gland at the same time may become enlarged by congestion, and by multiplication of its cells.
H^MOLYMPH GLANDS.
Haemolymph glands resemble small lymph glands, ranging in size
from a "pin-head to an almond." They occur especially in the retroperitoneal tissue near the origin of the superior mesenteric and renal arteries, but are found elsewhere, and it has been said that their distribution
coincides with that of ordinary lymph glands. They are darker than the
lymph glands, and on section yield blood in place of lymph. No lymphatic vessels are associated with them, when typically developed, and instead of a lymph sinus they possess a similar structure filled with blood,
the blood sinus. The lymphoid tissue with its blood supply, together with
the capsule and trabeculae, are like the corresponding structures in lymph
glands. The capillary blood vessels, however, are readily permeable,
so that their contents, both plasma and corpuscles, escape into the blood
sinus. The haemolymph gland is therefore a "blood filter." Many
2IO
HISTOLOGY
blood corpuscles fragment in passing through it, and are removed from the
circulation by phagocytic cells, which in consequence become pigmented.
The eosinophilic cells which are found in haemolymph glands have been
explained as due to the ingestion of haemoglobin products, but it has
been questioned whether these cells are more abundant than in ordinary
lymph glands. A second function of the haemolymph glands, depending
upon the lymphoid tissue around their arteries, is the production of
lymphocytes which may enter the blood vessels directly.
According to von Schumacher (Arch. f. mikr. Anat., 1912, vol. 81, pp. 92-150) the
haemolymph glands begin their development like ordinary lymph glands, but after the
formation of the peripheral sinus, the connections with afferent and efferent lymphatic
vessels are lost. He finds various intermediate forms between the lymph and haemolymph glands, depending upon the extent of atrophy of the lymphatic connections, and
the extent to which blood escapes from the intraglandular vessels. After accidents
accompanied by extravasations of blood, the sinuses of ordinary lymph glands may become filled with red corpuscles, conveyed to them by the afferent lymphatic vessels.
Such glands differ obviously from the true haemolymph glands, which structurally and
functionally are intermediate between lymph glands and the spleen.
SPLEEN.
The spleen, being five or six inches long and four inches wide, is much
the largest organ of the lymph gland series. It is the first of them to develop, appearing in rabbits of 14 days (10 mm.) as a condensation of the
mesenchyma in the dorsal mesentery of the stomach. At this stage the
art
FIG. 198. DIAGRAM OF A HAMOLYMPH GLAND. A; AND OF A PART OF THE SPLEEN, B.
The arteries are shown as slender lines (art.) and the veins as heavy ones (v.); c., capsule; b. s., blood
sinus, corresponding with the splenic pulp, p.; s . n., secondary nodule; sp. n., splenic nodule; tr.,
trabecula.
only lymphatic vessels in the embryo are those near the jugular vein.
Lymph glands are not indicated until six days later. The blood vessels
enter the spleen at its hilus and branch freely. In early stages they form
an ordinary capillary plexus, but subsequently their walls become so pervious that most of the blood escapes into the reticular tissue in passing
SPLEEN
211
from the artery to the vein. Surrounding the arterial branches there
is a zone of lymphoid tissue, which arises rather late in embryonic life.
In reptilian spleens it is so abundantly developed that the organs resemble
mammalian haemolymph glands. In the guinea-pig the lymphoid
sheath of the arteries is continuous, though narrow; in man it is so interrupted as to form a succession of spindle-shaped or spherical masses,
called splenic nodules (Malpighian corpuscles). An arterial branch
passes through each nodule. Thus, as compared with the haemolymph
Terminal vein
[Sheathed artery. Pulpartery.
Pulp vein.
Beginning of a
trabecular vein.
Capillaries of
a nodule.
Trabecula.
Penicillus.
, , Splenic
V 1 /obule.
Hilus. Reticulum. Splenic noudle.
Capsule.
FIG. 199. DIAGRAM OF THB BLOOD VESSELS OF THB HUMAN SPLEEN.
At x is shown the direct connection of terminal arteries with terminal veins (the existence of such a connection has been questioned). At xx and xxx are the free endings of the terminal veins in the pulp and
near the nodules respectively.
gland, the spleen is deficient in lymphoid tissue (Fig. 198). The bulk
of the spleen is composed of splenic pulp, which corresponds with the
blood sinus of the haemolymph glands. Its framework of reticular tissue
is continuous with that of the nodules, and it contains blood corpuscles
of all sorts, special phagocytic cells known as splenic cells, and the terminal
branches of both arteries and veins. There are no lymphatic vessels
within the spleen. The capsule and trabecular framework are highly
developed as in the largest lymph glands. The following features of the
spleen may be described in turn the blood vessels, the pulp, the nodules,
the capsule and trabeculae, and finally the nerves.
As shown in the diagram, Fig. 199, the splenic artery enters at the
hilus and, accompanied by veins, its branches are found in the largest
trabeculae. When about 0.2 mm. in diameter the arteries leave the trabec
212 HISTOLOGY
ulae, in which the veins continue further. The arteries, however, are still
surrounded by a considerable connective tissue layer, the outer portion of
which becomes reticular and is filled with the lymphocytes of the nodules.
The nodules occur near where the artery branches. Small arterial twigs
ramify in the nodules, in the periphery of which they anastomose before
passing into the pulp. When the main stems are about 15 n in diameter,
they lose their surrounding lymphoid layer and pass into the pulp, where
they form brush-like groups of branches (penicilli). These branches do
not anastomose. For a short distance before their termination the walls
of the branches possess ellipsoid thickenings, due to a longitudinal arrangement of closely applied fibers of reticular tissue. These "sheathed
arteries" are 6-8 M in diameter, and have been supposed to regulate the amount
of blood which enters the
terminal portion of the artery,
beyond them. Some authorities state that this distal
part connects with the terminal veins, meeting them
FIG. 200. CROSS SECTION (A) AND SURFACE VIEW (B) OF
TERMINAL VEINS FROM THE HUMAN SPLEEN a |- an acilt e angle. AcCOrd
e., Rod shaped endothelial cells, with projecting nuclei, n ; I., .
encircling reticular tissue; L, leucocytes passing between jngr tO Others SUCh COnnCC
the endothelial cells. (After Weidenreich.)
tions are infrequent, and still
others believe that the arteries empty only into the reticular tissue.
Numerous careful injections have shown the readiness with which the
arterial blood mingles with the pulp cells.
The terminal veins or splenic sinuses begin as dilated structures (sometimes unfortunately called "ampullae," the latter term being applied
also to the terminal arteries). Their endothelial cells are so long and
slender as to suggest smooth muscle fibers, and like certain other endothelial cells they are contractile. Their edges are not closely approximated,
so that corpuscles may pass between them freely (Fig. 200) . Around them
are encircling reticular tissue fibers, and a continuous basement membrane
has been described as stretching across the intervals between the endothelial
cells. The existence of such a membrane has recently been denied. The
endothelial cells project into the lumen of the vessel, and their nuclei are
at the summits of the elevations. Frequently the nuclei show one or
two longitudinal rod-like markings, said to be due to folds in the nuclear
membrane (Fig. 200, B) Several terminal veins unite to form a pulp vein,
which enters a trabecula in which it passes toward the hilus. The trabecular veins join to form the splenic vein.
For further details regarding the circulation see Weidenreich (Arch. f. mikr. Anat.,
1901, vol. 58, pp. 247-376) and Mall (Amer. Journ. Anat., 1903, vol. 2, pp. 315-332).
SPLEEN 213
The splenic pulp consists of a reticular tissue framework (Fig. 50,
p. 61). It supports the terminal arteries and veins, and in its meshes
are the white and red corpuscles passing between them.
The pulp appears as a diffuse mass of cells infiltrated with red corpuscles, and since the vessels within it are thin-walled and hard to follow,
likewise containing corpuscles, it is often impossible in ordinary sections
to determine which cells are inside and which are outside of the vessels
(Fig. 201). The nodules are not sharply separated from the pulp, so
that lymphocytes are abundant in their vicinity. These lymphocytes
enter the terminal veins and thus are removed from the spleen. In the
splenic vein the proportion of lymphocytes to red corpuscles is said to be
seventy times as great as in the splenic artery. One for every four red
Capsule.
Pulp.~| |f7 Trabeculae.
Spindle-shaped
nodule.
Sheathed artery.
Central arteries in
splenic nodules.
FIG. 201. PART OF A SECTION OF THE SPLEEN FROM AN ADULT MAN. X 15
corpuscles has been reported by two investigators, but later estimates are
lower. It seems evident that lymphocyte production is an important
function of the spleen. Another is the filtration of the blood passing
through the pulp. As in haemolymph glands, granular debris is found,
and there are phagocytic, pigmented, and eosinophilic cells. The phagocytes are cells with large round nuclei and considerable protoplasm. They
vary in size, but the small forms are most numerous; these are called splenic
cells. Some are described as multinucleate. Erythroblasts are not found
in the normal adult human spleen; they occur, however, in certain blood
diseases, and are normal in some adult mammals, as in the skunk. They
are abundant in the spleens of human embryos. Giant cells are numerous
in the spleens of young animals but are seldom found in the human adult.
214
HISTOLOGY
They are described as megakaryocytes, and are like those in bone marrow.
The formation of granular leucocytes, which has been asserted, presumably does not occur.
The splenic nodules are quite like the secondary nodules of lymph
glands. They consist of a reticular tissue framework continuous with
that of the pulp, but having coarser meshes. Fine elastic fibers are associated with it. It contains lymphocytes, and near the central arteries
**._ Surface blackened
by precipitate of
silver.
Nerve branches
tor the arterial
wall.
~ -- Nerves of the pulp.
\
Small nerve . .
bundle.
Branches for the , ,
arterial wall..'-' '
Fie. 202. GOLGI PREPARATION OF THE!SPLEEN OF A MOUSE. X 85.
The boundary between the splenic pulp and the lymphoid tissue is indicated by a dotted line.
The nerves are chiefly in the wall of an artery.
germinal centers are sometimes distinct. The nodules have been regarded
as varying in shape from time to time, being but transient accumulations
of lymphocytes.
The capsule of the spleen is divided into two layers. The outer is
the tunica serosa and the inner, the tunica albuginea. The serosa consists of the peritoneal mesothelium, which covers the spleen except at
the hilus, and of the underlying connective tissue. The albuginea is a
dense layer of connective tissue, containing elastic networks and smooth
muscle fibers. Similar tissue is found in the trabeculae. The muscle
SPLEEN 215
elements are less numerous in the human spleen than in those of many
animals. By contraction they force blood from the pulp and cause the
circulation to follow more definite channels. When they are paralyzed,
the pulp becomes filled with the blood corpuscles.
The nerves of the spleen, from the right vagus and the cceliac sympathetic plexus, are medullated and non-medullated fibers, chiefly the latter.
They form plexuses around the blood vessels (Fig. 202) and send fibers
into the pulp. Besides supplying the muscles of the vessels and trabeculae, (
some of them are thought to have free sensory endings. Lymphatic!
vessels are said to occur in the capsule and trabeculae, but not in the pulp
or nodules of the spleen.
The spleen is a large organ, without obvious subdivisions. On its surface, when
fresh, there is a mottled effect due to areas bounded more or less definitely by trabeculae. Such areas, about i mm. in diameter, have been described by Mall as ' 'lobules,"
and he states that they "can easily be seen on the surface of the organ or in sections."
A lobule, as he describes it, has a central artery, and its base is where the lymphoid
sheath of the artery terminates. It has peripheral veins, often three, enclosed in the
trabeculae. A lobule is composed of some ten structural (or histological) units, imperfectly separated from one another by branches of the trabeculae. Each unit contains a
central terminal artery (branches of the lobular artery) and has peripheral veins
(branches of those about the lobule). Apparently, therefore, the lobules shown in the
diagram, Fig. 199, except along its lower border, represent groups or pairs of Mall's
lobules. Stohr notes that "a division into lobules in the interior of the spleen is impossible." The arrangement of lobules at the periphery suggests an ill-defined cortex.
Lobes have also been described, corresponding with the main branches of the splenic
artery, but the lobes are not generally recognized. The spleen may present inconstant
subdivisions, which sometimes produce detached portions called accessory spleens.
THE ENTODERMAL TRACT.
DEVELOPMENT OF THE MOUTH AND PHARYNX.
In a previous section the early development of the fore-gut or pharyngeal pocket of entoderm has been described and illustrated (Figs. 27 and
28). This fore-gut of the young embryo is to produce the pharynx,
oesophagus, and stomach of the adult. Its anterior extremity encounters
the ectoderm at the bottom of a depression. The ectoderm and entoderm
there fuse to make the oral plate (Fig. 203) , which becomes thin, ruptures,
and disappears. Just anterior to the plate, in the median line, the
ectoderm sends a gland-like projection toward the brain. It branches
and becomes detached from the oral ectoderm, lying in the sella turcica
of the adult. It is known as the anterior lobe of the hypophysis, and will
be described with the brain, from which the posterior lobe develops.
The ectoderm in front of the oral plate forms also the epithelium of the
lips and of the peripheral part of the mouth, including the enamel organs,
as has already been described. The salivary glands are also considered
2l6
HISTOLOGY
ectodermal, but before they develop the oral plate has disappeared and
the boundary between ectoderm and entoderm cannot be sharply drawn.
The entoderm of the mouth and pharynx is a layer of epithelium lining
a broad, dorso-ventrally flattened cavity. From this cavity, a succession
of paired outpocketings grow out laterally to meet
the ectoderm on the side of the neck; these are
the pharyngeal pouches. They reach the ectoderm
at the bottom of furrows or clefts, corresponding
in number with the pharyngeal pouches, and there
the two germ layers fuse. The plates thus formed
FIG. 203. DIAGRAM SHOWING are comparable with the oral plate, and in fishes
THE RELATIONS BETWEEN / / ii
ECTODERM AND ENTO- they rupture producing the branchial clefts (gill
DERM IN THE MOUTH OF A J J
MAMMALIAN EMBRYO. clef ts)
a. 1., and p. 1., Anterior and
posterior lobes of the . , . , .
hypophysis; m. t., medui- Their arrangement in a young dog-fish is shown m Fig.
o* r p., t oraipte; x. and^ 204. The mouth, m, leads into a cavity, the pharynx, which
?hefip^nd enam p e?o d u the opens freely on the outer surface of the fish through five
&& defts S' c - Xt also P ens to the surface through the
spiracle, sp., a structure similar to the gill clefts, but anterior to them and having a more dorsal aperture. In respiration water is taken in
through the mouth and spiracle, and passes out through the gill clefts; but sometimes
water is ejected through the spiracle. In mammals the corresponding structure is
counted as the first gill cleft.
In mammalian embryos there are four well-defined pharyngeal
pouches on either side, which reach the ectoderm at the bottom of corresponding grooves; but if their closing plates ever rupture they are soon
restored, and permanent openings from the pharynx on the side of the
m gc
FIG. 204. HEAD OF A YOUNG DOG-FISH.
g. c., Gill cleft; m., mouth; n., nasal pit; sp., spiracle.
FIG. 205. HEAD OF HUMAN EMBRYO OF
IO MM.
c. s., cervical sinus; g. c. 2., second branchial
groove; h., hyoid arch; m., mouth; md.,
mandibular process; n., nasal pit; sp.,
auditory (spiracular) groove.
neck are not found. The first pouch, corresponding with the spiracle,
connects with the auditory groove (Fig. 205, sp}. Around it the external
ear develops, so that its position is always evident. The ectodermal
depression which connects with the second pouch disappears, except in
rare cases, where it forms a cervical fistula. This is a pit, or slender tube,
in the skin of the neck, situated primarily between the hyoid bone and
thyreoid cartilage. The third and fourth pouches connect with the
DEVELOPMENT OF THE PHARYNX
217
FIG. 206. DIAGRAM OF THE PHARYNX
OF A MAMMALIAN EMBRYO.
oesophagus; p. b., postbranchial body;
t., thyreoid; th., thymus; tr., trachea;
i> 2, 3, 4, the pharyngeal pouches.
S.t
ectoderm at the bottom of a single funnel-shaped depression known as
the cervical sinus (Fig. 205, c.s.). This also wholly disappears normally,
but it may remain as a cervical fistula low down on the neck, and its
deeper parts may give rise to branchial cysts. Thus all the ectodermal
branchial grooves except the first normally
disappear before birth.
The pharyngeal pouches, or entodermal
portions of the gill clefts, as they occur in
a mammalian embryo are shown in Fig.
206. The pharynx opens to the exterior
at the mouth, m, and divides posteriorly
into the trachea, tr, and CeSOphagUS, Oe.
In the median dorsal line it gives rise to
the anterior lobe of the hypophysis, cut off
at a. /., and in the median ventral line to
the thyreoid gland, t. This gland is a median structure, entirely separate from the pharyngeal pouches. It grows down through the hind
part of the tongue, acquiring a position in front of the trachea. Its
branching terminal part becomes separated from its outlet by the obliteration of its duct (called the thyreoglossal duct). A blind pit, the
foramen cacum, permanently retained at the back of the tongue,
marks the former outlet of the duct
(Fig. 207,7. c.}. Thus the thyreoid
gland is a detached clump of entodermal tubules in front of the
trachea.
The entodermal portions of the
gill clefts are four paired lateral
outpocketings. The first (Fig. 206,
i) extends to the auditory groove in
the ectoderm, and becomes the auditory tube (Eustachian tube). The
pharyngeal orifice of this tube in the
FIG. 207. A MEDIAN SB TION THROUGH THE . . . , . _,. , , ^
PHARYNX OF AN ADULT. (After Corning.) adult IS SnOWTl in rig. 2C>7 (0. pn.)\
P leal archre^^iglottisTfJ'cf.'fo'rame'n csecmn the OUter end of the tube expands tO
i. s-t., supratonsillar fossa; o. ph., pharyngeal r . , . j_ r , i
orifice of the auditory tube; pal., soft palate; r. f Orm the tympanic Cavity Of the Car,
ph., pharyngeal recess; s.t., sellaturcica (which i i i i
contains the hypophysis); t. 1., lingual tonsil; and Will be further Considered With
tons., palatine tonsil; t. ph., pharyngeal tonsil.
the sense organs.
The second pharyngeal pouch (Fig. 206, 2) loses its connection with the
ectoderm and becomes a relatively shallow depression on the side of the
pharynx. At a certain stage it is in close relation with the orifice of the
2l8 HISTOLOGY
auditory tube, and it has been thought to give rise to the pharyngeal
recess (fossa of Rosenmiiller), but according to Hammar such is not the
case. Instead, it produces only the sinus tonsillaris, into which a mound of
lymphoid tissue, the palatine tonsil, later projects (Fig. 207, tons.}. Above
the tonsil the supratonsillar fossa, which may readily be seen on looking
into the mouth, is to be regarded as a remnant of the original second
pouch (Hammar, Arch. f. mikr. Anat., 1903, vol. 61, pp. 404-458).
The lingual and pharyngeal tonsils, which are similar in structure to the palatine
tonsils, develop as median structures with no relation to the pharyngeal pouches.
Therefore the second pouches are to be regarded as the site rather than the source of
the palatine tonsils; there are no tonsils in the second pouches of the rat (Hammar).
The third pouch (Fig. 206, 3) near its junction with the ectoderm,
sends a tubular diverticulum (tti) down the neck behind the thyreoid
gland; it continues into the thorax, lying ventral to
the arch of the aorta (as seen in front view in Fig.
208). This diverticulum loses its lumen, becomes
detached from the pharynx, and unites with its fellow
on the opposite side to form the thymus. Besides
this elongated structure, each third pouch produces
_th an epithelial body, or nodulus thymicus, which is a
^ round clump of cells detached from the pouch at the
FIG. 208. upper end of the thymic diverticulum. Each epithe
The reouu"t, f ^f ' 29 d m^." ^ body becomes attached to the posterior surface
pa u r?h a y io e id br ^ikn p d of the thyreoid gland, forming the inferior pair of
pou"h> e ; p. grparath 3 y d parathyreoid glands (Fig. 208, p.}.
reoid gland (derived ,, , . .., ., ,_,. - N
from the 4th pouch)-. The fourth pouch on either side (Fig. 206, 4) gives
p. L, pyramidal lobe of .
the thyreoid; ao. rise to an epithelial body similar to the nodulus thy
aorta; v., vena cava x *
superior. (After Ver- micus. These likewise become detached as parathyreoid glands, and they constitute the superior pair
(Fig. 208, p. #.). Sometimes a parathyreoid gland degenerates and disappears, and in other cases one of them may become subdivided, but
typically there are four in the adult.
Behind the fourth pouch, on either side, there is a tubular prolongation of the pharynx variously known as the postbranchial, ultimobranchial or telobranchial body. As the fourth pouch becomes well formed,
the postbranchial body is so closely associated with it that together they
form a Y-shaped structure, attached to the pharynx by a common stalk
(Fig. 206). The postbranchial bodies then grow toward one another
across the front of the neck, after the manner of the thymic diverticula.
Their ventral ends become detached and imbedded in the thyreoid gland,
to the substance of which they were formerly believed to contribute.
There is, however, no satisfactory evidence that they produce thyreoid
tissue, and they are generally supposed to disintegrate.
DEVELOPMENT OF THE PHARYNX 2 19
The first recognition of the significance of the mammalian gill clefts is credited to
Rathke, in 1832, who published the following significant conclusions in his "Untersuchungen iiber den Kiemenapparat der Wirbelthiere."
"In all vertebrates without exception, in the earliest period of development, there
are formed the beginnings of a branchial apparatus. Its elements vary in number in
the different vertebrates, yet in tissue, form, position and connections they are very
similar to one another, and are built upon the same plan. Their development, however, proceeds along different lines in the various animals. In some it is partly regressive, bringing about the most manifold and divergent modifications of these
structures, not merely in form but also in tissue, type, and significance. Yet there
always remains an analogy between them; and through easy transitions, the forms and
types pass into one another from the bony fishes even to man. The branchial apparatus is most highly developed in fishes; in the other vertebrates its development is the
less complete, the further, in general, these vertebrates are removed from the fishes."
The mammalian gill clefts, although rudimentary as branchial organs,
are of the utmost anatomical importance. A single large artery passes
from the ventral aorta to the dorsal aorta between the successive pouches,
and also in front of the first and behind the last. These aortic arches
therefore number one more than the series of pouches; from them, portions
of the aorta, carotid and subclavian arteries are produced, as described
in works on embryology. The nerves send trunks down between the
pouches, the facial nerve being between the first and second, the glossopharyngeus between the second and third, and the superior laryngeal
branch of the vagus between the third and fourth. Thus these structures
determine the arrangement of the vessels and nerves.
On the basis of comparative studies the presence of a fifth pouch in mammals was
predicted, and the posterior arm of the Y-shaped outgrowth, including the postbranchial body, is often described as such. A branch of the superior laryngeal nerve is said
to pass between the arms of the Y, but a typical branchial relation between the nerves
and the fifth pouch has not as yet been established. A "fifth aortic arch" is often represented as passing between the fourth pouch and the postbranchial body, but it has
been shown that this arch differs from all the others in its order of development (forming only after the "sixth" is complete). Whereas the third, fourth, and last aortic
arches all produce very important vessels, the questionable "fifth arch" is an insignificant plexiform anastomosis, which disappears rapidly. Small vessels, however, are
always to be found near the postbranchial body in rabbit, pig and human embryos
measuring 5-10 mm. The most convincing evidence of the presence of a fifth pouch is
an actual contact with the ectoderm, posterior to the fourth pouch; this was recorded
by Hammar in a s-mm. embryo, but the contact on either side took place in only one
12 fi section. Grosser states that a closing membrane "is perhaps not always formed,
and is at all events very transitory" (Human Embryology, ed. by Keibel and Mall,
1912, vol. 2). There are as yet very few observations to show that it ever occurs in
mammalian embryos. The existence of a sixth pouch has been asserted on the basis
of slight elevations which are perhaps inconstant.
TONSILS.
The palatine tonsils are two rounded masses of lymphoid tissue, one
on either side of the throat, between the arches of the palate (Fig. 207.)
220 HISTOLOGY
Frequently they have been called amygdala (almonds), but the older
Latin term for them is tonsilla (a stake to which boats are tied). They
are covered by the mucous membrane or tunica mucosa, which throughout
the digestive tract consists of several layers. The soft moist entodermal
epithelium rests on a connective or reticular tissue layer, the tunica propria.
A structureless basement membrane, the membrana propria, is often present immediately beneath the epithelium. The epithelium, membrana
propria, and tunica propria together form the mucous membrane, which
in dissection would be stripped off as a single structure. Beneath it, and
sometimes not clearly separable from the tunica propria, is the submucous
FIG. 209. VERTICAL SECTION OF A HUMAN PALATINE TONSIL.
a, Stratified epithelium; b, basement membrane; c, tunica propria; d, trabeculae; e, diffuse lymphoid
tissue; f, nodules; h, capsule; i, mucous glands; k, striated muscle; 1, blood vessel; q, pits. (Prom
Radasch.)
layer, or tela submucosa. It is a vascular connective tissue, by which the
mucous membrane is attached to underlying muscles or bones. All the
layers named are involved in the tonsils which, however, are essentially
lymphoid accumulations in the tunica propria.
The epithelium of the palatine tonsils is a stratified epithelium of
many layers, with flattened cells on its smooth free surface, and columnar
cells beneath. Its attached surface is invaded by connective tissue elevations or papillae, so that it appears wavy in sections (Fig. 209). The
stratified epithelium lines from ten to twenty almost macroscopic depressions, called tonsillar pits or fossula (crypts). These are irregularly tubular and sometimes branched. Many lymphocytes penetrate between
the epithelial cells and escape from the free surface into the saliva, becoming "salivary corpuscles." In places the tonsillar epithelium is so
full of lymphocytes as to appear disintegrated, a condition which was
TONSILS
221
first described by Stohr (Biol. Centrabl., 1882, vol. 2). It occurs also in
the epithelium of the lingual tonsil as seen in Fig. 211. In the reticular
Pit.
Fibrous sheath.
Germinal center. Epithelium and pit containing lymphocytes.
FIG. 210. VERTICAL SECTION THROUGH A PIT IN THE LINGUAL TONSIL OF AN ADULT MAN. X 26.
Emigrating lymphocytes. Fragments of epithelium.
Emigrated
lymphocytes..
Stratified /
>ithelium.
epith
ra$
it
m>4
$&
$ . .
> i.
* **f
. 1:' *
> *
Lymphoid tissue I
I J
FIG. 211. FROM A THIN SECTION OF A LINGUAL TONSIL OF AN ADULT MAN. X 420.
On the left the epithelium is free from lymphocytes, on the right many lymphocytes, are wandering through.
tissue of the tunica propria, especially around the pits, there are many
lymph nodules, some of which are well defined, with germinative centers,
222 HISTOLOGY
but many others are fused in indefinite masses. The lymphoid tissue
constitutes the bulk of the tonsil.
The submucous layer forms a capsule for the organ, into which it
sends trabecular prolongations. It contains many blood and lymphatic
vessels, together with branches of the glossopharyngeal nerve and
spheno-palatine ganglion which supply the tonsil. It contains also the
secreting portions of small mucous glands, some of which empty into the
pits, but most of their ducts terminate in the mucous membrane surrounding the tonsil. They resemble other mucous glands of the mouth
which will be described presently. Beyond the submucosa is striated
muscle, belonging to the arches of the palate and to the superior constrictor
of the pharynx; striated muscle fibers are therefore readily included in
sections of the tonsil.
The pharyngeal tonsil is an accumulation of lymphoid tissue on the
median dorsal wall of the pharynx, between the openings of the auditory
tubes (Fig. 207). In childhood it is liable to become irregularly enlarged
so as to obstruct the inner nasal openings, thus forming the "adenoids"
of clinicians. It is covered with stratified epithelium, which is ciliated
in embryonic life; and in the adult, cilia may be found upon the epithelium
within the pits. The pits and lymphoid tissue are quite like those of
the palatine tonsils.
The lingual tonsil is an aggregation of pits surrounded by lymphoid
tissue (Fig. 210). It is found in the back part of the tongue (Figs. 207 and
220), the surface of which is very different in texture from the front part,
presenting low mounds with central depressions. Each depression is the
outlet of a pit. Lymphocytes pass through the epithelium (Fig. 211)
and become salivary corpuscles, which are said to produce substances
protecting the tissue from bacterial invasion.
THYMUS.
The thymus (Gr. 6vtw>, thymus) arises from the two tubular prolongations of the third pharyngeal pouches, which meet in the median line as
shown in Fig. 208, and become bound together by their connective tissue
coverings. The lumen is lost, and the cells proliferate. They form a
broad, flat, bilobed mass with a tapering prolongation up either side of
the neck. The bulk of the organ is in the thorax, beneath the upper part
of the sternum. At birth it weighs generally between 5 and 15 grams
(about half an ounce), and is relatively a large organ. Haller (1761)
described it in older embryos as "a huge gland, scarcely smaller than the
kidney; but in the adult it is diminished, and having become constricted,
dried up and much harder, it is almost buried in the surrounding fat."
Meckel found ordinarily no trace of it at twelve years, and according to
THYMUS
223
Hewson it gradually wastes until the child has reached between its tenth
and twelfth year, when ordinarily it is perfectly effaced, leaving only ligamentous remains. These older observations have been generally accepted, and the persistence of the thymus in the adult is regarded as of
Thymic
corpuscles.
J Connective tissue.
Transverse section
of blood vessel.
~- Medullary cord.
Cortex.
Medulla.
Blood vessel.
Thymic
corpuscle. >*,
FIG. 212. FROM A CROSS SECTION OF THE THYMUS OF A CHILD, ONE YEAR AND NINE MONTHS OLD. X 21.
considerable pathological importance. According to Waldeyer and Hammar, however, it persists for a much longer time. It increases in size
and weight for some years after birth, probably until puberty, and then
slowly atrophies. At fifteen years it is said to weigh 40-50 grams. It is
considered an active organ
even to the fortieth year,
losing its functions with
beginning old age (50-60
years). The duration of
the thymus has apparently
been underestimated. (See
Hammar. Arch. f. Anat. u.
Entw., 1906, Suppl.-Bd. pp.,
91-182; Anat. Anz., 1905,
vol. 27, pp. 23-89; and for
development, Anat. Hefte,
Abth. i, 1911, vol. 43, pp.
203-242).
The thymus is subdivided by connective tissue
layers into lobes from 4 tO FlG - 213. PART OF A SECTION OF THE THYMUS FROM A HUMAN
* EMBRYO OF FIVE MONTHS. X 50.
ii mm. in diameter, and
these are similarly subdivided into lobules of about i cu. mm. each. All
the lobules in the right and left halves of the thymus, respectively, are
attached to a cord of medullary substance, 1-3 mm. in diameter, as may
be seen if the gland is pulled apart. This axial structure suggests the
Tangential sections of lobules.
224
HISTOLOGY
original diver ticulum. Each lobule consists of a pale medulla, extending from the cord, and a darker peripheral cortex (Figs. 212 and 213).
The entire structure somewhat resembles a lymph gland, from which,
however, germinal centers are absent. It might be inferred that lymphoid tissue had developed in the mesenchyma surrounding the diverticulum, in the same way that such tissue forms about the tonsillar
pits, but careful study has shown that the thymus is largely of entodermal origin. Whether the cells of its cortex, which closely resemble
lymphocytes, are true lymphocytes or "deceptively similar epithelial
cells" has not been determined.
Vein.
Connective tissue
Thymic corpuscie
Entering Medullary
leucocytes. substance.
Cortical
substance.
FIG. 214. PART OF A SECTION OF THE THYMUS OF A CHILD AT BIRTH.
X 50.
According to Bell (Amer. Journ. Anat., 1905, vol. 5, pp. 29-62) the thymus is at
first a compact mass of entodermal cells. By vacuolization the cells form a reticulum,
and certain of them become lymphocytes. The lymphocytes pass into the cortex
where they are most abundant, and enter the vessels. The lymphoid transformation
of the thymus "is noticeable in pigs of 3.5 cm. and is well advanced at 4.5 cm.' Thus
lymphocytes appear in the thymus at about the time that lymph glands develop.
The first indication of lymph glands was found by Miss Sabin in pig embryos of 3 cm.
That the thymus cells are lymphocytes, however, is denied by Stohr, who regards
the cortex as composed of a network of stellate epithelial cells, containing in its meshes
THYMLS
225
small round epithelial cells deceptively similar to lymphocytes. Of true leucocytes
in the thymus he says, "In the places where the medulla is directly in contact with the
surrounding connective tissue and such places become constantly larger and more
numerous as the organ grows many leucocytes wander into the medulla; they lie
in the connective tissue surrounding the medulla but not in that around the cortex
(Fig. 214)." He considers that the cortex with its many mitotic figures represents a
zone of production, and the medulla, a zone of growth and degeneration (Anat. Hefte,
Abth. i, 1906, vol. 31, pp. 409-457). Hammar (1905, loc. cit.) is unable to determine
the source of the "thymus lymphocytes," but is confident that the reticulum is of
epithelial origin. He finds that in birds this reticulum produces cells resembling
striated muscle fibers, and these "myoid cells" he considers to be entodermal. In
his later work (1911, loc. cit.} he states that the lymphocytes enter the thymus chiefly
from the thymic blood vessels.
Not only lymphocytes, but other leucocytes, eosinophilic cells, and
multinuclear giant cells have been found in the medulla. Erythroblasts
are said to occur in its outer portion and in the cortex. The thymus
Degenerated epithelial cells.
Flat epithelial cells.
Degenerated nucleus.
FIG. 215. THYMIC CORPUSCLES, IN SECTION, FROM A MAN TWENTY-THREE YEARS OLD. X 360.
therefore is sometimes considered a blood-forming organ. Sometimes
the medulla contains cysts, which may be lined in part with typical
ciliated cells. The most characteristic structures in the thymus are the
thymic corpuscles (Hassall's corpuscles) which are found exclusively in
the medulla. They are rounded bodies, at first few in number and
small (i 2-20 ju in diameter), but they increase rapidly in size (to a diameter
of 1 8o/0 and new ones are constantly forming. They are said to be
present at about the fifth month, and at birth they are numerous, varying
in size as shown in Fig. 215. To produce them, the nucleus and protoplasm of an entodermal reticular tissue cell enlarge, and the nucleus
loses its staining capacity by changes in its chromatin. A layer of
deeply staining hyaline substance develops in the protoplasm. This increases until it fills the entire cell, often being arranged in concentric
layers, and the nucleus becomes obliterated. Neighboring cells are concentrically compressed by the enlargement of this structure, and by hyaline
is
226 HISTOLOGY
transformation they may become a part of the corpuscle. The larger
corpuscles are due to a fusion of smaller ones, or to hyaline changes occurring simultaneously in a group of cells. The central portion of a corpuscle
may become calcified. Sometimes it is vacuolated, containing fat. The
hyaline substance may respond to mucous stains, but generally it does not;
it has been considered similar to the 'colloid' of the thyreoid gland.
Leucocytes are said to become imbedded in the corpuscles, or to enter
them and assist in their disintegration. Thymic corpuscles have been
regarded not only as degenerative products of the entodermal epithelium
but also as concentric connective tissue masses, and as blood vessels
with thickened walls and obliterated cavities. Injections show that
they are not connected with the blood vessels. Although they have recently
been described as active constituents of the thymus, they are generally
regarded as degenerations.
The arteries of the thymus enter it along the medullary strand, and
extend between the cortex and medulla, sending branches into both but
chiefly into the cortex. The cortical branches empty into veins between
the lobules; the others into veins within the medulla. There are many
interlobular lymphatic vessels, beginning close to the surface of the gland
substance, and accompanying the blood vessels. There is nothing in the
thymus to correspond with a lymph sinus. The nerves, chiefly sympathetic fibers, with some from the vagus, terminate along the vessels; a
very few have free endings in the medulla.
THYREOID GLAND.
The thyreoid (i.e., shield-shaped) gland is a median, entodermal downgrowth from the tongue; its thyreoglossal duct becomes obliterated,
leaving the foramen caecum to mark its former outlet. The downgrowth
is joined by cells from the postbranchial bodies, which fuse with it.
This entire structure comes to lie beside and in front of the upper part of
the trachea. It consists of two lateral lobes, each about two inches long
and an inch wide, connected by an isthmus, about half an inch wide, which
crosses the median line ventral to the second and third tracheal rings.
An unpaired pyramidal lobe extends from the isthmus or adjacent part of
the lateral lobe toward the tongue (Fig. 208). Irregular detached portions of the gland, such as occur especially along the course of the thyreoglossal duct, are called accessory thyreoid glands.
The proliferating mass of entodermal cells forms at first a network of
solid cords. This becomes separated into small masses, within each of
which a lumen may appear. The lumen enlarges and becomes spheroidal;
the entodermal cells which surround it form a simple epithelium, either
coleumnar, cuboidal, or flat. Flat cells are said to occur especially in old
IHYREOLD GLAND
227
age; usually the cells are low columnar or cuboidal. The mature
thyreoid gland consists, therefore, of rounded, closed spaces, or follicles,
bounded by a simple entodermal epithelium (Fig. 216). The follicles
vary greatly in diameter. Generally they are rounded, but sometimes
they are elongated, and occasionally they branch or communicate with one
another. Among them are cords or clumps of cells which have not
acquired a lumen.
Within the follicles, and forming the most conspicuous feature of the
thyreoid gland in ordinary sections, is a hyaline material which stains
Flat epithelium.
Blood
Connective tissue. vessels.
Artery with
two thickenings.
Colloid with drops
of mucus.
Oblique section of a follicle.
FIG. 216. SECTION OF A LOBCLE OF THE THYREOID GLAND FROM AN ADULT MAN. X 220.
deeply with cosine and is named 'colloid.' The hyaline material in the
thymic corpuscles, the hypophysis, and in the coagulum in the cervical
blood and lymphatic vessels, has also been designated colloid. In sections
of the thyreoid gland it usually does not fill the follicle but has contracted,
producing a spiny border. Granules, vacuoles and droplets of mucus, detached cells, leucocytes, and crystalloid bodies may be found in it. It is a
product of the epithelial cells, in the protoplasm of which similar material
has been detected. It has been said that it is transferred to the blood and
lymphatic vessels, passing out between the epithelial cells.
As has been learned by experiment, the thyreoid gland produces an
internal secretion which is essential for the normal growth and development
of the body. It is, however, not known whether this secretion leaves the
basal or free surface of the thyreoid epithelium, and its relation to the
228
HISTOLOGY
colloid material is not clear. The finding of two sorts of thyreoid cells,
one of which produces colloid, and the other does not, lacks confirmation.
The cells may exhibit refractive, secretory granules, which are larger and
coarser toward the free surface. Eosinophilic granules have been reported, and in certain animals other granules of fatty nature have been
found, especially near the basal surface. Since the terminal bars are said
to be deficient at the angles where the epithelial cells meet, an opportunity
is afforded for the contents of the follicles to pass out between the epithelial
cells to the vascular tunica propria.
The thyreoid follicles are surrounded by loose elastic connective tissue,
said to be reticular near the follicles, which contains very many blood and
lymphatic vessels in close relation with the epithelium. Denser connective
tissue forms a capsule and lobular partitions. It contains small arteries,
the media and intima of which are said normally to present local thickenings (Fig. 216). The nerves from the cervical sympathetic ganglia form
perivascular plexuses, and pass to the follicles.
PARATHYREOID GLANDS.
It is generally stated that there are four parathyreoid glands in man,
the anterior or upper pair being derived from the fourth pharyngeal
^*/!V*'l.',J>#ll
FIG. 217. SECTION OF A HUMAN PARATHYREOID GLAND. (Huber.)
pouches, and the posterior or lower pair from the third (Fig. 208). They
are therefore entodermal structures. In the adult they are round or oval
bodies, said to measure from 3 to 13 mm., found on the dorsal or tracheal
surface of the thyreoid gland. They may be imbedded in its capsule or
attached to it by pedicles. Sometimes they (the lower pair?) are found
in the thymus. The parathyreoid glands may be lacking on one side,
where in other cases as many as four have been recorded; they may atrophy
PARATHYREOID GLANDS
229
c.t.
and disappear, or increase in number by subdivision. Both pairs possess
a similar structure unlike that of either the thyreoid gland or the thymus,
but resembling the corresponding epithelial bodies of the lower vetebrates.
They consist of masses and cords of polygonal, entodermal cells containing round nuclei with networks of chromatin. The protoplasm is pale,
''almost homogeneous" or "slightly granular," sometimes containing
vacuoles. Cell membranes are not prominent. Between these cells
and the large thin- walled blood vessels which pass among them (Fig. 217),
there is only a very small amount of connective tissue. A capsule surrounds the entire structure. The blood vessels are branches of those
which supply the thyreoid gland. Little is known of the lymphatics or
nerves.
GLOMUS CAROTICUM.
The glomus caroticum (carotid gland) is largely a knot of blood vessels
at the bifurcation of the common carotid artery. It is a reddish body
"5-7 mm. long, 2.5-4
mm. broad, and 1.5 mm.
thick." Between its
thin-walled, dilated
capillaries there are
strands of polygonal
chromaffine cells, which
are prone to disintegrate
(Fig. 218). Many nerve
fibers, both medullated
and non-medullated,
enter the glomus, and a
few multipolar ganglion
cells are associated with
them. Since the nature
of the glomus caroticum
is undetermined, the
three views regarding it MAN - < After
i i j b.v., Blood vessels; e.v., efferent vein; tr., trabecula; c.t., connective
may be mentioned. tissue septum.
First, it has been considered as derived from the third pharyngeal pouch. Since it has recently
been asserted that the "carotid gland" of Echidna comes from the
second pouch, the non-entodermal origin of the human glomus is perhaps not beyond question. Second, it has been considered ganglionic or
paraganglionic in nature, so that it is classed with nervous structures,
and this opinion is probably correct. Third, it is considered essentially
a vascular formation, containing strands of modified mesenchymal cells.
FIG. 218. SECTION OF A PART OF THE GLOMUS CAROTICUM OF
MAN.
230
HISTOLOGY
'
FIG. 219. FLOOR OF THE PHARYNX OF A IO-MM. HUMAN EMBRYO.
I-IV. Branchial arches; t 1 , anterior part of the tongue!
t*, second arch, joining the posterior part of the
tongue toward the median line. The thyreoid
gland is dotted. The epigk>ttis extends over the
fourth arch. (From McMurrich, after His.)
DEVELOPMENT AND STRUCTURE OF THE TONGUE.
The tongue consists of two parts, an anterior and a posterior, which
differ in origin and adult structure. Separating the branchial clefts from
one another there are columns
of tissue known as branchial
arches. They come together in
the median ventral line to form
the floor of the mouth (Fig.
219). In this figure the upper
jaw and roof of the pharynx
have been cut away; the branchial clefts are seen as dark depressions bounded laterally by
thin plates. The first branchial
arch (i) is between the oral
and auditory clefts. In the
median ventral line an elevation (tuberculum impar) arises
between this arch and the
second; it becomes continuous
with a larger elevated portion of the mandibular arch to form the anterior
part of the tongue (t 1 ). The second and third arches unite toward the
median ventral line and there produce the
posterior part of the tongue (t 2 ). Between
the anterior and posterior parts is the opening
of the thyreoglossal duct, later the foramen
caecum. . The epiglottis is an elevated part
of the third arch separated from the posterior part of the tongue by a curved groove.
In the adult (Fig. 220) the dor sum of the
anterior part of the tongue is roughened with
elevations or papilla. These are chiefly the
slender filiform papilla and conical papilla;
but knob-like forms, the fungiform papilla,
are scattered among them over the entire
surface, and in life they can be easily distinguished owing to their red color. Near the
junction of the anterior and posterior parts of
the tongue there is a V-shaped row of larger
papillae, generally six to twelve in number,
called vallate papilla. Their name refers to the deep narrow depression
which encircles them. Behind the apex of the V, which is directed
i.f,
L1
FIG. 220. THE UPPER SURFACE OF
THE ADULT TONGUE.
c., Conical papillae; ep., epiglottis; f.,
foliate papillae; f. c., foramen
caecum; f.f., position of the filiform and fungiform papillae; 1.,
lenticular papilla; 1. t., lingual
tonsil; p. t., palatine tonsil; v.,
vallate papillas.
TONGUE
231
Primary
papilla.
Secondary
papillae.
Filiform process.
toward the throat, is the foramen oecum. On either side of the tongue,
as indicated in the figure, there are from three to eight parallel vertical
folds (2-5 mm. long) occurring close together; these are the foliate
papillcs. In the foliate and vallate papillae the organs of taste are most
numerous. The under surface of the tongue is free from epithelial
papillae; its mucosa resembles that which lines the mouth. The posterior
part of the tongue has a nodular surface covered with soft epithelium
and contains the lingual tonsil, which has already been described. Laterally it presents fold-h'ke elevations called lenticular papillce.
Filiform papillae (Fig. 221) are slender cornified epithelial projections,
composed of pointed cells which are described as stacked like superimposed hollow cones. The cells
have undergone a horny hyaline
degeneration. These projections are arranged in clumps
which rest upon a group of from
five to twenty connective tissue
elevations, or secondary papillae ;
and these in turn are at the
summit of a cylindrical or conical primary papilla, composed
of vascular connective tissue
with numerous elastic fibers.
These primary papillae form the
basal portions of the filiform
papillae. They are well shown
in Fig. 222, along with the secondary papillae, but the cornified processes
of the thick epithelium above them have undergone post-mortem disintegration. Most of the papillae of the tongue are of the filiform type.
Fungiform papillae (Fig. 222) are rounded elevations with a somewhat
constricted base, varying in height from 0.5 to 1.5 mm. In life they are
red, since their epithelium is not cornified and transmits the color of the
blood beneath. They contain a primary connective tissue papilla, with
but few elastic fibers, beset on all sides with secondary papillae.
The vallate papillae resemble broad fungiform papillae. They are
from i to 3 mm. broad and i to 1.5 mm. tall, each being surrounded by a
deep groove (Fig. 223). Their connective tissue often contains longitudinal, oblique, or encircling smooth muscle fibers, the last named being
found near the lateral walls. Secondary papillae are confined to the upper
wall. Occasionally the epithelium sends branched prolongations into the
underlying tissue. These may become detached from the surface and
appear as concentric bulb-like bodies such as are generally known as
"epithelial pearls." There are also branched serous glands which grow
. Fat cells.
FIG. 221. FROM
Fascia linguae.
Muscle.
LONGITUDINAL SECTION OF THK
DORSUM OF A HUMAN TONGUE. X 12.
HISTOLOGY
down from the epithelium, having ducts which open into the deep grooves
(Fig. 223). The foliate papillae are parallel folds of mucous membrane,
in the epithelium of which there are many taste buds. These structures,
which occur also in the lateral walls of the vallate papillae (Fig. 223),
will be described with the nerves of the tongue.
The tunica propria of the mucous membrane is a loose connective
tissue layer containing fat. It is not sharply separated from the denser
submucosa. At the tip of the tongue, or apex lingua, and over the
dorsum, the submucosa is particularly firm and thick, forming the
fascia lingua. Three sorts of glands branch in the submucosa and
Cornified epithelium.
Secondary
papillae of a
fungiform
papilla.
Primary papilla.
Oblique section
of a filiform
papilla.
Secondary
papillae of a
filiform'
papilla.
Primary
papillae.
Nerves.
*m
- -- ;^ : W$fc :
Artery.
FIG. 222. FROM A LONGITUDINAL SECTION OF THE HUMAN TONGUE.
x, Epithelium showing post-mortem disintegration.
Fascia linguae.
.... Striated muscle fibers.
X 25.
may extend into the superficial part of the muscle layer. These are
the serous glands found near the vallate and foliate papillae; mucous
glands occurring at the root of the tongue, along its borders, and in
an area in front of the median vallate papilla; and the two mixed
anterior lingual glands, from half an inch to an inch long, each of which
empties by five or six ducts on the under surface of the apex. The
structure of these types of glands will be described in the section on
oral glands.
The muscular layer consists of interwoven bundles of striated fibers
which are inserted into the submucosa or into the intermuscular connect
TONGUE
233
ive tissue. Some of these striated fibers are branched. The musculature of the tongue is partly divided into right and left halves by a
dense median connective tissue partition, the septum lingua, which begins
low on the hyoid bone, attains its greatest height in the middle of the
tongue, and becomes lower anteriorly until it disappears. It does not
extend clear through the tongue since it ends 3 mm. beneath the dorsum.
The muscles of the tongue are partly vertical (Mm. genioglossus, hyoglossus, and verticalis lingua}, partly longitudinal (Mm. styloglossus,
chondroglossus, superior and inferior longitudinalis lingua} and partly
Tuica propna.
Secondary papillae. Taste bud.
Vallate papilla.
\
Groo
Orifice
of a Small
serous papilla,
gland.
Epithelium.
Tunica
propria
Striated
muscle.
Muscle fibers in cross Nerye with Fascia Mucous Vein.
and longitudinal section. ganglion cells, linguae. gland.
FIG. 223. VERTICAL SECTION OF A HUMAN VALLATE PAPILLA. X 25.
transverse (M. transversus lingua). The glossopalatine muscle of the
palatine group also enters the tongue. Some of the muscle fibers are oblique but many of the bundles cross at right angles. In the connective
tissue between them, medullated nerves are abundant. Some are sensory
nerves to the mucosa, but many of them are the lingual branches of the
hypoglossal nerve which supply all the tongue muscles except the inferior
longitudinal; the latter is supplied by fibers from the chorda tympani
Sensory spindles have been found in the lingual muscles.
Blood vessels are numerous in the submucosa and form extensive
capillary networks in the tunica propria of both primary and secondary
234
HISTOLOGY
papillae. Small lymphatic vessels also form a network in the tunica
propria, and this is continuous with a coarser net in the submucosa.
The sensory nerves are the terminations of the lingual branches of the
mandibular nerve anteriorly, and of the lingual branches of the glossopharyngeus posteriorly. In the submucous connective, tissue they form
a plexus of medullated and non-medullated fibers, and in some places,
notably beneath the vallate papillae, nerve cells are found, grouped in
small ganglia (Fig. 223). The terminal branches of these nerves probably
end in part in bulbous corpuscles, but most of them, as non-medullated
Taste bud.
Fibers between
the buds
Fibers overlying
a bud.
Connective tissue.
Epithelium.
Fibers within the buds.
Connective tissue.
Nerve.
p
FIG. 224. FROM A VERTICAL SECTION OF THE FOLIATE PAPILLA OF A RABBIT. X 220.
fibers, enter the epithelium and extend to the outer epithelial cells, generally without branching (as on the left of Fig. 224) . Others enter the groups
of specialized epithelial cells, known as taste buds, which are believed to
be the special organs of taste. Within the buds the nerves divide into
coarse varicose branches which end freely, without uniting with the cells
or anastomosing with one another (Fig. 224).
Taste buds are round or oval groups of elongated epithelial cells, most
of which extend from the basal to the free surface of the epithelium. In
embryos of from five to seven months they are more numerous than in the
adult, occurring in many filiform papillae, in all the fungiform, vallate and
foliate papillae, and also upon both sides of the epiglottis. Subsequently
they are destroyed with an infiltration of leucocytes except on the lateral
walls of the vallate and foliate papillae, on the laryngeal surface of the
TONGUE 235
epiglottis, and a small portion of those on the anterior and lateral fungiform papillae. These remain in the adult. In the outer half of each bud
the cells converge like the segments of a melon, so that their ends are
brought together in a small area. This area is at the bottom of a little pore
or short canal found among the outermost flat cells of the epithelium.
The taste pore opens freely to the surface, but in oblique sections it may
appear bridged as in Fig. 225.
Within the bud two sorts
of elongated cells may be distinguished, namely, supporting
cells which are chiefly peripheral, Taste pore. and taste cells which are central.
There are also certain cells which
lie wholly in the basal part of
the bud, and lymphocytes which P ceSu" 18 **.!
have entered the bud from below are frequently seen among
it AI 11 mi Taste cells. "*
the other cells. The supporting cells are paler than the Stratified
gustatory cells, and may be uni- e p ;thelium - '
form in diameter Or tapering FIG. 225. FROM A VERTICAL SECTION OF A HUMAN
FOLIATE PAPILLA. X 330.
toward their ends; they are
sometimes forked or branched below. The taste cells are darker and more
slender, being thickened to accommodate the narrow nucleus which is
usually near the middle of the cell. At the taste pore these cells end in
a stiff refractive process which is a cuticular formation. The processes
extend into the deeper part of the pore but do not reach its outlet.
These cells are believed to transmit the gustatory stimuli to the nerves
which branch about them. To a less extent the nerves are said to ramify
around the supporting cells, which perhaps have other functions than
their name implies.
MOUTH AND PHARYNX.
The lining of the mouth, like the covering of the tongue, consists of
epithelium, tunica propria, and submucosa. At the lips, toward the line
of transition from skin to mucous membrane, hairs disappear from the
skin. The epithelium becomes thicker but more transparent as it crosses
the line (Fig. 226). Its outer cells are still cornified, but they are not so
flat and compactly placed as in the skin. The deeper cells appear vesicular.
Within the mouth, except on the tongue, cornified cells are absent, but
granules of the refractive horny substance, keratohyalin, are said to occur in
the outer cells, even in the oesophagus. The free surface of the epithelium
236
HISTOLOGY
is generally smooth, but its under surface is indented by many connective
tissue papillae, which are particularly long and slender in the gums and
lips (Fig. 226). At the inner border of the lips at birth, there are free
papillary projections described as " true villi," but these later disappear.
Cilia are found on the oral, pharyngeal and cesophageal epithelia in the
embryo, but in the adult cilia persist only in certain parts of the pharynx.
The tunica propria in the mouth, as is generally the case in the digestive
tract, has few elastic fibers. Some of its tissue is reticular, and in it,
lymphoid accumulations are frequent; they may extend into the submucosa. On the oral surface of the soft palate there is a layer of elastic
Sebaceous gland
Tall papillae
Oblique sections of papillae.
Hair shafts
and sebaceous
glands.
Sebaceous
s gland.
Hair shaft.
Vein.
Artery.
Bulb of a hair.
Corium. Epidermis.
\ I
Epithelium. Tunica Submucosa. Orbicular Mimetic
propria. muscle. muscle.
FIG. 226. VERTICAL SECTION THROUGH THE LOWER LIP OF A MAN OF NINETEEN YEARS. X 10.
Epidermis and corium constitute the skin; epithelium, t. propria, and submucosa form the oral
mucous membrane.
tissue between the propria and submucosa. A similar layer is found in the
oesophageal end of the pharynx. It increases in thickness upward, at the
expense of the submucosa, so that it forms a thick layer in the back of the
pharynx in. contact with the muscles, among the fibers of which it sends
prolongations. This elastic layer, as the /asa# />^aryw#0&<m7am, is attached to the base of the skull.
In most of the oral region there is no sharp line of separation between
the propria and the submucosa. The latter may be a loose layer containing fat, and allowing considerable movement of the mucosa, or, as in the
gums and hard palate, it may be a dense layer binding the membrane
closely to the periosteum. In the submucosa are the branches of various
glands. On the inner border of the lips and the inner surface of the cheek,
MOUTH 237
there are sebaceous glands without hairs, which first develop during puberty.
This type is described with the skin. The other oral glands are considered
in the following section.
GLANDS OF THE ORAL CAVITY.
In the general account of glands (page 54) it has been stated that
serous gland cells which produce a watery albuminoid secretion should be
distinguished from the mucous gland cells which elaborate thick mucus.
When examined fresh, serous cells are seen to contain many highly refractive granules. In fixed preparations they may appear dark and granular
(empty of secretion) or enlarged and somewhat clearer (full of secretion),
as shown in Fig. 44, p. 54. The round nucleus is generally in the basal
half of the cell, not far from its center (Fig. 227). Mucous cells when
Man. Rabbit. Man.
Mucous glands. Serous glands.
TUBULES, FROM LINGUAL GLANDS, ILLUSTRATING THE DIFFI
Mucous AND SEROUS GLAND CELLS.
b, Empty mucous cells; c, mucous cells full of secretion; d, lumen of the tubule. X 240.
FIG 227. SECTIONS OF TUBULES, FROM LINGUAL GLANDS, ILLUSTRATING THE DIFFERENCES BETWEEN
Mucous AND SEROUS GLAND CELLS.
fresh are much less refractive than serous cells. In fixed preparations
they are typically clear, since the large area occupied by mucous secretion
stains faintly. Fully elaborated mucus, however, may be colored intensely
with certain aniline dyes, such as mucicarmine and Delafield's haematoxylin. In certain types of mucous cells the pale secretion area is large in
all stages of activity. When full of mucus, the nucleus is flattened against
the base of the cell, and when empty, the nucleus becomes more oval without essentially changing its position (Fig. 227). This differs from the type
of mucous cell found in the gastric epithelium, in which the secretion area
varies considerably with the elaboration and discharge of secretion (Fig.
45, P- 55)
Glands may consist entirely of serous or of mucous cells, but frequently
they include cells of both sorts and are called mixed glands. The mixed
glands contain some purely serous tubules or alveoli; the rest consist of
both mucous and serous cells, so arranged that the latter appear more or
less crowded away from the lumen. Often they form a layer outside of
the mucous cells, partly encircling the tubule or alveolus and constituting
a crescent (demilune), as shown in Fig. 237. The serous cells of the cres
~ ~/ Axial lumen.
238 HISTOLOGY
cent are connected with the lumen by means of secretory capillaries (p. 57)
which pass out to them between the mucous cells and branch around the
serous cells, ending blindly (Fig. 228). Sometimes the cells of the crescent
are directly in contact with the lumen. Since the serous crescents are
always associated intimately and somewhat irregularly with mucous cells,
they were naturally interpreted as a functional phase of the latter. It is probably
true that some crescents represent empty
mucous cells which have been crowded
from the lumen by those full of secretion.
No secretory capillaries lead to such
mucous crescents, which moreover are
not abundant. Another sort of crescentic figure is made by the basal protoplasm
Crescent. *
FIG. 228. FROM A SECTION OF THE SUBMAX- in mucous cells otherwise full of secre
ILLARY GLAND OF A DOG. X 320.
tion. Finally, in oblique sections, stellate cells associated with the basement membrane may resemble true
crescents.
The oral glands include serous glands, mucous glands, and mixed glands
to be described in turn.
Intercellular
secretory
capillary.
Serous Glands.
The serous oral glands are the parotid glands and the serous glands
of the tongue (v. Ebner's glands). The latter are branched tubular
glands limited to the vicinity of the vallate and
foliate papillae. Generally they open into the
grooves which bound these papillae. Their ducts
are lined with simple or with stratified epithelium,
which is occasionally ciliated. Their small tubules
consist of a delicate membrana propria or basement
membrane, which surrounds the low columnar or
conical serous cells. In this simple epithelium,
cell walls are lacking. With special stains and
high magnification, a dark granular zone toward
the lumen has been distinguished from the clear
basal portion of the cell which contains the nucleus (Fig 229). The lumen of the tubules is
very narrow and receives the still narrower intercellular secretory capillaries (Fig. 230).
The parotid glands are the largest oral glands. Each is situated in
front of the ear and is folded around the ramus of the mandible; its duct,
the parotid duct (Stenson's), empties into the mouth opposite the second
FIG. 229. TUBULE OF A SEROUS GLAND FROM THE HUMAN TONGUE. X 750.
Secretory granules toward the
lumen are finer than those
further out. The light intercellular lines represent the
secretory capillaries.
PAROTID GLAND
239
molar tooth of the upper jaw. The parotid gland is an organic, branched
serous gland, subdivided into lobes and lobules. The accessory parotid
gland appears as a lobe separated from the others. The parotid duct is
Intercellular
secretory
capillaries. ;
FIG. 230. SECTION OF A SEROUS GLAND
FROM THE TONGUE OF A MOUSE. X 240.
Prepared by Golgi's method, a precipitate
has formed in the ducts. The right
lower part of the figure has been completed by adding the cell outlines.
FIG. 231. PART OF A CROSS SECTION OF
THE SECRETORY DUCT FROM THE
PAROTID GLAND OF A MOUSE.
The basal rods (mitochondria) toward the
lumen break apart into secretorygranules.
Fat cells.
End piece. .
End piecees.
FIG. 232. DIAGRAM OF THE HUMAN PAROTID GLAND.
FIG. 233. SECTION OF THE PAROTID GLAND OF AN ADULT MAN
X 252.
The very narrow lumen of the alveolo-tubular end pieces is not
shown.
characterized by a thick membrana propria, and consists of a two-layered
columnar epithelium with occasional goblet cells. As the duct branches
repeatedly, the epithelium becomes a simple columnar epithelium, after
240
HISTOLOGY
being pseudostratified, with two rows of nuclei (cf. Fig. 39, p. 49)- Possibly the epithelium near the outlet of the duct is also pseudostratified.
This excretory portion of the duct is followed by the secretory part, formed
of simple columnar cells with basal striations, perhaps indicative of secretory activity (Fig. 231). As shown in the diagram (Fig. 232) and in the
section (Fig. 233) the secretory ducts become slender, forming the intercalated ducts. These are lined with flat spindle-shaped cells which are
continuous with the large cuboidal serous cells of the terminal alveoli.
The gland cells when empty of secretion are small and darkly granular,
Alveoli
FIG. 234. SECTION OF THE PAROTID GLAND FROM A MAN OF TWENTY-THREE YEARS. X 100.
Portions of three lobules are shown, which have drawn apart from one another in the process of preparation.
Note the abundance of secretory ducts.
and when full are larger and clearer. They rest upon a basement membrane containing stellate cells. Intercellular secretory capillaries end
blindly before reaching the basement membrane.
Between the alveoli, which are somewhat elongated and branched,
there is vascular connective tissue containing fat cells. In denser form
it surrounds the lobules and lobes of the gland, and the larger ducts. The
ducts which are found in the connective tissue septa are called interlobular
ducts, in distinction from those which are surrounded by the alveoli in
which they and their branches terminate. The latter are intralobular
ducts. They are smaller and have less connective tissue around them than
the interlobular ducts, of which, however, they are continuations.
PAROTID GLAND 241
Vessels and Nerves. The arteries generally follow the ducts from the
connective tissue septa into the lobules, where they produce abundant
capillary networks close to the basement membranes. The veins derived
from these soon enter the interlobular tissue, and may then accompany
the arteries. The lymphatic vessels follow the ducts, and branch in the
interlobular connective tissue, in which they terminate. Only tissue spaces
have been found within the lobules. The nerve supply is from several
sources. Sympathetic nerves from the plexus around the carotid artery
accompany the blood vessels into the parotid gland, and by controlling
the blood supply have an Important bearing upon secretion. The nerves
which reach the gland cells are in connection with the tympanic branch
of the glossopharyngeal nerve. This branch extends to the otic ganglion,
from which fibers pass to the parotid gland by way of an anastomosis
with the auriculo-temporal branch of the mandibular nerve. Within the
gland the nerves pass along the ducts, where they are associated with
microscopic ganglia, and form plexuses beneath the basement membranes
of the alveoli. From these plexuses, fibers penetrate the basement membranes and form simple or branched varicose endings in contact with the
gland cells. Other nerves enter the substance of the gland, either to pass
through it or to contribute to its nerve supply; these include branches
of the trigeminal, facial and great auricular nerves, the last coming from
the second and third cervical nerves. Free sensory endings of medullated
fibers are said to occur in the epithelium of the ducts
Mucous Glands.
The purely mucous glands of the mouth are simple branched alveolotubular glands found on the anterior surface of the soft palate and on the
hard palate (palatine glands), along the borders of the tongue (lingual
glands), and in greater numbers in the root of the tongue. There they
may open into the tonsillar pits through ducts lined with columnar epithelium, sometimes ciliated. The wall of the tubules consists of a structureless basement membrane and of columnar mucous cells, varying according to their functional condition as shown in Fig. 227, I-II. The
empty cells are narrower than the others, and the nuclei, though at
the base of the cell and transversely oval, are not as flat as in cells
full of secretion. Seldom can cells be found completely occupied by
unaltered protoplasm. A single gland, or even a single alveolus, may
contain cells in different phases of secretion, as is clearly seen when special
mucin stains are used. Secretory capillaries are not found in the purely
mucous glands.
Mixed Glands.
The mixed oral glands are the sublingual, submaxillary, anterior lingual, labial, buccal, and molar glands. They all possess crescents of
16
242
HISTOLOGY
serous cells such as are to be described in the largest glands of this group
the sublingual and submaxillary.
The sublingual glands are two groups of glands, one on either side of
the median line, under the mucous membrane in the front of the mouth.
The largest component is an alveolo-tubular structure emptying by the
ductus sublingualis major on the side of the
frenulum lingua. The main stem and the
principal branches of the large sublingual duct
are lined by a two-layered or pseudostratified
columnar epithelium, as in the parotid duct.
They are surrounded by connective tissue containing many elastic fibers. Ducts less than
.05 mm. in diameter have a simple columnar
epithelium, which in a few places becomes low
and basally striated to form the secretory
ducts. As shown in the diagram, Fig. 235, the
secretory ducts are very short, and they are
id pieces, accordingly infrequent in sections; the slender
intercalated ducts are absent. The terminal
FIG. 235. DIAGRAM OF THE HUMAN secreting portions of the gland are somewhat
tortuous structures, often presenting outpock
etings. They consist of mucous and serous cells quite evenly mixed, so
that the gland has a characteristic appearance under low magnification
(Fig. 236). The serous cells sometimes border upon the lumen, but often
they are separated from it by the mucous cells so that they form crescents
(Fig. 237). Only the serous cells are provided with the branched intercellular secretory capillaries. Around the tubules there is a basement
membrane including certain stellate cells. The interlobular connective
tissue contains many lymphocytes.
Near the gland just described, but apparently quite distinct from it,
there is a group of 5 to 20 alveolo-tubular glands which open by separate
ducts, the ductus sublinguales minores. These glands consist almost exclusively of mucous cells.
The sublingual gland as a whole receives fibers from the submaxillary
ganglion, and so from the chorda tympani, which passes to this ganglion
by way of an anastomosis with the lingual branch of the mandibular nerve.
Its ducts are said to have sensory fibers, probably derived from the lingual
nerve. Sympathetic fibers from the superior cervical ganglion, which
have ascended the neck as perivascular plexuses, extend to the sublingual
gland around its arteries.
The submaxillary glands are a pair of branched alveolar glands, in part
tubulo-alveolar, found in the floor of the mouth, each being drained by a
submaxillary duct (Wharton's) which opens on the sides of the frenulum
SUBLINGUAL GLAND
243
FIG. 236. SECTION OF THE SUBLINGUAL GLAND (GL. SUBL. MAJOR) FROM A MAN OF TWENTY-THREE
YEARS. X 100.
A crescent consisting of
eight serous cells.
Part of an excretory duct.
Lumen.
Tangential
section of serous
cells.
Mucous cells and
thick mernbrana
propria
FIG. 237. SECTION OF A HUMAN SUBLINGUAL GLAND. X 252.
244
HISTOLOGY
Excretory
duct.
linguae near its front margin. Sometimes this duct is joined by the ductus
sublingualis major so that the two have a common outlet. Its orifice may
be lined by stratified epithelium, but this soon gives place to the two layered
form. Secretory ducts are well developed (Fig. 238) and their basally
striated cells contain a yellow pigment. The
intercalated ducts, which are lined with simple cuboidal epithelium, lead to terminations
of two sorts. Most of these consist entirely
of serous cells. The others are mixed, but
the crescents are small, composed of only a
few or even of single serous cells (Figs. 239
and 240). Secretory capillaries such as have
already been described, are related only to
the serous cells. Elastic tissue surrounding
intercalated the alveoli has been thought to aid in expelling the secretion through the ducts. The
nerves have the same origin as those of the
sublingual gland.
In the oral glands, not infrequently degenerating lobules occur, characterized by
abundant connective tissue between tubules with wide lumens and low
gland cells. Sometimes they are surrounded by leucocytes.
Secretory
duct.
ducts.
End pieces.
FIG. 238. DIAGRAM OF THE HUMAN
SUBMAXILLARY GLAND.
-'
Serous gland cells.'
Intercalated duct.
Mucous
gland cells.
Secretory duct.
FIG. 239. SECTION OF THE SUBMAXILLARY GLAND OF AN ADULT MAN. X asa.
SUBMAXILLARY GLAND
245
Serous Intercalated Blood
cells. duct. vessels.
Secretory
duct
Connective tissu
Mucous cells.
w. Fat cells.
PIG. 240. SECTION OF THE SUBMAXILLARY GLAND FROM A MAN OF TWENTY-THREE YEARS. X 100.
Note that the serous cells predominate, and that secretory ducts are abundant. (A characteristic
crescent is shown at z.)
THE DEVELOPMENT OF THE DIGESTIVE TUBE.
The digestive tube of mammals arises as two outgrowths from the
yolk-sac the fore-gut and hind-gut respectively. They are shown in
Fig. 241, A, which represents a young rabbit embryo placed in a vertical
position. Most of the spherical yolk-sac has been cut away. Anteriorly
the fore-gut (pti) is seen extending from the yolk-sac to the oral plate;
posteriorly the sac has given rise to a short hind-gut from which a tubular
ventral outgrowth, the allantois, has begun to develop. The allantois
will be described with the membranes which surround the embryo. In
an older stage (Fig. 241, B) the fore-gut and hind-gut have elongated,
and the connection of the tube, which they form, with the yolk-sac is
becoming reduced to a slender stalk. The entodermal tube within the
stalk is called the mtelline duct. Posteriorly the intestine and allantois
unite and form the cloaca, which is closed to the exterior by the cloacal
membrane.. (The marked bend in the intestinal tube shown in Fig. 241, B,
which is often seen in human embryos, is exaggerated, if not produced
altogether, by a post-mortem sagging of the yolk-sac.)
246
HISTOLOGY
In the later stage (Fig. 241, C) both the fore-gut and hind-gut have
greatly elongated; together they form a loop of intestine extending out
into the cavity of the umbilical cord. Near the bend in this loop the
yolk-sac is still attached to the intestine by a stalk; the sac itself has been
cut away in the figure. In addition to the pharynx already described, the
al
FIG. 241. STAGES IN THE DEVELOPMENT OF THE DIGESTIVE TUBE. A. Rabbit of nine days. B. Man
2.15 mm. (after His). C. Pig. 12 mm. D. Man, 17.8 mm. (after Thyng). E. Man, about five months.
a., Anus; al., allantois; bl., bladder; cae., bulb of the colon; cl., cloaca; du., duodenum; 1. i., large intestine;
oe., oesophagus; p., penis; pe., perineum; ph., fore-gut; r., rectum; s. i., small intestine; St., stomach;
u. c., umbilical cord; ur., urethra; ura., urachus; u. s., urogenital sinus; v. p., vermiform process:
y. s., yolk-sac; y. St., vitelline duct within the yolk-stalk.
fore-gut has given rise to an expanded portion or stomach. Between the
stomach and pharynx it remains tubular and becomes the oesophagus;
posterior to the stomach it is likewise tubular and there it forms a part
of the small intestine. The first portion of the small intestine is called the
duodenum, and is followed by the jejunum which passes without demarcation into the ileum. The ileum includes the portion to which the yolkstalk is attached, and terminates at a bulbous enlargement (Fig. 241, C,
cae) which gives rise to the cacum and -vermiform process. This bulbus
coli (Johnson) marks the beginning of the large intestine or colon, and
the caecum and vermiform process are parts of the large intestine. Toward
the cloaca the colon becomes the rectum, and near its termination it forms
an elongated bulbous enlargement, the bulbus analis. As shown by F. P.
THE DIGESTIVE TUBE 247
Johnson (in a paper about to be published) this bulb forms essentially
the zona columnaris in the anal part of the rectum. The anus is produced
after the cloaca has separated into dorsal and ventral portions. The
ventral division, which carries with it the allantois, becomes expanded to
form the bladder, but its outlet remains relatively narrow and becomes the
urethra. The outlet of the rectum is the anus, which is at first closed by
the anal membrane; this membrane ruptures in embryos measuring from
20 to 30 mm., except in the occasional cases of imperforate anus. The
tissue which subdivides the cloaca reaches the surface and constitutes the
perineum.
In human embryos of about 10 mm. the intestinal loop becomes twisted
on itself (Fig. 241, D), and the large intestine is carried across the small
intestine in the duodenal region. The vermiform process thus comes to
lie on the right side of the body, and the colon, after it is withdrawn from
the umbilical cord into the body, is so bent as to form ascending, transverse,
and descending portions, below which, as the convoluted sigmoid colon,
it connects with the rectum. The disposition of the adult intestines depends chiefly upon this primary torsion of the intestinal loop, and upon
the subsequent elongation of tne small intestine, which forms many loops
and coils.
Meanwhile the yolk-sac has become detached, and its stalk has disappeared, usually leaving no indication of its former position. The stalk
does not become the vermiform process, as was once supposed, but occasionally it produces a blind pouch of the ileum, 3-9 cm. long, situated about
three feet above the beginning of the colon. This is the diverticulum ilei,
described and correctly interpreted by Meckel in 1812.
The division of the intestine into six parts is a heritage from the Arabians. Duodenum, jejunum, ileum, caecum, colon and rectum were well recognized in the fifteenth
century, when, following Hippocrates, they were counted from below upward. The
various names which have been applied to them are discussed by Hyrtl (Das arabische
und hebraische in der Anatomie, Wien, 1879). Those which are now adopted have the
following significance. The rectum is the straight terminal portion. "Colon is the
K<!>\OV of Aristotle, which according to Pliny is a great source of pain (colic)." The
caecum, or blind intestine, was so named by Galen, who did not practice human dissection and so referred to the more elongated pouch in lower animals. The name has
generally been considered inappropriate for the human caecum. The Greek synonym
rv<f>\bv (blind) is used in the medical term typhlitis (inflammation of the caecum).
The ileum (from eiAe'w) is the coiled portion, and is arbitrarily defined as the lower
three-fifths of the small intestine. The jejunum (Lat., fasting) is the portion generally
found void and empty (Avicenna), since food passes through it rapidly. The duodenum, which has no free mesentery, was originally considered a part of the stomach;
its name indicates that its length is twelve finger-breadths. Hyrtl notes that the same
term has sometimes been applied to the rectum.
Layers of the Digestive Tube. The wall of the digestive tube is com
248 HISTOLOGY
posed of four layers (i) tunica mucosa, (2) tela submucosa, (3) tunica
muscularis, and (4) tunica adventitia or tunica serosa. The parts which
are covered with peritoneum have a serous coat for their outer layer; the
parts imbedded in connective tissue have the adventitious coat instead.
The tunica mucosa consists of epithelium, tunica propria, and the
lamina muscularis mucosce. The epithelium is the entodermal lining of
the tube, and is folded and inpocketed so as to form innumerable pits and
glands, varying in their nature in different parts of the tube. The tunica
propria consists of reticular tissue, which in places becomes characteristic
lymphoid tissue. It is set apart early in development as a layer with
abundant nuclei, thus differing from the underlying mesenchyma. At a
later stage the lamina muscularis mucosce, or muscle layer of the mucous
membrane, develops beneath it, separating it from the submucosa. The
muscularis mucosae is a thin layer of smooth muscle fibers.
The tela submucosa (tela, tissue) is a connective tissue layer which
contains many blood and lymphatic vessels, and the ganglionated plexus
submucosus.
The tunica muscularis usually consists of an inner circular and an outer
longitudinal layer of smooth muscle fibers, separated by a thin layer of
connective tissue which contains the ganglionated plexus myentericus.
The tunica serosa is a connective tissue layer, covered by the peritoneal
epithelium.
The layers enumerated are to be examined in the oesophagus, stomach
and intestine, which differ from one another histologically, since these
layers are variously modified.
(ESOPHAGUS.
The oesophagus is a tube about nine inches long, the several layers
of which are continuous anteriorly with those of the pharynx, and posteriorly with those of the stomach. The mucous membrane is thrown into
folds, except when the tube is distended by the passage of food; but the
muscularis merely thickens on contraction, so that it always forms a smooth
round layer (Fig. 242).
The epithelium is thick and stratified like that of the pharynx. Its
outer cells are flattened in the adult, but in the embryo they include numerous islands of tall ciliated cells, some of which are found at birth. The
basal surface of the epithelium rests upon connective tissue papillae or
ridges.
The glands of the oesophagus are of two sorts, superficial and deep.
The deep glands (glandules msophagece produndci) develop as scattered
tubular downgrowths which pass through the tunica propria and muscu
(ESOPHAGUS
249
laris mucosae into the submucosa, where their blind ends expand and
branch, producing a cluster of tubulo-alveolar end pieces. The terminal
portions at birth are still poorly developed. The tubules are composed
wholly of mucous cells, although the basal protoplasm sometimes simulates
crescents. The ducts are slender tubes generally lined with simple epithelium. They tend to slant toward the stomach, and they enter the
epithelium where it dips down between the connective tissue papillae. The
cells of the ducts become continuous with the basal layer of the epithelium.
Large ducts are sometimes lined with stratified epithelium, often ciliated,
and they may present cyst-like dilatations. Lymphocytes tend to accumu
Stratified epithelium.
Tunica propria.
/ /Muscularis
/ mucosae.
Submucosae.
Mucous
membrane.
\
Group of
fat cells.
Circular muscles, f
Longitudina mus- VMuscularis.
cles. J
Mucous gland.
v Tunica adventitia.
Lymph nodule.
FIG. 242. TRANSVERSE SECTION OF THE UPPER THIRD OF THE HUMAN (ESOPHAGUS. X 5.
late around the ducts and occasionally they form nodules in the tunica
propria. The glands may show signs of infiltration and degeneration.
The number of deep glands varies greatly in different individuals. They
are usually more numerous in the upper half of the oesophagus.
The superficial glands (glandules cesophagea superficiales) are limited
to two rather narrow zones near the ends of the oesophagus. They are
always found at the entrance of the stomach, extending from i to 4 mm.
up the oesophagus; and generally (in 70% of the cases examined by S chaffer)
they occur between the level of the cricoid cartilage and fifth tracheal
ring. They develop in the embryo much earlier than the deep glands,
and appear as small areas of tall mucous cells which pass clear through
the stratified epithelium. These islands of simple epithelium become
depressed into shallow pockets from which a cluster of tubules grows
250 HISTOLOGY
out, but they never pass through the muscularis mucosae into the submucosa. In the adult the upper group may be seen with the naked eye as an
"erosion" of the mucous membrane. The glands produce a form of
mucus which stains less readily with the mucus-stains than that of the
deep glands. No special function has been assigned to this secretion.
Glands of the lower group are shown in Fig. 243. They are freely branching mucous glands, the ducts of which open at the tops of connective tissue
papillae. They very frequently show cystic enlargements.
d e f g
FIG. 243. LONGITUDINAL SECTION THROUGH THE JUNCTION OF THE HUMAN (ESOPHAGUS
AND STOMACH. X 60.
a, Duct of a superficial cesophageal gland; b, oesophageal epithelium; c, gastric epithelium; d, tubule of
the gland a; e, lymphoid nodule; f, lymphatic vessel; g, lamina muscularis mucosae.
The tunica propria in the oesophagus has fewer cells in its meshes than
that of the lower parts of the digestive tube. In places it includes solitary
lymph nodules. The muscularis mucosae is very wide in the oesophagus.
It is a layer of longitudinal smooth muscle fibers, which is thrown into longitudinal folds when the oesophagus is contracted. It begins anteriorly at
the level of the cricoid cartilage, arising as scattered bundles inside the
elastic layer of the pharynx. As the muscles increase to form a distinct
layer, the elastic lamina terminates. The submucosa is a loose connective
tissue layer, containing many vessels and nerves, groups of fat cells, and
the bodies of the deep mucous glands. The muscularis consists of an inner
circular and an outer longitudinal layer, as elsewhere in the digestive tube,
but in the upper part of the oesophagus the layers are composed of striated
CESOPHAGUS 251
muscle fibers. These fibers are not a downward extension of the striated
pharyngeal constrictors, but apparently develop from exactly such mesenchymal cells as produce smooth muscle further down. The striated
muscles in man are limited to the upper half of the oesophagus ; in the rabbit
they extend its whole length.
The adventitia is loose connective tissue, containing many vessels and
the plexiform branches of the vagus nerves. From these nerves, medullated and non-medullated fibers enter the oesophagus and form a ganglionated myenteric plexus between the muscle layers, and the plexus submucosus in the submucosa. Medullated fibers proceed from the vagus
trunks to the motor end plates of the striated muscles, which are thus
stimulated reflexly from the central nervous system. Other fibers pass
from the myenteric plexus to the plexus submucosus and thence to the
epithelium, in which free nerve endings have been found. Such fibers,
together with those to the smooth muscles, provide for local reflex action,
whereby the contents of the oesophagus causes contraction above, and relaxation below, the place of stimulation. This takes place independently
of the central system, and is the form of innervation characteristic of the
intestine.
STOMACH.
Form and Subdivisions. The opening through which the oesophagus connects with
the stomach is the cardia (Gr. KapSui, heart), and the opening from the stomach to the
intestine is the pylorus (Gr. 7rvA.o>pds, gate-keeper). The pylorus received its appropriate name from Galen (in the second century), who recognized that through its sphincter
muscle it controlled the exit of food. The significance of cardia was discussed by
Fabricius (1618) who cites Galen as stating that the upper orifice of the stomach is
called the heart because the symptoms to which it gives rise are similar to those which
sometimes affect the heart, sometimes even the brain; but for Fabricius, cardia, as
applied to this orifice, merely indicates a chief part of the body. The stomach as a
whole is termed gaster, from the Greek, but the Latin ventriculus was generally used
by the early anatomists. Although flaccid and shapeless when seen in the dissecting
room, the stomach has a very characteristic form. Its epithelium, from an embryo
of 44.3 mm., is shown in Fig. 244, and an adult stomach is seen in Fig. 250. It is a
tube which is greatly distended toward the left, where its border forms the greater
curvature; its right border is the lesser curvature. As a whole the stomach is divided
into two parts, the cardiac portion (pars cardiaca) and pyloric portion (pars pylorica).
This fundamental subdivision occurs in many animals, as was recognized by Sir
Everard Home in 1814. The pyloric part is relatively long in the embryo. It becomes
subdivided into the pyloric vestibule and the pyloric antrum. The latter is its smaller
part extending to the pylorus; between the two, on the greater curvature, is the sulcus
intermedius, well shown in Fig. 250. (The term pyloric antrum has been variously
employed, since in its original description by Willis (1674) the vestibule is not recognized; Cowper (1698) applies antrum to the terminal subdivision as above defined.)
The cardiac part of the stomach is divided into a main portion, or body of the stomach
(corpus gastri], and a blind pouch, formerly called the saccus caecus, but now less
252
HISTOLOGY
appropriately known as the fundus gastri (the bottom of the stomach). Recently
the gastric canal (canalis gastri) has been recognized along the lesser curvature of the
human stomach. It is a channel, highly developed in ruminants, which conveys liquids
from the cardia to the pars pylorica, when the stomach is filled with more solid contents.
Ordinarily open toward the interior like a groove, it may become closed as a tube
during its physiological activity. Beyond the cardia there is a conical expansion of
the oesophagus, not always well defined, known as the cardiac anlrum, and beyond
the pylorus is the first part of the duodenum, or duodenal antrum. (A further account
of the development of these subdivisiods will be found in the Amer. Journ. Anat.,
1912, vol. 13, pp. 477-503.)
(Esophagus.
Gastric canal.
Angular incisure.
Fundus.
Duodenal
antrum.
Corpus.
FIG. 244. MODEL OF THE GASTRIC EPITHELIUM IN A HUMAN EMBRYO OF 44.3 MM. X 18 diam.
The inner surface of the stomach presents macroscopic longitudinal
folds, which become coarse and prominent as the organ contracts. They
are sinuous, and anastomose in an irregular network. As finer markings,
there are rounded or polygonal areas, 2-4 mm. in diameter, which may
appear as elevations or depressions. They have been ascribed to the contraction of muscle fibers in the mucous membrane, to varying amounts of
lymphoid tissue, and to the varying height of the glands. Toward the
pylorus there are small leaf-like elevations, the plica villosce, which may
connect with one another in a network. The epithelium of the stomach is
thin enough to transmit the color of the underlying tissue, and appears
pinkish gray; whereas the color of the oesophagus, with a thicker epithelium,
is white.
The gastric epithelium, like that of the entire intestine, is a single layer
of columnar cells. In the stomach the cells are tall and contain mucus, but
they do not ordinarily acquire the bulging goblet shape, since the adjacent
cells likewise contain mucus. This simple layer of mucous cells is continuous at the cardia with the basal layer of the stratified epithelium of the
STOMACH 253
oesophagus, and the transition is abrupt. The outer strata of the cesophageal epithelium may form an overhanging wall (Fig. 243), or the number of
layers may have become reduced so that such a wall is absent. Sometimes
an island of stratified epithelium occurs just beyond the line of transition.
The gastric epithelium forms three types of glands, known as cardiac,
gastric, and pyloric glands respectively, none of which extend into the
submucosa.
The cardiac glands are like the superficial glands at the lower end of the
oesophagus, of which they may be regarded as a continuation. They extend only from 5 to 40 mm. into the stomach, and in the narrow zone which
they occupy, they present a gradual transition to the gastric glands.
Their branches, instead of continuing divergent, become groups of perpendicular tubes descending from epithelial pits; and deeply staining
eosinophilic cells and the granular chief cells become included in their
epithelium.
The cells characteristic of the cardiac glands contain a mucus which
does not respond readily to mucin stains. Like the superficial glands of
the oesophagus, the cardiac glands develop early, and they are found widely
distributed among mammals.
The gastric glands (sometimes inappropriately called fundus glands)
occur over the entire surface of the stomach, except near the cardia and pylorus. Each gastric gland is divided into an outer portion, or gastric pit
(foveola gastrica] , and a group of slender cylindrical tubules which empty
into the bottom of the pit. During development, as the lining of the stomach expands greatly, the number of pits increases. Toldt estimated that
there were 129,912 in the stomach at three months; 268,770 at birth and
2,828,560 at ten years. The increase is accomplished by division of the
pits from below upward. In spite of the fact that many new branches
develop, the average number of tubules emptying into each pit becomes
reduced as the pits become subdivided; and the average of seven per pit
observed at birth becomes three in the adult (Toldt, Sitz.-ber. Akad. d.
Wiss. Wien, 1881, vol. 82, pp. 57-128).
The pits are often described as if they were epithelial depressions separate from the glands, since the same sort of epithelium which lines them is
found on the free surface. Developmentally, however, they are to be regarded as parts of the glands, comparable with ducts. The epithelial cells
of the pits (Fig. 245) consist of a basal protoplasmic portion containing elongated, round, or sometimes flattened nuclei, and an outer portion containing the centrosome and secretion. The mass of mucus may cause the thin
top plate to bulge, and in preserved tissue to rupture, but this may be due
to reagents. The mucus first appears in granular form.
The gastric tubules are straight or somewhat tortuous slender structures, with narrow lumens. The portion which joins the pit constitutes
254
HISTOLOGY
the neck of the gland, and the slightly expanded basal end is the fundus.
Apparently the neck is the zone of growth, since it is the place where
Epithelium.
' '''-* ^ ' ''''-''
Gastric pit.
Neck. ,
Smooth muscle fibers.
Parietal cell.
%^ i..-;* \ i]ir Fundus Hi 'ii-'^ H'^-'Ml,
Ill; mm \'wd >'
Tubules
of the
gastric
glands.
i : v
FIG. 245. VERTICAL SECTION OF THE Mucous MEMBRANE OF A HUMAN STOMACH, SHOWING GASTRIC
GLANDS (GLANDUL* GASTRIOB PROPRL). X 220.
mitotic figures are found. Each tubule is composed of cells of two sorts,
chief cells and parietal cells.
The chief cells usually form the greater part of the tubules. They are
STOMACH
255
Gland lumen
FIG. 246. TRANSVERSE SECTION OF A
HUMAN GASTRIC GLAND. X 240.
Axial lumen.
Parietal cells with
intracellular se- \
cretory capillar- \
wedge-shaped cells, having a narrow contact with the lumen. In general
they have the aspect of serous cells, containing round nuclei and granular
protoplasm. The granules, which are coarser toward the lumen, do not
respond to mucin stains. They accumulate, and the chief cells enlarge, in
the absence of food from the stomach; but
during gastric digestion, the cells become .JftsSk Chief ceil.
small and the granules disappear. They Parietal ceii.
apparently give rise to the pepsin of the
gastric juice, and are called zymogen
granules. After death the chief cells
rapidly disintegrate, and the granules are seldom well preserved except in
special preparations.
The parietal cells, even in fresh tissue, may be readily distinguished
from the chief cells; the latter are dark and contain refractive granules,
whereas the parietal cells are clear.
They are large cells, containing one or
occasionally two round nuclei, and are
crowded away from the lumen like the
cells in the serous crescents (Figs. 245
and 246). They discharge their secretion through secretory capillaries which
produce basket-like networks within
the protoplasm; thus they differ from
the chief cells which have only intercellular secretory capillaries. The
secretory capillaries of the parietal cells
may be demonstrated by the Golgi
method, which produces a precipitate
wherever secretion is encountered (Fig.
247). After fasting, the parietal cells
are small and their intracellular capillaries have disappeared. Following
abundant meals, these cells enlarge and
may contain vacuoles due to the rapid
formation of secretion. They produce
the hydrochloric acid which is found in
the gastric juice.
In ordinary preparations they are
better preserved than the chief cells, and exhibit a finely granular structure,
being deeply stained with the anilin protoplasmic dyes. They differ so
markedly from the chief cells that they have been erroneously believed to
develop from the surrounding tunica propria. As seen in Fig. 245 they
occur chiefly along the body of the tubule, being infrequent at its fundus.
Intercellular secretory capillaries.
Chief cells./
FIG. 247. GOLGI PREPARATION, SHOWING THE
SECRETORY CAPILLARIES IN GASTRIC
GLANDS. X 230.
256 HISTOLOGY
The pyloric glands are found near the pylorus, but the area which they
occupy is not sharply set off; they pass over into gastric glands through
a "transition zone." Pyloric glands have very deep pits, from which
short, winding, branched tubules grow out. Their form in the adult is
shown in Fig. 248. The cells in the pits are mucous cells, and those in
Simple epithelium
cut obliquely, so
that it appears to
be stratified.
Tunica propria.
Pyloric gland.
Sections of pyloric
glands.
Solitary nodule.
Muscularis
mucosae.
FIG. 248. VERTICAL SECTION OF HUMAN PYLORIC GLANDS. X 90.
the tubules are also regarded as mucous cells. The latter are columnar,
with rounded nuclei in their basal part, and protoplasm which may closely
resemble that of the chief cells. Parietal cells are occasionally found, and
such cells have been reported in the duodenal glands and in the superficial
glands of the oesophagus. Slender dark cells, apparently due to com
STOMACH
257
Mucosa.
Muscularis
pression, are found in the pyloric glands of the dog. In certain respects
the pyloric glands are transitional between gastric and duodenal glands.
The tunica propria consists of the small amount of reticular and connective tissue which is found between the closely packed glands and immediately beneath them (Fig. 249). It is sufficient to support the numerous
capillaries branching about the glands, the terminal lymphatic vessels
and nerves, numerous wandering cells and a few vertical smooth muscle
fibers prolonged from the muscularis mucosae (Fig. 245). The lymphatic
vessels begin blindly
near the superficial epithelium and pass between the glands into
the submucosa where
they spread out and
are easily seen; they
continue across the
muscularis and pass
through the mesentery
to join the large lymphatic trunks. Solitary
nodules occur in the
gastric mucosa, especially in the cardiac and
pyloric regions (Figs.
243 and 248) ; they may
extend through the
muscularis mucosae into
the submucosa. The
muscularis mucosae may
be divided into two or three layers of fibers having different directions.
The submucosa contains its plexus of nerves and many vessels, together
with groups of fat cells. Its elastic fibers are said to be abundant toward
the pylorus.
The muscular coat of the stomach consists of three layers of smooth
muscle, an outer longitudinal, middle circular, and inner oblique layer
respectively. These layers can be recognized by dissection more readily
than by microscopic examination, and were found by Willis in 1674.
The middle layer is the one most highly developed. It not only surrounds the body of the stomach, but as the fundus pushes outward,
muscle fibers of this layer encircle its apex concentrically. Toward the
pylorus, along the antrum, the circular layer gradually thickens, thus
forming the sphincter pylori; it becomes abruptly thin in the duodenum.
There is no sphincter at the cardia, where the circular layer is continuous
17
Epithelium.
Tunica propria.
M uscularis mucosse. ^
Submucosa.
Smooth muscle cut
lengthwise.
Connective tissue.
Smooth muscle cut
transversely.
Serosa.
FIG. 249. VERTICAL SECTION OF THE WALL OF A HUMAN STOMACH.
The tunica propria contains glands standing so close together that
its tissue is visible only at the base of the glands toward the
muscularis mucosae.
258 HISTOLOGY
with that of the oesophagus, but elastic tissue in the muscularis is said to
be specially abundant and to "contribute to the tonus of the cardiac musculature." The outer longitudinal layer, continuous with the outer layer
in the oesophagus and duodenum, is an incomplete layer, being deficient
toward the greater curvature. As the body of the stomach bulges outward to form this curvature, the longitudinal fibers apparently become
separated into scattered bundles. In the pars pylorica, however, there
is a continuous longitudinal layer, and some of its fibers, which become
intermingled with those of the sphincter pylori, serve to dilate the pylorus. The innermost layer, composed of oblique fibers, is not represented
in the oesophagus and duodenum,
and is said to be absent from the
pars pylorica. The peculiar arrangement of its fibers is shown in Fig.
250, in which the outer longitudinal
layer has been almost entirely removed, and windows have been cut
through the circular layer; the oblique
fibers are seen against the submucosa.
& L/ TiiSl'l/^M Al^UMF* The y form a longitudinal strand par
allel with the lesser curvature, and
they pass from one side of the
' OF THE S !TOMACH. (Spaitehohs!" * stomach to the other across the notch
a X;^ n o&^aytr7 r ^/pyior^sfs^ between the oesophagus and fundus.
These fibers are important in the
activity of the gastric canal, but they do not produce the canal as some
have supposed. From these longitudinal bundles, fibers curve obliquely
toward the greater curvature, where, as transverse fibers they cross to
the opposite side. Thus the musculature of the stomach is so arranged
that it is very difficult to determine the plane of section in a small piece
of gastric mucous membrane, which is usually cut obliquely; but the
section shown in Fig. 249, with inner and outer layers cut lengthwise
and a middle layer cut across, is consistent with a longitudinal section
of the corpus gastri.
The tunica serosa consists of connective tissue with well-developed elastic nets, and a covering of peritoneal epithelium interrupted only along
the curvatures, at the mesenteric attachments. It contains the nerves and
vessels which supply the stomach. The right and left vagus trunks descend beside the oesophagus as the main stems in a plexiform network,
and then come together along the lesser curvature. From there they
send plexiform branches over both sides of the stomach, and the main
stems continue into the small intestine. Sympathetic nerves from the
coeliac plexus pass to the pyloric end of the stomach and join the vagus
DUODENUM
259
plexus. The further distribution of the nerves in myenteric and submucous plexuses is similar to that in the small intestine.
DUODENUM.
The duodenum contains branched mucous glands, the bodies of which
are found in the submucosa. These are called duodenal glands (B runner's
Intestinal gland
Epithelium. Villi.
Duodenal gland.
/'Plica, circularis.
Fat. Duodenal glands in the
submucosa.
Tunica propria
Muscularis
mucosas. '
Submucosa. r
Stratum of \ '
circular muscle. t-.
Stratum of longi- g
tudinal muscle. ~.
Connective tissue .
Intestinal glands
Longitudinal
section.
FIG. 251. LONGITUDINAL SECTION OF THE HUMAN DUODENUM. X 16.
glands) and they occur nowhere else in the small intestine (Fig. 251).
Their cells produce a mucus which stains with difficulty, thus contrasting
with the mucus of the goblet cells in the
tubular glands above them. The nature
of their epithelium is shown in Fig. 252,
which shows also that a portion of their
tubules may lie above the muscularis
mucosae, in the tunica propria. As in the
pyloric glands, occasional parietal cells
have been found, and also the dark cells,
due to compression. Secretory capillaries
extend out from the lumen between the
cells, and the tubules are provided with a
structureless basement membrane. The
ducts of the duodenal glands may open
on the free surface of the epithelium, or
into the lower ends of the tubular pits
situated in the mucous membrane and
known as intestinal glands. The duodenal
glands are so numerous toward the stomach that the submucosa may be filled
with their tubules. They are also abundant near the duodenal papilla where the
Transverse
section.
Longitudinal
section.
of the tubules of a duodenal gland.
FIG. 232. FROM A SECTION OF A HUMAN
DUODENUM. X 240.
Only the lower half of the mucosa and
upper half of the submucosa are
sketched.
260
HISTOLOGY
bile and pancreatic ducts enter the descending portion of the duodenum.
Beyond this point they become fewer, and disappear before the end of
the duodenum is reached. Except for these glands the duodenum is
essentially like the remainder of the small intestine, described in the
following section.
JEJUNUM AND ILEUM.
The lining of the small intestine, including the duodenum, has a velvety
appearance, due to the presence of innumerable cylindrical, club-shaped
or foliate elevations, known as mill (hairs or nap). True villi are found
in the large intestine of the embryo but they disappear before birth; they
are said to occur also in the pyloric end of the stomach, but it is questionable whether these are typical villi or merely irregular folds. Elsewhere
in the digestive tube, villi are absent. At the bases of the villi there are
simple tubular pits of glandular epithelium, which extend to the muscularis mucosse but do not penetrate it; these are the intestinal glands (glandul(B intestinales, formerly known as crypts of Lieberkiihn). An enlarged
^OV^ry* V :
$%i$m : to
FIG. 253.
A, Surface view of the hardened mucosa of the small intestine (after Koelliker). B, Side view of a wax
reconstruction of the epithelium in the human duodenum (Huber). i. g., Intestinal gland; v., villus.
surface view of the hardened mucous membrane is shown in Fig. 253, A
The orifices of the glands appear as round holes; the villi, which are from
0.2-1.0 mm. in height, have fallen over in various directions. Within
the duodenum the villi are low leaf-like folds, 0.2-0.5 mm. high, seen in
side view in the reconstruction, Fig. 253, B. Their shape cannot be determined from inspecting single sections (cf. Fig. 251).
It will be seen that villi are essentially circumscribed folds, and they
have been said to arise through the subdivision of longitudinal ridges
(Berry, Anat. Anz., 1900, vol. 17, pp. 242-249). According to Johnson
(Amer. Journ. Anat., 1910, vol. 10, pp. 521-561) they develop as low
knob-like elevations which increase in height. They may become subdivided, as indicated by bifid villi (Fig. 253).
The small intestine contains other elevations of its lining which are
much larger than the villi. These are the circular folds (plica circulares,
SMALL INTESTINE
26l
formerly known as Kerkring's valvula conniventes), which are seen conspicuously on opening the intestine. They are thin leaf -like membranes,
in places very close together, which, as their name implies, tend to encircle
the tube. Sometimes they form short spirals, and they may branch and
connect with one another. They begin in the duodenum, and beyond the
duodenal papilla they are tall and close together. They are highly
developed in the jejunum and form its most characteristic feature. In
the ileum they are lower and further apart; and they may come to an
end two feet above the colon. The villi correspondingly are taller and
Villi.
X X
Epithelium.
Plica circularis.
Intestinal glands
Submucosa
Circular muscle.
FIG. 254. VERTICAL LONGITUDINAL SECTION OF THE JEJUNUM OF AN ADULT MAN. X 16.
The plica circularis on the right supports two small solitary nodules, which do not extend into the submucosa; one of them exhibits a germinal center, x. The epithelium is slightly loosened from the
connective tissue core of many of the villi, so that a clear space, xx, exists between the two. The
isolated bodies lying near the villi (more numerous to the left of the plicaj circulares) are sections of
villi that were bent, so that their ends were cut off in sectioning.
more numerous in the jejunum than in the ileum, in the distal part of which
they are short and scattered, finally disappearing on the colic surface of
the valve of the colon (ileo-csecal valve). Thus few and short villi and
scattered plicae indicate that a section of the intestine is from the ileum.
As seen in sections, the plica circulares are elevations of the submucosa (Fig. 254) covered on both sides by the entire mucous membrane
villi, glands and the muscularis mucosae. A low plica of the duodenum
is shown in Fig. 251.
262
HISTOLOGY
The glands, villi, and plicae have usually been regarded as permanent structures,
serving to increase the secreting and absorbing surfaces of the intestine. In mammals
they apparently are not obliterated by the normal distention of the intestine, although
the villi may become shorter, the glands shallower, and the plicae may be partially
taken up like the folds of the oesophagus. In the guinea-pig, and to some extent in
the rabbit and cat, Heitzmann found that the villi change their shape with the intes
* C
FIG. 255. EFFECTS OF DISTENTION ON THE SMALL INTESTINE OF THE ADULT GUINEA-PIG. X so.
(Johnson.)
A, Strongly contracted; B, normally distended with food; C, distended with a pressure of 150 cm. of water.
tinal contractions and expansions associated with its physiological activity. Johnson
(Amer. Journ. Anat., 1913, vol. 14, pp. 235-250) has shown that in guinea-pigs the
villi and glands of the contracted intestine have the form seen in Fig. 255, A; with normal distention due to abundant food, they appear as in B ; and with extreme artificial
distention, the glands and villi are nearly obliterated as in C. The tube expands
to this limit, beyond which additional pressure has no effect until it ruptures. On
releasing the pressure, glands and villi return to their normal size. Interesting questions are suggested, as to how the muscle fibers become rearranged in the thin layer
when the intestine is distended, and what takes place in the blood and lymphatic vessels.
These problems are under investigation.
Finer Structure of the Glands and Villi. At the blind lower end or
fundus of the glands, there occur certain cells containing many coarse
granules in that part of their protoplasm which is toward the lumen (Fig.
256). These cells were first described by Paneth (Arch. f. mikr. Anat.,
1888, vol. 31, pp. 113-191) and are known as Paneth's cells. They are
found in the glands of the duodenum, jejunum and ileum, but not in those
of the large intestine. Although they may be observed with ordinary
stains, they are more strikingly demonstrated in iron-haematoxylin preparations. Apparently they produce a special secretion, which enters the
lumen of the gland in the form of fine granules when the digestion of fat
SMALL INTESTINE
263
is taking place, and may perhaps be concerned also with protein digestion
but not with that of carbohydrates (Miram, Arch. f. mikr.
Anat., 1912, vol. 79, pp. 105113). They do not contain
mucinogen granules, although
goblet cells occur in their immediate vicinity.
A short distance above the
fundus, the epithelial cells of
the glands exhibit mitotic figures. From this it is inferred
that the outer cells, including
those of the villi, are renewed
from below. The cells near the
bottom of the gland have
terminal bars, but they are not
as distinct as those of the villi.
FIG. 256. THE FUNDUS OF AN INTESTINAL GLAND FROM
THB3DUODENUM;OF A GUINEA-PIG. X 480.
a, Cell in mitosis; b, lymphocyte; c, Paneth's cell;
d, goblet cell.
During division, the cell seems to be
drawn up from the basement membrane, as if held in position by the
Epithelium.
Tunica propria.
Portion of a capillary
blood vessel.
Cuticula.
Nucleus of a lymphocyte.
Tangential section of a
goblet cell.
Mucus in a goblet cell.
Nucleus of a smooth muscle fiber. Central lymphatic vessel.
FIG. 257. LONGITUDINAL SECTION THROUGH THE APEX OF THE VILLUS OF A DOG. X 360.
The goblet cells contain less mucus as they approach the summit of the villus.
terminal bars (Fig. 256, a). The plane of division is at right angles with
the long axis of the gland (as shown on the right of Fig. 256), and after
264
HISTOLOGY
Epithelium.
FIG. 258. FROM A SECTION OF THE SMALL INTESTINE FROM
A KITTEN SEVEN DAYS OLD. X 250.
The epithelium on the left contains many wandering
leucocytes (lymphocytes). The epithelium on the right
contains but three.
mitosis the nuclei move back to the basal layer. Lymphocytes which
have made their way between the epithelial cells (Fig. 256, b), are
frequently seen, and when near the lumen and over-stained they may be
mistaken for mitotic figures.
The sides of the glands and surfaces of the villi are covered with simple
columnar epithelium, similar to that shown in Fig. 256. It contains
goblet cells separated from one another by cells free from mucus. The
cells of the villi are taller than
those in the glands, and the
goblet cells are somewhat
larger, but toward the tip of
the villus they become slender and empty (Fig. 257).
The top plates or cuticula
become thicker from the
fundus of the gland outward
to the tips of the villi, and
when well developed they
exhibit vertical striations which are considered to be protoplasmic
processes lodged in pores. The top-plate of the goblet cells is thin and
apparently ruptures to allow the escape of the mucus. Lymphocytes
may enter the epithelium in abundance as shown in Fig. 258.
Interest in the villi centers chiefly in their relation to the absorption
of nutritive material from the intestinal contents (chyme). Fat, chemically changed so that it does not
blacken with osmic acid, is conveyed through the cuticula.
Within the epithelial cells it forms
characteristic fat droplets, which
appear in abundance also between
the epithelial cells. Lymphocytes
ingest the droplets, and may then
enter the lymphatic vessel in the
central axis of the villus (Fig.
257), but apparently fat is conveyed to the lacteals also through intercellular spaces, without the intervention of leucocytes. Within the lymphatic vessel it forms the milky
lymph known as chyle.
In regard to the absorption of protein material, the observations of Pio Mingazzini,
which have been confirmed by some and denied by others, are of considerable interest.
As shown in Fig. 259, he found that the basal protoplasm of the resting epithelium
presented an ordinary appearance (A), but that after absorption had progressed, hyal ine spherules appeared iu it (B). As these became numerous they were detached from
FIG. 259. STAGES OF INTESTINAL ABSORPTION AS
SEEN IN EPITHELIAL CELLS OF VILLI FROM A
HEN. (After Mingazzini.)
A and D, The states of repose preceding and following the process, s., Spherules.
SMALL INTESTINE
265
the cells, forming a reticular mass between them and the tunica propria (C). After
the spherules had broken down and had probably been transferred to the blood vessels,
the tunica propria entered into its usual relation with the shortened epithelium (D).
The basal protoplasm was then restored. According to this interpretation protein
absorption is accomplished as a secretory process of the epithelium, the product being
eliminated from its basal portion. The spherules accumulate at and near the tips of
the villi, in spaces which many authorities describe as due to the artificial retraction
of the tunica propria (Fig. 260, a). The spherules have been considered a coagulum
of the fluid squeezed from the reticular tissue. In part they may be boundaries of
the basal ends of epithelial cells on the distal wall of the villus.
Sections of villi.
Ephitelium.
Muscularis
macosae.
**~
Submucosa. Intestinal glands. Oblique sections of intestinal glands.
PIG. 260. VERTICAL SECTION OF THE Mucous MEMBRANE OF THE JEJUNUM OF AN ADULT MAN. X 80,
The space, a, between the tunica propria and the epithelium of the villus is perhaps the result of the shrink.
ing action of the fixing fluid. At b the epithelium has been artificially ruptured. The goblet cells
have been drawn on one side of the villus on the right.
Outer layers of the small intestine. The tunica propria, which forms the
cores of the villi and extends between the glands, is a reticular tissue,
containing the usual types of free cells and also a large number of plasma
cells (see p. 68). Slender strands of smooth muscle extend up and down
the villi, being inserted into the reticulum, and by contraction they cause
the villi to shorten. The muscularis mucosa consists of an inner circular
and an outer longitudinal layer, thus duplicating on a small scale the tunica
muscularis. The submucosa is a connective tissue layer, such as has been
described in the stomach and oesophagus, and the muscularis is divided into
a thick inner circular layer of smooth muscle and a thinner outer longitudinal layer, between which is a thin stratum of intermuscular connective
266
HISTOLOGY
tissue. The intestine is covered externally by the tunica serosa. The
distribution of the vessels and nerves in these layers is as follows.
Blood vessels. The arteries pass from the mesentery into the serosa,
in which their main branches tend to encircle the intestine. Smaller
branches from these pass across the muscle layers to the submucosa, in
which they subdivide freely (Fig. 261, A). In crossing the muscle layers
they send out branches in the intermuscular connective tissue. These
and the arteries of the serosa and submucosa supply the capillary networks
found among the muscle fibers. The capillaries are mostly parallel with
the muscles. From the submucosa the arteries invade the mucosa, form
A B C
FIG. 261.
A f Diagram of the blood vessels of the small intestine; the arteries appear as coarse black lines; the capillaries as fine ones, and the veins are shaded (after Mall). B, Diagram of the lymphatic vessels (after
Mall). C, Diagram of the nerves, based upon Golgi preparations (after Cajal). The layers of the
intestine are m., mucosa; m. m., muscularis mucosae; s. m., submucosa; c. m., circular muscle; i. c.,
intermuscular connective tissue; 1. m., longitudinal muscle; s., serosa. c. 1., central lymphatic; n.,
nodule; s. pi., submucous plexus; m. pi., myenteric plexus.
ing an irregular capillary network about the glands, and sending larger
terminal branches into the villi. There is usually a single artery for a
villus, and it has been described as near the center, with the veins at the
periphery (Fig. 261), or sometimes on one side of the villus with the vein
on the other. The network of blood vessels in the villi is very abundant
as shown in Fig. 262. The veins branch freely in the submucosa and pass
out of the intestine beside the arteries. The muscularis mucosae has been
described as forming a sphincter for the veins which penetrate it; thus it
may control the amount of blood within the villi. No valves occur
until the veins enter the tunica muscularis; there they appear, and continue into the collecting veins in the mesentery. They are absent from
SMALL INTESTINE
267
the large branches of the portal vein which receive the blood from the
intestines.
Lymphatic vessels. The intestinal lymphatics (lacteals) appear as
Vein.
Tunica propria.
Muscularis mucosae. Submucosa.
FIG. 262. VERTICAL SECTION OF THE Mucous MEMBRANE OF THE HUMAN JEJUNUM. X so.
The blood vessels are injected with Berlin blue. The vein of the first villus on the left is cut transversely.
Villus.
Intestinal glands.
Submucosa.
Muscularis
mucosae. Lymph nodules.
Circular Longitudinal
layer. layer.
of the muscularis.
FIG. 263. TRANSVERSE SECTION OF AGGREGATE NODULES OF THE SMALL INTESTINE OF A CAT.
The crests of four nodules were not within the plane of the section. X 10.
central vessels within the villi (Fig. 261, B). Each villus usually contains
a single lacteal ending in a blind dilatation; sometimes there are two or
three which form terminal loops. In some stages of digestion the disten
268
HISTOLOGY
tion of these lymphatics is very great and their endothelium is easily
seen in sections. When collapsed they are hard to distinguish from the
surrounding reticulum. Small lateral branches and a spiral prolongation
of the central lymphatic have been found by injection, but these may be
tissue spaces into which the injected fluid has been forced. The lymphatics branch freely in the submucosa and have numerous valves. They
cross the muscle layers, spreading in the intermuscular tissue and the serosa,
and pass through the mesentery to the thoracic duct.
Lymphoid tissue. The lymphoid tissue of the intestine occurs primarily in the tunica propria, and in three forms diffuse lymphoid tissue,
solitary nodules, and aggregate nodules. Solitary nodules are seen in Fig.
254. The nodules are surrounded by small vessels, the lymphatics being
A B
FIG. 264.
A, Surface view of the plexus myentericus of an infant. X 50. g. Groups of nerve cells; r, layer of circular muscle fibers recognized by their rod-shaped nuclei. B, Surface view of the plexus submucosus
of the same infant. X So. g, Groups of nerve cells; b, blood vessel visible through the overlying
tissue.
drawn in Fig. 261, B. Blood vessels may make a similar net, and penetrate the outer portion of the nodule. The germinative centers are similar to those in the lymph glands.
Aggregate nodules (Peyer's patches) are oval areas, usually from i to
4 cm. long but occasionally much larger, composed of from ten to sixty
nodules in close contact (Fig. 263). The nodules may be distinct or
blended in a single mass. They distort the intestinal glands with which
they are in relation, and immediately above the nodules the villi are partly
or wholly obliterated. Thus they appear as dull patches in the lining
of the freshly opened intestine, and may be readily seen. There are from
fifteen to thirty of them in the human intestine (rarely as many as fifty
or sixty) and they occur chiefly in the lower part or the ileum on the side
SMALL INTESTINE
269
opposite the mesentery. A few occur in the jejunum and the distal part
of the duodenum. In the vermiform process, diffuse aggregate nodules are
always present, but they do not occur elsewhere in the large intestine.
Nerves. The small intestine is supplied by prolongations of the vagus
nerves, which are joined by branches of the superior mesenteric plexus
of the sympathetic system. The latter are regarded as the principal
supply. This plexus is ventral to the aorta, and sends branches through
the mesentery into the serosa. The manner in which they penetrate the
other layers, forming the myenteric plexus (Auerbach's plexus) between
the circular and longitudinal muscle-layers, and the submucous plexus
(Meissner's plexus) in the submucosa, is shown in Fig. 261, C. In surface
view, obtained by stripping the layers apart, these plexuses are seen in
Fig. 264. Their branches supply the smooth muscle fibers. From the
submucous plexus the nerves extend into the villi, where nerve cells have
been detected by the Golgi method (Fig. 261, C); it has been suspected,
however, that some of these "nerve cells" are portions of the reticular
tissue. The nerve fibers probably terminate in contact with epithelial
cells and provide for local reflex action, whereby the muscles contract in
response to stimulation of the epithelium. Most of the intestinal nerves
are non-medullated, but they include a few large medullated fibers said
to have free endings in the epithelium.
MESENTERY AND PERITONEUM.
The serous membrane which surrounds the intestinal tube and certain
other abdominal viscera is a part of the lining of the body cavity. Its
general relations are shown in the diagram, Fig.
265. After covering the ventral surface and the
sides of the intestinal tube, the two layers of serous
membrane come together to form the mesentery
and extend to the dorsal body wall; then, separating, they pass laterally as the lining of the abdominal walls and again come together in the midventral line. This serous membrane, or peritoneum, consequently forms a closed sac. It is divisible into the visceral peritoneum which covers the
viscera, and parietal peritoneum which lines the
body walls. In all cases its free surface is covered
with a single layer of flat polygonal cells, resembling endothelium (Fig. 266, B). Although quite
flat, the cells have a thin cuticular border which
is said to be striated, and the cuticulae of adjacent cells fit together closely.
The lateral walls of these flat cells are connected with one another by proto
FIG. 265. DIAGRAM OF A
MESENTERY AS SEEN IN
CROSS SECTION OF THE
ABDOMEN. (After Minot.)
a., Aorta; c. p., cavity of the
peritoneum; int., intestine; mes., mesentery; p.
m. and v. m., parietal and
visceral layers of mesothelium.
270
HISTOLOGY
plasmic bridges; thus in passing through the epithelium along the intercellular boundaries, one or two intercellular vacuoles would be encountered
(Fig. 266, A). Wandering cells pass readily across this epithelium,
between the cells, and substances in the peritoneal cavity are taken up
into the subserous lymphatics. It has long been thought that there are
permanent orifices or "stomata" between the epithelial cells (Fig. 266, B),
bounded either by modified protoplasm or by separate small cells, and that
lymphatic vessels open directly into the serous cavity through such stomata. This is contrary to recent investigations of the nature of lymphatic
vessels, and the existence of stomata as permanent apertures has
been denied. The stomata, so frequently found in a great variety of
animals may be shrinkage effects
caused by reagents, but their interpretation is not clear. In any
case, the transfer of material
through the epithelium takes place
readily, and the substances or cells
which pass through may be taken
up freely by the closed lymphatic
vessels in the underlying tissue.
In the mesentery, a thin layer of connective tissue with elastic networks
and interwoven bundles of white fibers fills the interval between the two
epithelial layers. In this connective tissue there are many lymphatic
and blood vessels, and nerves to the various organs. Mast cells may be
found along the vessels, especially in young animals (Fig. 55, p. 68) and
various other forms of wandering cells occur. The connective tissue layer
is denser in the parietal than in the visceral peritoneum. In places where
the peritoneum is freely movable there is a subserous layer of loose fatty
tissue, but there is no subserous layer in the intestine.
FIG. 266. SEROUS MEMBRANES.
A, Vertical section of the epithelium (after Heidenhain); B, Surface view, showing two stomata
(after Ludwig).
VERMIFORM PROCESS.
The vermiform process is a "worm-like" prolongation of the caecum.
Although small in size, in structure it more closely resembles the large
intestine, of which it is a part, than the small intestine. In embryos of
three and one-half to five months it is lined with villi, but with further
development the villi flatten out and disappear. Meanwhile the glands,
which are of the same type in both small and large intestines, have developed and are increasing in number and in length. Sometimes they pene
VERMIFORM PROCESS 271
trate the muscularis mucosae. In the adult (Fig. 267) they are simple
tubes, occasionally forked, thus indicating the way in which they multiply
in the embryo. As early as the fourth month, lymphoid tissue has been
found in the vermiform process, and at birth the lymphoid nodules in the
tunica propria are abundant and more or less confluent. The great development of lymphoid tissue is the most important histological feature of
the vermiform process in the adult (Fig. 267). It may invade and partly
FIG. 267. TRANSVERSE SECTION OF THE HUMAN VERMIFORM PROCESS. X 20. (Sobotta.)
Note the absence of villi and the abundance of nodules. Clear spaces in the submucosa are fat cells. Only
a part of the circular layer of the muscularis has been drawn.
break up the muscularis mucosae, and extend into the submucosa. The
latter, together with the inner circular and outer longitudinal muscle
layers, and the serosa, are similar to the corresponding layers of the small
intestine, already described.
During the fifth month of embryonic life, Stohr has found an interesting normal
form of degeneration in the glands of the vermiform process (Arch. f. mikr. Anat., 1898
vol. 51, pp. 1-55). The tunica propria around them appears to thicken, and the goblet
cells in the neck of the degenerating gland, after becoming flattened, produce a solid
strand. The strand then ruptures and the detached fundus becomes cystic. Subsequently it shrinks to a small nodule surrounded by dense connective tissue, and
ultimately disappears. This degeneration is said to be limited to the fifth and
sixth months.
The lumen of the normal vermiform process in the adult, when empty,
is thrown into folds, between which are deep pockets; but the normal con
272
HISTOLOGY
dition is found in scarcely 50% of individuals over forty years of age
(Stohr). Often the lumen is narrowed or even obliterated. The epithelium with its glands and the lymphoid nodules then disappear, and are
replaced by an axial mass of fibrous tissue. This is surrounded by the
unaltered submucosa and muscularis; the serosa may show the results of
inflammatory conditions.
CAECUM AND COLON.
The human caecum and colon contain villi only in the embryo. These
villi disappear at about the sixth month. The production of new cells
does not keep pace with the expansion of the epithelial tube, and the villi
Glands.
>>.:Wv.Vtfg3qp
Wwj$ii
'. ':' :-':;:. >:* :.'.'
l^e":^.^^/
?/% ^Miijf
-Tunica
propria.
Fat cells
Solitary nodule with germinal center.
FIG. 268. VERTICAL^SECTION OF THE Mucous MEMBRANE OF THE DESCENDING COLON OF AN ADULT
MAN. x 80.
The fat has been blackened with osmic acid. Compare the length of the glands with those of the small
intestine (Fig. 260), from the same individual and drawn under the same magnification.
therefore gradually flatten and disappear. In the parts of the embryonic
intestine distended with secretions and desquamated cells (constituting
the meconium), the villi disappear earlier than in the contracted portions
(Johnson).
After the villi have gone, the mucosa contains only tubular pits or
glands, lined with simple columnar epithelium (Fig. 268). These glands
are similar to those in the small intestine but are longer sometimes
twice as long (0.4-0.6 mm.). They contain more goblet cells, but cells
LARGE INTESTINE
273
of Paneth are absent. Striated cuticular borders appear near the outlets of the glands, and are well developed upon the columnar cells lining
the intestinal lumen. Solitary nodules are numerous, especially in the
caecum. They may extend through the muscularis mucosae and expand
in a flask-shaped manner in the submucosa (Fig. 268) ; in peripheral sections
of such a nodule the stalk by which it joins the tunica propria may not be
included, and the area of lymphoid tissue may seem to be wholly in the
submucosa. The latter is a connective tissue layer like that of the small
intestine.
The tunica muscularis of the colon and caecum has a characteristic
arrangement not found in the vermiform process. The longitudinal
smooth muscle fibers of the outer layer become gathered into three equidistant longitudinal bands or tanicz (Fig. 269); between
them the longitudinal fibers form a thin layer
which may be interrupted. The taeniae come
together at the root of the vermiform process
and are continuous with its outer muscle
layer. Since the longitudinal muscle layer
does not elongate as rapidly as the parts
within it, the inner layer of circular smooth
muscle, together with the mucosa and submucosa, become thrown into a succession of
transverse crescentic folds or plica semi- ,
FIG. 269. VERMIFORM PROCESS (V. p.).
lunar es. The horns of the crescents are op- 2SSS" fiS^AM? cSSJiTcSf.
posite the taeniae. Between the semilunar (After sobotta.)
h., Haustra; t., taema.
folds the wall of the large intestine bulges
outward, forming the haustra (Lat., buckets) as shown in Fig. 269. The
valve of the colon (valvula coli] is a pair of folds or labia, which resemble
the semilunar folds; that is, they include fibers of the circular muscle layer,
but the layer of longitudinal fibers passes directly from the ileum to the colon
without entering the valves. The serosa of the colon contains lobules of
fat which form pendulous projections known as appendices epiploica.
II
RECTUM.
The rectum is divided into two parts, an upper which extends from the
third sacral vertebra to the pelvic diaphragm, and a lower which continues
downward to the anus. The lining of the first part is thrown into several
folds, the plica transfer sales recti (valves of Houston). These are large
semilunar folds which usually extend only part way around the rectum,
but they have been described in some cases as having a spiral arrangement. The second part of the rectum, the pars analis recti (anal canal),
18
274
HISTOLOGY
presents on its inner wall a number of longitudinal folds, known as rectal
columns (columns of Glisson or Morgagni). At their lower extremities
the columns unite with one another, thus forming small transverse plicae
Rectal gland
Linea ano-rectalis
Zona columnaris
Linea'sinuosa analis
Zona intermedia.
Linea ano-cutanea.
Circular layer of
smooth muscle.
Longitudinal layer
of smooth muscle.
Levator ani.
Internal sphincter.
Intramuscular gland.
External sphincter.
-Sheath of a hair.
-Sebaceous gland.
Zona cutanea.
FIG. 270. LONGITUDINAL SECTION THROUGH THE PARS ANALIS RECTI.
From a human embryo of 187 mm. (about four months). (F. P. Johnson.)
or anal valves. The grooves between the columns extend downward behind the valves, forming a series of blind pockets, the sinus rectales.
The mucous membrane of the first part of the rectum is similar to
that of the colon, but its glands are somewhat longer (0.7 mm.). Soli
RECTUM 275
tary nodules are present. The muscularis mucosae, submucosa, and circular layer of smooth muscle also resemble those of the colon, but the
three taeniae spread out and unite so as to form a continuous layer of
longitudinal muscle. In the upper part of the rectum this layer is specially thickened dorsally and ventrally. As the rectum loses its mesentery, the tunica serosa is replaced by adventitious connective tissue.
The pars analis recti is the region of transition from mucous membrane to skin. This transition is not gradual but takes place in three
steps, thus forming three distinct superimposed zones. From above
downward these are the zona columnaris, zona intermedia, and zona cutanea
(Fig. 270). The last, however, does not belong to the pars analis, properly
speaking, but to the outside skin.
The zona columnaris is the region of the rectal columns, but these are
not always limited to this zone. They may extend upward into the first
part of the rectum for a short distance, and they may also be continuous
downward with the so-called anal skin folds. In the upper part of the
zona columnaris the simple columnar epithelium of the superior portion
of the rectum becomes two- or three-layered. Its outer cells are columnar,
with finely granular protoplasm. The transition takes place gradually
at the linea ano-rectalis . In the upper part of the zone there are usually
a few intestinal glands containing numerous goblet cells, and a few goblet
cells are found also in the surface epithelium. In the lower part of the
zona columnaris, arising from the rectal sinuses, there are a few branched
tubular gland-like structures, the intra-muscular glands (Fig. 270).
There are seldom more than six or eight in any one rectum. The main
ducts of these glands extend outward, and usually downward, and penetrate the internal circular muscle (internal sphincter). Here a flaskshaped swelling is usually met with. Extending beyond this ampulla there
are several tubular branches which continue through the internal sphincter
and end blindly in the intra-muscular connective tissue. Occasionally a
tubule is seen piercing the longitudinal muscle layer. Around the terminations of the tubules, which are sometimes swollen, there is a small amount
of lymphoid tissue. The epithelium lining the main ducts of these glands
consists of several layers of polygonal cells, but the ampullae and branches
are lined with one or two layers of cuboidal cells. Secretory cells are
present in the embryo and at birth, but are apparently wanting in the
adult.
The transition between the zona columnaris and zona intermedia is
marked by a rather abrupt change in the epithelium, which becomes many
layered and squamous. This transition takes place at the level of the
anal valves, but between the valves it extends upward on the rectal columns. Thus it follows a zig-zag line, the linea sinuosa analis (ano-cutaneous line of Hermann). Within the zona intermedia the epithelium, com
276 HISTOLOGY
posed of several layers of polygonal cells, is thicker than the epidermis.
Dermal papillae are present, but hairs and sweat glands are absent.
In the lower part of this zone there are a few isolated sebaceous glands
without hairs, and the epithelium is slightly cornined. Thus it gradually
goes over into skin, forming a true linea ano-cutanea, but this line is not
well marked. It has been denned as the place where the first sheaths of
the hairs appear.
The skin immediately surrounding the anus forms the zona cutanea.
Sweat glands are absent from the region bordering on the anus, but at a
distance of 1.0-1.5 cm. there is an elliptical zone, 1.25-1.5 cm. wide, containing simple tubular coiled glands, the circum-anal glands of Gay.
These are very similar to sweat glands but are considerably larger.
The outer layers of the pars analis recti include a very vascular tela
submucosa, which contains numerous nerves and lamellar corpuscles.
The muscularis mucosae terminates in slender longitudinal bundles which
extend for varying distances into the rectal columns (forming the M.
dilatator ani internus of Riidinger). The circular layer of the tunica muscularis becomes thickened at its termination, forming the M. sphincter
ani internus; it extends a little below the the linea sinuosa analis. Beyond
the internal sphincter, which is composed of smooth muscle, striated muscle
fibers surround the anus forming the M. sphincter ani externus. The
outer longitudinal layer of the tunica muscularis ends in relation with
connective tissue strands which diverge as they pass downward through
the external sphincter, to terminate in the subepithelial tissue of the zona
cutanea.
LIVER.
DEVELOPMENT AND GENERAL STRUCTURE.
The liver first appears in human embryos of about 2. 5 mm. as a diver ticulum of the ventral wall of the fore-gut, near its junction with the yolk-sac.
If the embryo is placed in an upright position (Fig. 271, A) the liver is
seen to be below the heart, and between the vitelline veins as they pass from
the yolk-sac to their cardiac termination. The diverticulum projects into
a mass of mesoderm, to which His gave the old anatomical term for diaphragm, namely septum transversum. The diaphragm develops in the
anterior or upper part of this septum; the lower or posterior part constitutes
the ventral mesentery, which extends from the fore-gut to the ventral
body wall. The hepatic diverticulum is in the mesenteric part of the septum, although it is always connected with the overlying diaphragmatic
shelf.
Very early the liver becomes divided into two parts, (i) the somewhat
rounded diverticulum proper, lined with columnar cells with pale proto
LIVER
277
plasm, and (2) a mass of anastomosing cords or trabeculae, composed of
deeply staining cells with round nuclei and abundant granular protoplasm.
These two parts are so unlike in appearance that they have been thought
to proceed from different germ layers, the trabeculae being described as
formed from mesenchyma in the septum transversum. This opinion is
erroneous; the entire structure is entodermal, and the trabeculae grow out
from the diver ticulum. They encounter the vitelline veins, which ramify
around them, producing the lacunar vessels or sinusoids already described
(Fig. 160, p. 167).
In an embryo of io-i2mm. (Fig. 271, B), the hepatic diverticulum
has elongated and is connected with the mass of anastomosing trabeculae
at several points. It shows also some detached ducts and round knob-like
FIG. 271. DIAGRAMS OF THE DEVELOPMENT OF THE LIVER.
A, From a 4.o-mm. human embryo. B, From a 12-mm. pig. C, The ducts in the human adult.
c.d.,
Cystic duct; c. p., peritoneal cavity; d., duodenum; d. c., ductus choledochus; dia., diaphragm; div.,
distal end of the diverticulum; f. 1., falciform ligament; g. b., gall bladder; g. o., greater pmentum;
h. d., hepatic duct; ht., heart; int., intestine; li., liver; 1. o., lesser omentum; m., mediastinum; oe.,
oesophagus; p. c., pericardial cavity; p. d., pancreatic duct; ph., pharynx; p. y., portal vein; s. t., septum
transversum; St., stomach; tr., trabecula; v. c. i., vena cava inferior; v. v., vitelline vein;;y. s., yolk-sac
swellings. The vitelline veins have given rise to the portal vein, which
enters the liver from below and breaks up into sinusoids among the
trabeculae. These reunite, and leave the liver above as the hepatic vein,
which was originally a part of the vitelline veins. In the lo-mm. embryo
the circulation of the liver is wholly venous. The trabeculae consist of
cells which are doubtless very active, taking up and transforming material
received from the blood, but it may be questioned whether bile is secreted
at this stage, since no complete system of ducts has been demonstrated.
In later stages the mass of anastomosing trabeculae is drained by a
system of ducts lined with clear cuboidal or columnar epithelium. These
all empty into a single hepatic duct, which represents one of the original
connections between the trabeculae and the diverticulum. (In the otter
there are said to be as many as seven persistent ducts.) The hepatic
duct (Fig. 271, C) is joined by the cystic duct which comes from the tapering pyriform gall bladder (vesica fellea). The latter is perhaps to be re
278 HISTOLOGY
garded as a special subdivision of the original diverticulum, rather than as
its expanded terminal portion. In certain mammals, as in the horse and
elephant, the gall bladder is lacking. After the hepatic duct has joined
the cystic duct, the common bile duct (ductus choledochus) thus formed
proceeds to the duodenum into which it opens, together with the pancreatic
duct, at the duodenal papilla. The common bile duct is an elongated
portion of the original hepatic diverticulum.
Ligaments of the Liver. At the time of its earliest formation the liver
bulges laterally from the ventral mesentery, on both sides, thus forming
right and left lobes. The lobes are covered with the peritoneal epithelium.
The mesenchyma beneath this epithelium produces loose connective tissue externally, and a
dense fibrous tissue, immediately surrounding
the trabeculae, internally; this latter becomes
the capsula fibrosa (or capsule of Glisson).
The part of the ventral mesentery extending
from the intestine to the liver is known as the
lesser omentum, and the part between the liver
and the ventral body wall is the falciform ligaFIG 272.-THE LEFT SIDE OF AN ment. These lie in the median plane (Fig. 272).
nfnfeSf ' Beneath the liver, the peritoneal cavity comes
d. c., Ductus choledochus; g. b., to extend across the median line so that the
gall bladder; I. L, falciform
bladder is covered with peritoneum, except
d p 'iigame P nt7 a v. V c? i r.: along its attachment to the under side of the
liver. On the upper surface of the liver, the
original broad connection with the septum transversum becomes relatively narrow dorso-ventrally, and forms a pair of lateral ligaments which
pass from the upper surface of the liver to the diaphragm. They extend
across the liver at right angles with the falciform ligament and lesser
omentum. The left lateral ligament retains these simple relations and is
known as the left triangular ligament. The right lateral ligament, except
at its tip (the right triangular ligament} , extends down over the posterior
surface of the liver as an extensive area of fusion with the diaphragm; this
is the coronary ligament (Fig. 275). The significance of this asymmetrical condition will be explained with the veins of the liver.
Development of the veins of the liver. The hepatic trabeculae are always
in close relation with the veins which are conveying nutriment to the
heart. These are (i) the vitelline veins conveying nutriment from the
yolk-sac, (2) the umbilical veins conveying nutriment from the placenta,
and (3) the portal vein conveying absorbed food from the intestine. The
liver also has important relations with the vena cava inferior.
The portal vein, which is the principal afferent vessel of the adult liver, is derived
from the vitelline veins. The latter; as they pass from the yolk-sac into the abdominal
LIVER
cavity, fuse with one another so as to form a single trunk (Fig. 271, B, r..). On
reaching the duodenum, the trunk separates into its components, and they pass into the
liver as the right and left vitelline veins (Fig. 273, A). Before entering the liver they
anastomose with one another dorsal to the duodenum, as shown in the figure. Thus
with the connections between the right and left veins within the liver, two complete
venous rings are formed around the intestine. Branches extend out from these rings,
notably the superior mesenteric vein which receives blood from the primary loop of
intestine, and the splenic vein which not only drains the spleen but receives the inferior
mesenteric vein together with pancreatic and gastric branches. The superior mesenteric
vein (Fig. 273, s.m.v.) is joined by the splenic (s.) to form the portal vein (p.v.), and the
portal vein is a persistent portion of the peri-intestinal rings formed by the vitelline
veins. Other parts of the rings atrophy, and as the yolk-sac degenerates and becomes
detached, the main vitelline trunk disappears. The
portal system of veins is therefore a derivative of
the vitelline system; its blood flows through the liver
in the vitelline sinusoids.
The formation of the rings as above described
takes place with great constancy, and apparently
the only variations observed in their atrophy are the
two cases described by Begg (Amer. Journ. Anat.,
1912, vol. 13, pp. 105-110).
The umbilical veins are at first a pair of vessels,
but they early unite in the umbilical cord. The p
single vein thus formed brings the embryonic blood The formation of the portal vein, p. v.,
back to the body after its excursion to the placenta. 5g SVTSd i^du ]dSfc
On reaching the body, the vein divides into right r^^Ma'SB*
and left vessels, which are contained in the ventral
body wall, and at first pass directly to the heart; later they anastomose with the
vitelline sinusoids in the liver, and the right umbilical vein then atrophies, leaving
the left vein to convey the blood to the liver. In Fig. 274, the left vein is larger
than the right, and is seen connecting with the hepatic sinusoids. Gradually it
shifts from the left side to the median line. It then passes from the umbilical
cord to the under surface of the liver along the free edge of the falciform ligament,
where, after the umbilical cord has been severed, it degenerates to form the round
ligament of the liver (Fig. 275). This extends to the porta or entrance to the liver,
where the portal vein goes in and the hepatic duct comes out. Beyond this point
the umbilical vein may be followed as the ductus venosus in the embryo, or the
ligament of the ductus venosus in the adult, to the vena cava inferior. The ductus
venosus may be defined as the channel made by the umbilical vein in passing to the
vena cava inferior across the under surface of the liver. It is sometimes completely
enfolded by the hepatic trabeculae, and it communicates with the hepatic sinusoids. It
follows the line of attachment of the lesser omentum, and empties into the vena cava
inferior.
The vena cava inferior apparently does not send much blood into the liver but passes
along its dorsal surface. An essential part of this great vein is formed from the hepatic
sinusoids. Before the vena cava inferior has developed, the blood in the dorsal body
wall flows to the heart through the posterior cardinal veins, one on either side of the
aorta. Each posterior cardinal vein shows a ventral subdivision, the right and left
subcardinal veins respectively, which are seen in section in Fig. 274. As shown in the
figure, the stomach prevents the liver from approaching the dorsal body wall (at the
root of the mesentery) on the left, but on the right there is no such obstruction, and the
280
HISTOLOGY
liver approaches and fuses with the body wall immediately in front of the right subcardinal vein. This fusion constitutes the coronary ligament (cf. Fig. 275) ; and across it,
the subcardinal vein anastomoses with the hepatic sinusoids. By a rapid enlargement
of this anastomosis, the trunk of the vena cava inferior is formed. It drains the
posterior cardinal system of veins, and the outlet of the vitelline veins into the heart becomes the terminal portion of the inferior vena cava; the main vessel from the liver, the
hepatic vein, is thereafter described as a branch of the vena cava inferior. The development of the posterior part of the vena cava inferior is described in connection with
the Wolffian body (p. 309) ; for a fuller account, see the Amer. Journ. Anat., 1902, vol.
i, pp. 229-244. Occasionally the trunk of the vena cava is entirely surrounded by a
band of hepatic tissue, as in Fig. 275.
v.c.i. o.b.
f.l. v'um.
FIG. 274. CROSS SECTION OF A MAMMALIAN
EMBRYO. TO SHOW THE ADHESION, x, BETWEEN THE RIGHT LOBE OF THE LIVER AND
THE DORSAL ABDOMINAL WALL.
ao., Aorta; f. c., fibrous capsule and serosa; f. 1.,
falciform ligament; g. o., greater omentum;
1. o., lesser omentum; 1. s-c. v., left subcardinal vein; o. b.. omental bursa; r. s-c. v.,
right subcardinal vein; St., stomach; v. um.,
left umbilical vein.
.r.t.l.
FIG. 275. DORSAL SURFACE OF THE ADULT
LIVER.
c. 1., Coronary ligament; f. 1., falciform ligament; g. b., gall bladder; 1. o., lesser
omentum; 1. 1. 1., left triangular ligament;
o. b., caudate lobe bounding the omental
bursa ventrally; p. v., portal vein; r. 1.,
round ligament; r. t. 1., right triangular
ligament; v. c. i., vena cava inferior.
Lobes of the liver. The structures already described form the boundaries of the lobes of the liver, which in man are few and not sharply marked
out. Right and left lobes have already been mentioned as the lateral
halves of the liver; they are not separated from one another by any internal
septum or indentation of the surface. The left lobe is relatively small, and
has a thin margin. It terminates in the appendix fibrosa at the extremity
of the left triangular ligament. This appendix represents a portion of the
liver from which the hepatic cells have degenerated and disappeared, leaving chiefly the anastomosing ducts. It indicates that in earlier stages the
left lobe was more extensively developed. Similar tissue containing
aberrant ducts (vasa aberrantia) may be found around the vena cava and
in some other parts of the liver. The quadrate lobe is marked out by the
porta, the round ligament, and the fossa containing the gall bladder. The
caudate lobe is bounded by coronary ligament, lesser omentum and porta.
The caudate process of this lobe extends to the right lobe over the foramen
epiploicum (of Winslow) between the vena cava and the porta.
The hepatic artery. The liver in an embryo of 10 mm. has no arteries,
but at that stage the hepatic artery can be followed to the porta. Later it
LIVER
28l
extends through the connective tissue around the gall bladder, so that the
cystic branch of the adult appears to be the main vessel in the young embryo. Still later, as the connective tissue which surrounds the structures
at the porta gradually extends into the liver around the branches of the
hepatic duct and portal vein, the hepatic artery sends branches in with it,
and they form capillaries which empty into the adjacent portal sinusoids.
Branches of the artery ramify also in the connective tissue capsule around
the entire liver. The quantity of blood supplied to the liver by the artery
always remains much smaller than that brought in by the portal vein, and
it is distributed to the connective tissue. There are no vessels between
the hepatic cells other than the "capilliform sinusoids" derived directly
from the embryonic lacunae of the vitelline veins.
MICROSCOPIC STRUCTURE.
Lobules. A section of the embryonic liver, or of the liver at birth, shows
great areas of anastomosing trabeculas, with intervening sinusoids and occasionally a larger vein. In the adult pig the hepatic tissue is arranged in
lobules bounded by connective tissue (Fig. 276). These subdivisions were
FIG. 276. LIVER OF A PIG. (Radasch.)
The lobules have artificially shrunken from the interlobular tissue, a; b, bile duct; c, hepatic artery;
d, interlobular vein (a branch of the portal); e, trabeculse; f, central vein.
first recognized in the liver of the pig (Wepfer, 1664), and in 1666 Malpighi
made the general statement that the entire liver is composed of a multiplicity of lobules. In the dog Mall finds that the lobules are short cylinders
282
HISTOLOGY
averaging 0.7 mm. high and 0.7 mm. in diameter, and that the entire liver
(of 175 c.c.) contains 480,000 of them (Amer. Journ. Anat, 1906, vol. 5,
pp. 227-308). There has been prolonged discussion as to whether the
lobules should be regarded as centering about the terminal branches of the
portal vein or around those of the hepatic vein, for, although it was frequently stated that they were arranged like a bunch of grapes, there was
no unanimity as to what formed the stem. If the human liver is examined
(Fig. 277) it is seen that the lobules are not definitely marked out as in the
pig, but the liver retains to a greater extent its embryonic appearance.
Scattered about through the section, but at quite uniform distances from
one another, there are islands of connective tissue containing branches
of the portal vein, hepatic artery, and bile duct. The strands of connective
Branch of portal vein.
Large interlobular bile duct.
Interlobular connective
tissue.
Central veins.
Central vein.
FIG. 277. FROM A TANGENTIAL SECTION OF THE HUMAN LIVER. X 40.
The three central veins in cross section mark the centers of three lobules, which are not sharply separated,
at the periphery, from their neighbors. Below and at the right the lobules are cut obliquely and their
boundaries are not seen.
tissue which conduct the portal branches were named portal canals by
Kiernan (Trans. Roy. Soc. London, 1833, pp. 711-770). If the connective
tissue should spread from one canal to another, connecting those nearest
together, it would mark out lobules like those in the pig's liver, and this
sometimes takes place pathologically in man. Normally the portal canals
stand as isolated "boundary stones."
Within each lobule thus marked out there is a central vein or enlarged
sinusoid, toward which the capilliform sinusoids between the hepatic
trabeculae converge. Occasionally there are two veins, side by side.
These central veins empty at right angles into sublobular veins (Fig. 278),
which come together to form the main branches of the hepatic vein. All
these veins, in contrast with the portal branches, have very little connective
tissue around them, and they are not associated with bile ducts or arteries;
LIVER
283
thus the hepatic veins are readily distinguished from the portal veins.
The flow of the blood (Fig. 279) is from the portal veins (in the portal
canals) through the capilliform sinusoids to the central veins, thence
Hepatic lobules.
Interlobular connective
tissue.
Central (intralobular)
veins.
Sublobular vein.
FIG. 278. FROM A VERTICAL SECTION OF A CAT'S LIVER. INJECTED THROUGH THE VENA CAVA INFERIOR
The central veins and the sublobular vein into which they empty are cut longitudinally. X 1 5.
Two bile ducts in
cross section.
Capilliform
sinusoids.
Central vein.
Interlobular vein
(branch of portal).
FIG. 279. FROM A SECTION OF THE HUMAN ADULT LIVER INJECTED
THROUGH THE PORTAL VEIN.
through the sublobular veins into the hepatic vein, which empties into the
vena cava inferior. The arteries empty through capillaries into the
capilliform sinusoids adjacent to the portal canals, and there is some
284 HISTOLOGY
evidence that the hepatic cells at the periphery of the lobule are better
nourished than those in its interior.
The recognition of the lobules above described, as the essential basis
of hepatic structure, would have been unquestioned except that, as Kiernan stated, " the essential part of the gland is undoubtedly its duct; vessels
it possesses in common with every other organ; and it may be thought that
in the above description too much importance is attached to the hepatic
veins." If the liver were divided into lobules comparable with those of
other glands, the portal canals with their ducts and adjacent afferent vessels would be the axial structures, and the efferent central veins would be
peripheral. By connecting the five central veins around the portal canal
in Fig. 277 (two of the central veins are not labelled and the one at the
lower edge of the figure is indistinct) , such a structural unit or secretory unit
would be marked out. It has been proposed to call it a portal lobule (from
its axial structure), in contrast with the hepatic lobules, which surround the
branches of the hepatic vein. In the seal it is said that the portal lobules,
or units, are bounded by connective tissue, but this must be regarded as
very exceptional. However, in attempting to picture the complex relations of the lobules in the liver, the morphologist must regard the portal
canals as axial, even though the term lobule is used for areas surrounding
the central veins. The bile flows from parts of several hepatic lobules
into a single portal canal.
Parenchyma. The parenchyma or essential tissue of the liver is found
in the anastomosing trabeculae of the lobules. The general arrangement
of the cells in these trabeculae is shown in Fig. 280, in which, however,
the slender lumens are rendered conspicuous by special treatment. These
lumens, or bile capillaries, are ordinarily inconspicuous, and the trabeculae
appear on superficial examination as solid cords of cuboidal cells, with
abundant granular protoplasm and large round central nuclei. Often
the hepatic cells contain two nuclei, and large cells with several nuclei,
produced by amitosis, have been reported. The general characteristics
of hepatic cells are shown in Fig. 281. They are arranged chiefly in double
rows which in certain positions appear single.
The hepatic cells have very delicate cell membranes, which are sometimes |aid to be absent. Their protoplasm often contains brown pigment,
especially toward the central vein. Near the periphery of the lobule the
cells may contain fat vacuoles of varying size, found normally in wellnourished individuals. Pathologically the vacuoles may be large and
widely distributed. Glycogen (p. 78) occurs in granules and larger
masses, especially after abundant meals. In the fasting condition, the
cells are relatively small, dark, and obscurely outlined, but during digestion they become larger with a clearer central part and a peripheral zone
of coarse granules. In man both conditions may be found in one liver.
LIVER
285
The bile, secreted by the hepatic cells, probably through granule formation, frequently contains granules and fat droplets such as are found
within the cells. It is eliminated through the bile capillaries.
The bile capillaries are minute tubes with continuous cuticular walls,
presumably formed by the local modification of the cell membranes of
two adjacent hepatic cells. The completed capillary, however, shows no
True meshes.
Lateral branches of bile capillaries.
Nuclei of
Sinusoids. Portion of a central vein.
FIG. 280. FROM A CROSS SECTION OF A HUMAN HEPATIC LOBULE. X 300.
Golgi preparation. The boundaries of the hepatic cells could not be seen. The black dots are precipitates of the silver.
indication of being formed of lateral halves which have fused. Cross
sections of the large bile capillaries in the liver of Necturus are shown in
Fig. 281, and their arrangement in the human liver is indicated in Fig. 280.
They extend through the axis of the two-rowed trabeculae of cells, giving
off short intercellular branches at right angles. Thus the bile capillaries
shown in Fig. 281 between the two sinusoids, may be separate axial
286
HISTOLOGY
capillaries, or they may be intercellular branches of an axial capillary
which is in the plane of the printed page. In some places the bile capillaries completely encircle an hepatic cell, forming " true meshes" (Fig. 280).
They may form larger meshes due to the anastomosis of trabeculae. Occasionally a bile capillary is in relation with three surrounding hepatic cells,
or even more, thus resembling the lumen of an ordinary gland-tubule.
In addition to intercellular capillaries there are said to be intracellular
branches, several of which may penetrate the protoplasm of a single cell
and end in knobs, as shown by the Golgi method. Since neighboring
capillaries may be free from these branches, they are regarded as tempo
FIG. 281. SECTION OF THE LIVER OF A SALAMANDER (Necturus). X 380.
a, Endothelial cell; b, endothelial reticulum; c, blood vessel; d, bile capillary; e, red corpuscle; f, hepatic cell.
rary phases of functional activity, accompanying the discharge of secretion.
They have been reported as forming baskets within the protoplasm,
similar to those found in parietal cells of the stomach.
The bile capillaries and their branches are generally separated from
the lining of the blood vessels by an appreciable portion of the hepatic cells
(cf. Figs. 280 and 281). Pathologically they may extend nearer the vessels
and may rupture, so that the bile escapes into the perivascular tissue and
is distributed over the body, causing jaundice.
Endothelium and Perivascular Tissue. The endothelium of the capilliform sinusoids which border upon the hepatic trabeculas is specially modified; it is well shown in the coarse-grained liver of Nectunis (Fig. 281),
but the same form occurs in the human liver. The endothelial cells, which
are phagocytic, produce a network of reticular fibers toward the hepatic
cells (Fig. 282). The reticulum contains no elastic elements, and the
only cell bodies associated with it are those of the endothelium. In the
reticular meshworkin the embryo, erythroblasts multiply in great numbers.
LIVER
287
Hepatic trabecula. Blood corpuscles. Reticulum.
and to some extent leucocytes are formed, but in the adult the recticulum
is free from cells. The endothelial cells, moreover, do not fit closely
together, and are known as the stellate cells of Kupffer. It is probable
that, whereas the blood flows through the capilliform sinusoids toward the
central vein, there is a current of tissue fluid in the reticulum taking the
reverse direction and passing toward the portal canal. This fluid is the
source of the great quantity of lymph which flows from the liver.
According to Schafer (Quain's Anatomy, 1912, vol. 2) the blood flowing
through the sinusoids comes into direct contact with the liver cells. He
states that blood corpuscles
may occasionally be found
normally within the hepatic
cells, into which they are
readily forced by injections
at low pressure; and he describes canaliculi within the
protoplasm of the hepatic
cells, which communicate
with the sinusoidal blood
vessels. These canaliculi
are presumably secretory
channels or canals of the
trophospongium, which have
been artificially invaded by
the injection. At the same time, the reticulum has been compressed and
its significance obscured.
Portal canals. The portal canals are strands of connective tissue
extending into the liver from the transverse fissure or porta (which is
essentially a hilus). They constitute the interlobular tissue of the liver,
and the ducts, arteries, and veins which they contain are often called interlobular. In addition to the structures already considered, the portal
canals contain lymphatics and nerves; these and certain features of the
ducts require further consideration.
The lymphatic vessels are abundant, forming plexuses around the ducts
and blood vessels, and receiving fluid from the perivascular reticulum
within the lobules; but no lymphatic vessels enter the lobules. They
pass out of the liver at the porta, where lymph glands are found. Certain
of the lymphatics in the capsule of the liver drain toward the porta;
others enter the diaphragm.
The nerves are chiefly non-medullated fibers from the sympathetic
system, but the liver also receives branches from the vagus. These
nerves are principally distributed to the blood vessels, but some are said
to penetrate the lobules and end in contact with the pehatic cells.
FIG. 282. SECTION OF THE LIVER PREPARED BY THE BIEL
SCHOWSKY METHOD. X 300.
288
HISTOLOGY
The interlobular ducts are lined with simple columnar or cuboidal
epithelium. They anastomose with one another, and have'blind pockets;
in the larger ducts, there are branched mucous glands. The connection
between the ducts and the hepatic trabeculae is difficult to observe, and it
was once thought that the ducts with their ramifications produced the bile,
leaving the parenchyma for the function of internal secretion. Through
injections, however, or by using the Golgi method, the connections between
the bile capillaries and the bile ducts can be readily demonstrated (Fig.
283). They are found at the periphery of the portal canals, and were
-vjr
Branch of portal
vein.
Small interlobular bile-duct,
continuing in
bile capillaries.
Large interlobular bile-duct.
Branch of hepatic artery.
Bile capillaries. '
Tf.'
Wall of the central vein.
FIG. 283. GOLGI PREPARATION OF THE LIVER OF A DOG. X24O.
described histologically by Hering (Strieker's Handbuch, Leipzig, 1871).
On the side toward the connective tissue these "canals of Hering," or
periportal ducts, exhibit a flat or cuboidal epithelium, like that of ordinary
ducts; but toward the lobule they are bounded by hepatic cells, or by flat
cells interrupted by hepatic cells (Fig. 284). Thus the hepatic trabeculae
are directly inserted into the walls of the ducts, and the bile capillaries connect with the lumen.
The hepatic, cystic and common bile ducts all have a simple columnar
epithelium, with occasional goblet cells and branching mucous glands.
Around the hepatic duct there is a wide zone formed by the ramifying
ducts of these mucous glands, as they extend into the surrounding connective tissue. The connective tissue layer is said to contain many elastic
LIVER
289
fibers. It is followed by a tunica muscularis consisting chiefly of circular
fibers. These form a sphincter around the common bile duct, at the duodenal papilla. In the cystic duct there are folds of mucous membrane, containing muscle fibers, and forming the "spiral valve."
The gall bladder is lined with a folded
mucous membrane covered with tall epithelial cells similar to those of the intestine (Fig. 285). They have elongated
basal nuclei and secretory granules
(mucin) in the outer part of their protoplasm. The free surface is covered
with a distinct cuticular border, and
terminal bars have been observed.
Goblet cells are absent and glands are infrequent. The muscularis consists of obliquely circular fibers arranged in a plexiform layer. Among
them are groups of sympathetic nerve cells which supply the muscle, and
medullated fibers which end in the epithelium. The subserous tissue is
highly developed and contains large lymphatic vessels.
FIG. 284. THE CONNECTION BETWEEN BILB
CAPILLARIES AND BILE DUCTS IN A HUMAN
EMBRYO OF FOUR AND A HALF MONTHS.
(After Toldt and Zuckerkandl.)
b. c., Bile capillary; h. c., hepatic cell; p. d.,
periportal duct.
Epithelium.
Muscularis.
Tunica
propria
FIG. 285. FROM A SECTION OF THB GALL BLADDER OF AS ADULT, A.
X S6o.
X too. B, the portion x of A
PANCREAS.
Development and General Features. Although the pancreas in the adult
is a single gland, it arises in the embryo as two entirely distinct entodermal
outgrowths, known as the dorsal and ventral pancreases respectively.
The dorsal pancreas grows out from the dorsal wall of the intestinal tube, a
little below the level of the common bile duct in most mammals, but a little
above it in man. The ventral pancreas grows down from the common bile
duct at its junction with the intestinal tube. As seen in Fig. 286, A and B,
the ventral pancreas may be more or less bi-lobed. Usually it grows to
the right of the intestine and there meets the dorsal pancreas, which approaches it in close relation with the portal vein.
19
2 go
HISTOLOGY
The left lobe of the ventral pancreas sometimes grows around the left side of the
intestine and joins the dorsal pancreas, so that the intestine is encircled by pancreatic
tissue (annular pancreas); sometimes it grows out beneath the gall bladder where it
ends in a cystic enlargement, as has been observed in adult cats (cf. Amer. Journ. Anat.,
191 2, vol. 1 2, pp. 380-400). Usually the left lobe is scarcely indicated. As a rather frequent abnormality, accessory pancreases of small size, but sometimes of very typical
D. ch.
p. d.
L. d.
L.s.
Int.
P d.
L. d.
Pr. v.
D. ch.
L. s.
Int.
A B
FIG 286. MODELS OF THE VENTRAL PANCREAS IN PIG EMBRYOS. X 120.
A, 5.1 mm., B, 6.0 mm. D. ch., ductus choledochus; Int., intestine; L. d., right lobe, and L. s., left lobe of
the ventral pancreas; P. d., dorsal pancreas; Pr. v., ventral process of the dorsal pancreas.
structure, are found along the intestine, or even in the wall of the stomach, especially
at the constriction between its cardiac and pyloric portions. Such glands may or may
not extend through the tunica muscularis.
After the dorsal and ventral pancreases have come in contact, they are
related to one another as shown in Fig. 287, A. The dorsal pancreas is
much larger than the
ventral pancreas, and
it grows across the
body toward the left
until it reaches the
spleen. Thus it gives
rise to the body and
tail of the pancreas
of the adult; and it
forms also the ventral
part of the head of
. p. d., Accessory pancreatic duct; c. d., cystic duct: d., duodenum; d. c., .-> i j i i /-ii
ductus choledochus; d. p., dorsal pancreas; h. d., hepatic duct; p., duo- trie gland. WlllCn nils
denal papilla; p. d., pancreatic duct; St., stomach; v. p., ventral pan- .
crea - the concavity in the
duodenal loop. In
the adult its duct opens into the duodenum 1-3 cm. above the orifice of the
common bile duct, but it has been tapped by an anastomosis with the ventral
pancreas. Its outlet persists as the accessory pancreatic duct, discovered by
Santorini (1775). It is shown in the dissection, Fig. 287, B, but a large
FIG. 287. A, DIAGRAM OF THE PANCREAS FROM A IS-MM. HUMAN EMBRYO. B, DISSECTION OF THE DUODENUM AND PANCREAS OF AN
ADULT. (After Schinner.)
PANCREAS
2QI
branch ordinarily found descending from it in front of the pancreatic duct,
p. d., is not included. In some cases the accessory duct becomes impervious, but it is generally functional, and if the outlet of the main duct were
blocked by gall-stones or otherwise, the presence of this accessory duct
would be of considerable importance. In some mammals, as in the pig, it
is normally the chief duct.
The duct of the ventral pancreas either opens into the duodenum close
beside the common bile duct (Fig. 287, B), or it retains its embryonic relation (Fig. 287, A) and opens into the common bile duct near its duodenal
orifice. The duct of the ventral
pancreas, by an anastomosis
with the duct of the dorsal pancreas, becomes the outlet of the
main pancreatic duct, which was
first figured by Wirsung (1642).
It will be noted that a large part
of the dorsal pancreatic duct,
extending through the body and
tail, becomes incorporated in
this main duct of Wirsung; the
ventral pancreas supplies only
its outlet.
In the adult no histological
distinction has ever been found
between the two pancreases,
but although alike in structure
and close together, there is no
general anastomosis between
them. Rarely they remain entirely separate. Usually, on injecting the ducts, only one connection is
found between the dorsal and ventral pancreases, but in an abnormal case
two connections have been observed. Moreover, anastomoses between
the smaller ducts and tubules in the separate glands have not been found
in human adults. Rings of pancreatic tissue occur in the embryo, and in
adult guinea-pigs Bensley has demonstrated a free anastomosis of the
ducts (Amer. Journ. Anat., 1911, vol. 12, pp. 297-388); such a condition
has not yet been found in man.
Microscopic structure. As a whole the pancreas somewhat resembles the
parotid gland. It is divided into lobes and lobules by connective tissue
septa containing blood and lymphatic vessels, nerves, and interlobular
ducts (Fig. 288). The lobules are composed chiefly of short tubules, or
alveoli, which in models appear pear-shaped; in sections they are cut
at all possible angles. Instead of exhibiting a well-defined lumen, the
PIG. 288. SECTION OF HUMAN PANCREAS, SHOWING SEVERAL ISLANDS. (Radasch.)
a, Interlobular connective tissue containing an interlobular duct, c; b, capillary; d, interlobular duct; e, alveoli;
f, pancreatic island.
292
HISTOLOGY
alveoli appear to be clogged with cells, known as centro-alveolar cells (or
centro-acinal cells). Irregularly distributed among the alveoli there are
round areas of paler cells, peculiar to the pancreas (Fig. 288). ^ They may
be at the center or periphery of the lobule, or occasionally in the interlobular connective tissue. These important structures were first described
in Langerhans' thesis in 1869 (Inaug. Diss., Berlin), and are known as the
pancreatic islands (islands of Langerhans).
The alveoli are composed chiefly of the secreting pancreatic cells (Fig.
289). Toward the lumen their protoplasm contains coarse granules of
zymogen, which accumulate while the cell is inactive and are eliminated
during secretion. Apparently they are transformed into fluid as they
Blood
capillary.
Cells oi
the al- ^"W Centro-aveolar eel'
Zymogen granules.
A B
FIG. 289. FROM SECTIONS OF A HUMAN PANCREAS. X 500.
In ction A the granules are wanting, the centro-alveolar cells are flat and dark; in section B the granules
are distinct, the centro-alveolar cells are cuboidal and clear.
are discharged, for they are not found free in the intestine. In fresh
specimens the granules are refractive and easily seen, but in preserved
tissue they are readily destroyed, so that the granular zone appears reticuiar. The granules are soluble in water, and are darkened by osmic acid.
The basal protoplasm of the pancreatic cells is vertically striated.
It contains the round nucleus which has coarse masses of chromatin.
Within the pancreatic cells there have been found "paranuclei" of unknown nature, thought to be functionally important. After the discharge
of secretion the cells become smaller and their boundaries more distinct.
The pancreatic cells rest upon basement membranes containing "basket
cells."
The centro-alveolar cells may be darker or lighter than the pancreatic
cells (Fig. 289), but they are always smaller, and may be readily identified
from their central position. They do not contain zymogen granules.
The intralobular intercalated ducts, which connect with the alveoli, are
very slender, and their walls are formed of flat cells (Fig. 289, A). They
PANCREAS
293
terminate in clusters of alveoli, which often present clover-leaf forms.
The centro-alveolar cells have been interpreted as due to the invagination
of these ducts into the alveoli, but apparently they do not develop in this
way; they are formed as an inner stratum of a two-layered epithelium.
The secretory capillaries of the alveoli are shown in Fig. 290. They extend between the centro-alveolar cells to the pancreatic cells, and may be
prolonged between the latter, but they do not reach the basement
membrane.
The intercalated ducts pass into excretory ducts lined with cuboidal
epithelium, without the intervention of secretory ducts such as are found
in the salivary glands. The plan of the pancreatic ducts is shown in
Fig. 291. The main pancreatic and accessory pancreatic ducts are composed of simple columnar epithelium surrounded by a connective tissue
Centro-alveolar cells.
Cells of the
alveolus.
Intercellular
secretory . '
capillary.
FIG. 200. A, FROM A SECTION OF THE PANCREAS OF AN ADULT MAN. X 320. B, AN INTERPRETATION OF
THE RIGHT LOWER PORTION OF A.
layer, outside of which is a zone of circular smooth muscle fibers. The
latter are gathered into sphincters at the major and minor duodenal
papillae, where the ducts open. Occasional goblet cells and small glands
resembling mucous glands have been found in the mucosa of the large
ducts.
The blood and lymphatic vessels and nerves of the pancreas resemble
those of the salivary glands. . The capillaries have notably wide meshes
so that considerable portions of the alveoli are not in contact with them.
The nerves end around the blood vessels, ducts and pancreatic cells.
They are chiefly non-medullated sympathetic fibers from the cceliac
plexus, associated with scattered nerve cells found within the pancreas.
Lamellar corpuscles occur in the connective tissue.
The pancreatic islands are usually not to be found in human embryos
under 50 mm. in length. Thus they develop only after the pancreatic
glands have come together and attained considerable size. They arise as
outgrowths from the smaller ducts, with which they may retain a solid
stalk-like connection, or they may become wholly detached. According
to Bensley, detached islands in the guinea-pig are infrequent. In the
294
HISTOLOGY
embryo, as in the adult (Fig. 292), they consist of coiled anastomosing
cords of cells, or irregular masses, which are in close relation with the
endothelium of dilated capillary blood vessels. The islands are composed
of pale cells with very delicate cell walls, and they contain finer granules
than those in the pancreatic cells. In fresh preparations Bensley observed that these granules exhibit the Brownian movement, and that
colorless spaces occur among them, representing the canals of Holmgren's
trophospongium. When preserved by special methods, two forms of
island-cells may be distinguished by the staining reactions of their gran
Tubule.
FIG. 291. DIAGRAM OF THE PANCREAS.
FIG. 292. AN ISLAND OF THE PANCREAS WITH THE SUR
ROUNDING ALVEOLI, FROM AN ADULT. X 400.
ules. In one type of cell the nucleus is oval, with finely granular chromatin; and in the other it is round, with large chromatin granules. Having
neither ducts nor lumen, the islands produce an internal secretion, which
is received by the blood vessels. There is evidence that this secretion
plays an important part in carbohydrate metabolism. If the pancreas
is removed, sugar appears in the urine; but if the ducts of the pancreas
are tied, the pancreatic alveoli degenerate, leaving the islands functional,
and sugar is not found in the urine. Thus the islands are regarded as
physiologically distinct from the remainder of the pancreas.
Morphologically the islands are likewise distinct, and Bensley finds
that the possibility of the transformation of alveolar tissue into island
tissue, or conversely of island tissue into alveolar tissue, "has not a single
well-established fact to support it" (Amer. Journ. Anat., 1911, vol. 12, pp.
297-388). The number of islands, however, is subject to great variation,
PANCREAS
295
there being from 13,00010 56,000 in the entire pancreas of guinea-pigs
(Bensley), the average being twenty-two islands per cubic millimeter. In
all stages, both in the guinea-pig and in man, they are usually most numerous in the tail of the pancreas, and least numerous in its head (Opie, Johns
Hopkins Hosp. Bull., 1900, vol. n, pp. 205-209).
RESPIRATORY APPARATUS.
Development. The respiratory apparatus, consisting of the larynx,
trachea, bronchi, and lungs, arises as a median ventral outgrowth of the
fore-gut, immediately behind the last pharyngeal pouches. It apparently
is in no way related to the branchial pouches, but it may correspond with
the air-bladder of the bony fishes. At
the stage when the lung-bud develops,
the fore-gut is laterally flattened, so that
its lumen is a dorso- ventral cleft. The
lung-bud develops as a pear-shaped swelling, directed downward, on the ventral
border of the fore-gut; and this swelling
becomes split off, from below upward, to
form the trachea, which is at first short
but which rapidly elongates. The upper
end of the trachea, with the cartilages
which develop around it, constitutes the
larynx. At the lower end of the trachea,
the pyriform dilatation spreads out on
either side to form the primary bronchi
(Fig. 293, A).
The tracheal and bronchial tubes are lodged in a mass of connective
tissue, situated above and behind the pericardial cavity, and since this
tissue stands in the middle of the thorax it is known as the mediastinum.
It is comparable with a broad mesentery. As the bronchi push out laterally they occupy right and left folds bulging from the mediastinum, called
by Ravn the pulmonary wings (ala pulmonales}. Into these the bronchi
extend and produce branches after the manner of a gland (Fig. 293, B).
The pulmonary wings consist of mesenchyma, covered by the epithelium
which lines the body cavity. At first they project into the part of the body
cavity which connects the peritoneal with the pericardial cavity; later, by
the development of the pleuro-pericardial and pleuro-peritoneal membranes
respectively (the latter being a part of the diaphragm) the chamber into
which the pulmonary wings project is entirely cut off from the rest of the
body cavity. On either side, it forms a pleural cavity (see Fig. 169, p.
175). The epithelium and underlying connective tissue covering the pul
FIG. 293. RECONSTRUCTIONS OF x H E
LUNGS OF YOUNG EMBRYOS, SEEN FROM
THE VENTRAL SURFACE. (His.)
A, A younger stage than B; ep, apical
bronchus; I, II, primary bronchi.
296 HISTOLOGY
monary wings, constitute the visceral pleura; and the similar layers toward
the thoracic wall form the parietal pleura. These layers are comparable
in development and structure with the corresponding layers of the peritoneum. Other subdivisions of the pleura are the mediastinal, pericardial,
and diaphragmatic pleurae. The lung is connected with the mediastinum
by a short and broad stem of connective tissue, across which the bronchi,
vessels and nerves extend. This is the root of the lung, and the vessels
enter at the hilus.
The branches which are given off by the stem-bronchus within the pulmonary
wings, are formed with great regularity, and they have been carefully studied in many
mammals. Very early in development, the human lungs become asymmetrical, and
at the stage shown in Fig. 293, B, the three lobes of the right lung and the two lobes
of the left lung are already indicated. In the pig the asymmetry is greater, since on the
right an unpaired lobe proceeds directly from the trachea; in certain animals, as in the
seal, the right and left lungs have symmetrical bronchi. Whether the symmetrical
condition is the primary one, and how the bronchi of one lung should be homologized
with those of the other, are questions which have been much discussed. For the comparative anatomy of the bronchi, see Huntington, Ann. N. Y. Acad. Sci., 1898, vol. n,
pp. 127-148; for their development, especially in the pig, see Flint, Amer. Journ.
Anat., 1906, vol. 6, pp. 1-137.
The blood vessels of the lungs are derived from several sources. They
include the large pulmonary arteries and veins, which are the principal
vessels of the lung, and the small but important bronchial arteries and veins.
The pulmonary vessels are shown in Fig. 294, which represents the trachea
and right lung of a human embryo, seen from the left side; the left lung has
been cut away at /. br.
The pulmonary arteries develop in connection with the pulmonary
arches, which are two vessels, one on either side, passing from the ventral
aorta to the dorsal aorta. Approximately midway in its course, each of
these arches sends a branch to the lung of the corresponding side. Subsequently the trunk of the ventral aorta becomes spirally subdivided by a septum, so that the portion leading to the pulmonary, arches is split off from
the rest; the way in which its root becomes connected with the right
ventricle only, has been described with the development of the heart. As
a result of this subdivision, the pulmonary artery leaves the heart and divides into right and left arches, each of which sends a branch to the lung on
the same side and then passes on to the dorsal aorta. The connection between the right arch and the right dorsal aorta is soon lost, however, so that
the vessel to the right lung (Fig. 294, r. r.} appears to be given off from the
main pulmonary artery. The left pulmonary arch enlarges, and until birth
it forms a great vessel, known as the ductus arteriosus, which conveys most
of the blood from the pulmonary artery into the aorta. The amount of
blood which goes to the inactive lungs may be inferred from the relative
size of the vessels shown in the figure. Soon after birth, when respiration
RESPIRATORY APPARATUS
2Q7
has begun, the ductus arteriosus closes, becoming a fibrous cord, and then
the volume of blood going through the pulmonary artery equals that in the
aorta. (For further details regarding the development of the pulmonary
arteries, see Bremer, Amer. Journ. Anat., 1902, vol. i, pp. 137-144).
The pulmonary veins are at first represented by a capillary plexus
around the lung-bud, which receives its blood in part from the pulmonary arteries already described, and in part from
branches of the dorsal aorta, some of which
persist as the bronchial arteries. The capillary
plexus is drained partly by branches of the
posterior cardinal or azygos veins, representing
the future bronchial veins, and partly by a
minute vein which has grown out from the left
atrium and is destined to become the great
pulmonary veins. At a certain stage these
veins, two from each lung, have a common
orifice in the left atrium; but in later stages, as
the heart enlarges, their short common stem is
taken up into the wall of the atrium, so that
the four pulmonary veins acquire separate
openings. The early stages in the development
of the pulmonary veins in the cat have recently
been studied by Brown (Anat. Rec., 1913, vol.
7, pp. 299-330).
The small bronchial arteries, one or two on
each side, are branches of the upper part of the
thoracic aorta (Fig. 294); sometimes one of
them proceeds from an intercostal artery.
The bronchial arteries enter the hilus of the
lung and pass into the fibrous tissue in the walls
of the bronchi. The main stems branch with
the bronchi. They produce capillary networks in the bronchial mucous membrane, and send branches to the peribronchial connective tissue, supplying it with capillaries and becoming
the vasa vasorum of the main branches of the pulmonary artery (Miller,
Anat. Anz., 1906, vol. 28, pp. 432-436). In some animals Miller finds
that the bronchial arteries pass on into the pleura, as in the horse; in others,
like the dog, terminal branches of the pulmonary arteries supply the pleura;
and in the human lung the pleura receives both pulmonary and bronchial
vessels (Amer. Journ. Anat., 1907, vol. 7, pp. 389-407).
The bronchial veins are small branches of the azygos vein. They do
not receive all the blood from the bronchial arteries, since some capillaries
from the latter are drained by the pulmonary veins.
thao.
FIG. 294. RECONSTRUCTION OF A
PART OF A HUMAN EMBRYO OF
13.8 MM. (Dr. F. W. Thyng.)
ao., Aorta; d.a., ductus arteriosus;
1., entodermal part of the lung;
1. at., left atrium; 1. br., left
bronchus; 1. r., left ramus of
pulmonary artery, p. a.; r. r.,
its right ramus; oe., oesophagus; p. c., pericardial cavity;
p. v., pulmonary vein; s. t.,
septum transversum; th. ao.^
thoracic aorta; tr., trachea.
298 HISTOLOGY
LARYNX.
The mucous membrane of the larynx is a continuation of that of the
pharynx, and accordingly consists of epithelium and tunica propria. A
submucosa connects it with the underlying parts. In most places the
epithelium appears to be stratified and columnar, but it is said to be
pseudo-stratified, with nuclei at several levels (Fig. 38, p. 49). It is
difficult to determine whether or not all the cells are in contact with the
basement membrane. This type of epithelium, which occurs also in the
trachea, is ciliated. The stroke of the cilia is toward the pharynx. A
stratified epithelium with squamous, non-ciliated outer cells is found on
the vocal folds (true vocal cords) , on the anterior surface of the arytaenoid
cartilages and on the laryngeal surface of the epiglottis. The distribution
of the two sorts of epithelium above the vocal folds is subject to individual
variation. The squamous epithelium often occurs in islands. The tunica
propria is composed of fibrous connective tissue with many elastic fibers,
and beneath the epithelium it forms a basement membrane (membrana
propria). It includes reticular tissue containing a variable number of
lymphocytes, which are gathered in solitary nodules in the wall of the
laryngeal ventricle (sinus of Morgagni). Connective tissue papillae are
found chiefly beneath the squamous epithelium. At the free border of the
vocal folds and on their under surface, the papillae unite to form longitudinal ridges. On the laryngeal surface of the epiglottis there are only
isolated papillae, against which rest the short taste buds.
The submucosa contains mixed, branched, tubulo-alveolar glands,
measuring from 0.2 to i.o mm.; they are abundant in the ventricular
folds but are absent from the middle part of the vocal folds. The
ventricular folds (false vocal cords) consist of a loose vascular fatty tissue,
often containing small bits of elastic cartilage about i mm. long, and
similar cartilages measuring 2-3.5 mm. are sometimes found in the anterior
ends of the vocal folds.
The cartilages of the larynx are mostly of the hyaline variety, resembling those of the ribs. To this class belong the thyreoid, cricoid, the
greater part of the arytaenoid, and often the small triticeous cartilages.
Elastic cartilage is found in the epiglottis, the cuneiform and corniculate
cartilages, the apex and vocal process of the arytaenoids, and generally the
median part of the thyreoid. In women this portion is not involved in
the ossification (chiefly endochondral) which begins hi the thyreoid and
cricoid cartilages between the twentieth and thirtieth years. The triticeous cartilages (nodules in the lateral hyothyreoid ligaments, named
from their resemblance to grains of wheat) are sometimes composed of
fibro-cartilage.
The blood vessels form two or three networks parallel with the surface.
RESPIRATORY APPARATUS 299
followed by a capillary plexus just beneath the epithelium. The lymphatic vessels similarly form two communicating networks, of which the
more superficial consists of smaller vessels and is situated beneath the
capillary plexus. The nerves form a deep and a superficial plexus which
are associated with microscopic ganglia. Non-medullated fibers end either
beneath the epithelium in bulbs and free endings with terminal knobs, or
within the epithelium in free ramifications and in taste buds. Below the
vocal folds, subepithelial nerve endings and buds are absent, but many
intraepithelial fibers occur, which surround individual taste cells. The
nerves and vessels of the larynx are numerous, except in the dense elastic
tissue of the vocal folds.
TRACHEA AND BRONCHI.
The trachea consists of a mucosa, submucosa, and a fibrous outer
layer containing the tracheal cartilages. The general arrangement of
the layers is the same as that found in the large bronchi (Fig. 295).
The mucosa consists of pseudo-stratified columnar epithelium with
cilia proceeding from distinct basal bodies (Fig. 38, p. 49). Exceptionally, the lining of the trachea, toward the oesophagus, has been found to
consist of stratified squamous epithelium resting on connective tissue
papillae. Beneath the epithelium there is a broad basement membrane,
followed by a layer of reticular tissue containing many lymphocytes,
forming a tunica propria. Beneath the reticular tissue there is a layer of
coarse longitudinal elastic fibers, which may readily be seen in haematoxylin and eosin preparations. This layer may be compared with the
muscularis mucosae of the intestine.
The submucosa is a layer of loose fatty connective tissue extending to
the perichondrium of the tracheal cartilages. It contains the bodies of
the tracheal glands, which include both serous and mucous cells, and are
beautiful objects for the study of serous crescents.
The outer layer of the trachea is continuous with the tissue of the
mediastinum. It contains abundant blood and lymphatic vessels, and
nerves, both medullated and non-medullated. Internally it forms the
perichondrium around the succession of C-shaped hyaline cartilages, the
free ends of which are toward the oesophagus. In the intervals between
these ends there is a layer of transverse smooth muscle fibers, usually
accompanied by outer longitudinal fibers. As in the intestine, elastic
fibers are abundant among the. muscle cells. In old age, the hyaline
cartilages show fibrous degenerative changes, and may become partly
calcified.
The primary bronchi have the same structure as the trachea, but in
their subdivisions changes occur, and the C-shaped rings of cartilage are
300
HISTOLOGY
replaced by irregular plates found on all sides of the tube (Fig. 295).
These diminish in size as the bronchi become smaller, and disappear in
those about i mm. in diameter. Usually the cartilages are hyaline, but
elastic cartilage is said to occur in places. The circular muscle fibers
form a layer completely surrounding the tube internal to the cartilages.
Branched tubulo-alveolar bronchial glands extend further down the tubes
than the cartilages. In the larger bronchi they are present in great numbers,
Tunica
Epithelium. propria. m
Connective tissue.
Bronchial gland.
Duct of gland.
FIG. 295. CROSS SECTION OF A BRONcays 2 MM. IN DIAMETER, FROM A CHILD.
and their bodies lie outside of the muscular layer and project into the spaces
between the cartilages. The mucosa is thrown into longitudinal folds;
it is covered with ciliated epithelium containing goblet cells and resembling
that of the trachea. Lymphocytes are numerous in the tunica propria,
sometimes collecting in solitary nodules and wandering into the epithelium.
The small bronchi, 0.5-1.0 mm. in diameter, are known as bronchioles.
They are free from cartilage and glands, and are lined throughout with
ciliated columnar epithelium.
RESPIRATORY APPARATUS
301
LUNGS.
The arrangement of the ultimate branches of a bronchiole is shown in
the diagram, Fig. 296. The respiratory bronchioles, 0.5 mm. or less in
diameter, at their beginning contain a simple columnar ciliated epithelium.
Further in their course the goblet cells disappear, cilia are lost, the cells
become cuboidal, and among them are found thin, non-nucleated plates of
different sizes. These plates constitute the respiratory epithelium. The
transition from the cuboidal to the respiratory epithelium occurs irregularly, so that a bronchiole may have cuboidal epithelium on one side and
Bronchial artery.
Pulmonary vein.
Pulmonary artery.
; ~ Respiratory'bronchiole.
Pleural capillaries
(Lobule.)
PIG. 206. DIAGRAM OF A LOBULE OF THE LUNG, SHOWING THE BLOOD VESSELS AND THE TERMINAL
BRANCHES OF A BRONCHIOLE.
respiratory epithelium on the other; or one sort of epithelium may form an
island in the midst of the other. Hence the respiratory bronchioles
contain a mixed epithelium (Fig. 297, A). The respiratory epithelium
steadily gains in extent until the cuboidal epithelium has disappeared.
At irregular intervals along the bronchioles the respiratory epithelium
forms hemispherical outpocketings or alveoli. The alveolar ducts, from i
to 2 mm. long, differ from the respiratory bronchioles in that they contain
only the respiratory epithelium and are thickly beset with alveoli. The
layer of smooth muscle fibers may be traced to the end of the alveolar
ducts, where it terminates. Since the muscles do not extend over the
302
HISTOLOGY
alveoli, but merely surround the main shaft of the duct, the layer is greatly
interrupted, and some consider that it ends in the course of the duct.
The respiratory bronchiole may be continued as a single alveolar duct or
may divide into two or more. The alveolar ducts branch to produce ahe
Pores. Cuboidal epithelial cells. Non-nucleated
\ ,__JbtfMUb Plates.
Cuboidal
epithelial Non-nucleated
cells. plates.
Border of an alveolus. B Fundus of an alveolus.
FIG. 297. FROM SECTIONS OF THE HUMAN LUNG. X 240.
A, Mixed epithelium of a respiratory bronchiole; B, an alveolus sketched with change of focus; the border
of the alveolus is shaded; it is covered by the same epithelium as that of the (clear) fundus of the
alveolus; the nuclei of the cells are invisible. (Silver nitrate preparation.)
olar sacs (infundibula) which are cavities in the center of clusters of alveoli.
The sacs resemble the ducts as shown in Fig. 296.
According to Miller (Arch. f. Anat. u. Entw., 1900, pp. 197-228) who has made
careful reconstructions of the terminal
branches in the human lung, an atrium, or
round cavity, should be recognized between
the alveolar duct and the alveolar sac. The
alveolar duct is said to terminate by opening into 3 to 6 atria, the entrances to which
are surrounded by smooth muscle fibers
forming "a sort of sphincter"; the atria possess no muscle fibers. Each atrium is connected with two or more alveolar sacs, and
is moreover beset with alveoli (Fig. 298).
Stohr states that the recognition of an
atrium between the alveolar duct and
alveolar sac seems to him superfluous; "in
good casts of the human lung it is not to
be distinguished, and in other animals it is
inconstant."
FIG. 298. CAMERA LUCIDA DRAWING FROM A
SECTION OF A CALF'S LUNG. (Miller.)
The stippling indicates smooth muscle and cuboidal epithelium ; the lines, respiratory epithelium. B. R., Respiratory bronchiole; D. A.,
alveolar duct; A., atrium; A. S., alveolar sac.
In sections, without resort to reconstructions, very little can be found
out concerning the relations of the alveoli to the bronchial ramifications.
The following structures are all which can easily be identified: (i) alveoli;
LUNGS
303
(2) spaces bounded by alveoli (alveolar sacs, atria and alveolar ducts, the
ducts having muscle fibers in their walls); (3) small bronchioles having
scattered alveoli along their walls, and therefore presenting a mixed epithelium (respiratory bronchioles); and (4) bronchioles with no respiratory
epithelium.
The study of sections of the adult lung is facilitated by comparison
with those from an embryonic lung. Comparable sections, including
the pleura, and drawn at the same scale of magnification, are shown in
Figs. 300 and 301. In the lung of the embryo of four months, the terminal
branches of the bronchioles are found in the centers of lobules, one of which
is shown in Fig. 300 (bounded by b. v. and lym.). The axial bronchioles
break up into ramifying tubules lined with cuboidal cells, and at birth
the alveoli which are found at the end of these structures are also lined with
cuboidal epithelium. The main arteries run with the axial bronchioles
in the centers of lobules; and the large veins and lymphatic vessels are at
their periphery. This arrangement is retained in the adult (Fig. 296).
Deep in the lung, the small bronchi are surrounded by considerable connective tissue, containing arteries, veins and large lymphatic vessels.
After respiration has been established, the alveoli become greatly
distended, so that the connective tissue containing the capillary vessels
is flattened out in very thin layers. These layers are bounded on either
side by the respiratory epithelium of adjacent alveoli (Fig. 301). In
producing this epithelium, the cells not only become flattened but they are
transformed into thin structureless plates, and those from several cells
may fuse to form large plates. In amphibia, nuclei in small amounts of
protoplasm are found attached to the basal or connective tissue side of the
plates, often associated in groups. In addition to these cells, the alveolar
walls contain the endothelial cells of the capillaries, connective tissue cells,
wandering cells, and many elastic fibers. These fibers surround the
alveoli and encircle their outlets; the alveolar walls are so elastic that in
inspiration they may expand to three times the diameter to which they
return during expiration (o.i to 0.3 mm.). Pores have been described
leading from one alveolus to another (Fig. 297, B).
The richness of the capillary network in the alveolar walls is seen in
injected specimens (Fig. 299). Respiration takes place by the transfer of
gases between the blood in these vessels and the air in the alveoli, therefore
through the endothelial cells and alveolar plates, together with the trivial
amount of connective tissue which may intervene.
The pulmonary and bronchial blood vessels have already been described, and their relations to the lobule of the lung are shown in Fig. 296.
The pulmonary arteries are axial vessels within the lobules, breaking up into terminal branches at the atria, and these branches become axial along the
alveolar sacs. Each terminal branch has been described as the center of
HISTOLOGY
an ultimate lobule or structural unit. The veins are peripheral both in
the units and larger lobules; between the latter they run through connective tissue septa.
The abundant lymphatic vessels are arranged in a superficial set draining into the pleura by way of the interlobular septa; and a deep set draining toward the hilus along the bronchi, accompanying the large vessels.
Lymphatics of the deep set do not extend into the lobules; they terminate
along the alveolar ducts. Around the larger bronchi and at the root of the
lung, lymph glands are numerous. A conspicuous feature of the sections
of the lung is the presence of black soot in the tissue around the lymphatic
vein vessels. It penetrates the pulmo
nary epithelium in the smallest
bronchioles, apparently passing
Artery. between the epithelial cells. Some
of it is taken up by phagocytes.
Having entered the lymphatic
FIG. 299. FROM A SECTION OF THE LUNG OF A vessels it is distributed along their
CHILD, INJECTED THROUGH THE PULMONARY ._ ., P /. ,,
ARTERY, x so. courses. On the surface of the
Of the five alveoli drawn^thejhree upper ones are l ung J t [ 5 seen j n tne interlobular
septa, marking out the boundaries
of the lobules. Because of the steady increase in this deposit, the color
of the lungs changes from birth until old age.
The nerves of the lung include the pulmonary plexus derived from the
sympathetic system. Its fibers enter at the root of the lung and spread
around the bronchi and vessels, to which they are chiefly distributed.
Small ganglia are found within these plexuses. The vagus also sends
branches to the lungs, including medullated and non-medullated fibers,
which join the sympathetic plexuses.
PLEURA.
The visceral pleura is a thinner layer than the parietal pleura, and is
closely attached to the lung. It is covered with a single layer of flat
mesothelial cells, which in the collapsed lung become thicker and shorter.
In specimens which have been handled, this layer is often lacking. It rests
upon a thin layer of fine-meshed fibrous tissue, beneath which is the coarse
subserous layer continuous with the interlobular septa of the lung (Fig.
301). This tissue is highly elastic. In the subserous layer, blood
vessels, derived from both pulmonary and bronchial arteries, form an
abundant capillary plexus. The superficial lymphatic vessels are very
evident, and in relation with them lymphoid tissue is found, and occasionally lymph nodules. Stomata, which have been described, are presumably artificial apertures in the epithelium and are not connected with
the lymphatic vessels.
b.v.
FIG. 300.
ep c.t. s.s.
al.s. al.
FIG. 301.
FIGS. 300 AND 301. SECTIONS OF THE LUNG DRAWN ON THE SAME SCALE OF MAGNIFICATION; FIG. 301
FROM A HUMAN EMBRYO OF FOUR MONTHS; FIG. 301, FROM AN ADULT.
al., Alveolus; al. s., alveolar sac; br., bronchiole; b. v., blood vessel; c. t., outer layer of pleural connective
tissue; ep., pleural epithelium; lym., lymphatic vessel; pi., pleura; s. s., subserous connective tissue;
t. b.. terminal branch of the bronchiole.
306
HISTOLOGY
. i-W.b.
The parietal pleura is a thicker and less elastic layer. Ventrally and
below, toward the pleuro-pericardial membrane, it exhibits folds containing
fat (plica adiposce); and sometimes it forms vascular elevations resembling
synovial villi the pleural mlli. Fat may be found in the pleura elsewhere.
The nerves of the pleura are derived from the phrenic, sympathetic
and vagus nerves. In the parietal pleura typical lamellar corpuscles may
be found, together with the smaller variety, known as the Golgi-Mazzoni
corpuscles.
URINARY ORGANS.
WOLFFIAN BODIES AND WOLFFIAN DUCTS.
On the twenty-eighth of November, 1759, Caspar Friedrich Wolff, then
in his twenty-sixth year, defended a thesis entitled "Theoriagenerationis"
and obtained the degree of doctor of medicine at Halle. In addition to
the fundamental principles which this renowned thesis set forth, it included
an account of the development of the kidneys in
chick embryos. From the diffuse substantia
cellulosa along the ventral side of the spinal
column, beginning on the third day of incubation, Wolff saw two elongated bodies gradually
take form, and become the kidneys, each being
connected with the cloaca by a ureter. These
structures, however, are not the kidneys of the
adult, and they are generally known as Wolffian
bodies', their ureters are the Wolffian ducts.
They are large and important organs in human
embryos, as shown in Fig. 302. The true or
permanent kidneys of mammals arise later, and
the Wolffian bodies degenerate, becoming vestigial in the female; in the male, however, they
acquire new functions, and are retained as a portion of the genital ducts (namely the duct of
the epididymis). In the embryo they are renal organs built upon the
same plan as the permanent kidneys, and moreover in the fishes and
amphibia they are the kidneys of the adult.
Still another renal organ develops in embryos, anterior to the Wolffian
body, and it has been found that the Wolffian duct is primarily the due of
this anterior kidney or pronephros; consequently the Wolffian duct is sometimes called the pronephric duct. The pronephros is the functional
kidney in only the lowest of vertebrates (myxinoids). Singularly it has
been found that " the human pronephros is by far the best developed within
the groups of mammals" (Felix, in Keibel and Mall's Human Embry
FIG. 302. DISSECTION OF A
HUMAN EMBRYO OF THIRTYFIVE DAYS. (After Coste.)
al., Bladder; 1., lung; St., stomach; s. tr., septum transversum; u. c., umbilical
cord; W. b., Wolffian body;
W. d., Wolffian duct.
WOLFFIAN BODIES
307
ology, Vol. 2). Except for its duct, it entirely disappears in very young
embryos (5 mm.). All the renal organs pronephros, Wolffian body (or
mesonephros) , and kidney (or metanephros) are developed from the
nephrotomes. They are all composed of mesodermal tubules, each of
which is in close relation with a knot of capillary blood vessels derived from
branches of the aorta. Such a knot of vessels is a glomerulus, and certain
products are eliminated from the glomerulus into the tubules to form the
urine.
Development of the Wolffian Body and Wolffian Duct. The general
relations of the neplirotome to the mesodermic somites and to the coelomic
mes.seg'.
,W.d.
neph.
nch.
B C
FIG. 303. A, TRANSVERSE SECTION OF A RABBIT EMBRYO OF NINE DAYS; B, HUMAN EMBRYO, 4 MM.;
C, HUMAN EMBRYO. 10 MM.
mo, Aorta; c., posterior cardinal vein; coe., ccelom; gl., glomerulus; g. r., genital ridge; int., intestine; mes.
mesentery; mes. seg., mesodermic somite; my., myotome; nch., notochord; neph., nephrotome; s-c. v.
subcardinal vein; si., sinusoid; sy., sympathetic nerves; u. v., umbilical vein; W. d., Wolffian duct
W. t., Wolffian tubule.
epithelium have already been briefly discussed (p. 41). A nephrotome
from a young rabbit embryo is seen in section in Fig. 303, A, together with
its elevation which contributes to the formation of the Wolffian duct. The
nephrotome here shown is from one of the anterior segments and belongs
with the pronephros.
In human embryos, according to Felix, pronephric tubules are formed
from the seventh to the fourteenth segments, and perhaps from those
further forward. The elevations to which these nephrotomes give rise
turn posteriorly and unite with one another to form the Wolffian duct.
This is at first a solid cord of cells which grows posteriorly in the trough
3 o8
HISTOLOGY
between the somites and somatic mesoderm. It lies near the ectoderm,
but it is now generally agreed that the ectoderm takes no part in its formation. Finally its growing extremity reaches the ventral portion of the
cloaca and fuses with it. Later this ventral part of the cloaca becomes cut
off to form the bladder, and the Wolffian duqts then empty into the neck of
the bladder. The pronephric tubules meanwhile become detached from
the ccelomic epithelium, but they remain rudimentary and degenerate
without having any glomeruli formed in connection with them.
The mesonephric tubules develop from the more posterior nephrotomes,
after the Wolffian duct has formed. They acquire openings into the
Wolffian duct, but do not contribute to its development. In producing mesonephric tubules, the
nephrotomic tissue becomes detached and separates into masses
which form vesicles (Fig. 303, B).
Each vesicle elongates and becomes an S-shaped tubule, one
end of which fuses with the
Wolffian duct and opens into it;
the other end remains blind. A
knot of capillaries, derived from a
branch of the aorta, develops in
the distal concavity of the S and
becomes a glomerulus ; a glomerulus
is formed in connection with every
Wolffian tubule. The tubules then elongate and become coiled, and
together they produce the rounded swellings on either side of the root of
the mesentery, which are the Wolffian bodies (Fig. 303, C). The genital
glands arise as mesodermal thickenings on the ventro-medial surface of
these bodies.
A single Wolffian tubule is shown in Fig. 304, and the way in which its
distal end envelops the glomerulus is clearly indicated. It is said to form
the capsule of the glomerulus. By passing through the inner layer of this
capsule, fluid from the blood vessels enters the tubule and is conveyed
through the Wolffian duct to the bladder. The tubules are generally
unbranched, and are lined with simple epithelium. The epithelium is in
part glandular, and contributes to the formation of the urine. Finally
it may be noted that a nephrotome may divide into several vesicles (sometimes perhaps as many as four), and therefore the number of Wolffian
tubules is greater than the number of corresponding segments. In man
the maximum number is 83 (Felix). The mesonephric tubules also
extend forward, so that some segments contain both mesonephric and
pronephric tubules.
c C a'
FIG. 304. RECONSTRUCTION OF A WOLFFIAN TUBULE
FROM A HUMAN EMBRYO OF 10.2 MM. (Except the
glomerulus, after Kollman.)
c., Inner layer, and c. a., outer layer of the capsule of
the glomerulus; div., diverticulum ; gl., glomerulus;
W. d., Wolffian duct.
WOLFFIAN BODIES
309
It is generally believed that the Wolffian bodies of mammalian embryos are active
renal organs, producing a form of urine which collects in the allantoic sac. In pig
embryos this sac and the Wolffian bodies are both unusually large. MacCallum (Amer.
Journ. Anat., 1902, vol. i, pp. 245^59) notes that the tubules of the Wolffian body in
the pig "show a very distinct division into a secretory and a conducting part." In
the human embryo, however, the allantois is very small and the Wolffian bodies degenerate early, before the kidney can become functional. Therefore Felix (Keibel and
Mall's Human Embryology, vol. 2) regards the question as settled. The Wolffian
body " does not function as an excretory organ"; but he adds, "This does not, of course,
imply that it may not have been active in another manner unknown to us."
Veins of the Wolffian Body. In determining the arrangement of the
large veins of the abdomen, the Wolffian bodies are of fundamental
importance. They are supplied by the posterior cardinal veins which
pass from the tail of the embryo, on either side of the aorta, to the heart.
I/, c. c.
h. a 2.
W.B
i I.'
il
FIG. 305- THE TRANSFORMATION OF THE POSTERIOR CARDINAL SYSTEM OF VEINS.
a. c., Anterior cardinal; as. L, ascending lumbar; az., azygos; c., caudal; c. s., coronary sinus; h., hepatic;
h. a. z., hemiazygos; h. az. a., accessory hemiazygos; il., common iliac; in., innominate; j., jugular;
K., kidney; I.e. c., left common cardinal; m. s., median sacral; p. c., posterior cardinal; r. c. c., right
common cardinal; s. c., subcardinal; scl., subclavian; sp., spermatic; sr., suprarenal; sup., supra cardinal;
T., testis; v. c. i., vena cava inferior; v. c. s., vena cava superior; W. B., Wolffian body.
Before entering the right atrium of the heart, they are joined by the
anterior cardinal veins from the head, thus forming the right and left
common cardinal veins, or "ducts of Cuvier." As each posterior cardinal
vein extends along the dorsal side of the Wolffian body, it sends branches
in among the tubules, and these unite ventrally on either side in the
subcardinal vein (Fig. 305, A). Thus each Wolffian body is lodged in a
venous loop formed by the posterior cardinal and subcardinal veins, and
3 io
HISTOLOGY
such a loop is found in all classes of vertebrates. Venous blood entering
the Wolffian body posteriorly flows out from it anteriorly, and circulates
among the tubules in lacunar vessels, closely resembling the hepatic sinusoids. This is the "renal portal system." It should be noted, however,
that the renal sinusoidal vessels are poorly developed in mammalian
embryos.
In sections these veins are readily recognized. The mesonephric
arteries pass from the aorta to the glomeruli of the Wolffian body, between
the subcardinal vein in front and posterior cardinal vein behind (Fig.
303, C). In places, the subcardinal veins form large anastomoses across
the mid-ventral line; the posterior cardinal veins are further apart, and
receive intersegmental branches from the dorsal musculature.
As the kidneys grow upward behind the Wolffian bodies, their ureters
become encircled by a branch from the posterior cardinal vein (Fig. 305,
A). The venous loop around the ureter was described by Hochstetter
(Morph. Jahrb., 1893, vol. 20, pp. 543-648), and its dorsal limb, together
with secondary anastomoses, has been named the supracardinal vein
(Huntington and McClure, Anat. Rec., 1907, vol. i, pp. 29-30). The
transformation of these veins into the branches of the inferior vena cava
is represented somewhat diagrammatically in Fig. 305, B, and may be
briefly described as follows:
The anastomosis between the subcardinal veins becomes a part of the left renal
vein. Above this anastomosis the right subcardinal vein connects with the veins of the
liver and forms a portion of the vena cava inferior. The left subcardinal vein, above
the renal anastomosis, becomes reduced to the left suprarenal vein (Hochstetter). The
subcardinal veins below the renal anastomosis are associated with lymphatic vessels to
which they apparently give rise; otherwise they disappear.
The posterior cardinal veins above the renal anastomosis, after they have been
tapped by the formation of the vena cava inferior, are known as the azygos and hemiazgos veins, and the outlet of the left common cardinal becomes cut off as the coronary
sinus (Fig. 305, B, which shows also the formation of the superior vena cava). Below
the renal anastomosis the posterior cardinal veins give rise to the genital veins (spermatic or ovarian), and the Wolffian body becomes reduced to an appendage of the genital glands. As the genital glands descend into the pelvis, their veins become elongated;
and the corresponding arteries, derived from the mesonephric arteries, are likewise
elongated. The supracardinal vein on the right side becomes a part of the vena cava
inferior; on the left it is probably represented by the ascending lumbar vein.
The kidneys are supplied by vessels which enter them after they have attained their
permanent position. Their arteries and veins consequently pursue a straight course
from the aorta and vena cava, respectively, to the hilus of the kidney.
KIDNEY.
Development. The kidney develops after the Wolffian body has been
formed. It arises in two parts, one of which is an outgrowth of the Wolffian duct; the other is a mass of dense mesenchyma surrounding this
outgrowth, and said to be derived from the posterior nephrotomes. Both
KIDNEY
parts are mesodermal. The part derived from the Wolffian duct may be
considered first.
Each Wolffian duct, near the place where it enters the cloaca, forms a
knob-like outpocketing which elongates rapidly, becoming the ureter.
The distal end of the outpocketing expands and becomes lobular, thus
producing the pelvis of the kidney. After the ventral part of the cloaca
W.d. M.d. Md
cl.
FIG. 306. THE DEVELOPMENT OF THE RENAL PELVIS AND URETER. (Keibel.)
A, Human embryo of n.s mm. (4i weeks); B, 25 mm. (8J-9 weeks), a., Anus; al. d., allantoic. duct
bL, bladder; cl., cloaca; M. d., Mullerian duct; p., pelvis of the kidney; r. t rectum; ur., ureter; u. s.,
urogenital sinus; W. d. Wolffiian duct.
has been split off to form the bladder, the ureter and Wolffian duct, on
either side, open into it by a common outlet (Fig. 306, A). Later, the
terminal portion of each Wolffian duct is taken up into the wall of the
expanding bladder, so that the ureters acquire openings separate from
FIG. 307. RECONSTRUCTION OF THE URETER,
RENAL PELVIS, AND
ITS BRANCHES IN A 20MM. HUMAN EMBRYO.
(Huber.)
FIG. 308. FROM A SECTION OF A KIDNEY OF AN 18MM. HUMAN EMBRYO. X 233. (Huber.)
a., Primary collecting tubule, with dilated extremity;
b,b'., inner layer, and c. ( outer layer of dense mesenchyma; d., loose mesenchyma; e., vesicle, the
beginning of a renal tubule.
those of the ducts. With further growth the orifices of the Wolffian ducts
are carried toward the median line and downward toward the outlet of
the bladder (Fig. 306, B), and this position is permanently retained.
Meanwhile the lobes of the renal pelvis have become deeper and formed
3 I2
HISTOLOGY
pouches known as the major and minor calyces. In the adult there are
usually two major calyces, one at either end of the pelvis, and from these
most of the minor calyces grow out; the others spring directly from the
main pelvic cavity. There are about eight in all. From the minor calyces the collecting tubules grow out. Each tubule has an enlarged extremity
FIG. 309.
-MODBLS SHOWING SUCCESSIVE STAGES IN THE DEVELOPMENT OF A URINIFEBOUS TUBULE
INCLUDING THE ASSOCIATED PORTION OF THE COLLECTING TUBULE. (Huber.)
From a human embryo of the seventh month. X iCo.
(Fig. 307) which divides into two branches with a U-shaped crotch, like
a tuning-fork. The branches subdivide repeatedly in the same manner,
so as to make pyramidal masses of straight tubules radiating from the
calyces. Thus the renal outgrowth from the Wolffian duct produces the
KIDNEY
313
p.C
A.
epithelial lining of the ureter, pelvis, calyces and collecting tubules, including all of their branches.
The second part of the kidney, which consists of dense mesenchyma,
becomes subdivided into masses enveloping the enlarged tips of the branching collecting tubules. Some of its cells
become arranged so as to form vesicles
(Fig. 308), one of which is shown in the
reconstruction, Fig. 309, A. The vesicles are at first entirely separate from
the collecting tubules. Each vesicle
becomes elongated, making an S-shaped
tubule (Fig. 309, B, C), and its outer
or upper end unites with the collecting
tubule (Fig. 309, D). A glomerulus
develops in the lower curve of the S,
and is gradually enveloped in the
terminal part of the tubule, which thus
forms its capsule. Between the capsule and the collecting tubule, the renal
tubules become greatly convoluted.
One of the loops in the coils thus formed
slongates downward, lying close beside
and parallel with the collecting tubule;
this is the loop of Henle (Fig. 309, J).
Three tubules of the adult kidney
are shown diagrammatically in Fig. 310.
Each capsule connects with a proximal
convoluted tubule, which, after extending
outward toward the surface of the kidney, turns downward as the descending
limb of Henle's loop. The descending
limb is a straight tubule, the lower portion of which is of small diameter owing
to the flatness of the cells in its walls;
its lumen is not reduced. This "thin
segment," as shown in the diagram,
does not form the entire descending
limb, but only its lower part. Frequently it passes around the bend into
the ascending limb. The tubule, after turning the bend, forms ihe ascending limb of Henle's loop. It returns to the vicinity of the capsule from
which it arose, and makes a few coils, thus constituting the distal convoluted tubule (intercalated or intermediate tubule). By means of the
functional tubule it joins the arched collecting tubule and this passes into
D.
.-ad.
FIG. 310. DIAGRAM OF THREE URINIFEROUS
TUBULES IN RELAT^N WITH A COLLECTING TUBULE. (Modified from Huber.)
a. 1., Ascending limb of Henle's loop; c., capsule; c. t., collecting tubule; d. c., distal
convoluted tubule; d. 1., descending limb;
j,, junctiona) tubule; p. c,, proximal convoluted tubule; p. d. .papillary duct.
A, cortex; B-D, medulla, subdivided into an
inner zone (D) and an outer zone (B-C) ;
the latter includes an inns' band or stripe
(C), and an outer band (I).
314 HISTOLOGY
the straight descending collecting tubule. From the capsule to the collecting tubule no branches occur; and this extent of the tubule represents the
part derived from mesenchyma. The collecting tubules receive many
branches. Traced toward their outlet in the pelvis they become larger,
finally forming the papillary ducts.
In the diagram (Fig. 310) the tubules are represented as much coarser than is actually the case. Their true proportions in the rabbit's kidney have been shown by Huber,
who, with extraordinary success, has isolated individual tubules, keeping them intact
from the capsule to the collecting tubule (Anat. Rec., 1911, vol. 5, pp. 187-194). They
are 20-30 mm. in length and less than o.i mm. in diameter. Huber's account of the
development of the kidney, from which Figs. 307-309 have been taken, is in the supplement to the Amer. Journ. Anat., 1905, vol. 4.
Surface Markings. Before studying sections of the kidney microscopically, the small subdivisions of the organ which may be seen upon its
cut surface should be examined. They are shown in transverse section
in Fig. 311, but appear equally well when the kidney is divided lengthcortex.
Pars convoluta. Pars radiata.
Pyramid
Pelvis.- "MMfcT :^i ''^ ^%^ A (Medulla).
Renal anery.
Ureter.
Renal vein.
Renal column.
^'< \
Calyx.
FIG. 311. THE SURFACE MARKINGS OF THE HUMAN KIDNEY. (After Brodel.)
wise. The ureter opens into the pelvis, which is prolonged into the cuplike calyces, two of which are shown in Fig. 311. Each calyx receives
a nipple-like projection of the substance of the kidney, known as a renal
papilla. Sometimes two of them project into one calyx. They are soft,
dark red structures, and it does not appear on inspection that the grayish
lining of the calyx is reflected over their outer surface; this is seen in sections. Toward the apex of each papilla there are from 15 to 20 foramina,
which are the orifices of as many papillary ducts; through them the urine
enters tte calyx. The foramina are barely visible without magnification. Each papilla forms the apex of a renal (or Malpighian) pyramid,
described by Malpighi (1666) in his treatise " on the structure of the viscera," which gave the first account of various almost microscopic "corpus
KIDNEY
3*5
cles" and surface markings. The base of the pyramid is toward the
periphery of the kidney, and may be lobular as in the figure. From two
to nine embryonic or primary pyramids are said to fuse to form a pyramid
of the adult kidney. In favorable specimens the pyramid is seen to be
divided into an inner and an outer zone, and the latter is composed of two
concentric bands. The significance of these markings will be considered
later. The pyramids collectively constitute the medulla of the kidney,
a term more fittingly applied to the kidneys of many animals which have
but a single pyramid. The base of each pyramid is surrounded by a
lighter zone, the cortex, which shows radial striations. With low magnification the striations are seen to taper outward. They constitute the
processes or pyramids of Ferrein and are known collectively as the radiate
part of the cortex (pars radiata). They consist of straight radial tubules
which are continuous with those in the medulla. Consequently they are
often called "medullary rays," but being in the cortex they may more
properly be designated "cortical rays." Between these rays is the convoluted part of the cortex (pars convoluta}\ it may be recognized by the
presence of many renal corpuscles (Malpighian corpuscles), which are
bodies consisting of a glomerulus and its surrounding capsule. They are
barely visible without magnification.
Over the outer surface of the kidney, there is a fibrous capsule (tunica
fibrosa) which may be readily stripped off when normal; and outside of
this there is a fatty layer (capsula adiposa) . The fat surrounds the pelvis
and extends into a hollow of the kidney known as the renal sinus; this is
the excavation which contains the pelvis and its calyces. In this fatty
tissue the large blood vessels enter the kidney, passing chiefly over the
anterior or ventral surface of the pelvis; having reached the boundary
zone between cortex and medulla they enter it, and pursue an arched
course, giving off both cortical and medullary branches. In certain places,
the cortex dips down to the renal sinus; this occurs between the Malpighian pyramids, and constitutes the renal columns (of Bertini); one of
them is shown in Fig. 311.
The arrangement of the renal tubules in relation to the cortex and
medulla is as follows. The convoluted part of the cortex contains the
capsules, and both proximal and distal convoluted tubules. The rays
contain the collecting tubules, together with the outer portions of Henle's
loops. The medulla contains the larger collecting tubules and the deeper
portions of Henle's loops; since these are all straight tubules, the medulla
resembles the radiate part of the cortex. Tubules which are connected
with capsules deep in the cortex, near the boundary zone, send their
Henle's loops much further into the medulla than those from the outer
capsules; and in the deeply placed tubules the thin segment of Henle's
loop is not limited to the descending limb but extends well up into the
3i6
HISTOLOGY
ascending limb. Thus it happens that a broad inner zone of the medulla
(i.e., toward the papilla) contains only thin segments of renal tubules
in addition to the large collecting tubules (Fig. 310, D); and the zone
so characterized may be distinguished macroscopically. The papilla contains only collecting tubules, but the loops of Henle turn back at different
levels, and therefore the papillary zone entirely free from loops is not well
defined. The outer zone of the medulla contains both thick and thin seg
Renal corpuscle. Convoluted tubules. Cortical ray.
Interlobular vein.
Rente's loop. Arciform vein. Arciform artery.
FIG. 312. PART OF A RADIAL SECTION OF A HUMAN KIDNEY. X 5.
At x a renal corpuscle has dropped out.
ments of Henle's loops, in addition to the collecting tubules. In the descending limbs the change to thin segments occurs at a more or less definite
level within this outer zone, thus subdividing it into a narrow outer band,
with few thin segments, and an inner band containing many of both sorts.
These zones have only recently been recognized (Peter, Untersuchungen
iiber Ban und Entwickelung der Niere, Jena, 1909).
The renal tubules which have their capsules close to the medulla are
the first to develop; the others are formed successively outward, the young
KIDNEY
317
est being immediately beneath the capsule. Thus a single section of an
embryonic kidney shows various stages in the development of the tubules.
Sections of the Kidney. Since a radial section of the kidney shows both
cortex and medulla, it is the form usually made for pathological examinations (Fig. 312). The tubules may be studied to better advantage,
however, in tangential sections, one through the cortex and the other
through the medulla. The tubules are then seen in cross section. The
Capsule of the
glomerulus
(outer layer.)
Thick segment of
the descending
limb of Henle's
loop.
Proximal convoluted
tubule.
\
I
Capillary.
Ascending limb of Henles' loop ;
Collecting tubule.
FIG. 313. TANGENTIAL SECTION OF THE CORTEX OF A HUMAN KIDNEY. X 200. (Schaper.)
The pars radiata is seen in the lower left corner. The line from "capsule of the glomerulus" passes between
two distal convoluted tubules.
rays of the cortex appear as islands of circular sections surrounded by the
irregular convoluted tubules, among which are the scattered renal corpuscles. The greater part of such an island is shown in the lower portion
of Fig. 313. The renal tubules are lined throughout with simple epithelium
and their characteristic features will now be considered, beginning with
the glomerular capsule.
The glomerular capsule (of Bowman) consists of two layers. Its inner
318 HISTOLOGY
layer is a flat syncytium blending with the perivascular tissue of the
glomerulus, and following its lobulations. The outer layer of the capsule
is smooth, and is composed of flat polygonal cells. Terminal bars, which
have been found in all other divisions of the renal tubules, have not
been demonstrated in the capsule. The flat epithelium of the outer layer
changes at the "neck" of the capsule to the low columnar epithelium
of the proximal convoluted tubule. The neck may occur in various
positions, generally being opposite the aperture through which the vessels
enter and leave. The space between the layers of the capsule is continuous
with the lumen of the convoluted tubule.
The proximal convoluted tubules are large (40-60 /* in diameter) , with
irregular lumens and indistinct cell walls. In some animals the walls
are folded so as to be vertically plaited. The cells show signs of secretory
activity and are believed to excrete urea and pigments; the fluid part of
the urine comes chiefly from the glomeruli. The nuclei are toward the base
Collecting Thin Thick
tubules. segment. segments.
FIG. 314. CROSS SECTION OF A CONVOLUTED TUBULE FROM A RABBIT.
(Szymonowicz.)
FIG. 315. TUBULES OF THE PARS RADIATA. FROM A RADIAL
SECTION OF A HUMAN KIDNEY. X 240.
of the cells, and the protoplasm contains granules arranged in vertical
rows which form basal rods (Fig. 314). Toward the lumen there is a
"brush border" suggestive of short non-motile cilia. It is uncertain
whether this is normal or due to post-mortem disintegration. Clear
spaces are sometimes seen in the outer part of the cells. The lumen is
wide and the cells are low after copious urine production; and the reverse
is true when the urine is scanty.
The upper segment of the descending limb of Henle's loop is similar
in structure to the proximal convoluted tubules. It is a straight tubule,
however, and is found in the radiate part of the cortex (Fig. 313).
The upper segments of the ascending limbs are also found in the pars
radiata. Their protoplasm is less granular than that of the descending
limbs, but closely resembles that of the distal convoluted tubules. The
latter are typically shown in Fig. 313 (there being one on either side of the
label line to the " capsule of the glomerulus"). Huber (loc. tit.) describes
these tubules as showing "an outer dark zone which is finely striated,
KIDNEY
319
and an inner zone which is lighter, the nuclei being placed at the junction
of the two zones." It is probable, from their position, that the distal
convoluted tubules in Fig. 313 are parts of the tubule which connects
with the glomerulus shown in the figure.
The arched collecting tubules, into which the distal convoluted tubules
empty, pass into the collecting tubules of the rays, which are readily
identified. They have round and clear-cut lumens; cell walls are distinct
(in all but the smallest), and the nuclei are regularly arranged. Thus
the collecting tubule resembles an excretory duct.
The structures seen in the radiate part of the cortex are therefore the
ascending and descending limbs of Henle's loops, and the collecting tubules;
Large collecting tubule.
Thick segments
of Henle's loop
(ascending).
Thin segments
of Henle's loop
(descending).
PIG. 316. TRANSVERSE SECTION THROUGH THE MEDULLA OF A HUMAN KIDNEY. X 320. (Schaper. )
they are shown in longitudinal section in Fig. 315. The convoluted part
of the cortex contains proximal and distal convoluted tubules and glomerular capsules.
The medulla (Fig. 316) contains the same elements as the rays. The
collecting tubules are larger, and their walls are more distinct. Among
their columnar cells a few are decidedly darker than the others. The thick
segments of Henle's loops are easily distinguished from the thin segments.
The latter are slender (9-16 A* in diameter) but have large lumens. Cell
walls are absent, and the cells are so flat that their nuclei cause elevations.
The thin segments are generally descending, but they may also ascend, as
seen in the inner zone of the medulla ; Fig. 3 1 5 is from the outer zone, in which
most, if not all, of the thin segments are descending. (In comparing
Fig. 316 with Fig. 313, it should be noted that the former is more highly
magnified, and the thick ascending limbs appear more granular than those
tubules of the cortex with which they are continuous.)
320
HISTOLOGY
Connective tissue. Between the renal tubules there is a small amount
of interstitial connective tissue. It is more abundant toward the papillae
and around the vessels and glomeruli than elsewhere. Beneath the
Lobule.
Lobule.
Arched colecting tubule.
Papillary du
>.-' Tunica fibrosa.
Stellate vein.
_ Interlobular
artery.
- Interlobular
vein.
, Arciforw artery
Arcif or vein.
Interlobar artery.
Interlobar vein.
FIG. 317. DIAGRAM OF THE COURSE OF THE RENAL BLOOD VESSELS.
epithelium of the tubules it forms basement membranes, apparently
homogeneous, but actually composed of fine fibrils. The normal amount
of interstitial tissue should be carefully studied, since its increase is indicative of an important pathological condition. This tissue is continuous
KIDNEY
321
with that of the fibrous capsule. The latter contains elastic fibers, which
increase in abundance with age, and also smooth muscle fibers.
Lobes and lobules. In embryonic life the kidney is divided into lobes,
bounded by the renal columns, and indicated by grooves
upon the outer surface (Fig. 318). The grooves become
obliterated during the first year. In the ox similar
grooves are permanent; in many mammals as in the cat
and rabbit, they never exist, since the kidney has but
one lobe, papilla and pyramid. The lobules or structural
units of the kidney are the areas centering around each
radiate division of the cortex, by which they are drained
(Fig. 317)septa.
Blood vessels. The kidney has a capillary circulation. The renal
artery passes from the aorta to the hilus, or notch on the medial border of
the kidney. It divides into several branches, most of which pass over the
FIG. 318. KIDNEY OF
They are not bounded by connective tissue * CHILD AT BIRTH.
J J ( After Hertwig.)
Partly injected glomerulL
Jwterlobular artery.
""Interlobular vein.
FIG. 319. FROM A SECTION OF THE INJECTED CORTEX OF AN ADULT HUMAN KIDNEY. X 30.
ventral surface of the pelvis into the fat around the calyces (Fig. 311).
Thence, as interlobar arteries, they extend to the boundary layer between
the cortex and medulla where they are known as arciform arteries (Fig.
317). These send interlobular arteries through the convoluted part of the
cortex and their terminal branches enter the fibrous capsule. It will be
noted that the kidney is exceptional in having its arteries at the periphery
of its lobules. From the interlobular arteries small stems pass to the glomeruli, each of which receives a single twig (Fig. 319). This is resolved into
a knot of capillary loops, the endothelium of which seems to blend with the
surrounding syncytium and indirectly with the inner layer of the capsule.
322
HISTOLOGY
The glomerulus often appears lobed, due to the arrangement of its vascular
loops. The capillaries unite to form a single efferent vessel which is smaller
in diameter than the afferent vessel; thus the pressure within the glomerulus is increased. The entire glomerulus is regarded as arterial. On leaving it, the efferent vessel divides into small branches. These spread
among the convoluted and straight tubules of the cortex, and some continue into the medulla. The latter is supplied also by straight branches (arteriola recta) from the interlobular, efferent and arcif orm arteries, as shown
in Fig. 317. The veins of the medulla begin
around the papillae, and as venula recta empty
. , . ,
into the arciform veins The cortical veins are
9 the interlobular vessels which are beside the
corresponding arteries. They arise from converging veins in the renal capsule, which on
surface view form stellate figures (vena
stellata). The interlobular veins drain the
capillaries of the cortex, but have no direct
relation with the glomeruli. Interlobar "veins
follow the arteries, passing out from the hilus of
the kidney over the ventral surface of the renal
Uriniferous tubules.
PIG. 320. FROM THE KIDNEY OF A P
MOUSE. GOLGI PREPARATION. Lymphatic vessels are said to occur within
the cortex and to follow the blood vessels out at
the hilus. The cortical lymphatics also pass through the tunica fibrosa
to connect with a network in the adipose capsule. They proceed to
neighboring lymph glands.
The nerves are medullated and non-medullated. There is a sympathetic plexus at the hilus associated with small ganglia, and from it
interlacing nerves extend into the kidney around the vessels (Fig. 320).
Fine branches supply the epithelial cells, especially those of the convoluted
tubules. They form plexuses beneath and above the basement membrane,
and have free intercellular endings.
RENAL PELVIS AND URETER.
The renal pelvis and ureter both consist of a mucosa (and submucosa),
muscularis and adventitia (Fig. 321). The mucosa includes the epithelium
and tunica propria, the latter blending with the submucosa. In sections
the epithelium resembles that of the moderately contracted bladder (Fig.
322), and its cells when found detached in urine are not distinguishable
from bladder cells. The epithelium is stratified but consists of few layers.
The basal cells are rounded, those of the middle layer are club shaped or
conical with rounded ends, and the outer cells are columnar, cuboidal,
URETER
323
or somewhat flattened. Their lower surface may be indented by the rounded
ends of several underlying cells, as is particularly the case in the contracted
bladder (Fig. 323). Two nuclei are often found in a superficial cell, and
Tunica adventitia.
Tunica
mucosa.
FIG. 321. TRANSVERSE SECTION OF THE LOWER HALF OF A HUMAN URETER. X 15.
e., Epithelium; t., tunica propria; 1, inner longitudinal muscle bundles; r, circular layer of muscle bundles
li, outer longitudinal muscle bundles.
in some animals they are known to arise by amitosis. Leucocytes frequently enter the epithelium. In some animals mucous glands have
been found extending into the tunica propria, and there are gland-like
pockets in man. Some of these have no lumen and it is said that none
FIG. 322. VERTICAL SECTION OF THE Mucous MEMBRANE OF A
HUMAN BLADDER. X 560.
a, Columnar cell with cuticular border; b, lymphocyte; c, tunica
propria.
FIG. 323- A SUPERFICIAL EPITHELIAL
CELL AND Two
C L U B-S H A P B D
CELLS FROM A CONTRACTED BLADDER.
(Koelliker.)
are true glands. Capillary blood vessels, which are abundant in the
mucosa, are found directly beneath the epithelium and present the deceptive appearance of becoming intra-epithelial. The tunica propria consists of
fine connective or reticular tissue, with few elastic fibers. It contains
324 HISTOLOGY
many cellular elements and some lymphocytes, and passes without a
definite boundary into the loose connective tissue of the submucosa.
The tunica muscularis has considerable connective tissue among its
smooth muscle bundles. The latter form an inner longitudinal and an
outer circular layer. In the lower half of the ureter there is a third, outer
longitudinal layer, specially thickened along the last 5 cm. Around the
papillae of the kidney the circular fibers form a "sphincter." The part
of the ureter which passes obliquely through the wall of the bladder has
only longitudinal fibers, ending in the tunica propria of the bladder. By
contracting they open the outlet of the ureter. The adventitia consists
of loose fibro-elastic connective tissue.
Lymphatics and blood vessels are numerous. There are sympathetic
nerves to the muscles, and free sensory endings in the tunica propria
and epithelium.
BLADDER.
The development of the bladder from the ventral part of the cloaca
has been described on page 245. Its epithelium is entodermal whereas
that of the ureters opening into it is mesodermal. There is however no
demarcation between the layers in the adult, since both produce the same
sort of "transitional epithelium." (This term, introduced by Henle
(Allg. Anat., 1841) as a designation for epithelia which are intermediate
between stratified squamous and simple columnar, such as occur at the
cardia and elsewhere, is now generally restricted to the peculiar epithelium
of the bladder, ureter and renal pelvis.)
The bladder consists of a mucosa, submucosa, muscularis and serosa.
The epithelium has been described as two-layered in the distended bladder,
the outer cells having terminal bars; in the contracted condition it becomes
several-layered and the bars form a net extending into the epithelium.
Thus it is not believed that during distention the layers shown in Fig. 322
merely flatten; they are thought to "slip by each other." The columnar
cells may, however, become extremely flat. The appearances of the
epithelium in the bladder and ureter of the dog under various conditions
of distention and contraction have been figured by Harvey (Anat. Record,
1909, vol. 3, pp. 296-307). The superficial cells have a cuticular border;
they often contain two nuclei, and their darkly granular protoplasm has been
considered suggestive of secretory activity. Round or oval pockets extend
into the tunica propria (Fig. 324). Some of them have no lumen, or are
detached from the epithelium, but others are pits containing a colloid
substance. The pits are rudimentary glands. In the adult, branched
tubules lined with cylindrical epithelium may sprout from the bottom of
the pits, thus forming true glands. Their occurrence is limited to the
BLADDER
325
fundus, which is the dorsally bulging lowest part of the bladder, and to
the neighborhood of the urethral outlet. In the latter position they have
been regarded as rudimentary prostatic glands.
The tunica propria sometimes contains solitary nodules. It blends
with the submucosa, as in the ureter, and contains lymphatic and blood
vessels, the latter extending very close to the epithelium.
The muscularis consists of smooth muscle fibers arranged in three
interwoven layers, which are seldom separable in sections. They are an
inner longitudinal, middle circular and outer longitudinal layer. The
Tangential sections of pits.
Gland.
mag.
Tunica propria. Smooth muscles.
FIG. 324- SECTION THROUGH THE FUNDUS OF THE URINARY BLADDER OF AN ADULT MAN. X 48.
circular fibers are strengthened at the beginning of the urethra to form
the "internal sphincter" of the bladder, a muscle not always distinct.
The serosa is a connective tissue layer covered with mesothelium.
In the non-peritoneal part of the bladder it is replaced by an adventitia
or fibrous layer.
Non-medullated nerves, with scattered groups of ganglion cells, are
found outside the muscles and also among them. Medullated fibers
terminate around the ganglion cells; others pass through the ganglia to
intra-epithelial sensory endings.
URETHRA (IN THE FEMALE).
The male urethra will be described with the genital organs; only its
upper portion is homologous with the urethra of the female. The latter
326 HISTOLOGY
is exclusively the outlet of the urinary tract. The epithelium has been
variously described as stratified, with outer squamous cells, or as pseudostratified, and columnar. It may be of different forms in different individuals. The lumen is irregularly crescentic, with longitudinal folds
(Fig- 325). Branched tubular urethral glands are found only in small
numbers, except near the outlet. Their secretion is mucoid, but is not
typical mucus. In the submucosa there are many thin-walled veins con
Fic. 323- CROSS SECTION OF THE FEMALE URETHRA. (Roelliker.)
d., Gland-like diverticulum; e., epithelium; L., lumen of the urethra; m., striated muscle; s., corpus spongiosum, containing venous spaces (v) and smooth muscle.
stituting the corpus spongiosum. This is comparable with the upper
part of the more highly developed corpus cavernosum urethras of the male.
(Compare with Fig. 349, p. 347.) The muscularis is a thick layer, consisting of inner longitudinal and outer circular smooth muscle fibers, among
which the veins extend, and connective tissue with many elastic fibers is
abundant. The striated constrictor urethra is outside of the smooth muscle
layer, as shown in the figure.
MALE GENITAL ORGANS.
DEVELOPMENT AND GENERAL FEATURES.
The discovery that the Wolffian bodies become a part of the genital system was
made by Oken, through dissections and injections of dog embryos (Beitrage, Heft II,
MALE GENITAL ORGANS 327
1807). Rathke studied these "Oken's bodies" further, and found more accurately
their relation to the epididymis and ductus deferens. Miiller (Bildungsgeschichte der
Genitalien, 1830) wrongly declared that they do not form the epididymis; but he discovered that "at the time when the Wolffian bodies are most highly developed, the
germ of the ovary or testis lies on their inner side; and on their outer side, extending
even to their upper end, there is a duct which does not connect with the Wolffian
bodies it appears to have arisen from their short and much stouter excretory duct."
He saw that this second duct, now known as the Mullerian duct, formed a part of the
uterine tubes. In fact it forms the entire tubes together with the uterus and vagina;
in the male it produces interesting vestigial structures which are constantly present in
the adult.
The Mullerian duct arises as an outpocketing of the ccelomic epithelium near the anterior end of the Wolffian body. The orifice into the
peritoneal cavity becomes surrounded by irregular folds known as
fimbrice. As the Mullerian duct grows posteriorly by the elongation
of its blind end, it lies in contact with the Wolffian duct as seen in Fig.
326, but the Wolffian duct does not contribute toward its formation.
The two Mullerian ducts reach the neck of the bladder side by side,
and acquire openings into it between those of the Wolffian ducts. Near
the bladder the two Mullerian ducts fuse with one another so that their
distal part is represented by a single median tube, on either side of which
is a Wolffian duct (Fig. 306, B, page 311). In the female the united
portion becomes the -vagina and uterus, and the separate parts are the
uterine (or Fallopian) tubes. In the male the united portion becomes a
small blind pocket, the prostatic utricle, opening into the prostatic urethra.
Each fimbriated extremity becomes transformed into the appendix testis,
and the remaining portion of the ducts, except for occasional fragments,
becomes obliterated. Thus only the two extremities of the Mullerian
ducts are ordinarily permanent in the male (Fig. 328).
The genital glands in either sex begin as a thickening on the ventromedial border of each Wolffian body (Fig. 326). A section of this genital
ridge is shown in Fig. 303, C, page 307. The ridge is a dense mass of
mesoderm covered by the peritoneal epithelium, which here consists
of a syncytium very closely connected with the underlying tissue. According to Felix (Keibel and Mall's Human Embryology, vol. 2) everything
that is later developed within the genital ridge has a common origin from
the peritoneal epithelium. The ridge becomes filled with an epithelial
mass which then separates from the peritoneal layer. Beneath the peritoneum this mass produces the dense connective tissue capsule which surrounds the testis, called, from its whiteness, the tunica albuginea; within
the genital ridge it is "quite suddenly" resolved into anastomosing cords
with looser tissue between them, and the cords become the tubules of the
testis. Allen, in an earlier account (Amer. Journ. Anat., 1904, vol. 3,
pp. 89-155), likewise finds that the cells of the peritoneum and the under
328
HISTOLOGY
lying mesenchyma appear to form a continuous protoplasmic network,
and "the stroma cells are practically identical with the peritoneal cells
from which they are originating." But Allen concludes that " the tubules
of the testis are formed as solid imaginations of the peritoneum, which
later become separated from it, and grow by the activity of their component cells." There is, then, a difference of opinion as to whether the
tubules of the testis are formed directly from the stroma within the
genital ridge (Felix), or as invaginations from the peritoneal epithelium
m
FIG. 326. FROM A RECONSTRUCTION OF A 13.6.
MM. HUMAN EMBRYO. (F. W. Thyng.)
bl., Bladder; f., fimbriae; g. g., genital ridge; g. p.
FIG. 327. DIAGRAM OF THE DEVELOPMENT OF
THE TESTIS, BASED UPON FIGURES BY MACCALLUM AND B. M. ALLEN.
genital papilla; M. d., foullerian duct; p. c., Glomerular capsule; i. c. f inner or sex cords;
M. d., Mullerian duct; o. c., outer or rete
cords; W. d., W. t., Wolffian duct and tubule.
renal pelvis; r., rectum; ur., ureter; u a.,
urogenital sinus; W. d., Wolffian duct.
(Allen). A figure of an n-mm. human embryo published by Felix appears to accord with Allen's interpretation, and such a condition is shown
diagrammatically in Fig. 327.
As the cords become detached from the peritoneum, they form arching
anastomoses, convex toward the periphery of the ridge; and with further
growth they become greatly convoluted. They acquire lumens, and
become the tubuli contorti, in the walls of which spermatogenesis takes
place. The shapes presented by these tubules in the embryo have been
carefully modelled by Bremer (Amer. Journ. Anat., 1911, vol. n, pp.
393-416).
Toward the interior of the genital ridge the cords become more slender
and converge toward the Wolffian body. There they are imbedded in a
considerable mass of tissue, which in the adult becomes the mediastinum
testis. The inner ends of the contorted tubules, toward the mediastinum,
remain straight, forming the tubuli recti; and these, further inward, become
thin-walled and anastomose freely, thus constituting the rete testis (Fig.
328).
MALE GENITAL ORGANS
329
All the tubules thus far considered are produced by the genital
ridge. Their inner ends, which form the rete, acquire openings into
the capsules of the degenerating Wolffian glomeruli, or sometimes directly
into a Wolffian tubule. From ten to fifteen Wolffian tubules thus become
connected with the rete testis, and serve to convey the genital products
to the Wolffian duct; these tubules are known as the ductuli efferentes.
In the adult each of them is a greatly convoluted tube which if straightened
measures 8 inches (20 cm.). When coiled, it forms a conical mass or
lobule of the epididymis, with its apex toward the rete, and its base toward
urethra
appendix epididymidis
appendix testis
convoluted tubule
straight ttibitlc
utriciilus prostaticus
biilbourethral gland
seminal vesicle
I prostatic gland.
dnctus defereits
para didytnis
due tu his efferent
rctc testis
ductnliis aberraus
dnctus epididymidis
FIG. 328. DIAGRAM OF THE MALE SEXUAL ORGANS. (Modified from Eberth, after Waldeyer.)
(The course of the Mullerian duct is indicated by dashes.)
the Wolffian duct which it enters (Fig. 328). The Wolffian duct, which
passes along the dorsal surface of the testis, is also greatly convoluted so
that it measures about 20 feet when straight (6-7 meters). Together with
the efferent ducts this coiled mass constitutes the epididymis (Gr. eVt,
upon; S6'Su/w>?> testis). Along the testis the Wolffian duct is called the
ductus epididymidis, and from the testis toward the urogenital sinus it
is named the ductus deferens. Near its termination a saccular outgrowth,
like a distended gland, develops from each Wolffian duct. It is called
the seminal vesicle, and that portion of the Wolffian duct between the
duct of the vesicle and the urethra is named the ejaculatory duct. Thus
330 HISTOLOGY
the Wolffian duct is arbitrarily divided in the adult into three parts, the
ductus epididymidis, ductus deferens, and ductus ejaculatorius.
It has been noted that only 10-15 f the Wolffian tubules persist as
efferent ducts; in man, according to Felix, these are the fifty-eighth to
seventieth out of a series of eighty-three which develop. Thus a great
many degenerate, and certain appendages of the epididymis are explained
as persistent remnants. The appendix epididymidis may represent a
part of the Wolffian duct or an anterior tubule (Fig. 328); its history
is still obscure. Other anterior tubules may be retained as appendages
of the rete. The paradidymis is "a functionless remnant of the Wolffian
body," situated behind the head or upper end of the epididymis and in
front of the cord of veins which accompany the ductus deferens.
Giraldes first described it (Bull. Soc. Anat. Paris, 1857) and Koelliker named it the
"organ of Giraldes"; Henle called it the paraepididymis (i.e., the organ beside the epididymis), and Waldeyer later shortened the term and changed its meaning. Felix
(loc. cit., 1912) contrary to the earlier descriptions, places the paradidymis "between
the epididymis and the testis, slightly below the head of the epididymis." Toldt
(Verb. Anat. Gesellsch, 1892, pp. 241-242) recognized two forms of paradidymis, but
both are behind the epididymis and in front of the veins of the spermatic cord. The
precise origin of these tubules from the Wolffian body has not been determined.
Other remains of the Wolffian body, apparently derived from the
tubules below those which become efferent ducts, are known as aberrant
ducts (ductuli aberr antes}. There may be two or three of them; usually
there is said to be but one. It proceeds from the duct of the epididymis,
or rarely from the ductus deferens at its junction with the duct of the
epididymis, and terminates in a coiled mass, sometimes having branches.
The length of the aberrant duct is "4-36 cm., generally 5-8 cm." (Henle).
The External Genital Organs. After the cloaca has been divided into
ventral and dorsal portions by the downward growth of the perineal septum, the ventral portion below the outlets of the Wolffian ducts is called
the urogenital sinus. It receives both urinary and genital products, and
in the male it forms all of the urethra below the orifices of the ejaculatory
ducts. In the young embryo, the distal part of the urogenital sinus becomes laterally compressed so that it forms an epithelial plate. This
plate reaches the external surface of the body along the mid-ventral line
of an elevation known as the genital papilla (or tubercle). The genital
papilla (Fig. 326) becomes very prominent in embryos of both sexes. In
the male it continues its development and forms the penis, along the under
side of which the urogenital sinus acquires a cleft-like opening (Fig. 329,
A). This elongated aperture closes from behind forward, along the line
permanently marked by a rap he (or seam). In the abnormal cases of
hypospadias, the urogenital sinus retains a more or less extensive opening
on the under side of the penis. A rounded terminal glans is early differen
MALE GENITAL ORGANS
331
tiated at the apex of the genital papilla. The epidermis is adherent to it,
but later becomes separated by the formation and splitting of an epithelial plate, thus producing the reflection of skin called the prepuce. The
urogenital sinus becomes secondarily prolonged through the glans so as
to form the terminal part and external orifice of the urethra. The entire
urethra is divided into three parts: (i) the prostatic portion (pars prostatica), which includes the outlet of the bladder together with the upper
end of the urogenital sinus, and receives the ejaculatory and prostatic
ducts; (2) the membranous part (pars membranacea) , which is the short
dilatable portion traversing the "pelvic diaphragm"; and (3) the long
cavernous portion (pars cavernosa), which is surrounded by the cavernous vascular tissue.
The scrotum develops as a median pouch at the dorsal end of the urogenital raphe. It is continuous above with the pair of large genital folds
which tend to encircle the base of the genital papilla, being deficient only
below (Fig. 329, A). At the stage when the testis and Wolffian body are
PIG. 329. A, DIAGRAM OF THE EMBRYONIC EXTERNAL GENITAL ORGANS IN THE MALE; B, C, D, DIAGRAMS
OF THE DESCENT OF THE TESTIS. (After Eberth.)
a., Anus; ep., epididymis; g., glans penis; g. f., lesser genital folds; g. g. f., greater genital folds; p. c., peritoneal cavity; p. v., processus vaginalis; r., raphe; t., testis; p. 1., parietal layer of the tunica vagmalis;
u. s., urogenital sinus; v. 1., visceral layer of the tunica vaginalis.
still within the abdomen, lying behind the peritoneum, the peritoneal
cavity sends a prolongation, the processus vaginalis, over the pubic bone
into each half of the scrotum (Fig. 329, B). A large retroperitoneal
column of connective tissue, the gubernaculum testis, extends from the
posterior end of each testis into the depth of the scrotum. For reasons
still obscure, such as unequal growth or the shortening of this cord, the
testes pass down in front of the pubic bones, into the scrotum (Fig. 329,
C). The Wolffian duct becomes bent over the ureter as shown in Fig. 328,
and this important relation is found in the adult. Except on its dorsal
border, the testis is closely invested by the peritoneum of the processus
vaginalis. Later the distal part of the processus becomes separated from
the abdominal cavity by the obliteration of its stalk. The part remaining about the testis is the tunica vaginalis, having a parietal and a visceral
layer as shown in Fig. 329, D. The descent of the testes is completed
shortly before birth, except in the occasional cases of "undescended testis."
332
HISTOLOGY
TESTIS.
Septa, Vessels, and Nerves. The general arrangement of the parts of
the testis, as they appear in cross section, is shown in Fig. 330. From
the tunica albuginea, small connective tissue septa (septula testis} pass to
the mediastinum, dividing the testis into "100-200" pyramidal lobules
with their apices toward the rete. The tunica albuginea is a dense connective tissue layer, containing numerous elastic fibers which increase in
abundance with age. Its outer surface is covered with the visceral layer
of the tunica vaginalis. The inner portion of the albuginea is very vascular, forming a distinct layer at birth (the tunica vasculosa) .
Ductus deferens.
Blood vessels.
Epididymis.
Mediastinum,
containing the
rete testis.
Straight tubules.
Tunica
vasculosa.
Tunica
albuginea.
FIG. 330. CROSS SECTION OF THE TESTIS OF A CHILD AT BIRTH. X 10.
Connective tissue extends from the septula among the convoluted
tubules. Immediately surrounding them there is a delicate basement
membrane, followed by a layer of closely interwoven elastic fibers and
flat cells. In the looser connective tissue between the tubules, there are
clumps of interstitial cells (Figs. 331 and 335), which arise from mesenchymal cells of the genital ridge. Sometimes they retain protoplasmic processes, but more often they are rounded or polygonal structures in close
contact, and without distinct cell boundaries. In their abundant protoplasm there are pigment and other granules, fat droplets, and rod-shaped
TESTIS
333
crystalloids, the significance of which is unknown. The nature of the
granules is discussed by Whitehead (Amer. Journ. Anat, 1912, vol. 14
pp. 63-71).
The interstitial cells, although not intimately related with the vessels,
are thought to produce an internal secretion, and certain observations
suggest that the sexual instinct is dependent on these cells rather than
upon the spermatozoa (cf. Whitehead, Anat. Rec., 1908, vol. 2, pp. 177182). During senile atrophy of the testis, the interstitial cells at first
increase; later they are destroyed. At the same time the basement membrane becomes thickened and hyaline, fat droplets accumulate, and the
sexual cells disappear from the tubules, leaving the sustentacular cells.
The arteries of the testis are branches of the internal spermatic artery,
which descends through the spermatic cord, beside the ductus deferens.
The branches enter the testis in part through the mediastinum, and in
Interstitial cells.
Connective tissue
PIG. 331. FROM A CROSS SECTION>F THE TESTIS OF A MAN TWENTY-TWO YEARS OLD. x so.
part through the tunica vasculosa. They extend through the septula,
and form capillary plexuses around the convoluted tubules. The veins
accompany the arteries. Lymphatic vessels are numerous in the tunica
albuginea and extend among the tubules. Nerves from the spermatic
plexus surround the blood vessels; the presence of intraepithelial endings
has not been established with certainty.
Convoluted Tubules. The shape of the tubules of the testis has been
repeatedly investigated, but whether blind ends occur has not been
established; generally the tubules end in loops. Anastomoses have been
recorded, not only between the tubules in a single lobule, but also between
adjacent lobules. The extent of the anastomoses among the closely
coiled tubules is difficult to determine.
For more than seventy years eminent anatomists have recorded their success or
failure in finding blind ends Krause, Kolliker, Sappey and LaValette St. George
state that they exist; Hyrtl, Henle, Mihalkovics and Eberth fail to find them. Two
334
HISTOLOGY
recent papers have dealt with the subject. Bremer (191 1) concludes that the tubules
may end blindly; Huber and Curtis (1913) state that the seminiferous ubules in the
rabbit present no blind ends.
The convoluted tubules are lined with a highly specialized stratified
epithelium (Fig. 332). The cells divide and differentiate as they pass
from the basal layer outward. Finally each outer cell produces a single
Spermatids.
Sustentacular cell.
Spermatogonium
Blood vessel with
blood corpuscles.
Fat
granules
Spermatids. '
PIG. 332. CROSS SECTIONS OF SEMINIFEROUS (CONVOLUTED) TUBULES OF A MOUSE. X 360.
Sustentacular cell. Spermatogonia, beneath Sustentacular cells,
large spermatocytes.
large cilium, or flagellum, projecting from the free surface, and becomes
detached as a spermatozoon. The process of transformation of the basal
cells, or spermatogonia, into spermatozoa is known as spermatogenesis.
Its cytological features, as observed in the testis of the grasshopper,
have already been described (p. 21). Ordinary sections of the human
testis present the following characteristics:
Each tubule is composed of cells of two sorts sexual cells and sustentacular cells. At birth the cords and developing tubules contain relatively
few sexual cells (Fig. 333). These are characterized by their large size,
clear protoplasm, and round vesicular nuclei. It is said that they retain
a primitive granular arrangement of their mitochondria. These cells
multiply by ordinary mitosis, producing the spermatogonia. Thus the
sexual cells in various forms eventually far outnumber the Sustentacular
cells.
TESTIS
335
The sexual or genital cells are apparently produced from the cords in the testis,
relatively late in embryonic development. It was suggested by Nussbaum, however,
that the sexual cells are set apart much earlier "they do not come from any cells
that have given up their embryonic character or gone into building any part of the
body." In accordance with this idea, it is considered by some authorities that in the
segmentation stages, a line of undifferentiated cells is set apart to become the sexual
cells, add that from the beginning they are distinct from the somatic cells which form
the rest of the body. As stated by Allen (Journ. Morph., 1911, vol. 22, pp. 1-36),
the sexual cells do not belong to any one germ layer; they are free to follow their own
path in their travels from the place of origin to the genital glands where they finally
come to rest. Thus the sexual cells have been reported as distributed somewhat
diffusely in the entoderm and mesoderm. (For papers on this subject, see Allen,
Anat. Anz., 1906, vol. 29, pp. 217-236.) In a human embryo of 2.6 mm. Felix found
seven of these large clear cells, all in the immediate vicinity of the cloaca. Another
embryo of 2.5 mm., showed twelve "primary genital cells." But he adds that they all
disappear in later stages, and when the genital glands are formed there are no genital
cells. At present it has by no means been demonstrated that the mammalian sexual
cells are not differentiated products of the testis or ovary, adapted for the special purpose of reproduction.
FIG. 333. CROSS SECTION OF A CONVOLUTED
TUBULE OF THE TESTIS AT BIRTH.
(Eberth.)
FlG. 334. SUSTENTACULAR CELLS.
a., Isolated (Koelliker); b., Gplgi preparations.
(B6hm and von Davidoff.)
The sustentacular or supporting cells, often called Sertoli's cells, are
at first indifferent cells forming a syncy tium (Fig. 333) . With the increase
in the number of spermatogonia, their protoplasm is resolved into a
network of strands, molded by the surrounding cells (Fig. 334). Their
nuclei are radially compressed into ovoid shapes, and lie in columns of
protoplasm extending from the periphery of the tubule toward its lumen.
Each nucleus has a distinct nucleolus, apart from which its chromatic
material is very scanty. Usually the nuclei are in the lower half of the
branching protoplasmic columns, the polygonal bases of which are in
contact with one another beneath the spermatogonia. Within the protoplasm fat droplets occur, together with brown granules; crystalloid
bodies in pairs may also be found. As seen in Fig. 334, a, the heads of
the spermatozoa appear attached to, or imbedded in, the protoplasm
of the sustentacular cells, which are supposed to nourish them. The
336
HISTOLOGY
spermatozoa may be gathered in characteristic clumps at their upper ends
(Fig. 332).
In ordinary sections of the testis, the sustentacular cells may be recognized by their distinctive nuclei (Fig. 335). The sexual cells in the basal
row are presumably spermatogonia. Above them are the spermatocytes,
which are larger; their nuclei usually show spiremes or other indications
of cell division. Secondary spermatocytes are further out than the primary spermatocytes; and above them are the spermatids in various
stages of transformation into spermatozoa. Since spermatogenesis occurs
in "waves," the outer cells in a tubule cut lengthwise form a succession of
zones, each of which shows gradations from young spermatids to mature
spermatozoa; a single zone is included in Fig. 335. In transverse sections
all the superficial cells may be of one stage, which differs from that in
the adjoining tubule (Fig. 332).
Heads of _
spermatozoa.
Spermatocyte.
Crystalloid, in
Spermatid.
Nuclei of sustentacular cells.
Spermatogonium.
Inter stitial con
necti ve tissue.
FIG. 33S- FROM A LONGITUDINAL SECTION THROUGH A CONVOLUTED TUBULE OF A HUMAN TESTIS. X 60
Stages in the transformation of a spermatid into a spermatozoon are
shown in the diagram Fig. 336. The chromosomes of the spermatid
disappear in a dense chromatic network which becomes apparently homogeneous. This deeply staining nucleus passes to one end of the protoplasm
of the spermatid. It becomes the essential part of the head of the spermatozoon, which in man is a flattened structure, oval on surface view,
and pyriform with its apex forward when seen on edge (Fig. 337). The
head is at the anterior end of the spermatozoon, which during its development is directed toward the basal layers of the convoluted tubule. The
anterior end of the head is probably covered by a thin layer of protoplasm,
known as the galea capitis. The archoplasm of the spermatid (known as
the idiozome) is said to leave the centrosome and to enter the protoplasm
of the galea capitis, where it forms the perforatorium. If this exists in
man, it is in the form of a cutting edge following the outline of the front
of the head; in other animals the perforatorium may be a slender spiral
or barbed projection, which enables the spermatozoon to penetrate the
ovum.
TESTIS
337
The protoplasm of the spermatid forms an elongated mass at the posterior end of the nucleus. It contains the centrosome which soon divides
in two. Of these the anterior forms a disc which becomes adherent to
the nuclear membrane. The posterior centrosome also becomes a disc
after giving rise to a motile axial
filament, which grows out from it like
a cilium. The disc-like centrosome
attached to the anterior end of the
filament becomes thin in such a way
that its peripheral portion is detached,
and as a ring surrounding the filament it passes to the posterior limit
of the protoplasm. The protoplasm
between the two parts of the posterior centrosome is reduced to a thin
layer in which a spiral filament develops, winding about the axial filament. Distal to the centrosome ring,
the axial filament, which consists of
fine fibrils, is surrounded by a thin
membrane, which terminates or becomes very thin near the extremity
of the filament. This membrane,
which in salamanders forms a conspicuous undulating frill, is thought to
be a product of the filament and not an extension of the protoplasm.
In man it is inconspicuous, and many of the details here described can
be made out only under most favorable conditions. The preceding account is based on studies of the guinea-pig (Meves,
Arch. f. mikr. Anat., 1909, vol. 73, pp. 751-792).
Mature spermatozoa are divided into three
parts the head, neck, and tail. The head (3-5 /*
long and 2-3 /* wide) includes the nucleus, galea
capitis and perforatorium. The neck consists of
the anterior centrosome and the substance, not
traversed by the axial filament, between it and
the posterior centrosome. The neck in man is
not constricted as in some forms, yet it is a place
where the head may become detached. The tail
includes three parts, the connecting piece, chief piece
and end piece. The connecting piece (6 ft long and scarcely i /* wide)
consists of protoplasm, axial and spiral filaments, and the two parts of
the posterior centrosome. The chief piece (40-60 /* long) is the axial
FIG. 336. DIAGRAMS OF THE DEVELOPMENT OF
SPERMATOZOA. (After Meves.)
a. c., anterior centrosome; a. f., axial filament;
c. p., connecting piece; ch. p., chief piece; g. c.,
galea capitis; n., nucleus; nk., neck; p., protoplasm; p. c., posterior centrosome.
FIG. 337- SPERMATOZOA: i,
2, 3, HUMAN 4, FROM A BULL.
a, Head; b, connecting piece,
and c, chief piece of the
tail, i, 3, and 4, Surface
views; 2, side view. X 360
338 HISTOLOGY
filament with its surrounding membrane; and the end piece (10 /*) is
a prolongation of the filament. When the spermatozoa become free they
float in the albuminous fluid secreted in small quantity by the tubules of
the testis. They pass through the straight tubules and rete to the epididymis, in which they accumulate, and where they first become motile.
Their motility is greater, however, in the seminal fluid, which is a mixture
of the products of the epididymis, seminal vesicles, prostate and bulbourethral glands. By an undulating movement of the tail, the head is
propelled forward, always being directed against such a current as is made
by cilia, at a rate of | of an inch in a minute. Water inhibits the motion,
which is favored by alkaline fluids; it occurs also in those faintly acid.
For three days after death spermatozoa may retain their activity in the
seminal passages; in the female urogenital tract they may live a week
or more. In addition to normal spermatozoa, giant forms, and some
with two heads or two tails occur, but these are probably functionless
abnormalities. The production of spermatozoa, beginning at puberty,
continues throughout life, but with advancing age the rate diminishes.
Since about 60,000 spermatozoa occur in a cubic millimeter of seminal
fluid, it has been estimated that 340 billions are produced in a lifetime.
The discovery of spermatozoa was reported to the Royal Society of London, in 1677
by Leeuwenhoek. They were first seen by Dr. Ham, "a man of singular modesty,"
to whom Leeuwenhoek gives full credit for the discovery in his letters to the Royal
Society. He wrote as follows:
"This discerning youth visited me and brought with him, in a small glass vial,
seminal fluid from a man who had cohabited with a diseased woman; and he stated that
after some minutes when the fluid had become so attenuate that it could be put in a
slender glass tube, he had seen living animalcules in it, which he thought were produced by some putrefaction. He added that those animalcules seemed to him to be
provided with tails, and that they did not survive the space of twenty-four hours.
Moreover he declared that when terebinth had been given to the patient internally, the
animalcules appeared to be dead.
"I poured this material in a glass tube and examined it in the presence of Dr. Ham,
and saw some live animalcules in it. But when after two or three hours, I examined
the material more carefully, by myself, I saw that all the animalcules were dead."
Leeuwenhoek diligently pursued the study of these animalcules, and found them in
enormous numbers in the semen of insects, fishes, birds and quadrupeds. He estimated
that there were 150,000,000,000 in the milt of one fish, or more than ten times the number of men then living (13,385,000,000 homines in orbe terrarum). Leeuwenhoek believed that the animalcules were of two sexes, and that the egg consisted of a fluid in
which they swam about and developed. To some it seemed not unreasonable that new
individuals should be enclosed in the spermatozoa, like an insect in its chrysalis, and
Dalenpatius (1699) thought that he could observe them. As quoted by Vallisneri, he
wrote as follows, illustrating his account with the figure here reproduced (Fig. 338).
"We have seen some animalcules having just the form of tadpoles such as are found
in brooks and muddy bogs in the month of May. The tail is four or five times as long,
as the body. They move with wonderful rapidity and by the strokes of their tails pro
TESTIS
339
duce little waves in the substance in which they swim. But who would believe that in
these a human body was hidden? Yet we have seen such with our own eyes. For
while we were observing them attentively, a large one threw off its surrounding membrane and appeared naked, showing distinctly two legs, thighs, breasts and arms. The
cast-off skin, drawn upward, covered the head like a cap, and it was a delightful and
incredible sight. Because of the minuteness of the object, the sex could
not be distinguished. After the little creature had lost its membrane it
soon died."
This is a gross presentation of the preformation theory, according to
which the various parts of the adult are represented in the very young
embryo. It was held by many who could not verify such observations.
An alternative theory is that of epigenesis, according to which the body FlG 8
and its parts arise out of formless substance. Descartes (1664) wrote
that the source of a new individual "seems to be only a confused mixture of
liquors, which, serving to leaven one another, become heated; some of their agitated
particles dilate, and press upon the others, gradually disposing them in the way necessary to form organs." Such physico-chemical speculations however, are quite as
imaginative as any views of the preformationists and Descartes's epigenesis was early
characterized as " a very lame account of the forming of an animal." Nevertheless,
the doctrine of epigenesis, as advocated by Harvey (1651) and Wolff (1759), prevailed
over the cruder ideas of preformation. If, however, the spermatozoon can contribute
to the production of only one of the myriad forms of animals, even the sex of which is
apparently predetermined, it is evident that the spermatozoon must possess a very
definite chemical composition, and perhaps a corresponding ultra-microscopic structure.
Doubtless there is a preformation no less remarkable than that expressed through
the active imagination of Dalenpatius.
Tubuli Recti and the Rete. The large convoluted tubules are 140 ft
in diameter. As they pass toward the epididymis they decrease in size;
they receive branches at acute angles and their windings diminish.
Sexual cells disappear, leaving only the sustentacular cells in the form of a
simple columnar epithelium. This flattens abruptly to form the lining of
the straight tubules. Both the straight tubules and the rete are lined
with a simple epithelium of low cells. In some places these are very flat,
suggesting endothelium; in others they are columnar. The characteristic
dilatations of the rete tubules are shown in Fig. 339. They contain
spermatozoa and immature sexual cells together with pigment granules
and broken down cells.
EproiDYMis.
The efferent ducts, which pass from the rete to the duct of the epididymis, are lined with an epithelium in which groups of columnar cells alternate with those which are cuboidal (Figs. 340 and 341). Thus the inner
surface of the epithelium has depressions suggesting glands, but the basal
surface is free from outpocketings. The epithelium is generally simple,
although in the tall parts it may appear two or three layered. The cells
contain fat, pigment, and other granules, and produce a secretion which
340
HISTOLOGY
P*^. ' /> ?' .? : - f . ' ji?s^>' -/^'
fe^^'/f*?'*:*^
' -%P+ *
:"^?J
FIG. 339. SECTION OF THE HUMAN RETE TESTIS. X 96. (Kfilliker.)
A, -Artery; C, rete tubules; L, lymphatic vessels; s, connective tissue partly surrounded by 'rete tubules
Sk, part of a convoluted tubule, to the left of which are sections, probably of straight tubules; V, vein.
.Tangential section
of a ductulus
efferens.
Connective tissue.
Blood vessel. Epithelium Circular muscles Transverse section of a
* ductulus efferens.
of the ductus epididymidis.
PIG. 340. FROM A SECTION OF THE HEAD OF A HUMAN EPIDIDYMIS, SHOWING SECTIONS OF THE DUCTUS
"'EPIDIDYMIDIS IN THE CENTER, AND OF DUCTULI EFFERENTES ON THE SIDES. X so.
EPIDIDYMIS
341
Cubical cells. Columnar cells.
may appear in vesicular masses on the surface of the cells. Often the tall
cells, and occasionally the short ones, are ciliated. The cilia vibrate so as
to produce a current toward the ductus epididymidis. The epithelium
rests on a striated basement membrane which is surrounded by a layer of
circular smooth muscle fibers, several cells thick. The muscle layer is
thickest toward the ductus
epididymidis. Among the
muscle cells there are elastic fibers, which, like those
of the ductus epididymidis
and ductus deferens, first
appear at puberty. There
are no glands in the efferent ducts, but the irregularities in the epithelium
are thought to be due to
glandular activity. Before
puberty and in old age these irregularities are slight.
Smooth muscle fibers. Connective tissue.
FIG. 341. TRANSVERSE SECTION OF A DUCTULUS EFFERENS,
FROM THE TESTIS OF AN ADULT MAN.
The right-band end of the illustration is schematic. No cilia
could be seen, although those of the epithelium of the epi
didymis were well preserved. X 360.
The ductus epididymidis consists of a two-rowed epithelium with
rounded basal cells and tall outer columnar cells. The latter contain
secretory granules and sometimes pigment, and have in the middle of
their upper surfaces long non-motile hairs, which in sections are usually
matted in conical processes (Fig. 41, &, p. 51). The epithelium may
contain round cavities opening into the lumen or forming closed cysts.
The delicate membrana propria and thick circular muscle layer complete
the wall of the ductus, the convolutions of which occur in a loose connective tissue. Toward the ductus deferens the muscle layer thickens.
There are no glands in the
ductus epididymidis, but its cells
produce considerable secretion
in which the spermatozoa become active.
The blood vessels of the epididymis, which are few in comparison with those of the testis,
lie in part so close to the efferent
ducts as to cause the membrana
propria to bulge toward the epithelium. The nerves, besides perivascular nets, form a thick plexus myospermaticus provided with sympathetic ganglia. It is found in the muscle layer, which it supplies,
sending fibers also into the mucosa. In the ductus deferens and seminal
vesicles this plexus is said to be more highly developed than in the
epididymis.
" Epithelium.
Membrana
propria.
Circular layer of
muscle fibers.
Loose connective
tissue.
FIG. 342. TRANSVERSE SECTION OF A HUMAN DUCTUS
EPIDIDYMIDIS. X So.
342
HISTOLOGY
.Epithelium.
.Tunica propria.
Inner longitu.-'dinal muscles.
Circular
'muscles.
Outer longitu'dinal muscles.
DUCTUS DEFERENS.
The ductus deferens begins as a convoluted tube continuous with
the ductus epididymidis; it becomes straight and passes to its termination in the ductus ejaculatorius. Shortly before reaching the prostate it
exhibits a spindle-shaped enlargement or ampulla about | inch long and f
inch wide (Fig. 344). The ductus deferens consists of a mucosa, muscularis and adventitia. The epithelium is generally in two rows, the
tall inner cells producing round masses of secretion. Toward the epididymis it may also have non-motile cilia. Toward the ampulla it may be
several rowed, resembling the
epithelium of the bladder.
It rests on a connective tissue
tunica propria, which is surrounded by the three layers
of the muscularis. The inner
and outer layers are longitudinal and generally less developed than the middle circular layer. The adventitia
is a loose elastic connective
tissue, blending with that
which forms the spermatic
The latter contains
arteries, veins,
and nerVCS, tO
gether with the striated mus
cle fibers of the cremaster muscle, and the rudiment of the processus
vaginalis. The veins are very numerous and constitute the pampiniform
plexus (i.e., tendril-like). Their walls are usually provided with a very
thick musculature including both circular and longitudinal fibers.
In the ampulla the longitudinal folds, which are low in the ductus
deferens, become tall and branched, so that they partly enclose irregular
spaces ( dive rticula) . Similar folds occur in the seminal vesicles. It is
doubtful whether in either place any of the spaces should be considered
glands. Around the ampulla the musculature is irregularly arranged; the
longitudinal layers separate into strands which terminate toward the
ejaculatory ducts.
SEMINAL VESICLES ANU EJACULATORY DUCTS.
The seminal vesicles grow out from the ductus def erentes at the prostatic
ends of their ampullae. Each consists of a number of saccular expansions
arranged along the main outgrowth, which is irregularly coiled. The
cord.
numerous
FIG. 343CROSs SECTION OFTHH ; HUMAN DUCTUS DEFERENS.
SEMINAL VESICLES
343
lining of the sacs is honeycombed with folds as shown in Figs. 344 and 345.
The epithelium is generally simple or two-layered, the height of the cells
varying with the distention of the vesicles by secretion. Granules occur
in the cells, which produce a clear gelatinous secretion in sago-like masses.
Spermatozoa are generally found in the human vesicles, but except during
sexual excitement they are absent from the vesicles of rodents; this and
other facts indicate that the function of the organ is primarily glandular.
The lumens of the various sexual glands are generally of very large caliber,
associated with the storing of secretions. Pigment granules in varying
-ep.
FIG. 344. SEMINAL VESICLE AND DUCTUS DEFERENS. (This is natural size.) (After Eberth.)
ad., Adventitia; am., ampulla; d., diverticulum;
d. d., ductus deferens; d. e., ductus ejaculatorius; m., muscularis; s. v., seminal vesicle;
t. p., tunica propria.
FIG. 345. VERTICAL SECTION OF THE WALL OF A
SEMINAL VESICLE. (After KSlliker.)
ep.,. Simple epithelium; g., gland-like depression; m., muscularis; t. p., tunica propria.
quantity occur in the epithelial cells and in the underlying connective
tissue. They may impart a brownish color to the secretion.
The ductus ejaculatorii, along their dorso-median sides, are beset with a
series of appendages, which do not project externally but are wholly enclosed in the connective tissue wall of the duct. Some of these appendages show the same structure as the seminal vesicles and therefore might
be described as accessory seminal vesicles; others are simply convolutions of alveolo-tubular glands which may be compared with prostate
glands. The mucous membrane of the ductus ejaculatorii is like that of
the seminal vesicles, except that its folds are not so complicated. Muscle
344 HISTOLOGY
fibers occur only around the appendages. The wall of the duct itself consists of an inner dense layer of connective tissue with circular strands, and
an outer loose layer (adventitia).
APPENDICES AND PARADIDYMIS.
The appendices are frequently called hydatids, which is a general term for watery
cysts. The appendix testis is a small lobule of connective tissue projecting from the
groove between the head of the epididymis and the testis (Fig. 346). It is quite
constant, having been reported in 90% of the testes examined. The projection
is covered with the peritoneum of the tunica vaginalis, which may be thickened around
it, or corrugated, suggesting the fimbriated orifice of the uterine tube. The appendix
consists of vascular connective tissue and encloses a canal, or fragments of canals,
lined with simple columnar epithelium which is sometimes ciliated. It is generally
not cystic, and it may be pedunculated, so that the terms "hydatid of Morgagni"
and "sessile hydatid," formerly applied to it, are inappropriate. Although its canal has been reported as connecting
with the seminal ducts, this is not now believed to be the
case; the structure is regarded as the degenerated end of the
Miillerian duct.
The appendix epididymidis (stalked hydatid) is not always
present. Among 105 cases examined by Toldt it was found
twenty-nine times. It consists of loose vascular connective
tissue covered by the vaginalis, and contains a dilated canal
lined with columnar epithelium, sometimes ciliated. The
canal generally has no connection with the tubules of the
. , r epididymis. It is regarded as a persistence of detached
FIG. 346. FRONT VIEW OF J , ,
A TESTIS, somewhat re- degenerating WolfEan tubules, or possibly of the terminal
duced. (AfterEberth.) , ...
a. e., Appendix epididymi- portion of the Wolffian duct.
iif* c t "e a , P capu n t ( epf" The P^adidymis, according to Toldt (Verh. Anat.
didymidis; 't, testis; t- Gesellsch., 1892, pp. 241-242), occurs in two forms. The
v., tunica vaginalis.
first is found frequently, but by no means regularly, in
older embryos and in children. It is a round or elongated structure, conspicuous
because of its white color, found on the ventral side of the spermatic cord, either behind
the head of the epididymis or higher up. Microscopically it is seen to be a thin, coiled,
blind canal, expanded in places, and lined with a simple columnar epithelium. Occasionally there are two to four such structures at varying distances from one another.
In later years they all disappear. They never contain spermatozoa.
The second form of paradidymis was found by Toldt in late childhood and in adults,
but it does not occur regularly. It is always immediately behind the head of the
epididymis and in front of the pampiniform plexus. It consists of a canal, sometimes
with saccular dilatations, which is easily followed with the naked eye. The tubule
may be closed at both ends, or one end may connect with the epididymis or testis;
sometimes one end connects with the testis and the other with the epididymis. These
tubules may contain spermatozoa, and they have been said to resemble the efferent
ducts in structure. They may be ciliated.
Toldt regards the first form of paradidymis as due to persistent WolfEan tubules,
and the second as a late separation of an efferent duct from its connection with the
epididymis. He notes that the second form may give rise to cysts of varying size.
Other cysts in the vicinity of the epididymis are said to arise from inpocketings of the
tunica vaginalis.
PROSTATE
345
PROSTATE.
The prostate is a group of branched tubulo-alveolar glands, imbedded
in a mass of muscular tissue, which stands before the outlet of the bladder.
The smooth muscle of the adult prostate forms a quarter of the bulk
of the organ, and together with an elastic connective tissue, it unites the
numerous glands in a compact mass. The development of these glands
up to the time of birth, has been studied by Lowsley (Amer. Journ. Anat.,
1912, vol. 13, pp. 299-349). He finds that the prostate includes from
fifty-three to seventy-four separate glands (the average number being
sixty-three) which are grouped in five
lobes. The middle lobe consists of nine
to ten large glands growing out from the
dorsal side of the urethra, between the
bladder and the openings of the ejaculatory ducts. The glands of the posterior
lobe grow out from the dorsal wall of the
urethra below the ejaculatory ducts;
those of the right and left lobes develop
from the sides of the prostatic urethra;
and those of the anterior lobe proceed
from its ventral surface. The anterior
lobe is well developed in young embryos,
but it "shrinks into insignificance at the
twenty-second week." It may persist in
the adult, but the great mass of the prostatic glands is at the sides and back of
the prostatic urethra. The number of
glands apparently becomes reduced. In
the adult it is said to be from thirty to
fifty.
The glandular epithelium is simple
and either cuboidal or columnar. It may appear stratified as it passes
over the folds in the walls of the tubules. Near the outlet of the larger
ducts the epithelium is like that of the bladder and prostatic urethra. In
the prostatic alveoli, of older persons especially, round or oval colloid
masses from 0.3 to i.o mm. in diameter occur; as seen in sections (Fig.
348) they exhibit concentric layers. Their reactions on treatment with
iodine solutions suggest amyloid. These concretions are probably deposited around fragments of cells. Octahedral crystals also occur in the
prostatic secretion, which is a thin milky emulsion, faintly acid; it has a
characteristic odor, whereas the other constituents of the seminal fluid are
said to be odorless.
FIG. 347. FROM A SECTION OF THE PROSTATE OF A MAN TWENTY-THREE YEARS
OLD. X so.
346
HISTOLOGY
The smooth muscle fibers are found everywhere between the prostatic lobules; toward the urethra they thicken to form the internal sphincter of the bladder. Smooth muscle is also abundant on the surface of
the prostate, and it borders upon the striated fibers of the sphincter of the
membranous urethra. The prostate is abundantly supplied with blood
and lymphatic vessels. The numerous nerves form ganglionated plexuses
from which non-medullated fibers pass to the smooth muscles; others
of the nerves have free endings; still others, both in the outer and inner
Red corpuscles in a
blood vessel.
Connective tissue
Epithelium.
Smooth
muscle fibers.
FIG. 348. FROM A SECTION OF THB PROSTATE OF A MAN TWENTY-THREE YEARS OLD. X 360.
The epithelium is cut obliquely at x, and has artificially separated from the connective tissue at xx.
parts of the gland in dogs and cats, end in cylindrical lamellar corpuscles.
The utriculus prostaticus (uterus masculinus, vagina masculina) is a
small pocket lined with stratified epithelium, opening into the dorsal wall
of the urethra midway between the orifices of the ejaculatory ducts, or a
little below them. It is sometimes absent, and is occasionally quite deep.
Lowsley failed to find any small prostatic tubules opening into it, such
as have been reported as occasionally present. The utriculus prostaticus
is the lower end of the Mullerian ducts, which have fused, and it corresponds with the vagina in the female.
URETHRA AND PENIS.
The form of epithelium found in the bladder extends through the
prostatic to the membranous part of the urethra. Its outer cells gradu
URETHRA 347
ally become elongated and it changes to the simple or few-layered columnar epithelium of the cavernous portion. In the dilatation of the urethra near its distal end, the fossa navicularis, the epithelium becomes
stratified with its outer cells squamous; the underlying papillae of the
tunica propria become prominent, and the whole is the beginning of the
gradual transition from mucous membrane to skin.
Glands. Small groups of mucous cells are scattered along the urethra,
and in the cavernous part, especially on the upper wall, they form pockets
called urethral glands (of Littre). Often these pockets are on the sides
of epithelial pits so that the glands are branched. Non-glandular pits
Mucous membrane of the urethra.
Epithelium. Tunica propria. Urethral glands. Submucosa.
Tunica
albuginea.
Arteries. Connective tissue Bundle of smooth Venous spaces,
trabeculae. muscle.
FIG. 349. TRANSVERSE SECTION OF THE PARS CAVERNOSA URETHRE.S OF MAN. X 28.
also occur, known as urethral lacuna, and the "paraurethral ducts" near
the external orifice are large lacunas of various sorts.
Two glands of considerable importance empty by irregularly dilated
ducts, i^ in. long, into the beginning of the cavernous urethra. The
bodies of these bulbo-urethral glands (Cowper's glands) are found one on
either side of the membranous urethra, in close relation with striated
and smooth muscle fibers. The end pieces, which are partly alveolar and
partly tubular, anastomose. They consist of mucous cells, with intercellular secretory capillaries, and produce a clear, glairy mucus, discharged
during sexual excitement. The ducts, surrounded by thin rings of smooth
348 HISTOLOGY
muscles, consist of simple low epithelium. They may connect directly
with the end pieces, or a secretory duct may intervene.
The muscularis of the prostatic part of the urethra consist of an
inner longitudinal and an outer circular layer of smooth muscles. Both
layers continue throughout the membranous part; the circular layer ends
in the beginning of the cavernous urethra leaving only oblique and longitudinal bundles in its distal part.
Corpus cavernosum urethra. In the submucosa of the cavernous
urethra there are many veins (Fig. 349) which become larger and more
numerous in and beyond the muscularis. This vascular tissue which
surrounds the urethra is limited by a dense elastic connective tissue layer,
the tunica albuginea, and the structure which is thus bounded is the corpus
cavernosum urethra. Toward the perineum it ends
in a round enlargement, the bulbus urethra, and
distally it terminates in the glans penis. The urethra
enters the upper surface of this corpus cavernosum
near the bulbus. Branches of the internal pudendal
(pudic) artery, namely, the arteriae bulbi and the
urethral arteries, penetrate the albuginea, and the
former pass the length of the cavernous body and
PIG. 35o.-CRosi SECTION end in the g lans - These arteries have particularly
thick walls of circular muscle, and in cross sections
the initma may be seen to form coarse rounded proEla8ti (AfterEberth S .) ain ' jections into the lumen. These projections contain
longitudinal muscles and subdivisions of the inner
circular elastic membrane (Fig. 350). The arteries in the corpus cavernosum produce capillaries found chiefly toward the albuginea. The
capillaries empty into thin-walled venous spaces which appear as endothelium-lined clefts in a connective tissue containing many smooth muscle
fibers. The cavernous body is permeated with these spaces which, at
times of sexual excitement, become distended with blood, reducing the
tissue between them to thin trabeculae. Such distensible vascular
tissue is known as erectile tissue. Some arteries connect directly with the
venous spaces, and such as appear coiled or C-shaped in a collapsed
condition are called arteries helicina. The vena cavernosa have such very
thick walls that they resemble arteries. They contain an abundance
of inner longitudinal muscle fibers, and since these are not evenly distributed but occur in columns, the lumen of the veins is usually crescentic
or stellate in cross section. Emissary veins pass out through the albuginea and empty into the median dorsal vein of the penis.
The corpora cavernosa penis are a pair of structures similar to the
cavernous body of the urethra, and are found side by side above it (Fig.
351). The septum between them is perforated distally so that they
PENIS 349
communicate with one another. Each is surrounded by a very dense
albuginea, i mm. thick, divisible into an outer longitudinal and an inner
circular layer of fibrous tissue. The septum is formed by the median
fusion of these layers. The cavernous or erectile tissue of which these
corpora are composed is essentially like
that around the urethra.
All three cavernous bodies are surrounded by fascia and subcutaneous
tissue containing blood vessels, lymphatics and nerves. The lymphatic
vessels form a superficial and a deep
set, the latter receiving branches from
the urethra. The principal sensory
nerves are the medullated dorsal nerves
of the penis. They terminate in many ^^^B^[[User:Z8600021|Mark Hill]] ([[User talk:Z8600021|talk]]) 16:16, 8 September 2017 (AEST)^~~ g
tactile corpuscles in the papillae beneath FIG. 351. CROSS SECTION OF A
the Skin, in bulboUS and genital COrpUS- 3 '' bug{nea; S d., C dorsatve S inri" e TO C rporTcave^
i . .-, , . . .. i nosa penis;!., urethra; g., corpus caver
cles in the deeper connective tissue, and nosum urethra. (Baiiey.)
in lamellar corpuscles found near or in
the cavernous bodies. Free endings also occur. The sympathetic nerves
are from a continuation of the prostatic plexus. They constitute the
cavernous plexus, which includes the major cavernous nerves accompanying
the dorsal nerves of the penis, and the minor cavernous nerves which enter
the roots of the corpora cavernosa penis. The sympathetic nerves supply
the numerous smooth muscles of the trabeculae and cavernous blood
vessels. They are said to be joined by fibers from the lower spinal nerves,
the nervi erigentes.
FEMALE GENITAL ORGANS.
DEVELOPMENT AND GENERAL FEATURES.
Although it is probable that sex is determined at the time of the fertilization of the ovum, and that it cannot be modified by subsequent conditions of any sort, the sex of young embryos cannot be recognized. All
human embryos of 13 mm. possess a prominent genital papilla; they have
both Wolffian and Miillerian ducts, in so far as the latter have developed;
and they contain genital ridges which are still in an "indifferent stage"
it cannot be said whether they will become ovaries or testes (cj. Fig.
326, p. 328). In the female the Miillerian ducts become highly developed,
the Wolffian ducts degenerate, and the genital ridges produce ovaries.
The Miillerian Ducts. Before reaching the urogenital sinus, the lower
ends of the Miillerian ducts are in contact, being situated between the
Wolffian ducts (Fig. 352). The figure here reproduced represents a por
350
HISTOLOGY
W.d.
M.d. Wd.
tion of the genital apparatus shown in Fig. 306, B, p. 311, both being
sketched from the beautiful lithographs accompanying Keibel's fundamental account of the development of the human urogenital tract, which
students should consult in its original form (Arch. f. Anat. u. Entw., 1896,
pp. 55-156). A fusion of the Miillerian ducts begins just above their
lower termination and extends downward to the urogenital sinus. Thus
the entire ducts form a Y-shaped structure, and the lower part of the stem
becomes the vagina. It is at first a solid cord of cells, but those in the
center break down and a lumen appears, "first in embryos of 150-200
mm." The lower end of the vagina remains
closed by epithelium for some time longer, and as
the vagina enlarges, a transverse fold, the hymen, is
formed at this point. With the breaking down of
the central cells, the hymen becomes perforate; it
then usually forms a crescentic fold on the dorsal
side of the entrance to the vagina (Fig. 353). Its
remains permanently mark the orifice of the Miillerian ducts.
Above the vagina the Mullerian ducts form
the lining of the uterus, which develops from the
upper part of the stem of the Y, and from the
inner ends of its arms. This region of junction
becomes surrounded by a very thick layer of
smooth muscle. The occasional occurrence of a
median septum in the uterus or vagina, dividing
them into right and left halves, is due to imperfect fusion of the Mullerian
ducts.
The outer portions of the Mullerian ducts retain relatively thin walls
and become the uterine (or Fallopian) tubes. Each opens freely through
its fimbriated extremity into the abdominal cavity.
The Wolffian Bodies and Wolffian Ducts. In the female these structures become functionless and degenerate. Their principal derivative
is a group of blind tubules, which may readily be seen in the translucent
mesentery-like membrane extending between the ovary and tube. These
tubules were named the "organ of Rosenmuller" after their discoverer,
who described them in 1802, and were called the "parovarium" (later
corrected to paroophoron) because of their position beside the ovary;
but when it was shown that these tubules were homologous with the
epididymis, they were given a corresponding name, and are now known
as the epoophoron ( Vt, upon; <Lo<6pos, ovary). The epoophoron consists
of "8 to 20" transverse ducts, which begin with blind ends in or near the
upper end of the ovary and follow a more or less convoluted course to the
longitudinal duct, into which they empty (Fig. 353). They are lined with
U.S. -/
FIG. 352. RECONSTRUCTION
SHOWING THE FUSION OF
THE MULLERIAN DUCTS.
(After Keibel.)
bl., Bladder; M.d., Mulleriae
duct; u., ureter; ur., uruthra; u.s., urogenital
sinus; W.d., Wolffian duct.
FEMALE GENITAL ORGANS
351
uterine lube
epoophoron
paroophoron
appendix
vesiculosa
ovary
simple cuboidal or columnar epithelium, sometimes ciliated, and are
surrounded with muscle fibers. Occasionally there are detached solid
cords in their vicinity, and sometimes the tubes become cystic. Obviously
they correspond with the efferent ducts of the testis, and the longitudinal
duct, into which they empty, represents the duct of the epididymis. Some
of the transverse tubules, or the main duct itself, may extend into soft
round nodules of tissue projecting from the mesentery, to which they may
be attached by slender pedicles. These appendices vesiculosce correspond
with the appendix of the epididymis. Frequently there
is a vesicular appendix entirely separate from the
epoophoron, situated near the
fimbriated orifice of the uterine tube, and said by Felix
to develop around an accessory Miillerian duct. Although accessory ducts have
not been found in the male,
the relations of this structure
to the Miillerian duct suggest
a comparison with the appendix testis. Both in the
female and the male the
appendages have been described as of two sorts, connected with the Miilleria n and
Wolffian ducts respectively.
vagina
'land
clitoris
^*<^22*^
* vestibule
FIG. 353. DIAGRAM OF THE FEMALE GENITAL ORGANS.
The paroophoron is a remnant of the Wolffian tubules corresponding with the
paradidymis. It was first described as nearer the uterus than the epoophoron, and
situated as in the diagram, Fig. 353. The tubules there shown, however, are presumably a part of the epoophoron; the paroophoron is now said to be on the opposite
side of the ovary (toward the right of the diagram), in relation with the ovarian vessels.
It disappears by the fifth year.
The lower end of the Wolffian duct, which corresponds with the ductus deferens,
may remain as the canal of Gartner. This canal terminates near the hymen. It may
extend upward beside the vagina, and be enclosed in the musculature of the lower part
of the uterus; usually it is entirely obliterated.
Development ol the Chary. Like the testis, the ovary is formed from the
middle portion of the genital ridge. The peritoneum which covers it
gives rise to the mass of cells in its interior, and deep within, the cells
become arranged in medullary cords and a rete ovarii. These are rudimentary structures. The rete cords do not connect with the Wolffian tubules.
They are said to acquire lumens toward birth, so that they are bounded
352
HISTOLOGY
by simple epithelium; they remain in the adult and may become cystic.
Sexual cells disappear from the cords in the central part of the ovary,
which becomes filled with vascular connective tissue and forms the medulla
in the adult. The peripheral part of the ovary, or cortex, contains great
numbers of sexual cells, which instead of being lodged in tubules as in
the testis, are arranged in small groups surrounded by indifferent cells.
The entire structures are primary follicles, and they are imbedded in a
stroma likewise derived from the peritoneum. Felix considers that the
follicles develop, for the most part at
least, directly from the tissue of the
genital ridge, and states that tubes or
cords growing in from the peritoneal
epithelium, as described by Pfliiger, do
not exist in the human ovary. Generally it has been said that the primary
follicles arise by the subdivision of such
cords (Fig. 354).
Ligaments. As the Miillerian ducts
come together below, they occupy ridges
FIG. 354SECTioN OF THE OVARY AT BIRTH, covered with peritoneum. These ridges
.. Epithelium; STSSSw cord; c, sexual coalesce so as to form a partition which
cfes : : f aH^feS'fahS^^wSS crosses the pelvis from side to side and
rises upward from its floor. Ventral to
the partition is the bladder, separated from it by the vesico-uterine pouch;
dorsal to it is the rectum, separated by the deeper recto-uterine pouch;
and within it are the uterus and tubes. In the adult these folds of peritoneum extending laterally from the uterus constitute its broad ligaments.
The Wolffian bodies and ovaries, which at first occupy vertical ridges on
either side of the root of the mesentery, appear to slip down or descend
into the interior of the broad ligaments, from the dorsal surfaces of which
the ovaries later project.
Above each ovary there is a band of fibrous tissue which extends to
the orifice of the tube, and running along this band there is a fimbria known
as the fimbria ovarica; this arrangement apparently serves to keep the
orifice of the tube in close relation with the ovary. Below the ovary,
between the laminae of the broad ligament, a cord of fibrous tissue
passes from it to the musculature of the uterus, lying just below the
uterine tubes; this is the ovarian ligament. The round ligaments start
from the uterine musculature not far from the ends of the ovarian
ligaments. They pass downward, one on either side within the broad
ligament, and terminate in the folds which correspond with those of the
scrotum. The ovarian and round ligaments are believed to be subdivisions of a single structure equivalent to the gubernaculum testis.
FEMALE GENITAL ORGANS
353
The External Genital Organs. The urogenital sinus, which receives the
urethra and vagina, becomes a shallow space called the vestibule (Fig. 353).
The genital papilla, with the glans at its apex, becomes relatively shorter as
the female embryo develops. It forms the clitoris, analogous with the
penis, and is covered by the lesser genital folds, the
labia minor a. (Compare Fig. 355 with Fig. 329, A,
page 331.) The labia form a prepuce for the clitoris
but do not unite beneath it to make a raphe; they
remain separate, as parts of the lateral boundaries
of the vestibule. The larger genital folds, labia
majora, likewise remain separate. They receive the
ends of the round ligaments of the uterus which pass ^
. FIG. 355- DIAGRAM OF
into them over the pubic bones, sometimes accom- THE EXTERNAL GENITAL ORGANS OF A
panied by a prolongation of the peritoneal cavity FEMALE EMBRYO.
. . . a., anus; g., glans clitori
formmg a processus vagmalis. In late stages of de- dis: g . f lesser genital
folds (labia mmora) ;
velopment the labia majora become large enough to foids^iahlTmwffiu
conceal the clitoris and labia minora, which previously (v't?buil i * a ! sinus
project between them.
OVARY.
The ovary is an oval body about an inch and a half long, covered by
a modified portion of the peritoneum. Along its hilus it is attached to a
mesentery, the mesovarium, which is a subdivision of the broad ligament
of the uterus. The epithelium of the mesovarium is continuous with that
of the ovary, and its connective tissue joins the mass which forms the
ovarian medulla. This tissue, rich in elastic fibers and containing strands
of smooth muscle, surrounds the vessels and nerves. The blood vessels
are abundant, and they pursue a very tortuous course both in the mesovarium and within the ovary. This is strikingly shown in Clark's injections (Johns Hopkins Hosp. Rep., 1900, vol. 9). They are derived
in part from branches of the uterine vessels, but are chiefly the terminations
of the ovarian artery and vein. Large stems traverse the medulla and
form capillary plexuses around the follicles in the cortex. Thin-walled
lymphatic vessels arise in the cortex below the rather dense sub-peritoneal
layer (or tunica albuginea} and pass out at the hilus. The nerves are
chiefly non-medullated sympathetic fibers, derived from the plexus which
accompanies the ovarian artery, and distributed to the blood vessels.
Ganglion cells have been found near the hilus, and a few medullated fibers
occur. It is said that certain fibers end in contact with the cells of the
follicles.
The relation of the cortical stroma to the looser tissue of the medulla
is so characteristic that sections of the human ovary containing few ova
23
354 HISTOLOGY
and no active follicles may be readily identified. Usually a section of the
ovary may be recognized as such without magnification, owing to the presence of the large cysts or follicles in which the maturing ova are contained.
These extend from the cortex into the medulla, and are numerous even in
childhood (Fig. 356).
Growth of the Follicles. It is probable that all the sexual cells which are
to be produced in a life-time are present in the ovaries at birth. At that
stage, at least, many of those previously formed have already degenerated;
and the ovaries contain a great excess of ova, all but a few hundred of
which are destined to atrophy within the limits of the genital glands. In
so far as the sexual cells have ceased to multiply and have entered upon
the growth period, they represent the last generation of oogonia, and are
being transformed into primary oocytes. During this transformation
FIG. 356. CROSS SECTION OF THB OVARY OF A CHILD EIGHT YEARS OLD. X 10.
Germinal epithelium; 2, tunica albuginea; 3, peripheral zone with primary follicles: 4, vesicular follicle;
5, stroma ovarii; 6, medulla; 7, 8, peripheral section of vesicular follicles; 9, hilus, containing large
veins.
they increase greatly in size, finally becoming about 0.3 mm. in diameter.
These egg cells have already been described in detail (p. 29). They are
conspicuous in sections as large, round, deeply staining cells, with round or
oval vesicular nuclei, each containing a prominent uucleolus. The cells
become so large that frequently they are cut into several sections, and
portions of protoplasm without nuclei are to be expected. The larger
oocytes are surrounded by the clear, radially striated zona pellucida (Fig.
22, p. 30); their protoplasm may contain the viteUine bodies previously
described.
The follicles are composed of the cells which surround the oocytes.
After the groups of egg cells and indifferent cells become subdivided, each
oocyte is typically surrounded by a single layer of flat follicular cells, and
this primary follicle lies isolated in the stroma of the cortex, beneath the
tunica albuginea (Figs. 357 and 358). As the follicle enlarges, the follic
OVARY
355
ular cells become columnar and then stratified (Fig. 358). A crescentic
cleft filled with fluid appears in the midst of the stratified epithelium on
one side of the follicle, and by the accumulation of fluid, or liquor folliculi,
this cleft becomes a spherical cavity (Fig. 359). The fluid is regarded by
some as a transudate from the blood vessels, which are abundant in the
stroma outside of the follicle. Others consider that it is actively secreted
by the cells of the follicle,
certain of which undergo
liquefaction. Spaces containing a stainable fluid,
differing from that in the
main cavity, may appear
Follicular cells.
Egg cells in
an island.
Egg cell.
^
FIG. 357. FROM A SECTION OF THE OVARY OF A CHILD FOUR
WEEKS OLD. X 240.
in the epithelium (CallExner bodies), around
which the cells are radially
arranged. By the development of the main cavity, the stratified epithelium becomes a relatively thin layer, the stratum granulosum, which
decreases in width as the follicle enlarges. The oocyte is on one side of
the follicle, contained in a heap of cells known as cumulus oophorus (formerly the discus proligerus) . This is connected with the wall of the follicle,
but in certain sections it may appear completely detached (e.g., in a sec
Germinal
epithelium.
Tunica
albuginea.
Primary
follicle.
A degenerating
follicle.
Follicular cells.
Nucleus.
Xucleolus. Protoplasm. Zona pellucida.
FIG. 358. FROM A SECTION OF A RABBIT'S OVARY. X 240.
don at right angles with the plane of the page, near the top of the cumulus
in Fig. 359).
Surrounding the follicle, even in early stages, there is a connective
tissue sheath, the theca folliculi (Fig. 358). This later becomes differentiated into a vascular tunica interna, and a fibrous tunica externa (Fig. 359).
The tunica interna contains many cells with abundant protoplasm. It is
separated from the epithelium of the follicle by a delicate membrana
propria.
356 HISTOLOGY
In distinction from the solid primary follicles, those with cavities are
known as vesicular follicles (Graafian follicles). They increase in diameter
from 0.5 to 12.0 mm., and are then ready to discharge the contained oocyte.
Occasionally a single follicle has two oocytes, and rarely more. Arnold
(Anat. Rec., 1912, vol. 6, pp. 413-422) describes the ovaries of a negress,
in which he found forty-three follicles containing four oocytes or more,
including one which contained eleven. It cannot be stated whether the
additional oocytes develop by division of the oogonium within a primary
follicle, or by the failure of a group of primitive sexual cells to become
separated from one another.
Theca
folliculi.
Tunica externa.
Tunica interna.
Stratum granulosum
Cumulus ofiphorus.
Egg cell with zona
pellucida, nucleus
and nucleolus.
FIG. 359- SECTION OF A LARGE VESICULAR FOLLICLE OF A CHILD EIGHT YEARS OLD. X 90.
The clear space within the follicle contains the liquor folliculi.
Owlation and the Corpus Luieum. Around the mature vesicular
follicle, the tunica interna becomes very thick and cellular, forming elevations toward the stratum granulosum. At this stage the follicle is large,
being about half an inch in diameter, and one surface of it is so close to the
ovarial epithelium as to cause it to bulge and then to rupture. Through the
opening thus made the liquor folliculi escapes, together with the oocyte.
The latter is said to become detached by the formation of fluid-filled spaces
between the cells of the cumulus; it generally carries with it more or less
of the innermost layer of the cumulus, and these cells, because of their
radial arrangement, are termed the corona radiata. As the oocyte leaves
the follicle there is apparently a chance for it to become lost in the abdominal cavity, but the fimbriated orifice of the tube is near at hand, and the
stroke of its cilia produces a current toward its entrance. In a guinea-pig
OVARY
357
Hensen observed that the fimbriae were in very active motion, sweeping
here and there over the surface of the ovary so powerfully that the effect
of ciliary action must have been trivial. The ova, surrounded by the
mucoid cells of the follicles, adhered more closely to the fimbriae than to
the smooth surface of the ovary. Except toward the time of ovulation,
Hensen found that the fimbrias were relatively inactive (Zeitschr. f. Anat. u. Entw.,
1875, pp. 213-270). The discharge of the
ovum from the follicle is known as ovulation.
It may be noted that in approaching the
peritoneal epithelium, through which the
rupture occurs, the follicle must push aside
or distend the connective tissue of the tunica
albuginea. This is ordinarily a rather weak
layer, but it has been suggested (by Reynolds) that in some cases it is more highly
developed and acts as an obstruction to ovulation.
After ovulation, blood escapes from the capillaries of the tunica interna
and forms a clot within the empty follicle (Fig. 360). This clot is sometimes called the corpus hcsmorrhagicum. On all sides it is surrounded by
the cells of the stratum granulosum, which enlarge and produce a yellow
fatty pigment. They form a yellow convoluted zone which may easily
FIG. 360. OVARY, CUT ACROSS
SLIGHTLY REDUCED.
i., Aperture through which the
ovum escaped; c. a., corpus
albicans; cl., blood clot in a
corpus luteum of ovulation;
th., theca folliculi; v. f., vesicular follicle. (After Rieffel.)
Connective tissue septa.
Fibrous connective tissue.
Vacuoles.
Lutein cells.
A B
FIG. 361. A, PORTION OF A CORPUS LUTEUM OF A RABBIT. B, PORTION OF A CORPUS LUTEUM OF A
CAT. X 260. r
In B the lutein. 'cells have become fatty and contain large and small vacuoles.
be seen without magnification; the entire structure is then known as the
corpus luteum. Vascular strands of connective tissue extend between the
lutein cells (Fig. 361) and enter the central clot. The extravasated blood
breaks down into granules and haematoidin crystals, and is gradually
358 HISTOLOGY
absorbed. It is replaced by gelatinous connective tissue which finally
contracts into a dense white fibrous nodule, and this scar is known as the
corpus albicans. Meanwhile the lutein cells undergo hyaline degeneration
and become resorbed. The surface of the ovary, which is smooth in childhood, becomes pitted and irregular with the increasing formation of these
corpora albicantia.
Provided that pregnancy does not take place, the corpus luteum reaches
its maximum development in about two weeks after ovulation, and it becomes reduced to a scar in about two months. If pregnancy occurs, it
enlarges further and persists at the height of its development until the
fifth or sixth month. Its diameter is then 1.5-3.0 cm., and at the end of
pregnancy it is still quite large and yellow. If the corpus luteum is removed, the ovum fails to become attached to the wall of the uterus. There
is both experimental and histoiogical evidence that it produces an internal
secretion which is probably received by the blood vessels invading it from
the theca. In order to distinguish between the corpus luteum of pregnancy
and that of unproductive ovulation, the former is called the true corpus
luteum; the latter is the corpus luteum spurium.
Many follicles degenerate at various stages in their evolution without
discharging their ova. Leucocytes and cells from the stratum granulosum are said to invade the protoplasm of the oocytes, in which they disintegrate. The zona pellucida, which surrounds the oocyte, may become
conspicuously folded and persist for some time (Fig. 358). The basement
membrane of the stratum granulosum may also thicken and become convoluted. These degenerating or atretic follicles are finally reduced to
inconspicuous scars. After the menopause the degeneration of the oocytes
becomes general.
Within the stroma of the cortex, interstitial cells are found, which resemble lutein cells but are smaller. They have been compared with the
interstitial cells of the testis, and are said to contain secretory granules.
Apparently they are derived from the thecae of atretic follicles (Cohn,
Arch. f. mikr. Anat, 1903, vol. 62, pp. 745-772; Allen, Amer. Journ. Anat.,
1904, vol. 3, pp. 89-153).
UTERINE TUBES.
Each uterine tube is about 5 inches long and extends from its orifice
in the abdominal cavity to its outlet in the uterus. It is divided into the
fimbriated funnel or infundibulum; the ampulla or distensible outer twothirds, the lumen of which is about a quarter of an inch in diameter; the
isthmus or narrow inner third, not sharply separated from the ampulla;
and the uterine portion which extends across the musculature of the uterus
to the uterine orifice. The wall of the tube is composed of three layers, a
OVARY
359
mucosa, muscularis. and serosa (in addition to which a tela submucosa is
enumerated in the Basle nomenclature). The mucosa is thrown into thin
longitudinal folds, which are low in
the isthmus, but tall and branched in
the ampulla (Fig. 362). Occasionally the branches anastomose, enclosing a pocket, but glands are
absent. The epithelium is chiefly
simple columnar, and ciliated, the
stroke of the cilia being toward the
uterus; but there are areas of nonciliated cells which are said to produce
a mucoid fluid. The two types of
cells are connected by intermediate
forms. Mucous cells are absent.
The folds of the mucous membrane are occasionally indented or overhanging, so that in transverse sections detached fragments may appear,
suggestive of villi (Fig. 363) ; but the fact that almost all of the many pro
A
FIG. 362. THE MUCOSA OF THE UTBRINB TUBE.
A, Near ITS FIMBRIATED END; B., NEAR THE
UTERUS. (After Orthmann.)
\ Longitudinal muscles.
\
Blood vessels.
\
Circular muscles.
Mucosa.
FIG. 363. CROSS SECTION; NEAR THE AMPULLA, OF A UTERINE TUBE FROM AN ADULT WOMAN.
jections connect with the submucous layers indicates that they are elongated folds. Each of them contains a thin layer of cellular connective
tissue, in which there are small arteries and veins running chiefly length
360
HISTOLOGY
wise of the tube. Lymphocytes occur in the meshes of the tissue and lymphatic vessels have been reported. Occasionally strands of smooth muscle
fibers are found within the folds.
The mucous membrane rests directly upon the tunica muscularis, and
Schafer considers that " the larger part of the muscular layer must probably be regarded as a much thickened muscularis mucosae." The muscle
coat consists of a thick inner circular layer and a thin outer longitudinal
layer of smooth muscle fibers, but both layers are resolved into coarse
bundles by the abundance of intermuscular connective tissue.
Since the uterine tubes are imbedded in the broad ligaments, they are
not closely invested by the peritoneum. There is a considerable layer of
loose vascular connective tissue outside of the muscularis, and toward the
ovary this tissue may include sections of the tubules of the epoophoron.
It contains the branches of the ovarian and uterine blood vessels which
supply the tube. These are accompanied by lymphatic vessels and
nerves. The latter innervate the tubal musculature and the mucous
membrane.
/Tube
, Fuudus
UTERUS.
The uterus is a pyriform, muscular organ, flattened dorso-ventrally.
It is about two and a half inches long, receiving the uterine tubes at its
upper end or fundus, and ending below in the vagina. It is divided into
fundus, corpus and cervix. The corpus and fundus
together have a triangular cavity, which opens
into the canal of the cervix through the internal
orifice; the canal communicates with the vagina
through the external orifice of the uterus. The
lining of the cervix presents a feather-like arrangement of folds on its dorsal and ventral surfaces;
these are the plica palmatcs. The walls of the
uterus consist of a mucosa, muscularis and serosa
(constituting the endometrium, myometrium, and
perimetrium, respectively).
The uterus is lined with simple columnar epithelium, some areas of which are ciliated. The
cilia have been described as diflicult to preserve, and their absence
from certain cells has been attributed to faulty fixation. According
to Gage the uterine cilia are as readily preserved as those which occur
elsewhere, and he finds that only one cell among fifteen or twenty is actually ciliated. Mucous cells are absent. The epithelium forms slender
tubular pits, the uterine glands, but these produce no definite secretion.
They are branched tortuous tubes extending through the broad mucosa
-Vagina
FIG. 364. THB DORSAL HALF
OF A VIRGIN UTERUS. Twothirds natural size. (After
Rieffel).
UTERUS
361
(which is i mm. thick) , and invading to a slight extent the muscular tissue
beneath. They have been carefully modelled by Hedblom, whose studies
are not yet published; he finds that occasionally they anastomose with one
another, and that in their deeper portion they have long horizontal
branches, at right angles with the main tube. Sometimes a small group
of glands opens into a single depression of the surface epithelium (Fig. 365).
In older persons the glands degenerate, losing their connections with the
surface and becoming cystic. Each gland is surrounded by a delicate
basement membrane, and between them there is an abundant tunica
/*&-, Epithelium.
Gland.
>._ Mucosa.
FIG. 365. Mucous MEMBRANE OF THE RESTING UTERUS OF A YOUNG WOMAN.
(After B6hm and von Davidoff.)
X 35
propria, containing many blood vessels. These form capillary networks
around the glands and especially beneath the free surface. The propria
contains also many lymphocytes, and its lymphatic vessels form a widemeshed plexus with blind extensions. These structures are supported by
a reticular tissue framework containing many nuclei.
The upper and larger part of the cervix of the uterus is likewise lined
with simple columnar ciliated epithelium, but its cells are taller than those
of the corpus (60 n as compared with 20 /*) Mucous cells occur, especially
in the outpocketings of epithelial pits which constitute the branched
cervical glands. They discharge a secretion which occludes the canal of
the cervix during pregnancy. Often they produce macroscopic retention
cysts, named "ovules of Naboth," after the Leipzig anatomist who first
362
HISTOLOGY
Mucosa.
Muscularis. <(
described them. Toward the external orifice of the uterus the epithelium
becomes stratified and squamous, and rests on connective tissue papillae.
Thus it resembles the lining of the vagina of which it is a continuation,
and after the first child-birth it extends further up into the cervix than
before.
The musculature of the uterus is a thick investment of interwoven
bundles which cannot be subdivided into well-defined layers (Fig. 366).
It begins immediately outside the tunica propria, and its inner portion
has been regarded as "an immensely hypertrophied muscularis mucosae."
Further out there is a zone
containing many blood
vessels, which according to
this interpretation marks
the position of the submucosa (Schafer). Accord'
ing to Henle and Stohr,
these vessels belong with
the middle of three muscle
layers, which is named,
therefore, the "stratum
vasculare." It is the
thickest of the layers and
its fibers are chiefly circular. The innermost layer
or "stratum submucosum"
(Stohr) consists principally
of longitudinal fibers. The
outermost layer or "stratum supravasculare" contains circular fibers internally and longitudinal
fibers externally. Some of the latter are continuous with the longitudinal fibers of the uterine tubes; others are said to enter the round ligaments, which contain also some striated fibers; and still others spread
into the broad ligaments.
In the cervix the three strata of muscle fibers are found to be very
distinct inner and outer longitudinal, and middle circular. Although
the uterus generally contains few elastic fibers, found only in its peripheral
layers and running perpendicular to the plane of contraction of the muscles,
elastic fibers are abundant in this position in the lower segment of the
corpus and vaginal portion of the uterus. During the first half of pregnancy both elastic and muscular fibers increase in size and number;
in the second half, the elastic fibers decrease in the musculature, but
increase in the perimetrium (Stohr). The way in which the thick layer
of muscles in the resting uterus becomes arranged in the thin layer of
Serosa.
FIG 366. FROM A TRANSVERSE SECTION OF THB MIDDLE OF
THE UTERUS OF A GIRL FIFTEEN YEARS OLD. X 10.
a, Epithelium; b, tunica propria; c, glands; i, inner muscular
layer; 2. middle muscular layer; 3, outer muscular layer.
UTERUS 363
late pregnancy is an unsolved problem, similar to that presented by the
musculature of the bladder and intestine during distention.
The serosa covering the dorsal and ventral surfaces of the uterus is in
part a well-defined layer, but it blends with the connective tissue of the
broad ligaments laterally and below; and this tissue, from its position
beside the uterus, is known as the "parametrium." Imbedded in the
parametrium the main trunks of the uterine vessels run along the lateral
margins of cervix and corpus, both artery and vein showing many kinks
and convolutions. The vessels are thus apparently adapted to the future
expansion of the uterus, but when it retracts after pregnancy they are
said to show more pronounced bendings, as if they had been permanently
elongated. The parametrium contains also numerous lymphatic vessels,
together with the ganglionated sympathetic utero-vaginal plexus. Nerves
from this plexus and from the third and fourth sacral nerves supply the
uterus.
MENSTRUATION.
Menstruation is the periodic degeneration and removal of the superficial part of the mucosa of the uterus, accompanied by haemorrhage from
the vessels of the tunica propria. Three successive stages may be distinguished, namely (i) the stage of congestion, lasting four to five days; (2)
the stage of desquamation and hamorrhage, four days; and (3) the stage of
regeneration and repair, seven days. Thus the entire process requires
about sixteen days, and after an interval of twelve days the cycle begins
anew.
For four or five days before the discharge occurs, the thickness of the
mucosa increases greatly, due to the congestion of its vessels and the
proliferation of the reticular tissue. The glands become wider, longer, and
more tortuous, opening between irregular swellings of the superficial
epithelium. Red corpuscles pass out between the endothelial cells of the
distended veins and capillaries, and form subepithelial masses. This
stage of congestion and tumefaction is followed by one of haemorrhage and
desquamation. The epithelium of the surface and outermost parts of the
glands becomes reduced to granular debris, or it may be detached in shreds.
The underlying vessels rupture and add to the blood which had escaped by
diapedesis. In the stage of regeneration, the epithelium spreads from the
glands over the exposed reticular tissue, the congestion diminishes, and
the mucosa returns to its resting condition. The cervix takes no part in
menstruation except that the secretion of its glands may increase during
the stage of congestion.
Beginning at puberty (13-15 years) menstruation takes place normally
once in 28 days for 33 years, more or less. During pregnancy it is interrupted,
364
HISTOLOGY
although the time when it should occur may be indicated by slight uterine contractions and finally by those which cause the delivery of the child. Thus the
duration of pregnancy is described as ten menstrual cycles. The significance of menstruation is suggested by conditions in those mammals in which sexual seasons are
annual or infrequent. In them a period of congestion, accompanied by uterine changes
which are sometimes closely comparable with those of menstruation, precedes sexual
intercourse and ovulation. Thus in the bitch ovulation takes place when the external
bleeding "is almost or quite over," and this is the time of coitus. Domestication in
Disintegratin ^^vZX., ? ft.i.
epithelium. V^|gLffii&ifesBt
Blood vessel v^^v^fi,''^^
^^''ll^wi^^^^^t <*' v
r - J V,i'^ '3 life I ">; feglES^- l^-O'^'vi. Disintegrating
ne gland "^^f^^^-.'^-^^^'j^ P^J^.~ e P ithehum
^i^','::\ll^l frffiVwOfi** ""0-^i>C J " p it- lik e depression.
r ___________________ Surface epithelium.
Uteri
Bifurcating tubule.
Cystic t
Blood vessel." -'"4?
FIG. 367. Mucous MEMBRANE OF A VIRGIN UTERUS DURING THE FIRST DAY OF
MENSTRUATION. X 30. (Schaper.)
various animals causes an increased frequency of the congestive cycles, sometimes
unaccompanied by ovulation. It is generally accepted that human menstruation
may take place without ovulation, and that ovulation may occur between menstrual
periods, and also during pregnancy. It may even occur in children before menstruation has begun. Nevertheless ovulation probably occurs usually and normally at the
close of menstruation. Coitus is not considered to be a factor in inducing ovulation,
but it is said that in the rabbit and ferret, and in pigeons, ovulation may fail to occur
in the absence of the male.
MENSTRUATION 365
The following considerations are also important in establishing the age of young
embryos. The time required for spermatozoa to travel to the upper end of the tube,
where fertilization takes place, is probably about twenty-four hours. There they
may fertilize the ovum at once if ovulation has just occurred. They retain their
vitality and are capable of fertilizing the ovum during a period of ten days in the
rabbit, and this may be true also of man. Thus it is probable that if coitus has occurred
shortly before menstruation, the spermatozoa may remain active in the tube, and
fertilize the ovum discharged at the close of the following menstruation.
THE DISCOVERY OF MAMMALIAN OVA.
During the seventeenth century the ovary was called the testis muliebris, or testis
foemineus. It was believed to produce the mucoid secretion which escapes from the
genital orifice, and this was regarded as seminal fluid. The uterine tubes were accordingly the vasa deferentia mulierum, serving to convey this fluid to the uterus, where,
through a mixture and interaction of the male and female semina, an embryo was
produced. Aristotle had argued to the contrary, but his opinion was summarily disposed of by Bartholin, who discussed the ovaries as follows (Anatomia, 1666):
"Their function is to produce semen in their own way, which Aristotle, against all
reason and observation, has dared to deny to women, contrary to the express teaching
of Hippocrates. "
The ancient doctrine of Aristotle, expounded in his treatise on the generation of
animals, was based upon the familiar facts that menstruation marks the beginning,
and ceases at the end, of the child-bearing period; and moreover menstruation is
interrupted while the embryo is being formed. Therefore he concluded that the menstruum supplies the substance and material for the new body, which arises like the
curd in milk, through the agency of the semen. The semen engenders; the menstruum
nourishes. The theory had already been advanced that the semen comes from all
parts of the body, and that its particles reproduce the structures from which they are
derived. This enticing speculation, revived by Darwin in his theory of pangenesis,
was discussed at length and rejected by Aristotle.
Generation, therefore, was considered to result from the mixing of two fluids, and
would have remained a barren physico-chemical problem until recent times, if further
morphological observations had not been made. The view of Bartholin had at least
the merit of definitely associating the ovary with the reproductive function. Vesalius
and Fallopius had seen the follicles and corpora lutea; Fallopius described them as
"vesicles filled with water or aqueous humor, some limpid and others yellow (Observationes, 1588). Many others had observed them, and from their resemblance to the
ova of birds they had even been called "ova," when in 1672 a young Dutch physician,
Regnerus de Graaf , made his thorough study of the female genital tract.
De Graaf concluded that the "semen muliebre" is not produced by the "testes
muliebres," but that the general function of the latter is "to produce and nourish
ova, and bring them to maturity." Consequently he proposed to substitute the name
ovary, and to call the tubes oviducts. He declared that the ova escaped from the
follicles through minute apertures (in the rabbit admitting a bristle) and made their
way through the tubes to the uterus, in which they developed. The abnormal formation of a human embryo within the tube was figured and, to a certain extent, explained.
De Graaf studied many mammals, and especially rabbits* He found minute ova in
the oviducts and observed the follicles from which they had escaped. In older stages
he recorded a general agreement between the number of corpora lutea and embryos.
366 HISTOLOGY
Since, however, he frequently referred to the entire follicles as ova, his results were not
promptly accepted; the diameter of the isthmus of the tube is so small that the entrance
of the follicles into the uterus was considered impossible. It was a matter of easy
observation to determine more precisely the relation of the ova to the follicles. After
many years this was done by Von Baer, an eminent embryologist, whose studies
of the chick are regarded as " the most profound, exhaustive and original contribution
to embryology which has ever been made" (Minot). This work bears the famous
subtitle "Beobachtung und Reflexion" the German expression of Haller's "Observations suivies de Reflexion" and De Graaf's " Cog itationes atque observationes." After
describing the condition of the ova in the tubes of the bitch, Von Baer writes:
"It remained for me to ascertain the condition of ova in the ovary, for it seemed
clearer than day that ova so small as those found in the tubes did not represent Graafian
follicles expelled from the ovary; and I did not consider it probable that such solid bodies
had been coagulated from the fluid of the vesicles. Now, contemplating the ovaries
before making an incision, I clearly distinguished in almost every vesicle, a yellowishwhite point unattached to the walls, which swam about freely in the fluid when the
vesicle was pressed upon with a probe. Led on by a certain curiosity, rather than
moved by hope that with the naked eye I had seen ovules in the ovaries through all
the coats of the Graafian follicle, I opened a vesicle, and taking out a point in question
on the blade of a knife, I placed it under the microscope. I was overcome with amazement when I saw the ovule, now recognized outside of the tubes, so clearly that a blind
man could hardly doubt it. Surely it is strange and unexpected that an object so persistently sought for, and endlessly described as inextricable, in every physiological
compendium, could so easily be placed before the eyes" (De ovi genesi, Lipsiae, 1827).
Thus the ova in mammalian ovaries, which had long been believed to exist, were
first definitely seen within the follicles one hundred and fifty years after the discovery
of the microscopic spermatozoa, the existence of which had never been suspected.
THE DECIDUAL MEMBRANES OF THE UTERUS AND EMBRYO.
DEVELOPMENT AND GENERAL FEATURES.
Before describing the mucous membrane of the uterus during pregnancy, it is necessary to consider the membranes which envelop the
embryo. Although these are in contact with the lining of the uterus and
in part intimately blended with it. they are portions of the embryo itself.
The external membrane, toward the uterus, is known as the chorion; the
inner membrane, toward the embryo, is the amnion. Since the embryo
receives its nutriment from the wall of the uterus through blood vessels
in the chorion, these membranes develop very early and thus provide for
rapid growth. They are already present in the youngest human embryos
which have yet been obtained.
Of the fertilization and segmentation of the human ovum, which
doubtless take place in the upper part of the uterine tube, nothing is
known except by inference from lower animals. The four-celled stage
has been observed once in a monkey, but the youngest known human
embryo is already provided with ectoderm, mesoderm and entoderm, and
has entered the uterus. As a purely hypothetical figure, we venture to
DECIDUAL MEMBRANES
367
present the diagram Fig. 368, A, followed by the diagrams B and C which
include many features actually observed.
In Fig. 368, A, a mass of cells (ect.} represents the ectoderm which will
later cover the body and line the inner membrane or amnion. This ectoderm probably arises in connection with the layer (tr.} which covers the
entire vesicle and becomes the epithelium of the outer membrane or
chorion. The layer in question has been named the trophoblast (or
trophoderm} .
eel.
cho.
FIG. 368. DIAGRAMS ILLUSTRATING THE EARLY DEVELOPMENT OF THB HUMAN EMBRYO (A is HYPOTHETICAL).
al., Allantois; am. c., amniotic cavity; cho., chorion; coe., ccelom; ect., ectoderm; m, mesodertn; tr.
trophoderm (trophoblast); z, entodermal cyst; y. s., yolk-sac.
The term trophoblast (i.e., nutritive layer) was introduced by Hubrecht to correspond with the terms epiblast, mesoblast and hypoblast, which he used for the other
germ layers. Since these are now generally called ectoderm, mesoderm and entoderm,
the outer layer should be trophoderm, and the substitution of this name is therefore
recommended. Trophoderm has, however, been used by Minot for the proliferating
part of Hubrecht's trophoblast. It may be noted that a similar difficulty is encountered in His's angioblast which, as a germ layer, should be angioderm. Schafer
applies angioblast logically to the individual cells which become the endothelial
lining of vessels. Consistency requires the use of "-derm" for germ layers, leaving
" -blast" for formative cells.
In addition to the trophoderm and ectoderm, the hypothetical stage
shown in Fig. 368, A, exhibits a yolk-sac completely lined with entoderm.
Between the trophoderm and entoderm, the mesoderm has appeared and
is separating into somatic and splanchnic layers, with the body cavity
between them. The somatic mesoderm is closely applied to the trophoderm,
and together they form the chorion; the splanchnic mesoderm is against
the entoderm of the yolk-sac, and forms the outer layer of its wall. The
early and rapid development of the mesoderm is characteristic of human
embryos, as may be inferred from the later stages.
In the diagram Fig. 368, B, the amniolic cavity has appeared in the ectoderm. It is believed to arise as a cleft in a solid mass of cells, and not by
the coalescence of ectodermal folds as in the chick; however, in the youngest human embryos observed, it is completely formed. The entoderm
368 HISTOLOGY
shows an outpocketing extending into the mesoderm at the future caudal
end of the embryo; this is the allantois, which soon becomes a slender tube
(Fig. 368, C). The mesoderm in which it is lodged later produces the
"body stalk."
The allantois develops very early in human embryos, being present in most if not
in all of the specimens thus far obtained. Possibly there is no allantois in the very
imperfect embryo described by Bryce and Teacher (Contributions, etc, Glasgow,
1908), and there is uncertainty as to its presence in Peters's embryo (Ueber die Einbettung des menschlichen Eies, Leipzig, 1899); but in other very young specimens it is
well defined. According to Keibel, the allantois first appears in chicks of about
twenty segments; in rabbits of eleven segments; in pigs of four to five segments;
and in the apes and man, before any segments have formed. Its very early appearance
in human embryos is probably correlated with the rapid establishment of the placental
circulation, for the umbilical vessels are primarily the vessels of the allantois.
In Fig. 368, B and C, the entoderm of the yolk-sac is represented as giving rise to a
detached cyst (x). There is a cyst of this sort within the chorionic cavity of the
somewhat damaged Herzog embryo in the Harvard Collection, and a smaller detached
cyst in the very perfect Minot embryo. (These will be further described by the writer
in a subsequent publication.) It is possible that such cysts are of regular occurrence,
although destined to atrophy. They may be lodged in a strand of mesoderm extending from the lower pole of the yolk-sac downward to the chorion (Grosser, Anat.
Hefte, 1913, Abt. I, vol. 47, pp. 653-686), and they may arise as indicated in the
diagrams (Fig. 368).
As the body cavity develops between the somatic and splanchnic
layers of mesoderm, it is at first bridged by strands of mesenchymal tissue,
forming the "magma reticulare." These strands become attenuate and
break down, so that the yolk-sac is then suspended in a well-defined
"extra-embryonic ccelom." This part of the ccelom, although within the
embryonic membranes, is outside of the body proper of the embryo, as will
appear in the following diagrams.
The arrangement of the membranes surrounding human embryos of
about 2 mm. is shown in Fig. 369, A. The chorion has become covered
with branching elevations or villi, which contain a vascular core of chorionic mesoderm. not shown in the diagram. The body of the embryo is
connected with the chorion by the mesodermic body stalk containing the
allantois. This has become relatively slender. On one side it is covered
by the ectoderm of the amnion. The ectoderm, as in preceding stages,
may be divided into two parts. Toward the yolk-sac it is thickened and
there it forms the axial medullary tube and gives rise ultimately to the
epidermis covering the body. Continuous with this epidermal ectoderm
is the thinner portion which lines the amnion, as shown in the figure.
The amnion forms a membranous sac attached to the ventral side of the
embryo, leaving an aperture through which the yolk-sac projects downward
into the extra-embryonic ccelom. The ccelom now extends between the
amnion and chorion, except at the narrow body stalk. The yolk-sac has
DECIDUAL MEMBRANES
369
given rise to the fore-gut and hind-gut, and the allantois now appears as
an appendage of the latter.
In Fig. 369, B, the embryo is represented as rotated so that its head is
downward and its ventral side toward the left. It is now connected with
the membranes by an umbilical cord, the composition of which may be seen
by comparing A and B. Its principal constituent is the elongated body
stalk, containing the allantois and covered above and on the sides with
adherent amnion. Below, the amnion also forms the covering of the cord,
but here it is separated from the body stalk by an extension of the body
cavity. The yolk stalk passes from the primary loop of intestine through
the cavity of the umbilical cord to the yolk-sac, in which it terminates.
cho
cho
A B
FIG. 369. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE EMBRYONIC MEMBRANES AND THE FOR
MATION OF THE UMBILICAL CORD.)
al., Allantois; am., amnion; am. c., amniotic cavity; cho., chorion; coe., coelom; y. s., yolk-sac.
This sac is now lodged in its permanent position between the amnion and
chorion. Ultimately the parts of the allantois, yolk stalk and body
cavity within the cord are obliterated.
The appearance of a human embryo at a stage intermediate between
those shown in Fig. 369 is reproduced in Fig. 370. An irregular piece cut
out from the chorionic vesicle forms the background of the picture.
Around the cut edges of this piece the shaggy chorionic villi are seen,
directed toward the wall of the uterus. At the top of the figure is the
spherical yolk-sac lodged between chorion and amnion, between which
the yolk stalk passes to the distal end of the umbilical cord, which it enters.
The amnion is a membranous sac completely enclosing the embryo; in
the figure, half of it has been cut away to show the embryo within. The
skin of the embryo is continuous with the covering of the umbilical cord,
and distally this covering is reflected and becomes continuous with the
amnion.
In later stages the umbilical cord is greatly elongated. It contains
the umbilical vessels which pass between the embryo and the chorion,
24
37
HISTOLOGY
through the persistent body stalk. The amniotic cavity greatly enlarges
to accommodate the growing embryo, and the mesoderm of the amnion
comes in contact with that of the chorion, to which it adheres more or less
firmly. The embryo is bathed in the amniotic fluid (liquor amnii) of
uncertain derivation, once thought to be sweat from the embryo, and later
considered to contain the products of the Wolffian body, and urine from
the permanent kidneys. Occasionally toward birth the meconium from
PIG. 370. A NORMAL HUMAN EMBRYO OF 10.0 MM., REMOVED SURGICALLY WITH THE UTERUS. Six, WEEKS
AFTER THE LAST MENSTRUATION.
the intestine mingles with it and discolors it. It is now generally believed
to be secreted by the amniotic epithelium.
Relation between the Embryonic Membranes and the Uterus. When the
embryo within its chorionic vesicle passes from the tube into the uterus,
it is probably in a stage comparable with that shown in Fig. 368 (B or C).
By the activity of the proliferating trophoderm, the uterine mucosa is
partially destroyed and the chorionic vesicle becomes imbedded in its
substance. This process is known as the implantation of the ovum. The
walls of the vessels in the tunica propria of the uterus are broken down,
and the maternal blood flows over and around the chorionic villi, in con
DECIDUAL MEMBRANES
371
tact with which it does not clot. Elsewhere in the body, except in reticular tissue, blood clots on escaping from the endothelial tubes. Toward
the uterine cavity, however, there is a clot which completes the encapsulation of the chorionic vesicle in the mucosa. The mucous membrane itself
later passes entirely around the vesicle as shown in Fig. 371, A.
The greater part of the mucosa of the uterus becomes cast off at the end
of pregnancy; thus it forms a membrana decidua, which may be subdivided
into three parts (i) the decidua basalis (or serotina) on which the implanted chorionic vesicle rests, and which forms the maternal part of the
placenta; (2) the decidua capsularis (or reflexa) which spreads over the
part of the vesicle which is toward the uterine cavity; and (3) the decidua
vera, which lines the remainder of the uterus. These subdivisions of the
decidua are indicated in Fig. 371, A.
FIG. 371. THE UTERUS AND DECIDUAL MEMBRANES IN EARLY PREGNANCY. A, AND IN LATE PREGNANCY
B. THE CORD HAS BEEN CUT AND THE EMBRYO REMOVED FROM B.
am., Amnion; am. c., amniotic cavity; c., cervix; ch., chorion; c. u., cavity of the uterus; d. b., decidua
basalis; d. c., decidua capsularis; d. v., decidua vera; m., amnion and chorion laeve drawn as one
line; pi., placenta; u. c., umbilical cord; y. s., yolk-sac.
Soon after the ovum becomes implanted, the chorion ceases to be
uniformly covered with villi. The villi toward the decidua basalis elongate and branch freely, producing the shaggy chorion frondosum; this is the
embryonic portion of the placenta. As the chorionic vesicle enlarges, the
villi directed away from the wall of the uterus, toward the decidua capsularis, become shorter and disappear, so that a large portion of the
chorion becomes smooth the chorion Iceve. Usually the umbilical cord
passes to a nearly central position in the chorion frondosum; rarely it has
a "marginal attachment" at the periphery of the frondosum, and it may
have a "velamentous insertion" in the adjacent part of the chorion laeve,
through which the umbilical vessels then extend to the frondosum.
With the growth of the embryo, which fills and distends the uterine
cavity, the decidua capsularis becomes thin, degenerates, and is resorbed,
372 HISTOLOGY
so that in the last half of pregnancy the chorion laeve rests directly against
the decidua vera (Fig. 371, B).
The placenta at birth is a discoid mass of spongy vascular tissue, about
7 in. in diameter and i in. thick, weighing a pound. It is composed of two
parts, the placenta uterina and placenta fetalis, which in certain lower
mammals can be readily separated, but in others, and in man, they cannot,
be disengaged. The uterine portion, as already stated, is the decidua
basalis, and the embryonic or fetal portion is the chorion frondosum. At
the margin of the placenta, the chorion frondosum is continuous with the
chorion laeve, which is adherent to the decidua vera. Lining the chorionic
cavity and spreading from the distal end of the umbilical cord, the amnion
forms a complete sac, with a smooth and glistening surface toward the
embryo. It is lightly adherent to the chorion laeve and to that surface of
the placenta which is toward the embryo. From the way in which the
chorion Iseve and chorion frondosum become differentiated, the fact that
small accessory placentas sometimes occur near the main mass may be
readily understood; detached groups of chorionic villi continue their growth,
and their vessels communicate with those of the adjacent placenta. Such
small accessory structures are known as succenturiate (i.e., recruited)
placentas.
Fate of the Membranes at Birth. Shortly before birth, the cervix of
the uterus dilates and the sac of membranes containing the liquor amnii
bulges into it. The membranes thus exposed are ruptured, and the
amniotic fluid escapes. The birth of the child follows, and the umbilical
cord then extends from the navel through the vagina to the placenta. The
cord is so short in some mammals that it ruptures with the expulsion of
the embryo; in other forms it is bitten off or otherwise severed, setting free
the embryo. Occasionally the membranes rupture in such a way that the
head of the infant remains more or less covered with a cap of amnion and
chorion laeve, formerly known as the "caul." After the birth of the child
the uterine musculature contracts quite rapidly, and in about half an hour
the after-birth is expelled, the sac of membranes being turned inside out
in this process. The part from the fundus of the uterus is forced out first,
and that from the lower segment of the uterus follows. Thus the amnion
and the amniotic surface of the placenta are on the outside of the afterbirth. The denuded uterine mucosa is gradually restored to its normal
condition. As after menstruation, the epithelium spreads from the glands
over the tunica propria.
The entire after-birth, since its delivery follows that of the child, was called the
secunda or secundina by the ancient anatomists. The round flat mass which is its
principal part was named the placenta by Fallopius, from its fancied resemblance to
a pan-cake. Long before this, the membranes enveloping the embryo were known as
the chorion, allantois. and amnion, and were described as the outer, middle and inner
DECIDUAL MEMBRANES
373
layers respectively. These ancient terms are of obscure derivation. Chorion (Gr.,
x6pu>v) is the same as the Latin corium, which is applied to the vascular layers of the
skin. In its Greek form it is used to designate the vascular chorioid coat of the eye,
and the chorioid plexuses of the brain, but it refers particularly to the vascular embryonic
membrane. Amnion is derived indirectly from d/wos (a sheep) and Hyrtl reasonably asks "How came the sheep to have his name enrolled in anatomy?" Whether
the amnion was first observed in the sheep, or was so named because of its softness,
or for some very different reason, is discussed by the early commentators. The
allantois was first observed in the lower mammals in which it attains great size. For
example, in the sheep and pig it forms an elongated sac filled with fluid and attached
like the arms of a "T" to the distal end of the allantoic duct. This duct, which
corresponds with the entire human allantois, issues from the ventral abdominal wall
and divides into its two branches, as seen indistinctly through the chorion in Fig.
372 (over the body of the embryo). The allantoic sac extends almost the entire length
FIG. 372. A PIG EMBRYO REMOVED FROM THE UTERUS, "SURROUNDED BY ITS THREE MEMBRANES.
(Fabricius ab Aquapendente. 1687.)
of the chorion, and its ends break through the chorionic membrane, projecting freely
as the allantoic appendages. In Fig. 372, the one at the right extends upward, and
the one at the left, downward. Such an allantois was sought for in man, between the
amnion and chorion, where a corresponding structure should be located. Hale (1701)
was among those who thought that he found one, but he declared that "most of the
ancients allow a human allantois not from their experience of it, but because they
took it for granted that men and other animals were alike in their viscera." It was
not until 1885 that it was clearly and finally stated that the human allantois was
merely a blind tube in the body stalk, never being free or vesicular (His, Anatomic
menschlicher Embryonen).
As to the appropriateness of the term allantois (sausage-like, from the Gr. dAAas
there is difference of opinion. Fabricius (De formato fcetu, 1600) one of whose
drawings is reproduced in Fig. 372, considers that the word really means "intestinal,"
or like a sausage skin.
DECIDUA VERA, AMNION, AND CHORION L^EVE.
The three structures named above may readily be included in a single
vertical section of the wall of the uterus, in the latter part of pregnancy.
Care must be taken, however, not to detach the amnion. In Fig. 373 the
374
HISTOLOGY
amnion is seen on the upper surface of the section, having its simple cuboidal or flat epithelium toward the embryo, and its mesodermic connec
Amnion.
Com pa
Cave-nous layer. <j*if^'
land
Vein
Muscularis.
Pic. 373. VERTICAL SECTION THROUGH THB WALL OF A UTERUS ABOUT SEVEN MONTHS PREGNANT
WITH THE MEMBRANES IN SITU. X 3o. (Schaper.)
live tissue toward the chorion. Adhesions in the form of slender strands
bind it to the connective tissue of the chorion. The chorionic epithelium
forms a layer over the surface of the vera; it
presents slight irregularities but is without
villi. The superficial uterine epithelium has
degenerated; it disappeared in an earlier
stage. The modified mucosa. or decidua
vera, is divisible into a superficial compact
layer and a deep cavernous layer. After the
epitheb'um of the glands in the compact
layer had degenerated and was resorbed, the
connective tissue came together obliterating
the gland cavities. The compact layer is
therefore without glands. The cells of the
tunica propria have enlarged, and become
decidual cells (Fig. 374). These cells, which
occur only in pregnancy, are flattened, round,
oval or branched structures of large size (0.03
to o.i mm.). Usually they contain a single nucleus but often there are
two or more, and in giant forms there may be thirty or forty. The
FIG. 374. DECIDUAL CELLS FROM
THE Mucous MEMBRANE OF A
HUMAN UTERUS ABOUT SEVEN
MONTHS PREGNANT. One cell
shows a mitotic figure. X 250
(Schaper.)
DECIDUAL MEMBRANES 375
cavernous layer of the mucosa contains slender clefts parallel with the
muscularis. These are glands which have been stretched laterally; some
of them retain areas of normal epithelium, but in many the epithelium has
degenerated, and from some it has wholly disappeared. The connective
tissue is but slightly modified. Throughout the decidua, but especially
in the superficial portion, the vessels are greatly distended.
PLACENTA.
The chorionic villi, the interlacing branches of which form the fetal
portion of the placenta, are shaped as in Fig. 375. The finding of such
structures in a uterine discharge or curetting is diagnostic of pregnancy. The villi in the earliest stages are composed entirely of epithelium, but they soon acquire a core of the chorionic mesenchymal tissue,
in which are the terminal branches of the umbilical vessels. The epithelium is very early divisible into two layers. The outer layer consists
XI9
FIG. 37S- ISOLATED TERMINAL BRANCHES OF CHORIONIC VILLI; THAT ON THE LEFT is FROM AN EMBRYO
OF TWELVE WEEKS; ON THE RIGHT, AT FULL TERM. (Minot.)
of densely staining protoplasm, said to contain fat granules and to present a brush border; it has dark, round or flattened nuclei. Since cell
boundaries are lacking, this is called the syncytial layer (Fig. 376).
Mitotic figures are seldom seen in it. Generally its nuclei are in a single
layer at varying distances from one another, but they may accumulate
in "knots" or "proliferation islands," especially in late stages (Fig. 377).
The knots project from the surface of the villi, so that in certain planes
of section they appear completely detached and suggest multinucleate
giant cells. The syncytial layer perhaps completely invests the villi at
first, but later it is interrupted in many places.
HISTOLOGY
The deeper layer of the chorionic epithelium consists of distinct cells
with round nuclei and clear protoplasm. Although this is a single layer
at the base of young villi, it produces great masses of cells at their tips.
These columns or caps of cells in which the villi terminate fuse with one
-Syncytium.
Cuboidal cells ofjthe
basal layer. ^f
Connective tissue.
Blood vessel containing
nucleated red corpuscles.
Oblique section of the epithelium.
FJG. 376. CROSS SECTION OF A HUMAN CHORIONIC VILLUS OF THE FOURTH WEEK OF PREGNANCY.
Epithelium
Epithelial nucleus -Capillaries <=' -
Syncytial knot. -.
Small artery. K
Syncytial knot
Epithelium.
;"/" "Small vein.
Capillary.
Syncytial knot.
FIG. 377. CROSS SECTION THROUGH A SMALLER (A) AND A LARGER (B) CHORIONIC VILLUS OF A HUMAN
PLACENTA AT THE END OF PREGNANCY. X 250. (Schaper.)
another next the decidua, and the uterine tissue seems to be dissolved as
this mass of epithelium proliferates. All the superficial epithelium of the
decidua basalis degenerates and disappears, and the underlying parts of
the blood vessels in the tunica propria are destroyed. The uterine blood
DECIDUAL MEMBRANES
377
escapes into the intervillous spaces, bounded by the syncytium, or where
this is deficient, by the basal cells. The maternal blood circulates in the
intervillous spaces as shown in the diagram Fig. 378, and does not clot.
So extraordinary is this, that attempts have been made to detect an endothelial covering for the villi, but without success. (The syncytial layer
has been considered endothelial or otherwise of maternal origin, but this
view is not accepted.) It is said that the products of the disintegration
of the maternal tissue, including haemoglobin and even entire red corpuscles, are taken up by the syncytium and used for the nutrition of the
embryo.
Chorionic villi..
Decidua
basalis.
Compact
layer.
Cavernous ,
layer.
Muscularis.
Vein.
FIG. 378. DIAGRAM OF THE HUMAN PLACENTA AT THE CLOSE OF PREGNANCY. (Schaper.)
The placenta at birth, being an inch thick, presents in cross section a
vast number of the branches of villi cut in various planes. A small fragment is shown in Fig. 379. On the left, there is a section of a large villus,
containing fibrous tissue of the loose embryonic type, in some cases forming a thin basement membrane beneath the epithelium. Each villus
contains a branch of the umbilical artery which ends in capillaries of very
large but varying caliber. They are situated close beneath the epithelial
layer, through which nutriment is transferred from the maternal blood
in the intervillous spaces to that of the embryo in the vessels of the villi.
Maternal and fetal blood never mingle, as may readily be seen in early
stages when the embryonic blood contains nucleated red corpuscles.
The two primary layers of the chorionic epithelium are difficult to
recognize in many parts of the placenta at birth. Thus in the villi shown
in Fig. 377 it is seen that the epithelium is in places hardly distinguishable
from the connective tissue. This thin portion may represent the basal
378 HISTOLOGY
layer and the dark clumps of nuclei scattered over its surface may arise
from the syncytium, but the reverse relation of the two types of epithelium to the original layers is sometimes stated. Frequently the villi
are covered in part with very conspicuous masses of hyaline material,
apparently derived from epithelial degeneration (Fig. 379). Deposits of a
substance staining deeply with eosin and resembling the fibrin of blood
clots may also be observed. This material is often in the form of layers,
with intervals between them, and is known as "canalized fibrin." It is
believed to be derived from the blood, but the origin of these deeply
staining masses is "not yet fully understood" (Stohr).
Connective tissue.
Hyaline substance in tangential section.
.cytium.
Blood vessel. Hyaline substance. . Proliferation islands.
FIG. 379- FROM A SECTION OF A HUMAN PLACENTA AT TERM. X 260.
The surface of the placenta toward the embryo is covered with amnion,
which has remained in place in the section shown in Fig. 380. Sometimes
it becomes detached in preparing the specimen. It consists of homogeneous connective tissue toward the chorion, and is covered on its free surface
by simple low columnar epithelium, sometimes containing fat droplets and
vacuoles. The chorionic membrane is a much thicker layer, consisting
of vascular connective tissue, and covered with epithelium continuous with
that of the villi. The root of a villus is cut tangentially in Fig. 380. The
epithelium at term is often in relation with the hyaline material or " canal
DECIDUAL MEMBRANES
379
ized fibrin" which partially replaces it. In Fig. 380, cells of the deeper
layer of the chorionic epithelium may still be recognized, but these are
often lacking.
Toward the uterine wall the placenta is formed by the decidua
basalis, which, like the decidua vera, includes a superficial compact layer
and a deeper cavernous layer. The compact layer, which is detached
with the placenta at birth, consists of connective tissue, blood vessels,
giant cells and decidual cells (Fig. 381). Some of the chorionic villi
Amniotic epithelium.
Homogeneous layer. SS
,v :*.
a % ...^ %
r
'
.1
Leucocytes. c^_
Connective tissue of <.
the chorion.
Chorionic'epithelium. *
Hyaline substance.
Syncytium.
Connective tissue...
Red corpuscles.
frfrsf*"
Blood vessels.
FIG. 380. FROM A SECTION OF THE HUMAN PLACENTA AT TERM. X 200.
^Chorionic villus.
have free endings toward this layer; others are extensively fused with
it, forming such masses as shown on the right of Fig. 381.
The decidua basalis extends out among the villi in the form of septa,
which subdivide the mass of villi into lobes or cotyledons. (In the ruminants, the cotyledons are widely separated by areas of smooth chorion, but
in man they are closely adjacent, with septa between them.) The septa
end before reaching the chorionic membrane, except at the placental
margin, where they form an enclosing wall. As the uterine arteries
approach the intervillous spaces of the chorion, they pursue a coiled course,
so that they may be cut several times in one section (Fig. 378). They pass,
3 8o
HISTOLOGY
without branching, into the septa of the placenta, and before they empty
into the intervillous spaces, their walls are reduced to mere endothelium.
The veins which drain the intervillous spaces are not found in the septa,
except at the placental margin. They pursue an oblique course downward
from the floor of the cotyledons, beginning as large thin-walled tubes, into
which free ends of villi may project (Fig. 378).
^Decidual cells.
"_ .Connective tissue
Cell knots.
FIG. 381. FROM A SECTION OF THE HUMAN PLACENTA AT TERM. Xa6o.
UMBILICAL CORD.
The umbilical cord is a translucent, glistening, white or pearly rope
of tissue about 2 feet in length, extending from the umbilicus to the
placenta. It consists of mucous tissue (p. 62) covered with epithelium,
and contains at birth three large blood vessels, two umbilical arteries and
one umbilical vein (Fig. 382, B). The parallel arteries generally wind
around the vein making sometimes forty revolutions. The surface of the
cord shows corresponding spiral markings and often irregular protuberances
called false knots. (True knots, tied by the intrauterine movements of the
embryo, are very rare.) There are no lymphatic vessels or capillaries in
the cord, and the large blood vessels do not anastomose. The walls of the
arteries contain many muscle fibers but very little elastic tissue, and they
are usually found collapsed in sections; their contraction is of interest since
nerves have been traced into the cord for only a very short distance. The
vein generally remains open.
UMBILICAL CORD 381
The umbilical arteries arise in young embryos as the main terminal branches into
which the dorsal aorta bifurcates. These vessels curve ventrally on either side of
the pelvis and pass out through the cord to the chorion; they are equidistant from the
allantois which they accompany. In the adult the parts of these vessels near the
aorta are known as the common iliac arteries, and the small offshoots from them which
FIG. 382. CROSS SECTION OF UMBILICAL CORDS.
A, from an embryo of two months, X 20; B, at birth, X 3. aL, Allantois; art., artery; coe., coelom; ?..
vein; y. s. t yolk stalk.
have grown down the limbs, have become the external iliac arteries. The distal course
of the original vessels may still be followed through the hypogastric arteries (internal
iliacs) up on either side of the median line to the navel; toward the navel the vessels
have become reduced to slender cords. The umbilical vein, within the cord, represents
the fusion of a pair. On entering the body it conveys the blood from the placenta,
through the persistent left umbilical vein, directly to the under side of the liver, which it
crosses as the ductus venosus, and then empties
into the vena cava inferior. In the adult, its
former course is marked by the round ligament of
the liver and the ligament of the ductus venosus.
The allantois, which the umbilical vessels accompany, at first extends the entire
length of the cord as a slender epithelial
tube. Its condition at three months is
shown in Fig. 383. At birth, it has become reduced to a very slender, and generally interrupted, solid strand of epithelial cells. That it may retain its continuity is stated by Ahlfeld (Arch. f.
Gynak., 1876, vol. 10, p. 81). This remnant may be sought for near the
body of the embryo, and its tendency to retain its original position equidistant from the umbilical arteries is the best guide for locating it. By
the use of Mallory's connective tissue stain, the epithelial cells may be
FIG. 383. CROSS SECTION OF THE ALLANTOIC DUCT, FROM THE UMBILICAL
CORD OF A HUMAN EMBRYO OF THREE
MONTHS. X34O. (Minot.)
Ent., Entodermal epithelium; mes., mesenchyma.
382
HISTOLOGY
stained red in contrast with surrounding blue fibrils. Within the body of
the embryo the allantois is prolonged to the upper end of the bladder,
with which it is continuous; this intra-abdominal part has long been
called the urachus (i.e., vas urinarium). If it remains pervious at birth,
which is abnormal, urine may escape at the umbilicus.
The yolk stalk, surrounded by an extension of the body cavity, is
found in young umbilical cords (Fig. 382, A). This stalk is a slender strand
of mesoderm, containing the entodermal vitelline duct, and the vitelline
vessels which accompany it to the yolk-sac. The loop of intestine
from which the yolk stalk springs may also extend into the cavity of the
cord, and if it has not been drawn into the abdomen at birth, umbilical
hernia results. If the cavity of the vitelline duct remains pervious at
birth, the intestinal contents may escape at the umbilicus. (Such a condition is known as a fecal fistula, whereas the pervious urachus constitutes
a urinary fistula.) Ordinarily the yolk stalk and its vitelline vessels, together with the ccelom of the cord, have been obliterated before birth, so
that no trace of them remains in sections of the cord.
FIG. 384. YOLK-SAC A NO PERSISTENT
VITELLINE VESSELS, EXPOSED BY REFLECTING THE AMNION AT THE DISTAL
END OF THE CORD. (Lonnberg.)
FIG. 385. PART OF A HUMAN AMNIOTIC
VlLLUS. X 330.
Ep., Epitrichium; S. C., stratum corneum;
S. g., stratum granulosum; S. G.,
stratum germinativum; M. B., homogeneous layer; F. T., fibrous tissue;
. T., areolar tissue.
ge
A.
The yolk-sac may be found with almost every placenta, as a very small
cyst adherent to the amnion in the placental area. If the distal end of
the cord is gently stretched, a wing-like fold appears (Fig.. 384), differing
from all others by containing no large vessels; the fold indicates the direction of the yolk-sac, which may be exposed by stripping the amnion from
the chorion. It may be beyond the limits of the placenta. Further
details will be found in Lonnberg's admirable Studien tiber das Nabelblaschen, Stockholm, 1901.
Amniotic villi are irregular, flat, opaque spots on the amnion near the
Distal end of the cord. They are often present and may suggest a diseased
UMBILICAL CORD 383
condition. As seen in Fig. 385 they are areas of imperfectly developed
skin, and as shown in this case (Lewis, Art. "Umbilical Cord," Buck's
Hdb., 2nd ed.) they present all of its fundamental layers. Frequently
these cornified areas are less fully developed. They have been compared
with the pointed epithelial elevations which cover the surface of the
umbilical cord in ruminants, but the latter do not appear as areas of
imperfect skin, and probably are entirely different structures. They may
appropriately be called villi, but the human "villi" scarcely rise above
the surface. Their significance is unknown.
VAGINA AND EXTERNAL GENITAL ORGANS.
The vagina consists of a mucosa, submucosa, muscularis and fibrosa.
Its epithelium is thick and stratified, its outer cells being squamous and
easily detached. It rests upon the papillae of the tunica propria, and
is thrown into coarse folds or ruga. Glands are absent. The tunica
propria is a delicate connective tissue with few elastic fibers, containing
a variable number ^ of lymphocytes. Occasionally there are solitary
nodules, above which numerous lymphocytes wander into the epithelium.
The submucosa consists of loose connective tissue with coarse elastic
fibers. The muscularis includes an inner circular and a small outer
longitudinal layer of smooth muscle. The fibrosa is a firm connective
tissue, well supplied with elastic elements. Blood and lymphatic vessels
are found in the connective tissue layers, and wide veins form a close
network between the muscle bundles. There is a ganglionated plexus of
nerves in the fibrosa.
The mucous membrane of the vestibule differs from that of the vagina
in possessing glands. The numerous lesser vestibular glands. 0.5-3 mm. in
diameter, produce mucus; they occur chiefly near the clitoris and the
outlet of the urethra. The pair of large vestibular glands (Bartholin's)
also produce mucus; they correspond with the bulbo-urethral glands in
the male and are of similar structure. The hymen consists of fine-fibered,
vascular connective tissue covered with mucous membrane. The clitoris
is an erectile body; resembling the penis. It includes two small corpora
cavernosa. The glans clitoridis contains a thick net of veins. It is not,
as in the male, at the tip of a corpus cavernosum urethrae which begins
as a median bulb in the perineal region; the bulbus in the female exists
as a pair of highly vascular bodies, one on either side of the vestibule.
Each is called a bulbus vestibuli. The labia minora contain sebaceous
glands, 0.2-2.0 mm. in size, which are not connected with hair follicles;
they first become distinct between the third and sixth years. The labia
majora have the structure of skin.
384
HISTOLOGY
ectoderm
epidermis
SKIN.
The skin (cutis) consists of an ectodermal epithelium, the epidermis,
and a mesodermal connective tissue, the corium (Fig. 386). The ectoderm is at first a single layer but it soon becomes double, the outer cells
staining more deeply, and being notably larger than the inner cells. Their
characteristic dome shape is
seen in the figure. The outer
layer has been named the
epitrichium, since the hairs
which grow up through the
underlying epithelium do not
penetrate it, but cause it to be cast off. The epitrichium has been
found on the umbilical cord and in places on the amnion. It may
possibly be related to the chorionic syncytium. The deeper layer of
ectoderm becomes stratified, and gives rise to the hairs, nails, and
FIG. 386. SKIN FROM THE OCCIPUT OF AN EMBRYO OF Two
AND ONE-HALF MONTHS. (After Bowen.)
The outer layer of dark cells is the epitrichium.
Duct of a sweat
gland.
Coil of a sweat
gland.
Stratum corneum.
Stratum lucidum
Stratum granulosum.
Stratum germinativum
Stratum
papillare.
Corium.
Stratum
reticulare. J
Epidermis.
Stratum subcutaneum.
FIG. 387. VERTICAL SECTION FROM THE SOLE OF THE FOOT OF AN ADULT. X 25.
enamel organs. It also produces two types of glands, the sebaceous
glands which are usually connected with hairs, and the sweat glands.
These are widely distributed; locally the ectoderm forms the mammary glands, ceruminous glands of the ear, ciliary glands of the eyelids,
SKIN
385
Epidermis.
and other special forms. The greater part of the surface of the skin
presents many little furrows, the sulci cutis, which intersect so that they
bound rectangular spaces. On the palms and soles the furrows are parallel
for considerable distances, being separated from one another by slender
ridges, the cristce cutis, along the summits of which the sweat glands open.
The ridges are most highly developed over the pads of tissue at the finger
tips, where they present the familiar spiral and concentric patterns.
These pads of connective tissue, the toruli tactiles, must not be confounded
with elevations due to underlying muscles.
In the pentadactylous mammals, each extremity typically presents five digital
toruli, at the tips of the fingers or toes; four interdigital toruli, near the metacarpo- or
metatarso-phalangeal joints; and two or three proximal cushions a tibial and an
elongated fibular; or a radial and two ulnar, one behind the other. Often the interdigital cushions fuse, as in the paw of the cat and the ball of the human foot, and the
one between the thumb and fingers may be suppressed. These toruli are very prominent in the embryo. According to
Miss Whipple (Zeitschr. f. Morph. u.
Anthr., 1904, vol. 7, pp. 261-368)
they are primarily walking pads,
witfi ridges at right angles to the slipping force. Usually they are considered primarily tactile. The extensive literature pertaining to them
has been reviewed by Schlagenhaufen
(Anat. Hefte, 1906, Abt. II, vol. 15,
pp. 628-662).
Corium.
Corium. The corium is a
layer of densely interwoven
bundles of connective tissue extending from the epidermis to
the fatty, areolar subcutaneous
tissue (Fig. 387). Toward the
epidermis the corium forms papilla, which vary considerably in size and number in different parts of the
body. They are tallest (even 0.2 mm. high) and most numerous, often
being branched, in the palms and soles. Beneath the epidermal ridges
they may occur quite regularly in double rows (Fig. 388), as long since
observed by Malpighi. In the skin of the face the papillae are poorly
developed, and in advanced age they may wholly disappear. The papillae
are composed of cellular connective tissue, which forms a tunica propria;
and each papilla contains a terminal knot of capillary blood vessels, or a
tactile corpuscle (Fig. 152, p. 159). The corpuscles are most numerous
in the sensitive finger tips, where they may be found in one papilla in
every fur.
25
Papillae under Tactile
the ridge A. corpuscle.
Papillae under
the ridge D.
PIG. 388. VERTICAL SECTION FROM THE SOLE OF THB
FOOT OF AN ADULT, SHOWING FOUR RIDGES (A-D)
WITH A PAIR OF PAPILLA BENEATH EACH. Between
the papillae of D is the duct of a sweat gland. X 25.
386
HISTOLOGY
The entire corium is somewhat arbitrarily subdivided into an outer
stratum papillare and an inner stratum reticulare (Fig. 387). These layers
blend with one another, but the outer portion consist of finer bundles of
connective tissue, more closely interwoven than those in the coarse network characteristic of the stratum reticulare. Beneath the skin, but inseparable from it, is the stratum subcutaneum, which is composed of areolar
tissue with large areas of fat cells; where the fat forms a continuous layer,
it is known as the panniculus adiposus. Finally the bundles of the stratum
subcutaneum connect more or less intimately with the fascia around the
muscles, or, in places, with the periosteum.
The elastic fibers of the corium form evenly distributed networks,
which are finer in the stratum papillare and coarser in the stratum
reticulare. There is said to be a subepithelial network, and a layer of
Depressions which
were occupied by
papillae.
Ridge corresponding
to a furrow of the
corium.
Portion of the duct of
a sweat gland.
FIG. 389- EPIDERMIS DETACHED FROM THE DORSUM OF THE HUMAN FOOT, SEEN
FROM THE LOWER SURFACE. Xi2o.
The dark epithelial network between the papilla is the rete Malpighii.
numerous coarse fibers immediately above the general layer of fascia.
In old age a notable decrease in the elastic fibers has been recorded.
The muscle fibers of the corium are chiefly the small bundles of smooth
muscle attached to the sheaths of the hairs, forming the arrectores
pilorum. Smooth muscle is diffusely distributed in the nipple, and in the
scrotum it forms a layer pervaded by elastic tissue, known as the tunica
dartos. Striated muscle fibers derived from the muscles of expression
terminate in the skin of the face. The vessels and nerves of the corium
are described on page 399.
Epidermis. If a piece of skin is boiled, the epidermis may be stripped
off, carrying the tunica propria with it; and the epidermis itself may be
separated into two layers. The outer layer is the stratum corneum; the
inner is the stratum germinativum.
The stratum germinativum was formerly called the stratum mucosum or rete
Malpighii. It was first described by Malpighi who recognized its soft or "mucous"
SKIN
387
nature, and referred to it as a rete since it forms a network between the papillae of
the corium (Fig. 389). Malpighi considered that the color of the Ethiopian skin was
confined to this layer.
The stratum germinativum and stratum corneum are subdivisions of a
single thick stratified epithelium. The basal cells, which rest directly
upon the papillae of the corium, constitute a single row of columnar cells,
with elongated nuclei and no cell walls (Fig. 390). Through mitotic
division these cells multiply and give rise to the outer polygonal cells,
but it is noteworthy that mitotic figures are seldom seen. The polygonal
cells which form the bulk of the stratum germinativum are connected with
one another by slender intercellular bridges (Fig. 43, p. 53), through which
fibrils pass from cell to cell. Because of this striking feature, the stratum
germinativum was formerly called the stratum spinosum.
Stratum
corneum.
Stratum
germinativum
FIG. 390. THE DEEPER PART OF THE EPIDERMIS, FROM THE SOLE
OF THE FOOT OF AN ADULT MAN. X 3&O.
The transition from the stratum germinativum to the stratum corneum
is abrupt. It may be marked by an incomplete layer of coarsely granular
cells, such as are highly developed in the skin of the palms and soles, where
they form the stratum granulosum (Fig. 390). In the stratum corneum
the cells acquire a horny exoplasmic membrane; the bridges become
short stiff spines; the protoplasm and nucleus are dry and shrunken; ane
in the outermost cells the nucleus wholly disappears. The cells becomd
flatter toward the surface, from which they are constantly being desquamated.
388
HISTOLOGY
The process of cornification presents a more elaborate picture in
sections of the palms and soles. Outward from the stratum germinativum
there is a darkly staining, coarsely granular layer, one or two cells thick,
which is followed by a clear, somewhat refractive band in which the cell
outlines are indistinct. This layer seems saturated with a dense fluid
formed by dissolution of the underlying granules. In haematoxylin
and eosin specimens, the granular layer or stratum granulosum is followed
by a pink and then by a bluish band, which are subdivisions of the clear
stratum lucidum. These' are followed by a very thick stratum corneum.
Except in the palms and soles, the granulosum is thin and the lucidum is
absent. Chemically the coarse granules of the stratum granulosum resemble the horny substance keratin (from which they differ by dissolving in
caustic potash) ; they are therefore called kerato-hyalin granules. Their
diffuse product in the stratum lucidum is named eleidin. In the corneum
it becomes pareleidin, which, like fat, blackens with osmic acid, but the
reaction occurs more slowly. The pareleidin is not due to fat entering
the skin from oily secretions on its outer surface. Further information
regarding these substances is supplied by Pinkus (Keibel and Mall's
Human Embryology, vol. i).
The color of the skin is due to fine pigment granules in and between
the lowest layers of the epidermal cells. Underlying cells of the corium
sometimes contain groups of finer pigment granules, but such cells are
absent from the palms and soles and are infrequent elsewhere. They
may be found in the deeply pigmented circum-anal tissue, and in the
eyelids.
NAILS.
The nails are areas of modified skin consisting of corium and epithelium. The corium is composed of fibrous and elastic tissue, the bundles
Nail.
Corium.
Stratum
germinativum.
Nail wall.
Nail groove.
Bone of third
phalanx.
FIG. 391. DORSAL HALF OF A CROSS SECTION OF THB THIRD PHALANX OF A CHILD. X 15.
The ridges of the nail bed in cross section appear like papillae.
of which in part extend vertically between the periosteum of the phalanx
and the epithelium, and in part run lengthwise of the finger. In place of
papillae, the corium of the nail forms narrow longitudinal ridges, which
NAILS
389
are low near the root of the nail but increase in height toward its free
distal border; there they abruptly give place to the papillae of the skin.
The epithelium consists of a stratum germinativum and a stratum corneum.
The latter, according to Bo wen (Anat. Anz., 1889, vol. 4, pp. 4 2 1-4 50) ,
represents a greatly thickened stratum lucidum, but this opinion requires
confirmation. In the embryo the horny substance is entirely covered by
a looser layer, the eponychium, and this name is applied in the adult to
the skin-like tissue which overlaps the root and sides of the nail (Fig. 391).
The eponychium is the stratum corneum of the adjoining skin.
It is now generally considered that the cells of the stratum germinativum covering the greater part of the "nail bed" do not produce any of the
overlying horny material. This function is reserved for the germinative
cells at the root of the nail, beneath the crescentic white area, the lunula,
and its extension backward under the nail fold. The
latter is a fold of skin which is deep at the root of the
nail, but becomes shallower as it extends forward on
either side, bounded by the nail wall (Fig. 391). It is
now stated that cornification in the nails takes place without the formation of kerato-hyalin granules, and a fibrillar
arrangement of the keratin has been thought to account
for the whiteness and opacity of the lunula. The cornified cells of the nail may be separated by placing a fragment in a strong solution of caustic potash and heating to boiling. The
cells differ from those in the outer layers of the skin by retaininig their
nuclei (Fig. 392).
FIG. 392 CELLS
OF A HUMAN
NAIL. X 240.
Development.
Epidermis.
Epithelial column.
HAIR.
The hairs arise as local thickenings of the epidermis.
They soon become round columns of
ectodermal cells extending obliquely
downward into the corium (Fig. 393).
As the columns elongate the terminal
portion becomes enlarged, forming the
bulb of the hair, and a mesodermal
papilla occupies the center of the bulb.
On that side of the epithelial column
which from its obliquity may be called
the lower surface, there are found two
Mesenchyma. .... , . N .
FIG. 393-VEKTicAL SECTION OF THE SKIN OF swellings (Figs. 394-396). The upper
MoNTHs K x F 2 A 3 " UMANEMBRYOOFFlVE is to become a sebaceous gland, discharging its secretion into the epithelial
column; the lower or deeper swelling is called the "epithelial bed," and
390
Epidermis.
HISTOLOGY
Cells of the hair canal.
Epithelial Part of a
bed. hair
column.
Tip of the
inner
sheath.
, V Arrector
u muscle.
FIG. 394. VERTICAL SECTION OF THB
SKIN OK THE GLUT^AL REGION
OF A HUMAN EMBRYO OF FIVE
MONTHS. X 230.
Root of
the hair.
FIG. 395. VERTICAL SECTION OF THE
SKIN OF THE BACK OF A HUMAN
EMBRYO OF FIVE AND A HALF
MONTHS. X 230.
Sebaceous
gland,
Outer
sheath.
Epithelial
bed.
Hyaline membrane.
Papilla.
Tangential section of the outer sheath. ,- ' ~+*f *?** **^
Cornified inner sheath.
Cell nuclei of the sheath cuticle
of
FIG. 396. VERTICAL SECTION OF THE SKIN OF THE FOREHEAD OF A HUMAN
EMBRYO OF FIVE MONTHS. X23O. Differentiation of the sheaths of the hair.
HAIR
391
Blood vessel.
Hair canal.
Epithelium.
its cells, which increase by mitosis, contribute to the growth of the column. (The lower swelling is often described as the place of insertion
of the arrector pili muscle). Beginning near the bulb, the core of the
column separates from the peripheral cells; the latter become the outer
sheath of the hair. The core forms the inner sheath and the shaft of the
hair. The cells of the shaft become cornified just above the bulb, and
they are surrounded by the inner sheath as far as the sebaceous gland.
Beyond this point the inner sheath degenerates, so that in later stages
the distal part of the shaft is immediately surrounded by the outer
sheath. As new cells are
added to the hair from
below, the shaft is pushed
toward the surface. The
central cells in the outer
end of the column degenerate, thus producing a
"hair canal" which is prolonged laterally in the epidermis (Fig. 397). The
shaft enters the canal,
breaks up the overlying
epitrighium, and projects
from the surface of the
body. That portion of the
hair which remains beneath the epidermis is
- -"
--.-"- Degenerating inner
*^"-C sheath.
Epithelial bed.
I ',." , Outer sheath.
called its root,
tion to the
In addiepithelial
FIG. 397. VERTICAL SECTION OF THE SKIN OF THE BACK'OF A
HUMAN EMBRYO OF FIVE AND A HALF MONTHS. X 120.
The staining with iron hsematoxylin has made the horny parts so
black that their details are invisible.
sheaths, the root in all
larger hairs possesses a connective tissue sheath, derived from the corium.
This serves for the insertion of a bundle of smooth muscle fibers, the other
end of which is connected with the elastic and fibrous elements in the superficial part of the corium. Since this muscle by contraction causes the
hair to stand on end, it is called the arrector pili. Its insertion is always
below the sebaceous gland and on the lower surface of the hair, as shown
in Fig. 398. The hairs which cover the body of the embryo, persisting
after birth to a variable extent, are soft and downy, and are known
as lanugo. Arrector muscles are absent from the lanugo of the nose,
cheeks and lips, and also from the eyelashes (cilia) and nasal hairs
(vibrissae) .
Adult Structure. The general appearance of hairs in sections of the
adult skin is shown in Fig. 398, which includes also the sebaceous glands
emptying into the sheaths of the hairs, and sweat glands which are usually
392
HISTOLOGY
entirely separate structures. Occasionally a sweat gland opens into the
sheath of a hair near its outlet. Each hair consists of a papilla, bulb and
shaft, together with sheaths around the root, namely an inner and outer
epithelial sheath and, external to these, a connective tissue sheath.
These structures, together with the arrector pili muscle which is inserted
into the connective tissue sheath, are indicated in Fig. 398, but they are
Shaft of a hair. --Stratum corneum.
Stratum germinativum. 'V~
Corium. - i_
(': ft
Sebaceous gland.
M. arrector pili.
Sweat gland.
Outer epithelial sheath.
Inner epithelial sheath. Medulla.
Cortex.
Conn, tissue sheath.
Bulb.
Papilla.
Stratum subcutaneum.
Epicranial tendon. --
FIG. 398. THICK SECTION OF THE HUMAN SCALP. X 20. .
shown in detail in the longitudinal section, Fig. 399, and in the transverse
sections, Figs. 401-405. They may be described as follows:
The connective tissue sheath, derived from the corium, is found around
the roots of the coarser hairs, but is absent from the lanugo. It may be
subdivided into three concentric layers. The outermost consists of loose
connective tissue with longitudinal fibers, and contains elastic tissue and
numerous vessels and nerves. The middle layer, which is thicker, consists
of circular bundles of connective tissue without elastic fibers. The inner
HAIR
393
layer, also free from elastic tissue, is sometimes longitudinally fibrous, and
sometimes homogeneous. It forms the outer stratum of the hyaline (or
vitreous) membrane, and is continuous below with the thin but distinct
Cortical substance.
Shaft of the hair.
Longitudinal fiber
layer.
Circular fiber layer.
Outer layer of the hya' line membrane.
Inner layer of the hyaline membrane.
Outer epithelial
sheath.
Henle's layer.
Huxley's layer
Cuticle of the inner
'sheath.
Papilla
FIG. 309. LONGITUDINAL SECTION OF THE LOWEST PART OF THE ROOT OF A HAIR. (From a section of
the human scalp.) X 200.
The kerato-hyalin granules are colored red.
394
HISTOLOGY
Cortex.
Medulla.
layer which covers the papilla (Fig. 399) . An inner stratum of the hyaline
membrane is formed, according to Stohr, from the epithelial cells of the
root sheath. This inner stratum is provided with fine pores, and is
always clear and homogeneous. It may unite with the connective tissue
stratum so that both may appear as a single membrane. The connective
tissue sheath is found fully developed only around the lower half of the root.
The outer epithelial sheath is an inpocketing of the epidermis. The
stratum corneum extends to the sebaceous gland; the stratum granulosum
continues somewhat deeper, but only a thinned stratum germinativum
can be followed to the bulb. All of these are included in the outer epithelial sheath (Figs. 401-405, I, II, and 5).
The inner epithelial sheath extends from the sebaceous gland to the bulb.
It begins as a layer of cornified cells below the termination of the stratum
granulosum, but it is not a continuation of
that layer. Toward the bulb the inner
sheath is divisible into two layers. The
outer or Henle's layer consists of one or two
rows of cells with occasional atrophic nuclei;
for the most part they are non-nucleated.
The inner or Huxley's layer is a row of
nucleated cells. The inner surface of Huxley's layer is covered by a membrane, the
cuticula of the sheath, composed of nonnucleated cornified scales. Traced downward, the elements of the inner epithelial
sheath and its cuticula all become nucleated
cells, but the layers may be distinguished
almost to the neck of the papilla. There
they lose their sharp boundaries, but may
still be distinguished from the pigmented
cells of the bulb. Traced upward, it is
found that kerato-hyalin granules appear in Henle's layer at the level of the
papilla, and in Huxley's layer somewhat higher (Fig. 399) ; still higher these
granules disappear and the cells of the inner sheath become cornified.
The shaft of the hair is entirely epithelial; it consists of cuticula,
cortex and medulla (Fig. 400). The cuticula, which covers its surface, is a
thin layer formed of transparent scales directed from the center of the shaft
outward and upward, thus overlapping like inverted shingles. This
arrangement is readily seen in wool and the hairs of various mammals, but
is much less evident in human hair. The cuticula is composed of nonnucleated cornified cells.
The greater portion of the shaft is included in the cortex. Toward the
bulb, the cortex consists of soft round cells; distally these cells become corni
Cuticula.
FIG. 400. PART OF A WHITE HUMAN
HAIR. X 240.
HAIR
395
/ ' / *)* " ' . N
- '/< S'* ^ ***^ .*
' "^ ^*Tz:^' i f V* ' v*
^xS?^^/V..\
Bulbus pili. -^
Papilla
-..
FIG. 401.
FIG. 405.
FIGS. 401-405. FOUR CROSS SECTIONS OKA HAIR OF THE HEAD (X 160). WITH A DIAGRAMMATIC LONGITUDINAL VIEW FOR ORIENTATION.
A, Cuticula; B, cortex; C, medulla. I, Str. corneum; II, str. germinativum; HI, corium. 1-3, Connectivetissue sheath; i, longitudinal fiber layer; a, circular fiber layer; 3, conn, tiss hyaline membrane; 4;
epithelial hyaline membrane; 5, outer epithelial sheath; 6, inner epithelial sheath; 6a, Henle's layer,
6b, Huxley's layer; 7, cuticula of the sheath; Muse., arrector pili; Seb., sebaceous gland.
396
HISTOLOGY
fied, elongated and very closely joined together. Their nuclei are then
linear. The cortex of colored hairs contains pigment both in solution
and in the form of granules. These
granules are partly within the cells,
and partly between them. Moreover
every fully developed hair contains
minute intercellular air-spaces, found
within both cortex and medulla. But
a medulla is lacking in many hairs, and
when present, in the thicker hairs, it
does not extend their whole length. It
consists of cuboidal cells containing
kerato-hyalin (Fig. 399), and generally
arranged in a double row. Their nuclei are degenerating.
Growth and Replacement of Hairs. The growth of the shaft, and of the
inner epithelial sheath with its cuticula, takes place through continued
Remains of inner
sheath.
Epithelial bed.
FIG. 406. FOUR STAGES IN THE SHEDDING OF
A HAIR. FROM THE SKIN OF THE NOSE
OF A SEVEN AND ONE-HALF MONTHS
EMBRYO. X 50.
Parts of A and B are shown enlarged in Figs.
407 and 408.
Cornified bulb.
Remains of inner
sheath.
Cornified
bulb.
Epithelial cord.
Atrophic papilla.
Connective tissue.
FIG. 407. LOWER PART OF FIG. 406, A. X230. FIG. 408. LOWER PART OF FIG. 406, B. X 230.
mitotic division of the epithelial matrix cells of the bulb of the hair. These
become cornified, and are added from below to the cells previously cornified. Accordingly the oldest cells are at the tip of the hair and the young
HAIR
397
est are immediately above the bulb. The outer epithelial sheath grows in
a radial direction from the inner surface of the hyaline membrane toward
the shaft.
Shortly before and after birth, there is a general shedding of hair, subsequent to which the loss and replacement of individual hairs is constantly
taking place. A hair of the scalp is said to last 1600 days, but the duration
of other hairs has not been definitely determined. The process of removal
begins with a thickening of the hyaline membrane and circular fiber
sheath. The matrix cells cease to produce, first the inner epithelial sheath,
and then the cuticulae and shaft. The hollow bulb becomes a solid cornified "club." The matrix cells increase without differentiating into hair
cells or sheath cells, and the clubbed hair, with its inner sheath, is forced
outward to the level of the orifice of the sebaceous gland, where it may
remain for some time (Fig. 406, D). The lower part of the outer epithelial
sheath, which has become empty, forms an epithelial strand which shortens
and draws the papilla upward; but the connective tissue sheath remains
behind, forming the "hair stalk." After some time, the columnar cells
of the epithelial bed proliferate, causing the epithelial cord to return to its
former depth (Figs. 407 and 408), and a new hair develops in the old sheath
upon the old papilla. The new hair in growing toward the surface completes the expulsion of its predecessor, which is dislodged together with
cells of the adjacent epithelial bed.
SEBACEOUS GLANDS.
The sebaceous glands are simple, branched or unbranched alveolar
structures situated in the superficial layer of the corium and usually ap
Epidermis. J f, ' =
Corium.
Cell with shrunken
nucleus.
Cell with well-developed drops of secretion.
l{_ Cell with developing
drops of secretion.
Cuboidal cell
Pig. 400. A, PROM A VERTICAL SECTION THROUGH THE ALA NASI OF A CHILD. X 40. C. Stratum corneum; M, stratum genninativum; t, sebaceous gland consisting of four sacs, a, duct of the same; w,
lanugo hair, about to be shed; h, sheath of the same, at the base of which a new hair, z, is forming.
B, FROM A VERTICAL SECTION OF THE SKIN OF THE ALA NASI OF AN INFANT. X 240. Sac of a sebaceous
gland containing gland cells in various stages of secretion.
pended to the sheath of a hair (Fig. 398). In connection with the lanugo,
a large gland may be associated with a very small bair (Fig. 409), and in
398 HISTOLOGY
exceptional cases as at the margin of the lip or on the labia minora, they
occur independently of hairs. They vary in size from 0.2 to 2.2 mm., the
largest being found in the skin of the nose where the ducts are macroscopic. None are found in the palms or soles, where hairs also are absent.
The short duct is a prolongation of the outer epithelial sheath of the hair
and is formed of stratified epithelium, the number of layers of which decreases toward the alveoli. The alveoli consist of small cuboidal basal
cells, and of large rounded inner cells in all stages of fatty metamorphosis.
As the cell becomes full of vacuoles, the nucleus degenerates, and the cell
is cast off with its contained secretion. In life the product of the glands is
a semi-fluid material, composed of fat and broken-down cells.
Glandules prceputiales are sebaceous glands without hairs which are
sometimes, but not always, found on the glans and praeputium penis.
The designation "Tyson's glands" is not justified since Tyson described
the epithelial pockets ^ to i cm. long which regularly occur near the frenulum praeputii. Praeputial glands and crypts are not found in the embryo.
The praeputium is united to the outer surface of the glans by an epithelial
mass, which often persists after birth and is broken up by the formation
of concentric epithelial pearls. Glands and crypts are absent from the
praeputium and glans of the clitoris.
SWEAT GLANDS.
The glandula sudoriparce are long unbranched tubes terminating in a
simple coil (described by Oliver Wendell Holmes as resembling a fairy's
intestine, Fig. 410). The coil is found in the deep part of the corium or
in the subcutaneous tissue (Fig. 387). The duct pursues a straight or
somewhat tortuous course to the epidermis
which it enters between the connective tissue
papillae. Within the epidermis its spiral windings are pronounced (Fig. 387); it ends in a
pore which may be detected macroscopically.
The epithelium of the ducts consists of two
or three layers of cuboidal cells; it has an inner
cuticula, and an outer basement membrane
FIG. 410. MODEL OF THE COILED j i_ i -j. j- i
PART OF A SWEAT GLAND covered by longitudinal connective tissue fibers.
FROM THE SOLE OF THE FOOT. tTT-, 1 ,1 i . .
(After Huber.) Within the epidermis its walls are made of cells
of the strata through which it passes. The
secretory portion of the gland (3.0 mm. long according to Huber) forms
about three-fourths of the coil, the duct constituting the remainder. The
secretory epithelium is a simple layer of cells, varying from low cuboidal
to columnar, according to the amount of secretion which they contain.
Those filled with secretion present granules, some of which are pigment and
SWEAT GLANDS
399
fat. The product is eliminated through intra- and intercellular secretory
capillaries. It is ordinarily a fatty fluid for oiling the skin, but it becomes
the watery sweat under the influence of the nerves. The gland cells are
not destroyed by either form of activity. The secretory tubule is surrounded by a distinct basement membrane, within which there is a row
of small longitudinally elongated cells described as muscle fibers. They
do not form a complete membrane, and they appear as a continuation of
the basal layer of cells of the ducts.
Sweat glands are distributed over the entire skin, except that of the
glans and the inner layer of the praeputium penis. They are most numerous in the palms and soles. In the axilla there are branched sweat glands
and large forms with 30 mm. of coiled tube. They acquire their large size
at puberty and have been considered as sexual "odoriferous" glands. In
the vicinity of the anus there are also branched sweat glands, together
with the large unbranched "circum-anal glands."
A. Duct in
cross section.
Nuclei of Muscle
gland cells, fibers.
Membrana propria.
Cuticula.
Muscle fibers
B. Columnar epithelium
from the coiled tubule.
C. Surface view
of the coiled tubule.
D. Low epithelium from
a coiled tubule.
Membrana propria.
Muscle fibers.
Muscle nucleus.
Cuticula.
>.-^\l*/ Membrana propria
Muscle fiber.
E. Cross section of
coiled tubule.
FIG. 411. A-D, FROM A SECTION OF THE SKIN OF THE AXILLA; E, FROM THE FINGER TIP OF A MAN OF
TWENTY-THREE YEARS. X 230. E is not a true cross section.
VESSELS AND NERVES or THE SKIN.
The arteries proceed from a network above the fascia, and branch as
they ascend toward the surface of the skin. Their branches anastomose,
forming a cutaneous plexus in the lower portion of the corium. From
this plexus branches extend to the lobules of fat and to the coils of the
sweat glands, about which they form "baskets" of capillaries. Other
branches pass to the superficial part of the corium where they again anastomose, forming a subpapillary plexus, before sending terminal arteries
into the papillae. The subpapillary plexus sends branches also to the
sebaceous glands and hair sheaths, but the papilla of a hair receives an
independent artery. The veins which receive the blood from the superficial capillaries form a plexus immediately beneath the papillae, and
sometimes another just below the first and connected with it. The veins
from these plexuses accompany the arteries and the ducts of the sweat
400
HISTOLOGY
glands to the deeper part of the corium, where they branch freely, receiving the veins from the fat lobules and sweat glands. Larger veins continue into the subcutaneous tissue where the main channels receive specific
names.
The lymphatics form a fine- meshed plexus of narrow vessels beneath
.Epidermis.
Branches of the subpapillary arterial plexus.
Veins of the second superficial plexus.
Veins along the duct of a
sweat eland.
Large vein. Vessel to the Vessel to the
fat tissue. sweat gland.
FIG. 412. PART OF A VERTICAL SECTION OF THE INJECTED SKIN OF THE SOLE OF THE FOOT. X 20.
The veins are not completely filled by the injection.
the subpapillary network of blood vessels, receiving tributary loops from
the papillae. This plexus empties into a wide-meshed subcutaneous
plexus. There are lymphatic vessels around the hair sheaths, sebaceous
glands, and sweat glands.
The nerves form a wide-meshed plexus in the deep subcutaneous tissue,
and secondary plexuses as they ascend through the skin. The sympathetic,
CUTANEOUS NERVES
401
non-medullated nerves supply the numerous vessels, the arrector pili
muscles, and the sweat glands; an epilamellar plexus outside of the basement membrane sends branches through the membrane to terminate
in contact with the gland cells. Medullated sensory nerves end in the
various corpuscles already described, and in free terminations, some
being intraepithelial. Medullated fibers to the hairs lose their myelin
and form elongated free endings with terminal enlargements in contact
with the hyaline membrane. (The nerves to the tactile hairs of some
animals penetrate the hyaline membrane and terminate in tactile menisci
among the cells of the outer epithelial sheath.) Small, round or discoid
elevations of the epidermis, visible with the naked eye, occur close to the
hairs as they emerge from the skin, being on the side toward which the
hairs slope. These "hair discs" (Pinkus) are said to be abundantly supplied with nerves. The corium beneath the nails is rich in medullated
nerves, the non-medullated endings of which enter the Golgi-Mazzoni
type of lamellar corpuscle (having a large core and few lamellae), or they
form knots which are without capsules. Elsewhere the skin contains
tactile corpuscles in its papillae and lamellar corpuscles in the subcutaneous
tissue, together with free endings in the corium and epidermis (as far out
as the stratum granulosum) .
MAMMARY GLANDS.
In young mammalian embryos generally, the mammary glands are
first indicated by a thickened line of ectoderm extending from the axilla
to the groin. Later much of the line disappears, leaving a succession of
nodular thickenings corresponding with the nipples. In some mammals
FIG. 413. SECTION THROUGH THE MAMMARY GLAND OF AN EMBRYO OF 25 CM.
i. Connective tissue of the gland. (After Basch, from McMurrich.)
this row of nipples remains, in others only the inguinal thickenings, and
in still others only those toward the axilla. Thus in man there is normally
only one nipple on each side, but structures interpreted as accessory nipples
are frequent; they are not always situated along the mammary line.
In an embryo of 25 cm. (Fig. 413) several solid cords have grown out from
26
402
HISTOLOGY
the ectodermal proliferation. There are ultimately from 15 to 20 of these
tn each breast, and they branch as they extend through the connective tissue. At birth the nipple has become everted, making an elevation, and
at that time the glands in either sex may discharge a little milky secretion
similar to the colostrum which precedes lactation. The glands grow in both
sexes until puberty, when those in the male atrophy and only the main
ducts persist. In the female enlarged terminal alveoli are scarcely evident until pregnancy. The glands until then are discoid masses of connective tissue and fat cells, showing in sections small scattered groups of^ductlike tubes.
Toward the end of pregnancy each of the fifteen or twenty branched
glands forms a mammary lobe, and its alveolo-tubular end pieces are
Branch of an excretory duct. Connective tissue.
tii
*. ;.'!.',*.: *. ?:,/' vt '--- 'VV /-"Si r * v ' ~- > v .n >;
^OOiiMif^. ^i^SSff^'H ^i^;Sr
;M^8^!l
" - : - ' ^>
.14. SECTION OF A HUMAN MAMMARY GLAND AT THE PERIOD OF LAI
FIG. 414. SECTION OF A HUMAN MAMMARY GLAND AT THE PERIOD OF LACTATION. X so.
grouped in lobules. The secretory epithelium is a simple cuboidal or
flattened layer, in which fat accumulates at the seventh or eight month of
pregnancy. The fat first appears as small granules at the basal ends of
the cells, where it is taken up from the surrounding tissue. It is not produced by the gland cells. Leucocytes, derived from the connective tissue,
make their way between the epithelial cells of the alveoli and enter the
gland lumen, where some of them degenerate; others receive fat from the
gland cells, either in solution, or in drops which are devoured by phagocy tic
action. These fatty leucocytes grow to considerable size and are called
colostrum corpuscles. Beneath the alveolar epithelium there are basal or
basket cells, which have been compared with the muscle fibers of sweat
MAMMARY GLANDS
403
glands. A basement membrane separates them from the connective tissue
which contains many lymphocytes and eosinophilic cells.
After the birth of the child, the gland cells become larger and are filled
with stainable secretory granules and fat droplets; the latter are near
the lumen and are often larger than the nucleus (Fig. 415). After two
days of lactation, some of the gland cells are flat
and empty of secretion. Others are tall and
columnar, with a rounded border toward the
lumen; often they contain two nuclei. The
fat within them is not the result of degeneration as in sebaceous glands, nor a secretion produced by the nucleus; it accumulates through
protoplasmic activity, and the cell may be filled
several times before it perishes. Transitions
between low empty cells and columnar forms
occur, but mitoses are absent from the lactating gland,
sions are numerous during pregnancy.
Milk consists of fat droplets, 2-5 /x in diameter, floating in a clear fluid
which contains nuclein derived from degenerating nuclei, and occasionally
a leucocyte or colostrum corpuscle. Free nuclei may be found, and some
cells which undoubtedly are to be interpreted as detached from the alveoli
of the gland.
Gland cell. Membrana Oil drops,
propria.
FIG. 415. FROM A SECTION OF
THE MAMMARY GLAND OF A
NURSING WOMAN. X 250.
Mitotic divi
O
o.'O
FIG. 416. A., MILK GLOBULES FROM
HUMAN MILK. X 560. B., ELEMENTS OF THE COLOSTRUM OF A
PREGNANT WOMAN. X 560.
i, Cell containing uncolored fat globules; 2, cell containing minute
colored fat globules; 3, leucocyte;
4, milk globules.
FIG. 417. FROM A THICK SECTION OF THB MAMMARY GLAND OF A WOMAN LAST PREGNANT Two
YBARS BEFORE. X 50.
i, Large excretory duct; 2, small excretory duct; 3.
gland lobules, separated from one another by connective tissue.
At the end of lactation, the connective tissue, which has become greatly
reduced owing to the enlargement of the glands, increases in quantity and
the leucocytes reappear; as during pregnancy, they form colostrum corpuscles. The lobules become smaller and the alveoli begin to degenerate.
404 HISTOLOGY
In old persons all the end pieces and lobules have gone and only the
ducts remain.
The ducts are lined with simple columnar epithelium, surrounded by a
basement membrane and generally by circular connective tissue bundles.
Toward the nipple each duct forms a considerable spindle-shaped dilatation, the sinus lactiferus. The epithelium near the outlet of the ducts
is stratified and squamous.
The skin of the nipple, and of the areola at its base, contains abundant
pigment in the deepest layers of its epidermis. The corium forms tall
papillae and contains smooth muscle fibers, some of which extend vertically
through the nipple and others are circularly arranged around the ducts.
There are tactile corpuscles in the nipple, and lamellar corpuscles have been
found beneath its areola. It is particularly sensitive, and upon irritation
becomes rapidly elevated, due both to muscular and vascular activity.
There are many sweat and sebaceous glands in the areola, and occasional
rudimentary hairs. The areolar glands (of Montgomery) are branched
tubular glands having a lactiferous sinus and otherwise resembling the
constituent mammary glands. Their funnel-shaped outlets are surrounded
by large sebaceous glands. The areolar glands are regarded as transitions
between sweat glands and mammary glands.
Blood vessels enter the breast from several sources and form capillaries
around the alveoli. Lymphatic vessels are found in the areola, around
the sinuses, and in the interlobular tissue. The collecting lymphatics pass
chiefly toward the axilla; a few penetrate the intercostal spaces toward
the sternum. The nerves are mostly those which supply the blood vessels,
but fibers are said to extend to the glandular epithelium.
SUPRARENAL GLANDS.
Development and General Features. The suprarenal glands are two
flattened masses of cells, without lumen or ducts, situated in the retroperitoneal tissue above the kidneys. They vary considerably in size and
shape, but are usually about a quarter of an inch thick and between i and
2 inches tall, sometimes being wider and sometimes narrower than their
height. The right suprarenal gland is generally described as triangular
and the left as crescentic.
The gland resting upon the kidney (Glandula Rent incumbens) was first described
by Eustachius (Tractatio de Renibus, 1564). It was apparent from the outset that
the relation of the suprarenal glands to the kidneys was merely that of juxtaposition,
nevertheless most anatomists still find it convenient to describe them with the urinary
organs Certain early writers supposed that they were renal structures and named
them "succenturiate kidneys." Bartholin (Anatomia, 1666) perceived the medulla,
which he described as a cavity containing a black humor; and he published an extraordinary figure in which the gland resembles a cocoanut cut across "with the lid lifted.
SUPRARENAL GLANDS
405
In accordance with this conception he named the structures "atrobiliary capsules,"
and the name capsule is still often applied to them. Diemerbroeck (Anatome, 1672),
following Wharton, states that "the glands are found at a place where there is a
plexus of nerves, to which they are firmly united." In reviewing the various "conjectures" as to their function, he writes, "Wharton thinks that in these capsules a
certain juice is removed from the plexus of nerves on which they lie, useless indeed to
the nervous system, but which, flowing thence into the veins, may serve some useful
purpose." The intimate relation of these glands to the nervous system, and the
production of an internal secretion received by the veins, have since been demonstrated; in certain recent works the glands have even been described as parts of the
nervous system. Diemerbroeck concludes by hoping that physicians, through
many autopsies, may find out to what diseases these glands give rise. In 1855,
Addison described the disease, usually fatal, which is thought to depend upon the
loss of function of these glands. Their physiological importance has been amply
demonstrated, but they still present fundamental problems, both as to function and
structure.
A section through a fresh suprarenal gland reveals at once the division
into cortex and medulla. The cortex is yellowish, owing to the presence
of lipoid substance, and the medulla is dark brown, due in part to the large
amount of blood which it contains. The color contrast is usually very striking, and it is shown also
in unstained sections of tissue preserved in chromic
acid solutions (Fig. 418), although the medulla may
then be lighter than the cortex. Not only do the
cortex and medulla differ in gross appearance, but
they are radically different in embryonic origin, and
in the sharks they exist as separate organs. In sharks
the medulla is represented by groups of chromaffin
cells associated with the sympathetic ganglia, and the
cortex takes the form of an "interrenal gland,"
composed of cords of mesodermal cells with a sinu
Cortex. Medulla.
soidal circulation. Inhuman embryos correspond- FIG. 418. SECTION OF THE
SUPRARENAL GLAND OF
ing parts arise separately, but they come together to * CHILD, x is.
form a single gland.
The cortex appears first, and is formed from cells which develop as buds
of the ccelomic epithelium, growing into the mesenchyma on either side of
the root of the mesentery, medial to the Wolffian bodies. In embryos of
8-12 mm., the buds or cords have become detached from the peritoneal
epithelium (Zuckerkandl), and in cross sections they appear as round
masses of cells penetrated by a network of slender veins. The cells
of these masses rest directly against the vascular endothelium, so that
the vessels are described as sinusoids.
Meanwhile cells from the sympathetic ganglia grow ventrally along
the medial side of these masses, where they are conspicuous because of
their dark stain (Fig. 419). These cells, which give rise to the medulla of
406
HISTOLOGY
the suprarenal gland, do not appear like nerve cells and may be radically
different from them, although always closely associated with the sympathetic ganglia. Because of their affinity for chromium they are known
as chromaffin cells. They produce the important internal secretion,
adrenalin, which on injection causes contraction of the musculature of
the blood vessels, with consequent rise in blood pressure. The chromaffin
cells are not confined to the suprarenal glands, as already stated (p. 152).
In embryos of 15-20 mm., strands of chromaffin cells are seen penetrating
the cortical portion of the gland, but it is not until much later that they
gather in a central mass which constitutes the medulla; even at 190 mm.
the invasion is not complete
(Zuckerkandl) . As a whole, the
gland acquires a relatively very
great size in embryos.
From this mode of development,
it is seen that islands of medullary
substance may occasionally occur in
the cortex, and that outlying portions of the gland may not contain
any medulla. Moreover portions of
the gland frequently become detached, forming accessory suprarenal
glands. These may remain near the
main glands or may be carried down,
with the descent of the adjacent
sexual glands, into the broad ligament, or epididymis (cf . Wiesel, Sitzb.
kais. Akad. Wiss., Wien, 1899, vol.
108, pp. 257280). Such glands usually consist entirely of cortex, but they may
contain medullary substance. Isolated paraganglia, consisting entirely of medullary
substance, are not regarded as suprarenal glands. There is no evidence that accessory suprarenal glands may arise from the ccelomic epithelium at a distance from the
main glands (Zuckerkandl, Keibel and Mall's Human Embryology, vol. 2).
Adult Structure. The cortical substance may be divided into three
layers or zones the zona glomerulosa, zona fasciculata, and zona reticularis
(Fig. 420). The zona glomerulosa, found just beneath the capsule, is
said to develop between the second and third years after birth, "reaching
its characteristic structure only in the later years of childhood." It consists of round masses of cells which in man are much like those of the zona
fasciculata; in some animals they are distinguished by their columnar shape.
The zona fasciculata is composed of cords of rounded or cuboidal cells,
containing secretory granules and an abundance of fat vacuoles (Fig. 421).
There is no lumen within these cords and they are not surrounded by basement membranes. Thin-walled vessels pass between them, sometimes
lodged in connective tissue strands proceeding from the capsule. The
FIG. 419. SECTION OF THE SUPRARENAL GLAND OF AN
EMBRYO OF 17 MM. (Wiesel.)
A, Aorta; R, cortical portion; S, chromaffin tissue,
penetrating to form the medulla at SB. (From
McMurrich's Development of the Human Body.)
SUPRARENAL GLANDS
407
cords of the zona fasciculata are perpendicular to the surface; they end
below in a network, the zona reticularis. In this deeper portion the cells
Capsule
Zona glomerulosa.
\ Cortex
Medulla.
Zona fasciculata
Zona reticularis.
Cell cords of the medulla.
Nerve in cross section
Ganglion cells ....
Bundles of smooth muscle
fibers in cross section.
Veins.
FIG. 420. SECTION OF A HUMAN SUPRARENAL GLAND, x 50.
become pigmented, so as to form a dark brown band visible without
magnification. Fat vacuoles are here smaller or absent, as seen in Fig.
421, which shows also the close
relation between the cells and
the vascular endothelium. In
portions of the suprarenal gland
where the medulla is lacking,
the zonae reticulares of the opposite sides come together, forming the core of the organ.
The medulla is composed of
chromaffin cells arranged in
strands and masses which unite
to form a network, with lacunar
veins filling the interstices (Fig.
420). The cells contain an
abundant granular protoplasm,
but they tend to shrink, even in
well-preserved specimens, so
armour crpllatP CFicr FIG. 4- FROM A SECTION OF THE SUPRARENAL GLANB
appear steiiaie ^r ig. OF AN ADULT, x 360.
HISTOLOGY
Long meshed
capillary net of
the cortex.
421). These are the cells which are believed to produce adrenalin; the
function of the cortical cells remains unknown.
The capsule of the suprarenal glands is a connective tissue layer, said
to contain smooth muscle fibers and small ganglia, in addition to vessels and
nerves. Around the blood vessels especially, it contains elastic tissue.
The capsule sends slender prolongations into the gland, and elastic tissue
occurs in the medulla. The cortex contains very few if any elastic fibers,
and its framework appears to consist of reticular tissue.
The arteries supplying the suprarenal glands are from several sources.
They divide into many small branches in the capsule, and these penetrate
the cortex, forming a long- meshed capillary network (Fig. 422). In the
medulla the meshes become round and the
T^^^M^^H^^^^^I ^HH
Artery. sfl vessels collect to form
veins, the larger of which
are accompanied by longitudinal strands of
smooth muscle fibers.
Some arteries are said to
pass directly from the
capsule to the medulla,
without branching in the
cortex. Within the medulla the veins unite to
form the central veins,
which are the main stems
of the suprarenal veins
(Fig. i68,p. 173). They
emerge at the hilus; the right empties into the inferior vena cava and the
left joins the left renal vein.
Lymphatic vessels have been found in the capsule, where they may
drain the cortex, and also in the medulla, emerging at the hilus.
The numerous, mostly non-medullated nerves, of which a human
suprarenal gland receives about thirty small bundles, proceed chiefly from
the cceliac plexus and pass with the arteries from the capsule into the medulla. Within the capsule they form a plexus, from which branches descend into the zona glomerulosa and zona fasciculata; there they end on the
surface of groups of epithelioid cells, without penetrating between the individual cells. The plexus in the zona reticularis is more abundant, and is
formed from fibers which descend directl thryough the outer zones; its
fibers likewise terminate on the outer surface of groups of cells. In the
medulla, the nerves are extraordinarily abundant and each cell is surrounded by nerve fibers. Groups of sympathetic ganglion cells are found
Round meshed
net of the
medulla.
J
Vein of the
medulla.
FIG. 422. FROM A SECTION OF AN INJECTED SUPRARENAL GLAND OF
A CHILD. X 50.
SUPRARENAL GLANDS
here and there in the medulla but only rarely in the cortex,
nerves terminate in the walls of the vessels.
409
A part of the
CENTRAL NERVOUS SYSTEM.
SPINAL CORD.
Development and General Features. The formation of the medullary
tube, which gives rise to the spinal cord and brain, has already been described (cf. Fig. 125, p. 133); in the following section, the differentiation which takes place in its wall will be considered, together with the
general features of the spinal cord in the adult.
Very early in development, the cells of the medullary tube form a
syncytium. Those nuclei of the syncytium which border upon the lumen
FIG. 423. DIAGRAMS SHOWING THE DIFFERENTIATION OF THE CELLS IN THE WALL OF THE MEDULLARY
TUBE. (Schaper.)
The germinal cells are stippled, and the indifferent cells are empty circles. Circles with dots represent
neuroglia cells, and the black cells are neuroblasts. Circles containing an z are germinal cells in mitosis.
of the tube, or central canal, divide repeatedly by mitosis, and many of
them are forced outward laterally, so that the sides of the tube become
greatly thickened. In the floor and roof of the tube a corresponding thickening fails to take place, as shown in Fig. 423.
The lateral walls of the tube very early become divisible into three
layers (Fig. 423). The inner layer consists of germinal or prolif crating
cells and is wide only in the embryo. In the adult it becomes reduced
to a single layer of inactive cells, which surround the central canal like
a simple epithelium and constitute the ependyma (Gr., r/8v/*a, a cloak).
The middle layer is composed of cells derived from the germinal layer, and
in the adult it constitutes the gray substance of the cord. Its cells early
differentiate into two types the supporting cells, or neuroglia, and the
4io
HISTOLOGY
nerve cells. The processes of the nerve cells, in so far as they are within
the limits of the gray substance, are non-medullated. The outer layer is
at first entirely free from nuclei, and later it contains only a few cell
bodies, belonging with the neuroglia and with the endothelium of vessels
which penetrate the cord; it contains no nerve cells. This layer consists
of a network of neuroglia fibers through which nerve fibers extend in various directions, but chiefly up and down the cord. As these fibers become
medullated, the layer becomes white macroscopically, and it forms the
white substance of the adult cord. In preparations in which myelin is
Dorsal Median 1 Portion of
Entrance median Dorsal [ dorsal
zone. septum, funiculus | Lateral j root. Dorsal root.
Dorsal column.
Groups of nerve cells.
Central canal.
Ventral root. Whjte Ventral Ventral funiculus.
commissure. median
fissure.
FIG. 424. CROSS SECTION OF THE LUMBAR ENLARGEMENT OF THE HUMAN SPINAL CORD. X8.
deeply stained, the white substance appears darker than the gray substance (Fig. 424). From what has been said, it appears that the medullary tube early becomes divisible into inner, middle, and outer
layers, which give rise to ependyma, gray substance and white substance
respectively.
As the medullary tube enlarges, ventral swellings are formed on either
side of the median line (Fig. 423). These later project so far ventrally
that the flloor of the medullary tube is found at the dorsal end of a ventral
SPINAL CORD
411
median fissure, which is bounded on either side by the bulgings just described. Into each of these two swellings the gray substance projects,
forming the ventral "horns" or columns (columna anterior or ventralis}.
The term "horn" refers to the appearance in sections, and "column"
applies to their true form, taken as a whole. Corresponding with the
ventral columns of gray substance, there are two dorsal columns, which
arise somewhat later, and cause the gray substance, as seen in sections,
to assume the form of a letter H. With many variations this appearance
is characteristic of the entire spinal cord in mammals generally. As
seen in Fig. 424, there are secondary swellings on the sides of the "H"
which are called lateral columns; at certain levels they are ill-defined or
absent.
Instead of forming a dorsal median fissure, the medullary tube produces a dorsal median septum. The lower or ventral part of the septum
is apparently formed by the coalescence of the lateral walls of the medullary tube, thus leaving the ventral portion of the original lumen as the
central canal of the adult. Occasionally this small cavity, 0.5-1.0 mm.
wide, is entirely obliterated. The dorsal portion of the septum consists
of neuroglia fibers extending from the roof of the central canal to the periphery of the cord. Thus in the adult the cord is divided into right and
left halves, except for the transverse connections or commissures near the
central canal. These include a dorsal commissure, a ventral gray commissure, and a ventral white commissure.
The white substance of each half of the cord is subdivided into three
longitudinal juniculi, each of which includes several smaller bundles or
fasciculi, otherwise known as "fiber tracts." The funiculi are dorsal,
lateral, and ventral respectively, and their boundaries are seen without
magnification. The dorsal or sensory roots enter the cord along a groove
known as the dor so-lateral sulcus, and the ventral or motor roots emerge
along the ventro-lateral sulcus. All the white substance between these
two sulci is included in the lateral funiculus. The dorsal funiculus extends from the dorso-lateral sulcus to the median dorsal septum; and the
ventral funiculus extends from the ventro-lateral sulcus to the midventral fissure.
The fasciculi of which each funiculus is composed cannot be studied
profitably in normal specimens. They have been followed chiefly by
observing the effects of local injury and disease, for if a group of nerve cells
is destroyed, all the fibers proceeding from it will degenerate. In this
way it has been shown that the fibers of the funiculi are not arranged indiscriminately, but occur in definite tracts, which in some respects are
radically different in different animals. Thus the fibers of voluntary
motion which descend from the cerebral hemispheres to the motor cells of
the cord, forming the cerebro-spinal fasciculi, are found in the dorsal fun
412 HISTOLOGY
iculi of rodents but in the lateral and ventral funiculi of the human cord.
In man most of these fibers, in descending from the brain, cross to the
opposite side in the medulla oblongata and complete their descent in the
lateral funiculus of the cord, where they form the lateral cerebro-spinal
fasciculus; they terminate in relation with motor cells on the same side
of the cord. A smaller number of these fibers fail to cross in the medulla,
and descend in the ventral funiculus as the ventral cerebro-spinal fasciculus; these fibers cross to the opposite side in the cord, passing through
the ventral commissure, and then terminate in relation with the motor
cells. Thus the cerebro-spinal fibers all cross, but the decussation may
take place either in the medulla or in the cord.
The fibers which convey tactile stimuli to the brain enter by the
dorsal roots and pass into the gray substance of the cord, where they
terminate in relation with small cells dorsally placed. Fibers from these
cells cross to the opposite side of the cord through the gray commissure,
and then enter the white substance of the lateral funiculus in which they
ascend to the brain. One of these fibers and a descending fiber of the
lateral cerebro-spinal fasciculus are shown in the diagram, Fig. 123, p. 131.
In addition to fibers of the long tracts, such as pass between the spinal
cord and the hemispheres, cerebellum and other parts of the brain, the
ventral and lateral funiculi contain fibers which emerge from the gray
substance of the cord at one level and re-enter it at another, thus placing
the cells at different levels in communication. The fibers of these "ground
bundles" or fasciculi proprii generally remain close to the gray substance.
Their entrance and exit along the lateral concavity of the gray substance
causes it to be broken up into a formatio reticularis (Fig. 424).
The dorsal funiculi in the upper part of the cord are each subdivided
into a slender medial fasciculus gracilis (column of Goll) and a wider
lateral fasciculus cuneatus (column of Burdach), which are partially separated from one another by a septum. These fasciculi are composed
chiefly of the fibers of "muscle sense," which enter by the dorsal roots
and divide into ascending and descending branches. Many of these pass
into the gray substance of the cord after traveling varying distances in
the dorsal funiculi. Some of the ascending fibers, however, are very long
and extend to the medulla oblongata, gradually approaching the median
septum in their ascent. The gracile fasciculi are composed of these long
ascending fibers, and since they are not segregated in a distinct bundle in
the lower portion of the cord, this fasciculus is absent from the lumbar
region. In addition to the fibers of muscle sense, the dorsal funiculi contain some fibers of general sensation, a limited number of association
fibers, and others.
The description of the fiber tracts in the spinal cord and brain is the
subject of special text-books; they are briefly and clearly described by
SPINAL CORD
413
Villiger (Brain and Spinal Cord, translated by Piersol, 1912). The form
of the cord at different levels is considered in works on gross anatomy.
In general, the white substance increases toward the brain, since the
cervical cord contains the fibers to and from all the lower levels in addition to those for the cervical region itself. In levels which supply the
nerves to the upper and lower limbs, there is a general increase in both
gray and white substances, producing the cervical and lumbar enlargements, respectively. The lower end of the cord tapers into the rudimentary filum terminate.
Adult Structure. The spinal cord and brain are surrounded by two
membranes or meninges, of which the outer is dense and fibrous, and is
known as the dura mater; and the inner is thin and vascular, forming the
pia mater.
Curiously they are not called membranes, and the term meninx (in the singular) is
not employed in anatomy. They retain the ancient Arabic designation of "mother of
the brain," following, according to Hyrtl, a general Arabian tendency to name things
"mothers," "fathers," etc. (The vena cava was the mater venorum, and the pupil, the
filia oculi.} Carrying the figure further, the adjectives of double meaning, dura and
pia, were substituted for dense and thin. In the fifteenth century it was said that
these membranes were called matres because they produce the membranes surrounding
the nerves, the coats of the eye, and the periosteum of the skull, with which they
are continuous; but Hyrtl denies that the term has any such significance.
The dura mater spinalis, or dura mater of the cord, consists of compact
fibrous connective tissue with many elastic fibers, flat connective tissue
cells and plasma cells. Its inner surface is covered by a layer of flat cells
forming a mesenchymal epithelium. It has few nerves and blood vessels.
Anteriorly it is continuous with the dura mater of the brain at the foramen
magnum. It does not fill the vertebral canal, and is not continuous with
the vertebral periosteum. Around it externally there is a layer of vascular
fatty connective tissue; and internal to it there is a capillary cleft containing a very small amount of fluid. This subdural space connects with
tissue spaces in the dura and with those which extend out in the perineurium of the peripheral nerves. It communicates freely, but probably
indirectly, with the lymphatic vessels.
The pia mater spinalis, as described by Stohr, is a two-layered sac.
The outer layer is covered on its free outer surface with a simple layer of
flat cells, which is lightly connected with the dura, and forms the inner
wall of the subdural space. The inner layer, or pia proper, is a delicate
and very vascular connective tissue, closely connected with the spinal
cord, into which it sends prolongations accompanying the blood vessels.
The arteries of the spinal cord are primarily two pairs, situated as shown
in Fig. 125, E (p. 133) and in Fig. 424. One pair is ventral to the dorsal
roots, and the other is near the mid-ventral fissure; their branches supply
both the white and gray substance, and the collecting veins branch freely
414
HISTOLOGY
White External limiting
substance. membrane.
,
Cross sections of
medullated
nerve fibers
consisting of
"Axis cylinder
and
^Medullary sheath.
in the pia mater. Between the two layers of the pia, as described by Stohr,
there is a wide space filled with cerebro-spinal fluid and traversed by many
strands and membranes which pass from one layer of the pia to the other.
These strands constitute the arachnoid membrane, so-called from its
cobwebby texture. Often the name is restricted to the subdural membrane (following Henle), so that the spaces between the meshes of the
arachnoid are described as subarachnoid. They are preferably termed
arachnoid spaces and they are of great importance. The fluid which they
contain has access to that within the central canal of the cord and the
ventricles of the brain, through an aperture in the thin roof of the medulla
oblongata. Whether the arachnoid spaces open directly into
lymphatic vessels may be questioned, but undoubtedly they are
freely drained by the lymphatic
system.
On either side .of the cord, between the successive spinal nerves,
there is a frontally placed triangular plate of fibrous connective
tissue, which passes from the pia
to the dura and serves to support
the cord. The succession of these
pointed projections, with their
bases attached to the pia, constitutes the denticulate ligament.
White Substance. The white
substance of the cord consists essentially of medullated nerve fibers
supported by a network of neu
roglia. Toward the outer surface, the neuroglia fibers become felted together, forming an external limiting membrane just within the pia mater
(Fig. 425); this is an ectodermal tissue, which must be distinguished
from the adjacent connective tissue penetrating the cord with the blood
vessels. Although in transverse sections the neuroglia fibers appear to
be radially arranged (Fig. 425), longitudinal sections show that they extend also up and down the cord (Fig. 426), and in fact they form a diffuse syncytial network. The protoplasm of this network is characterized
by the presence of stiff neuroglia fibrils, imbedded in the peripheral
exoplasm, and passing freely from one cell territory to another. They are
well shown in specimens stained with Mallory's phosphotungstic acid
haematoxylin, and resemble the myoglia and fibroglia fibrils both in form
and staining reaction.
7 Neuroglia cells.
,. i
- \fr~~-"-\. Connective tissue.
Blood vessels.
FIG. 425. FROM A CROSS SECTION OF THE HUMAN
SPINAL CORD IN THE REGION OF THE LATERAL
FUNICULUS. X 1 80.
SPINAL CORD
415
As the nerve fibers which occupy the interstices of the neuroglia network increase in number and acquire myelin sheaths, thus becoming larger,
the protoplasm of the neuroglia is compressed into stellate accumulations,
often surrounding a nucleus (Fig. 428, A). In Golgi preparations they
appear as in Fig. 427, and are described as long rayed, and short rayed
or mossy cells. These forms represent clumps of neuroglia fibers, sometimes clogged with precipitate, in the center of which there may or may
not be a nucleus.
The nerve fibers of the white substance vary in diameter, the coarsest
being found in the ventral funicali and lateral parts of the dorsal funiculi;
FIG. 426. NEUROGLIA CELLS AND FIBERS FROM THE SPINAL CORD OF AN ELEPHANT. (Hardesty.)
c-i, Successive stages in the transformation of neuroglia cells, ending with disintegrating nuclei (i) ; 1, a
leucocyte. Benda's stain. X 940.
the finest are in the medial parts of the dorsal and lateral funiculi. Elsewhere coarse and fine fibers are intermingled. Their general direction
is parallel with the long axis of the cord. Like other nerve fibers they
consist of fibrillae imbedded in neuroplasm. Most of them are medullated,
and in cross section the myelin often forms concentric rings. Although a
few observers have described nodes, it is generally considered that there
are no nodes in the central nervous system. During the development of
the myelin, fibers have been found encircled by sheath cells, Fig. 428, B,
as described by Hardesty (Amer. Journ. Anat., 1905, vol. 4, p. 329-354).
In longitudinal view, these sheath cells are seen in depressions of the
myelin, where they greatly resemble the neurolemma cells of peripheral
416
HISTOLOGY
nerves. With the increase of myelin the sheaths become very slender and
can seldom be detected in the adult. It is ordinarily stated that the medullated fibers of the central nervous system are without a neurolemma.
Gray Substance. The gray substance consists of neuroglia, nerve cells,
and a confused mass of non-medullated nerve fibers running in all directions. The nerve
Bloodvessels. *. , , cells ^ Q f
types: (i) large
motor cells with processes which enter the
peripheral nerves; (2)
cells with processes
limited to the central
nervous system and
extending through its
white substance from
one part to another;
and (3) small cells
with processes confined to the gray substance. The neurax
ons of cells of the third type branch freely, and they may cross to the
gray substance on the opposite side of the cord.
The motor cells occur in groups in the ventral columns (horns). In
the cervical and lumbar enlargements there are two groups, a ventro
medial and a dorso-lateral (Fig.
424), which unite in the upper cervical and thoracic portions of the
cord; less well defined are the
dorso-medial and ventro-lateral
groups. In all of these groups
the motor cells are large (67-135 ju
Short rayed cells. Long rayed cells.
FIG. 427. NEUROGLIA CELLS FROM THE BRAIN OF AN ADULT MAN
Golgi Method. X 280.
my
C ac
FIG. 428.
in diameter), with round or oval
nuclei and prominent nucleoli
(Figs. 429 and 430). Their protoplasm appears densely granular
in ordinary preparations, but when
specially treated it is seen to contain an abundance of neurofibrils;
if preserved in alcohol and stained with methylene blue, the groups of
granules known as Nissl's bodies may be demonstrated. As already
noted, these are abundant in vigorous cells but become reduced or disappear in various conditions of exhaustion. Granules of brownish
A, Neuroglia cells and nerve fibers from a crosa
section of the spinal cord of an elephant. B,
Neuroglia cells, nerve fibers and sheath cells, from
the spinal cord of a pig, 2 weeks after birth. C,
Isolated fiber from the cord of 21 cm. pig embryo,
stained with osmic acid. (After Hardesty.) a.
c., Axis cylinder; my., myelin; n., neuroglia
nuclei; n. f., neuroglia fibrils; s. c., sheath cell.
SPINAL CORD 417
pigment are sometimes conspicuous. All of these features may be observed in the smaller nerve cells, but they are most evident in the large
motor cells. The dendrites of the motor cells extend far into the dorsal
columns (horns), and they even pass out of the gray substance into the
ventral and lateral funiculi. The neuraxon begins as a slender nonmedullated fiber at the tip of a clear "implantation cone" and acquires
its myelin sheath as it crosses the white layer. Ordinarily it has no collaterals; when present they are very small. None of the neuraxons cross
to the opposite side of the cord before entering the motor roots.
The nerve cells of the second type, usually smaller than the motor
cells but more abundant, are distributed throughout the gray substance
either singly or in groups. Definite groups of nerve cells in the spinal
cord and brain are known as nuclei, and at the root of the dorsal column
(horn) near its junction with the gray commissure, there is the important
FIG. 429. NERVE CELL OF THE SPINAL CORD FIG. 430. NERVE CELL OF THE SPINAL CORD OF
OF A DOG. X 600. A CHILD. X 430.
dorsal nucleus (column of Clarke). It is composed of cells which send
their neuraxons into the lateral funiculus, in which they ascend to the
cerebellum. The dorsal nucleus is limited to the thoracic portion of the
cord, and adjacent parts of the lumbar and cervical regions.
The fibers of the ground bundles are derived from scattered cells of
the second type. Their dendrites are long but sparingly branched.
The neuraxons give off collaterals in the gray substance, and enter the
ventral and lateral funiculi (rarely the dorsal) of the same or opposite
side. In the white substance most of them divide into ascending and
descending fibers, which send collaterals back into the gray, either singly
or in bundles, and the main branches finally terminate like the collaterals.
After re-entering the gray substance they ramify freely around the motor
cells.
In transverse sections the dorsal column appears capped by the zona
spongiosa which covers the substantia gelatinosa (Fig. 424). The former
contains spindle-shaped "marginal cells" which send fibers into the
white substance. The substantia gelatinosa contains a limited number
of very small nerve cells which send processes into the zona terminalis
27
4i8
HISTOLOGY
(Fig. 424); it contains also stellate neuroglia cells, the processes of which
are said to become transformed into a granular substance.
Ependyma. The ependyma is that part of the neuroglia which lines
the central canal. It appears like a simple columnar epithelium, but its
cell-like bodies are the ends of strands which primarily extend clear
across the spinal cord to the external limiting membrane. A nucleus is
generally found in the strand near the central canal, and there may be
others further out (Fig. 431). Although in the embryo strands may
readily be traced from the central canal to the periphery, in the adult
they are generally broken up into stellate cells, or forms retaining a chief
From the substantia gelatinosa of a newborn rat.
Neuroglia cell.
Central canal.
Ependymal cells.
Neuroglia cell of the
white substance from
a cat 6 weeks old.
Concentric- neuroglia cell from a cat
six weeks old.
Chief process.
Neuroglia cell of the gray substance of the base
of the dorsal column of a human embryo.
FIG. 431. NEUROGLIA CELLS FROM THE SPINAL CORD. X 280.
process directed either toward the central canal or the periphery (Fig. 431).
All these cells are parts of a general syncytium, as already described.
The ependymal cells at birth, and for sometime afterwards, possess
cilia projecting into the central canal, but in the adult these disappear.
It is questionable whether or not they are motile. Single bodies have
been found at their bases, but not diplosomes.
Surrounding the central canal, outside of the ependymal layer, there
is a zone of central gray substance, characterized by concentrically arranged neuroglia cells, one of which is shown in Fig. 431.
BRAIN.
Development and General Features. If a human embryo of 4 mm. is
placed in such a position that the spinal portion of the medullary tube is
BRAIN
419
approximately vertical, the anterior end of the tube, from which the
brain develops, is bent as shown in Fig. 432, A. The first portion, beginning at the anterior extremity where the neuropore is still open, passes
vertically upward. At the head-bend it turns backward and passes to the
neck-bend, where it curves downward, becoming continuous with the
part of the tube which forms the spinal cord. The anterior ascending
portion is the fore-brain (prosencephalon) ; the part where the head-bend
occurs is the mid-brain (mesencephalori) ; and the remainder is the hindbrain (rhombencephalon) . These three fundamental parts have become
more distinct and exhibit
subdivisions in the 10 mm.
embryo shown in Fig. 432, B.
Prosencephalon. The
fore-brain becomes subdivided into the telencephalon
anteriorly, and the diencephalon posteriorly; the latter
connects with the mid-brain.
In very early stages the forebrain produces two lateral
outpocketings, one on either
side, called the optic vesicles.
Each expands distally to
form the retina of an eye,
and its connection with the
fore-brain becomes reduced
to a slender stalk. In later
stages, the depression on the
irmpr wall of tViP brain which FlG - 432. A, THE BRAIN OF A 4.0 MM HUMAN EMBRYO
(after Bremer); B, THE BRAIN OF A 10.2 MM. EMBRYO
marks the pOSltlOn Of the Except the isthmus, is. the principal subdivisions of the brain
sp.c.
stalk is called the optic reIt is shown in the me
cess.
are indicated by prefixes of the term encephalon; sp. c.,
spinal cord; h., hemisphere; o. v., optic vesicle; r., rhinencephalon; v., roof of the fourth ventricle.
dian sagittal sections of the bran of an embryo of three months and of
an adult, in Figs. 433 and 434 respectively.
Telencephalon. The principal derivatives of the telencephalon are a
pair of lateral outpocketings which arise somewhat later than the optic
vesicles and are known as the cerebral hemispheres. Each contains a
cavity, or lateral ventricle, which opens into the medullary tube through
the interuentricular foramen (foramen of Monro). In later stages this
foramen is relatively small, and it appears in Figs. 433 and 434 as a darkly
shaded cleft in front of the thalamus (th.). As the hemispheres expand,
they approach one another in the median line above the brain, being separated by a thin plate of connective tissue. They grow backward, cover
420
HISTOLOGY
ing all the hind part of the brain. Their outer walls (constituting the
pallium, or mantle) become convoluted, forming gyri, with intervening
FIG. 433. SAGITTAL SECTION OF THE BRAIN OF AN EMBRYO OF THREE MONTHS. (After His.)
bl.JCerebellum; hem., hemisphere; hy., hypophysis (posterior lobe) ; isth., isthmus; med., medulla oblongata; mes., mesencephalon; ol. b., olfactory bulb; o. r., optic recess; p., pons; p. b., pineal body; p. s.
pars subthalamica; th., thalamus.
o.b.
cbl.
FIG. 434- MEDIAN SAGITTAL SECTION OF AN ADULT BRAIN.
cbl.. Cerebellum: c. c., corpus callosum; c. q., corpora quadrigemina; f., body of the fornix; hy., posterior
lobe of the hypophysis; med., medulla oblongata; o. b., olfactory bulb; o. r., optic recess; p., pons;
p. b., pineal body; p. 8., pars subthalamica; s. p., septum pellucidum; th., thalamus.
sulci, and each hemisphere as a whole is divided into frontal, parietal,
occipital and temporal lobes, as described in works on gross anatomy. A
BRAIN 421
more independent subdivision of the hemisphere is the olfactory lobe,
which terminates anteriorly in the olfactory bulb an expansion which
receives the olfactory nerves. The entire olfactory portion of the brain
is called the rhinencephalon.
Connecting the hemispheres with one another, there is a great transverse
commissure known as the corpus callosum (Fig. 434, c.c.). Below this is
the arched body of the fornix (f ) , representing a median fusion of two longitudinal bundles of commissural fibers, only small parts of which are included in a median section. Between the corpus callosum and the fornix, there is a thin septum pellucidum which consists of two vertical plates
with a closed cleft-like cavity between them.
It is probable that the corpus callosum and body of the fornix develop in a thickening of the front wall of the telencephalon, where it crosses the median line. The cavity
of the septum pellucidum is, accordingly, a secondary cleft in the thickened wall.
A fusion between the adjacent medial walls of the hemispheres, to provide a path for
the fibers of the corpus callosum and to account for the cavity in the septum, has been
described, but not confirmed.
In addition to the hemispheres with their commissures and olfactory
lobes, and the optic vesicles which are not counted as a part of the brain,
the telencephalon produces the pars o plica hypothalami. This "optic
portion of the region below the thalamus" includes the optic recess, and
in the mid-ventral line it forms a funnel-shaped depression, the infundibulum, terminating below in the posterior lobe of the hypophysis. (The
anterior lobe of the hypophysis is derived from the pharynx.) The
median cavity of the telencephalon is a laterally compressed space which
forms the front part of the third ventricle. The lateral ventricles, which
open from it, are counted as the first two.
Diencephalon. In the mid-dorsal line the diencephalon produces a
cone-like body, the corpus pineale. Laterally, in its thick walls, there
is a mass of gray substance called the thalamus (bed). External to the
thalamus are the great bundles of fibers passing from the hemispheres
to the spinal cord. The sensory fibers ascending from the cord terminate
in the thalami, where there is a relay of nerve cells to convey the impulses
to the hemispheres. The thalami have other connections of equal importance. They come in contact with one another across the cleft-like cavity
of the diencephalon (which is a part of the third ventricle) and may fuse,
forming the massa intermedia. The ventricle surrounds this mass. Beneath the thalamus the diencephalon forms the pars mammillaria hypothalami, which is represented on the under surface of the brain by the pair
of rounded mammillary bodies, one on either side of the median line
(Fig. 435, B).
Mesencephalon. The mid-brain remains undivided, and its walls
become very thick. Dorsally it forms four rounded elevations, the
422
HISTOLOGY
corpora quadrigemina (Fig. 435, A). These are arranged in pairs, the
anterior pair being known as the superior colliculi, and the posterior as
the inferior colliculi; the former have important relations with the optic
tracts, and the latter with the auditory tracts. On the under side of the
mid-brain there are two great bundles of fibers, the cerebral peduncles
(pedunculi cerebri) , which diverge as they pass forward from the hind-brain,
and swing upward on the sides of the mid-brain to connect with the hemispheres (Fig. 435). Between the cerebral peduncles on the under side of
the mid-brain, the oculomotor nerves emerge. They are derived from
groups of motor cells situated just beneath the floor of the cavity of the
mid-brain. This cavity remains a slender tube and is known as the
cerebral aqueduct (aquaductus cerebri).
i : iS^3^ oc.
FIG. 433. A, DORSAL AND B, VENTRAL VIEW OF THE POSTERIOR PART OF THE ADULT BRAIN. THK
CEREBELLUM AND ROOF OF THE FOURTH VENTRICLE HAVE BEEN REMOVED FROM A.
b. c., Brachium conjunctivum; b. p., brachium pontis; c. m., corpus mamillare; c. p., cerebral peduncle;
c. q. a., and c. q. p., anterior and posterior corpora quadrigemina; inf., infundibulum; med., medulla;
ol., olive; p., pons; p. b., pineal body; pyr., pyramid; r. b., restiform body; ven., floor of fourth ventricle. The nerves are oc., oculomotor; tr., troclear; tri., trigeminal; abd., abducens; int., intermedius,
fa., its facial portion; ac., acoustic; glo., glossopharyngeal ; va., vagus, ace., its accessory portion;
hy., hypoglossal.
Between the mid-brain and the hind-brain there is a marked constriction, known as the isthmus (Fig. 432, B). From the dorsal surface of the
isthmus the trochlear nerves make their exit (Fig. 435, A); they are
processes of nerve cells situated beneath the floor of the cavity, but they
pass to the dorsal surface and cross to the opposite side before emerging.
Rhombencephalon. The rhombencephalon (or hind-brain) receives its
name from the diamond shape which it presents when seen from above.
This form is established in young embryos and persists in the adult (Fig.
435, A). The roof of the rhombic cavity becomes a thin membrane and
is readily torn away, but the sides and especially the floor are greatly
thickened. The form of the hind-brain may be imitated, as described
by His, by cutting a short slit in the upper side of a piece of rubber tubing
BRAIN
423
and forcing the ends toward one another; the region with the weakened
dorsal wall buckles downward and bulges toward either side. The most
prominent part of the embryonic hind-brain, as it buckles downward, becomes the pons in the adult. From the dorsal part of the front end of
the hind-brain, the cerebellum develops, overhanging the thin roof of the
posterior portion. Pons and cerebellum are thus both derived from the
anterior part of the rhombencephalon, which is set apart as the metencephalon; the remainder of the hind-brain is included in the myelencephalon
(Fig. 432), which becomes the medulla oblongata and is continuous with
the spinal cord.
Before considering the subdivisions of the hind-brain in further detail,
the relation of the principal parts of the adult brain to the primary
vesicles may be reviewed in the following table:
Fore-brain. .
Telencephalon.
Hemisphere :
Pallium.
Rhinencephalon.
Corpus callosum.
Optic part of the hypothalamus.
Hypophysis (posterior lobe).
Pineal body.
Thalamus.
Mammillary part of the hypothalamus.
Mid-brain. . . { Mesencephalon . . . { Corpora quadrigemina.
[ Cerebral peduncles.
Diencephalon ,
Hind-brain.
[ Isthmus Isthmus.
Metencephalon...( CerebeUlim . I Pons.
Myelencephalon. . Medulla oblongata.
Metencephalon. The pons, as seen from the under side of the brain
(Fig. 435, B), appears as a broad bundle of transverse fibers interrupted
for the passage of the motor and sensory roots of the trigeminal nerve.
The superficial fibers of the pons pass dorsally around the wall of the
brain-tube, forming a pair of arms, the brachia pontis, which enter the
cerebellum. In addition to these large bundles, the cerebellum receives
fibers through the brachia conjunctiva which extend into it from the
isthmus, and also from the restiform bodies (i.e., rope-like) which ascend
from the posterior part of the hind-brain (Fig. 435, A). Thus on either
424 HISTOLOGY
side the cerebellum connects with three bundles of fibers, which come
together to form its medulla (corpus medullare). The medulla is surrounded by the gray cortical substance, and the entire cerebellum is
divided into many lobes and lobules.
The cavity of the hind-brain, which is continous posteriorly with the
central canal of the cord, and anteriorly with the cerebral aqueduct,
is known as the fourth ventricle. It extends upward toward the medulla
of the cerebellum, forming a tent-like recess, the apex of which is the
fastigium.
Myelencephalon. The myelencephalon becomes the medulla oblongata, continuous without demarcation with the medulla spinalis or spinal
cord. The ventral median fissure becomes shallow, but it may be traced
to the pons (Fig. 435, B). On either side of its upper portion, there is an
elongated swelling, the pyramid, corresponding in position with the
ventral funiculus of the cord. Each pyramid is bounded laterally by
the ventro-lateral groove, from which the motor roots of the hypoglossal
nerve emerge; this groove is continuous with the ventro-lateral groove of
the cord, from which the motor roots of the spinal nerves proceed. Near
the pons the abducent nerve comes out close beside this groove. The
dorso-lateral groove of the cord likewise extends to the pons; and in line
with the dorsal, roots of the cord, the sensory roots of the vagus, glossopharyngeal, acoustic and facial nerves enter this groove. The lateral
roots of the accessory, glossopharyngeal and facial nerves emerge just
below them. The space between the ventro-lateral and dorso-lateral
grooves corresponds with the lateral funiculus of the cord. Toward the
upper end of the medulla, it presents a rounded swelling known as the
olive (Fig. 435, B).
The dorsal funiculus of the upper part of the cord is divided into the
medial gracile and lateral cuneate fasciculi; these may be followed into
the medulla where they become broader (Fig. 435, A). Some of their
fibers enter the restiform body, and pass to the cerebellum; others pass
downward on either side of the central canal and continue beneath the
floor of the fourth ventricle to the hemispheres. Where the central canal
expands to become the thin-roofed fourth ventricle, all nerve fibers either
pass downward into its floor, or turn aside to enter the restiform body.
MEDULLA OBLONGATA.
The study of the medulla oblongata requires full consideration of the
fiber tracts of the cord and anterior portion of the brain, which cannot here
be taken up; only a few of the most fundamental features of the medulla
are to be mentioned. Sections through the lower end of the medulla resemble those of the cord, and the gray substance retains the form of
MEDULLA OBLONGATA
425
an H. The fibers from the hemispheres, which descend to the motor
cells of the cord, run mostly in the lateral funiculi, as previously stated.
They descend from the brain, however, in the ventral funiculi, in which
they form the pyramids in the upper part of the medulla (Fig. 437).
In the lower part of the medulla they decussate, crossing to the lateral
funiculus of the opposite side, as shown in Fig. 436; they appear to cut
off the ventral columns (horns) from the remainder of the gray H.
Then they descend in the spinal cord as the lateral cerebro-spinal tract
(also called crossed pyramidal). A few fibers, however, descend in the
ventral funiculi of the cord without having crossed in the medulla. Such
fibers of the ventral cerebro-spinal tract (direct pyramidal) cross to the op
t.s.n.t.-/
f.c.l
FIG. 436. SECTION AT THE LEVEL OF THE
FIRST CERVICAL NERVE. (After Dejerine.)
The right half of the section shows the effect
of Weigert's stain, the myelinated portions
being dark; the left half shows the gray
substance stippled; the white is blank, f.
c., Fasciculus cu neat us; f. c. 1., fasciculus
cerebro-spinalislateralis; f . c. v., fasciculus
cerebro-spinalis ventralis; f. g., fasciculus
gracilis; d. c., dorsal column; d. p., decussation of the pyramids; d. r., dorsal root of
first cervical nerve; v. c., ventral column.
FIG. 437. SECTION OF THE MEDULLA. (After
Dejerine.)
d. c., Dorsal column; d. L, decussation of the
lemnisci; f. c., fasciculus cuneatus; n. ace.,
nucleus of the accessory nerve; n. c., cuneate
nucleus; n. g., gracile nucleus; py. f pyramid;
t. s. n. t., spinal tract of the trigeminal
nerve; v. c., ventral column.
posite side in the cord before terminating in contact with the motor cells
of the ventral columns.
The fibers in the cerebro-spinal tracts are the neuraxons of the pyramidal cells in the outer layers of the hemispheres, which will be described
in a following section. They descend through the internal capsule (which
in a layer of white substance lateral to the thalami), thence through the
cerebral peduncles, pons, medulla oblongata and spinal cord, without interruption. This motor path from the hemispheres to the voluntary
muscles includes, therefore, only two neurones or nerve cells, one from the
cortex to the motor cells of the ventral column of the cord, and the other
from the ventral column to the end plate on the muscle fiber. Other
motor fibers from the hemispheres to the cord terminate in the red nucleus
deep within the substance of the mid-brain; cells of the red nucleus send
neuraxons to the opposite side, and these descend in the lateral funiculi
of the cord as the rubro-spinal tract. They terminate in relation with
426 HISTOLOGY
motor cells on the same side, and thus is formed a motor path composed
of three neurones. Other tracts to the cord proceed from the cerebellum.
The motor nerves of the medulla oblongata, pons, and mid-brain arise
from groups of cells, or nuclei, which are typically near the median line and
only a short distance below the floor of the ventricle or cavity. Fig. 438
includes the nucleus of the hypoglossal nerve, which is in this position.
The lateral motor roots are further below the ventricle and are more lateral. The nucleus ambiguus, which is an elongated structure containing
the motor cells of the accessory, vagus and glossopharyngeal nerves, is of
this sort (Fig. 438). These
ts. n.h. v motor nuclei correspond with
cell groups in the ventral
columns of the cord, and
they are similarly in connection with fibers from the
pyramidal cells of the hemispheres. In so far as the
latter pass to these cerebral
nerves, they form the corticobulbar tract, "bulb" being a
general term for the expanded part of the hindbrain. The cortico-bulbar
fibers decussate at different
levels.
Somewhat higher in the
lem.
FIG. 438. SECTION OF THK MEDULLA. (After Dejerine.)
c. i., Corpus restiforme; f. c. o., cerebello-olivary fibers; lem.'
lemniscus or fillet; n. am., nucleus ambiguus; 'n. h., nucleus hypoglossi; ol., olive; py., pyramid; t. s., tractus
solitarius; t. s. n. t., tractus spinalis nervi trigemini; v.,
fourth ventricle.
medulla than the decussation
of the descending motor fibers or pyramids, the sensory fibers ascending in
the gracile and cuneate fasciculi terminate in relation with groups of cells
known as the gracile and cuneate nuclei respectively (Fig. 437). They appear as additional horns of gray substance. The neuraxons from the cells
in these nuclei pass ventrally and decussate beneath the central canal, as
shown in Fig. 437. The bundles to which they give rise are known as the
medial lemnisci or fillets. In their course through the upper part of the
medulla, they are vertically placed bands of longitudinal fibers, on either side
of the median line (Fig. 438). The fillets not only receive fibers of muscle
sense through the gracile and cuneate fasciculi, but they are joined by the
spino-thalamic fasciculi of fibers of cutaneous sense, which pass up the
cord in the lateral funiculi. Moreover, they receive accessions from the
cerebral sensory nerves. The fibers of the latter enter the medulla and
divide into ascending and descending branches, like the dorsal root fibers
of the spinal nerves, but the descending fibers are relatively longer. The
position of the descending fibers of the trigeminal nerve (tractus spinalis
MEDULLA OBLONGATA 427
nervi trigemini] is shown in Fig. 438, and the tractus solitarius, containing
sensory fibers from the vagus and glossopharyngeus, is shown in the same
figure. In connection with these bundles of sensory fibers, there are
groups of nerve cells forming the nucleus of the tractus solitarius, and
nucleus of the spinal tract of the trigeminal nerve. These correspond with
the gracile and cuneate nuclei, and send fibers into the fillets. The fillets
continue through the pons and cerebral peduncles to the thalami, in
which they terminate. Nerve cells of the thalami convey the impulses
received onward to the hemispheres. Thus the sensory tract is composed of three neurones, the first being in the ganglia of the sensory
nerves, outside of the central nervous system; the second begins in the
gracile and cuneate nuclei, or in the gray substance of the cord in case
the impulse travels by the spino-thalamic tract, or in the nuclei associated with central tracts of the sensory cerebral nerves, and in all three
cases extends to the thalamus; the third begins in the thalamus and
extends to the cerebral cortex.
CEREBELLUM.
The medullated nerve fibers of the restiform bodies, brachia pontis,
and brachia conjunctiva come together to form the medulla of the cerebellum, and place the cerebellum in connection with spinal and cerebral
nerves and with the hemispheres. The medulla contains several paired
nuclei, the largest being the dentate nuclei, which have convoluted gray
capsules resembling those of the olivary nuclei (shown in Fig. 438).
The restiform bodies include the fibers derived from the dorsal nuclei or columns
of Clarke in the spinal cord; these fibers ascend in the lateral funiculi, within which they
form the dorsal spino-cerebellar tract (of Flechsig). The restiform bodies contain also
fibers from certain cells in the gracile and cuneate nuclei, and many fibers from the
olivary nuclei, mostly of the opposite side. The brachia pontis contain fibers passing
to the cerebellum from the numerous nuclei pontis. The latter are in connection with
fibers descending from the hemispheres, thus forming cerebro- or cortico-cerebellar
tracts. Some fibers pass in the reverse direction. The brachia conjunctiva contain
fibers of the ventral spino-cerebellar tracts (of Gowers), which arise from central or
lateral cells in the gray substance of the cord, and pass through the lateral funiculi to
the brachia conjunctiva, through which they turn back to enter the cerebellum. The
main part of the brachia conjunctiva consists, however, of fibers passing outward
from the cerebellum and its dentate nucleus, to end, after decussating, in the red nuclei
of the mid-brain. Thence fibers pass on to the thalami and hemispheres, and also
downward to the medulla and spinal cord.
The medulla of the cerebellum extends into the small peripheral lobules,
where it is covered by the cortical substance (Fig. 439). The latter
consists of three strata an inner granular stratum, which is rust-colored
in the fresh condition; a middle ganglionic stratum, composed of a single
row of large cell bodies ; and an outer gray stratum.
428
HISTOLOGY
Gray stratum.
Ganglionic stratum .
The inner granular stratum consists of many layers of small cells which
by ordinary methods show
relatively large nuclei and
very little protoplasm. With
the Golgi method it appears
that besides neuroglia cells,
two sorts of nerve cells are
present, the small and large
granule cells; the former (Fig.
440) are multipolar ganglion
cells with short dendrites
having claw-like terminations, and slender nonmedullated neuraxons which
ascend perpendicularly to the
gray layer and there divide
in T-form into two branches.
The branches run lengthwise
of the transverse folds or
convolutions of the cerebellum and have free unbranched endings. In sagittal sections (Fig. 442) the
terminal branches of the neuraxons are cut across. The small granule
cells form the bulk of the granular stratum. The less frequent large
granule cells (Fig. 442) are
more than twice the size of the
small ones; their branched dendrites penetrate the gray stratum and their neuraxons, going
in the opposite direction, are
soon resolved into very numerous branches which ramify
throughout the granular stratum.
The granular layer contains
also a thick network of medullated fibers which enter it
chiefly from the white substance. A part of these fibers
end in the "eosin bodies" of
the granular stratum, which
are heaps of stainable particles found between the small cells (Fig. 441).
FIG. 439. FROM A SAGITTAL SECTION OF THE CEREBELLUM
OF AN ADULT MAN. X 12.
FIG. 440. DIAGRAM OF A SECTION OF THE CEREBELLUM
LENGTHWISE OF THE TRANSVERSE CONVOLUTIONS.
GOLGI'S METHOD. (Koelliker.)
gr., Cells of the granular stratum; n. their neuraxons in
the granular layer and n'., in the gray stratum; p., p'.,
Purkinje's cells. (From Bailey's "Histology.")
CEREBELLUM
429
Eosin bodies.
!\
Nuclei of small cells of
the granular stratum.
C
Some of the fibers form bundles parallel with the surface, running between the granular and ganglionic strata in the sagittal direction; they
send branches into the gray layer. A small portion of the granular stratum is formed by the medullated neuraxons of the cells in the ganglion
layer.
The middle ganglionic stratum consists entirely of a single layer of
very large multipolar ganglion cells, called Purkinje's cells. Their oval
or pear-shaped bodies send two large dendrites into the gray stratum,
where they form an extraordinary arborization (Fig. 442) Their many
branches do not extend in all directions but are confined to the sagittal
plane, that is, to a plane at right angles with the long axes of the convolutions. When the convolutions are cut lengthwise, Purkinje's cells appear
as in Fig. 440. The neuraxons arise
from the deep surface of the cell bodies,
and as medullated fibers they pass
through the granular stratum to the
white substance. Within the granular
layer they produce collateral fibers
which branch and in part run back into
the ganglionic layer, ending near the
bodies of other Purkinje's cells (Fig.
442).
The outer gray stratum, of gray
color, contains two sorts of nerve cells,
the large and the small cortical cells.
The large cortical or basket cells are
multipolar ganglion cells, the dendrites of which pass chiefly toward the
surface. Their long neuraxons, thin at first but later becoming thicker,
run parallel with the surface in the sagittal plane. They send occasional
collaterals toward the surface, and at intervals produce fine branches which
descend and terminate in baskets around the bodies of Purkinje's cells
(Fig. 442), often surrounding also the beginning of their neuraxons.
The small cortical cells, distinguishable from the basket cells since
their neuraxons are not in relation with Purkinje's cells, may be divided
into two types, connected by intermediate forms. The cell bodies of the
first type are nearly or quite as large as those of the basket cells. Their
two to five dendrites lie in the sagittal plane like those of Purkinje's cells;
the slender neuraxons, i mm. long or more, sometimes form loops and
are characterized by abundant branches in their proximal parts. The
terminal branches are few. Cells of the second type are in general somewhat smaller; their shorter neuraxons branch in the immediate vicinity
of the cell bodies. The elements of the first type form the bulk of the
relatively numerous small cortical cells, and are found throughout the
FIG. 441. FROM A THIN SECTION OF THE
CEREBELLUM OF AN ADULT. X 400.
43
HISTOLOGY
gray stratum, though they are more abundant in its superficial part.
The second type likewise appears throughout the gray stratum.
The medullated nerve fibers found in the gray layer are prolongations of those in the granular stratum. In part they proceed toward the
surface, where, after losing their myelin, they end in branches among the
Purkinje's cell.
Neuraxon of a basket cell.
Short-rayed cell
Neuroglia cell.
Collaterals of a Purkinje cell.
Long-rayed
cell.
w --*^ _ ^-ki ^-~>jf,e j v s ' \ *--~\ i . V flmr
Collaterals of a \.
cortical cell. /
>v->_/ ^ } f n- '^Y~O"L ' ^ ^ " "* ?
Neuraxon of a large
cell of the granular.
stratum.
Fibers to the cortex
Small cells of the granular stratum.
FIG. 442. DIAGRAM OF A SAGITTAL SECTION OF THE CEREBELLUM.
Except the large granule cell, which is from a kitten, the cells are drawn from Golgi preparations from an
adult man. K, large cortical or basket cell.
dendrites of Purkinje's cells; in part they run between the bodies of
Purkinje's cells lengthwise of the convolutions.
The neuroglia of the cerebellum consists of short-rayed stellate cells
found in all the layers; of long-rayed cells in the white substance; and of
epculiar cells with small bodies at the outer boundary of the granular layer
CEREBELLUM
431
(Fig. 442). These send only a few short processes inward, but many
long processes straight out to the free surface, where they end in triangular
expansions. In this way a thick peripheral neuroglia layer is produced.
As long as the cerebellar cortex is not fully developed, it presents a
series of peculiarities which are lacking in the adult. Thus in embryos
and young animals the partly developed gray stratum is covered by a
superficial granular layer, the cells of which later become more deeply
placed.
HEMISPHERES.
The ascending sensory fibers from the thalamus and the parts below,
and the descending motor fibers which pass out of the hemispheres are
contained in the internal capsule, which is a layer of white substance between the thalamus medially and the basal
nuclei of the hemispheres laterally. The path
by which these fibers enter and leave the deep
white substance of the hemispheres is indicated in Fig. 443. Surrounding the inner
white substance is the peripheral layer of gray,
which forms the cerebral cortex. The cortex
is divided into four ill-defined layers an outer
molecular or neuroglia layer; a layer of small
pyramidal cells; a layer of large pyramidal
cells ; and next the white substance, a layer of a .t
polymorphous cells. From the pyramidal
... , .. FIG. 443. TRANSVERSE SECTION OF
cells the fibers of the descending motor THE BRAIN. About j natural
size.
tract arise. The layers are shown in Figs. 444 The gray substance is stippled; the
white is blank, a. t., Ascending
and 44 v tract, including the fillet; c. c.,
corpus callosum ; d. t., descendin g
The molecular layer, which in ordinary sec- tract, leaving the hemisphere to
enter the cerebral peduncle; n. 1.,
tions appears finely punctate or reticular, con
tains besides many neuroglia cells, a network
of medullated tangential fibers, which are parallel with the surface. Other
fibers, as shown by the Golgi method, are partly neuroglia, and partly
dendrites of pyramidal cells. The "cells of Retzius" found in this layer
have bodies of irregular shape, which send out processes parallel with the
surface, and these processes send short branches outward; other processer
descend into the deeper layer (Fig. 446). They are probably neuroglia
cells.
The layer of small pyramidal cells contains a special form of nerve cells,
with pyramidal bodies measuring 10-12 P.. Since they taper into a dendritic process, their length cannot be definitely determined. The chief
dendrite, after producing small lateral branches, enters the molecular layer
432
HISTOLOGY
Supra
Iradial
network.
Inter- i
radial
etwork
Radial
bundles
Medulla
or white
substance.
SSfc^-^r*T^^|$Ui-^7;!?r,V^
' i- ' iX- ifh\ *\ *~7 , i !-; l i '
^itU^w&? -^ l ^Vv
S^^^ilW'^^
?5^1^^r^^U. b\_-*^^
j^^;.v:'^S^^ ti f ^
^^r^i---^^-^"-^''-^-!' i-v
?^S^^f ' : '' :
Molecular
layer.
Layer of
small
pyramidal
cells.
Blood
vessel.
Layer of
large
pyramidal
cells.
Layer of
polymorphous
nerve cells.
i. / .. r' s. (
'- ' ! f/ . ." : . '-\' /r ,".
../':,
^ t ~^ ^ ^LJ_I "^ ^ *
FIG. 445.
FIGS 444 and 445 are from vertical :
tions of the cortex (central convoli
tion) of an adult. Fig. 444 is a Weige
preparation; Fig. 445 is from a sectip
stained with hsematoxylin and X 45.
FlG. 444
CEREBRAL CORTEX
433
Cell of Retzius.
Short-rayed neuroglia cell.
Blood vessel.
--.. Small
pyramidal
cell.
Large
/ pyramidal
cell.
Collateral.
Xeuraxon of a
polymophous
nerve cell.
Long-rayed
neuroglia
cell.
FIG. 446. DIAGRAM OF THE CEREBRAL CORTEX. The cells on the right are drawn from Golgi preparations of an adult man. X 120. The left portion of the diagram is X 60.
where it arborizes freely; its terminal branches often show small irregular
projections. Lesser dendrites proceed from the sides and basal surface
of the pyramidal cell body. The neuraxon always arises from the basal
surface, and after producing branched collaterals, it generally enters the
white substance where it may divide in two (Fig. 446, 3). Sometimes the
neuraxon turns toward the molecular layer, joining the tangential fibers;
28
434 HISTOLOGY
infrequently an inverted pyramidal cell is found. The neuraxons and
collaterals are medullated.
The layer oj large pyramidal cells contains those with bodies 20-30 /*
long (the "giant pyramidal cells" of the anterior central convolution
measure even 80 /*). The very large neuraxon always goes to the white
substance, after sending out several collaterals in the gray.
The layer of polymorphous cells includes oval or polygonal cells which
lack a chief dendrite directed toward the surface; their slender neuraxons
produce collaterals, and enter the white substance where they may divide
into two branches in T-form (Fig. 446, 4). Polymorphous cells with
branched neuraxons limited to the vicinity of the cell body, are found in
this layer and in the pyramidal layers also. The neuraxon may branch in
the molecular layer (Fig. 446, 6).
Many medullated fibers are found in the deeper layers of pyramidal
and polymorphous cells. They are grouped in tapering radial bundles
which terminate toward the layer of small pyramidal cells, as seen in
Fig. 444. The bundles include the descending medullated neuraxons
of the pyramidal and polymorphous cells, and the ascending medullated
sensory fibers from the white substance. The latter branch repeatedly,
forming the supra-radial and tangential networks. The medullated collaterals of the pyramidal cells run at right angles with the radial bundles;
they form an inter-radial network, the outer part of which is so thick in
the region of the calcarine fissure that it can be seen without magnification, and is there known as the "stripe of Vicq d* Azyr." Similar bands
may be detected elsewhere in thick sections (Baillarger's stripes).
In the gyrus hippocampi and gyrus uncinatus the tangential fibers
are so abundant as to form a considerable layer, the substantia reticularis
alba. The hippocampus (Ammon's horn), olfactory bulb, and some other
areas of the cortex, differ in details from the central region which has been
described; these pecularities are considered in the larger special works on
the nervous system.
The neuroglia of the hemispheres, like that of the cord, is at first a
syncytium with strands extending from the ventricle to the periphery.
Later, the syncytium is divisible into short-rayed neuroglia cells found
chiefly in the gray substance, long-rayed cells found chiefly in the white,
and ependymal cells lining the ventricles. The ependymal layer is continuous through the aqueduct with that of the fourth ventricle and central
canal. In early stages its cells have cilia-like processes which are in part
retained in the adult. The short-rayed cells, which are characterized by
knotted branching processes, are often in close relation with the blood
vessels; they may serve to transfer the nutritive and myelin-forming
material from the vessels to the nerve fibers. The outer surface of the
cerebral cortex is covered with a feltwork of neuroglia fibers.
HYPOPHYSIS
435
HYPOPHYSIS.
The hypophysis (i.e., a growth beneath the brain) is a rounded mass,
about half an inch wide and a quarter of an inch thick, attached to the
tip of the infundibulum, and lodged in the sella turcica of the sphenoid
bone. Its stalk of attachment to the infundibulum extends through the
fibrous membrane fastened to the four posts or corners of the sella, and
in removing the brain, the hypophysis is therefore often torn from its
stalk and left in the bony excavation. It is now known to be a most
important organ of internal secretion, consisting of two parts which are as
distinct from one another as the cortex and medulla of the suprarenal
gland. The anterior lobe is formed from Rathke's pouch (Rathke, Arch. f.
Anat.> Phys., u. wiss. Med., 1838, pp. 482-485) which grows upward from
the oral ectoderm and encounters the knob-like posterior lobe which is a
part of the brain (Fig. 203, p. 216). The anterior lobe then sends up a
short process on either side of the posterior lobe, like the thumb and first
finger of a hand, and in later stages Gushing ventures to describe the pos
FIG. 447. DIAGRAMS OF THE HYPOPHYSIS CEREBRI. (From Morris's Anatomy, after Testut.)
A, Posterior surface. B, Transverse section. C, Sagittal section, i, Anterior lobe; 2, posterior lobe;
3, infundibulum; 4, optic chiasma; 5, infundibular recess; 6, optic recess.
terior lobe as resting in the anterior lobe like a ball in a catcher's glove
The anterior lobe becomes separated from the roof of the mouth by the
obliteration of its duct, which is reduced to a slender solid epithelial strand
and ruptures in embryos of about 20 mm. A depression marking its former
outlet has sometimes been found in the vault of the pharynx, and there
may be a canal through the sphenoid bone, the craniopharyngeal canal,
which follows the course of the former duct. It is said that a small
"pharyngeal hypophysis," having the structure of the anterior lobe, is
constantly found near the pharyngeal end of this canal, on the under
surface of the sphenoid bone.
The posterior surface of both lobes, as they appear in the adult, is
shown in Fig. 447, A, and a sagittal section is shown in C; the orientation
of the latter may readily be understood by comparing it with the region
of the optic recess in Fig. 434.
The hypophysis can hardly be overlooked in examining the brain, and its existence
is recorded by the earliest writers. The epiphysis, on the top of the brain, was called
436
HISTOLOGY
the pineal body from its resemblance to a pine cone, and according to Hyrtl the hypophysis below, being a round structure attached to a stem, was named the "rose hip"
by the Mohammedan physician Avicenna (ca. A. D. 1000). Vesalius introduced
the name pituitary gland. The pituita or phlegm was believed to be excrementitious
material, eliminated by the brain and received by the naso-pharynx, and its possible
origin by way of the olfactory nerves had been discussed. Vesalius and his followers
believed that it was collected by the infundibular funnel and eliminated by the pituitary
gland. If the sella turcica of a prepared skull is examined, four grooves may be traced
from it, two passing forward to the optic foramina, and two passing backward to the
lacerated foramina. Vesalius pictured these four channels as outlets for the pituitary
gland, the two latter (which in life are closed by cartilage) being in relation with the
naso-pharynx. Bartholin recorded another function of the pituitary gland, namely,
"to close the infundibulum lest vital spirits should escape," and finally V. C. Schneider
showed conclusively that the pituitary gland is not the source of phlegm. According
to Hyrtl this was accomplished in five classic but lengthy books, De catarrhis, 16401642, and he adds "No physician and no anatomist should leave this fundamental
and learned work unread if he has time for it."
The anterior lobe consists of solid branched epithelial cords, of irregular
caliber, connected with one another by frequent anastomoses. Between
Portion of the,
anterior lobe.\
Epithelial cord.
Epithelial follicle.
Blood vessel containing blood
corpuscles.
Portion of the
posterior lobe.
Multipolar cell.
Connective tissue
fibers.
FIG. 448. PORTION OF A HORIZONTAL SECTION OF A HUMAN HYPOPHYSIS, showing the boundary line
between the anterior and the posterior lobes. Two gland follicles on the left each contain a dark
epithelial cell. X 220.
the cords and in close relation with them, there are wide lacunar capillaries derived from several arterioles which descend along the stalk of the
infundibuhim. The wide terminal vessels are arterio-venous connections
having a sinusoidal structure. Along their margins, especially in the
central part of the lobe, the cords are covered with eosinophilic cells,
having round nuclei; the axial cells of the cords are neutrophilic and less
granular. Although the nature of the marginal cells has not been
fully determined, they are usually described as glandular, and their granules presumably represent an internal secretion which is discharged into
the adjacent vessels. At the periphery of the anterior lobe, basophile
cells occur.
HYPOPHYSIS 437
Like the cortex of the suprarenal gland, the anterior lobe of the hypophysis is the
larger part, and has a characteristic epithelial structure, whereas the portion associated
with the nervous system is smaller, with less striking morphological characters. Nevertheless the latter, in both cases, produces the more active extracts, and its products are
better understood. The anterior lobe of the hypophysis appears to "preside more
intimately over skeletal growth;" and overgrowth, acromegaly and gigantism are attributed to its excessive activity. The administration of extracts of the posterior lobe
causes a rise in blood pressure, owing to the contraction of the vascular musculature,
thus resembling adrenalin in its action. Repeated injections cause emaciation; and
deficient secretion, or the removal of the gland, leads to a high tolerance for sugars
with the resultant accumulation of fat. "Thus normal activity of the posterior lobe
is essential for effective carbohydrate metabolism" (Gushing, The Pituitary Gland and
its Disorders, 1912).
The posterior lobe consists of a mass of neuroglia cells, the pars nervosa,
and an epithelial investment, the pars intermedia. The latter is of special
interest since its cells, sometimes ciliated, tend to become arranged in
cysts containing a hyaline or colloid secretion. According to Stohr, these
cysts belong with the anterior lobe, and since the two lobes are in contact
near the anterior part of the infundibular stalk, it is possible that its elements have grown around and invested the pars nervosa, thus producing
the pars intermedia. Except anteriorly, however, the two lobes of the
hypophysis are generally separated by a cleft.
The pars nervosa contains ependymal and neuroglia cells, but no
nerve cells and only a few nerve fibers. The ependyma lines the cavity
which extends downward into the lobe from the inf undibulum. According
to Tilney, "very often in the human hypophysis the lumen is not only seen
to be distended by large masses of colloid, but its walls are evaginated so
as to give rise to cysts of varying sizes, all containing colloid" (Mem. of
the Wistar Inst., No. 2, 1911). The colloid material is believed to be
evidence of a secretion which is eliminated into the third ventricle, and
which finds its way into the cerebro-spinal fluid. Possibly it may be given
off from the outer surface of the lobe, for the inconstant cavity or lumen
is not a typical duct; but the secretion apparently does not enter the blood
vessels, which in this lobe are neither abundant nor sinusoidal. Eosinophilic cells are generally absent.
PINEAL BODY.
The pineal body (sometimes called the epiphysis) is a median dorsal
outpocketing of the diencephalon (Figs. 434 and 435), terminating in a
small nodule composed of neuroglia and round or polygonal epithelial
cells. The human pineal body contains no nerves (Kolliker) but below it
there is the commissura habenularum. A connective tissue capsule sends
prolongations into its interior and surrounds groups of epithelial cells and
follicles.
438 HISTOLOGY
It is generally considered that the pineal body is a function! ess rudiment. In lower vertebrates an eye-like structure develops just in front of
it, sometimes being found beneath a transparent cornea, but the extent
of the visual functions of this organ remains undetermined. The corpus
pineale immediately behind this eye may take its place to some extent,
and "often shows, as in certain lizards, traces of visual structure"
(Kingsley). The unimportant position to which this organ has been
relegated, contrasts with the familiar conjecture of Des Cartes that all
ideas which proceed from the five senses are perceived in the pineal body
as a center, and that from it all nervous impulses irradiate. In man not
the slightest function is now assigned to it.
Within the pineal body, acervulus cerebri or "brain
sand" is usually found, consisting of round or mulberry-like concretions, 5 n to i mm. in diameter (Fig.
449). In specimens preserved in glycerin or balsam
these concretions show distinct concentric layers. They
consist of an organic matrix containing calcium carbonate and magnesium phosphate, and are sometimes
FIG 449 ACERVULUS surrounded by a thick connective tissue capsule.
BODY OF A WOMAN Not infrequently, especially in old age, the brain
OU> ENT X so YEARS substance contains round or elongated bodies, distinctly
stratified, which are colored violet by tincture of iodine
and sulphuric acid, and therefore are related to amyloid. These cor
puscula amylacea are found almost always in the walls of the ventricles,
and also in many other places in both gray and white substance, and
in the optic nerve. They have a homogeneous capsule with occasional
processes, and are evidently neuroglia cells transformed by amyloid
infiltration.
MENINGES.
The dura mater cerebralis or dura mater of the brain, includes the
periosteum of the inner surface of the cranium and consists, therefore, of
two lamellae. The inner is like the dura mater of the cord but contains
more elastic fibers; the outer corresponds with the periosteum of the vertebral canal. It contains the same elements as the inner layer, but its fibers
run in a different direction. In order that the dura of the brain and cord
may be strictly comparable, some anatomists count the vertebral periosteum and the considerable layer of vascular fatty tissue beneath it, as
a part of the dura of the cord. In relation with the brain, the dura forms
reduplications extending between the cerebellum and the hemispheres,
and between the right and left hemispheres. Its two layers separate to
enclose large, thin-walled veins, the sinuses of the dura. These receive
MENINGES 439
veins from the substance of the brain, but the arteries of the dura, or
meningeal arteries, supply the cranial periosteum. The dura has many
nerves, some with free endings, and others supplying the musculature of
the vessels.
The arachnoid membrane, as in the cord, is separated from the dura
by a cleft-like sub-dural space. In certain places, especially along the
sides of the superior sagittal sinus, there are found arachnoid villi (Pacchionian bodies or granulations), which project into the cavity of the venous sinus. They are elevations of the arachnoid covered with a thin portion of the dura and venous endothelium, and possibly facilitate the transfer of fluid between the arachnoid (or subarachnoid) spaces and the veins.
These spaces contain the cerebro-spinal fluid, and are continuous with
the corresponding spaces around the cord. Through apertures in the
thin roof of the fourth ventricle, they communicate with the central
cavity of the cord and brain.
The pia is a delicate and highly vascular layer, containing arteries
which send branches into the cortex from all points on its surface. These
cortical arteries arise from the anastomoses between the internal carotid
and vertebral arteries at the base of the brain, which produce the arterial
circle of Willis. Other branches from these vessels enter the substance of
the base of the brain, supplying the basal nuclei, thalamus and internal
capsule. Because of the effects of haemorrhage in relation with the motor
and sensory tracts in this region, these small arteries are of very great
importance. The vascular membranes which cover the thin portions of
the roof of the third and fourth ventricles are in places invaginated into
the ventricles, forming the chorioid plexuses. These networks of small
vessels, covered only by thin membranes, are found in the lateral ventricles, as well as in the third and fourth; their position is described in
text-books of gross anatomy. The simple layer of cuboidal epithelium,
which covers the plexuses, contains pigment granules and fat droplets,
and may perform secretory functions.
EYE.
Development and General Anatomy. The eyes first appear as a pair of
optic vesicles, which are lateral out-pocketings of the fore-brain (Fig.
451, A). They enlarge rapidly, but their connections with the wall of
the brain remain relatively slender, forming the optic stalks. The epidermal ectoderm immediately overlying the vesicles thickens and becomes invaginated (Fig. 451, B and C). The invaginated portion is then
detached in the form of a vesicle, the inner wall of which is distinctly
thicker than the outer; this "lentic vesicle" becomes the lens of the eye.
Meanwhile, as seen in B and C, that layer of the optic vesicle which is
44 HISTOLOGY
toward the epidermis sinks in upon the deeper layer, transforming the
vesicle into the optic cup. At first the cup is not complete, being deficient
on its lower side (Fig. 450). The arteria centralis retina is seen passing
through this indentation, which begins on the lower surface of the stalk
and extends to the free margin of the cup; the cleft is sometimes called
the "chorioid fissure." Distal to the point of entrance of the artery into
the optic cup, the edges of the fissure fuse; the artery then appears to
perforate the base of the cup, and it retains this relation in the adult.
The artery is shown in section in Fig. 451, D.
In a remarkable series of experiments upon tadpoles, Warren Lewis has shown
that "the lens is dependent for its origin on the contact influence or stimulus of the
optic vesicle." If the optic vesicle is removed, the epithelium in the region of the
normal lens does not become thickened or invaginated; but if an optic vesicle is
transplanted by detaching it from its stalk and pushing it caudally through the
mesenchyma, it will cause the formation of a lens from any portion of the epidermal
epithelium which happens to be above it. Moreover, if an area of skin from the abdomen of a frog of one species is grafted over the optic vesicle of another species, a
lens may be produced from the grafted epithelium. Thus there is no predetermined
area for lens formation, and its development depends upon the presence of the vesicle
beneath (Amer. Journ. Anat., 1904, vol. 3, pp. 505-536, 1907, vol. 7, pp. 145-169).
The two layers of the optic cup, the inner of
which is toward the lens, are normally in contact
with one another, although in sections they have
often become more or less separated. They constitute the retina, which includes a thin outer pigmented layer, and a thick inner visual layer; the
FIG. 450. OPTIC CUP AND latter is composed of several strata of nerve cells and
STALK OF A HUMAN
avfter Y Konmann ) ""' n t> ers - The stimulus of light is received by tapering
projections extending from the outer surface of the
visual layer toward the pigmented layer; to reach them the rays of
light must traverse the strata of the visual layer. In explanation of
the fact that the sensory processes are turned away from the light, it
may be said that the outer surface of the skin ordinarily receives stimuli,
and that through the infolding which makes the medullary tube and the
outpocketing which makes the optic vesicle, the sensory surface of the
retina is continuous with the outer surface of the skin. Since in mammals
the optic vesicles begin to form before the related portion of the medullary
groove has closed, they first appear as depressions in a thickened epidermal
ectoderm.
Nerve fibers grow from the inner surface of the visual layer toward
the central artery and vein of the retina, around which they pass out of
the optic cup (Fig. 451, D). They grow beneath and among the cells of
the optic stalk to the brain, which they enter. These fibers, which constitute the optic nerve, cause the obliteration of the optic stalk. It is
EYE
441
shown in the figure that the optic nerve at its origin interrupts the retinal
layers, producing a "blind spot." The part of the nerve which forms the
blind spot, with the vessels in its center, is called the papilla o1 the optic
nerve.
The lens (Fig. 451, D) loses its central cavity by the elongation of the
cells in its posterior layer. These become the fibers of the lens. The
anterior layer remains throughout life as a simple epithelium, called the
epithelium oj the lens. The lens becomes covered by an elastic capsula
i.f.
FIG. 451. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OF THE EYE. A, 9} days. 3.0 mm.
B, 10} days, 5.4 mm.; C, n days, 5.0 mm.; D, 14 days, 18 hours, 12.0 mm.; E, 20 days, 29 mm.
a c. r., Arteria centralis retinae; c., conjea; c. a., anterior chamber; conj., conjunctiva; c. p., posterior
chamber; c. v., corpus vitreum; e. 1., eyelid; f. b., fore-brain; 1., lens; 1. e., lens epithelium; 1. f., lens
fibers; o. c., optic cup; o. n., optic nerve; o. v., optic vesicle; r. p., pigmented layer of the retina; r. v.,
visual layer of the retina.
lentis, and in embryonic life it possesses a -vascular capsule (Fig. 451, E)
containing branches of the central artery. The vascular layer covering
the anterior surface of the lens is designated the pupillary membrane, and
it disappears shortly before birth. Its occasional persistence interferes
with vision.
Between the lens and the retina there is a peculiar tissue, mucoid in
appearance and resembling mesenchyma in form. Since processes from
442 HISTOLOGY
the retina and from the lens have been found extending into it, it is considered to be essentially ectodermal. Its blood vessels become obliterated and it forms the vitreous body of the adult, consisting of a stroma and
a humor. Extending through it, from the papilla of the optic nerve toward
the lens, is the hyaloid canal, which in the embryo lodged the hyaloid
artery (a prolongation of the central artery). Sometimes this artery is
represented in the adult by a strand of tissue. The vitreous body is
surrounded by a fibrous layer called the hyaloid membrane.
A cavity forms in the tissue in front of the lens and becomes filled with
a watery tissue fluid (aqueous humor). It is bounded by a mesenchymal epithelium. The portion of the cavity which is anterior to the
retinal cup and lens is called the anterior chamber of the eye; the smaller
part within the retinal cup but in front of the lens and the fibrous covering
of the vitreous body, is the posterior chamber (Fig. 451, E, c.p.).
The retinal cup is surrounded by two layers of mesenchymal origin.
The inner tunica vasculosa corresponds with the pia mater and forms the
chorioid coat of the eye; the outer tunica fibrosa corresponds with the
dura mater and forms the sclera, into which the muscles of the eye are
inserted. The portion of the retinal cup which forms a curtain, circular
in front view, between the anterior and posterior chambers, is called the
iris. It consists of tunica vasculosa with a thin pigmented prolongation
of the retina over its posterior surface (Figs. 451, E, and 452). This pars
iridica retina is rudimentary and without visual function. The iris is
covered by the mesenchymal epithelium of the chambers. At the attached border of the iris the vascular coat contains important muscle
fibers, and is there thickened to form the ciliary body. This is also covered
by a rudimentary pigmented layer on its inner surface, the pars ciliaris
retina. At the ora serrata (Fig. 467) an abrupt thickening of the visual
layer of the retina marks the boundary between its ciliary and optic
portions. The pars optica retina extends from the ora to the optic
nerve, covered externally by the chorioid and sclera.
As a relatively frequent congenital anomaly, the chorioid fissure fails to close
normally and the resulting defect is known as coloboma. If the closure has been
nearly complete, so that there is merely a notch at the free margin of the optic cup,
it appears in the adult as a median ventral cleft in the iris, so that the pupil is shaped
like an inverted pear. If the deeper parts of the chorioid fissure fail to unite, there
will be a median ventral gap in the optic portion of the retina, which may seriously
interfere with vision.
The cornea is the tissue in front of the anterior chamber, consisting of
a non-vascular mesenchymal tissue, bounded posteriorly by mesenchymal
epithelium and anteriorly by the epidermal ectoderm. It is extremely
transparent. The epidermal ectoderm extends from the cornea and front
of the eye over two folds which form the eyelids. They have met in
EYE
443
Fig. 451, D, and fused temporarily. Externally the lids are covered by
skin, and internally by the conjunctiva palpebrarum, or conjunctiva of the
lids. The latter is continuous with the conjunctiva bulbi which forms
the opaque vascular "white of the eye." It surrounds the cornea, the
epithelium of the two structures being continuous.
The parts of the eye to be examined histologically are therefore the
retina, optic nerve, lens, and vitreous body, all of which are ectodermal;
pithelium
-Anterior basal lamina
Substantia propria
.Posterior basal lamina
Mesenchymal epithelium
of the cornea.
Sphincter muscle
Stroma
Pars iridica retinae
Angle of the iris.
of the iris.
Sinus venosus sclerae
/
.. Epithelium "I o f the
L conjunctiva
- Tunica f bulbL
J
propria
Sclera.
Zonula. Ciliary process muscle fibers.
Circular Meridional Pars ciliaris retinae.
FIG. 452. MERIDIONAL SECTION OF A PART OF THE EYE. X 15.
The radial fibers of the ciliary muscle cannot be distinguished with this magnification.
then the tunica vasculosa, including the chorioid, the ciliary body, and
iris; next the tunica fibrosa, including the sclera and cornea; and finally
the accessory structures the lids, conjunctivas and glands.
RETINA.
The retina extends from the papilla of the optic nerve to the pupillary
border of the iris, and is divisible into three parts; the pars optica retina
includes all which is actually connected with the optic nerve and which
therefore is sensitive to light. It covers the deeper portion of the optic
444
HISTOLOGY
cup, ending near the ciliary body in a macroscopic, sharp, irregular line
bounding the ora serrata. The pars ciliaris and the pars iridica retina
are the rudimentary layers covering the ciliary body and iris respectively.
The pars optica retinae in a fresh condition is a transparent layer
colored reddish by the "visual purple." In sections it presents many
layers arranged as seen in Fig. 453, the cells of which are related to one
another as in the diagram, Fig. 454. The outer layer of the optic cup
forms the pigmented epithelium of the retina, which consists of a simple
layer of six-sided cells. Toward their outer surface (that next the chorioid,
where the nucleus lies) they are poor in pigment, whereas in their inner
portion they contain numerous rod-shaped (1-5 p, long) brown granules of
r
Chorioid. {
Pigmented _
epithelium.
f
Layer of rods and
cones.
Membrana^imitans _
externa.
f
Outer nuclear I
layer.
Henle's fiber layer j
Outer reticular
layer.
Inner nuclear
layer.
Inner reticular ,'
layer.
Ganglion cell layer
Nerve fiber layer.
Membrana limitans
interna.
Vessels of the
S chqriocapil
laris.
Lamina basalis.
Rods 1 Outer.
- Cones J segment.
Cones
s Rods
Inner
segment.
t&4
\ ^jy
\
\
.
Mi%
1 ,. Base of a cone fiber.
Nucleus of a radial
fiber.
Nucleus of an
amakrine cell.
Pyramidal base of
a radial fiber.
Blood vessels.
FIG. 453. VERTICAL SECTION OF A HUMAN RETINA. X 36.
the pigment " fuscin." In albinos the pigment is lacking. From the inner
surface of the pigmented epithelium, numerous processes extend between
the rods and cones.
The visual cells, which are found along the outer surface of the inner
retinal layer, are of two sorts, rod cells and cone cells. In both, the nucleus
is found in the inner half of the cell, and the outer non-nucleated half
projects through a membrane, the membrana limitans externa. This
causes the visual cells to appear divided into layers, their nucleated
parts beneath the limiting membrane constituting the outer nuclear layer
(or outer granular layer), and the non-nucleated parts outside of the
membrane forming the layer of rods and cones.
The rods are four times as numerous as the cones. They are regularly
placed so that three or four rods are found between every two cones (Fig.
EYE
445
453). The rods are elongated cylinders (60 n long and 2 ju thick) consisting
of a homogeneous outer segment, in which the visual purple is found
exclusively, and a finely granular inner segment. In the outer third of
the inner segment there is said to be an ellipsoidal, vertically striated
structure (which in some lower vertebrates is very distinct). The portion of the rod cells below the limiting membrane is a slender thread,
expanding to surround the nucleus which is characterized by from one
to three transverse bands. Beneath the nucleus the protoplasm again
becomes thread-like, and this basal prolongation of the cell terminates
in a small club-shaped enlargement, without processes (Fig. 454).
Cone cell
Rod cell.
Stellate ganglion cell.
Bipolar cells.
Amakrine cells.
Centrifugal nerve fiber.
Multipolar ganglion cell.
11 Layer of rods and cones.
Membrana limitans
' externa.
Outer nuclear layer.
Henle's fiber layer.
1 Outer reticular layer.
Dinner nuclear layer.
Inner reticular layer.
Ganglion cell layer.
==^-_ = Nerve fiber layer.
Collateral. Pyramidal bases
of radial fibers.
FIG. 454. DIAGRAM OF HUMAN RETINA. SUPPORTING SUBSTANCE RED.
The cones likewise consist of an outer and an inner segment. The
conical outer segments are shorter than those of the rods. The inner
segments are thick and somewhat dilated so that the entire cone is flaskshaped. Moreover, the inner segment contains a vertically striated
"fiber apparatus." The nuclei of the cone cells are situated just beneath
the limiting membrane; below the nuclei the protoplasm forms a fiber, ending in an expanded pyramidal base.
The entire visual cells therefore form three layers of the retina, namely,
(i) the layer of rods and cones; (2) the outer nuclear layer, containing the
nuclei of the rod and cone cells; and (3) Henle's fiber layer, composed
of the basal processes of these cells. The three layers next beneath are
formed essentially of superposed parts of the radially arranged bipolar
nerve cells, which constitute the ganglion retina. Immediately beneath
Henle's fiber layer, dendritic processes of these cells form an outer reticular
layer, whereas their nuclei are situated in an inner nuclear layer, and their
centripetal processes, or neuraxons, enter an inner reticular layer. There
446 HISTOLOGY
they terminate in relation with dendrites and cell bodies of large ganglion
cells which constitute the ganglion of the optic nerve. Cell bodies of this
ganglion form the ganglion cell layer, and their neuraxons, traveling toward
the papilla of the optic nerve, are the principal elements in the nerve fiber
layer. The latter is separated from the vitreous body by an internal
limiting membrane. Thus visual stimuli, received by the rods and cones,
are transferred by means of the bipolar cells of the ganglion retinae, to the
ganglion cells of the optic nerve, through the neuraxons of which they
proceed to the brain. These layers may be described in further detail as
follows:
Henle's fiber layer contains not only the fiber-like basal ends of the
rod and cone cells, but also the slender unbranched dendritic processes
of the bipolar cells of the ganglion retinae. Each bipolar cell sends one
such process through Henle's layer to terminate in a little thickening
near the membrana limitans externa. In the outer reticular layer, however, these dendrites of the bipolar cells send out branches which bifurcate
repeatedly, becoming reduced to the finest fibrils; they form a close subepithelial felt- work (Fig. 454).
Occasionally nuclei are found in the outer reticular layer. Most of
these belong with bipolar cells displaced outward (Fig. 454, x). Toward
the inner nuclear layer, however, there are stellate ganglion cells with
neuraxons which pursue a horizontal course and then turn inward to join
the optic nerve fibers, as shown in Fig. 454. The existence of such fibers
has been denied by some writers. The neuraxons of other stellate ganglion cells in this region end in relation with the bases of the visual cells
(Fig. 454, +).
Toward the inner reticular layer, the inner nuclear layer contains
the bodies of ganglion cells, which appear to lack a chief or large process,
and are therefore called "amakrine" cells. They send branching fibers
into the inner reticular layer, where they interlace with the fine varicose
branches of the bipolar cells, and with the ramifications of the dendrites
from the ganglion nervi optici.
The ganglion cell layer consists of a single row of large multipolar
cells containing Nissl's bodies. Certain of these cells because of exceptional size are known as "giant ganglion cells," and they occur at quite
regular intervals. "Twin cells" have been found, consisting of two cell
bodies united by a short bridge; only one of the pair has a neuraxon.
The nerve fiber layer consists chiefly of the non-medullated neuraxons
of the ganglion cells, arranged in plexiform bundles. Occasionally the
neuraxons send collaterals back to the ganglion cell layer, where they
branch about the cell bodies (Fig. 454). The fiber layer contains also
neuraxons which have come out from the brain to terminate in free
branches among the cells of the inner nuclear layer.
EYE
447
In addition to the nervous elements, the retina contains blood vessels
and a supporting framework of neuroglia cells. The largest vessels are
toward the fiber layer (Fig. 453), in thich they travel to and from the central vessels in the papilla. The neuroglia framework consists chiefly of
radial (or Miiller's) fibers, which are elongated cells extending from the
internal to the external limiting membrane. Beyond this membrane they
send short processes between the rods and cones, forming "fiber baskets"
(Fig. 455). The radial fibers are not isolated cells but are parts of a
general syncytium, being connected by a network of processes which penetrate all the layers of the retina (Fig. 454). The external limiting membrane, through which the rods and cones pass, is formed by the coa
Fiber basket.
Nucleated part of the
fiber.
__. Basal pyramid.
- Precipitate.
FIG. 455. GOLGI PREPARATION OF RADIAL FIBERS IN A THICK SECTION OF THE HUMAN RETINA.
The fine processes of the fibers in the outer nuclear layer appear as a compact mass. X 360.
lescence of these processes, and the internal limiting membrane is made up
of the closely adjacent basal expansions of the radial fibers. The nuclei
of the fibers are found in the inner nuclear layer. In addition to the radial
fibers there are neuroglia cells with horizontal or tangential branches
(Fig. 454, "oo"). As in the central nervous system, some of the stellate
groups of fibers do not contain nuclei.
Two modifications of the retina require special description, namely,
the fovea centralis, which is the region of most acute vision, and the pars
ciliaris, which is the rudimentary peripheral portion.
Macula lutea and fovea centralis. When vision is centered upon a
particular object, the eyes are so directed that the image of the object falls
upon the macula lutea, or yellow spot of the retina, within which there is
a depression, the fovea centralis. The macula sends straight slender
448
HISTOLOGY
EYE 449
fibers to the papilla of the optic nerve, which is close by on its median
side; other coarser optic fibers diverge as they pass the macula, forming
an ellipse around it. The retinal layers of the macula are arranged as
shown in Fig. 456. At its border the number of rod cells diminishes, and
within the macula they are entirely absent. The nuclei of the numerous
cone cells, which are here somewhat smaller than elsewhere, form an inner
nuclear layer of twice the usual thickness. The basal portions of the
cone cells make a broad Henle's fiber layer, and slope away from the
fovea. The bipolar cells of the ganglion retinae are so numerous that
their nuclei may form nine rows. The ganglion cells of the optic nerve
are also abundant. All of these strata become thin toward the fovea, the
deepest part of which contains scarcely more than the cone cells. In
some individuals the slope of the sides of the fovea is less steep than in the
figure; its depth is variable. The macula and fovea are saturated with a
yellow pigment soluble in alcohol.
Pars ciliaris retina. The optic nerve fibers and their ganglion cells
disappear before reaching the ora serrata. The cone cells extend further
toward the ora than the rods, but the last of them appear to lack outer
segments. By the thinning of the reticular layer, the nuclear layers
become confluent (Fig. 457). Near the ora serrata large clear spaces
normally occur in the outer nuclear layer, and they may extend into the
deeper layers (Fig. 457). The radial sustentacular cells form a simple
columnar epithelium as the other layers disappear, and they constitute
the visual layer of the pars ciliaris. The pigmented epithelium is apparently unmodified as it extends from the optic to the ciliary portion. Along
the inner surface of the ciliary part of the retina, the cells of the visual
layer produce closely packed horizontal fibers, which form a refractive
hyaline membrane.
Zonula ciliaris. Some of the fine homogeneous fibers arising from the
pars ciliaris immediately in front of the ora serrata enter the vitreous body,
but a much larger number pass between the ciliary processes to the lens.
They are attached to the borders of its capsule, overlapping slightly its
anterior and posterior surfaces. Thus they form the zonula ciliaris (suspensory ligament) which holds the lens in place (Fig. 452). The zonula
is not a continuous layer, nor does it consist of two laminae, one to the anterior and the other to the posterior surface of the lens, with a space between them. It consists rather of numerous bundles, between which and
the vitreous body, and among the bundles themselves, there are zonular
spaces (canals of Petit) which communicate with the posterior chamber.
OPTIC NERVE.
In its intraorbital portion the optic nerve is surrounded by prolongations of the meninges. On the outside is the dural sheath, consisting of
29
45
HISTOLOGY
_ ^- "Vacuole.
: K-V
:w
*^-,V *:*>'
-Radial fibers of
Muller.
11
EYE
451
dense connective tissue with many elastic fibers. The outer connective
tissue bundles tend to be longitudinal and the inner circular. Internally
the outer sheath is connected with the arachnoid layer by a few dense
strands of tissue, and the arachnoid joins the pial sheath by many branched
trabeculae. The pia surrounds the entire nerve and sends anastomosing
septa among the bundles of nerve fibers. The latter are slender and
medullated, but without a neurolemma; they are supported by longrayed neuroglia cells, which are found between the individual fibers, but
are most numerous at the periphery of the bundles and around the entire
nerve. Thus the optic nerve differs from the peripheral nerves, and
resembles a cerebral commissure.
At the posterior surface of the eye-ball (or bulbus oculi), the dura
blends with the sclera. Continuous with both is the dense elastic lamina
cribrosa which is perforated by the optic nerve fibers. The chorioid and
the pia are also in relation with this lamina (Fig. 458). As the optic
Central artery.
Fibers of the lamina cribrosa. | Central vein.
Hyaloid membrane t
loosened.
Dural sheath
FIG. 458. LONGITUDINAL SECTION OF THB OPTIC ENTRANCE OF A HUMAN EYE. X 15.
Above the lamina cribrosa is seen the narrowing of the optic nerve, due to its loss of myelin. The central
artery and vein have been for the most part cut longitudinally, but above at several points transversely.
nerve penetrates the lamina, its fibers lose their myelin and radiate into
the nerve fiber layer of the retina. The central artery and vein of the
retina enter the optic nerve in its distal half, and appear at the fundus of
the eye in the center of the optic papilla. Their branches spread in the
inner layers of the retina, which are covered by the membrana limitans
interna (Fig. 453).
LENS.
The lens is a biconvex structure having an anterior and a posterior
pole, and a vertical equatorial plane. It is enclosed in a thick transparent
452
HISTOLOGY
elastic capsule, 6.5-25 /* thick in front and 2-7 n thick behind, which is
apparently derived from the lens itself. Within the capsule the anterior
surface of the lens is formed by the lens epithelium, a single layer of cells
2.5/1 thick at the pole, but becoming taller toward the equator. There
they are continuous with the elongated lens fibers of the posterior layer,
which collectively are called the substantia lentis.
Originally the fibers multiply throughout the lens, but in later stages
the formation of new fibers, as indicated by the presence of mitotic figures,
is limited to the region of transition between the lens epithelium and
the mass of lens fibers (Figs. 451, E, and 460). When first formed the
fibers are short, but they increase in length and become six-sided prisms,
somewhat enlarged at one or both ends. The first fibers extend from one
surface of the lens to the other. Later these become buried in by the new
C
FIG. 459. LENS FIBERS OF A NEW-BORN INFANT
A, Isolated lens fibers; three with smooth and one
with dentate borders. X 240. B, Human
lens fibers cut transversely; c, section
through club-shaped ends. X 560.
IG. 460. CAPSULE AND EPITHELIUM OF A LENS OF
ADULT MAN.
C, Tangential section. D. Meridional section across
the equator of the lens; i, capsule; 2, epithelium;
3, lens fibers. X 240.
fibers formed at the periphery, and thus they constitute the nucelus of the
lens. This is a dense mass of somewhat shrunken fibers, which have lost
their nuclei and have acquired wavy or notched margins (Fig. 459).
The outer fibers of the cortical substance are softer. They have smooth
borders, and nuclei which are chiefly in the equatorial plane. Their protoplasm is transformed into a clear fluid substance, said to be chiefly a
globulin. The fibers are united to one another by a small amount of
cement substance, which is more abundant at the poles, at each of which it
forms a "lens star," usually with nine rays.
When the fibers formed at the periphery of the original nucleus elongate so as to
cover it in, they do not extend from one pole to the other. Those that reach the anterior pole fall short of the posterior pole, terminating along a horizontal suture of
cement substance; and conversely those that reach the posterior pole terminate
anteriorly along a linear vertical suture. As the lens becomes larger, the linear sutures
at either pole are replaced by tri-radiate or Y-shaped stars, one of which is inverted.
EYE 453
Lens fibers starting near the center of one star end near the tips of the rays of the
other, and vice versa. When the stars become nine-rayed the arrangement of the
fibers is very intricate. Without crossing one another, and without any of them being
long enough to pass from pole to pole, they cover the lens with even layers. The
development of the stars is described by Rabl (Ueber den Bau und Entwicklung der
Linse, Leipzig, 1900). As a result of its structure the lens may be separated into concentric lamellae, but Rabl considers that the meridional segments, or "radial lamellae,"
of which the lens contains about two thousand, are its essential subdivisions.
VITREOUS BODY.
The corpus vitreum consists of the fluid vitreous humor, together with
looser or denser strands of fibrous stroma which stretch across it in all
directions. Although it is difficult to recognize any definite arrangement
in the stroma, certain pathological cases suggest that it is distributed like
the septa of an orange. The cells of the vitreous body are round forms,
probably leucocytes, and stellate or spindle-shaped connective tissue
cells, sometimes degenerating and vacuolated, which invaded the vitreous
body with the blood vessels. The latter have atrophied and been resorbed,
except for occasional shreds and filaments. Such opacities, which occur
normally, are observed when looking at a bright light, and are frequently
troublesome to those beginning to use the microscope; because of their
erratic motion they are known to physiologists as muscce volitantes. In
old age, in eyes otherwise normal, crystals may form in the vitreous
humor and float about, " falling like a shower to the bottom of the eye when
the eye is held still." Surrounding the vitreous body there is a very resistant thick fibrous layer, which is continuous anteriorly with the hyaloid
membrane of the ciliary part of the retina.
TUNICA VASCULOSA.
Chorioid. Between the sclera and the chorioid there is a loose tissue
containing many elastic fibers and branched pigment cells, together with
flat non-pigmented cells. In separating the sclera from the chorioid, this
layer is divided into the lamina fusca of the sclera and the lamina suprachorioidea. Internal to the latter is the lamina vasculosa, which forms the
greater part of the chorioid. It contains many large blood vessels imbedded in a loose elastic connective tissue, some of its cells being branched
and pigmented; others without pigment are flat and arranged in layers surrounding the vessels. A thin inner layer of blood vessels, the lamina choriocapillaris, consists of a very close network of wide capillaries. The choriocapillaris is separated from the pigmented epithelium of the retina by a
structureless elastic lamella which may be 2 n thick. This lamina basalis
shows the imprint of the polygonal retinal cells on its inner surface, and is
associated with fine elastic networks toward the choriocapillaris.
454
HISTOLOGY
Between the vascular lamina and the choriocapillaris, there is a boundary layer
consisting of a fine elastic network, generally without pigment. Here in ruminants
and horses there are many wavy bundles of connective tissue, which give to the eyes
of those animals a metallic luster. Such a layer is known as the tapetum fibrosum.
The similarly iridescent tapetum cellulosum of the carnivora is formed of several
layers of flat cells which contain numerous fine crystals.
The ciliary body encircles the eye as a muscular band, attached to the
inner surface of which there are from 70 to 80 meridional folds, the ciliary
processes (Fig. 452). The equator of the eye is vertical, like that of the
lens, and the meridians are antero-posterior. The processes begin low at
the ora serrata and rise gradually to a height of i mm., terminating
abruptly near the border of the lens. Each process consists of fibrillar
connective tissue containing numerous elastic fibers and blood vessels,
and is bounded toward the pars ciliaris retinae by a continuation of the
Cross and longitudinal
sections of bundles
of scleral fibers.
Lamina supra\ chorioidea.
Lamina vasculosa.
Boundary zone.
Choriocapillaris.
'' Basal membrane.
Pigment layer of the
retina.
FIG. 461. VERTICAL SECTION THROUGH A PART OF THE HUMAN SCLBRA AND THE ENTIRE THICKNESS OF
THE CHORIOID. X 100.
g, Large vessels; p, pigment cells; c, cross section of capillaries.
lamina basalis, which is thrown into intersecting folds. The ciliary processes, which are compressible, may serve to prevent the increase of intraocular pressure during the contraction of the ciliary muscle; and the
fluid within the eye is derived from the vessels which they contain. The
ciliary muscle is a band of smooth muscle fibers about 3 mm. broad and
0.8 mm. thick anteriorly; it arises beneath the sinus venosus of the sclera
and tapers toward the ora serrata (Fig. 425). It consists of three sets
of fibers, the meridional, radial, and circular. The meridional fibers (Fig.
452, p. 443) are next to the sclera, grouped in numerous bundles with elastic
tissue intermingled. They extend to the smooth part of the chorioid,
and constitute the tensor chorioidea. The radial fibers are directed to
EYE
455
ward the center of the eye-ball. They form a middle layer of curving
fibers which blend with the meridional fibers externally. The circular
fibers, which vary in number in different individuals, form that part of the
ciliary muscle which is nearest to the equator of the lens. The contraction of these muscles affects
the shape of the lens, which is
attached to the adjacent tissue
by the zonula.
The iris consists of its stroma
anteriorly, and the pars iridica
retina posteriorly, and is covered
by the mesenchymal epithelium
of the chambers of the eye.
The anterior epithelium is a
simple layer of flat polygonal
cells (sometimes called "endothelium"). It rests upon a
loose network of stellate cells,
in part pigmented, resembling
the reticulum of a lymph gland. This is followed by the loose connective tissue of the stroma, likewise containing networks of stellate cells,
which in blue eyes are not pigmented. The very few elastic fibers are
limited to the posterior layers, where they are radially arranged in rela
Mesenchymal
epithelium.
Loose connective
tissue.
p
FIG. 462. A, FROM A TEASED PREPARATION OF A HUMAN
CHORIOID. X 240. p, Pigment cells; e, elastic fibers;
k, nucleus of a flat non-pigmented cell; the cell body
is invisible.
B, PORTION OK A HUMAN CHORIOCAPILLARIS AND THB
ADHERENT LAMINA BASALIS. X 240. c, Wide
capillaries, some of which contain (b) blood-corpuscles; e, lamina basalis, showing a fine lattice work."
r/t
Vascular layer.
Spindle cell layer.
JBi
FIG. 463. VERTICAL SECTION OF THE PUPILLARY PORTION OF A HUMAN IRIS. X 100. About one-fifth
of the entire width of the iris is shown.
g, Blood vessel, with thick connective tissue sheath; m, sphincter pupilla muscle cut transversely;
pupillary border of the iris.
tion to the pupil. The stroma contains numerous radial blood vessels with thick connective tissue coats, but (in man) without musculature or elastic fibers. In the vascular layer, toward the pupillary
border of the iris, there is a band of circular smooth muscle fibers, i
mm. deep; this is the sphincter pupilla. It is invested with many
456 HISTOLOGY
prolongations of the stromatic network, the polygonal meshes of which
are radially elongated. The dilatator pupilla is a peculiar membrane
of smooth muscle fibers on the posterior surface of the vascular
layer, stretching from the connective tissue between the muscle
bundles of the sphincter, to that between the ciliary muscles. Its fibers
consist of an anterior contractile portion, and a posterior nucleated and
pigmented portion. The anterior parts form a continuous layer, readily
seen in radial sections as "Henle's spindle cell layer," which is a clear
non-nucleated stripe, 2-5 p wide (Fig. 463). The nucleated portions of
the fibers appear to blend with the pigmented retinal layer of the iris,
from which they are derived. These muscles are therefore ectodermal.
The two layers of the optic cup are intimately blended in the thin
stratum which forms the posterior layer of the iris. Except in albinos,
this pars iridica retinae is deeply pigmented. Posteriorly it is covered
by a continuation of the hyaline membrane of the pars ciliaris.
TUNICA FIBROSA.
The sclera, toward the chorioid, is bounded by the pigmented lamina
fusca. This is a loose tissue containing branched pigment cells and
flattened connective tissue cells. Except for this boundary layer, the
sclera consists of densely interwoven bundles of connective tissue, chiefly
meridional and longitudinal. Elastic fibers accompany the bundles, and
are especially abundant at the insertions of the ocular muscles. The flat
irregular cells of the connective tissue are surrounded by tissue spaces as
in the cornea, and anteriorly the cornea and sclera are continuous with one
another. The transition, however, is quite abrupt and the boundary is
oblique, so that the rim of the cornea is bevelled at the expense of its anterior surface.
The cornea (Fig. 464) consists of an outer epithelium, external basal
membrane, substantia propria, internal basal membrane, and mesenchymal epithelium bounding the anterior chamber. The corneal epithelium, about 0.03 mm. thick, is stratified and consists of a basal layer of
clearly outlined columnar cells followed by three or four rows of cuboidal
cells and several layers of flattened superficial cells. The outer cells
retain their nuclei. Peripherally the epithelium is continuous with that
of the conjunctiva bulbi. The anterior basal membrane (Bowman's) is
an almost homogeneous layer, sometimes as much as o.oi mm. thick.
Superficially it connects with the epithelial cells by spines and ridges.
Beneath, it blends with the substantia propria, of which it is a modification.
Since it is not formed of elastic substance the name "anterior elastic membrane" is not justified.
The substantia propria consists of fine straight fibrils of connective
EYE
457
tissue, bound together in bundles of almost uniform thickness by an
interfibrillar substance, perhaps fluid; these bundles are joined to one
another by an interfascicular cement, so that they form a succession of
Epithelium.
Anterior basal membrane.
Substantia propria.
Posterior basal membrane.
Mesenchymal epithelium.
FIG. 464. VERTICAL SECTION OF A HUMAN CORNEA. X 100.
Corneal canaliculus. Corneal space. Corneal cells.
FIG. 465. CORNEAL SPACES AND CANALICULI (IN FIG. 466. CORNEAL CELLS FROM A HORIZONTAL
WHITE) FROM A HORIZONTAL SECTION OF SECTION OF THE CORNEA OF A RABBIT. X 240.
THE CORNBA OF AN Ox. Silver preparation.
X 240.
superposed flat lamellae, parallel with the corneal surface. Oblique bundles, the so-called arcuate fibers, are found especially in the anterior
layers, where they pass from one lamella to that next above or below.
458 HISTOLOGY
Numerous tense elastic fibers are found especially in the deeper layers,
where they form a fine network over the posterior elastic membrane.
Within the cement substance, there is a system of branched canaliculi,
dilated in places to form oval spaces. The latter are between the lamellae,
but the canaliculi extend also among the constituent fiber-bundles.
Within the spaces, there are flat stellate anastomosing cells or "corneal
corpuscles," the branches of which extend into the canals and tend to
unite with those of neighboring cells, at right angles (Fig. 466). The cells
and their processes are more or less surrounded by serous fluid. Leucocytes enter the canals, and are normally found in the cornea; if the cornea
is inflamed they become abundant. Blood vessels and lymphatic vessels
are absent.
The posterior basal or elastic membrane (Descemet's membrane) is a
structure clear as glass, 6 n thick. Its posterior surface is covered by a
simple layer of flat polygonal cells (Fig. 464), which form a part of the
lining of the anterior chamber. Toward the periphery of the cornea in
adults, the posterior surface of the elastic membrane presents rounded
elevations, and the posterior epithelium becomes continuous with the
anterior epithelium of the iris (Fig. 452). In this "angle," the cornea
receives connective tissue prolongations from the iris, which form the
pectinate ligament of the iris a structure highly developed in the horse
and cow, but rudimentary in man.
BLOOD VESSELS.
The central vessels of the retina supply a part of the optic nerve, and
the retina; the ciliary vessels supply the rest of the eye. These two sets
of vessels anastomose with one another only at the entrance of the optic
nerve (Fig. 467).
The ciliary arteries include (i) the short posterior ciliary arteries; (2)
the long posterior ciliary arteries; and (3) the anterior ciliary arteries. The
three groups will be considered in turn.
1. After supplying the posterior half of the surface of the sclera, some twenty
branches of the short posterior ciliary arteries penetrate the sclera around the optic
nerve. They form the capillaries of the lamina choriocapillaris. At the entrance of
the optic nerve they anastomose with branches of the central artery of the retina (c)
and thus form the circulus arteriosus nervi optici. At the ora serrata they anastomose
with recurrent branches of the long posterior ciliary and the anterior ciliary arteries.
2. The two long posterior ciliary arteries also penetrate the sclera near the optic
nerve (Fig, 467, i). They pass, one on the nasal and the other on the temporal side
of the eye, between the chorioid and sclera to the ciliary body. There each artery
divides into two branches which follow the ciliary border of the iris, and connect with
the corresponding branches from the artery of the opposite side, thus encircling the
iris with an arterial ring. This is the circulus iridis major (Fig. 467, 2), from which
EYE
459
numerous branches extend to the ciliary processes (3) and to the iris (4). Near the
pupillary border of the iris, the arteries form an incomplete ring, the circulus iridis
minor.
3. The anterior ciliary arteries proceed from those supplying the recti muscles,
penetrate the sclera near the cornea, and in part join the circulus iridis major, in part
supply the ciliary muscle, and in part through recurrent branches, connect with the
Branches Branches
to the to the
Sinus corneal conjunctiva
venosus border, bulbi.
sclerae. . V > Connection with the lamina choriocapillaris.
en" 53 } ciliaris anterior.
Venous ] episcleral
I branches of the
( anterior
Arterial J ciliary vessels.
Capillaries of the lamtna choriocapillaris.
Vena vorticoia.
"^ Venous \ episcleral branches
Arterial / of the short posterior
ciliary vessels.
1
y e " a . | ciliaris posterioris brevis.
Ik
__, Outer)
v vessels of the sheath.
* Inner J
Short posterior ciliary arteries.
Vena Arteria
centralis retina.
FIG. 467. BLOOD VESSELS OF THE EYE. (After Leber.)
The retina, optic nerve and tunica fibrosa are stippled; the tunica vasculosa is blank. V, Connection of
the anterior ciliary artery with the circulus iridis major (2).
lamina choriocapillaris. Before penetrating the sclera, the anterior ciliary arteries
give off posterior branches for the anterior half of the sclera, and anterior branches for
the conjunctiva bulbi and the corneal border. The cornea itself is without vessels,
but at its border, between the anterior lamellae of the substantia propria, there are
terminal loops.
460 HISTOLOGY
The veins generally proceed toward the equator, uniting in four (less
often in 5 or 6) vents vorticosa. These pass directly through the sclera
and empty into one of the ophthalmic veins. Besides the venae vorticosae
there are small veins accompanying the short posterior and the anterior
ciliary arteries. The short ciliary veins receive branches from the ciliary
muscle, the episcleral vessels, the conjunctiva bulbi and the periphery of
the cornea. The episcleral veins also connect with the venae vorticosae.
Within the sclera, near the cornea, there is a circular vein, receiving small
branches from the capillaries of the ciliary muscle. This sinus venosus
sclerce (canal of Schlemm) connects with the anterior ciliary veins.
Arteria centralis retina. The central artery of the retina enters the
optic nerve 15-20 mm. from the eye-ball, passes to its center and proceeds
to the optic papilla. There it divides into two branches directed upward
and downward respectively, and these by further subdivision supply the
entire pars optica retinae. Within the optic nerve the artery sends out
numerous little branches which anastomose with small vessels that have
entered the sheaths from the surrounding fat; and also with branches of
the short posterior ciliary arteries (Fig. 467, &).
The central vein of the retina receives two main branches at the optic
papilla and follows the artery along the axis of the optic nerve.
CHAMBERS AND TISSUE SPACES OF THE EYE.
The eye contains no lymphatic vessels, but is provided with communicating tissue spaces, bounded by loose cells or mesenchymal epithelia.
They include the corneal and scleral canaliculi, and the anterior and posterior chambers; the latter connect with one another through the capillary interval between the lens and iris. The posterior chamber extends into the
zonular spaces; and there are irregular extensions of the anterior chamber,
associated with the pectinate ligament of the iris, called spaces of the angle
of the iris (spaces of Fontana). The latter are but slightly developed in
man. Posteriorly the tissue spaces include the hyaloid canal of the
vitreous body; the very narrow perichorioideal space between the chorioid
and sclera; the subdural and arachnoid spaces of the optic sheaths
named the intravaginal spaces; and finally the interfascial space (of Tenon)
which surrounds most of the sclera and is prolonged as a supradural space
around the optic nerve. These spaces may be filled from the arachnoid
space about the brain. They contain a "filtrate from the vessels." The
interfascial and perichorioideal spaces hold but little fluid; acting as
bursae, they facilitate the movements of the eye.
NERVES.
Apart from the optic nerve, the eye is supplied by the short ciliary
nerves from the ciliary ganglion, and the long ciliary nerves from the naso
EYE
461
ciliary branch of the ophthalmic nerve. The ciliary nerves penetrate the
sclera near the optic nerve and send branches containing ganglion cells
to the vessels of the chorioid. The main stems pass forward between the
chorioid and sclera to the ciliary body, where they form a circular ganglionated plexus, the plexus gangliosus ciliaris. Its branches extend to
the ciliary body, the iris and the cornea, and are described as follows:
The nerves of the ciliary body form a delicate network on its scleral
surface; they supply its muscle fibers and those of the vessels with slender
motor endings; and between the
ciliary muscle bundles they have
branched free endings, perhaps
sensory.
The medullated nerves of the
iris lose their myelin and form
plexuses as they pass toward the
pupillary margin. A sensory
plexus is found just beneath the
anterior surface, and motor fibers
supply the sphincter, dilator and
vascular muscles. The existence of ganglion cells in the human iris
is denied.
The nerves of the cornea enter it from the plexus annularis in the
sclera just outside. The annular plexus also sends fibers into the conjunctiva, where they end in networks, and in bulbous corpuscles (Fig. 154,
p. 160) situated in the connective tissue close to the epithelium. Such
corpuscles may be found i or 2 mm. within the corneal margin. The
corneal nerves become non-medullated and form plexuses between the
lamellae throughout the stroma. They extend into the epithelium and
there form a very delicate plexus with free intercellular endings.
Substantia \
propria. I
FIG. 468. FROM A SECTION OF THE HUMAN CORNEA.
X 240.
n, A branching nerve penetrating the anterior basal
membrane; s, subepithelial plexus beneath the
cylindrical cells; a, fibers of the intraepithelial
plexus ascending between the epithelial cells.
EYELIDS.
The eyelids or palpebrce (Fig. 469) are covered with thin skin provided with fine lanugo hairs; small sweat glands extend into the corium,
which here contains pigmented connective tissue cells. The subcutaneous
tissue is very loose, having many elastic fibers and few or no fat cells.
Near the edge of the lid there are two or three rows of large hairs, the
eyelashes or cilia, the oblique roots of which extend deep into the corium.
Since they are shed in from 100 to 150 days they occur in various stages of
development. They are provided with small sebaceous glands, and the
ciliary glands (of Moll) open close beside or into their sheaths. The ciliary
glands are modified sweat glands, with simpler coils, which may show
successive constrictions ; " a branching of the tubules has been observed."
462 HISTOLOGY
The central portion of the eyelids is muscular. Beneath the subcutaneous tissue there are bundles of the striated orbicularis palpebrarum
extending lengthwise of the lid. A subdivision of this muscle, found behind
the roots of the cilia, is called the musculus ciliaris Riolani. Posterior to
the obicularis muscle are found the terminal radiations of the tendon of
the levator palpebra. A part of these are lost in connective tissue; another
part, associated with smooth muscle fibers, are inserted into the upper
border of the tarsus and form the superior tarsal muscle. This occurs in
the upper lid, but correspondingly in the lower lid the radiations from the
inferior rectus muscle contain smooth muscle fibers, forming the inferior
tarsal muscle.
The inner portion of the lids consists of the conjunctival epithelium
and the underlying connective tissue, including the tarsus. This is a plate
of dense connective tissue which gives firmness to the lid. It begins at the
free edges and extends over the adjacent two-thirds of the lid, close to the
conjunctiva. Imbedded in its substance in either lid, there are about 30
tarsal (or Meibomian) glands, which open along the palpebral border.
Each of them cnsists of a wide excretory duct, surrounded on all sides
by small acini, which empty into the duct through short stalks. In
structure they resemble sebaceous glands. At the upper end of the
tarsus and partly enclosed in its substance, there are branched tubular
accessory lachyrmal glands. They occur chiefly in the medial (nasal)
half of the lid.
The tunica propria of the palpebral conjunctiva contains plasma and
lymphoid cells; the latter invade the epithelium, beneath which in some
animals they form nodules. The stratified epithelium of the skin gradually changes to that of the conjunctiva, which has several basal layers of
cuboidal cells and a superficial layer of short columnar cells. The latter
are covered by a thin cuticula, and goblet cells are found among them.
The transition from the superficial squamous cells to the columnar form
may occur at the posterior edge of the lid, or quite high on the conjunctival surface. Toward the arch where the palpebral conjunctiva becomes
continuous with that of the bulb, the epithelium is so folded that in sections it may seem to form glands.
The conjunctiva bulbi is similar to that of the lid. Its outer epithelial cells, however, become squamous toward the cornea and over the
exposed portion of the eye, and its basal cells contain pigment. The
yellow appearance of the exposed portion, often most pronounced near
the medial border of the cornea, and known as pinguecula, is said not to
be due to fat or to an epithelial pigment; it accompanies a thickening of
the connective tissue layer. The tunica propria forms well-marked
papillae near the cornea. Its lymphocytes may form nodules, as many as
twenty having been found in the human conjunctiva bulbi. Occasional
EYE
463
mucous glands occur. (It may be noted that the entire anterior covering of the bulb of the eye is named by some the conjunctiva bulbi, which
accordingly is divided into the conj. sclera and the conj. cornets.}
Conjunctiva
;arsal Radiations
mscle. from the tendon Orbicularis
of the levator palpebrae. palpebrarum. Skin.
Epithelium.
Tunica
propria.
Arcus
tarseus
ezternus.
Papilla
Tarsus
Sweat gland.
Oblique section of
a hair sheath.
Cross section of the
bundles of the
orbicularis
palpebrarum.
Epidermis.
Corium.
Stratum
subcutaneum.
Tarsal gland
Arcus tarseus
Part of a
ciliary gland.
Cilium.
Posterior edge of the lid. '
Musculus ciliaris (Riolani).
FIG. 469. SAGITTAL SECTION OF THE UPPER LID OF A CHILD OF Six MONTHS. The outlet of the tarsal
gland was not in the plane of section. X 15.
At the medial angle of the lids there is a thin fold of connective tissue
covered with stratified epithelium; this plica semilunaris is a rudimentary
third lid. The nodular elevation of tissue at the medial angle, the caruncula lacrimalis, resembles skin except that a stratum corneum is lacking;
464
HISTOLOGY
it contains fine hairs, sebaceous and accessory lachrymal glands, and in its
middle part, small sweat glands.
The blood vessels of the lids proceed from branches approaching the
lateral and medial angles of the eye. They form an arch, the arcus tarseus externus, at the upper border of the tarsus, and a second arcus tarseus
near the free margin of the lid (Fig. 469). They extend also into the conjunctiva bulbi, and near the margin of the cornea they pass inward to
unite with the anterior ciliary vessels (Fig. 467). The lymphatic vessels
form a close network beneath the palpebral conjunctiva, and a loose one
in front of the tarsus. Whether the lymphatic vessels of the conjunctiva
bulbi end blindly toward the cornea or connect with the canaliculi, has
not been determined. The nerves form a very thick plexus in the tarsus
and supply the tarsal glands. There are free endings in the conjunctival
epithelium, and bulbous corpuscles in the connective tissue beneath.
LACHRYMAL GLANDS.
The lachrymal glands are groups of compound tubular glands, and are
therefore provided with several excretory ducts. These are lined with a
double row of epithelial ceils, the superficial layer being columnar. The
excretory ducts pass gradually into long intercalated
ducts with a low epithelium.
These terminate in tubules,
surrounded by a membrana
propria, and containing two
sorts of cells. Certain cells
are tall when filled with secretion, which occupies the
superficial half of the cell;
when empty they are shorter.
The cells of the other form
are low when full of secretion, which gathers in a large
FIG. 470. From A SECTION OF A HUMAN LACHRYMAL GLAND.
X 420.
A, Gland body; a, tubule cut across; a', group of tubules cut
obliquely; s, intercalated tubule; s', intercalated tubule
in cross section; b, connective tissue. B, cross section
of an excretory ^uct; e, two-rowed cylindrical epithelium; b, connective tissue.
round mass, leaving only a
thin basal layer of protoplasm. Intercellular secretory capillaries and
secretory granules have been demonstrated. Between the gland cells and
the basement membrane there are occasional flat cells, which are a continuation of the deeper layer of the epithelium of the duct. The blood
vessels and nerves are similar to those of the oral glands.
At the medial angle of either eye there are two lachrymal ducts which
have no connection with the lachrymal glands, but serve to convey the
secretions which pass across the front of the eye to the lachrymal sacs.
EYE 465
From these sacs it passes through the naso-lachrymal ducts into the nasal
cavity. The lachrymal ducts are lined with stratified squamous epithelium, resting upon a tunica propria containing an abundance of cells and
elastic fibers. Externally these ducts are surrounded by striated muscle
fibers, chiefly longitudinal. The lachrymal sac, which is provided with
small branched tubular glands, and the naso-lachrymal duct are both
lined with two-rowed columnar epithelium, surrounded by a lymphoid
tunica propria. They are separated from the underlying periosteum by
a thick plexus of veins.
EAR.
Development and General A natomy. The ear is divided into three parts :
(i) the external ear, which includes the auricles projecting from the surface
of the body, and the external acoustic meatus leading from the surface to
the tympanic membrane; (2) the middle ear, including the tympanic cavity
or "drum" and the chain of three bones extending across it; and (3) the
internal ear, which is a system of epithelial ducts and surrounding tissue
spaces, imbedded in the temporal bone, and connected with terminal
branches of the acoustic nerve.
On either side of the body, the internal ear first appears as a local
thickening of the epidermal ectoderm near that portion of the medullary
tube which later becomes the pons. The thickened areas are invaginated
as shown in Fig. 471 A and B, and the pockets thus produced become
separated from the epidermis in the form of auditory vesicles (otocysts).
The place where they become detached from the epidermis is marked
by a slight elevation on the medial surface of the vesicle, which soon
elongates, producing the tubular endolymphatic duct (Fig. 471, C). The
blind upper end of the duct becomes enlarged to form the endolymphatic
sac, which, however, is only slightly developed in man; it appears in the
models of the embryonic vesicle shown in side view in Fig. 472, A-C. ID
the adult the endolymphatic duct is a very slender tube, terminating
blindly (or perhaps with secondary apertures) just beneath the dura.
In two places the medial and the lateral walls of the upper half of the
vesicle approach one another, and after fusing, the epithelial plates thus
produced become thin and rupture, so that two semicircular ducts are formed
(Fig. 472, B and C). The space encircled by each duct may be regarded
as a hole through the vesicle. The two ducts are the superior and posterior
semicircular ducts respectively . Th e third or lateral semicircular duct forms
soon afterward. In Figs. 471, D and 472, B it is a horizontal shelf-like
projection of the vesicle, the center of which is to become perforated so
that its rim will become the duct. The portion of the vesicle which receives the terminal openings of the three semicircular ducts is called the
30
466
HISTOLOGY
utriculus. Since at one of their ends the superior and posterior ducts unite
in a single stalk before entering the utriculus, there are but five openings
for the three ducts (Fig 472, D). Near one end of each duct there is a
dilatation or ampulla, where nerves terminate.
/m.t.
FIG. 471. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OF THE EAR. X 9.
A, 9 days, 3.8 mm.; B, 10 days, 3.4 mm.; C, I2j days, 7.5 mm.; D, 14 days, 10 mm. a., Ectoderma
epithelium which forms the membranous internal ear; a. bas., basilar artery; ch. t., chorda tympanj.
d. c., cochlear duct; d. e., endolymphatic duct; d. s. 1., lateral semicircular duct; d. s. s., superior semi;
circular duct; ep., epidermis, fa., facial nerve; meten., metencephalon; m. t. medullary- tube; phpharynx.
d.S.p.
FIG. 472. LATERAL OR EXTERNAL SURFACES OF MODELS OF THE MEMBRANOUS PORTION OF THE LEFT
INTERNAL EAR FROM HUMAN EMBRYOS. Different enlargements. (After His, Jr.)
A, from an embryo of 6.9 mm.; B, 10.2 mm.; C, 13.5 mm.; and D, 22 mm. am., ampulla; c T., caecum
vestibulare of d. c., cochlear duct; d. e., endolymphatic duct; d. s. 1., d. s. p., and d. s. s. lateral,
posterior, and superior semicircular ducts; sac., sacculus; ut., utriculus.
While the formation of the semicircular ducts is occurring in the upper
part of the auditory vesicle, the lower portion elongates and its end becomes coiled, eventually making two and a half revolutions. The coiled
EAR
467
tube is the ductus cochlearis; its distal end is the cacum cupulare, and at its
proximal end is the ccecum vestibulare (Fig. 472, D, c. v.). A dilated sac
formed at its proximal or upper end, opposite the caecum vestibulare, is
known as the sacculus; in the adult the connection between the sacculus
and ductus cochleae is relatively narrow, and is called the ductus reuniens
(Fig. 481). The portion of the original vesicle between the sacculus and
utriculus, from which the endolymphatic duct arises, becomes a comparatively slender tube, the ductus utriculo-saccularis (Fig. 481).
The ectodermal vesicle thus produces a complex system of connected
epithelial ducts, namely the superior, posterior, and lateral semicircular
Semicircular duct.
^'' f^^-^^^^W~ * Epithelium of the duct.
''':.' {'^ : .Ivr;^^;':'^ym
Blood vessel. k t3
Wall of the semi- r^g
circular duct.
/ "" Ligament of the duct.
. Bone of the semicircuS3f / lar canal,
me '
Ligament.
Perilymph spaces ..
Blood vessel.
FIG. 473. CROSS SECTION OF A SEMICIRCULAR DUCT AND THE ADJACENT PERILYMPH SPACES TOGETHER
WITH THE SEMICIRCULAR CANAL OF BONE IN WHICH THEY ARE LODGED. From a human adult. X so.
(B6hm and von Davidoff.)
ducts; the utriculus, and utriculo-saccular duct with the endolymphatic
duct connected with it; the sacculus, ductus reuniens and ductus cochleae.
They all contain a fluid called endolymph. The acoustic nerve sends
branches between the epithelial cells in certain parts of the ducts. Round
areas of neuro-epithelium, in which the nerves terminate, are called
macula acustica; there is one in the sacculus and another in the utriculus.
Elongated areas are crisia, and there is one in each of the three ampullae.
The axis, or modiolus, about which the cochlear duct is wound, contains
the nerves which send terminal fibers to the spiral organ of the adjoining
epithelium. In this they form a line of terminations along the medial
wall of the cochlear duct, following its windings from base to cupola.
468
HISTOLOGY
The mesenchyma immediately surrounding the entire system of ducts
becomes mucoid in appearance, and cavities lined with mesenchymal
epithelium are formed within it. They contain a tissue fluid called perilymph. Around the semicircular ducts the perilymph spaces are so large
that the tissue between them is reduced to strands as shown in Fig. 473 ;
these are sometimes called ligaments. The perilymph spaces around the
semicircular ducts are irregularly arranged and communicate with one
another at various points; they connect also with the perilymph cavities
of the vestibule, which is the central part of the internal ear, from which
the semicircular, cocblear and endolymphatic ducts proceed outward.
All of these structures are surrounded by spaces, connecting with those of
the vestibule which enclose the sacculus and utriculus. At the distal
Modiolus.
Bone.
Scala vestibuli.
of the nervus
acusticus.
Meatus acusticus interims.
FIG. 474. HORIZONTAL SECTION OF THE COCHLEA OF A KITTEN. X 8.
The winding ductus cochlearis, x, crossed the plane of section five times. Above it in every case is the
scala vestibuli, and below it is the scata tymponi.
end of the endolymphatic duct, the spaces communicate with those of
the cerebral arachnoid, and the perilymph mingles with cerebro-spinal
fluid.
Around the cochlear duct the perilymph spaces form a single tube.
Starting from the vestibule, it ascends to the cupola, following the windings
of the cochlear duct, to which it is closely applied. It is known as the
scala vestibuli (i.e., "staircase of the vestibule," from which it passes out)
At the apex of the cochlea it turns and becomes the descending scala
tympani, which ends blindly at the base of the cochlea, close against
the wall of the tympanum. The two scalae bear a constant relation to the
coils of the cochlear duct. If the cochlea is so placed that its apex is
upward, the scala vestibuli is always found on the upper side of the
duct, and the scala tympani on the lower side, as shown in Fig. 474. In
the body, the apex of the cochlea is directed forward and outward.
The temporal bone develops from the mesenchyma surrounding the
EAR 469
ducts and their perilymph spaces, so that when the membranous labyrinth
which they form, is removed by maceration, the bone still contains a
corresponding arrangement of cavities and canals. These constitute the
bony labyrinth. Casts of it, made in soft metal, may be seen in all anatomical museums. Instead of subdivisions to correspond with the utriculus,
sacculus, and utriculo-saccular duct, the bony labyrinth has a single space,
already referred to as the vestibule. Into it the semicircular and cochlear
canals open, together with the aquaductus vestibuli which contains the
endolymphatic duct.
The middle ear and external ear arise in connection with the first or
spiracular gill cleft. In common with the other clefts, this includes an
entodermal pharyngeal outpocketing (Fig. 206, p. 217) and an ectodermal
depression (Fig. 205, sp.}. At an early stage these meet one another and
fuse, but later, the primary epithelial connection breaks down, and
mesenchyma intervenes. In the adult, however, the two parts are still
close together, being separated by only the drum membrane, which is
covered on one side with ectoderm and on the other with entoderm.
The ectodermal groove becomes surrounded by several nodular elevations of skin, which coalesce in a definite manner to make the projecting
auricle (pinna). Its depression deepens, becoming the external acoustic
meatus, which extends inward to the tympanic membrane. The entodermal portion of the spiracular cleft becomes in the adult an elongated
outpocketing of the pharynx, known as the auditory tube (Eustachian
tube). As seen in the section Fig. 475, the tube is separated from the
bottom of the meatus by a very thin layer of mesenchyma, which is later
included in the drum membrane.
In the mesenchyma behind the spiracular cleft, a chain of three small
bones (the malleus, incus, and stapes} develops; it extends from the
meatus to the vestibule. The bony wall of the vestibule is deficient
at the small oval area where the stapes reaches it, so that the chain of bones
comes directly in contact with the fibrous covering of the perilymph space.
This area of contact is thejenestra veslibuli (i.e., window of the vestibule).
When the chain of bones vibrates back and forth, the motion of the stapes
is transmitted through the fenestra vestibuli to the perilymph, and waves
may pass up the scala vestibuli and down the scala tympani, stimulating
the nerves of hearing in the cochlear duct. The blind termination of
the scala tympani rests against the lateral wall of the vestibule, where also
the bone fails to develop; the round fenestra cochlea is thus produced. Its
fibrous membrane may yield somewhat to the perilymph waves, thus
relieving tension in the cochlea.
In Fig. 475 the fragments of the chain of bones together with neighboring nerves are imbedded in a mass of mesenchyma. In a later stage
the outer end of the auditory tube expands, filling all the space between
470
HISTOLOGY
the vestibule and the bottom of the meatus. Thus it forms the tympanic
cavity. It encounters the chain of bones and the chorda tympani, and
wraps itself around them so that they lie in its folds or plica. Thus all
structures which extend into the tympanic cavity, or appear to cross it,
are covered with a layer of entodermal epithelium derived from the auditory tube. The original contact between the ectoderm and entoderm
of the spiracular cleft forms only an insignificant part of the tympanic
membrane. The latter becomes greatly enlarged, extending somewhat
along the upper surface of the ectodermal auditory meatus. The portion
of the malleus lying near it becomes imbedded in its mesenchymal layer,
d.s.p.
d.c.' /:;.;:.;.
FIG. 475. HORIZONTAL SECTION THROUGH THE EAR OF A HUMAN EMBRYO OF ABOUT 5 CMS.
an., Auricle; au.t., auditory tube; ch.t., chorda tympani; d.c., cochlear duct; d.s.l., and d.s.p., lateral
and posterior semicircular ducts; e.a.m., external acoustic meatus; fa., facial nerve; f.c., fenestra
cochlea; p.s., perilymphatic space; St., stapes; s.tr., transverse sinus; t.b., temporal bone.
and its inner entodermal layer is made by the expansion of the tympanic
cavity. The enlargement of the tympanic cavity continues after birth,
when it invades the spaces formed within the mastoid part of the temporal
bone.
In spite of these modifications the course of the spiracular cleft is
retained in the adult. The ectodermal depression and its surrounding
elevations constitute the external ear; the pharyngeal outpocketing persists as the auditory tube and the tympanic cavity of the middle ear. It
opens freely into the pharynx and contains air.
SACCULUS, UTRICDLUS, AND SEMICIRCULAR DUCTS.
The walls of all these structures consist of three layers. On the outside there is connective tissue with many elastic fibers and occasional pig
EAR 471
ment cells. This is followed by a narrow basement membrane said to
form small nodular elevations toward the third and innermost layer, the
simple flat epithelium. Near the maculae and cristae the connective tissue
and the basement membrane become thicker, and the epithelial cells are
columnar with a cuticular border. In the neuro-epithelium of these
areas there are two sorts of cells, sustentacular and hair cells. The sustentacular or fiber cells extend clear across the epithelium and are somewhat expanded at both ends; they contain oval nuclei. Hair cells, which
receive the stimuli, are columnar cells limited to the superficial half of the
epithelium; they have large spherical nuclei near their rounded basal ends,
and a clump of fine agglutinated filaments projecting from their free surface. The nerves lose their myelin as they enter the epithelium and ascend to the bases of the hair cells. There **
they bend laterally, forming a dense network which ^
appears as a granular layer in ordinary preparations; ^ ^
the granules are optical sections and varicosities. ^y*
The horizontal fibers terminate like their occasional F IG . 47 6. OTOCONIA
branches, by ascending between the hair cells, on the LUSO/AN INFANT"
sides of which they form pointed free endings. They
do not reach the free surface of the epithelium. This surface is covered by
a continuation of the cuticula, a "membrana limitans," which is perforated
by the hairs. Over the two maculae there is a soft substance containing
very many crystals of calcium carbonate, 1-15 ju long, which are named
otoconia. (Large "ear stones" of fishes are called otoliths.) Over the
cristae of the semicircular ducts there is a gelatinous substance, transparent
in fresh preparations, but coagulated and rendered visible by reagents.
The "ligaments" of the ducts, the thin periosteum of the bony semicircular canals, and the perilymph spaces lined with mesenchymal epithelium are seen in Fig. 473.
COCHLEA.
The relation between the ductus cochleae and the scalae tympani and
vestibuli is shown in Fig. 474. The ductus is triangular in cross section,
being bounded on its peripheral surface by the thick periosteum of the
bony wall of the cochlea; on its apical surface (toward the cupola) by the
membrana vestibularis (Reissner's membrane) ; and on its basal or medial
surface by the lamina spiralis. These three walls may be described in
turn.
The peripheral wall of the cochlear duct is formed by the dense fibrous
periosteum attached to the bone, together with a large mass of looser
tissue crescentic in cross section, the ligamentum spirale (Fig. 477). The
spiral ligament is covered by a layer of cuboidal epithelial cells belonging
472
HISTOLOGY
to the cochlear duct. Close beneath the epithelium there are blood vessels
which are said to give rise to the endolymph. The thick plexus which
they form is described as a band, the stria vascularis, which terminates
more or less distinctly with the vas prominens. The latter occupies a low
elevation of tissue which has its maximum development in the basal coil
of the cochlea (Fig. 477).
The apical wall, or membrana vestibularis, consists of a thin layer of
connective tissue bounded on one side by the mesenchymal epithelium
of the scala vestibuli, and on the other by the simple flattened ectodermal
epithelium of the cochlear duct.
Blood vessels.
Labium
Membrana
Ganglion spirale.
Scala vestibuli.
Ductus
cochlearis.
Vas prominens
Ligamentum
spirale.
Scala tympani
Lamina spiralis ossea
Lamina spiralis membranacea.
FIG. 477. THE PORTION OF FIGURE 474 MARKED "SCALA VESTIBULI" AND "SCALA TYMPANI." X 50
The basal wall or lamina spiralis extends outward from the modiolus
to the bony wall of the cochlea. Near the modiolus it lies between the
two scalae, but peripherally it is between the cochlear duct and the scala
tympani. Toward the modiolus it contains a plate of bone perforated
for the passage of vessels and nerves; this part is the lamina spiralis ossea.
The peripheral portion is the lamina spiralis membranacea. Both parts
are covered below by the mesenchymal epithelium of the scala tympani,
and above by the epithelium of the cochlear duct, including its complex
neuro-epithelium known as the spiral organ (of Corti).
Where the membrana vestibularis meets the osseous spiral lamina, there
is an elevation of tough connective tissue called the limbus spiralis (Fig.
477). It consists of abundant spindle-shaped cells, and blends below with
the periosteum of the spiral lamina. Superficially it produces irregularly
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473
hemispherical papillae covered with simple flat epithelium, found within
the cochlear duct near the vestibular membrane. Further within the
cochlear duct the papillae give place to a single row of flat ridges or plates,
directed peripherally. These are "Huschke's auditory teeth" (Fig. 480).
Beneath them the limbus terminates abruptly in an overhanging labium
vestibulare, which projects over an excavation the sulcus spiralis (Fig.
478). The basal wall of the sulcus is the labium tympanicum, found at
the peripheral edge of the osseous spiral lamina. As the epithelium of
the limbus passes over the labium vestibulare into the sulcus, it becomes
cuboidal. A remarkable non-nucleated structure projects from the labium
Membrana tectoria.
Capillaries of the
Labium vestibulare.
Nerve bundle.
FIG. 478. PORTION OF FIGURE 477.
Deiter's Membrana Connective
- cells. basilaris. tissue.
Pillar cells.
X 240. x, Intercellular "tunnel" traversed by nerve fibers.
vestibulare over the neuro-epithelium of the membranous spiral lamina.
It is called the membrana tectoria and is considered to be a cuticular formation of the labial cells to which it is attached. Hardesty describes
it as composed of "multitudes of delicate fibers of unequal length, embedded in a transparent matrix of a soft, collagenous semi-solid character,
with marked adhesiveness" (Amer. Journ. Anat., 1908, vol. 8, pp. 109-179).
The lamina spiralis membranacea, or lamina basilaris, consists of four
layers. The mesenchymal epithelium of the scala tympani is followed by
a layer of delicate connective tissue, prolonged from the periosteum of the
scala. Its spindle cells are at right angles with the fibers of the overlying
membrana basilaris. This membrane, which is beneath the epithelium of
the cochlear duct, consists of coarse straight fibers extending from the labium
tympanicum to the ligamentum spirale. They cause it to appear finely
striated (Fig. 479). Peripherally (beyond the bases of the outer pillar
cells) the fibers are thicker, and are called "auditory strings"; they are
474 HISTOLOGY
shortest in the basal part of the cochlea and longest toward the apex,
corresponding in length with the basal layer of the cochlear duct. These
fibers have been thought to vibrate and assist in conveying sound waves
to the nerves, but theories which assume that the basilar membrane is
a "vibrating mechanism" are considered untenable by Hardesty; he finds
it more probable that the membrana tectoria vibrates and transmits
stimuli to the neuro-epithelium.
The epithelial cells covering the basilar layer occur in rows of highly
modified forms, which extend up and down the cochlear. duct, constituting
the spiral organ (organ of Corti). Next to the cuboidal epithelium of
the sulcus spiralis there is a single row of inner hair cells
(Fig. 480). These are short columnar cells which do
not reach the bottom of the epithelium; each has about
forty long stiff hairs on its free surface. The inner
hair cells are followed peripherally by two rows of
pillar cells, the inner and outer, which extend the whole
length of the cochlear duct. As seen in cross section
F IG . 479. SURFACE they are in contact above, but are separated below by
VIEW OF THE . . , ., , . - .
LAMINA SPIRALIS a triangular intercellular space or tunnel, which is
MEMBRANACEA OF .....
A CAT. x 240. filled with soft intercellular substance. Thus they rest
Drawn with
change of focus. upon the basilar membrane in A -form. Each pillar
e, Epithelium ( cells of r ,,..,,. 11 , j ,
Claudius) of the cell may be subdivided into a head, a slender body,
ductus cochleans J
o" thTmembrana an ^ an ex P an< ied triangular bas'e. The greater portion
bf s nuc?ei n of oc the ^ eac ^ ce ^ nas been transformed into a resistant band,
nec d t!ve y t^ue con " at the base of which, within the tunnel, there is a mass
of protoplasm containing the nucleus. A protoplasmic
sheath extends up from the base around the body of the cell. Dark round
structures which may be found in the heads of the pillars, and at the
foot of the outer ones, are not nuclei, but are "probably of horny nature."
The heads of the pillars interlock. Both pillars produce "head-plates"
directed outward, and so arranged that the plate from the inner pillar
overlies that from the outer pillar (Fig. 480). Moreover, the round head
of the outer pillar is fitted into a concavity in the head of the inner pillar,
as shown in the figure.
On the peripheral side of the outer pillars there are several rows
(usually four) of outer hair cells separated from one another by sustenlacular cells (Deiter's cells). The outer hair cells have shorter hairs than the
inner ones, and are characterized by the presence of "Hensen's spiral
bodies," one of which occurs in the outer half of each cell. These bodies,
shown as dark spots in Fig. 480, probably represent a trophospongium.
The centrosomes of the hair cells are always in their upper ends. Like
the inner hair cells, the outer ones do not extend to the basilar membrane,
thus leaving unoccupied the communicating intercellular spaces between
EAR
475
the deeper portions of the sustentacular cells. These Nuel's spaces connect
with the tunnel.
Deiter's sustentacular cells are slender bodies, each containing a stiff
filament, and having at its free end a cuticular formation referred to as a
"phalanx." The phalanges come between the outer hair cells, separating
them from one another (Fig. 480), and the inner hair cells are similarly
separated by short processes the inner phalanges, derived from the inner
pillars. (The inner phalanges are not shown in the figure.) The phalanges
of Deiter's cells connect with one another, forming a trim reticular membrane. As a whole Deiter's cells resemble the pillar cells, but their transformation into stiff fibers has not proceeded so far; the cuticular border is
comparable with the head plate.
Nerve.
Tun- | \IT Membrana Tympanal
nel. Nuel's Deiter's basilaris. lamella.
Vas spirale space. cells.
Inner Outer
Pillar cells.
FIG. 480. DIAGRAM OF THE STRUCTURE OF THE BASAL WALL OF THE DUCT OF THE COCHLEA.
A, View from the side. B, View from the surface. In the latter the free surface is in focus. It is evident
that the epithelium of the sulcus spiralis. lying in another plane, as well as the cells of Claudius, can
be seen distinctly only by lowering the tube. The membrana tectoria is not drawn. The spiral
nerves are indicated by dots.
The most peripheral of the sustentacular or Deiter's cells are followed
by elongated columnar cells (cells of Hensen), which gradually shorten,
and are succeeded by the low "cells of Claudius" which extend to. the
limit of the membrana basilaris. In both the columnar and the low forms
there are single stiff filaments which are less developed than in the sustentacular cells. The centrosomes of all these cells lie near their free surfaces.
Beyond the basilar membrane the epithelium is continued over the
ligamentum spirale as a layer of cells with branching basal processes
extending deep into the underlying tissue.
476 HISTOLOGY
NERVES OF THE LABYRINTH.
The acoustic nerve is a purely sensory nerve passing between the pons
and internal ear through a bony canal, the internal acoustic meatus. It is
divided into vestibular and cochlear portions (Fig. 474). The vestibular
nerve proceeds from the vestibular ganglion and has four branches the
utricular nerve and the superior, lateral, and posterior ampullary nerves;
according to Streeter (Amer. Journ. Anat., 1906, vol. 6, pp. 139-165) it
produces also the branch to the saccuhis, usually regarded as derived
from the cochlear nerve. If this is true, the cochlear nerve supplies
only the spiral organ of Corti. The ganglion of the cochlear nerve is
lodged within the modiolus at the root of the lamina spiralis, and is known
as the spiral ganglion (Figs. 474 and 477). The ganglion cells remain
bipolar, like those of embryonic spinal ganglia. They are surrounded by
connective tissue capsules; and their neuraxons and single peripheral
dendrites receive myelin sheaths not far from the cell bodies.
The peripheral fibers extend through the lamina spiralis ossea, within
which they form a wide-meshed plexus, and after losing their myelin they
emerge from its outer border in the labium tympanicum through the
foramina nervosa. In continuing to the spiral organ they curve in the
direction of the cochlear windings, thus producing spiral strands. Those
nearest the modiolus are on the axial side of the pillar cells; the middle
ones are between the pillars, in the tunnel; and the outer ones are beyond
the pillar cells. From these bundles, delicate fibers pass to the hair cells,
on the sides of which they terminate.
VESSELS OF THE LABYRINTH.
The internal auditory artery is a branch of the basilar artery. It
arises in connection with branches which are distributed to the under
side of the cerebellum and the neighboring cerebral nerves, and passes
through the internal acoustic meatus to the ear. It divides into vestibular
and cochlear branches (Fig. 481). The vestibular artery supplies the
vestibular nerve and the upper lateral portion of the sacculus, utriculus
and semicircular ducts. The cochlear artery sends a vestibulo-cochlear
branch to the lower and medial portion of the sacculus, utriculus, and ducts.
This branch also supplies the first third of the first turn of the cochlea.
The capillaries formed by the vestibular branches are generally wide
meshed, but near the maculae and cristae the meshes are narrower. The
terminal portion of the cochlear artery enters the modiolus and forms three
or four spirally ascending branches. .These give rise to about thirty radial
branches distributed to three sets of capillaries (Fig. 482); i, those to the
spiral ganglion; 2, those to the lamina spiralis; and 3, those to the outer
walls of the scalae and the stria vascularis of the cochlear duct.
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477
The veins of the labyrinth form three groups (Fig. 481). i. The vena
aqu&ductus vestibuli receives blood from the semicircular ducts and a part
of the utriculus. It passes toward the brain in a bony canal along with
the ductus endolymphaticus, and empties into the superior petrosal sinus.
2. The vena aquaductus cochlea receives blood from parts of the utriculus,
sacculus and cochlea; it passes through a bony canal to the internal
jugular vein. Within the cochlea it arises, as shown in Fig. 482, from
Ductus semicircularis
superior.
Ampulla lateralis.
f Arteria vestibulans.
[ Arteria cochlearis.
/ I
Superior Inferior
Anterior
\
Posterior
Vena spinalis.
Vena vestibularis.
Ampulla
posterior.
Ductus semicircularis
posterior.
Arteria
cochlearis.
Vestibulo-cochlear
branch of the
arteria cochlearis.
Vena aquseductus cochlea.
PIG. 481. DIAGRAM OF THE BLOOD VESSELS OF THE RIGHT HUMAN LABYRINTH. MEDIAL AND POSTERIOR ASPECT.
D, c., Ductus cochlearis; S., sacculus; U., utriculus; i, ductus reuniens; 2, ductus utriculo-saccularis. The saccus
endolymphaticus is cut off.
small vessels including the vas prominens (a) and the vas spirale (6).
Branches derived from these veins pass toward the modiolus. (There are
no vessels in the vestibular membrane of the adult, and the vessels in the
wall of the scala tympani are so arranged that only veins occur in the part
toward the membranous spiral lamina; thus the latter is not affected by
arterial pulsation.) Within the modiolus the veins unite in an inferior
spiral vein, which receives blood from the basal and a part of the second
turns of the cochlea, and a superior spiral vein which proceeds from the
478
HISTOLOGY
apical portion. These two spiral veins unite with vestibular branches to
form the vena aquaeductus cochleae (Fig. 481). 3. The internal auditory
vein arises within the modiolus from the veins of the spiral lamina; these
anastomose with the spiral veins (Fig. 482). It receives branches also
from the acoustic nerve and from the bones, and empties "in all probability, into the vena spinalis anterior."
Lymphatic spaces within the internal ear are represented by the perilymph spaces, which communicate through the aquaeductus cochleae with
Scala tympani. Scala vestibuli.
Stria vasculars.
A
Cross section of a spiral
artery of the modiolus.
--Vena laminae spiralis.
Ganglion spirale.
Vena spiralis superior.
Cross section;of a spiral
artery of the modiolus.
Vena laminae spiralis.
Anastomosis.
Vena spiralis inferior.
FIG. 482. DIAGRAM OF A SECTION OF THE FIRST (BASAL) AND SECOND TURNS OF THE COCHLEA.
a, Vas prominens; b, vas spirale.
the arachnoid space; the connecting structure, or "ductus perilymphaticus," is described as a lymphatic vessel. The saccus endolymphaticus, which is the dilated distal end of the endolymphatic duct, is in contact with the dura, and there are said to be openings between it and the
subdural space. In the internal ear perivascular and perineural spaces
are found, and they probably connect with the arachnoid spaces.
MIDDLE EAR.
The tympanic cavity, which contains air, is lined with a mucous membrane closely connected with the surrounding periosteum. It consists of
EAR
479
a thin layer of connective tissue, covered generally with simple cuboidal
epithelium. In places the epithelial cells may be flat, or tall with nuclei
in two rows. Cilia are sometimes widely distributed and are usually to
be found on the floor of the cavity. In its anterior part, small alveolar
mucous glands occur very sparingly. Capillaries form wide-meshed networks in the connective tissue, and lymphatic vessels are found in the
periosteum.
The auditory tube includes an osseous part toward the tympanum, and
a cartilaginous part toward the pharynx. Its mucosa consists of fibrillar
connective tissue, together with a ciliated columnar epithelium which
Cartilage
Cartilage.
M ucosa of the
pharynx.
Glands.
Glands. ---ZIZ~
FIG. 483. CROSS SECTION OF THE CARTILAGINOUS PART OF THE AUDITORY TUBE.
(Bohm and von Davidoff.)
X 12.
becomes stratified as it approaches the pharynx. The stroke of the cilia
is toward the pharyngeal orifice. In the osseous portion, the mucosa is
without glands and very thin; it adheres closely to the surrounding bone.
Along its floor there are pockets containing air, the cellules pneumatics.
In the cartilaginous part the mucosa is thicker; near the pharynx it contains many mucous glands (Fig. 483). Lymphocytes are abundant in
the surrounding connective tissue, forming nodules near the end of the
tube, which blend with the pharyngeal tonsil. The cartilage, which only
partly surrounds the auditory tube, is hyaline near its junction with the
bone of the osseous portion; it may contain here and there coarse fibers
which are not elastic. Toward the pharynx the matrix contains thick
nets of elastic tissue, and the cartilage is consequently elastic.
480
HISTOLOGY
EXTERNAL EAR.
Between the middle ear and the external ear is the tympanic membrane, which consists, from without inward, of the following strata: the
cutaneum, radiatum, circular e and mucosum (Fig. 484). The stratum
cutaneum is a thin skin without papillae in its corium, except along the
handle or manubrium of the malleus. There it
is a thicker layer, containing the vessels and
nerves which descend along the manubrium and
spread from it radially. In addition to the venous
plexus which accompanies the artery in that situation, there is a plexus of veins at the periphery
of the membrane, receiving tributaries from both
the stratum cutaneum and the less vascular stratum mucosum. The radiate and circular strata
consist of compact bundles of fibrous and elastic
tissue, arranged so as to suggest tendon. The
fibers of the radial layer blend with the perichondrium of the hyaline cartilage covering the manubrium. Peripherally the fiber layers form a fibro-cartilaginous ring which
connects with the surrounding bone. The stratum mucosum is a thin layer
of connective tissue covered with a simple non-ciliated flat epithelium
Epidermis.
FIG. 484. CROSS SECTION OF
THE MEMBRANA TYMPANI
BELOW THE MANUBRIUM.
X 450. (After Kdlliker.)
a, Stratum cutaneum (showing _the corneum and
germinativum) ; b, stratum radiatum, its fibers
cut across; c, stratum circulare; d, stratum mucosum.
Hair sheath.
Corium
Excretory duct
Young hair
Coil of ceruminous gland .
FIG. 485. FROM A VERTICAL SECTION THROUGH
THE SKIN OF THE EXTERNAL AUDITORY
MEATUS OF AN INFANT. X 50. The excretory duct opens into the hair follicle.
Membrana propria.
Nuclei of smooth muscle fibers.
Secretion.
Gland cells.
Secretion.
, Cuticular border.
. Gland cells.
Nuclei of smooth muscle
fibers.
' Membrana propria.
FlG. 486. TUBLES OF THE CERUMINOUS GLANDS.
A, Cross section, from an infant; B, longitudinal section, from a boy 1 2 years old.
continuous with the lining of the tympanic cavity. Peripherally, in
children, its cells may be taller and ciliated. As a whole the tympanic
membrane is divided into tense and flaccid portions. The latter is a relatively small upper part in which the fibrous layers are deficient.
EAR 481
The external acoustic meatus is lined with skin continuous with the
cutaneous layer of the tympanic membrane. In the deep or osseous portion the skin is very thin, without hair or glands except along its upper
wall. There and in the outer or cartilaginous part, ceruminous glands are
abundant. "They are branched tubulo-alveolar glands" (Huber) which
in many respects resemble large sweat glands. Their ducts are lined with
stratified epithelium. The coils consist of a single layer of secreting cells,
general cuboidal, surrounded by smooth muscle fibers and a well-defined
basement membrane. They differ from sweat glands in that their coils
have a very large lumen, especially in the adult; and their gland cells,
often with a distinct cuticular border, contain many pigment granules and
fat droplets. Their narrow ducts in adults end on the surface of the skin
close beside the hair sheaths; in children they empty into the sheaths
(Fig. 485). It has not been shown that the ceruminous glands are more
directly concerned in the production of cerumen than the sebaceous
glands. The cerumen obviously is an oily rather than a watery secretion, and it contains fatty cells and pigment.
The cartilage of the external acoustic meatus and of the auricle is
elastic.
NOSE.
The nasal cavities are formed by the invagination of a pair of epidermal thickenings similar to those which give rise to the lens and auditory
vesicle. The pockets thus produced in the embryo are called "nasal pits"
(Fig. 205, n, p. 216). Their external openings
remain as the nares of the adult, but temporarily, from the third to the fifth month of embryonic life, they are closed by an epithelial
proliferation. Each nasal pit acquires an internal opening, the choana, in the roof of the
pharynx. The choanae are at first situated
near the front of the mouth, separated from
one another by a broad nasal septum (Fig. 487).
As the latter extends posteriorly, it is joined by
FIG. 487. THE ROOF OF THB
the Palate processes which grow toward it from MOUTHOF A HUMAN EMBRYO
OF 8 WEEKS. X 4. (After
the sides of the maxillae. Thus the choanae re- Koiimann.)
na, Nans; ch., choana; al. p., i. p.,
Cede tOWard the back Of the mOUth While and pa. p., alveolar, intermax
illary, and palate processes.
the embryonic condition of cleft palate is being
removed (Fig. 488). The lateral walls of the nasal cavities produce three
curved folds one above another; they are concave below, and in them
the concha (turbinate bones) develop. The nasal mucosa covers these
and extends into excavations in the adjacent bones, forming the sphenoid,
31
482
HISTOLOGY
maxillary, and frontal sinuses, and the ethmoidal cells. The boundary between the epithelium of the nasal pit and that of the pharynx early disappears, and the extent of each in the adult is uncertain. Presumably the olfactory neuro-epithelium is derived from the nasal pit. In man the olfactory region is limited to the upper third of the nasal septum and nearly the
whole of the superior concha (Read). This regio olfactoria is covered by a
yellowish-brown membrane, which may be distinguished macroscopically
from the reddish mucosa of the regio respiratoria. The latter includes the
remainder of the nose. The two regions may be considered in turn.
The vestibule, or cavity of the projecting cartilaginous portion of the
nose, is a part of the respiratory region which is lined with a continuation
Cartilaginous
nasal septum.
Dental ridge
of the
upper iaw.
Oral
epithelium.
Dental ridge
of the
lower jaw.
Nasal cavity.
Maxilla.
Oral cavity.
Tongue.
Mandible.
FIG. 488. FRONTAL SECTION OF THE HEAD OF A /JO-MM. SHEEP EMBRYO, x 15.
The palate processes have united with the nasal septum. The conch are developing along the lateral
walls of the nasal cavity. In the lower part of the nasal septum the vomero-nasal organs are seen as a
pair of tubes, each of which is partly surrounded by a crescentic cartilage.
of the skin. Its stratified epithelium has squamous outer cells and rests
upon a tunica propria with papillae. It contains the sheaths of coarse
hairs (vibrissa) together with numerous sebaceous glands. The extent
of the squamous epithelium is variable; frequently it is found on the
middle concha, less often on the inferior concha.
The remainder of the respiratory mucosa consists of a pseudo-stratified
epithelium with several rows of nuclei. It may contain few or many
goblet cells. The tunica propria is well developed, being even 4 mm.
thick on the inferior concha (Fig. 489). It consists of fibrillar tissue with
many elastic elements, especially abundant in its deeper layers. Beneath
the'epithelium, it is thickened to form a homogeneous membrana propria,
NOSE
483
perforated with small holes. Lymphocytes are present in variable quantity, sometimes forming solitary nodules and often entering the epithelium
in great numbers. Branched alveolo-tubular mixed glands extend into
the tunica propria. Their serous portions have intercellular secretory
capillaries, and both mucous and serous cells contain a trophospongium.
The glands often empty into funnel-shaped depressions, which are macroscopic on the inferior concha, and are lined with the superficial epithelium.
Epithelium
Bone.
FIG. 489. VERTICAL SECTION THROUGH THE MUCOSA OF THE INFERIOR CONCHA OF MAN. X 48. On
the left is a funnel-shaped depression receiving an excretory duct; near-by on the right is the section
of a large vein.
The mucosa of the several paranasal sinuses is thin ( o.o2mm.), with
less elastic tissue and but few small glands. A pocket which extends
into the lower part of the median septum has already been described
as the vomero-nasal organ (Jacobson's organ). In man it is the rudimentary remnant of an important sense organ, supplied by special branches
of the olfactory nerves and by the nervus terminalis (cf. p. 141). It
is lined with a tall columnar epithelium, and contains, at least in the
484
HISTOLOGY
cat, "sensory cells apparently identical with those of the olfactory
mucosa." In man sensory cells are said to be lacking in the adult and
in embryos older than five months.
In the regio olfactoria the mucosa includes a tunica propria and an
olfactory epithelium. The latter consists of
sustentacular cells and olfactory cells. The
superficial halves of the sustentacular cells
are cylindrical, and contain yellowish pigment, together with small mucoid granules
often arranged in vertical rows (Fig. 490) .
The more slender lower halves have dentate or notched borders, and branched
basal ends which unite with those of neighboring cells, thus forming a protoplasmic
network. Their nuclei, generally oval, are
in one plane and in vertical sections they
form a narrow "zone of oval nuclei"
(Fig. 491). The olfactory cells generally
have round nuclei containing nucleoli. They occur at different levels
and so form a broad "zone of round nuclei." From the protoplasm
FIG. 490. ISOLATED CELLS OF THE OLFACTORY MUCOSA OF A RABBIT.
X 560.
st, Supporting cells; s, extruded mucus
resembling cilia; I, olfactory cells, from
r' the lower process has been torn off; f,
ciliated cell; b, cells of olfactory glands.
Excretory duct
Wandering cell. Mucus.
\
rT.y _ Pigment
granules.
Oval nucleus of
a sustentacular
cell.
Round nucleus
of an olfactory
cell.
Basal celL
Nerve.
tf
Sections of olfactory glands.
FIG. 491. VERTICAL SECTION THROUGH THE OLFACTORY REGION OF AN ADULT. X 400.
Dilated duct. Mucus.
which is gathered immediately about the nucleus, each olfactory cell
sends a slender cylindrical process toward the surface, where it ter
NOSE 485
minates in a variety of ways. It may end in a small knob-like swelling,
or in a single slender spine; sometimes the terminal knob sends out a small
cluster of divergent olfactory hairs or spines. Basally the olfactory cells
pass directly into the axis cylinders of the olfactory nerves (Fig. 492).
Thus they are ganglion cells, their basal processes being neuraxons. Cells
intermediate between the olfactory and sustentacular forms may be found,
and these are doubtless imperfectly developed sensory cells. At the free
surface of the olfactory epithelium there are terminal bars, and small
projecting strands of mucus, sometimes suggesting cilia (Fig. 490, s).
Central ependymal
cells.
Fibers of the olfactory tract.
Mitral cells.
Glomeruli.
Olfactory nerves.
Olfactory fibers in
the nasal mucous
membrane.
Olfactory cells.
PIG. 492. CHIEF ELEMENTS OF THE OLFACTORY BULB. (Gordinier, after Van Gehuchten.)
The mucus, which is the product of the sustentacular cells, may appear to
form a continuous superficial membrane (Fig. 491). Near the tunica
propria there is a network of so-called "basal cells" (Fig. 491).
The tunica propria is composed of fibrous tissue and fine elastic
fibers, associated with many connective tissue cells. In some animals
(for example, the cat) it forms a structureless membrane next to the epithelium. It surrounds the numerous olfactory glands (Bowman's glands).
In man these consist of excretory ducts extending through the epithelium,
and of branching gland bodies beneath. They have the appearance of
serous glands, but sometimes contain mucus, generally in small quantities.
They are found not only in the olfactory region, but also in the adjoining
part of the respiratory region.
486 HISTOLOGY
The deeper layers of the tunica propria contain the arteries of the
mucous membrane, which send branches toward the epithelium, and
form a thick sub-epithelial plexus of capillaries. The veins are very
numerous, especially at the inner end of the inferior concha, where the
tunica propria resembles cavernous tissue. Lymphatic vessels form a
coarse meshed network in the deeper connective tissue. Injections of the
arachnoid spaces around the olfactory bulbs follow the perineural sheaths
of the olfactory nerves into the nasal mucosa, but these tissue spaces are
not lymphatic vessels.
The olfactory nerves, as already stated, are formed of the basal processes of the olfactory epithelial cells, which become non-medullated nerve
fibers. This is a primitive type of nervous apparatus (cf. p. 132), such as
is not found elsewhere in the human body. After a tangential course
beneath the epithelium, the fibers unite in bundles, and pass through the
cribriform plate of the ethmoid bone to the olfactory bulb just above it,
which they enter. They spread tangentially and branch, finally terminating in the glomeruli. The glomeruli are round or oval groups of
arborizing fibers, in which the processes of the olfactory cells end in
relation with the dendrites of the mitral cells. The latter are nerve cells
with triangular bodies, which form a characteristic layer of the olfactory
bulb, and send their neuraxons through the olfactory tracts to make
various connections within the hemispheres.
In addition to the olfactory nerves, the nasal mucous membrane contains medullated branches of the trigeminal nerve, distributed both to the
olfactory and respiratory regions.
PART II.
MICROSCOPICAL TECHNIQUE.
I. THE PREPARATION OF MICROSCOPICAL
SPECIMENS.
REVISED BY LAWSON G. LOWREY.
The methods of fundamental importance, which are likely to be employed by students who are beginning their histological studies, are here
given. Further information may be obtained from "The Micro tomist's
Vade Mecum" by A. B. Lee (Blakiston, Philadelphia) and from Mallory
and Wright's "Pathological Technique" (Saunders, Philadelphia). The
former deals with the subject from the point of view of general biology;
the latter is particularly adapted to the needs of medical students.
FRESH TISSUES.
Certain tissues may be studied advantageously in a fresh condition.
They are simply spread on a clean glass slide, covered and examined.
Desquamated epithelial cells, spermatozoa, blood, and other fluids containing cells, may be treated in this way. But structures such as muscles,
tendons, nerves, connective tissue, etc., must first be "teased" that is,
torn into very small fragments or spread into a thin layer with a pair of
fine needles.
The "parenchymatous" organs, or other structures which cannot be
investigated satisfactorily by the above methods, must be sectioned or
macerated. The old methods of making free-hand sections of the object
held between pieces of pith, or of making sections with a double bladed
knife, have been superseded in most laboratories by the freezing method.
This method is often serviceable in histology, and is indispensable in
the rapid diagnosis of pathological conditions.
Blocks of tissue not over 5 mm. thick are moistened with water, placed
on the carrier of a special form of microtome and frozen by a jet of carbon
dioxide proceeding from a tank of the compressed gas. Sections 10 to 15 //
thick may be chiselled from the block of tissue and unrolled by transferring
to a dish of 0.6 per cent, sodium chloride solution. They are floated on a
slide, covered and examined.
487
488 HISTOLOGY
Sections or teased preparations must be kept moist during examination.
In order to avoid distortion, they are not mounted in water, but in socalled indifferent fluids, such as the lymph, aqueous humor, serous fluids,
amniotic fluid, etc. Of the artificial indifferent media, a 0.6 per cent,
solution of sodium chloride in distilled water has been found to cause less
distortion than the stronger fluids formerly recommended.
Ringer's Solution. An indifferent fluid which is perhaps more satisfactory than the 0.6 per cent, salt solution is a modification of Ringer's
solution adapted to the tissues of warm-blooded animals. It is to be made
in large quantities.
Sodium chloride 90. o
Potassium chloride 4.2
Calcium chloride (anhydrous) 2.4
Potassium bicarbonate 2.0
Distilled water 10,000.0
Examination of fresh tissues reveals but little of the fine details of
structure. Since the indices of refraction of the different tissue elements
have much the same value, outlines are usually dim and there is very little
optical differentiation. The method of handling is prone to produce distortion and with many tissues and organs it is difficult to separate their
constituent elements. It is generally necessary to employ more complex
methods of treatment to gain an adequate idea of the histological details.
One of the simplest methods is to add one or two drops of i to 5 per cent,
acetic acid solution to the fresh preparation. The nuclei then appear more
distinctly. Albuminous granules are dissolved, but fat and myelin are not
affected. The white fibers of connective tissue swell and disintegrate,
leaving the elastic fibers unaffected.
Nuclei may be rendered distinct by allowing a few drops of stain to act
upon the tissue for a few minutes. A i per cent, aqueous solution of methylene blue, or a i per cent, solution of methyl green in 20 per cent, alcohol,
or the haematoxylin solutions, may be used.
ISOLATION.
Some tissues cannot properly be separated into their elements in the
fresh condition, but may be shaken or teased apart after preliminary
treatment. The reagents employed in maceration have the property of
softening or removing certain constituents of the tissues, at the same time
fixing or hardening other elements. Usually the intercellular portions are
softened or removed, while the cellular elements undergo fixation.
Ranvier's Alcohol. This is a mixture of one volume of 95 per cent, alcohol
and 2 volumes of distilled water. The cells of small pieces of epithelium
(5-10 mm. square) are separable in 24 to 48 hours. They are examined
in the same fluid, or washed in water and examined in glycerin.
MACERATING FLUIDS 489
Nitric Acid and Potassium Chlorate. About 5 gm. of potassium
chlorate are dissolved in 20 c.c. of the acid. Muscle cells are separable in
one to six hours. Wash thoroughly in water and examine in water or
glycerin.
Potassium Hydrate. Muscle cells may be teased apart after immersion
for about an hour in a 35 per cent, aqueous solution. They may be examined in the same solution or transferred to a saturated aqueous solution
of potassium acetate, which prevents further maceration. The solution
of potassium hydrate may also be used for isolating epithelial cells.
Concentrated Sulphuric Acid. The elements of the epidermis, hair and
nails may be separated after immersion in this fluid. They should be
thoroughly washed in water.
PERMANENT PREPARATIONS.
None of the methods described above yield much information respecting the finer structure of tissues and organs, nor do they yield permanent preparations. For ease of reference, the various steps in the production of a permanent preparation have been grouped under the following five headings.
1. Fixation. Under this heading are given formulae for the best fixing
fluids, with directions for their use and for the subsequent handling of the
tissue until it is placed in 80 per cent, alcohol, in which tissues may be
kept for a considerable time.
2. Imbedding. This includes the various steps for preparing the
tissues to be sectioned in paraffin or celloidin, starting from 80 per cent,
alcohol.
3. Cutting and handling sections. Brief directions are given for cutting
sections and handling them, until they are ready for staining.
4. Staining. Formulae and directions for the use of stains, and the
after treatment until the preparation is in the appropriate clearing fluid.
5. Clearing and mounting. The choice of a clearing agent for paraffin
and celloidin sections rs discussed, together with the methods and media
for mounting.
Since each of the fixing, imbedding and staining methods is considered
as a unit, each starting where the previous step ends, the student can
easily prepare specimens according to any desired possible combination
by referring to the directions for the selected fixative, imbedding method,
and stain.
i. Fixation.
A good fixative should penetrate and kill tissues quickly; preserve the
tissue elements, particularly the nuclei, in the condition in which they are
4QO HISTOLOGY
found at the moment of its action; render structures insoluble, and harden
them so that they will not be altered by the various after-steps; and give
a certain degree of optical differentiation.
No single compound has yet been found which successfully fulfills all
of these conditions, nor are any of the recommended fixatives adequate
in all cases or for all special studies. Only the fluids commonly employed,
which have proven most useful, are here given.
Small pieces of tissue, preferably less than i cm. in thickness, should
be dropped into a considerable amount of fluid. The tissue should be
handled as little as possible, in order that delicate structures may not be
destroyed. For example, contact between the fingers and the peritoneum
is sufficient to destroy the thin epithelium.
In order to insure uniform action of the fixing fluid, it is often advisable
to place a little absorbent cotton in the bottom of the vessel. Frequent
gentle mechanical agitation will serve the same end. Tubular organs
should be washed out, or cut open and their contents and any adherent
blood washed away, with salt solution. Membranes may be kept flat
and smooth by tying them across the end of a short tube or detached
bottle neck.
Alcohol. Small or thin pieces of tissue are supported on a little absorbent cotton in absolute alcohol, for 12 to 24 hours, changing after 3
or 4 hours. Large pieces are fixed by successive immersion in 70 per cent.,
80 per cent., and 95 per cent, alcohol for 24 hours each.
Alcohol is a valuable dehydrating and hardening agent, but its fixing
qualities are inferior, so that it is rarely used alone as a fixative. Small
embryos or blocks of tissue obtained in an emergency should be preserved
in 10 per cent, formalin, rather than in alcohol.
Bouin's Fluid.
Picric acid, saturated aqueous solution 75 '
Formalin 20
Glacial acetic acid 5
This fluid is particularly recommended for the fixation of embryos,
for which it is unexcelled. Small embryos are fixed in 4 to 6 hours. Larger
objects may be fixed 24 to 48 hours or longer. For washing out the fixing
fluid, alcohol, first 70 per cent., then 80 per cent., should be employed.
Renew the alcohol as often as discolored.
Carney's Mixtures.
No. i Absolute alcohol 6
Chloroform 3
Glacial acetic acid i
This is a very rapid fixative, even large pieces being fixed in $ to X
hour. Wash in absolute alcohol until the odor of acetic acid is lost, changing every 12 hours, and imbed; or grade through 95 per cent, to 80 per
cent, alcohol.
FIXING FLUIDS 49 1
No. 2. Saturate mixture No. i with mercuric bichloride (about 20
parts). This is the most rapid and penetrating fixative known, and it
affords a very delicate cytological fixation. Immersion for 30 minutes
to i hour is sufficient even for the larger pieces. Subsequent treatment
as with No. i, except that the crystals of sublimate must be removed from
the tissue, either by placing the block in 80 per cent, alcohol and iodine
(see Zenker's fluid) ; or after the block has been cut, by treating the sections with iodine (see p. 497).
Flemming's Fluid.
Osmic acid, i % aqueous solution 10
Chromic acid, i % aqueous solution 25
Glacial acetic acid, i % aq. solution 10
Distilled water 55
This solution should be mixed only at the time of using. Only very
thin pieces (not over 2 mm. thick) should be used. Fix for 24 hours or
longer (sometimes even for weeks). Wash in running water 24 hours.
Pass through 50 per cent., 70 per cent. (12 hours in each), to 80 per cent,
alcohol.
Formaldehyde. The gas is soluble in water to the extent of 40 per
cent., and solutions of this strength are obtainable under the trade names
of formalin, formol, and formalose.
For fixing tissues, 10 c.c. of the commercial product are added to 90
c.c. of water. It penetrates very quickly, but specimens may be left in
it for a considerable time without apparent harm. Ordinary blocks are
sufficiently fixed in from 12 to 24 hours. Transfer directly to 80 per cent,
alcohol.
Histologically, its chief use is for the preservation of nervous tissue,
the fixation of tissue to be cut with the freezing microtome, and the preservation of embryos. Small human embryos obtained by practitioners should
be put at once into 10 per cent, formalin and forwarded to an embryological
laboratory.
March!' s Fluid.
Potassium bichromate 2.5
Sulphate of sodium i . o
Water 100 . o
Osmic acid, i% aqueous solution 50.0
Small pieces are fixed for 5 to 8 days in the dark. Wash 24 hours in
running water; 50 per cent, and 70 per cent, alcohol (24 hours each);
80 per cent, alcohol. Used for demonstrating degenerated nerve fibers
and in making damar mounts of fat and myelin, since the osmium reduced
by fat is insoluble in alcohol. Sections must not be treated with xylol,
but chloroform should be used instead.
Orth's Fluid.
Potassium bichromate 25
Sodium sulphate 10
Water. . . . 1000
492 HISTOLOGY
At the time of using mix 10 c.c. of formalin with 90 c.c. of the above
solution (which is known as Miiller's fluid). Small pieces are fixed in
about 48 hours. Wash in running water for 12 to 24 hours. Then 50
per cent, alcohol and 70 per cent, alcohol, 12 to 24 hours each; 80 per cent,
alcohol. This is useful as a fixative for the central nervous system, and
as a general fixative.
Zenker's Fluid. This is kept in the form of the following stock solution, in preparing which the water is heated and the ingredients are stirred
with a glass rod (metal instruments must not be put into this fluid) .
Potassium bichromate 25
Sodium sulphate 10
Mercuric bichloride 50
Water 1000
At the time of using, add 5 c.c. of glacial acetic acid to 95 c.c. of the
above solution. The tissues, which float for a short time, are fixed for
6 to 24 hours, after which they are washed in running water 12 to 24
hours. Then they are transferred to 50 per cent, alcohol for 12 to 24
hours; 70 per cent, alcohol, 12 to 24 hours; 80 per cent, alcohol.
Corrosive sublimate forms crystalline deposits in the tissues, and
these must be removed before the preparation is stained. They may be
removed by adding enough tincture of iodine to give a port-wine color
to the 70 per cent, and 80 per cent, alcohols in which the block of tissue
is immersed. More iodine is added as the solution becomes colorless
(or nearly so) and the treatment must be continued until the color no
longer changes. The tissues are then to be placed in fresh 80 per cent.,
renewed two or three times in order to remove completely the mercuric
iodide. The crystals of sublimate may be removed after the tissue has
been sectioned, as described on p. 497.
Zenker's fluid is an excellent fixative, which penetrates easily and
does not decrease the staining qualities. It is probably the best "general
fixative."
DECALCIFICATION.
Specimens which contain bone or calcareous material cannot be sectioned until they have been decalcified. The tissues are fixed, according
to the directions given above, in Zenker's fluid, Orth's fluid, or formaldehyde, and hardened. After several days in 80 per cent, alcohol, they are
put into a considerable quantity of 3 to 5 per cent, aqueous solution of
nitric acid. This should be renewed at intervals for 3 or 4 days, until
the bone can be penetrated easily with a needle. Wash in running water
for a day, and return to 80 per cent, alcohol. Imbed in celloidin.
Phloroglucin is sometimes added to the decalcifying fluid to protect
the tissue. The following solution has been recommended. It is to be
used in the same manner as the aqueous solution of nitric acid.
DECALCIFYING FLUIDS 493
Phloroglucin i
Nitric acid 5
Alcohol, 95% 70
Water 30
The addition of i or 2 per cent, of nitric acid to the 80 per cent, alcohol
will decalcify small embryos. The specimen should then be thoroughly
washed in fresh 80 per cent., in order to remove the acid.
2. Imbedding.
Most of the fixatives employed are in aqueous solution. After fixation and the removal of the fixative by washing in water or alcohol, as
directed, the specimen must not be left in water, but must be dehydrated.
Dehydration has a double purpose: (i) to remove the water, which especially favors post-mortem decomposition, and (2) to prepare the tissue
for infiltration with the imbedding substance or, in the case of objects
to be mounted whole, for infiltration with the mounting medium.
All the fixation methods given above end with placing the block of
tissue in 80 per cent, alcohol. Here they may be left until wanted,
although immersion for a considerable time causes a gradual loss in staining qualities. Stronger alcohol causes an overhardening, while maceration may occur in weaker alcohols.
Dehydration is accomplished by immersing the specimen in gradually
increasing strengths of alcohol. Those commonly employed are 50 per
cent., 70 per cent., 80 per cent., 95 per cent, and absolute. The lower
grades may be prepared from the ordinary barrel alcohol, of about 95 per
cent, strength, as follows:
80 per cent. 425 c.c. 95 per cent, alcohol mixed with 75 c.c. distilled water
70 per cent 370 c.c. " 130 c.c.
50 per cent. 265 c.c. " 235 cc.
The specimen is left in each grade long enough to be saturated. The
time required varies from 3 to 24 hours. Objects of average size require
about 6 to 1 2 hours. Prolonged immersion in 95 per cent, or absolute is
very injurious to the tissues.
In imbedding, the tissue is surrounded and infiltrated with a firm substance which can be cut into thin sections, supporting and holding firm the
fragile tissue. Celloidin, which is solid upon the evaporation of its solvent,
and paraffin, which is solid at ordinary temperatures, are the substances
used, each having its particular advantages.
Paraffin Imbedding. Specimens cannot be passed directly from alcohol to paraffin, since alcohol dissolves only a very little paraffin and the
specimen would not be thoroughly infiltrated. So the specimen must first
be passed through some fluid which mixes with absolute alcohol and will
dissolve paraffin. Of a host of reagents possessing this property, chloroform is recommended for general use.
After thorough dehydration (12-24 hours in absolute alcohol), the
494 HISTOLOGY
specimen is passed from absolute to a mixture of equal parts of absolute
and chloroform for 2 to 6 hours, and then to pure chloroform for an equal
length of time. It is then transferred to a saturated solution of paraffin in
chloroform, kept warm by placing on top of the paraffin bath, for 2 to 4
hours, and is then put into melted filtered paraffin.
The melting point of the paraffin used varies with the temperature in
which it is to be cut. During the winter, paraffin melting at 5o-52 C.
should be used, while during the summer paraffin with a melting point of
56-58 is best. Harder paraffin is required for thin than for thick sections. The melted paraffin should be kept in a paraffin bath or thermostat
maintained at a temperature but slightly higher than the melting point of
the paraffin.
The specimen should be left in the melted paraffin for the shortest time
which will allow thorough infiltration, as heat is very injurious to the tissue.
For average specimens, 3 hours is sufficient. Transfer to fresh paraffin
at the end of i| or 2 hours. At the end of the full time, the specimen is
to be imbedded.
The imbedding frame consists of a glass plate and two L-shaped pieces
of metal. By sliding the latter back and forth on one another, the size
of the enclosed space or box may be varied. Before using the frame,
the inner surfaces of the metal pieces and that part of the glass plate on
which they rest are rubbed with glycerin. It should form a thin film
over the surfaces, but not accumulate in drops. Melted paraffin is
poured into the box and the specimen is transferred to it with a spatula.
The specimen sinks to the bottom, and may be arranged in any desired
position by means of needles warm enough to prevent the paraffin solidifying over their surfaces. The paraffin must be quickly cooled by lowering
the frame into a basin of cold water so that the water comes up on the
sides of the metal pieces. As soon as a resistant film has formed over
the surface of the paraffin, the entire frame may be submerged, and in
a few minutes the glass plate and metal pieces may be detached from the
solid paraffin. The block may be sectioned as soon as it is thoroughly
cooled.
When a number of specimens are to be imbedded; a flat dish of suitable
size may be used. After a thin layer of glycerin has been coated over
the interior, the dish is filled with a sufficient quantity of melted paraffin
and the blocks are put into position. The mass is cooled and removed
as before, and the large mass is cut into smaller parts, each containing a
specimen.
One or several specimens may be imbedded in paper boxes of suitable
size. The tabs at the ends may be labelled and the specimens kept in
the boxes until wanted; otherwise labels may be scratched in the paraffin
with needles.
PARAFFIN IMBEDDING 495
Paraffin imbedding is to be chosen when very thin sections or serial
sections are desired. Material imbedded in paraffin may be kept for
years without any apparent deterioration.
Celloidin Imbedding. Thick celloidin is prepared by dissolving 30
gm. of Schering's granular celloidin in 300 c.c. of a mixture of equal
parts of ether and absolute alcohol. It has a thick syrupy consistency,
and becomes constantly denser by evaporation of the solvent. It should
be kept in a tightly closed preserve jar. Thin celloidin is prepared by
mixing equal volumes of the thick celloidin and the absolute and ether
mixture.
The hardened and dehydrated block of tissue, trimmed to the size
and shape desired, is transferred from absolute alcohol to a mixture of
equal parts of ether and absolute for 24 hours. From this it is transferred
to thin celloidin, in which it remains from 24 hours to a week or longer,
and then to thick celloidin for the same length of time. The success
of the process depends largely upon the thorough infiltration of the
tissue with the celloidin. The time required in the celloidin varies with
the penetrability of the tissue and the size of the piece.
After remaining for a sufficient length of time in the thick celloidin,
the tissue is taken out with a mass of adherent celloidin and is pressed
gently against the roughened surface of a block of vulcanized fiber.
As soon as a film has formed upon the surface, the block and attached
specimen are dropped into 80 per cent, alcohol, in which the mass becomes
firm. It is ready for sectioning in about 6 hours.
In case it is desired to secure sections through the entire thickness
of the specimen, the following method is recommended. A sufficient
quantity of thick celloidin is poured into a flat dish (or paper box) and
the specimen is put into it. The entire mass is hardened as before and
then a block of celloidin containing the specimen is cut out. This is
trimmed to leave only a thin rim around the specimen. The block is
placed for a few moments in the ether-absolute mixture, and then dipped
in thick celloidin and pressed against the surface of a fiber block, which
has also been dipped in the ether-absolute mixture and in thick celloidin.
The mass is allowed to harden somewhat, and then is placed in 80 per
cent, alcohol.
The imbedded specimen is kept in 80 per cent, alcohol until wanted
for sectioning. Celloidin imbedding is recommended for large objects,
or for those from which very thin sections are unnecessary.
Rf SUME" OF IMBEDDING METHODS.
Assuming that the tissues have been fixed and carried into 80 per
cent, alcohol, the steps in imbedding are as follows:
496 HISTOLOGY
Paraffin Celloidin
95 per cent alcohol 12-24 hr. 95 per cent, alcohol 12-24 hr.
Absolute 1 2-24 hr. Absolute 1224 hr.
Absolute and chloroform, equal parts. 2 6 hr. Absolute and ether, equal
Chloroform 2- 6 hr. parts 12-24 hr.
Chloroform saturated with paraffin. 2- 4 hr. Thin celloidin 24 hr. to a week
Melted paraffin 2- 4 hr. Thick celloidin 24 hr. to a week
Imbed in fresh melted paraffin and cool quickly. Mount on fiber block; harden and preserve in
80 per cent, alcohol.
3. Cutting and Handling Sections.
Paraffin Sections. Two kinds of microtomes are in general use for
sectioning objects imbedded in paraffin. In one form, the "precision
microtome," the knife is horizontally placed and the object is moved
backward and forward on a carrier. In the rotary microtome, the knife
is vertically placed and the object is moved up and down, being cut on
the down stroke. In both forms, the knife edge is at right angles to the
carrier and the object.
For sectioning with the precision microtome, the object is mounted on
a fiber block which is then clamped in the microtome; with the rotary
form, it is mounted on a special metal disc. Before attaching the imbedded object, superfluous paraffin is cut away, leaving the tissue rising
from a broad base and completely surrounded by a thin layer of paraffin.
The block should be trimmed so as to give a rectangular or square surface
to be cut, and there should be a considerable layer of paraffin between the
object and the block or disc to which it is to be attached. The base is
placed upon a heated spatula which rests upon the fiber block. When
the paraffin is somewhat melted, the spatula is withdrawn and the base
is pressed down upon the block, to which it adheres when the paraffin
solidifies. In mounting upon the metal disc, the disc is heated, the block
pressed \ipon it and the whole quickly cooled by immersing in water.
If the paraffin on each side of the object is trimmed parallel with the
knife edge, the successive sections adhere to one another, forming ribbons.
As they are taken from the knife, the ribbons are laid in a shallow box.
By placing them in order, they may later be attached to the slide in perfect series, one after the other. The first one cut is attached to the upper
left hand corner, and the others follow like lines on a printed page. Sections mounted in this way are called serial sections. The sections should
be from 5 to 10 p in thickness, but under favorable conditions thinner
sections may be secured.
Before they can be stained, paraffin sections must be attached to the
slide and the paraffin must be removed. To attach them to a slide, a
mixture of equal parts of white of egg and glycerin is used. The white
of egg is thoroughly stirred and filtered. An equal volume of glycerin is
added, the two thoroughly mixed and a small lump of camphor added as
PARAFFIN SECTIONS 497
a preservative. The mixture is kept in a glass-capped bottle, with a
glass rod for a dropper.
A drop is placed upon a thoroughly clean slide and rubbed evenly
with the ringer (freed from oil) over all the area upon which sections may
be placed. It should be free from bubbles and should make a very thin
layer, just thick enough to allow the finger to glide easily over the surface
of the slide. A few drops of water are placed upon it, forming a layer over
the albumen deep enough to float the paraffin sections, strips of which
are placed upon the water. The shiny side of the ribbon should rest upon
the water. The slide is then held for a moment over the flame of an alcohol lamp so that the water is heated. Repeat until the sections become
perfectly smooth and flat, but the paraffin must not be melted. The water
should not come in contact with the fingers holding the slide. If the albumen layer ends abruptly before reaching the border of the slide, the
water will not so readily spread beyond it. After the flattening process,
the water is cautiously drained off by a moist sponge held at the corner
of the slide. The sections settle down upon the albumen and may be
arranged in straight lines with needles applied to the paraffin, but not to
the tissues of the sections. The slide is then held vertically in contact with
filter paper to drain off any water which may remain, and the portions
of the slide which are free from sections are wiped off with a cloth free
from lint. The slide is next placed in a drying oven which is not warm
enough to melt the paraffin. It is well to let the slides remain over night,
but a few hours may be sufficient to dry them thoroughly.
In preparing large numbers of slides, each bearing only one or two sections, fragments of the ribbon containing the desired number of sections
are floated in a basin of water warm enough to flatten but not to melt
them. Slides rubbed with albumen are dipped into the water beneath the
sections, which are held in place with a needle. The slides are drained
and dried in the usual way, care being taken to have the sections in the
center of the slide. Or the ribbons may be floated on warm water and
cut into fragments with a heated knife, proceeding then as before.
To remove the paraffin, the slides are immersed in xylol for about 5
minutes. The slide is then transferred in turn to a mixture of equal parts
of xylol and absolute alcohol, then through absolute, 95 per cent., 80 per
cent., 70 per cent, and 50 per cent, alcohols, remaining about i minute
in each, to water. In case the stain is in alcoholic solution, the transfers
may be stopped at that grade of alcohol which corresponds to the solvent
of the stain.
In case the sectioned tissue was fixed in a fluid containing corrosive
sublimate and has not previously been treated with iodine for the removal
of mercurial deposits, enough tincture of iodine to give a port-wine color
may be added to the 80 per cent, alcohol. The slide is immersed in this
32
498 HISTOLOGY
for about a minute and then washed in fresh 80 per cent, to remove the
iodine.
All of the reagents and stains to be used for paraffin sections may be
kept in a series of tube-like vials, in which the slides may be placed in
pairs back to back, being transferred from one vial to another. The vials
are kept tightly corked, and the reagents can be used for some time
before they must be renewed.
In handling a large number of slides, grooved rectangular boxes are useful. Each reagent is allowed to act, poured out and another substituted.
Celloidin Sections. Objects imbedded in celloidin are cut with
either a sliding or a precision microtome, the knife edge meeting the block
obliquely. The block and knife are kept wet with 80 per cent, alcohol.
Sections are cut 10 to 15 n in thickness, and are transferred, by means
of a camel' s-hair brush wet with alcohol, to a dish of 80 per cent, alcohol,
in which they remain until wanted for staining.
Celloidin sections are stained in a series of small, shallow staining
dishes. The sections are taken from 80 per cent, alcohol and transferred
through graded alcohols to water or the solvent of the stain. If deposits
of corrosive sublimate are present and were not removed before imbedding,
the sections should be treated as directed for paraffin sections. The sections are transferred from dish to dish with bent metal or glass needles.
Celloidin sections are not treated with absolute alcohol, since the celloidin
would be softened.
The handling of large numbers of celloidin sections is facilitated if
they are placed in a perforated cup which fits into another ordinary cup.
The ordinary cups contain the various reagents and the sections are transferred from one to another in the perforated cups. The latter may be
obtained as "Hobb's Tea Inf users," and lemonade cups are of proper
size to receive them.
Wright's Method for Frozen Sections. This method gives permanent
preparations which are adequate for most routine purposes in histological
examination, and saves much time, labor, skill and expense. The success
of the method depends to a considerable extent upon the frozen sections
being as thin as good celloidin sections. Special automatic microtomes
may now be purchased, or the older form in which the sections are "chiselled" from the block will give good results if properly used.
The tissues are fixed in 10 per cent, formalin for 12 to 24 hours or longer.
The piece is then trimmed so that it will present a thickness to be frozen
of not over 5 mm. The other dimensions of the block may be as large
as the freezing box of the microtome will permit. The block is rinsed in
water, placed on the freezing box with a few drops of water beneath it;
frozen and cut into sections, which should not be over 10 or 15 p- in
thickness.
FROZEN SECTIONS 499
The sections are floated into water, in which they unroll. Select a
good section and spread it smoothly on a slide which has been coated
with a thin, even layer of albumen-glycerin. Superfluous water is
drained off and the section pressed upon the slide with a piece of smooth
blotting paper, by exerting an even but not great pressure with the ball
of the thumb. The section will adhere to the slide.
Now quickly cover the section with a small quantity of 95 per cent,
alcohol, followed in a few seconds by absolute. Pour from a drop bottle
quickly and evenly over the section and adjacent surface of the slide a
very thin solution of celloidin dissolved in equal parts of ether and absolute alcohol. Drain off immediately, blow the breath once or twice on
the surface of the section and immerse the slide at once in water for a
few seconds. The thin film of celloidin thus formed fastens the section
to the slide. The solution of celloidin should be almost watery in consistence, and so thin that it will form drops readily without stringing.
If it is too thin, it will not hold the section on the slide, while if it is too
thick, the layer on the slide will become white when it is immersed in
water. The film of celloidin should be so thin as to be almost invisible.
The section may now be stained by any of the usual methods applied
to sections affixed to the slide. The thin layer of celloidin offers no obstruction to the staining. After staining, the section is dehydrated by
covering with 95 per cent, alcohol for a few seconds. Absolute is now
poured on and allowed to remain for a few seconds. This removes most
of the celloidin, but, unless the action is unduly prolonged, the section
will not be loosened. Clear in xylol.
The microtomes and knives mentioned in this section are described
and directions for their use are given in Mallory and Wright's "Pathological Technique." Their use, however, is seldom learned except by personal
demonstration in the laboratory.
4. Staining.
The purpose of staining is to differentiate the tissue elements. The
staining of tissues is in a measure a micro-chemical color reaction, the
differential staining being due to the fact that certain elements take up
more of the stain than others.
Stains used in microscopic work may be divided into two general
classes according to their chemical properties (i) basic stains, which
show especial affinity for the nuclei of cells and are called nuclear stains,
and (2) acid stains, which affect the cytoplasm more readily and are called
cytoplasmic stains. Certain so-called selective stains (either acid or basic)
affect one tissue element especially, or even exclusively. Preparations
may therefore be stained with several dyes, each affecting certain tissue
500 HISTOLOGY
elements only. Certain stains may be applied to the tissue before it is
imbedded and sectioned. They are seldom used except in the preparation of embryos.
GENERAL STAINS.
THE STAINING OF SECTIONS.
Haematoxylin and Eosin. Haematoxylin is a basic dye obtained from
logwood, which stains nuclear structures blue. Eosin is an acid anilin
dye which stains cytoplasm red. (It is recommended that all anilin
dyes used in histological work be those prepared by Griibler in Germany.)
There are many formulae for the preparation of haematoxylin solutions,
of which the two following are especially useful.
Alum haematoxylin.
Haematoxylin crystals i gm.
Saturated aqueous solution of ammonia alum 100 c.c.
Water 300 c.c.
Dissolve the haematoxylin in a little water with the aid of heat, and
add to the remainder of the solution. Put the mixture in a bottle and
add a small lump of camphor or thymol to prevent the growth of mould.
Stopper the bottle with a loose plug of cotton and set in the light for about
10 days to ripen. It changes to a deep blue color during this process of
oxidation, after which it is ready for use and is kept tightly stoppered.
It deteriorates and must be renewed after a few months. The solution
may be ripened immediately by the addition of 2 c.c. of hydrogen peroxide
solution, neutralized by a crystal of sodium chloride.
For use after Zenker fixation, the water in the above formula may be
omitted.
Delafield's Haematoxylin.
Hsematoxylin crystals 4 gm.
Alcohol, 95 per cent 25 c.c.
Sat. aq. sol. of ammonia alum 400 c.c.
The haematoxylin is dissolved in the alcohol and added to the alum
solution. This is exposed to the light in an open vessel to ripen for about
4 days and then is filtered. To the filtrate is added:
Methyl alcohol (or 95%) 100 c.c.
Glycerin 100 c.c.
This mixture is exposed to the light in a cotton-plugged bottle for
about a week, after which it is again filtered and tightly stoppered. The
solution keeps for a considerable time. It may be used in this strength,
but preferably it is diluted with one or two volumes of water.
Eosin is sold in two forms, one soluble in water, the other in alcohol.
In connection with haematoxylin, a T V to i per cent, aqueous solu
HjEMATOXYLIN AND EOSIN 501
tion may be used, but it is preferable to use a solution made by adding
to 95 per cent, alcohol enough of a stock solution, consisting of 5 gm. of
eosin dissolved in 300 c.c. of 50 per cent, alcohol, to give a deep yellowishred color to the alcohol.
Sections are transferred from water to the haematoxylin solution for
about 5 minutes. They are then washed in several changes of tap water
until the sections are deep blue in color. If this change does not occur
rapidly, a few drops of ammonium hydrate may be added to the water
They are then examined with the microscope. If the sections are overstained (a condition recognizable in the staining of the cytoplasm as well
as the nuclei) the sections are washed in a 0.5 per cent, solution of hydrochloric acid in 70 per cent, alcohol until the section is reddish-brown,
usually about i minute. Wash in water made alkaline with a few drops
of ammonia and re-examine. If the nuclei are not well stained, the sections are returned to the haematoxylin solution for 5 to 10 minutes longer,
after which they are examined as before.
When the stain is sharply limited to the nuclei and is of satisfactory
depth, the sections are washed for 15 to 30 minutes in tap water and then
are passed through 50 per cent., 70 per cent., 80 per cent, and 95 per cent,
alcohol, remaining about i minute in each. Stain in the alcoholic solution
of eosin for i to 5 minutes. Wash in 95 per cent, until red clouds no longer
leave the section.
Paraffin sections are then passed through absolute; absolute and xylol
(equal parts) ; and xylol, in which they remain for about 5 minutes, and
are then ready for mounting. Celloidin sections are transferred from
95 per cent, to oil of origanum for about 5 minutes, before mounting.
Eosin and Methylene Blue. This method is highly recommended,
especially for tissues fixed in Zenker's fluid and sectioned in paraffin.
Stain the sections for 20 minutes or longer in a 5 or 10 per cent, aqueous
solution of eosin. The tissue must be overstained, as the eosin is partially
extracted in the subsequent treatment. Wash out the excess of the stain
in water.
Stain for 10 to 15 minutes in Unna's alkaline methylene blue (i gm.
of methylene blue and i gm. of potassium carbonate dissolved in 100 c.c.
of water) diluted i : 3 or i : 5 with water. Wash in water. Differentiate
and dehydrate in 95 per cent, alcohol, keeping the section in constant
motion so that the decolorization is uniform. When the pink color returns to the section and when, as seen under the microscope, the blue is
limited to the nuclei, the section is quickly washed in absolute, passed
through absolute and xylol, and put in xylol for about 5 minutes.
Heidenhain's Iron Haematoxylin. The best results are obtained with
very thin paraffin sections. From water the sections are transferred to
a 2-2.5 P er cent - aqueous solution of ferric ammonium sulphate for 4 to
502 HISTOLOGY
8 hours. Wash quickly in water. Stain for 12 to 24 hours in a wellripened solution consisting of 0.5 gm. haematoxylin dissolved in 10 c.c. of
absolute alcohol and added to 90 c.c. of water. Wash in water. Differentiate in the iron alum solution, controlling the result under the microscope. The section should be washed in water before each examination.
When the stain is limited to the nuclei, and these are sharp, wash in running water for 15 to 30 minutes; 50 per cent.; 70 per cent.; 80 per cent.;
95 per cent.; absolute alcohol; absolute and xylol; xylol.
If a counterstain is desired, add to 95 per cent, alcohol enough of a
i per cent, solution of Orange G in 50 per cent, alcohol to give a deep
orange color. Transfer the section from 95 per cent, to the stain for a
few minutes. Wash well in 95 per cent., and pass through absolute, absolute and xylol, to xylol.
Weigert's Iron Haematoxylin.
A. Haematoxylin crystals i gm.
Alcohol, 95 per cent 100 c.c.
B. Liquor ferri sesquichlorati 4 c.c.
Water 95 c.c.
Hydrochloric acid i c.c.
At the time of using, mix equal parts of A and B. Transfer the sections from water to the stain for 2 to 5 minutes. Wash in water. If
the section is overstained, add a few drops of hydrochloric acid to the
water. To stop the decolorization, dip in water made alkaline with a
little ammonia. This is an excellent stain and gives brilliant results.
If a counterstain is desired, place sections for about i minute in Van
Gieson's mixture:
Picric acid, sat. aq. sol too
Acid fuchsin, i per cent. aq. sol 10
Wash in water; 95 per cent.; absolute; absolute and xylol; xylol. For
celloidin sections, 95 per cent.; oil of origanum.
Safranin.
Safranin i gm.
Absolute alcohol 10 c.c.
Anilin water 90 c.c.
Anilin water is prepared by shaking up 5 c.c. of anilin oil in 95 c.c. of
distilled water and filtering through a wet filter. Dissolve the safranin
in the alcohol and add to the anilin water.
Safranin is to be used after fixation in Flemming's fluid. Stain thin
paraffin sections for 24 hours; wash in water; decolorize in absolute alcohol,
to which has been added hydrochloric acid in the proportion 1:1000,
until only the nucleus retains the stain; fresh absolute; absolute and
xylol; xylol.
COCHINEAL AND CARMINE 503
STAINING IN BULK.
Embryos which are to be mounted whole or cut into serial sections
are commonly stained before they are imbedded. The time given for
the action of the following stains is that required in the preparation of
i2-mm. pigs. Larger or smaller objects should, of course, receive a
longer or shorter treatment.
Alum cochineal.
Cochineal 60 gm.
Potassium alum 60 gm.
Water 800 c.c.
Boil vigorously for 20 minutes. Cool and filter. Boil the filter
paper and contents with more water for the same length of time. Cool
and filter. Repeat the boiling and filtering until the powder disappears
from the residue. Then put all of the filtrate together, boil for about
20 minutes and make the volume up to 800 c.c.
Stain 1 2 -mm. pigs for about 36 hours. Wash in water for 10 to 15
minutes; 50 per cent, alcohol, 20 to 30 minutes; 70 per cent., i hour; 80
per cent. Imbed in paraffin and cut in serial sections.
Counterstaining the sections in the solution of Orange G described
with Heidenhain's iron hasmatoxylin will bring out the nerves beautifully.
The paraffin is removed in xylol, the sections passed through absolute
and xylol, absolute, and 95 per cent, to the stain. Wash in several changes
of 95 per cent.; and pass back through absolute, and absolute and xylol,
to xylol.
Borax carmine.
Borax 20 gm.
Carmine 30 gm.
Water 500 c.c.
Boil until everything is dissolved. Cool and add 500 c.c. of 70 per
cent, alcohol. Let stand 24 hours and filter.
Stain a i2-mm. pig about 36 hours; water 10 to 15 minutes; 0.5 per
cent, hydrochloric acid in 70 per cent., 30 minutes to i hour; 70 per cent,
changed several times, i hour; 80 per cent, changed twice. Imbed in
paraffin and cut in serial sections. The sections may be counterstained
as directed under alum cochineal.
SELECTIVE STAINS.
Mallory's Phosphotungstic Acid Haematoxylin.
Haematoxylin o. i gm.
Water 80.0 c.c.
10 per cent. aq. sol. of phosphotungstic acid (Merck) 20.0 c.c.
Dissolve the haematoxylin in a little water with the aid of heat; cool,
and add to the rest of the solution. If the solution does not stain, it may
504 HISTOLOGY
be ripened by the addition of 10 c.c. of | per cent. aq. sol. of potassium
permanganate.
Tissues must be fixed in Zenker's fluid. Sections are transferred
from water to | per cent, aqueous solution of potassium permanganate
for 3 to 5 minutes, washed in water, and put for 5 to 10 minutes in 5 per
cent, aqueous solution of oxalic acid. Wash thoroughly in several changes
of water, and stain in the haematoxylin solution for 12 to 24 hours. Transfer directly to 95 per cent, alcohol for not more than i or 2 minutes, followed by absolute for paraffin sections. Clear in xylol, using the filterpaper blotting method for celloidin sections (see p. 508).
Neuroglia, myoglia, and fibroglia fibrils and fibrin are stained blue;
collagen fibrils reddish-brown; mitotic figures well shown.
Mallory's Connective Tissue Stain.
Anilin blue soluble in water (Griibler) 0.5 gm.
Orange G (Griibler) * 2.0 gm.
Phosphomolybdic acid, i per cent. aq. solution. 100.0 c.c.
Paraffin or celloidin sections of material fixed in Zenker's fluid are
transferred from water to a 0.2 per cent, aqueous solution of acid fuchsin
for 5 to 20 minutes. Transfer directly to the anilin blue solution and stain
for 20 minutes or longer. Wash in several changes of 95 per cent, alcohol.
Clear celloidin sections in oil of origanum. Paraffin sections are passed
through absolute, absolute and xylol, to xylol.
Fibrils of connective and reticular tissue, amyloid, and mucus stain
blue; nuclei, cytoplasm, muscle, axis cylinders, and neuroglia fibers stain
red; red corpuscles and myelin, yellow.
Weigert's Resorcin-fuchsin. Boil, in an evaporating dish, 2 gm.
of fuchsin and 4 gm. of resorcin in 200 c.c. of water. When it is boiling
briskly, add 25 c.c. of liquor ferri sesquichlorati. Stir and boil for 5
minutes. Cool and filter. Allow the precipitate to dry; return the filter
paper with precipitate to the dry dish; add 200 c.c. of 95 per cent,
alcohol and boil, stirring constantly. Fish out the paper. Cool and
filter; add alcohol until the volume of 200 c.c. is reached and add 4 c.c.
of hydrochloric acid.
From 95 per cent, alcohol the sections, preferably fixed in alcohol or
formaldehyde, are transferred to the stain for 20 minutes to an hour.
Wash in 95 per cent. Clear in xylol by the blotting method (p. 508).
The elastic fibers are stained a deep purple. The remainder of the
tissue should be nearly or quite colorless. If other parts are affected, the
sections should be washed in alcohol containing 0.5 per cent, of hydrochloric acid. A light nuclear stain with alum haematoxylin after the elastic tissue has been stained will increase the value of the specimen.
Scharlach R. Frozen sections of fresh or formalin fixed material
are stained from 15 minutes to over night in a saturated solution of the
SCHARLACH 505
dye in 70 per cent, alcohol. The sections are transferred from water to
the stain, which has been freshly filtered into a tightly closing vessel.
Evaporation of the alcohol causes a precipitation of the staining material.
Wash in water, stain the nuclei lightly with alum haematoxylin, and
mount in glycerin. Fat and lipoids stain red.
Nile Blue. Frozen sections of fresh material or material fixed in
formalin for not more than 12 hours are stained for 15 minutes to 2 hours
in a saturated aqueous solution of nile blue (Griibler). Wash in distilled
water for 5 minutes or more, and transfer to tap water. If after 5 minutes
in the tap water the section does not assume a reddish hue, add a small
amount of alkali to the tap water. When the section is reddish, transfer
to distilled water. Mount in glycerin or glycerin jelly and examine at once.
Neutral fat red; lipoids blue.
Osmic Acid. Fat and myelin in fresh tissues may be blackened in a
i per cent, aqueous solution of osmic acid. The myelin sheaths of teased
nerve fibers may be so treated, the fragments dehydrated, cleared in chloroform, and mounted in chloroform damar. Sections may be prepared from
tissues fixed in Marchi's fluid (p. 491), showing the fat blackened by the
osmium. Use chloroform to remove paraffin from the sections, and
mount in chloroform damar.
Wright's Blood Stain. After 0.5 gm. of sodium bicarbonate has been
completely dissolved in 100 c.c. of distilled water, add i gm. of Griibler's
methylene blue (either the form called BX, Koch's, or Ehrlich's rectified).
"The mixture is next to be steamed in an ordinary steam sterilizer at
100 C. for i hour, counting the time after steam is up. The heating
should not be done in a pressure sterilizer, or in a water bath, or in any
other way than as stated." The mixture is then removed from the sterilizer and allowed to cool, the flask being placed in cold water if desired.
When cold, it is poured into a large dish or flask. Add to each 100 c.c.
of the methylene blue solution, stirring or shaking meanwhile, about
500 c.c. of a o.i per cent, solution of Griibler's yellowish eosin soluble in
water. The eosin solution should be added until the mixture, losing its
blue color, becomes purple, and a scum with yellowish metallic luster forms
on the surface, "while on close inspection a finely granular black precipitate appears in suspension." The solution is then filtered and the precipitate allowed to become perfectly dry on the filter paper. The stain
is made by dissolving 0.3 gm. of the precipitate in 100 c.c. of pure methyl
alcohol. The stain need not be filtered, and like the precipitate, it keeps
indefinitely. If by evaporation of the alcohol it becomes too concentrated,
as shown by the formation of a precipitate when it is used, it should be
filtered and a small quantity of methyl alcohol added.
Blood is obtained usually from a needle puncture in the lobule of the
ear. A drop of blood is caught in the center of a perfectly clean dry
506 HISTOLOGY
cover, and another clean dry cover is immediately dropped upon it. The
blood should spread between the two cover glasses, forming a film which
cannot be too thin. The covers are then drawn rapidly apart (they should
slide along one another and not be lifted apart). The blood film dries
from exposure to the air, and remains stainable for weeks.
To stain the blood film, the cover glass is to be held in cover-glass
forceps with the film side uppermost. The stain is applied as follows:
1. Cover the preparation with a noted quantity of the stain by means
of a drop-bottle or medicine dropper.
2. After i minute add to the staining fluid on the preparation the
same quantity of distilled water, by means of the dropper, and allow the
mixture to remain i\ minutes, not longer. Longer staining may produce
a precipitate. The total quantity of diluted fluid on the preparation
should not be so much that some runs off. A metallic scum forms when
the stain is properly diluted, but the stain should not become transparent.
3. Wash the preparation in tap water for 30 seconds, or until the
thinner portions of the film become yellow or pink. Disregard the thick
parts, which are blue. The process of decolorizing may be watched
through the microscope by placing the cover glass with film side uppermost on a slide.
4. Dry and mount directly in damar.
Silver Nitrate. Intercellular cement spaces and the boundaries of
endothelial cells may be blackened by a i to i per cent, aqueous solution
of silver nitrate, which acts chiefly upon free surfaces. The fresh tissue
should be kept flat, the mesentery, for example, being tied over a detached
bottle neck, while it is immersed in the solution for i to 10 minutes.
Transfer to distilled water, and expose to direct sunlight. As soon as it
becomes brown, usually in 5 to 10 minutes, it is washed in 0.6 per cent,
salt solution. If desired, the nuclei may be lightly stained in alum haematoxylin. Examine in glycerin, or dehydrate clear in xyol and mount in
damar.
Blood vessels may be injected through glass tubes with the silver
solution. Sections are made and exposed to the light, and the outlines
of the endothelial cells become dark.
5. Clearing and Mounting.
CLEARING.
Before satisfactory permanent preparations can be obtained, the
sections or object must be cleared,. This is accomplished by infiltrating
the tissues with substances which, by reason of their high index of refraction, render the tissues more or less transparent. Structures to be studied
are previously stained and thus easily rendered prominent.
CLEARING AGENTS 507
A variety of reagents with widely different chemical properties are
used. Glycerin and acetate of potash are commonly used for frozen
sections or teased preparations which, for any reason, cannot be mounted
in damar.
The choice of a clearing agent for damar mounts depends chiefly on
two factors, the kind of stain employed and the imbedding medium.
Xylol. This is the best clearing agent for use after aniline dyes. It
clears only from absolute alcohol, through which celloidin sections cannot
be passed, since it dissolves celloidin. However, it can be used for celloidin or other sections dehydrated in 95 per cent, alcohol by the following method. Blot the section on the slide with smooth, soft filter paper
and pour on a few drops of xylol. Repeat the blotting, followed by xylol
two or three times and the section will be perfectly clear. Paraffin sections attached to the slides are cleared by immersion in a vial of xylol;
this has already been mentioned as the last step in the staining processes.
Oleum Origani Cretici. This clears readily from 95 per cent, alcohol
without dissolving celloidin and affects aniline colors slowly. Although
particularly recommended for clearing after celloidin imbedding, it is
useful for all kinds of sections.
Carbol-xylol.
Carbolic acid crystals i
Xylol 3
Used for clearing thick sections of the central nervous system after
carmine and haematoxylin stains. Clears from 95 per cent, alcohol without affecting celloidin, but extracts the basic aniline dyes.
Chloroform. Since osmic acid reduced by fat is soluble in xylol,
chloroform is used in cases where permanent mounts of such preparations
are desired (as after fixation in Marchi's fluid).
MOUNTING.
Frozen sections which cannot be mounted in damar are mounted
in glycerin, potassium acetate or glycerin jelly.
Glycerin Jelly. Soak 7 gm. of gelatin for about 2 hours in 42 c.c.
of distilled water. Add 50 gm. of glycerin. Warm, stirring constantly
for icr-i5 minutes. Filter hot through moistened cotton.
Bring sections on a slide and blot off excess water. Put on a small
piece of glycerin jelly and warm gently until it melts. Cover and cool.
Preparations mounted in any of these three substances may be
rendered more or less permanent by coating the edges of the cover and
adjacent surfaces of the slide with paraffin or wax.
Gum Damar. Of the two substances most commonly employed for
permanent mounts, namely, Canada balsam and damar, the latter is
preferable, since balsam turns yellow with age.
508 HISTOLOGY
Colorless pieces of damar are dissolved in xylol and filtered. If
the solution is too thin, evaporate to the proper consistence; if too thick,
add more xylol. The proper consistence is that of a thin syrup.
A solution in chloroform should be prepared in similar manner for
use with osmium preparations.
After paraffin sections have been cleared (3 to 5 minutes), the excess
of clearing agent is drained away and the surface of the slide outside the
sections is wiped off. The section must not be allowed to become dry.
A drop of damar solution is placed at once upon the section and a cover
glass is carefully lowered over it. With all preparations, whatever the
mounting medium may be, the cover glass should be lowered in the
following manner. It is held over the specimen and its left edge is first
brought into contact with the slide; a needle held in the left hand holds
this edge in position. Another needle held in the right hand with its
point beneath the right edge of the cover enables one to have perfect
control of the cover glass while it is being lowered. The contact between
the cover glass and the mounting medium spreads gradually from left
to right as the cover is lowered, expelling the air as it advances. If air
bubbles are caught in the medium, the cover may be alternately raised
and lowered a little until they escape, but once the cover is flat upon
the specimen it should not be lifted. The cover glass should be somewhat larger than the specimen so as to extend beyond it on all sides.
Celloidin sections which have been cleared in oil are floated over
the blade of a spatula placed in the oil, and are spread out flat upon it
with the aid of a needle. They are then transferred to a clean glass
slide, being pulled from the spatula with the needle. They should be
moved to the exact center of the slide, if the preparations are to look
well, and then the oil is removed by placing two thicknesses of filter
paper over the section and pressing upon it quite firmly; at the same
time the section is made smooth. A drop of damar is then placed upon
the section before it dries, and the cover glass is applied as described in
the preceding paragraph.
Damar mounts are then labelled and may be placed in a drying
oven with a temperature of 35-40 C. They may be used after a few days,
but the damar is not thoroughly hardened for a considerable time.
SLIDES AND COVER GLASSES.
Slides should be of colorless glass with ground edges. For ordinary
use, slides measuring i X3 inches (26 X 76 mm.) are preferable. Occasionally, as in mounting serial sections, or large sections of the central nervous
system, wider slides are needed. Thick slides are preferable to thin
ones.
COVER GLASSES 509
For ordinary use, cover glasses 18-22 mm. square are sufficient.
Occasionally, as in mounting serial sections or large specimens, oblong
covers may be needed. If possible, no covers ranging outside of 0.150.18 mm. in thickness (No. i grade) should be used, since thicker covers
(No. 2 grade) often prevent the oil immersion lens from being brought
into focus. Many valuable sections have been destroyed in attempting
to focus through thick cover glasses.
Clean the slides and covers by dipping in alcohol and drying with a
soft crash towel or old linen handkerchief. Sometimes it may be necessary to wash them in 10 per cent, nitric acid, followed by a thorough
washing in water and then in alcohol. Slides which remain hazy after
thorough washing must be discarded.
INJECTIONS.
The courses of blood and lymphatic vessels and of ducts are studied
by means of injections. Transparent, deeply colored fluid mixtures,
which will harden in the vessels, are used. So-called "warm" injection
masses, which contain gelatin, give more perfect results but are more
difficult to use than "cold" injection masses.
A tapering glass or metal cannula is inserted into the vessel
or duct, which is then tied securely around it. From a syringe connected
with the cannula by a short rubber tube, the mass is then forced into the
vessel. Pressure may also be obtained by having the injection mass in a
receptacle which is connected with the cannula by a long flexible tube.
The pressure is varied to suit the needs of the moment by raising and
lowering the receptacle.
When a warm injection mass is being used, the bottle containing the
mass must be placed in a water-bath and kept at a temperature of about
45 C. The organ or animal to be injected must also be placed in a
water- bath of the same temperature.
Organs to be injected must be perfectly fresh; they may be left within
the body or removed and injected separately. It is advisable to wash
out blood vessels with warm salt solution or Ringer's solution before the
injection.
It is important that in connecting the end of the tube carrying the
injection mass with the cannula inserted into the vessel, no air bubbles be
allowed to enter.
COLD INJECTION MASSES.
i. Blue Injection Mass.
Soluble Berlin blue i
Distilled water. . . .20
510 HISTOLOGY
2. Carmine Injection Mass. Dissolve i gm. of carmine in i c.c. of
strong ammonia plus a little water; dilute with 20 c.c. of glycerin. Add i
gm. of common salt dissolved in 30 c.c. of glycerin. To the whole solution add an equal quantity of water.
WARM INJECTION MASSES.
1. Berlin Blue. Allow clear sheets of best French gelatin to swell
up for i or 2 hours in double the quantity of water. Dissolve by warming
gently over a water-bath and add, stirring constantly, an equal volume
of a warm solution of Berlin blue prepared as directed above. Filter
through flannel wrung out in hot water.
2. Carmine. This is the best injection mass to use, but it is very
difficult to prepare. Dissolve 2 to 4 gm. of the best carmine in the required amount of ammonia. Filter and stir into 10 to 50 gm. of a filtered
warm solution of gelatin, prepared over the water-bath as described
above. Then add 25 per cent, acetic acid, drop by drop, stirring .constantly, until the mass becomes bright red and loses its ammoniacal odor.
If too much acetic acid is added a precipitate forms and the mass is spoiled.
Filter through warm flannel.
Organs injected with a cold mass are put into 80 alcohol. After a
few hours they may be cut into pieces. After injection with a warm
mass, the specimen is put into cold water to hasten the solidification of
the gelatin, and then transferred to 80 per cent, alcohol. Imbed in celloidin. Thick sections are necessary in order to follow the course of the
vessels.
Prepared injection masses for use, cold or warm, are sold by dealers
in microscopical supplies.
Many ingenious injection methods have been devised, such as the
injection of small living embryos by allowing ink to enter the veins and
be distributed through the body by the action of the heart; and vessels
have been injected with milk, after which frozen sections were stained
with Scharlach R.
SPECIAL METHODS.
The following special methods are included because of their fundamental importance. For the many other special methods which are occasionally of service, reference should be made to the works on technique
mentioned at the beginning of this section.
Weigert's Method for Staining Myelin Sheaths. This is a method for
the differential staining of the myelin sheath of nerve fibers and is much
used in the study of the normal and pathological histology of the central
WEIGERT'S MYELIN STAIN 511
nervous system. As a result of some chemical reaction between myelin
and a chrome salt, the myelin is fixed so that it does not dissolve in alcohol and ether, and at the same time is mordanted so that it stains deeply
with haematoxylin.
1. Fix in a 10 per cent, aqueous solution of formalin for several days
to several weeks, or indefinitely. Use a large quantity of fluid; change
at the end of 24 hours and thereafter whenever it becomes cloudy.
2. Cut the tissue into pieces not over i cm. thick, and place in a 2.5
per cent, aqueous solution of potassium bichromate renewed each day
for 3 or 4 days; then in a 5 per cent, solution renewed each day for 3 to
4 days.
3. Wash in running water for 24 hours (large pieces several days).
4. Transfer the tissues to the following solution (Weigert's second
mordant) for 24 to 48 hours.
Acetate of copper 5.0 gm.
Acetic acid, 36 per cent, solution. . 50 c.c.
Fluorchrom 2.5 gm.
Water 100. o c.c.
Boil the fluorchrom and water in a covered dish; turn off the gas and
add the acetic acid and then the acetate of copper. Stir until the latter
dissolves, and cool.
5. Wash in running water 24 hours or longer.
6. Dehydrate in graded alcohols and imbed in celloidin. Cut sections 20-25 p thick.
7. Stain sections for 12 to 24 hours in:
Ripened 10 per cent, solution of hsmatoxylin in absolute alcohol 10
Saturated aqueous solution of lithium carbonate i
Water ; 90
The haematoxylin is kept as a stock solution and combined with the
carbonate of lithium and water at the time of using. Beautiful results
may also be obtained by staining over-night or longer in Weigert's ironhaematoxylin (p. 502).
8. Wash in water.
9. Differentiate in the following solution:
Borax 2.0
Potassium ferricyanide 2.5
Water 100.0
It is advisable to dilute this solution with i or 2 volumes of water.
After the first staining method, the gray substance of the sections appears
yellow; after the iron-haematoxylin it is colorless.
10. Wash in running water 4 hours to over-night.
u. 95 per cent, alcohol, 3-5 hours (may be left over-night)
12. Fresh 95 per cent., 5 minutes.
13. Clear in carbol-xylol.
14. Mount in xylol damar.
512 HISTOLOGY
Pal's Modification of Weigert's Stain. The tissue is fixed, mordanted
and imbedded as directed in i, 2, 3 and 6 above. Sections may be very
much thicker.
7. Sections are placed for several hours in a per cent, aqueous solution of chromic acid, or for a longer time in a 2.5 per cent, solution of
potassium bichromate. (May be omitted.)
8. Stain for 24 to 48 hours in:
Ripened 10 per cent, solution of haematoxylin in absolute alcohol 10
Water 90
9. Wash in water plus i to 3 per cent, of a saturated aqueous solution
of lithium carbonate until the sections appear of a uniform deep blue
color.
10. Differentiate for 20 seconds to i minute in a \ per cent, aqueous
solution of potassium permanganate.
11. Place for a few seconds in the following solution, until the gray
substance is colorless or nearly so:
Oxalic acid i
Potassium sulphite i
Water 200
12. Wash in water.
Repeat steps 10, n and 12 until the differentiation is complete. Then
wash 4 hours or longer in running water.
13. 95 per cent, alcohol, 3 to 5 hours.
14. Fresh 95 per cent., 5 minutes.
15. Carbol-xylol. Mount in xylol damar.
Golgi's Method for the Impregnation of Nerve Cells. This method
depends on the formation of a fine precipitate in certain tissue elements
or in pre-existing spaces when the tissues are treated with a solution of
potassium bichromate and then with a solution of silver nitrate or
mercuric bichloride. The value of the method lies on the fact that it
picks out here and there a cell and stains it with its processes more
or less completely. This same fact renders the method very uncertain.
Of the several modifications of this stain, only one the so-called
rapid method is here given.
1. Pieces of fresh tissue about 5 mm. thick are placed for 3 to 8 days
in the following solution:
Osmic acid, i per cent, solution i part
Bichromate of potassium, 3 . 5 per cent, solution 4 parts
2. Transfer to a large quantity of 0.75 per cent, solution of silver
nitrate for 2 or 3 days.
Keep the tissues in the dark during treatment with both fluids.
The length of time the tissues should remain in the first solution
depends on the elements it is desired to impregnate.
WEIGERT'S MYELIN STAIN 513
For the human cord, the time is approximately as follows: for neuroglia,
2-3 days; nerve cells, 3-5 days; for nerve fibers and collaterals, 5-7 days.
3. Cut sections 50 to 100 p thick. They may be made free-hand or
with a microtome. Blocks may be quickly imbedded by dehydrating for
a few minutes in absolute alcohol, and placing in a thick solution of celloidin for about 5 minutes. Harden in 80 per cent.
4. Sections are dehydrated quickly in alcohol.
5. Clear in oil of cloves or bergamot.
6. Mount without a cover glass in xylol damar and dry quickly at
40 C. Protect the sections from the light and dust as much as possible.
If the method is unsuccessful, the specimens may be transferred back
to an osmic acid and bichromate mixture containing less osmic acid, and
after several days again placed in the silver nitrate solution for 24 to 48
hours.
II. THE EXAMINATION OF MICROSCOPICAL
SPECIMENS.
THE MICROSCOPE.
It is unfortunate that the price of a microscope is prohibitive to
many medical students, and that some who might purchase instruments
at the beginning of their work wait until later. The cost is now so
reduced that an increasing proportion of students can enjoy the advantage
of having a microscope of their own.
Microscopes of a certain grade are required, and if they cannot be
afforded, no instrument should be bought. The necessary equipment, as
shown in the figure, is a stand with fine and coarse adjustments ("micrometer screw" and "rack and pinion"), and a large square stage. The
more expensive round and mechanical stages are not necessary, and
since mechanical stages are detachable, they may be obtained later if
desired. There should be an Abbe condenser (with iris diaphragm), a
triple revolver, a high and a low eyepiece or ocular, and the following
objectives: a i6-mm. (f-inch) and a 4-mm. (|- or f-inch) which must
be parfocal; together with a 2-mm. (yV-inch) oil immersion, for cytological
and bacteriological work; and a 48-mm. (2-inch), which is a very low
power, for embryological work. The figures indicate the distance of the
section from the objective when the specimen is in focus; the higher the
power, the nearer the objective is brought to the object. The 2-mm.
oil immersion is an expensive objective, and its purchase may be postponed .
The 2-inch is a cheap objective which is very useful in obtaining a view of
an entire section, and for embryological reconstructions it is essential.
It may be noted that microscopes are now being finished more extensively
in black enamel than in lacquered brass; the former is not damaged by
alcohol and is more desirable. Improvements have also been made in
the post and fine adjustment, so that the form shown in the figure, although
good, is not the best.
Satisfactory microscopes of American manufacture are now made
but all agree that the Zeiss microscopes (German) are the best (and most
expensive). If the microscope is purchased by a student unfamiliar
with its use, it is well to have the lenses' iacamined by a disinterested
microscopist.
For a description of the nature and use of the microscope, the student
is referred to the nth edition of "The Microscope," by Professor S. H.
Gage (Comstock Pub. Co., Ithaca, N. Y.).
THE MICROSCOPE
515
For the sake of emphasis it may be said that the microscopist works
with his right hand upon the fine adjustment and his left hand upon the
Eye-piece (Ocular)
Tube
Draw-tube
Back and pinion adjustment
Triple revolver
Objective
Micrometer Screw
FIG. 493.
slide. As the latter is moved about, bringing different fields into view,
the focussing is done with the adjustment and not with the eyes. It is
impossible to study even a single field without constantly changing the
516 HISTOLOGY
focus, and the continuous use of the fine adjustment distinguishes an
experienced microscopist from a beginner. Both eyes should be open
(as will be natural after becoming accustomed to the instrument).
Often one acquires the habit of using only the right or the left eve for microscopic work, but it is better to learn to use both.
Always examine a specimen first with a low power objective and then
with a high power. In focussing the microscope, have the objective
drawn away from the slide and focus down. This should be done cautiously, with a portion of the specimen actually beneath the lens; if there
is only cover glass and damar there, the objective will probably be driven
down upon the slide. Unless one is sure that stained tissue is in the field,
the slide should be moved back and forth as the objective is being lowered.
In working with the Abbe condenser, the flat surface of the mirror
should be uppermost, provided that it is used in daylight and the rays
falling upon it are therefore parallel; but for the divergent rays of an artificial light near at hand, the concave mirror may be used, and the light
may advantageously be made to pass through a blue glass, which lessens
the yellow glare.
The objectives must never be scratched. Lens paper or fine linen
should be used to wipe them. If they are soiled with damar they should
be wiped with a cloth moistened with xylol, but since the lenses are
mounted in balsam, xylol must be applied to them cautiously. A microscope of the kind shown in the figure should never be lifted by any
part above the stage, lest the fine adjustment be damaged; the pillar
should be grasped below the stage.
RECONSTRUCTIONS.
There is an important arrangement of mirrors (Abbe's camera lucida)
for drawing the outlines of sections. It is attached to the microscope
above the eye-piece, and on looking into it one can see the image of the
section beneath the objective apparently spread upon the drawing paper
beside the microscope. Thus the pencil point can be seen as it is made to
trace the outline on the paper. With a little practice the same result
may be obtained more or less perfectly without the camera, by looking
into the microscope with one eye and at the same time upon the paper
with the other. This possibility was noted by the early microscopists,
and it is a useful accomplishment. More satisfactory than the camera
lucida is the projection apparatus of Edinger, arranged with an arc light,
whereby the image of the section is projected through an inverted microscope upon the drawing paper beneath. With the camera, or projection
apparatus, a succession of serial sections may be drawn with the uniform
magnification essential for reconstructions. The magnification is deter
RECONSTRUCTIONS 517
mined by substituting a stage micrometer for the slide of sections. The
micrometer is a slide upon which i mm., with subdivisions into twentieths
or hundredths, has been marked off by scratches in the glass; the subdivisions may be drawn with the camera, under the same conditions as
the sections, and the enlargement of the subdivisions may then be measured.
From the camera-drawings of serial sections, wax reconstructions of
various embryonic organs or small structures in the adult can be built
up. If the sections are 10 /* thick and alternate sections have been drawn,
magnified 50 diameters, then, on the scale of the drawings, these alternate
sections are i mm. apart. Wax plates i mm. thick are therefore to be
made, either by rolling beeswax, or by spreading a weighed amount of
melted wax in a pan of hot water. It floats and spreads in an even layer,
solidifying as the water cools. The outlines of the drawings are then
indented upon the wax plates, and the desired portions are cut out and
piled up to make the model. In this way reconstructions like those of
the ear (p. 466) may be made. This method was first employed by Born.
Further details of the process should be learned from demonstrations in
the laboratory.
Graphic reconstructions (first used by His) are generally side views of
structures, made from measurements of their transverse sections. Fig.
176, p. 185, is from such a reconstruction. A camera drawing of the side
of an embryo (or other structure) is made before it is sectioned. The
outline of this drawing is enlarged, and parallel lines, equally spaced, are
ruled across it, corresponding in number and direction with the sections
into which it was cut. Often only every other section, or every fourth
section, is used for the reconstruction, and the number of lines to be ruled
across the drawing is correspondingly reduced. Camera drawings of a
lateral half of every section to be used in the reconstruction are then made,
and across each drawing two lines are ruled. The first follows the median
plane of the body; and the second is at right angles with it, being drawn
so as to touch the dorsal or ventral surface of some structure to be included
in the reconstruction. Provided that the camera drawings and side
view have been enlarged to the same extent, the perpendicular distance
from the middle of the back to the junction of the two lines is marked off
in the side view, on the line corresponding with the section in question.
The perpendicular distances from the second line to the dorsal and to the
ventral surfaces of all structures to be reconstructed, are also marked off
upon the line on the side view. The same is done in the following section, and the points belonging with a given structure are connected from
section to section. Thus the outlines of the organs are projected upon
the median plane; two dimensions are accurately shown but the third is
lost.
Often it is undesirable to attempt to make the magnification of the
518 HISTOLOGY
sections and of the side view identical; the measurements may be enlarged or reduced as they are transferred for plotting, by means of the
draughtsman's proportional dividers, an indispensable instrument for
this method of reconstruction. The corrections for unequal shrinkage
of the sections in paraffin, and other details, can best be explained in the
laboratory with the drawings at hand.
In addition to making side views, this method may be used in reconstructing ventral or dorsal views, by plotting outward from the median
line.
DRAWINGS.
Since anatomy, both gross and microscopic, is a study of forms and
relations, that is of things seen, it finds natural expression in drawing;
and the volumes of wood-cuts, copper-plates, and lithographs, together with
the cheaper process-drawings and half-tones of the present day, form
almost as important a part of anatomical literature as the accompanying
text. Often there may be shown in a figure at a glance what pages of
writing fail to make clear; and it is significant that the great books of
Vesalius, which marked a new era in anatomy, were illustrated by Jean
de Calcar, a pupil of Titian. Burggraeve believes that Vesalius doubtless
supplied preliminary sketches and adds "Almost all the great anatomists were no less excellent draughtsmen Scarpa and Cuvier furnish
us remarkable examples and one can hardly imagine an anatomist who
is not deeply sensitive to the beauty and harmony of contours and forms."
Selenka (1842-1902) drew the ape embryos, which he collected and described, with consummate skill, and "always impressed upon his students
the great value of a ready pencil." Robert Hooke (1635-1703) was far
less successful with his drawings. In the preface to his fully illustrated
microscopical observations, he makes the following explanation of the
defects of his plates, and in conclusion sets an example which all students
should follow. He says
"What each of the delineated Subjects are, the descriptions annext
to each will inform, of which I shall here, only once for all, add, that in
divers of them the Gravers have pretty well followed my directions and
draughts: and that in making of them, I indeavoured (as far as I was able)
first to discover the true appearance, and next to make a plain representation of it."
To discover the true appearance of each section and to make a plain
representation of it, is by far the best method for beginning the study
of histology, and conscientious attempts to represent what is seen invariably lead to deeper and more valuable observations. Thus drawings
are unhesitatingly required of all students, and every effort should be
DRAWINGS 519
made to acquire some skill in this direction. The problem of the microscopist, who has but little to do with the third dimension, is relatively
simple. A few suggestions may be given.
Generally sections are stained in different colors, and the question
at once arises how to represent these with the pencil. The accompanying
FIG. 494. DIAGRAMS SHOWING THE WAY IN WHICH THE SHADE VALUES OF THE PRIMARY
SECONDARY AND TERTIARY COLORS MAY BE REPRESENTED IN TERMS OF BLACK AND WHITE
(Lee, in Hardesty's "Laboratory Guide;" Blakiston, 1908.)
figures indicate the way in which this is done, the primary colors being
shown in the inner ring, and their combinations in the outer rings. Red
being a brighter color than blue is to be made lighter. Orange, a combination of the two brightest of the primary colors, should be lighter than
520 HISTOLOGY
purple a combination of the darkest and lighter than pure red since
it has the brighter yellow mixed with it. Thus the various colors may be
suggested in black and white, and the contrast between blue nuclei and
red protoplasm can be carefully preserved in the drawing. This is facilitated by the use of pencils of varying degrees of hardness "H" and
"3 H" for dark structures, and "6 H" for pale areas. Soft pencils, which
rub, should not be used.
Before beginning a drawing, the specimen should be carefully looked
over, to find the place most worthy of such attention. The time which
the drawing is to take must be considered, and a small area may be
found which combines features elsewhere scattered about the specimen.
The entire field is rarely, if ever, to be drawn; and the figures should not
be encumbered with surrounding circles.
The magnification of the drawing is next to be decided upon. The
form of a gastric gland and the structure of its cells, for example, cannot
profitably be included in a single drawing. General features, such as the
forms of glands, should be represented in "low power" sketches. "Low
power" as here used does not necessarily refer to the lenses employed,
but means that the drawing is on such a scale that the nuclei appear merely
as spots, round or elongated as the case may be. Often, however, such
a drawing shows features which can be clearly observed only with high
power lenses. "High-power drawings" are those which present details
of nuclear and protoplasmic structure.
Usually in studying an organ, it is desirable to make a general lowpower sketch showing the arrangement of its lobules or layers, and to
supplement this by high-power drawings of the most significant cells or
tissues. In these, which are the final test of a student's keenness of observation, no details of cellular structure are too minute for careful representation, and "the difficulty of observing them proves not the merit of
overlooking them."
Having selected a field and decided upon the magnification, the outlines of the parts should be sketched lightly, with a soft pencil, and corrected until accurate. As finally made, they should be definite clean lines,
not pieced out, representing the boundaries of layers, nuclear membranes,
cell walls when present, cuticular surfaces, and the like. Having completed the outline, shading should be undertaken, to differentiate substance
from empty space, and to indicate the nature of the substance. In highpower drawings protoplasmic texture must be faithfully reproduced homogeneous, finely granular or coarsely granular; if the granules are not distinct enough to be counted, they should not be readily countable in the
drawing. If definite walls are absent from the specimen, they should
not be drawn, but the shaded areas of the finished drawing should end
abruptly without a bounding line.
DRAWINGS
521
Drawings consist, therefore, of two parts outline, and shaded texture
or finish. Ruskin observes that the real refinement of the outline depends
on its truly following the contours, and in regard to finish be offers suggestions which may be applied to the drawings of the wall of the medullary
tube here reproduced. He states that if we are to "finish" farther, we
1
FIG. 495. THE WALL OF THE MEDULLARY TUBE, AS DRAWN BY Six STUDENTS.
must know more or see more about the object. These sketches are not
finished in any sense but this, that the paper has been covered with lines.
A piece of work is more finished than others, not because it is more delicate or more skillful, but simply because it tells more truth. " That which
conveys most information, with least inaccuracy, is always the highest
finish."
INDEX
Abducent nerve, 141, 424
Absorbents, 183
Absorption, intestinal, 264
Accessory chromosome, 23
duct of pancreas, 290
nerve, 142, 424
Acervulus cerebri, 438
Acetic acid, 488
Acidophiles, 197
Acinus, 58
Acoustic meatus, external, 481; internal, 477
nerve, 141, 476
Adamantoblasts (ameloblasts) , 103
Addison, on suprarenal glands, 405
Adelomorphous cells (chief cells), 254
Adenoid tissue, 207
Adipose tissue, 72
Adrenalin, 406
Aggregate nodules, 268
Agminated nodules (aggregate nodules), 268
Ahlfeld, on the allantois, 381
Albumen, for attaching sections to slides, 497
Alcohol, for fixation, 490
Allantois, 245, 368, 373, 381
Allen, on tubules of the testis, 327
sexual cells, 335
ovary, 358
Alum cochineal, 503
haematoxylin, 500
Alveolar ducts, 301
sacs, 302
Alveolus, 58
of the lungs, 301
of the pancreas, 291
of the teeth, 99
Amakrine cells, 446
Ameloblasts, 103
Amtanthoid fibers, 80
Amitosis, 13
Amnion, 366, 373
Amniotic cavity, 367
fluid, 370
villi, 382
Amoeboid motion, n
Amphiaster, 17
Ampulla, of the ductus deferens, 342
of the semicircular ducts, 466
of the uterine tubes, 358
Ampullary nerves, 476
Anal canal, 273
membrane, 247
Anaphase, 18
Angioblast (angioderm) 43, 367
Anisotropic substance in muscle, 123
Annuli fibrosi, 176, 178
Anterior neuropore, 37
Anus, 247
Aorta, 165
in young embryos, 44
Aortic arches, 219
Apdthy, on myofibrils, 128
neurofibrils, 146
Appendices epiploicae, 273
Appendix epididymidis. 300, 344
fibrosa hepatis, 280
testis, 327, 344
vesiculosa, 351
vermiformis (processus vermiformis), 270
Aponeuroses, 77
Aquasductus cerebri, 422
cochleae, 477
vestibuli, 469
Aqueous humor, 442
Arachnoid membrane, 414, 439
granulations, 439
Archoplasm, 6
Areola, 404
Areolar glands, 404
tissue, 65
Aristotle, on blood vessels, 163
generation, 365
Arnold, on ovarian follicles, 356
Arrector pili, 386, 391
Arteria centralis retinae, 440, 460
Arteriae helicinae, 348
Arteries, 168
Arterioles, 168
Articular cartilage, 92, 97
discs, 98
Aschoff, on the atrio- ventricular bundle, 180
Aselli, on lymphatics, 182
Association fibers, 132
Aster, 15
Atretic follicles, 358
Atria, of the heart, 175
of the lungs, 302
Atrio- ventricular bundle and node, 180
Attraction sphere, 6
Auditory nerve (acoustic nerve) 141, 476
teeth, 473
tube, 217, 469, 479
vesicle, 465
Auerbach's plexus, 138, 248, 249
Auricle, 469
of the heart, 175
Automatic system, 139
Autonomic system, 139
Avicenna, on the intestine, 247
hypophysis, 436
Axial filament, 337
Axis cylinder, 145
Axolemma, 156
Axon, 134
Azygos veins, 310
B
Badertscher, on eosinophiles, 198
v. Baer, on ova, 366
Baldwin, on muscle cells, 122, 124
tendon, 127, 128
Balsam, 507
523
524
INDEX
Bardeen, on the development of muscle, 119
myelin, 156
sheath cells of non-medullated nerves, 154
Bartholin, on the ovary, 365
suprarenal glands, 404
pituitary gland, 436
vestibular glands of Bartholin, 383
Basal body, 7
Basement membrane, 53
Basophile cells, 69
Begg, on the vitelline veins, 279
Bell, on the nerves, 134
Bell, E. T., on the thymus, 224
Bensley, on the pancreas, 291, 294
Berlin blue, for injections, 510
Berry, on intestinal villi, 260
Bertini, columns of (renal colums), 315
Bichat, on the nervous system, 139
Bicuspid valve, 176
Bidder, on bone, 92
Bile capillaries, 285
ducts, 278, 288
Bipolar nerve cells, 143
Bladder. 324
development, 245
Blast, 67, 367
Blood, 1 88
crystals, 194
destroying organs, 202
forming organs, 202
islands, 43
pigments, 194
plasma, 188
plates (or platelets), 188, 199
red corpuscles, 188
stains for, 505
vessels, 163
white corpuscles, 188, 195
Blue injection masses, 509
Body cavity, 36
stalk, 369
Bone, 83
blood vessels, 96
cartilage replaced by bone, 87
cells, 84
compact, 87
corpuscles, 85
decalcification, 492
development, 84
endochondrial, 90
growth, 91
lacunae, 85
lamellae, 92
marrow, 202
membrane bones, 83
nerves, 96
perichondrial, 90
spongy, 87
Borax carmine, 503
Border fibrils, 64, 114
Bern's method of reconstruction, 517
Bouin's fluid, 490
Bowman, on the sarcolemma, 121, 122
Bowman's capsule (of the renal glomeruli),
317
glands (olfactory glands'), 485
membrane (of the cornea), 456
Brachium conjunctivum, 423, 427
pontis, 423, 427
Brain, 418
cerebellum, 427
early development, 37; later development, 418
hemispheres, 431
hypophysis, 435
medulla oblongata, 424
meninges, 438
pineal body, 437
pons, 423
Branchial arches, 230
clefts, 216, 217
Bremer, on development of blood vessels, 43
186
pulmonary arteries, 297
tubules of the testis, 328, 334
Bridges, intercellular, 53
Bronchi, 299
Bronchial arteries, 297
glands, 300
veins, 297
Bronchioles, 30x3
respiratory, 301
Brown, on pulmonary veins, 297
Brownian movement, 12
Brunner's glands (duodenal glands'), 259
Brush border, 50, 318
Bryce and Teacher's embryo, 368
Bulbous corpuscles, 161
Bulbo-urethral glands, 347
BuJbus analis, 247
coli, 246
urethrae, 348
vestibuli, 383
Bullard, on granules in muscle, 124
Bundle of His, 180
Burdach, on sympathetic system, 139
column of, 412
Bursae, 77
Caecum, 246, 272
cupulare, 467
vestibulare, 467
Cajal, on the growth of nerves, 136
Call-Exner bodies, 355
Calvert, on lymph glands, 209
Calyx, renal, 312
Camera lucida, 516
Canada balsam, 507
Canal of Gartner, 351
of Schlemm, 460
Canaliculi, of bone, 85
of the cornea, 458
Canalized fibrin, 378
Capillaries, 164, 167
bile, 285
secretory, 57
Capsula fibrosa hepatis, 278
Capsule, Bowman's, 317
of cartilage cells, 78
Glisson's, 278
internal, 431
of joints, 97
of the kidney, 315
of the lens, 441, 452
of the liver, 278
of the spleen, 214
Tenon's (interfascial space) 460
INDEX
525
Carbol-xylol, 507
Cardia, 251
Cardiac ganglion, 182
glands, of the oesophagus, 248
of the stomach, 253
muscle, 113, 128
plexus, 138
Cardinal veins, 309
Carmine, for injections, 510
Carnoy's mixtures, 390
Caitilage, 77
articular, 92, 97
elastic 80
epiphyseal. 91
nbro-cartilage, 81
growth, 78, 79
hyaline, 80
Caruncula lacrimalis, 463
Celloidin sections, 498
imbedding in, 495
Cells, i
amakrine, 446
amoeboid motion, n
basket, 292, 402, 429
basophile, 69
centro-alveolar (or centro-acinal), 292
chromamn, 152, 406
chief (of gastric glands), 254
Claudius's, 475
decidual, 374
Deiter's (of the cochlea), 474
differentiation, 9
division, direct, 13; indirect, 14
egg, 354
eosinophilic, 69
ependymal, 413
epithelial, 48
fat, 73
form, 8
formation, 12
germ, 20, 334
giant, of the bone marrow, 200, 202
glia (neuroglia)/64, 414, 434
goblet, 56
Hensen's (in the cochlea), 475
Kupffer's (in the liver), 287
lutein, 358
mast, 69, 197
mitral, 486
mucous, 237
neuroglia, 64
olfactory, 484
Paneth's, 262
parietal, 255
pigment, 71, 72
pillar (of the cochlea), 474
plasma, 68, 70
polymorphonuclear, 196
Purkinje's, 429
pyramidal, 431
"resting wandering," 71
Retzius's, 431
serous, 237
Sertoli's, 335
sexual, 20, 334
size, 9
squamous, 49
tactile, 157
taste, 235
Cells, visual, 444
vital phenomena, n
wall or membrane, 7
Cellulae pneumaticae, 479
Cement (subslantia ossea denlis), 99, in
intercellular, 10
Central nervous system, 130, 409
Centro-alveolar cells, 292
Centrosome, 6, 17, 33
Cerebellum, 423, 427
Cerebral hemispheres, 431
nerves, 139, 424
Cerebro-spinal fluid, 414
tracts, 425
Ceruminous glands, 481
Cervical glands (of the uterus), 361
sinus, 217
Chambers of the eye, 460
Chief cells (of the stomach), 254
Chloroform, 507
Choana, 481
Chondrioconta, 63, 121
Chondriosomes, 63
Chondromucoid, 78
Chorda dorsalis, 38
Chordae tendineae, 176, 178
Chordoid tissue, 82
Choriocapillaris, 453
Chorioid (coat of the eye), 442, 453
plexuses, 439
Chorion, 366, 371, 373
frondosum, 371, 375
laeve, 371, 373
vilJi, 375
Chromamn cells, 152, 406
organs, 152
Chromatin, 5
Chromatocytes, 72
Chromatolysis, 10
Chromatophores, 71
Chromosomes, 15, 19
accessory, 23
individuality, 20
number, 19
Chyle, 264
Chyme, 264
Cilia, 7, 50
(eyelashes), 461
Ciliary arteries, 458
body, 454
glands, 461
muscle, 454; (of Riolanus), 462
nerves, 460
processes, 454
Circumanal glands, 276
Circumvallate papillae (or vallate), 230, 231
Cisterna chyli, 183
Clark, E. F., on amitosis, 13
E. R., on lymphatics, 184
J. G., on ovarian vessels, 353
Clarke, column of, 427
Claudius's cells, 475
Clasmatocytes, 71
Clearing sections, 506
Clitoris, 353, 383
Cloaca, 245
Cochlea, 471
scala tympani, 468
vestibuli, 468
526
INDEX
Cochlear artery, 476
duct, 467, 471
nerve, 476
Coelom, 36
extraembryonic, 368
Cohn, on ameloblasts, 103
on interstitial cells of the ovary, 358
Cohnheim's areas, 120
Coil glands (sweat glands), 398
Collagen, 62
Collateral nerve fibres, 131, 144
Collecting tubules, of the kidney, 312, 319
Colliculi, 422
Colloid, 227
Coloboma, 442
Colon, 246, 272
. Colostrum, 402
Column, of Bertini, 315
of Burdach, 412
of Clarke, 427
of Glisson, 274
of Gcll, 412
of Morgagni, 274
rectal, 274
renal, 315
spinal cord, 411
Commissural fibers, 132
Commissures of the spinal cord, 411
Common bile duct, 278, 288
Conchas, 481
Concretions, prostatic, 345
Cone cells, 445
Conical papillae, 230, 231
Conjunctiva bulbi, 443, 462
corneas, 463
palpebrarum, 443
sclerae, 463
Conklin, on cleavage centrosomes, 34
Connective tissue, 43, 65
cells, 67
fibers, 65
stains, 504
Contour lines, in dentine, 108
in enamel, 105
Convoluted tubules, of the kidney, 313
of the testis, 334
Corium, 384, 385
Cornea, 442, 456
Corona radiata, 356
Coronary ligament, 278
sinus, 177
sulcus, 175
Corpora cavernosa clitoridis, 383
cavernosa penis, 348
mammillaria, 421
quadri gemma, 422
Corpus albicans, 358
callosum, 421
cavernosum urethras,
hasmorrhagicum, 357
luteum, 356; luteum spurium, 358
spongiosum, 326
Corpuscles, articular, 151
blood, red, 188; white, 188, 195
bone, 85
bulbous (of Krause), 161
colostrum, 402
corneal, 458
cylindrical, 161
Corpuscles, genital, 161
Golgi-Mazzoni, 401
Hassall's, 225
lamellar, 161
Malpighian (renal) 315; (splenic), 211
nerve, 160
Pacinian, 161
renal, 315
salivary, 220
splenic, 211
tactile (of Meissner), 160
thymic, 225
Corpuscula amylacea, 438
Cortex, 205
cerebral, 431
Corti,. organ of, 472, 474
Cotyledons of the placenta, 379
Councilman, on plasma cells, 70
Cover glasses, 508
Cowper, on the stomach, 251
glands of, 347
Cranial nerves, 139, 424
Crenated red corpuscles, 194
Crescents of serous cells, 237
Crile, on Nissl's bodies, 3
Cristae ampullares, 467
Crypts of Lieberkiihn, 260
Cumulus oSphorus, 355
Curran, on the atrio- ventricular bundle, 180
Gushing, on the hypophysis, 435, 436
Cuticula, 7, 50
dentis, 105
Cutis, 384
Cuvier, duct of, 309
Cylindrical corpuscles, 161
Cystic duct, 288
Cytoblastema, 12
Cytogenetic glands, 58
Cytomorphosis, 9
Cytoplasm, 2
Dalenpatius, on spermatozoa, 338
Damar, 507
Davis, on spermatogenesis, 21-23, 2 5> 2 7
Decalcification, 492
Decidua basalis, 371
capsularis, 371
reflexa, 371
serotina, 371
vera, 371, 373
Decidual cells, 374
membranes, 366
Decussation of the lemnisci (sensory), 426
of the pyramids (motor), 425
Deiters, on nerve cells, 145
cells of cochlea, 474
Dekhuyzen, on red corpuscles, 191
Delafield's haematoxylin, 500
Demilunes, 237
Dendrite, 143
Dental canaliculi, 107
cavity, 99
fibers, 107
groove, 102
lamina, 100
papilla, 101, 107
pulp, 99, no
sac, no
INDEX
527
Dentine, 99, 107
contour lines, 108
Dermomyotome, 41, 118
Descartes, on epigenesis, 339
pineal body, 438
Descemet's membrane (of the cornea), 458
Deuteroplasm, 29
DeWitt, on the atrio-ventricular bundle, 180
Diaphragm, 174
Diaphysis, 91
Diarthrosis, 96
Diaster, 18
Diemerbrceck, on the suprarenal gland, 405
Diencephalon, 421
Digestive tube, 245
layers, 248
Dilatator muscle of the pupil, 456
Diplosome, 7
Discus proligerus, 355
Dispireme, 18
Diverticulum ilei, 247
Downey, on plasma cells, 71
Drawing of specimens, 518
Ducts, 56
aberrant, of epididymis, 330
aberrant, of the liver, 280
alveolar, 301
Bartholin's (sublingual), 242
cochlear, 467, 471
common bile, 278, 288
Cuvier's, 309
cystic, 288
efferent, 329
ejaculatory, 329, 343
endolymphatic, 465
Gartner's, 351
hepatic, 288
intercalated, 58, 240, 292
Mlillerian, 327, 349
perilymphatic, 478
Santorini's (accessory pancreatic), 290
semicircular, 465, 470
Stenson's (parotid), 238
thoracic, 183
utriculo-saccular, 467
Wharton's (submaxillary), 242
Wirsung's (pancreatic), 291
Wolffian, 306, 307
Ductuli efferentes, 329
Ductus arteriosus, 296
deferens, 329, 342
epididymidis, 329
reuniens, 467
venosus, 279
Dujardin, on red corpuscles, 191
Duodenum, 246, 259
Dura mater cerebralis, 438
spinalis, 413
Dyads, 24
E
Ear, 465
external, 480
internal, 470
middle, 478
nerves, 476
stones, 471
vessels, 476
v. Ebner, on enamel prisms, 107, 108
red corpuscles, 191
Ectoderm, 35, 36
derivatives, 45
Efferent ducts (of the testis), 339
Egg cells, 354
Ejaculatory ducts, 329, 343
Elastic cartilage, 80
fibers, 66
stain for elastic tissue, 504
Elastin, 66
Eleidin, 388
Embryos, human, earliest stages 366
preservation of, 491; see also Bouin's
fluid, 490
Enamel, 99, 104
organs, 100, 102
prisms, 104
pulp, 102
End bulbs (Krause's), 161
discs, 149
organs (of Ruffini), 160
Endocardium, 178
Endochondrial bone, 90
Endolymph, 467
Endolymphatic duct, 465
sac, 465, 478
Endometrium, 360
Endoneurium, 154
Endoplasm, 3
Endosteum, 91
Endothelium, 43, 46
Entoderm, 35, 38
derivatives of, 45
Entodermal tract, 215
Eosin, 500
bodies, 428
Eosinophiles, 69, 197
Ependyma, 409, 418
Epicardium, 178, 181
Epidermis, 36, 384, 386
Epididymis, 329, 339
Epigenesis, 339
Epiglottis, 230
Epincurium, 154
Epiphysis, of bone, 90
of the brain, 437
Epithelioid glands, 58
Epithelium, 42, 46
basement membrane, 53
bridges, 53
cilia, 50
columnar, 48
cuboidal, 48
cuticular border, 50
differentiation of cells, 50
false, 47, 98
glands, 54
membrana propna, 53
mesenchymal, 47 ; 98
neuro-epithelium, 130
pseudo-stratified, 50
simple, 48
secretion, 54
stratified, 48
terminal bars, 52
transitional, 324
Epitrichium, 384
Eponychium, 389
528
INDEX
Epoophoron, 50
Equational division, 24
Equatorial plate, 15
Erasistratus, on lymphatics, 182
Erectile tissue, 348
Ehrlich, on mast cells, 69
Erythroblast, 189, 204
Erythrocyte, 188, 202
Eustachian tube, 217, 469
Eustachius, on lymphatics, 182
suprarenal gland, 404
Evans, on development of blood vessels, 166
perilymphatic blood vessels, 188
Exoplasm, 3
External acoustic meatus, 481
Eye, 439
blood vessels, 458
chambers, 442, 460
cornea, 442
iris, 442, 455
lachrymal glands, 462, 464
lens, 451
lids, 461
nerves, 460
optic nerve, 450
retina, 443
tunica fibres a, 456
tunica vasculosa, 453
vitreous body, 453
Fabricius, on valves of the veins, 164
stomach, 251
allantois, 373
Facial nerve, 141
Falciform ligament, 278
Fallopian tube, 327
Fallopius, on ovarian follicles, 365
placenta, 372
Farmer and Shove, on cell division, 14, 19, 20
Fascia, 77
linguae, 232
pharyngo-basilaris, 236
Fasciculus cerebro-spinalis, 411, 425
cuneatus, 412, 424
gracilis, 412, 424
proprius, 412
rubro-spinalis, 425
spino-thalamicus, 426
Fat cells, 73
crystals, 74
stains, 504
Felix, on the pronephros, 306
Wolffian tubules, 308
genital glands, 327
paradidymis, 330
sexual cells, 335
Female genital organs, 349
Fenestra cochleae, 469
vestibuli, 469
Fenestrated cells, 150
membrane, 66, 170
Ferrein, pyramids of, (in the kidney), 315
Fertilization, 32
Fiber cells (in the internal ear), 471
layer of Henle, 445
tracts, 132 (see also Fasciculi).
Fibers, elastic, 66
Miiller's (in the retina), 447
muscle, 113, 116, 126
nerve, 132
osteogenic, 84
Sharpey's (in bone), 92
white, 62
Fibrils, in connective tissue fibers, 65
in muscle fibers, 113
in smooth muscle, 114
in striated muscle, 122
in nerve fibers, 144
Fibrin, 191
canalized, 378
Fibroblasts, 67
Fibro-cartilage, 81
Fibroglia, 64
Filiform papillae, 230, 231
Fillets (lemnisci), 426
Fimbria ovarica, 352
Fixation of tissues, 489
Flack, on the sino-atrial node, 181
Flagella, 52
Flemming, on the origin of white fibers, 63
Flemming's fluid, 491
Foliate papillae, 230, 231
Follicles, 58
atretic, 358
Graafian (vesicular ovarian), 354
primary ovarian, 352
thyreoid, 227
Fontana, spaces of (in the iris), 460
Foramen apicis dentis, 99
epiploicum (of Winslow), 280
interventriculare (of Monro), 419
interventriculare, of the heart, 176
ovale, 176
Fore-brain, 419
Fore-gut, 39, 245
Formaldehyde, 491
Formalin, 491
Fornix, 421
Fossa of Rosenmuller, 218
Fovea centralis, 447
Fresh tissues, examination of, 487
Frozen sections, 498
Fungiform papillae, 230, 231
Funiculi of the spinal cord, 411
Gartner's duct, 351
Gage, on glycogen, 78
Galea capitis, 336
Galen, on the intestine, 247
stomach, 251
Gall bladder, 277, 289
Ganglia, 113, 147
cardiac, 182
cervical, 137
coeliac, 138
of the cerebral nerves, 141
of the sympathetic nerves in the head
142
retinal, 445
spinal, 134, 147
spiral, 476
sympathetic, 137, 142, 150
INDEX
529
Gastric canal, 252
glands, 253
Gelatin, 62
Genital corpuscles, 161
organs (female), 349
organs (male), 326
papilla, 330, 353
ridge, 327
Germ cells, 20, 334
layers, 36; origin of tissues from, 45
Giant cells, of the bone marrow, 200, 202
Gill clefts, 216, 219
Giraldes, organ of, 330
Glands, 54
anterior lingual, 232
areolar, 404
Bartholin's (major vestibular), 383
Bowman's (olfactory), 485
bronchial, 300
Brunner's (duodenal), 259
buccal, 241
bulbo-urethral, 347
cardiac, 248, 253
ceruminous, 481
cervical, of the uterus, 361
ciliary, 461
circumanal, 276
classification, 56, 59
compound, 57
Cowper's (bulbo-urethral), 347
cytogenic, 58
duodenal, 259
ducts, 56, 58
v. Ebner's (serous lingual), 238
end-pieces, 56
epithelial, 54
epithelioid, 58
fundus (gastric), 253
gastric, 253
intestinal, 259, 260
labial, 241
lachrymal, 462, 464
Lieberkiihn's (intestinal), 259, 260
lingual, 241
Littrd's (urethral), 347
lumen, 57
lymph, 205
mammary, 401
Meibomian (tarsal), 462
mixed, 241
molar, 241
Moll's (ciliary), 461
Montgomery's (areolar), 404
mucous, 54, 241
cesophageal, 248
olfactory, 485
oral, 237
palatine, 241
peptic (gastric), 253
praeputial, 398
pyloric, 256
sebaceous, 237, 397
secretory capillaries, 57
serous, 54, 238
simple, 57
sublingual, 242
submaxillary, 242
sudoriparous, 398
suprarenal, 404
34
Glands, sweat, 398
tarsal, 462
tracheal, 299
Tyson's, 398
unicellular, 56
urethral, 326, 347
vestibular, 383
Glans penis, 330
Glia cells (neuroglia), 64, 414, 434
Glisson's capsule, 278
columns, 274
Glomerulus, 307
of the kidney, 313
of olfactory nerves, 486
of WolfEan body, 307
Glomus caroticum, 229
Glossopharyngeal nerve, 142, 424
Glycerin jelly, 507
Glycogen, 78
Goblet cells, 56
Golgi preparations, 145
method, 512
Golgi-Mazzoni corpuscles, 401
Goll, column of, 412
Gower's tract, 427
Graafian follicles, 356, 365
Granules, in protoplasm, 3
metachromatic, 70
Grasshopper, sex determination, 28
spermatogenesis, 21
Gray matter (substance), 416
nerves, 153
rami, 137
Gr6goire and Wygaerts, on cell division, 15
Gregor, on muscle spindles, 127
Grosser, on pharyngeal pouches, 219
yolk-sac, 368
Gubernaculum testis, 331
Gum damar, 507
Gums (gingivec), 112
Gustatory organ (taste-buds), 232, 234
Haematoidin, 194
Haematoxylin, 500
Haemin, 194
Haemoglobin, 188, 194
Haemolymph glands, 209
Haemolysis, 195
Haemosiderin, 194
Hair, 389
bulb, 389
lanugo, 391
papilla, 389
root, 389, 391
shaft, 394
shedding, 396
Hair-cells, of the cochlea, 471, 474
Halban, on muscle fibers, 126
Hale, on the allantois, 373
Haller, on epithelium, 46
thymus, 222
Hammar, on pharyngeal pouches, 218
thymus, 222, 225
Hardesty, on nervous tissue, 415
membrana tectoria, 473
Harrison, on the growth of nerves, 135, 146
Harvey, on epigenesis, 339
530
INDEX
Harvey, R. W., on epithelium of the bladder,
3?4
Hassall's corpuscles (thymic), 225
Haustra, 273
Haversian systems, 94
Head-bend, 419
Heart, 174
endocardium, 178
epicardium, 178, 181
myocardium, 178, 179
nerves, 181
pericardium, 178
valves, 176
Hedblom, on uterine glands, 361
Heidenhain, on muscle contraction, 124
intercalated discs, 129
Heidenhain's iron haematoxylin, 501
Hemiazygos veins, 310
Hemispheres, cerebral, 431
Henle, on collagenous fibers, 62
transitional epithelium, 324
aberrant ducts, 330
Henle's fiber layer of the retina, 445
layer of the hair sheath, 394
loops in the kidney, 313
spindle cell layer in the iris, 456
Hensen, on the number of ova, 29
ovulation, 357
Hensen's cells of the cochlea, 475
membrane in striated muscle, 123
Hepatic arteries, 280
cells, 284
diverticulum, 276
duct, 277, 288
lobules, 281
trabeculae, 277
veins, 277, 282
Hertwig, on mesenchyma, 59
origin of nerves, 132
Hertzog's embryo, 368
Hewson, on the thymus, 222
Hilus, 205
Hind-brain, 422
Hind-gut, 39, 245
Hippocrates, on the intestine, 247
His, on endothelia, 46
development of nerves, 136
allantois, 373
Hochstetter, on the cardinal veins, 310
Home, on the stomach, 251
Hooke, on cells, 8
drawing, 518
Horns of the spinal cord (columns), 411
Houston's valves, 273
Howship's lacunae, 86
Howell, on red corpuscles, 191
white corpuscles, 195
Huber, on the notochord, 38
end-discs, 149
tubules of the kidney, 314, 318
tubules of the testis, 334
ceruminous glands, 481
Hubrecht, on the trophoblast, 367
Huntington, on lymphatics, 186
bronchi, 296
supracardinal vein, 310
Huschke's auditory teeth, 473
Huxley, on the cuticula dentis, 105
Huxley's layer of the hair sheath, 394
Hyaline cartilage, 80
Hyaloid artery, 442
canal, 442
membrane, 453
Hyaloplasm, 4
Hydatid of Morgagni, 344
sessile, 344
stalked, 344
Hymen, 350
Hypogastric plexus, 139
Hypoglossal nerve, 142, 424
nucleus, 426
Hypophysis, 435
Hypospadias, 330
Hypothalamus, 421
Hyrtl, on the intestine, 247
amnion, 373
meninges, 413
pituitary gland, 436
Idiozome, 336
Ileo-caecal valve (vahe of the colon), 261
Ileum, 246, 260
Imbedding, 493, 495
Implantation of the ovum, 370
Incisures of Lantermann (in myelin sheaths)
155
Inclusions, 5
Incus, 469
Infundibulum of the fore-brain, 421
of the lungs, 302
of the uterine tubes, 358
Injections, 509
Inner cell mass, 35
Intercalated discs, 129
ducts, 58, 240, 292
Intercellular bridges, 53
secretory capillaries, 57
substance, 10
Interfascial space, 460
Interglobular spaces in dentine, no
Intermediate nerve, 141
Internal acoustic meatus, 477
secretions, 58
Interstitial cells of the ovary, 358
cells of the testi?, 332
granules in sarcoplasm, 124
lamellae of bone, 94
Interventricular foramen, of the heart, 176
of the brain (Monro), 419
Intestinal absorption, 264
glands, 259, 260
villi, 260
Intestine, large, 270
small, 246
IntraceUular secretory canals, 57
Involuntary muscle, 113, 128
Iris, 442, 455
Iron haematoxylin, 501, 502
Islands of Langerhans, 292
Isolation of tissues, 488
Iso tropic substance in muscle, 123
Isthmus, 422
Jacobson's organ (vomero-nasal organ), 141,
483
INDEX
531
Jejunum, 246, 260
Johnson, on the rectum, 247, 273
intestinal villi, 260, 272
distention of the small intestine, 262
Johnston, on the nervus terminalis, 141
Joints, 96
Jordan, on intercalated discs, 129
Karyokinesis, 14
Karyolysis, 10
Karyoplasm, 2
Karyorrhexis, 10
Keibel, on the development of the urogenital
tract, 350
allantois, 368
Keith and Flack, on the sino-atrial node, 181
Kent, on the atrio-ventricular bundle, 180
Keratohyalin, 388
Kerkring's valvulae conniventes, 261
Kidney, 310
calyces, 312
capsule, 315
columns, 315
corpuscles, 315
cortex, 315
labyrinth (pars convoluta), 315
lobes, 321
medulla, 315
medullary rays (pars radiate), 315
pelvis, 311, 322
pyramids, 314
vessels and nerves, 321, 322
zones of the medulla, 316
Kiernan, on the portal canals, 282
Kingsbury, on mitochondria, 4
Kingsley, on the pineal body, 438
Kling, on lymph glands, 205
K611iker, on osteoclasts, 86
growth of bone, 91
aorta, 172
paradidymis, 330
v. Korff, on dentinal fibers, 108
Krause's corpuscles, bulbous and cylindrical,
161
membrane in striated muscle, 123
Kupffer's stellate endothelial cells, 287
Labia majora, 353, 383
minora, 353, 383
Labium tympanicum, 473
vestibulare, 473
Labra glenoidalia, 98
Labyrinth, of the ear, 469
of the kidney (pars convoluta}, 315
Lachrymal glands, 464; accessory, 462
sac, 464
Lacteals, 201, 267
Lactiferous sinus, 404
Lacunae, of bone, 85
of cartilage, 78
Howshtp's, 86
urethra!, 347
Lamellae of bone, 92
Lamellar corpuscles, 161
Lamina choriocapillaris, 453
cribrosa, 451
fusca, 453
spiralis, 471
suprachorioidea, 453
Langer, on lymphatics, 186
Langerhans, cells of in epidermis, 159
islands of, 292
Lantermann's incisures, 155
Lanugp, 391
Large intestine, 270
Larynx, 298
Leeuwenhoek, on the teeth, 105
red corpuscles, 191
spermatozoa, 338
Lens, 439,^451
Lentic vesicle, 439
Lesser omentum, 278
Leucocytes, 188, 195, 199
Lewis, F. T., on lymphatics, 185
shape of red corpuscles, 191
stomach, 252
vena cava, 280
ventral pancreas, 290
amniotic villi, 383
Lewis, W. H., on muscle, 118
development of the lens, 440
Lewis, W. H. and M. R., on the growth of
nerve fibers, 146
Lieberkiihn, crypts of (intestinal glands), 259,
260
Ligaments, 77, 97
denticulate, 414
hepatic, 278
ovarian, 352
pectinate, 458
suspensory, of lens, 449
Ligamentum spirale, 471
Limbus spiralis, 472
Lingual glands, 232, 241
tonsil, 218, 222
Linin, 5
Lips, 235
Liquor amnii, 370
folliculi, 355
Littr6, glands of (urethral glands), 347
Liver, 276
artery, 280
bile capillaries, 285
capsule, 278
ducts, 288
hepatic cells, 284
ligaments, 278
lobes, 280
lobules, 281, 284
perivascular tissue, 286
portal canals, 282, 287
structural units, 284
veins, 278
Lode, on the number of spermatozoa, 29
Loeb, on fertilization, 34
Long, on maturation, 31, 32
Lowsley, on the prostate, 345
Lumen of glands, 57
Lungs, 301
alveoli, 301
atria, 302
development, 295
lobules, 304
532
INDEX
Lungs, pigment, 304
pleura, 296, 304
structural units, 304
vessels and nerves, 303, 304
Lunula, 389
Lutein cells, 358
Lymph, 201
follicles (nodules), 204, 205
glands, 204
nodes (lymph glands), 204
nodules, solitary, 207; aggregate, 207
sacs, 185
sinuses, 205, 208
Lymphatic vessels, 182
development, 183, 185
stomata, 270
valves, 188
Lymphocytes, 70, 196, 202
Lymphoid tissue, 207
M
MacCallum, on lymphatics, 184
Wolman bodies, 309
McClung, on spermatpgenesis, 21, 23
McClure, on lymphatics, 186
supracardinal vein, 310
McCotter, on the nervus terminalis, 141
McGill, on smooth muscle, 114, 116, 118
Macula acustica, 467
lutea, 447
Magma reticulare, 368
Mall, on reticular tissue, 61
cartilage, 77
endocardial connective tissue, 178
spleen, 212
lobules of the liver, 281
Malleus, 469
Mallory, on fibroglia, 64
Mallory's connective tissue stain, 504
phospho-tungstic acid haematoxylin, 503
Malpighi, on capillaries, 164
lobules of the liver, 281
skin, 386
Malpighian corpuscles (renal), 315; (splenic),
211
pyramids, 314
Mammillary bodies, 421
Mammary glands, 401
Marchi's fluid, 491
Mark, on maturation, 31, 32
Marrow, 202
Mascagni, on lymphatics, 183
Mast cells, 69, 197
Maturation, 20
Maurer, on capillaries in epithelium, 53
Maximow, on the centrosome in amitosis, 13
mast cells, 69
clasmatocytes, 71
lymphocytes, 195
Meckel, on the thymus, 222
Mecke!' s diverticulum, 247
Meconium, 272
Mediastinum, of the testis, 328
of the thorax, 295
Medulla, 205
oblongata, 424
ossium, 202
spinalis, 409
Medullary groove, 36
tube, 36, 409
Medullated nerve fibers, 144
Megakaryocytes, 202
Megaloblasts, 189
Meibomian glands (tarsal glands'), 462
Meigs, on the contraction of smooth muscle,
117
contraction of striated muscle, 124
Meissner's corpuscles, tactile, 160
plexus, 269
Melanin, 72
Membrana basilaris (of the cochlea), 473
limitans externa (of the retina), 444
limitans interna, 446
propria, 53
vestibularis (of the cochlea), 471
Membrane, Bowman's (of the cornea), 456
Descemet's (of the cornea), 458
hyaloid, 453
pupillary, 451
Reissner's (membrana vestibularis), 471
tympanic, 469, 480
Meninges, 413, 438
Menstruation, 363
Merkel, on the origin of white fibers, 63
Mesencephalon, 421
Mesenchyma, 59
Mesenchymal epithelium, 47, 98
tissues, 59
Mesentery, 269
Mesoderm, 35, 39
derivatives, 45
Mesodermic somites, 39, 118
Mesonephros (Wolffian body), 307
Mesothelium, 47
Mesovarium, 353
Metachromatic granules, 70
Metaphase, 17
Metencephalon, 423
Methylene blue, 501
Meves, on the origin of white fibers, 63
fibroglia, 114
spermatozoa, 337
Micron, 9
Microscope, 514
Microtome, 496
Mid-brain, 421
Milk, 403
Miller, on peri vascular lymphatics, 188
pulmonary arteries, 297
lungs, 302
Mingazzini, on intestinal absorption, 264
Minot, on cytomorphosis, 9
mesothelium, 47
sinusoids, 166
blood corpuscles, 195
trophoderm, 367
Miram, on Paneth's cells, 263
Mitochondria, 4, 54, 63, 239
Mitosis, 14
Mitral cells, 486
valve, 176
Mixed glands, 241
Modiolus, 467
Moenkhaus, on fertilization, 34
Moll, glands of (ciliary glands), 461
Mononuclear leucocytes, 196
Monophyletic theory of blood formation, 195
INDEX
533
Monro, foramen of, 419
Montgomery's glands (areolar glands), 404
Morgagni, hydatid of (appendix testis), 327,
344
sinus of (Ventricles oj the larynx), 298
Morpurgo, on muscle fibers, 126
Morula, 35
Motor cells, 132, 134
endings, 163 .
nerves, 134
plate, 163
Mounting sections, 507
Mouth, 215
Mucins, 62
Mucous bursae, 77
glands, 54, 241
tissue, 62
Mucus, 62
M tiller, on the Wolffian bodies, 327
Miillerian duct, 327, 349
Miiller's fibers, of the retina, 447
Multipolar ganglion cells, 143
Muscle, 113
cardiac, 113, 128
columns, 120
contraction, 117, 124
fibrils, 113, 114, 120
involuntary, 113
skeletal, 118
smooth, 113
spindles, 127, 159
striated, 113, 122, 125, 128
voluntary, 113
Myelencephalon, 424
Myelin, 144, 155
Myelocytes, 202, 203
Myenteric plexus, 138, 248, 269
Myoblasts, 114
Myocardium, 178, 179
Myofibrils, 113
Myoglia, 64
Myometrium, 360
Myotome, 118
N
Naboth, ovules of, 361
Nagel on oogenesis, 30
Nails, 389
Nares, 481
Nasal pits, 481
septum, 481
Nasolachrymal ducts, 465
Nasmyth's membrane (cuticula dentis), 105
Neck-bend, 419
Nfimec, on mitosis, 14, 18, 19
Nephrotome, 41
Nerve cells, 130, 132
bipolar, 143
dendrites, 143
multipolar, 143
neuraxon, 134, 143
in spinal ganglia, 134
in sympathetic ganglia, 150
unipolar, 143
Nerve corpuscles, 160
Nerve endings, free, 157
motor, 163
sensory, 157
tactile menisci, 158
Nerve fibers, 132
afferent, 132, 134
association, 132
axis cylinders, 145
axolemma, 156
collateral, 134, 144
commissural, 132
efferent, 132, 134
growth, 135
incisures, 155
motor, 132
neuraxon, 134, 143
neurofibrils, 144
neurolemma, 144
neuroplasm, 156
nodes of Ranvier, 144, 156
reflex path, 131
Remak's fibers, 145
sensory, 132
sheath of Schwann, 144
in the spinal cord, 415
structure, 153
Nerves, 153
automatic, 139
autonomic, 139
cerebral, 139
gray, 153
medullated, 154
non-medullated, 153
spinal, 133
sympathetic, 137
visceral, 139
white, 153
Nervous system, 130
central, 130, 409
peripheral, 130
sympathetic, 137, 139
Nervus terminalis, 141, 483
Neumann's sheath, 109
Neural crest, 133
tube (medullary tube), 36
Neuraxon, 134, 143
Neuroblasts, 134
Neuro-epithelia) cells, 130
Neurofibrils, 144
Neuroglia, 64, 114, 434, 451
Neurokeratin, 155
Neurolemma, 144
Neurone, 145
Neuroplasm, 156
Neuropores, 37
Neutrophiles, 199
Nile blue, 505
Nissl's bodies, 3, 416
Nitric acid, 489
Nodes of Ranvier, 144, 156
Nodules, aggregate, 207, 268
solitary, 207
Nodulus thymicus, 218
Non-medullated nerves, 153
Normoblasts, 189
Nose, 481
Notochord, 38, 83
Notochordal tissue, 82
Nuclear membrane, 5
sap, 5
Nucleolus, 6
Nucleus, ambiguus, 426
of cells, 5
534
INDEX
Nucleus cuneatus, 426
dorsal, 417, 427
gracilis, 426
of the nervous system, 417
pulposus, 38
pycnotic, 10,189
Nuel's spaces, 475
Nussbaum, on sexual cells, 335
Nutrient artery, 96
Oculomotor nerve, 141, 422
Odontoblasts, 107
Odoriferous glands, 399
(Esophagus, 246, 248
Oil of origanum, 507
Oken, on the Wolffian bodies, 326
Olfactory bulb, 421
cells, 483
epithelium, 483
glands, 485
nerves, 141, 486
Olive, 424
Oocytes, 28, 354
Odgenesis, 20, 28
OOgonia, 28
Opie, on the islands of the pancreas, 295
Optic cup, 440
nerve, 141, 449
recess, 419
stalk, 439
vesicle, 439
Ora serrata, 442, 449
Oral plate, 215
Organ of Corti (spiral organ), 472, 474
of Rosenmiiller (epoophoron), 350
of Zuckerkandl, 152
Organs, 46
Orth's fluid, 491
Osmic acid, 505
Ossification, 90
Osteoblast, 84
Osteoclast, 86
Osteo-dentine, no
Osteogenic fibers, 84
Otoconia, 476
Otocyst, 465
Otoliths, 471
Ova, discovery of, 365
maturation, 31
mature, 28
number in human female, 29
Oviduct, 365
Ovulation, 31, 356
Ovules of Naboth, 361
Oxy haemoglobin, 194
Oxyphiles, 197
Pacchionian bodies (arachnoid granulations),
439
Pacinian corpuscles (lamellar corpuscles), 161
Palate processes, 481
Palatine glands, 241
tonsils, 218, 219
Pallium, 420
Palpebrae, 461
Pal's modification of Weigert's stain, 512
Pancreas, 289
centro-alveolar cells, 292
dorsal, 289
islands, 292
ventral, 289
Paneth, cells of, 262
Panniculus adiposus, 386
Papilla, duodenal, 259, 278
genital, 330
of hair, 389
of the optic nerve, 441
renal, 314
Papillae, of the corium, 385
of the tongue, 230
Paradidymis, 330, 344
Paraffin, imbedding in, 493
sections, 496
Paraganglia, 152
Parametrium, 363
Paranucleus, 26
Parathyreoid glands, 218, 228
Parenchyma, 46
of the liver, 284
Parietal cells, 255
Parker, on cilia 52
on the nervous system, 132
Paroophoron, 350, 351
Parotid gland, 238
Parovarium (epoSphoron), 350
Pavement epithelium, 48
Pecquet, on lymphatics, 183
Pectinate ligament, 458
Peduncles of the cerebrum, 422
Pelvis of the kidney, 311, 322
Penicilli, 212
Penis, 346
Peptic glands (gastric glands), 253
Perforating fibers, of Sharpey, 92
Perforatorium, 336
Pericardial cavity, 174
Pericardium, 178
Perichondrial bone, 90
Perichondrium, 79
Periodontal membrane, (aheolar periosteum),
in
Perilymph, 468
Perilymphatic duct, 478
Perimetrium, 360
Perimysium, 124
Perineum, 247
Perineurium, 154
Peripheral nerves, syncytial interpretation of,
US
Peripheral nervous system, 130
Periosteal lamellae, 93
Periosteum, 91, 92
Peritoneum, 269
Permanent preparations, 489
Peter, on the zones of the renal medulla, 316
Peters's embryo, 368
Petit, canal of (zonular spaces), 449
Peyer's patches (aggregate nodules), 207, 268
Pfluger's egg tubes, 352
Phagocytes, 12, 197
Pharyngeal pouches, 216, 217
recess, 218
tonsil, 218, 222
Pharynx, 215
INDEX
535
Phospho-tungstic acid haematoxylin, 503
Pia mater, 413, 439
Pigment cells, 71, 72
Pillar cells of spiral organ, 474
Pineal body, 421, 437
Pinguecula, 462
Pinkus, on the nervus terminalis, 141
on the skin, 388
Pinna, 469
Pituitary gland (hypophysis), 436
Placenta, 372, 375
succenturiate, 372
Plasma, 188, 201
Plasma cells, 68, 70
Plates, blood, 188, 199
Pleura, 296, 304
Pleural villi, 306
Plexus annularis, 461
Auerbach's, 138, 248, 269
cardiac, 138
chorioid, 439
cceliac, 138
ganglicsus ciliaris, 461
hypogastric, 139
Meissner's, 269
myenteric, 138, 248, 269
pulmonary, 304
solar, 139
submucous, 139, 248, 269
Plica semilunaris of the eyelid, 463
Plicae adiposae, of the pleura, 306
circulares, of the small intestine, 260, 261
palmatae, of the uterus, 360
semilunares, of the large intestine, 273
trans versales, of the rectum, 273
villosae, of the stomach, 252
Polar bodies, 31
field, 20
radiations, 17
Polykaryocyte, 202
Polymorphpnuclear leucocytes, 196
Polyphyletic theory of blood formation, 195
Pons, 423
Porta hepatis, 279
Portal canals, 282, 287
lobules, 284
vein, 277, 278
Potassium chlorate, 489
hydrate, 489
Praeputial glands, 398
Prasspermatid, 22
Praespermid, 22
Precartilage, 77
Predentine, 107
Preformation theory, 339
Premyelocytes, 202, 203
Prenant, on amianthoid fibers, 80
Prepuce, 331, 398
Primitive knot, 35
streak, 35
Prisms, enamel, 104
Processus vaginalis, 331
vermiformis, 270
Pronephros, 306
Pronucleus, 32
Prosencephalon, 419
Prostate, 345
Prostatic utricle, 327
Protoplasm, 2


Protoplasmic processes, 145
FIG.. 6. RETICULAR NETWORK (Fig. 3).
Protovertebrae (mesodermic somites), 39, 118
Prowazek, on cilia, 51
Pseudopodia, 7


Pseudostratified epithelium, 50
3. Vacuoles. Protoplasm often contains large or small drops of clear fluid, fat, or some other substance less highly organized than the surrounding material (Fig. 4). In preserved cells the spaces which were occupied by these droplets appear clear and empty, and are known as vacuoles.
Pulmonary arches, 296


arteries, 296
tiey vary greatly in size, and one or several of them may be found in a single cell.


plexus, 304
4. Canals. The protoplasm of certain cells is said to contain fine tubes or clefts which communicate with lymphatic spaces outside of the cell (Fig. 5). Prolongations from the surrounding capsule-cells have been described as entering these canals and as performing, together with the lymph, a nutritive function. Hence the network of canals has been called trophospongium. But it has not been shown conclusively that these canals open to the exterior of the cell. They may be similar to the closed networks or "reticular apparatus" lying wholly within the protoplasm, shown in Fig. 6. Such networks have been described in nerve cells, cartilage cells and gland cells. The network is said to be of a thick fluid consistency. In certain gland cells there are canals within the protoplasm, which convey the secretion to the free surface of the cell. These may be simple, branched, or arranged in a network. Like the other forms of intracellular canals, they can be studied only in special preparations.


veins, 297
5. Inclusions. Various foreign bodies, such as other cells or bacteria, which may have been ingested by the protoplasm, are grouped as inclusions. This term is applied also to crystalloid substances formed within the protoplasm (Fig. 7), and to coarse masses of pigment granules which appear extraneous.


Pulp of teeth, 99, no
===Nucleus===
Pupil, dilatator muscle of, 456


sphincter muscle of, 455
The nucleus (Latin, nucleus, "the kernel of a nut"; Greek, Kdpvov, "a nut") is typically a well-defined round body, situated near the center of the cell, appearing denser or more coarsely granular than the surrounding protoplasm (Fig. i). There are characteristic variations in the shapes of nuclei, in their position within the cells and in their structure.
Pupillary membrane, 441
Purkinje's cells, 429


fibers, 181
Ordinarily the karyoplasm, or nuclear substance, is sharply marked off from the cytoplasm by the nuclear membrane. Sometimes, in preserved tissues, the cytoplasm has shrunken away from the nuclear membrane, so as to leave a narrow space partially encircling it; and in certain living cells, the nucleus migrates through cytoplasm, as if it were an independent body. But there are phases of cell-development in which the nuclear membrane disappears and no line can be drawn between karyoplasm and cytoplasm. At all times they have a common structural basis. The ground substance of the nucleus, corresponding with the hyaloplasm, is the nuclear sap; and it contains, for spongioplasm, a meshwork of delicate linin fibrils. These help to form the nuclear membrane, in which they terminate. The nuclear membrane, nuclear sap, and linin reticulum do not stain deeply, and are therefore grouped together as the achromatic constituents of the nucleus.


Pycnotic nuclei, 10, 189
The principal chromatic constituent of the nucleus is known as chromatin. It stains deeply, since it contains a large amount of nucleic acid, which has a marked affinity for basic stains. Chromatin occurs in the form of granules, which are bound together in strands or masses by the lining fibers (Fig. i). The masses, known as chromatin knots, occur especially at the points of intersection in the linin meshwork. Sometimes they are attached to the nuclear membrane, or so distributed over its surface that it appears to consist of chromatin. It forms morphologically the most important part of the nucleus.
Pyloric glands, 256
Pulorus, 251
Pyramidal cells, 431


tracts, 425
Certain nuclei contain one or more round bodies, which belong with the chromatic elements because of their deep staining, but which are chemically different from chromatin. These bodice, known as nucleoli, are stained with acid or neutral dyes. They are said to be composed of paranuclein, whereas chromatin is composed of nuclein. In distilled water the structures formed of nuclein disappear, but those consisting of paranuclein remain. The nuclei of nerve cells contain typical nucleoli (Figs. 3 and 5). Sometimes a nucleolus, lodged in the nuclear reticulum, is more or less covered with chromatin (Fig. 9, A), but the term should not be applied to irregular knots of chromatin, even when most of the chromatic material within a nucleus is gathered into one or two such bodies. These are the so-called false nucleoli (pseudonucleoli).
Pyramids of Ferrein (in the kidney), 315


of Malpighi (in the kidney), 314
Every nucleus, therefore, consists of ground substance or nuclear sap, a network of linin, and granules and masses of chromatin. Usually it is surrounded by a membrane, and sometimes it contains a nucleolus. Most cells contain a single nucleus; but occasionally a single cell contains two nuclei, as is frequent in the liver, or even several nuclei, as in certain cells associated with bone. Non-nucleated bodies, like the mammalian red blood corpuscles, and the dead outer cells of the skin, have lost their nuclei in the course of development.


of the medulla oblongata, 424
Functionally the nucleus is regarded as a center for chemical activities necessary for the life of the cell. It is believed to produce substances which pass out into the cytoplasm, where they may be further elaborated. Evidences of nuclear extrusions into the cytoplasm have been frequently recorded. But the interactions between nucleus and cytoplasm, of such nature that they cannot be observed under the microscope, are presumably of far greater biological importance.


R


Rabl, on the development of the lens, 453
===Centrosome===
Radial fibers of the retina, 447
Rami of spinal nerves, 136, 137
Ranson, on ganglion cells, 150
Ranvier, on clasmatocytes, 71


on lymphatics, 184
The centrosome is typically a minute granule in the center of a small sphere of differentiated protoplasm. Often the term is applied to this entire structure, but it refers particularly to the central granule; the enveloping sphere is known as the attraction sphere, and it is composed of archoplasm^ When a cell is about to divide, delicate fibrils, either rearranged from the protoplasmic reticulum or formed anew, radiate from the archoplasm toward the periphery of the cell. The central granule becomes subdivided into two, which then move apart. In resting cells, or those which are not undergoing division, the centrosome may already have divided into a double body or diplosome preparatory to the next division of the cell (Fig. i).


nodes of, 144, 156
Ranvier's alcohol, 488
Raphe of the penis, 330
Rathke, on gill clefts, 219


on the Wolffian bodies, 327
Centrosomes have been detected in many forms of resting cells, and it is assumed by some authorities that the centrosome is an invariable constituent of the cells of the higher vertebrates. According to this opinion the centrosome may become inconspicuous but it never loses its identity. Often they are found very close to the nuclear membrane, which may be indented to accommodate them; and rarely, as in certain cancer cells and in one form of the worm Ascaris, they have been reported as within the nucleus. They may occur near the free surface of certain cells, usually in the form of diplosomes, as shown in cell a, Fig. 8. Just above the diplosome, such cells may send out contractile projections of protoplasm (pseudopodia), with the activity of which the! diplosome may be in some way associated.j Pseudopodia, with an underlying diplosome, have been observed in the columnar cells of the human large intestine. In cell b of Fig. 8 there are four diplosomes, one of which lies beneath the protoplasmic projections. It is believed that the diplosomes may multiply by fission, and that thus they may give rise to the numerous motile hairs, or cilia, which project from certain cells. Of these they form the basal bodies (Fig. 8, c). In many gland cells the centrosome lies in the midst of the protoplasm where the secretion accumulates. The discharge of the secretion is accomplished by the contraction of the protoplasmic strands in which the centrosome is lodged. In all these relations the centrosome appears to be a center for motor activities, and it is described as the kinetic or dynamic center of the cell.
Rathke's pouch, 435
Reconstructions, 516
Rectal columns, 274
Rectum, 247, 273
Red corpuscles, 188


color, 1 88


dimensions, 193


number, 193


shape, 191
FIG. 8. CELLS OF THE EFFERENT DUCTS OF THE TESTIS OF A MOUSE. (After Puchs.)
Red nucleus, 425
Reductional division, 24
Reflex path of spinal cord, 131
Reflexa (decidua capsularis) , 371
Reissner's membrane, 471
Remak on nerves, 145
Remak's fibers, 145
Renal columns, 315


corpuscles, 315
To show diplosomes, and (in c) cilia with basal bodies.


lobules, 321


papilla, 314
===Cell Wall===


pelvis, 322
The protoplasm at the surface of certain cells floating in the blood or lymph forms a thin pellicle, apparently as a result of protoplasmic concentration, or other reaction to the surrounding medium. Cells which line the greater part of the digestive tube, and have only one surface directed toward the intestinal contents, are provided with a thick wall on the exposed surface. Such a wall is called a cuticular border, or cuticula. On the other sides of these cells, the membrane is much thinner, and on the basal surface it is sometimes lacking. In such cases the protoplasm appears to be continuous with that of the underlying cells. In other cases the entire cell is devoid of any membrane. The cell membrane, therefore, is not an essential part of a cell; if present it ranges from a thin pellicle, on the border line of visibility, to a well-defined wall, which may be formed as a secretion of the underlying protoplasm. If the several surfaces of the cell are in relation to different environments, there is often a corresponding difference in the structure of their walls.


portal system, 310
In examining a group of cells, it will be important to determine whether they are merely in contact, or actually continuous. Sometimes cells are so completely fused that their nuclei are irregularly distributed through a single mass of protoplasm. Such a formation is a syncytium in which the position of the nuclei is the only means of estimating the territory of a single cell. A syncytium may arise from the fusion of cells, or, as in striated muscle fibers, it may be due to the multiplication of nuclei in an undivided mass of protoplasm. Instead of being completely fused, cells are often joined to one another by protoplasmic processes of varying length and width, thus forming cellular networks. Fibrils within such a syncytium may pass continuously from the protoplasm of one cell into that of another.


pyramids, 314
Although cell membranes are often inconspicuous in animal cells, they cannot be overlooked in plants. Thus cork is a mass of dead cells from which nuclei and protoplasm have disappeared, leaving only the cell walls. In describing cork, Robert Hooke introduced the name "cell," in 1664. He wrote: "I took a good clear piece of Cork and with a Pen-knife sharpen'd as keen as a Razor, I cut a piece of it off, and thereby left the surface of it exceeding smooth, then examining it very diligently with a Microscope, me thought I could perceive it to appear a little porous. . . . These pores, or cells, were not very deep, but consisted of a great many little Boxes ."


tubules, 312-316
In this way one of the briefest and most important of scientific terms was introduced.
Resorcin-fuchsin, 504
Respiratory apparatus, 295


bronchioles, 301
Restiform body, 423
Resting wandering cells, 71
Rete Malpighii (stratum germiiutivum), 386


ovarii, 351


testis, 328, 339
===Form and Size of Cells===


Cells are regarded as primarily spherical in form. Spherical cells are comparatively numerous in the embryo, and in the adult the resting white blood corpuscles, which float freely in the body fluids, assume this shape. Such cells are circular in cross section. When spherical cells are subjected to the pressure of similar neighboring cells, they become polyhedral and usually appear six-sided in cross section. Such cells, as a whole, may be cuboidal, columnar, or flat. Certain cells become fusiform (spindle-shaped) or are further elongated so as to form fibers; others send out radiating processes and are called stellate. Thus the form of cells is extremely varied. The shape of the nucleus tends to correspond with that of its cell. It is usually an elliptical body in elongated cells, and spherical in round or cuboidal cells. In stellate cells it is either spherical or somewhat elongated. Crescentic nuclei, and others more deeply and irregularly lobed, are found in some of the white blood corpuscles and in giant cells.




536
The size of cells ranges from that of the yolks of birds' eggs which are single cells, at least shortly before being laid down to microscopic structures four thousandths of a millimeter in diameter. The thousandth of a millimeter is the unit employed in microscopic measurements. It is called a micron, and its symbol is the Greek letter /*. The small cells referred to are therefore four microns (4 ju) in diameter. The size of any structure in a section of human tissue may be roughly estimated by comparing its dimensions with the diameter of a red blood corpuscle found in the same section. These red corpuscles are quite uniformly 7.5 p. in diameter.


==Cytomorphosis==


Cytomorphosis is a comprehensive term for the structural modifications which cells, or successive generations of cells, undergo from their origin to their final dissolution. 1 In the course of their transformation, cells divide repeatedly, but the new cells begin development where the parent cells left off. Cell division, therefore, is an unimportant incident in cytomorphosis.


INDEX
Cytomorphosis is a continuous advance in which four successive stages are recognized first, the stage in which the cells are undifferentiated ;_ second, the stage of specialization or differentiation: third, the stage of degeneration; and fourth, the stage in which the cells die and are removed. These may be considered in turn.


Undifferentiated cells, as can be seen in sections of young embryos, are characterized by large nuclei and little protoplasm. They multiply rapidly, but the rate of division declines with the gradual increase of the protoplasm and the consequent functional differentiation of the cell. In the adult, relatively undifferentiated cells are found in many situations, as, for example, in the deepest layer of the epidermis. As the cells at the surface die and are cast off, new ones come up from below to take their places. But since the basal cells can produce only epidermal cells, they are themselves partly differentiated. From this point of view the fertilized ovum, which can produce all kinds of cells, must be regarded, in spite of its size and great mass of yolk-laden protoplasm, as the least differentiated cell.


Differentiated cells may preserve a round or cuboidal form, but usually they are elongated, flattened, or stellate. The cytoplasm usually contains coarse granules, fibrils, masses of secretion or other special formations. As a result of their own protoplasmic activity, the cells of many tissues become surrounded by intercellular substances, which may far exceed in bulk the cells which produced them. Intercellular substances may be solid or fluid. When present in small amount they form thin layers of cement substance between closely adjacent cells; in large amount these substances constitute a ground work in which the cells are imbedded, as, for example, in cartilage and bone.


Reticular tissue, 26
* 1 The term cytomorphosis was introduced by C. S. Minot in 1901 in a lecture entitled "The Embryological Basis of Pathology" (Science, 1901, vol. 13, p. 494). Cytomorphosis is further discussed by Professor Minot in "The Problem of Age, Growth, and Death," published by G. P. Putnam's Sons, 1908.
Reticulin, 62
Retina, 443


cones, 445


development. 440


fovea centralis, 447


macula lutea, 447


pars ciliaris, 449
Although the differentiation of cells is chiefly cytoplasmic, there is some evidence of corresponding nuclear changes. Thus while the muscle


pars iridica, 455, 456
cells of the salamander are elaborating complex fibrils, the nuclei become modified as shown in Fig. 9. The significance of the nuclear changes is unknown.


pars optica, 443
Degeneration is the manifestation of the approaching death of the cell. In nerve cells this process normally takes place very slowly. These cells remain active throughout life, and if destroyed, they can never be replaced. In many glands, in the blood and in the skin, however, the cells are constantly dying and new ones are being differentiated. In a few organs the cells perish, but no new ones form, so that the organ to which they belong atrophies. Thus a large part of the mesonephros (Wolffian body) disappears during embryonic life; the thymus becomes vestigial in the


pigment layer, 440, 444
adult; and the ovary in later years loses its chief function through the degeneration of its cells.


rods, 444
The optical effects of degeneration cannot at present be properly classified. In a characteristic form, known as "cloudy swelling," the cell enlarges, becoming pale and opaque. In another form the cell shrinks and stains deeply, becoming either irregularly granular or homogeneous and hyaline. The nucleus may disappear as if in solution (karyolysis, chromatolysis) ; or it may become densely shrunken or pycnotic, and finally break into fragments and be scattered through the protoplasm (karyorhexis). If the process of degeneration is slow, the cell may divide by amitosis. It may be able to receive nutriment which it cannot assimilate, and thus its protoplasm may be infiltrated with fat and appear vacuolated. It may form abnormal intercellular substances, for example, amyloid; or the existing intercellular substances may become changed to mucoid masses, or have lime salts deposited in them. Thus an impairment or perversion of function is often associated with optical changes in the cell substance.
Retzius, cells of, 431


lines of, 105


Reynolds, on ovulation, 357
Rhinencephalon, 421
Rhombencephalon, 422
Rhomboidal sinus, 37
Richards, on mitosis, 14, 18, 19
Rindfleisch, on red corpuscles, 191
Ringer's solution, 488
Riolanus, ciliary muscle of, 462
Rod cells, 444


Rollett, on muscle striations, 123
FIG. 9. NUCLEI OF STRIATED MUSCLE FIBERS FROM YOUNG SALAMANDERS (NECTURUS). (Eycleshymer.)
Rose, on tooth-development, 101
Rosenmuller, fossa of, 218


organ of (epo'dphorori), 350
A, From a 7 mm. embryo; B, from one of 26 mm.; ch, chroma tin knot; g. s, ground substance; 1, linin fibril; n, nucleolus; n.m, nuclear membrane.
Rouleaux, 191
Round ligament, of the liver, 272


of the uterus, 352
The removal of dead cells is accomplished in several ways. Those near the external or internal surfaces of the body are usually shed or desquamated, and such cells may be found in the saliva and urine. Those which are within the body may be dissolved by chemical action or devoured by phagocytes.
Ruffmi's terminal cylinders, 160
Russel's bodies, in plasma cells, 70
Ruysch, on epithelia, 46


Every specimen of human tissue exhibits some phase of cytomorphosis. In some sections a series of cells may be observed from those but slightly differentiated, to those which are dead and in process of removal. Because of the similarity and possible identity of this normal "physiological" regression, with that found in diseased tissues, such specimens should be studied with particular care.


==Vital Phenomena==


Sabin, on lymphatics, 184, 185
The vital properties of cells are fully treated in text-books of physiology. They include the phenomena of irritability, metabolism, contractility, conductivity, and reproduction. Under irritability may be grouped the response of cells to stimuli of various sorts, such as heat, light, electricity, chemical reagents, the nervous impulse, or mechanical interference. Metabolism, in a wide sense, includes the ingestion and assimilation of food, the elaboration and secretion of desirable products, together with the elimination of waste products. Contractility may be changes in form observed during ten minutes;


lymph glands, 224
Sacculus, 467, 470
Saccus endolymphaticus, 465, 478
Safranin, 502
Salivary corpuscles, 220
Sappey, on lymphatics, 183
Sarcolemma, 121
Sarcomeres, 123
Sarcoplasm, 124
Sarcostyles, 120
Scala media (cochlear duct), 467, 471


tympani, 468
FIG. 10. LEUCOCYTES OF A PROG. * au.


vestibuii, 468
Schafer, on striated muscle, 120, 122, 124


non-medullated nerve fibers, 153
, at the beginning of the observation;


development of blood vessels, 166
manifest m the locomotion Of the J.a half minute later, etc.


shape of red corpuscles, 191
entire cell, in the vibratile action of


blood plates, 200
slender hair-like processes, the cilia, or in contraction of the cell body. Conductivity is the power of conveying impulses from one part of the cell to another. Reproduction is seen in the process of cell division. Many phases of these activities are observed in microscopic sections and as such they will be referred to in later chapters. A few which are of general occurrence will be described presently.


sinusoids of the liver, 287
===Amoeboid Motion===


musculature of the uterine tube, 360
The unicellular animal, Amoeba, exhibits a type of motility known as amoeboid, which has been observed in many sorts of cells in the vertebrate body. In marked cases, as in certain white blood corpuscles (the leucocytes), the cell protoplasm sends out fine or coarse processes which divide or fuse with one another, causing the cell to assume a great variety of forms. The processes may be retracted, or they may become attached somewhere and draw the remainder of the cell body after them, the result of which is locomotion or the so-called wandering of the cell. Such wandering cells play an important part in the economy of the animal body. Their processes can flow around granules or cells and thus enclose them in protoplasm. Some of these ingested bodies may be assimilated by the cell as a result of complex chemical and osmotic reactions. Cells which feed on foreign particles and can alter or digest them are known as phagocytes. Amoeboid movements take place very slowly. In preparations from warm-blooded animals they may be accelerated by gently heating the object.
Schaffer, on chordoid tissue, 82
Scharlach R, 504


Schlagenhaufen, on the tactile toruli, 385
Another form of motion is that which occurs within the protoplasm of fresh cells, whether living or dead, and consists in a rapid oscillation of minute granules, due to diffusion currents. Although these movements were first observed within protoplasm, it was soon shown that they occurred when various inert particles were suspended in a liquid. Robert Brown described the motion in 1828, in an essay entitled "On the General Existence of Active Molecules in Organic and Inorganic Bodies," and the phenomenon is called the molecular or Brownian movement. It may often be seen in salivary corpuscles.
Schlemm, canal of (sinus venosus sclerce), 460
Schneider, on the pituitary gland, 430
Schreger's lines, 106, 109
Schultze, M., on nerve cells, 130


white corpuscles, 197
Schultze, O., on muscle and tendon, 127
Schulz, on bone, 92
v. Schumacher, on lamellar corpuscles, 162


haemolymph glands, 210
==Formation and Reproduction of Cells==


In the past, two sorts of cell formation have been recognized, namely the spontaneous generation of cells, and the origin of cells through the division of pre-existing cells. According to the theory of spontaneous generation it was once thought that animals as highly organized as intestinal worms came into existence from the fermentation of the intestinal contents. After this had been disproved, it was still thought that unicellular animals arose spontaneously and that cells might be formed directly from a suitable fluid, the cytoblastema. Something of the sort may have occurred when life began, and it is the expectation of certain investigators that conditions may yet be produced which shall lead to the formation of organic bodies capable of growth and reproduction. At present, however, only one source of cells is recognized the division of existing cells. "Omnis cellula e cellula." A nucleus likewise can arise only by the division of an existing nucleus; it cannot be formed from nonnucleated protoplasm.




Schwann, sheath of, 144
===Amitosis===


on striated muscle fibers, 121
The simplest form of cell division is one which rarely occurs. Ordinarily the division of the cell is accompanied with the production of protoplasmic filaments, and the process is therefore called mitosis (Greek, /uVos, a thread). But in direct division or amitosis these filaments are not developed. The nucleus merely becomes increasingly constricted at the middle until divided in two; or it may be bisected by a deep cleft or fissure. Preceding the division of the nucleus, the nucleolus, if present, may subdivide and supply each half of the nucleus with a nucleolus (Fig. n). Cells which divide by this method are usually degenerating, and the process may terminate with the multiplication of nuclei. If carried to completion, the protoplasm also divides, and a cell membrane develops between the daughter nuclei. The role of the centrosome in amitosis has not been determined. Maximow finds it in a passive condition between the two halves of the nucleus, or beside the stalk connecting these halves if the division is not complete (Anat. Anz., 1908, vol. 33, p. 89). He states that certain mesenchymal cells which divide by amitosis in the rabbit embryo are not degenerating, but may later divide by mitosis, and thus he confirms Patterson's similar conclusion in regard to certain cells in the pigeon's egg. These instances are regarded as exceptional. In the human body the detachment of a portion of the lobate nucleus of certain leucocytes has been described as amitotic division, but the superficial cells of the bladder furnish more typical examples. E. F. Clark has found many cells dividing by amitosis hi the degenerating parts of a human cancer. The occurrence of two nuclei within one cell by no means indicates this form of division. Associated with such cells, others containing nuclei of the dumb-bell shape, or those partially bisected by clefts must be found, in order to prove that amitotic division is taking place.
Schweitzer, on the lymphatics of the teeth,  


no


Sclera, 456
Sclerotome, 118
Scrotum, 331


Sebaceous glands, 237, 397
Secretion, 54


internal, 58
FIG. ii. AMITOSIS ix EPITHELIAL CELLS FROM THE BLADDER OF A MOUSE. Xs6o. Such preparations as that shown in the figure are made by pressing the lining of a freshly obtained piece of the bladder against a clean cover-glass. Certain of the superficial cells adhere to it, and they are then fixed and stained.


Secretory capillaries, 57, 238
Sections (cutting and handling), 496
Segmentation of the ovum, 35
Semicircular canals (ducts), 465, 470
Seminal vesicles, 329, 342
Sensory decussation, 426


nerve cells, 132, 134
===Mitosis===


endings, 157
Mitosis, also called indirect division and karyokinesis, is the ordinary mode of cell division. Although it is a continuous process, it has been conveniently divided into four successive phases the prophase, metaphase, anaphase, and telo phase. During the prophase the chromatic material of the nucleus prepares for division and collects in the center of the cell. It is divided in halves in the metaphase, and the two halves move apart during the anaphase. The chromatic material becomes reconstructed into resting nuclei during the telophase. The various patterns which the chromatic material and protoplasmic fibrils present during these phases are known as mitotic figures.
Septa, in glands, 57
Septula testis, 332
Septum membranaceum, 177


pellucidum, 421
Mitotic figures are found hi all rapidly growing tissues, but especially favorable for preliminary study are the large cells in the root tips of plants. In longitudinal sections of root tips, the cells are cut at right angles to the plane of cell division, which is desirable; and often in a single section 5 mm. long, all the fundamental stages may be quickly located. The following general description of mitosis is based upon such easily obtained preparations, and the plant selected is the spiderwort (Tradescantia virginiana). 1 They may be satisfactorily stained with saffranin, or with iron haematoxylin and a counter stain such as orange G. There are many descriptions of mitosis in root tips, among them the following:


trans versum, 276
Rosen, (Hyacinthus oriental/is) Beitr. zur Biol. der Pflanzen, 1895, vol. 7, pp. 225-312; Nemec, (Allium cepa} Sitz.-ber. kon. Ges. der Wiss. Prag, 1897, No. 33, pp. 25-26, and Jahrb. fur wiss. Bot., 1899, vol. 33, pp. 313-336; Schaffner, (Allium cepa) Bot. Gaz., 1898, vol. 26, pp. 225-238; Hof, (Ephedra major) Bot. Centralbl., 1898, vol. 76, pp. 63-69, 113-118, 166-171, 221-226; Gregoire and Wygaerts, (Trillium grandiflorum) La Cellule, 1904, vol. 21, pp. 1-76; Farmer and Shove, (Tradescentia mrginiana) Quart. Journ. Micr. Sci., 1905, vol. 48, pp. 559-569; Richards, (Podophyllum peltatum] Kansas Univ. Sci. Bull., 1909, vol. 5, p. 87-93.
Serotina (decidua basalis), 371
Serous glands, 54, 237
Sertoli's cells, of the testis, 335
Serum, 201
Sexual cells, 20, 334
Sharpey's fibers, 92
Sigmoid colon, 247
Silver nitrate, 506
Silvester, on lymphatics, 185
Simple epithelium, 48
Sino-atrial (or sino-auricular) node, 181
Sinus, coronary, 177


lactiferous, 404
The cells to be described are found in the interior of the root tip, just back of the protecting cap of cells which covers its extremity. They are oblong in shape and their long axis corresponds with that of the root. The walls are very distinct, and the cells consist of granular vacuolated protoplasm, which in preserved specimens is generally irregularly shrunken.


rectalis, 274
The resting cells (Fig. 12, A) contain large round nuclei in which the chromatin is in the form of fine granules evenly distributed throughout the nucleus. A nucleus usually contains from two to five round nucleoli, each of which, when in focus, is seen to be surrounded by a clear zone. The nuclear membrane is distinct.


tonsillaris, 218
* 1 Good specimens may be obtained from any rapidly growing root tip. Those starting from hyacinth bulbs placed in water are very favorable. Onion root tips have been extensively used, and also those of bean and corn seedlings. The pointed ends are snipped off and dropped into Flemming's stronger solution


transversus pericardii, 175


urogenital, 330, 353


venosus, 177
Prophase. The first indication of approaching division is a change in the chromatin, which becomes gathered into fewer and coarser granules and takes a deeper stain. Portions of the linin network break down, so that the chromatin granules come to be arranged in long convoluted threads. Such threads are developing in the cell, Fig. 12, B, but are more perfectly formed in C. It is possible that at a certain stage the nucleus contains only a single continuous thread, but this condition cannot be demonstrated in Tradescantia. The stage of nuclear division in which the chromatic material appears to be arranged in a coiled thread or skein is called a spireme. The "close spireme" (B) is succeeded by the "loose spireme" (C). Successive stages in the development of the spireme in animal cells are seen in Fig. 20, D, E, and F.
sclerae, 460
Sinuses of the dura mater, 438


in lymph glands, 205, 208
As the spireme develops, the nuclear membrane becomes less distinct, and the clear zones disappear from around the nucleoli. The nucleoli become apparently less regular in outline, and forms which suggest that two of them have fused (Fig. 12, B) are perhaps more frequently seen than in resting cells. Usually it is stated that the nucleoli break up into smaller bodies toward the time of their dissolution, and that some of these escape into the cytoplasm after the disappearance of the nuclear membrane. Farmer and Shove believe that the nucleoli contribute to the chromatin; Richards regards them as a store of food material for the rest of the cell; and others believe that they form the achromatic "spindle" which will be described presently. Their function in animal cells is equally uncertain.


in haemolymph glands, 209
In the stage shown in Fig. 12, D, which may be regarded as the end of the prophase, the nuclear membrane and the nucleoli have disappeared, and the spireme thread has become divided into a number of segments or chromosomes. These are straight or curved rods of different lengths. Sometimes they appear as bent V-shaped bodies, but these often represent two chromosomes with their ends together. J-shaped forms, with one long and one short arm, have been described in various plants. The chromosomes become so arranged that one end of the rods, or the apices of the V's, are situated in the equatorial plane, which extends transversely across the middle of the cell. Often it is temporarily tilted (as in D and E) as if the mitotic apparatus had shifted to a position in which it obtained more space. It may do this mechanically if the contents of the cell are under pressure. When the chromosomes are gathered at or in the equatorial plane, they constitute collectively the equatorial plate. Because of their stellate arrangement at this stage, which is best seen in transverse sections of the cell, this mitotic figure is known as the aster.
Sinusoids, 166
Skin, 384


corium, 384, 385
The manner in which the chromosomes are formed from the spireme thread is difficult to determine. According to Gregoire and Wygaerts, the linin and chromatin, which have often been regarded as closely related


epidermis, 384, 386


hair, 389


nails, 388
i6


sebaceous glands, 397


sweat glands, 398


vessels and nerves, 399
HISTOLOGY
Slides and cover glasses, 508
Small intestine, 246


blood vessels, 266


distention of, 262


duodenum, 246
substances, are identical, and linin is merely a name for slender filaments of chromatin. Accordingly the chromatin simply draws together to


glands, 262


Ueum, 246


jejunum, 246


lymphatics, 267


mesentery, 269


nerves, 269


villi, 262


Smreker, on enamel prisms, 107
Sobotta, on fertilization, 33
Solar plexus, 139






INDEX
FIG. 12. MITOTIC CELL DIVISION IN THE ROOT TIP OF Tradescanliavirginiana. Xias diam. A, resting cell; B, C, D, prophase; E, metaphase; F, anaphase; G, H, I, telophase.


form chromosomes, and the beaded appearance of the spireme thread is due to alternate enlargements and constrictions of one substance. Others consider that a different substance connects the granules of chromatin




537


INDIRECT DIVISION




Solitary nodules, 207
Somatopleure, 36
Spermatic cord, 342
Spermatid, 22, 26, 336
Spermatocytes, 22, 26, 336
Spermatogenesis, 21
Spermatogonia, 22, 23, 334
Spermatozoa, 22, 26, 337


discovery of, 338
with one another; and Rosen states that each chromatin granule is completely imbedded in a broad strand of linin. Davis similarly interprets the spireme shown in Fig. 20, F. Whatever the actual structure may be, the chromatin granules in the spireme thread early divide in two, so that the thread appears double. When the thread shortens and condenses to form the chromosomes, the rows of granules may coalesce so as to produce a rod already divided lengthwise, although its halves are in close apposition. Occasionally the ends of the chromosomes are seen to be slightly separated.


number in man, 29
Metaphase. In the metaphase (Fig. 12, E), the two longitudinal halves of each chromosome are being drawn apart toward the opposite poles of the cell. If the chromosome is V-shaped, the separation of the two halves begins at the apex of the V.
Spermid, 22
Spermium, 22
Sphincter pylori, 257
Spinal cord, 409


central canal, 409
At this stage an achromatic figure, known as the spindle, is evident in plant cells, but it is more sharply defined in those of animals. As seen in the diagram (Fig. 13), it consists of fibrils which pass from the equatoria


columns, 411
Polar radiation. Nuclear spindle.


commissures, 411


dorsal nucleus, 417


ependyma, 418


fasciculi, 411


funiculi, 411
FIG, 13. EARLY METAPHASE.


gray substance, 416


horns, 411


membranes, 413
FIG. 14. LATE METAPHASE.


white substance, 414
Spinal ganglia, 134


nerves, 133
Spindle, 17


muscle-spindle, 159
plate toward either pole, where, in animal cells, there is a well-defined granule, the centrosome. Around each centrosome there are radiating protoplasmic fibrils, forming the polar radiation (Figs. 13 and 14). The polar radiation is also called an aster, and the two asters connected by the spindle are known as the amphiaster. Some of the spindle fibers are attached to the chromosomes and appear to pull their halves apart; others pass from pole to pole without connecting with the chromosomes. In animal cells the spindle arises as the two centrosomes, lying beside the nucleus, move apart (Fig. 20, A). As they pass to the opposite poles of the nucleus, the spindle forms between them, either from the nuclear reticulum, or the cytoplasmic reticulum, or hi part from both. These conditions appear to vary in different animals.
Spiracle, 216
Spiral ganglion, 476


organ, 467, 472
In the cells of root tips, a condensation of protoplasm forms a cap at the poles of the nucleus at the time when the nuclear membrane and nucleoli are disappearing. From the "polar cap," spindle fibers develop
Spireme, 15
Splanchnic nerves, 138
Splanchnopleure, 36
Spleen, 210


capsule. 214


cells, 211, 213


lobules, 215
l8 HISTOLOGY


nodules, 211, 214
which invade the nucleus, and also radiations which have been traced even to the cell walls. But as Rosen states, sun-like figures, such as certain botanists have pictured, do not occur. Schaffner has described a distinct centrosome or central granule in the root tip of the onion, but Richards finds that in Podophyllum there is no such structure, and the weight of evidence appears to be against the existence of a definite centrosome in the higher plants.


pulp, 211, 213
Anaphase. In the anaphase the halves of each chromosome move to the opposite poles (Fig. 12, F). The figure thus produced is known as a double star or diaster. Since each chromosome has divided into two, the original number of chromosomes is preserved, and an equal number of rods will be found in either star. They cannot all be brought into focus together, and because of overlapping, they are hard to count. Sometimes one chromosome, longer than the others, remains for a time as a continuous bar from one aster to the other. Between the asters there are always straight spindle fibers, but they vary in distinctness. (The anaphase in an animal cell is well shown in Fig. 21, D.)
Spongioplasm, 4
Squamous cells, 49
Stains, general, 500


selective, 503
Telophase. After the chromosomes have reached the opposite poles, they form two dense masses. They are generally said to unite end to end, thus forming a spireme thread. But in the root tips of Trillium, Gregoire and Wygaerts state that they come into contact with one another laterally; and as they separate, transverse connections are retained, which, with the vacuolization of the chromosomes, restore the nuclear reticulum. This may not be the correct interpretation, but immediately after the anaphase the chromosomes form a very compact mass, easily overstained so that it appears solid. Subsequently the mass enlarges (Fig. 12, H), and the chromosomes become coarsely granular, taking the form of wide bands. Nucleoli reappear, and according to Richards, "it is a general rule that they arise on the side of the nucleus nearest the new cell wall." This accords with Nemec's statement that they form from the outer fibers of the spindle. Nemec and Rosen agree that they first appear outside of the nucleus, which they enter before the nuclear membrane develops. These are details which require confirmation.
Staining methods, 499
Stapes, 469


Steele, on intercalated discs, 129
The new cell wall arises in plants as a series of thickenings of the interzonal spindle fibers, which at this stage form a barrel-shaped bundle (Fig. 12, G). The thickenings coalesce to form a membrane which does not at first reach the sides of the cell. While this wall is developing the nuclei are in a condition resembling the spireme stage of the prophase. The entire mitotic figure is therefore called the double spireme or dispireme. The cell wall is soon completed and the nuclei return to the resting condition (Fig. 12, I).
Stenson's duct (parotid duct), 238
Stohr, on peripheral nerves, 145


thymus, 224
The time required for mitotic cell division varies from half an hour (in man) to five hours (in amphibia). After death, if the tissues are not


glands of the vermiform process, 271
Stomach, 246, 251


glands, 253


musculature, 257
INDIRECT DIVISION 19


subdivisions, 251
hardened by cold or reagents, it is thought that mitoses go on to completion. Forty-eight hours may elapse before they entirely disappear from the human body.
Stratified epithelium, 48
Streeter, on the acoustic nerve, 476
Striated muscle, 113, 128
Stroma. 46


Studnic'ka, on dentinal fibers, 108
Pluri-polar mitosis. Under abnormal conditions, as in the cancer cells shown in Fig. 15, spindles may develop simultaneously in connection with three or four centrosomes. Similar pluri-polar spindles have been produced experimentally, by treating cells with various poisonous solutions. An unequal distribution of chromatin may occur under such conditions, and this may happen also with bipolar spindles, as shown in Fig. 15, a.
Subarachnoid space, 414, 439
Subcardinal veins, 309
Subcutaneous tissue, 385
Subdural space, 413, 439
Sublingual glands, 242
Submaxillary glands, 242
Substantia adamantina, 99


alba, 414
Number and individuality of the chromosomes. It is now generally believed that every species of plant or animal has a fixed and characteristic number of chromosomes, which regularly recurs in the division of all






Substantia eburnea, 99


gelatinosa, 417
FIG. 15. MITOSES IN HUMAN CANCER CELLS. (From Wilson, after- Galeotti.) a, Asymmetrical mitosis with unequal distribution of chromatin; b, tripolar mitosis; c, quadripolar" mitosis.


grisea, 416


lentis, 452


ossea, 99
its cells, with the exception of the germ cells, in which the number is reduced. In certain species, however, the two sexes regularly differ from one another in the number of their chromosomes, and one sex may contain an odd number. Usually the number of chromosomes is believed to be even.


Sulphuric acid, 489
There is considerable difficulty in counting the chromosomes. Generally it is possible that some have been cut away in the process of sectioning, so that, if the number is believed to be invariable, the highest number found in any cell is assumed to occur regularly. Another source of error lies in the fact that a bent chromosome may be counted as two, or rods with their ends overlapping may appear as one. Farmer and Shove have ventured to state that the number in Tradescantia "varies from about twenty-six to thirty- three." Nemec found that twelve chromosomes occur regularly in young tissues of the onion, but that in older tissues the number diminishes even to four. Sixteen have been recorded in the onion by other botanists. Podophyllum is said to have sixteen (Mottier), but Richards records counts of fourteen. In man the number has been placed at 16 and 32, but it is now believed to be 24. Gutherz, with particularly favorable material, emphasizes the difficulty of counting
Supracardinal vein, 310
Suprarenal gland, 404
Supratonsillar fossa, 218
Sustentacular cells, of the inner ear, 471, 475


of the olfactory epithelium, 484


of the taste buds, 235


of the testis, 334
20 HISTOLOGY
Sweat glands, 398
Sylvius, aqueduct of, 422
Sympathetic ganglia, 137, 142, 150


ganglionated trunk, 138
the chromosomes in man. He found only two cells in which a count could be made, in neither case with absolute certainty. But he agrees with Duesberg that the reduced number is twelve, according to which the whole number should be twenty-four. Recently, however, Wieman has found cells in the brain of a 9-mm. human embryo which contained 33 chromosomes. Some cells in the nasal epithelium and mesenchyma of this specimen contained 34, and others 38. Thus Wieman concludes that the number in man is certainly greater than 24 and is perhaps variable (Amer. Journ. Anat., 1913, vol. 14, pp. 416-471).


nervous system, 137, 139
In the grasshoppers, which are among the most favorable objects for the study of mitosis, not only is the number of chromosomes for a given species believed to be constant, but each cell appears to contain a definite series of chromosomes, the members of which vary somewhat in shape and size. Recent studies of such cells favor Rabl's hypothesis of the individuality oj the chromosomes, according to which the chromosomes persist in the resting nucleus, although disguised by their lateral branches and diffuse granular form. If this hypothesis is correct, when a nucleus prepares for division the same chromosomes which entered it will reappear. Sometimes in the prophase the bands of chromatin are arranged hi a polar field such as is seen in the telophase (Fig. 12, H). This arrangement has been observed by Farmer and Shove in the prophase of Tradescantia, and by others in various plants and animals. It is regarded as evidence that the chromosomes are "independent and continuously perpetuated organs of the cell." Nevertheless it is generally true that in resting nuclei no trace of individual chromosomes can be made out. The great importance of accurate knowledge of the chromosomes is shown by the following considerations.
Synapsis, 23
Synarthrosis, 96
Synchondrosis, 91 96
Syncytium, 8
Synizesis, 23
Synovia, 99
Synovial membrane, 98


As a result of mitotic cell division, it is evident that every new cell regularly receives one-half of each chromosome found in the parent cell, and thus the number of chromosomes remains constant. But in the germ cells the number is invariably reduced, and hi some animals it becomes exactly one-half of the number found elsewhere in the body. In such a case, when the male sexual cell, or spermatozoon, unites with the female sexual cell, or mature ovum, in the process of fertilization, the original number is restored. Each parent thus contributes one-half of the chromosomes found in the cell which gives rise to a new individual; and since each of these divides with every subsequent cell division, it is evident that one-half of the chromatin in every cell of the adult body is of maternal origin and one-half of paternal origin. The process by which the sexual cells acquire the reduced number of chromosomes and become ready for fertilization is known as maturation. The production of the sexual cells in the male is called spermatogenesis and in the female oogenesis.




Tactile cells, 157


menisci, 158
Taenias coli, 273
Tapetum cellulosum, 454


fibrosum, 454
Tarsal glands, 462
Taste buds, 232, 234


cells, 235
FIG. 16. DORSAL (a) AND LATERAL (b) VIEWS OF THE ABDOMEN OF GRASSHOPPERS. (After Hyatt and Scudder.)


Technique, microscopical, 487
Teeth, 99


cement, 99, in
===Spermatogenesis===


dentine, 99, 109
In its essential features, the process of spermatogenesis in insects corresponds with that in mammals, and very favorable material can be obtained in abundance from grasshoppers of various genera.


enamel, 99, 104
The males may be distinguished from the females by the shape of the abdomen. In males it is more rounded (Fig. 16) with various appendages directed dorsally. The abdomen of the female is pointed, terminating in the ovipositor, the parts of which as seen from the side may be together, or widely separated dorso-ventrally. The genital glands can be readily removed by dissecting as follows: Male grasshoppers, which have been chloroformed, are opened by a mid-ventral incision. The abdominal walls are pinned out on a wax plate under normal salt solution (0.6 per cent.). The intestinal tube, which is usually black or green, is taken out with forceps, and the yellow or orange testes are seen close together at the upper end of the abdomen, attached to the back. Each testis consists of a number of separate cylindrical lobes, and it should be worked loose from the surrounding tissue with forceps in such a way that these lobes remain together. The tissue may be preserved in Flemming's strong solution or in Hermann's fluid, and stained with iron haematoxylin.


pulp, 99, no
Tela submucosa, 220, 248
Telencephalon, 419
Tendon, 75


spindles, 159
Among the many publications upon spermatogenesis in the grasshoppers, the following may be cited: McClung, C. E., The accessory chromosome sex determinant? Biol. Bull., 1902, vol. 3, pp. 43-84; Sutton, W. S., On the morphology of the chromosome group in Brachystola magna, Biol. Bull., 1902, vol. 4, pp. 24-39; McClung, C. E., The chromosome complex of orthopteran spermatocytes, Biol. Bull., 1905, vol. 9, pp. 304-340; Robertson, W. R. B., The chromosome'complex of Syrbula admirabilis, Kansas Univ. Sci. Bull., 1908, vol. 4, pp. 273-305; Davis, H. S., Spermatogenesis in Acrididae and Locustidae, Bull. Mus. Comp. Zool., 1908, vol. 53> PP- 57-IS7; Wilson, E. B., The sex chromosomes, Arch, fur mikr. Anat., 1911, vol. 77, pp. 249-371.
Tenon's capsule, 460
Terminal bars, 52
Testis, 332


cells, 332, 334


convoluted tubules, 334
As seen in sections, each lobe of the testis of the grasshopper contains a considerable number of closed sacs or cysts, which are filled with sexual cells; and all the cells within a cyst are in approximately the same stage of development. The cysts are shown in Fig. 17, which represents a longitudinal section of a single lobe. Usually the testis is sectioned as a whole, and the specimen consists of a group of lobes cut transversely or obliquely. Cross sections from the apical portion, furthest from the outlet, will contain younger stages than the sections lower down in the lobe, since the cysts form at the apex and gradually move downward. At the apex, according to Davis, there is an apical cell which is surrounded by young sexual cells known as spermatogonia (Fig. 17, a). The spermatogonia move away from the apical cell, and each becomes enclosed in a cyst-wall derived from the surrounding tissue. Within the cysts thus formed, the spermatogonia multiply, and the cysts in the upper part of the lobe are filled with spermatogonia (Fig. 17, b). After repeated divisions the spermatogonia pass through a period of growth, accompanied by a rearrangement of their nuclear contents. The large cells with characteristic nuclei which are thus produced, are known as primary spermatocytes. They fill the cysts further down in the lobe (Fig. 17, c). Each primary spermatocyte divides into two secondary spermatocytes, and each of these divides into two spermatids, after which no further cell division is possible until fertilization takes place. But each spermatid becomes transformed from a round cell into a linear body with a whip-like tail, and is then capable of independent motion. Since in this form these cells were once thought to be parasitic animals living in the spermatic fluid, they received the name spermatozoa, which they still retain. 1 Cysts containing spermatozoa occur near the outlet of the lobe, or if the grasshoppers are collected late in the season, they may be found throughout most of the testis. Specimens from young grasshoppers, in which the spermatocyte divisions are abundant, are more desirable, even though no spermatozoa have become fully developed.


descent of, 331


development, 328


interstitial cells, 332
FIG. 17. LOBE OF THE TESTIS OF A GRASSHOPPER. Xso. (After Davis.)


muliebris, 365
a, apical cell.


rete, 328, 339
b, spermatogonia.


vessels and nerves, 333
c, spermatocytes.
Tetrads, 23
Thalamus, 421
Thebesius, veins of, 179
Theca folliculi, 355
Thoma, on the development of blood vessels


1 66
d, spermatocytes dividing.


Thoracic duct, 183
e, spermatids.
Thrombocytes, 199
Thymus, 213, 222
Thymic corpuscles, 225
Thyreo-glossal duct, 217
Thyreoid gland, 217, 226


f, spermatozoa.




538


Spermatogonia




INDEX
The succession of cell divisions described in the preceding paragraph is shown in tabular form in Fig. 18. Except for the number of chromosomes within the various cells, this diagram is quite as applicable to man as to the grasshopper. In this figure, however, only two spermatogonial divisions have been included. The number of times which the spermatogonia may divide before becoming spermatocytes is considerable and


1 It has been proposed to substitute the term spermium for spermatozoon; and consequently spermiocyte, spermid, etc., for spermatocyte and spermatid. The secondary spermatocytes are sometimes called praespermatids or praespermids; but these changes in names are of questionable value




Tilney, on the hypophysis, 437


Tissues, 35


Toldt, on gastric glands, 253
Secondary Spermatocytes


paradidymis, 344
Tomes's fibers, 107


processes, 104
Tongue 230
Tonsils, lingual, 218, 222


palatine, 218, 219
Spermatozoa


pharyngeal, 218, 222
Top-plate 50
Toruli tactiles 385
Trabeculae, 205
Trachea, 299


Tradescantia, cell division in, 14
Transitional epithelium, 324


leucocytes, 196
FIG. 1 8. DIAGRAM OF THE CELL DIVISIONS IN SPERMATOGENESIS. The figures indicate the number of chromosomes found in the cells of certain grasshoppers.


Triangular ligaments of the liver, 278
Tricuspid valve, 177
Trigeminal nerve, 141, 423, 426
Trochlear nerve, 141, 422
Trophoblast, 367
Trophoderm, 367
Trophospongium, 5
Tuberculum impar. 230
Tunica albuginea, 327, 349 353


propria, 220
Tympanic cavity ; 470, 478


membrane, 469. 480
presumably indefinite. As seen in sections, the spermatogonia, spermatocytes, and spermatids may be described as follows, using for illustrations Davis's figures of a common grasshopper Dissosteira Carolina.
Tyson's glands, 398


Spermatogonia. The nucleus of each spermatogonium contains the full number of chromosomes, which in most of the grasshoppers (Acrididee) is 23. With every spermatogonial division, each chromosome is split lengthwise. In this and other respects the mitotic figures are quite like those occurring elsewhere in the body. They are shown in Fig. 20, A, B, and C. When the twenty- three chromosomes have formed the equatorial plate, it is sometimes possible to see all of them in a single transverse section of the cell (Fig. 19, A). It then appears, as found by Montgomery (1901) in certain Hemiptera, and a year later by Button in grasshoppers, that the chromosomes vary in size, but the "gradations in volume are not between individual chromosomes but between pairs, the two members of which are of approximately equal size." In Fig. 19, A, twelve forms of chromosomes have been identified by Davis; and all of these are paired except the one numbered 4. The members of a pair are often, but by no means invariably, side by side. In some cases, owing to foreshortening, their resemblance in size is not apparent in the drawing. The behavior of the odd or accessory chromosome is of special interest, since according to McClung's hypothesis, now well established, this accessory chromosome is the bearer of those qualities which determine sex.


Primary spermatocytes. After the last spermatogonial division, the cells begin their "growth period." At this time the chromatin tends to collect on one side of the nucleus, in a condition known as synapsis (or more recently as synizesis). This distribution of the chromatin has been frequently observed, but it has not been shown to be of special significance. In the primary spermatocytes drawn in Fig. 20, D, E, and F, the chromatin is evenly distributed. All of the chromosomes, except the accessory chromosome, have become filamentous, but the accessory chromosome remains as a compact, darkly staining body close to the nuclear membrane. It resembles a nucleolus, for which in fact it has been mistaken. True nucleoli may occur in these cells, together with the accessory chromosome, but they stain differently.


Umbilical arteries, 381
As the primary spermatocytes prepare for the next division, the spireme becomes resolved into eleven loops, each of which represents the two members of a pair of chromosomes joined end to end. The granules imbedded in the linin thread divide as usual, so that each loop contains a double row of granules (Fig. 20, F). These loops contract to form eleven chromosomes, which, because of their four parts, are known as tetrads. The structure of the tetrads is shown in Fig. 19, B-G. The filaments seen in the upper row of drawings contract into corresponding solid forms of chromosomes seen in the lower row, in which the place of attachment to the spindle fibers has been indicated.


cord, 369, 380
Each tetrad represents two chromosomes joined end to end and split lengthwise. The simplest forms are shown in Fig. 19, B and C, which illustrate respectively two ways in which the tetrad may later divide. The two component chromosomes may simply be pulled apart, as indicated in Fig, 19, B, in which the spindle fibers are attached to the ends of the rod. If this takes place, each secondary spermatocyte will receive one member of every pair of chromosomes which occurred in the spermatogonium, but no part of the other member. Such a division, which eliminates one-half of the chromosomes from the daughter cell, is known as a reductional division. The other form of chromosome division is known as equational. When it takes place, every chromosome divides lengthwise, and the daughter cells receive one-half of every chromosome in the parent cell. This occurs in ordinary cell division, and also in the division of the tetrads provided that the spindle fibers are attached to the place where the two component chromosomes come together (Fig. 19, C).


veins 279, 381
Unipolar cells, 143
Units, structurla, of kidney, 321


of the liver, 284


of the spleen, 215
Unna, on plasma cells, 70
Urachus, 382
Ureter. 311, 322
Urethra, female, 325


male, 346
FIG. 19. A, POLAR VIEW OF THE METAPHASE OF A SPERMATOGENIAL DIVISION IN Dissosteira Carolina. X 1450 (After Davis.) The pairs of chromosomes have been numbered. B-G, various forms of tetrads, rom primary spermatocytes. (After Davis and Robertson.)


Urinary organs, 306
As a stage in the separation of the two halves of a rod-shaped tetrad, crossshaped forms are produced (Fig. 19, D). If the separation is almost complete, such shapes are seen as in Fig. 19, E. The arms of the tetrad which are not attached to the spindle fibers may bend toward one another and unite, so as to form rings (F), or they may twist about like a figure 8, as shown in G. If the spindle fibers are attached to the points xx in the upper figure in G, the division will be equational; if as shown in the lower figure it will be reductional.
Uriniferous tubules, 313
Urogenital sinus, 330, 353
Uterine tubes, 327, 358
Uterus, 327, 360


masculinus, 346
Usually it is considered that the division of the tetrads into double bodies or dyads, is equational, and that the division of the dyads, which takes place when the secondary spermatocytes divide, is reductional. According to Davis, however, the first division of the tetrads is reductional and the second division is equational. In either case the end-result is the same. Each spermatid will contain one of the four parts of each tetrad, and thus one member of every pair of chromosomes will be eliminated from any given spermatid.


menstruating, 363


pregnant, 366
Since in the testis tetrads occur only in the primary spermatocytes, the cells shown in Fig. 20, G-J, are easily identified. These are success ive stages in the division of the primary spermatocyte. In G the accessory chromosome is seen as a rod-shaped body above and to the right; in H it is below and to the right. In J it is obliquely placed just above the equatorial plate and in K it is passing to the upper pole of the spindle. In the spermatogonial divisions the accessory chromosome always divides with the others; but in the division of the primary spermatocyte it passes undivided into one of the daughter cells. Thus one secondary spermatocyte will receive eleven chromosomes (dyads) and the other will receive twelve (eleven dyads and the accessory chromosome) . In the late anaphase shown in Fig. 20, L, the accessory chromosome cannot be recognized.
Utriculus, 466, 470


prostaticus, 346




FIG. 30. SPERMATOGENESIS IN Dissosleira Carolina A-FXI4SO; G-LX966. (Davis.)


Vacuoles, 4
A, B, C, prophase. metaphase. and telophase of a spennatogonial division. D-L, successive stages in the
Vagina, 327, 383


masculina, 346
.division of a primary spermatocyte into secondary spermatocytes.
Vagus nerve, 142, 424, 426
Vallate papillae, 230, 231
Valves, of the colon, 261, 273


of the heart, 176


of Houston, 273


of lymphatic vessels, 188
Secondary spermatocytes. The secondary spermatocytes pass rapidly from the condition shown in Fig. 20, L, to that of Fig. 21, A. A nuclear membrane has developed, and the dyads have become somewhat filamentous. Without passing through a complete resting stage they proceed to divide as shown in Fig. 21, B-F. The dyads separate into their component halves. In those secondary spermatocytes which received the accessory chromosome, that body will be seen dividing with the dyads, and each spermatid will receive one-half of it. It has been questioned whether the division of the accessory chromosome is longitudinal and therefore equational, or transverse and reductional. Many cytologists consider that if a chromosome splits lengthwise, all of its parts will be represented in the resulting halves, but if it divides transversely, essential elements will be lost. This conception lends importance to the question of transverse or longitudinal division of the accessory chromosome. By the division of this chromosome it comes about that one-half of the spermatids contain twelve chromosomes, and one-half contain eleven, as indicated in the diagram, Fig. 18. The spermatids shown in Fig. 21, F, contain the accessory chromosome.


of the veins, 164, 174
Spermatids and Spermatozoa. In forming spermatozoa, the spermatids become elongated, passing from the condition shown in Fig. 21, F, to that of Fig. 21, G. The chromatin within the nucleus is distributed in fine granules throughout the linin reticulum. Close to the nuclear membrane a small dark body has appeared, from which a slender filament has grown out. This body is usually described as the centrosome. A condensation within the cytoplasm, seen also in F, is known as the paranucleus. It is of uncertain origin, but may proceed from the cytoplasmic structure called mitochondrium. The paranucleus forms a sheath about the axial filament.
Valvulae conniventes (circular folds) , 261


Successively later stages are shown in Fig. 21, H, I, and J. The chromatin within the nucleus becomes homogeneous and very dense; at the same time the nucleus elongates and forms the head of the spermatozoon. This is enveloped by the cell membrane, but there is no appreciable layer of protoplasm around it. The centrosome elongates and forms the middle piece of the spermatozoon; and the axial filament, with a covering derived from the paranucleus and cytoplasm, constitutes the tail. Only a portion of the tail is included in the figure. The human spermatozoon likewise consists of a head, which is essentially the nucleus, a middle piece containing the centrosome, and a tail; but the form of the head is very different from




Vas deferens (ductus deferens), 324, 342
Vasa aberrantia of the liver, 280
Vasa vasorum, 171
Vascular tissue, 44, 163
Veins, 172


cardinal, 309


portal, 277, 278
FIG. 21. SPERMATOGENESIS IN Dissosteira Carolina. Xi4So. (Davis.)


pulmonary, 297
A-F, successive stages in the division of a secondary spermatocyte into spermatids. G-J, successive stages in the transformation of spermatids into spermatozoa.


umbilical, 279, 381
that in the grasshopper. It will be described in a later chapter


vitelline, 44, 278, 279
Although the spermatozoa of the grasshopper appear alike, it has been shown that one-half of them contain eleven chromosomes, and one-half contain twelve. The mature ova all contain twelve chromosomes. If a spermatozoon with eleven chromosomes unites with an ovum with twelve, a male animal will be produced, in every cell of which there will be twentythree chromosomes. But if the spermatozoon contains twelve chromosomes, a female animal is formed, containing twenty-four chromosomes in every cell. Thus sex appears to be determined by the presence or absence of a chromosome within the spermatozoon.
Vena cava inferior, 177, 279
Venae minimae, 179
Ventral aorta, 165
Ventricles, of the brain, 419, 421, 424


of the heart, 175
In some cases, as in several Hemiptera described by Wilson, the accessory chromosome is paired, but its mate is of small size. Thus the spermatozoa all have the same number of chromosomes; but half of them contain the large member of the pair and will produce females, and the other half contain the small member and will produce males. The mature ova all contain the large member. In tfyese insects, therefore, both sexes contain the same number of chromosomes, but the cells of the male contain a small chromosome, whereas the corresponding one in the female is large. From these observations it is reasonable to conclude that sex may be determined by a difference in the nature of certain chromosomes in those animals in which there are no appreciable differences in size or number.
Vermiform process, 270
Vesalius, on blood vessels, 164


pituitary gland, 436
In man, a difference in the number of chromosomes in the sexes has been reported, but the observations have not been confirmed. It is supposed that the spermatogonia contain twenty-four chromosomes, but it has not been shown that they exist as pairs. The spermatocytes, spermatids and spermatozoa apparently contain twelve. As the principal constituents of the spermatozoon, the chromosomes are believed to be the essential agents in the transmission of all qualities inherited from the male parent, and certain of them may determine sex.
Vesica fellea, 277
Vesicular follicles of the ovary, 354


supporting tissue, 82
Vestibule, of the labyrinth, 468


of the nose, 482


of the vagina, 353
===Oogenesis===
Vibrissae, 482
Villi, amniotic, 382


chorionic, 375
Mature ova result from a succession of cell divisions closely comparable with those which produce spermatozoa. The primitive female sexual cells correspond with the spermatogonia, and are called oogonia. They are provided with the full number of chromosomes, and divide an indefinite number of times. After a period of growth they become primary oocytes, in which the number of chromosomes is reduced one-half. The primary oocytes divide to form secondary oocytes; and these again divide to produce the mature ova, which are incapable of further division unless fertilization takes place. (The term {{ovum}} is ordinarily loosely applied, so that it includes not only the mature cells, but also {{oocyte}}s, and the clusters of cells resulting from the division of the fertilized ovum.)


intestinal, 260


pleural, 306
Although the mature ovum and the spermatozoon are closely similar in their nuclear constitution, they differ radically as to form, size, and cytoplasmic structure. The ova are very large cells, stored with nutriment for the embryo which each one may later produce. In the higher vertebrates they are formed in relatively small numbers. According to Hensen's estimate, about two hundred, ready for fertilization, are produced by the human female in a life-time. But the male, according to Lode, produces 340 billion spermatozoa, or, as stated by Waldeyer, nearly 850 million per ovum. A large number must be produced, since many will fail to traverse the uterus and tube so as to find the ovum at the time of its discharge from the ovary. The ova of lower vertebrates, which are fertilized and develop outside of the body, are discharged in great numbers; in certain fishes from three to four million are produced annually.


synovial, 98
Visual cells, 444


purple, 444
The multiplication of oogonia in the human ovary takes place before birth, and about fifty thousand are produced. At birth, or shortly thereafter, all the oogonia have become primary ob'cytes (Keibel). At first the oocytes are small, but they enlarge at varying rates, and the largest are indistinguishable from mature ova except by their nuclear contents. Since some grow more rapidly than others, the ovary in childhood contains primary oocytes of many sizes. Each oocyte becomes enclosed in a cyst or follicle. The way in which these follicles develop, and the. manner in which the oocyte escapes into the uterine tube by the rupture of these follicles, will be described in connection with the ovary. Between the cells of the follicle and the oocyte, there is a broad, radially striated membrane, known as the zona pellucida or zona radiata (Fig. 2 2) . This zona has sometimes been regarded as a cell membrane, but the oocyte divides within it as if enclosed in a capsule. It does not invest the daughter cells like a membrane. The radial striations have been interpreted as slender canals containing processes of the f ollicular cells, and the zona has been considered as a product of these cells. In certain cases a perivitelline space has been described as encircling the oocyte and thus separating it from the zona, but this space has been considered as artificial, or as a refractive line wrongly interpreted as a space.
Vitelline duct, 245


veins, 44, 278, 279
The cytoplasm of the oocyte becomes charged with yolk granules or spherules. They constitute the deuteroplasm (or deutoplasm), but this term is equally applicable to fat droplets and other secondary products of the protoplasm. In the human oocyte the granules are centrally placed (Fig. 22), and they are so transparent, when fresh, as to cause only a slight opacity. In the eggs of many animals the yolk is more highly developed, and it may be evenly distributed or gathered at one pole. Within the cytoplasm of the developing oocyte, a large dark body of radiate structure is sometimes conspicuous. It is inappropriately known as the yolk nucleus, and is probably a derivative of the centrosome and surrounding archoplasm. Other "vitelline bodies," of uncertain origin and significance, have been described. Some have been considered as nuclear extrusions.
Vitreous body, 442, 453


humor, 442
The nucleus of the oocyte is very large and vesicular. The chromatin occurs chiefly along the nuclear membrane and about the nucleolus. The nucleous is also very large, and Nagel stated that in the fresh condition it exhibits amoeboid movements, but this observation has not been verified. The nuclei of the oocytes ordinarily show no signs of mitosis, and they may remain in the resting condition for thirty years or more and then divide. Many of them, however, will degenerate without division.
Volkmann's canals, 93
Vomero-nasal organ, 141, 483






W
FIG. 22. THE OVUM AS DISCHARGED FROM A VESICULAR FOLLICLE OF AN EXCISED OVARY OF A WOMAN THIRTY YEARS OF AGE. Examined fresh in the liquor folliculi. (Nagel.)


c. i., Corona radiata composed of cells of the follicle; n., nucleus; p., granular protoplasm; p. s., perivitelline space; y., yolk; z. p., zona pellucida. (From McMurrich's "Embryology.")




Waldeyer, on spermatozoa, 29
The cell divisions which give rise to the secondary oocyte and the mature ovum respectively, have never been observed in man. Some of the cells within the ovary may be secondary oocytes and the cell shown in Fig. 22 may be of this sort, or possibly a mature ovum, but this cannot be determined. From what is known of other mammals, however, it may confidently be assumed that the cell divisions take place as shown in the diagram, Fig. 23.


oogenesis, 34
When the primary oocyte divides, the chromosomes, reduced in number, also divide and are equally distributed to the daughter cells; but the great mass of cytoplasm remains with one of these cells, namely, the secondary oocyte. The other cell, which is relatively very small, is known as the first polar body, or polar cell. It has the same nuclear contents as the secondary oocyte, and may divide into two other polar bodies, equivalent to mature ova. More often it degenerates without division. When the secondary oocyte divides, it likewise produces one large cell, the mature ovum, and one small cell, the second polar body. The latter is said to be capable of fertilization, but to what extent it may develop is unknown. Functionally the production of polar bodies serves to prevent the subdivision and distribution of the nutritive material elaborated within the primary oocyte. One mature ovum with abundant yolk is provided at the expense of three ova (polar bodies) which degenerate.


plasma cells, 68
Although the maturation of the ovum has not been observed in man, nor even the presence of definite polar bodies, the entire process has been carefully studied in other mammals, notably in the mouse. 1 It has been shown that the maturation of the ovum of the mouse takes place rapidly, both of the oocyte divisions being accomplished within from four to fifteen hours. The first polar body usually forms before the oocyte is discharged from the ovarian follicle in other words, before ovulation takes place. The second polar body is usually formed in the uterine tube, after the spermatozoon has entered the oocyte. Long and Mark have found that the chromosomes of the primary oocyte are tetrads, or bodies showing transverse and longitudinal divisions; and that those of the secondary oocyte are dyads. They believe that the first division is transverse or reductional, and that the second is equational.
Wax reconstructions, 516
Weidenreich, on pigment cells, 72


shape of red corpuscles, 191


white corpuscles, 195


eosinophiles, 198


the spleen, 212
Weigert's iron haematoxylin, 502


method for myelin sheaths, 510
FIG. 23. DIAGRAM OF THE CELL DIVISIONS IN OOGENESIS. (Compare with Fig. 18.)


resorcin-fuchsin, 504
Wepfer, on the lobules of the liver, 281
Wharton, on the suprarenal glands, 405
Wharton's duct (submaxittary duct), 242


jelly, 62
* 1 Among the most important papers are: Sobotta, J., Die Befruchtung und Furchung des Eies der Maus. Arch. mikr. Anat., 1895, vol. 45, pp. 15-91. Long, J. A., and Mark, E. L. The maturation of the egg of the mouse. Carnegie Inst. Publ. No. 142, 1911, pp. 1-72.


Whipple, on the tactile toruli, 385
White corpuscles, 188, 195


fibers, 62


nerves, 153
The difficulty of counting chromosomes is apparent from the varying numbers which have been reported in the mouse After reduction the number has been placed at 8, 12, 16, 18 and 20 by different observers.


rami, 137
The polar bodies in the mouse are relatively large. In the upper part of Fig. 24, A, a polar body is about to be formed, and it is completely cut off from the oocyte in Fig. 24, C. In D and G, two polar bodies are shown.


substance of the spinal cord, 414
===Fertilization===
Wiesel, on the suprarenal gland, 406
Williams, L. W., on the notochord, 38, 82


somites of the chick, 118
In the mouse, from six to ten hours after coitus, spermatozoa have made their way to the distal end of the uterine tube, where fertilization takes place. According to Long and Mark, the maturation of ova usually occurs at some time during the period from "13! to 28^ hours " after the mouse has given birth to a litter; and during the process of their maturation, the oocytes are discharged from the ovary and enter the distal end of the tube. Here, if fertilization takes place, a single spermatozoon penetrates the zona pellucida. In a section obtained by Sobotta, the entrance of the spermatozoon has been partially accomplished (Fig. 24, B). Its tail lies outside of the zona, and appears to have become thickened. In another specimen Sobotta found the head, middle piece and a part of the tail within the cytoplasm of the oocyte. The tail had broken as it crossed the zona, and the portion remaining outside had drawn together and was disintegrating. In some animals it is said that the entire spermatozoon enters the ovum, but in others only the head and middle piece. In any case the tail appears to be a propelling apparatus which becomes functionless after the head and middle piece have passed through the zona. It has entirely disappeared in the stage shown in Fig. 24, A, in which the head of the spermatozoon is seen within the oocyte on the right side of the figure. Meanwhile the oocyte is becoming a mature ovum by undergoing divisions and producing the second polar body; and the anaphase of this division is shown in Fig. 24, A. Sobotta stated that no centrosomes occur in connection with the spindles of the maturation divisions, and Long and Mark have likewise failed to find any "typical centrosomes."
Williams, S. R., on anomalous vessels, 166
Willis, on the intercostal nerve, 136


stomach, 251
In Fig. 24, C, the second polar body has become a separate cell. The chromosomes of the ovum, which is now mature, have formed a compact mass. They next become resolved into a chromatic reticulum, and a resting nucleus is produced, provided with a nuclear wall and distinct nucleoli (Fig. 24, D and E). This nucleus, which becomes large and moves toward the center of the cell, is known as the female pronucleus. Meanwhile the head of the spermatozoon has enlarged and formed the male pronucleus, as shown in Fig. 24, C, D and E.




The two pronuclei, which are very similar, develop rapidly, "probably within a few minutes after the entrance of the spermatozoon." Simultaneously they prepare for division, and the chromatic reticulum of each becomes resolved into the reduced number of chromosomes which it received during maturation (Fig. 24, F). A centrosome with astral radiations is now seen between the two groups. In Fig. 24, G, it has divided in two, and the spindle has developed. There has been much discussion as to the origin of these centrosomes. Since in this case they arise by the division of a single body, the possibility that one comes from the spermatozoon and one from the ovum has been eliminated. Moreover in the mouse they cannot be derived from the surviving centrosome of the last maturation division of the ovum, for that division takes place without centrosomes. Therefore the centrosome must either be brought in by the spermatozoon as a constituent of its middle piece, or it must be a new formation. Sobotta accepted the former alternative, and he observed a centrosome in connection with the head of the spermatozoon in certain stages (Fig. 24, C) but not in all. It is probable, according to Conklin, that "the source of the cleavage centrosomes may differ in different animals, or even in the same animal under different conditions."


INDEX 539


Wilson, on fourth molars, 101 Y


Wilson, E. B., on sex chromosomes, 21, 28
FIG. 34. MATURATION AND FERTILIZATION OF THE OVUM OF THB MOUSE .A,C-J,Xsoo; 8X750.


Winslow, foramen of, 280 Yolk nucleus, 29
(After Sobotta.)


Wolff, on the kidney (WolfBan body), 306 sac 39, 382
A-C, entrance of the spermatozoon and formation of the second polar body. _ D-E, development of the pronuclei. F-J, successive stages in the first division of the fertilized ovum.


epigenesis, 339 stalk, 382


Wolffian body, 306, 307 Z


duct, 41, 306, 307 ,  
Later stages in the division or "cleavage" of the fertilized ovum into two cells are shown in Fig. 24, H-J. The two groups of chromosomes come together upon the spindle so that the full number, characteristic of the species, is restored. Each chromosome then divides lengthwise, and thus each daughter cell receives one-half of its chromosomes from the male parent and one-half from the female parent. This is strikingly evident when the eggs of the fish Fundulus, which have long rod-shaped chromosomes, are fertilized with the sperm of Menidia, which has shorter rods. Moenkhaus, who performed this experiment (Amer. Journ Anat., 1904, vol. 3, pp. 29-64), states that the two kinds of chromosomes remain grouped and bilaterally distributed on the spindles during the first and second divisions of the fertilized ovum, but that later they become gradually mingled.


tubules, 308 Zenker's fluid, 492
Important information in regard to the nature of fertilization has been obtained by experiments upon unfertilized eggs. Changes in the concentration or composition of the sea water in which the eggs of marine animals have been placed, mechanical agitation, or, in the case of frogs' eggs, puncturing the outer layer with a needle, have led to repeated cell divisions. In this way embryos or larvae have been produced from unfertilized eggs, and, in a few instances, adult animals. Loeb, who has been a foremost investigator in this field, concludes that the spermatozoon causes the development of the egg by carrying a substance into it which liquefies the cortical layer of the egg, and thereby causes the formation of a membrane. "This membrane formation, or rather the modification of the surface of the egg which underlies the membrane formation, starts the development." At the same time there is an acceleration of the oxidations in the egg. "What remains unknown at present is the way in which the destruction of the cortical layer of the egg accelerates the oxidations."


Wright, on blood plates, 200 Zona columnaris, 247
For the physicist and chemist, the production of mitotic figures and the process of fertilization, have been subjects of great interest, and discussions of their significance will be found in various text-books of physiology and biological chemistry. For further morphological details the student is referred to "The Cell in Development and Inheritance," by E. B. Wilson (2nd ed., New York, 1900) and to the chapters on "Die Geschlechtszellen" and "Eireife, Befruchtung und Furchungsprozess," by W. Waldeyer and R. Hertwig respectively, in vol. i of Hertwig's "Handbuch der vergl. u. exp. Entwickelungslehre der Wirbeltiere," (Jena, 1906).


Wright's blood stain, 505 pellucida, 29


method for frozen sections, 498 radiata, 29


Zonula cmans, 449


Zuckerkandl, on chromaffin bodies, 152
'''Next :'''  [[Book - A text-book of histology arranged upon an embryological basis (1913) 1-2|1.2. General Histology]]
suprarenal glands, 406
organs of Zuckerkandl, 152
Xylol, 507 Zymogen granules, 255 ,


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Lewis FT. and Stöhr P. A Text-book of Histology Arranged upon an Embryological Basis. (1913) P. Blakiston’s Son and Co., 539 pp., 495 figs.

   Histology with Embryological Basis (1913):   Part I. 1.1. Cytology | 1.2. General Histology | 1.3. Special Histology
Part II. 2.1. The Preparation of Microscopical Specimens | 2.2. The Examination of Microscopical Specimens
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Part I. Microscopic Anatomy

I. Cytology

The Cell

Since 1838 it has been known that all plants and animals are composed of small structural elements called cells (Latin, cellula; Greek, *vros). The lowest forms of animals and of plants are alike in being single cells throughout life. The more complex organisms are groups of cells, which have been derived by process of repeated division from a single cell, the fertilized ovum. Thus the human body, which begins as one cell, becomes in the adult an aggregation of cells variously modified and adapted to perform special functions. Since the liver is a mass of essentially similar cells, the problems of its functional activity are the problems of the functions of a single one of its cells. The diseases of the liver are the result of changes occurring in these cells, which must be restored to a normal condition to effect a cure. As this is equally true of other organs, it is evident that cytology, the science of cells, is a basis for both physiology and pathology.

A cell may be defined as a structural element of limited dimensions, which under certain conditions can react to external stimuli and perform the functions of assimilation, growth, and reproduction. Because of these possibilites a cell may be considered an elementary organism. It is described as a mass of protoplasm containing a nucleus. A third element, the centrosome, is found in the cells of animals, but it is doubtful whether it exists in the cells of the higher plants. It becomes prominent when a cell is about to divide. Some authorities regard the centrosome as a temporary structure, which forms shortly before division begins and disappears after it is completed. Others consider it as a permanent and essential part of a cell, which accordingly consists of protoplasm, nucleus, and centrosome.


Protoplasm

Protoplasm is the living substance of which cells are composed. More specifically the term is applied to this living substance exclusive of the nucleus, or to the corresponding dead material, provided that death has not changed its physical properties. It has been proposed to substitute the name cytoplasm for protoplasm in the restricted and earlier sense of the term, to call the nuclear substance karyoplasm, and to consider both cytoplasm and karyoplasm as varieties of protoplasm. Although these names are often employed, the cell substance apart from the nucleus is ordinarily called protoplasm.

Protoplasm is a heterogeneous mixture of substances forming a soft viscid mass of slightly alkaline or neutral reaction. ("The terms may be used interchangeably for an alkalinity which is so slight" Henderson.) It is ordinarily more than three-fourths water, and the remainder consists of salts and organic substances, some in solution and some in a colloidal state. The organic bodies are classed as proteins, glycogen or some allied carbohydrates, and lipoid (fat-like) bodies. Protoplasm may exist in a numberless variety of forms.



FIG. r. DIAGRAM OF A CELL.

In the four quadrants different types of protoplasmic structure are represented namely, homogeneous, granular, foam-like, and fibrillar.


On microscopic examination, even with lenses of the highest power, the protoplasm of certain living cells appears homogeneous and structureless. But most of the cells which the histologist examines are not living.


They have been killed by various reagents, selected as causing the most rapid fixation possible. The protoplasm of such cells usually exhibits granules, fibrils, or networks with closed or open meshes. Whether these structures are wholly due to precipitation and coagulation is difficult to determine, but indications that they preexist have been observed in certain living cells. In any case, the various forms of coagulation occur with such constancy that their study is of the utmost importance to the histologist.

Even the ground substance of protoplasm, in which the fibrils or granules are imbedded, is not necessarily homogeneous. According to Biitschli's interpretation it has the structure oj foam or of an emulsion that is, it consists of minute droplets of one substance completely surrounded by walls of another substance. In these walls, granules and filaments may be lodged, as seen at the margins of the upper right quadrant of Fig. i. The complex chemical activities of a cell are said to be manifestly impossible in any homogeneous mass; but in such a heterogeneous medium as an emulsion, they are conceivable (Alsberg). In other words, the vital qualities of protoplasm may not depend so much on hypothetical complex and unstable living molecules, as upon the interaction of various substances, made possible by their arrangement in droplets and investing films.

The various structures commonly observed in protoplasm may be grouped as follows:

i. Granules. Ultra-microscopic granules doubtless exist in protoplasm, since the smallest of those observed approach the limit of visibility. The minute granules, if abundant, give the Nissl's bodies.

protoplasm a dark color. Often they are absent from the peripheral layer of protoplasm, or exoplasm, which is then clear, somewhat firmer, and chemically different from the inner endoplasm (Fig. i). In addition to minute granules such as may be found in most preserved protoplasm, certain cells contain larger granules, which are important secretory products elaborated by the cell. In active gland cells these granules are well defined and abundant, and they diminish as the cell becomes exhausted.


FIG . 2 . CLUMPS OF~GRANULES (NissL's BODIES) IN A NERVE


Various forms of white blood corpuscles may be distinguished by the size and staining reaction of the granules imbedded in their protoplasm. In certain nerve cells (Fig. 2) granules occur in large groups, known as Nissl's bodies. As Crile has shown, these become disorganized as a result of surgical shock or muscular fatigue. It is evident, therefore, that the careful observation of protoplasmic granules is of very great importance.



FIG. 3. 'FIBRILS IN A NERVE CELL.


FIG. 4. VACUOLES IN A YOUNG FAT CELL.


FIG. 5. CANALS (TROPHOSPONG IUM) IN A NERVE CELL.


2. Fibrils. Protoplasm may be permeated with a delicate meshwork of fibrils, which collectively constitute the spongioplasm, or filar mass. This is imbedded in the clear hyaloplasm, or interfilar mass (Fig. i). In certain cells there are filaments, known as mitochondria, which are formed by the coalescence of rows of granules. The relation between these structures and the reagents used is discussed by Kingsbury (Anat. Rec., 1912, vol. 6, pp. 39-52). The spongioplasm may form an irregular network, or its constituent fibrils may be parallel, passing from one end of the cell to the other. In oblique and transverse sections of such cells, the filaments are cut across, so that they appear as short rods, or even as granules. Fibrils may be extremely slender, as in the case of those which radiate through the protoplasm at the time when the cell divides; or they may be quite coarse, like the permanent fibrils characteristic of certain muscle and nerve cells

Reticular apparatus.

FIG.. 6. RETICULAR NETWORK (Fig. 3).

3. Vacuoles. Protoplasm often contains large or small drops of clear fluid, fat, or some other substance less highly organized than the surrounding material (Fig. 4). In preserved cells the spaces which were occupied by these droplets appear clear and empty, and are known as vacuoles.

tiey vary greatly in size, and one or several of them may be found in a single cell.

4. Canals. The protoplasm of certain cells is said to contain fine tubes or clefts which communicate with lymphatic spaces outside of the cell (Fig. 5). Prolongations from the surrounding capsule-cells have been described as entering these canals and as performing, together with the lymph, a nutritive function. Hence the network of canals has been called trophospongium. But it has not been shown conclusively that these canals open to the exterior of the cell. They may be similar to the closed networks or "reticular apparatus" lying wholly within the protoplasm, shown in Fig. 6. Such networks have been described in nerve cells, cartilage cells and gland cells. The network is said to be of a thick fluid consistency. In certain gland cells there are canals within the protoplasm, which convey the secretion to the free surface of the cell. These may be simple, branched, or arranged in a network. Like the other forms of intracellular canals, they can be studied only in special preparations.

5. Inclusions. Various foreign bodies, such as other cells or bacteria, which may have been ingested by the protoplasm, are grouped as inclusions. This term is applied also to crystalloid substances formed within the protoplasm (Fig. 7), and to coarse masses of pigment granules which appear extraneous.

Nucleus

The nucleus (Latin, nucleus, "the kernel of a nut"; Greek, Kdpvov, "a nut") is typically a well-defined round body, situated near the center of the cell, appearing denser or more coarsely granular than the surrounding protoplasm (Fig. i). There are characteristic variations in the shapes of nuclei, in their position within the cells and in their structure.

Ordinarily the karyoplasm, or nuclear substance, is sharply marked off from the cytoplasm by the nuclear membrane. Sometimes, in preserved tissues, the cytoplasm has shrunken away from the nuclear membrane, so as to leave a narrow space partially encircling it; and in certain living cells, the nucleus migrates through cytoplasm, as if it were an independent body. But there are phases of cell-development in which the nuclear membrane disappears and no line can be drawn between karyoplasm and cytoplasm. At all times they have a common structural basis. The ground substance of the nucleus, corresponding with the hyaloplasm, is the nuclear sap; and it contains, for spongioplasm, a meshwork of delicate linin fibrils. These help to form the nuclear membrane, in which they terminate. The nuclear membrane, nuclear sap, and linin reticulum do not stain deeply, and are therefore grouped together as the achromatic constituents of the nucleus.

The principal chromatic constituent of the nucleus is known as chromatin. It stains deeply, since it contains a large amount of nucleic acid, which has a marked affinity for basic stains. Chromatin occurs in the form of granules, which are bound together in strands or masses by the lining fibers (Fig. i). The masses, known as chromatin knots, occur especially at the points of intersection in the linin meshwork. Sometimes they are attached to the nuclear membrane, or so distributed over its surface that it appears to consist of chromatin. It forms morphologically the most important part of the nucleus.

Certain nuclei contain one or more round bodies, which belong with the chromatic elements because of their deep staining, but which are chemically different from chromatin. These bodice, known as nucleoli, are stained with acid or neutral dyes. They are said to be composed of paranuclein, whereas chromatin is composed of nuclein. In distilled water the structures formed of nuclein disappear, but those consisting of paranuclein remain. The nuclei of nerve cells contain typical nucleoli (Figs. 3 and 5). Sometimes a nucleolus, lodged in the nuclear reticulum, is more or less covered with chromatin (Fig. 9, A), but the term should not be applied to irregular knots of chromatin, even when most of the chromatic material within a nucleus is gathered into one or two such bodies. These are the so-called false nucleoli (pseudonucleoli).

Every nucleus, therefore, consists of ground substance or nuclear sap, a network of linin, and granules and masses of chromatin. Usually it is surrounded by a membrane, and sometimes it contains a nucleolus. Most cells contain a single nucleus; but occasionally a single cell contains two nuclei, as is frequent in the liver, or even several nuclei, as in certain cells associated with bone. Non-nucleated bodies, like the mammalian red blood corpuscles, and the dead outer cells of the skin, have lost their nuclei in the course of development.

Functionally the nucleus is regarded as a center for chemical activities necessary for the life of the cell. It is believed to produce substances which pass out into the cytoplasm, where they may be further elaborated. Evidences of nuclear extrusions into the cytoplasm have been frequently recorded. But the interactions between nucleus and cytoplasm, of such nature that they cannot be observed under the microscope, are presumably of far greater biological importance.


Centrosome

The centrosome is typically a minute granule in the center of a small sphere of differentiated protoplasm. Often the term is applied to this entire structure, but it refers particularly to the central granule; the enveloping sphere is known as the attraction sphere, and it is composed of archoplasm^ When a cell is about to divide, delicate fibrils, either rearranged from the protoplasmic reticulum or formed anew, radiate from the archoplasm toward the periphery of the cell. The central granule becomes subdivided into two, which then move apart. In resting cells, or those which are not undergoing division, the centrosome may already have divided into a double body or diplosome preparatory to the next division of the cell (Fig. i).


Centrosomes have been detected in many forms of resting cells, and it is assumed by some authorities that the centrosome is an invariable constituent of the cells of the higher vertebrates. According to this opinion the centrosome may become inconspicuous but it never loses its identity. Often they are found very close to the nuclear membrane, which may be indented to accommodate them; and rarely, as in certain cancer cells and in one form of the worm Ascaris, they have been reported as within the nucleus. They may occur near the free surface of certain cells, usually in the form of diplosomes, as shown in cell a, Fig. 8. Just above the diplosome, such cells may send out contractile projections of protoplasm (pseudopodia), with the activity of which the! diplosome may be in some way associated.j Pseudopodia, with an underlying diplosome, have been observed in the columnar cells of the human large intestine. In cell b of Fig. 8 there are four diplosomes, one of which lies beneath the protoplasmic projections. It is believed that the diplosomes may multiply by fission, and that thus they may give rise to the numerous motile hairs, or cilia, which project from certain cells. Of these they form the basal bodies (Fig. 8, c). In many gland cells the centrosome lies in the midst of the protoplasm where the secretion accumulates. The discharge of the secretion is accomplished by the contraction of the protoplasmic strands in which the centrosome is lodged. In all these relations the centrosome appears to be a center for motor activities, and it is described as the kinetic or dynamic center of the cell.



FIG. 8. CELLS OF THE EFFERENT DUCTS OF THE TESTIS OF A MOUSE. (After Puchs.)

To show diplosomes, and (in c) cilia with basal bodies.


Cell Wall

The protoplasm at the surface of certain cells floating in the blood or lymph forms a thin pellicle, apparently as a result of protoplasmic concentration, or other reaction to the surrounding medium. Cells which line the greater part of the digestive tube, and have only one surface directed toward the intestinal contents, are provided with a thick wall on the exposed surface. Such a wall is called a cuticular border, or cuticula. On the other sides of these cells, the membrane is much thinner, and on the basal surface it is sometimes lacking. In such cases the protoplasm appears to be continuous with that of the underlying cells. In other cases the entire cell is devoid of any membrane. The cell membrane, therefore, is not an essential part of a cell; if present it ranges from a thin pellicle, on the border line of visibility, to a well-defined wall, which may be formed as a secretion of the underlying protoplasm. If the several surfaces of the cell are in relation to different environments, there is often a corresponding difference in the structure of their walls.

In examining a group of cells, it will be important to determine whether they are merely in contact, or actually continuous. Sometimes cells are so completely fused that their nuclei are irregularly distributed through a single mass of protoplasm. Such a formation is a syncytium in which the position of the nuclei is the only means of estimating the territory of a single cell. A syncytium may arise from the fusion of cells, or, as in striated muscle fibers, it may be due to the multiplication of nuclei in an undivided mass of protoplasm. Instead of being completely fused, cells are often joined to one another by protoplasmic processes of varying length and width, thus forming cellular networks. Fibrils within such a syncytium may pass continuously from the protoplasm of one cell into that of another.

Although cell membranes are often inconspicuous in animal cells, they cannot be overlooked in plants. Thus cork is a mass of dead cells from which nuclei and protoplasm have disappeared, leaving only the cell walls. In describing cork, Robert Hooke introduced the name "cell," in 1664. He wrote: "I took a good clear piece of Cork and with a Pen-knife sharpen'd as keen as a Razor, I cut a piece of it off, and thereby left the surface of it exceeding smooth, then examining it very diligently with a Microscope, me thought I could perceive it to appear a little porous. . . . These pores, or cells, were not very deep, but consisted of a great many little Boxes ."

In this way one of the briefest and most important of scientific terms was introduced.


Form and Size of Cells

Cells are regarded as primarily spherical in form. Spherical cells are comparatively numerous in the embryo, and in the adult the resting white blood corpuscles, which float freely in the body fluids, assume this shape. Such cells are circular in cross section. When spherical cells are subjected to the pressure of similar neighboring cells, they become polyhedral and usually appear six-sided in cross section. Such cells, as a whole, may be cuboidal, columnar, or flat. Certain cells become fusiform (spindle-shaped) or are further elongated so as to form fibers; others send out radiating processes and are called stellate. Thus the form of cells is extremely varied. The shape of the nucleus tends to correspond with that of its cell. It is usually an elliptical body in elongated cells, and spherical in round or cuboidal cells. In stellate cells it is either spherical or somewhat elongated. Crescentic nuclei, and others more deeply and irregularly lobed, are found in some of the white blood corpuscles and in giant cells.


The size of cells ranges from that of the yolks of birds' eggs which are single cells, at least shortly before being laid down to microscopic structures four thousandths of a millimeter in diameter. The thousandth of a millimeter is the unit employed in microscopic measurements. It is called a micron, and its symbol is the Greek letter /*. The small cells referred to are therefore four microns (4 ju) in diameter. The size of any structure in a section of human tissue may be roughly estimated by comparing its dimensions with the diameter of a red blood corpuscle found in the same section. These red corpuscles are quite uniformly 7.5 p. in diameter.

Cytomorphosis

Cytomorphosis is a comprehensive term for the structural modifications which cells, or successive generations of cells, undergo from their origin to their final dissolution. 1 In the course of their transformation, cells divide repeatedly, but the new cells begin development where the parent cells left off. Cell division, therefore, is an unimportant incident in cytomorphosis.

Cytomorphosis is a continuous advance in which four successive stages are recognized first, the stage in which the cells are undifferentiated ;_ second, the stage of specialization or differentiation: third, the stage of degeneration; and fourth, the stage in which the cells die and are removed. These may be considered in turn.

Undifferentiated cells, as can be seen in sections of young embryos, are characterized by large nuclei and little protoplasm. They multiply rapidly, but the rate of division declines with the gradual increase of the protoplasm and the consequent functional differentiation of the cell. In the adult, relatively undifferentiated cells are found in many situations, as, for example, in the deepest layer of the epidermis. As the cells at the surface die and are cast off, new ones come up from below to take their places. But since the basal cells can produce only epidermal cells, they are themselves partly differentiated. From this point of view the fertilized ovum, which can produce all kinds of cells, must be regarded, in spite of its size and great mass of yolk-laden protoplasm, as the least differentiated cell.

Differentiated cells may preserve a round or cuboidal form, but usually they are elongated, flattened, or stellate. The cytoplasm usually contains coarse granules, fibrils, masses of secretion or other special formations. As a result of their own protoplasmic activity, the cells of many tissues become surrounded by intercellular substances, which may far exceed in bulk the cells which produced them. Intercellular substances may be solid or fluid. When present in small amount they form thin layers of cement substance between closely adjacent cells; in large amount these substances constitute a ground work in which the cells are imbedded, as, for example, in cartilage and bone.

  • 1 The term cytomorphosis was introduced by C. S. Minot in 1901 in a lecture entitled "The Embryological Basis of Pathology" (Science, 1901, vol. 13, p. 494). Cytomorphosis is further discussed by Professor Minot in "The Problem of Age, Growth, and Death," published by G. P. Putnam's Sons, 1908.



Although the differentiation of cells is chiefly cytoplasmic, there is some evidence of corresponding nuclear changes. Thus while the muscle

cells of the salamander are elaborating complex fibrils, the nuclei become modified as shown in Fig. 9. The significance of the nuclear changes is unknown.

Degeneration is the manifestation of the approaching death of the cell. In nerve cells this process normally takes place very slowly. These cells remain active throughout life, and if destroyed, they can never be replaced. In many glands, in the blood and in the skin, however, the cells are constantly dying and new ones are being differentiated. In a few organs the cells perish, but no new ones form, so that the organ to which they belong atrophies. Thus a large part of the mesonephros (Wolffian body) disappears during embryonic life; the thymus becomes vestigial in the

adult; and the ovary in later years loses its chief function through the degeneration of its cells.

The optical effects of degeneration cannot at present be properly classified. In a characteristic form, known as "cloudy swelling," the cell enlarges, becoming pale and opaque. In another form the cell shrinks and stains deeply, becoming either irregularly granular or homogeneous and hyaline. The nucleus may disappear as if in solution (karyolysis, chromatolysis) ; or it may become densely shrunken or pycnotic, and finally break into fragments and be scattered through the protoplasm (karyorhexis). If the process of degeneration is slow, the cell may divide by amitosis. It may be able to receive nutriment which it cannot assimilate, and thus its protoplasm may be infiltrated with fat and appear vacuolated. It may form abnormal intercellular substances, for example, amyloid; or the existing intercellular substances may become changed to mucoid masses, or have lime salts deposited in them. Thus an impairment or perversion of function is often associated with optical changes in the cell substance.


FIG. 9. NUCLEI OF STRIATED MUSCLE FIBERS FROM YOUNG SALAMANDERS (NECTURUS). (Eycleshymer.)

A, From a 7 mm. embryo; B, from one of 26 mm.; ch, chroma tin knot; g. s, ground substance; 1, linin fibril; n, nucleolus; n.m, nuclear membrane.

The removal of dead cells is accomplished in several ways. Those near the external or internal surfaces of the body are usually shed or desquamated, and such cells may be found in the saliva and urine. Those which are within the body may be dissolved by chemical action or devoured by phagocytes.

Every specimen of human tissue exhibits some phase of cytomorphosis. In some sections a series of cells may be observed from those but slightly differentiated, to those which are dead and in process of removal. Because of the similarity and possible identity of this normal "physiological" regression, with that found in diseased tissues, such specimens should be studied with particular care.

Vital Phenomena

The vital properties of cells are fully treated in text-books of physiology. They include the phenomena of irritability, metabolism, contractility, conductivity, and reproduction. Under irritability may be grouped the response of cells to stimuli of various sorts, such as heat, light, electricity, chemical reagents, the nervous impulse, or mechanical interference. Metabolism, in a wide sense, includes the ingestion and assimilation of food, the elaboration and secretion of desirable products, together with the elimination of waste products. Contractility may be changes in form observed during ten minutes;


FIG. 10. LEUCOCYTES OF A PROG. * au.


, at the beginning of the observation;

manifest m the locomotion Of the J.a half minute later, etc.

entire cell, in the vibratile action of

slender hair-like processes, the cilia, or in contraction of the cell body. Conductivity is the power of conveying impulses from one part of the cell to another. Reproduction is seen in the process of cell division. Many phases of these activities are observed in microscopic sections and as such they will be referred to in later chapters. A few which are of general occurrence will be described presently.

Amoeboid Motion

The unicellular animal, Amoeba, exhibits a type of motility known as amoeboid, which has been observed in many sorts of cells in the vertebrate body. In marked cases, as in certain white blood corpuscles (the leucocytes), the cell protoplasm sends out fine or coarse processes which divide or fuse with one another, causing the cell to assume a great variety of forms. The processes may be retracted, or they may become attached somewhere and draw the remainder of the cell body after them, the result of which is locomotion or the so-called wandering of the cell. Such wandering cells play an important part in the economy of the animal body. Their processes can flow around granules or cells and thus enclose them in protoplasm. Some of these ingested bodies may be assimilated by the cell as a result of complex chemical and osmotic reactions. Cells which feed on foreign particles and can alter or digest them are known as phagocytes. Amoeboid movements take place very slowly. In preparations from warm-blooded animals they may be accelerated by gently heating the object.

Another form of motion is that which occurs within the protoplasm of fresh cells, whether living or dead, and consists in a rapid oscillation of minute granules, due to diffusion currents. Although these movements were first observed within protoplasm, it was soon shown that they occurred when various inert particles were suspended in a liquid. Robert Brown described the motion in 1828, in an essay entitled "On the General Existence of Active Molecules in Organic and Inorganic Bodies," and the phenomenon is called the molecular or Brownian movement. It may often be seen in salivary corpuscles.


Formation and Reproduction of Cells

In the past, two sorts of cell formation have been recognized, namely the spontaneous generation of cells, and the origin of cells through the division of pre-existing cells. According to the theory of spontaneous generation it was once thought that animals as highly organized as intestinal worms came into existence from the fermentation of the intestinal contents. After this had been disproved, it was still thought that unicellular animals arose spontaneously and that cells might be formed directly from a suitable fluid, the cytoblastema. Something of the sort may have occurred when life began, and it is the expectation of certain investigators that conditions may yet be produced which shall lead to the formation of organic bodies capable of growth and reproduction. At present, however, only one source of cells is recognized the division of existing cells. "Omnis cellula e cellula." A nucleus likewise can arise only by the division of an existing nucleus; it cannot be formed from nonnucleated protoplasm.


Amitosis

The simplest form of cell division is one which rarely occurs. Ordinarily the division of the cell is accompanied with the production of protoplasmic filaments, and the process is therefore called mitosis (Greek, /uVos, a thread). But in direct division or amitosis these filaments are not developed. The nucleus merely becomes increasingly constricted at the middle until divided in two; or it may be bisected by a deep cleft or fissure. Preceding the division of the nucleus, the nucleolus, if present, may subdivide and supply each half of the nucleus with a nucleolus (Fig. n). Cells which divide by this method are usually degenerating, and the process may terminate with the multiplication of nuclei. If carried to completion, the protoplasm also divides, and a cell membrane develops between the daughter nuclei. The role of the centrosome in amitosis has not been determined. Maximow finds it in a passive condition between the two halves of the nucleus, or beside the stalk connecting these halves if the division is not complete (Anat. Anz., 1908, vol. 33, p. 89). He states that certain mesenchymal cells which divide by amitosis in the rabbit embryo are not degenerating, but may later divide by mitosis, and thus he confirms Patterson's similar conclusion in regard to certain cells in the pigeon's egg. These instances are regarded as exceptional. In the human body the detachment of a portion of the lobate nucleus of certain leucocytes has been described as amitotic division, but the superficial cells of the bladder furnish more typical examples. E. F. Clark has found many cells dividing by amitosis hi the degenerating parts of a human cancer. The occurrence of two nuclei within one cell by no means indicates this form of division. Associated with such cells, others containing nuclei of the dumb-bell shape, or those partially bisected by clefts must be found, in order to prove that amitotic division is taking place.



FIG. ii. AMITOSIS ix EPITHELIAL CELLS FROM THE BLADDER OF A MOUSE. Xs6o. Such preparations as that shown in the figure are made by pressing the lining of a freshly obtained piece of the bladder against a clean cover-glass. Certain of the superficial cells adhere to it, and they are then fixed and stained.


Mitosis

Mitosis, also called indirect division and karyokinesis, is the ordinary mode of cell division. Although it is a continuous process, it has been conveniently divided into four successive phases the prophase, metaphase, anaphase, and telo phase. During the prophase the chromatic material of the nucleus prepares for division and collects in the center of the cell. It is divided in halves in the metaphase, and the two halves move apart during the anaphase. The chromatic material becomes reconstructed into resting nuclei during the telophase. The various patterns which the chromatic material and protoplasmic fibrils present during these phases are known as mitotic figures.

Mitotic figures are found hi all rapidly growing tissues, but especially favorable for preliminary study are the large cells in the root tips of plants. In longitudinal sections of root tips, the cells are cut at right angles to the plane of cell division, which is desirable; and often in a single section 5 mm. long, all the fundamental stages may be quickly located. The following general description of mitosis is based upon such easily obtained preparations, and the plant selected is the spiderwort (Tradescantia virginiana). 1 They may be satisfactorily stained with saffranin, or with iron haematoxylin and a counter stain such as orange G. There are many descriptions of mitosis in root tips, among them the following:

Rosen, (Hyacinthus oriental/is) Beitr. zur Biol. der Pflanzen, 1895, vol. 7, pp. 225-312; Nemec, (Allium cepa} Sitz.-ber. kon. Ges. der Wiss. Prag, 1897, No. 33, pp. 25-26, and Jahrb. fur wiss. Bot., 1899, vol. 33, pp. 313-336; Schaffner, (Allium cepa) Bot. Gaz., 1898, vol. 26, pp. 225-238; Hof, (Ephedra major) Bot. Centralbl., 1898, vol. 76, pp. 63-69, 113-118, 166-171, 221-226; Gregoire and Wygaerts, (Trillium grandiflorum) La Cellule, 1904, vol. 21, pp. 1-76; Farmer and Shove, (Tradescentia mrginiana) Quart. Journ. Micr. Sci., 1905, vol. 48, pp. 559-569; Richards, (Podophyllum peltatum] Kansas Univ. Sci. Bull., 1909, vol. 5, p. 87-93.

The cells to be described are found in the interior of the root tip, just back of the protecting cap of cells which covers its extremity. They are oblong in shape and their long axis corresponds with that of the root. The walls are very distinct, and the cells consist of granular vacuolated protoplasm, which in preserved specimens is generally irregularly shrunken.

The resting cells (Fig. 12, A) contain large round nuclei in which the chromatin is in the form of fine granules evenly distributed throughout the nucleus. A nucleus usually contains from two to five round nucleoli, each of which, when in focus, is seen to be surrounded by a clear zone. The nuclear membrane is distinct.

  • 1 Good specimens may be obtained from any rapidly growing root tip. Those starting from hyacinth bulbs placed in water are very favorable. Onion root tips have been extensively used, and also those of bean and corn seedlings. The pointed ends are snipped off and dropped into Flemming's stronger solution


Prophase. The first indication of approaching division is a change in the chromatin, which becomes gathered into fewer and coarser granules and takes a deeper stain. Portions of the linin network break down, so that the chromatin granules come to be arranged in long convoluted threads. Such threads are developing in the cell, Fig. 12, B, but are more perfectly formed in C. It is possible that at a certain stage the nucleus contains only a single continuous thread, but this condition cannot be demonstrated in Tradescantia. The stage of nuclear division in which the chromatic material appears to be arranged in a coiled thread or skein is called a spireme. The "close spireme" (B) is succeeded by the "loose spireme" (C). Successive stages in the development of the spireme in animal cells are seen in Fig. 20, D, E, and F.

As the spireme develops, the nuclear membrane becomes less distinct, and the clear zones disappear from around the nucleoli. The nucleoli become apparently less regular in outline, and forms which suggest that two of them have fused (Fig. 12, B) are perhaps more frequently seen than in resting cells. Usually it is stated that the nucleoli break up into smaller bodies toward the time of their dissolution, and that some of these escape into the cytoplasm after the disappearance of the nuclear membrane. Farmer and Shove believe that the nucleoli contribute to the chromatin; Richards regards them as a store of food material for the rest of the cell; and others believe that they form the achromatic "spindle" which will be described presently. Their function in animal cells is equally uncertain.

In the stage shown in Fig. 12, D, which may be regarded as the end of the prophase, the nuclear membrane and the nucleoli have disappeared, and the spireme thread has become divided into a number of segments or chromosomes. These are straight or curved rods of different lengths. Sometimes they appear as bent V-shaped bodies, but these often represent two chromosomes with their ends together. J-shaped forms, with one long and one short arm, have been described in various plants. The chromosomes become so arranged that one end of the rods, or the apices of the V's, are situated in the equatorial plane, which extends transversely across the middle of the cell. Often it is temporarily tilted (as in D and E) as if the mitotic apparatus had shifted to a position in which it obtained more space. It may do this mechanically if the contents of the cell are under pressure. When the chromosomes are gathered at or in the equatorial plane, they constitute collectively the equatorial plate. Because of their stellate arrangement at this stage, which is best seen in transverse sections of the cell, this mitotic figure is known as the aster.

The manner in which the chromosomes are formed from the spireme thread is difficult to determine. According to Gregoire and Wygaerts, the linin and chromatin, which have often been regarded as closely related


i6


HISTOLOGY


substances, are identical, and linin is merely a name for slender filaments of chromatin. Accordingly the chromatin simply draws together to






FIG. 12. MITOTIC CELL DIVISION IN THE ROOT TIP OF Tradescanliavirginiana. Xias diam. A, resting cell; B, C, D, prophase; E, metaphase; F, anaphase; G, H, I, telophase.

form chromosomes, and the beaded appearance of the spireme thread is due to alternate enlargements and constrictions of one substance. Others consider that a different substance connects the granules of chromatin


INDIRECT DIVISION


with one another; and Rosen states that each chromatin granule is completely imbedded in a broad strand of linin. Davis similarly interprets the spireme shown in Fig. 20, F. Whatever the actual structure may be, the chromatin granules in the spireme thread early divide in two, so that the thread appears double. When the thread shortens and condenses to form the chromosomes, the rows of granules may coalesce so as to produce a rod already divided lengthwise, although its halves are in close apposition. Occasionally the ends of the chromosomes are seen to be slightly separated.

Metaphase. In the metaphase (Fig. 12, E), the two longitudinal halves of each chromosome are being drawn apart toward the opposite poles of the cell. If the chromosome is V-shaped, the separation of the two halves begins at the apex of the V.

At this stage an achromatic figure, known as the spindle, is evident in plant cells, but it is more sharply defined in those of animals. As seen in the diagram (Fig. 13), it consists of fibrils which pass from the equatoria

Polar radiation. Nuclear spindle.



FIG, 13. EARLY METAPHASE.


FIG. 14. LATE METAPHASE.


plate toward either pole, where, in animal cells, there is a well-defined granule, the centrosome. Around each centrosome there are radiating protoplasmic fibrils, forming the polar radiation (Figs. 13 and 14). The polar radiation is also called an aster, and the two asters connected by the spindle are known as the amphiaster. Some of the spindle fibers are attached to the chromosomes and appear to pull their halves apart; others pass from pole to pole without connecting with the chromosomes. In animal cells the spindle arises as the two centrosomes, lying beside the nucleus, move apart (Fig. 20, A). As they pass to the opposite poles of the nucleus, the spindle forms between them, either from the nuclear reticulum, or the cytoplasmic reticulum, or hi part from both. These conditions appear to vary in different animals.

In the cells of root tips, a condensation of protoplasm forms a cap at the poles of the nucleus at the time when the nuclear membrane and nucleoli are disappearing. From the "polar cap," spindle fibers develop


l8 HISTOLOGY

which invade the nucleus, and also radiations which have been traced even to the cell walls. But as Rosen states, sun-like figures, such as certain botanists have pictured, do not occur. Schaffner has described a distinct centrosome or central granule in the root tip of the onion, but Richards finds that in Podophyllum there is no such structure, and the weight of evidence appears to be against the existence of a definite centrosome in the higher plants.

Anaphase. In the anaphase the halves of each chromosome move to the opposite poles (Fig. 12, F). The figure thus produced is known as a double star or diaster. Since each chromosome has divided into two, the original number of chromosomes is preserved, and an equal number of rods will be found in either star. They cannot all be brought into focus together, and because of overlapping, they are hard to count. Sometimes one chromosome, longer than the others, remains for a time as a continuous bar from one aster to the other. Between the asters there are always straight spindle fibers, but they vary in distinctness. (The anaphase in an animal cell is well shown in Fig. 21, D.)

Telophase. After the chromosomes have reached the opposite poles, they form two dense masses. They are generally said to unite end to end, thus forming a spireme thread. But in the root tips of Trillium, Gregoire and Wygaerts state that they come into contact with one another laterally; and as they separate, transverse connections are retained, which, with the vacuolization of the chromosomes, restore the nuclear reticulum. This may not be the correct interpretation, but immediately after the anaphase the chromosomes form a very compact mass, easily overstained so that it appears solid. Subsequently the mass enlarges (Fig. 12, H), and the chromosomes become coarsely granular, taking the form of wide bands. Nucleoli reappear, and according to Richards, "it is a general rule that they arise on the side of the nucleus nearest the new cell wall." This accords with Nemec's statement that they form from the outer fibers of the spindle. Nemec and Rosen agree that they first appear outside of the nucleus, which they enter before the nuclear membrane develops. These are details which require confirmation.

The new cell wall arises in plants as a series of thickenings of the interzonal spindle fibers, which at this stage form a barrel-shaped bundle (Fig. 12, G). The thickenings coalesce to form a membrane which does not at first reach the sides of the cell. While this wall is developing the nuclei are in a condition resembling the spireme stage of the prophase. The entire mitotic figure is therefore called the double spireme or dispireme. The cell wall is soon completed and the nuclei return to the resting condition (Fig. 12, I).

The time required for mitotic cell division varies from half an hour (in man) to five hours (in amphibia). After death, if the tissues are not


INDIRECT DIVISION 19

hardened by cold or reagents, it is thought that mitoses go on to completion. Forty-eight hours may elapse before they entirely disappear from the human body.

Pluri-polar mitosis. Under abnormal conditions, as in the cancer cells shown in Fig. 15, spindles may develop simultaneously in connection with three or four centrosomes. Similar pluri-polar spindles have been produced experimentally, by treating cells with various poisonous solutions. An unequal distribution of chromatin may occur under such conditions, and this may happen also with bipolar spindles, as shown in Fig. 15, a.

Number and individuality of the chromosomes. It is now generally believed that every species of plant or animal has a fixed and characteristic number of chromosomes, which regularly recurs in the division of all



FIG. 15. MITOSES IN HUMAN CANCER CELLS. (From Wilson, after- Galeotti.) a, Asymmetrical mitosis with unequal distribution of chromatin; b, tripolar mitosis; c, quadripolar" mitosis.


its cells, with the exception of the germ cells, in which the number is reduced. In certain species, however, the two sexes regularly differ from one another in the number of their chromosomes, and one sex may contain an odd number. Usually the number of chromosomes is believed to be even.

There is considerable difficulty in counting the chromosomes. Generally it is possible that some have been cut away in the process of sectioning, so that, if the number is believed to be invariable, the highest number found in any cell is assumed to occur regularly. Another source of error lies in the fact that a bent chromosome may be counted as two, or rods with their ends overlapping may appear as one. Farmer and Shove have ventured to state that the number in Tradescantia "varies from about twenty-six to thirty- three." Nemec found that twelve chromosomes occur regularly in young tissues of the onion, but that in older tissues the number diminishes even to four. Sixteen have been recorded in the onion by other botanists. Podophyllum is said to have sixteen (Mottier), but Richards records counts of fourteen. In man the number has been placed at 16 and 32, but it is now believed to be 24. Gutherz, with particularly favorable material, emphasizes the difficulty of counting


20 HISTOLOGY

the chromosomes in man. He found only two cells in which a count could be made, in neither case with absolute certainty. But he agrees with Duesberg that the reduced number is twelve, according to which the whole number should be twenty-four. Recently, however, Wieman has found cells in the brain of a 9-mm. human embryo which contained 33 chromosomes. Some cells in the nasal epithelium and mesenchyma of this specimen contained 34, and others 38. Thus Wieman concludes that the number in man is certainly greater than 24 and is perhaps variable (Amer. Journ. Anat., 1913, vol. 14, pp. 416-471).

In the grasshoppers, which are among the most favorable objects for the study of mitosis, not only is the number of chromosomes for a given species believed to be constant, but each cell appears to contain a definite series of chromosomes, the members of which vary somewhat in shape and size. Recent studies of such cells favor Rabl's hypothesis of the individuality oj the chromosomes, according to which the chromosomes persist in the resting nucleus, although disguised by their lateral branches and diffuse granular form. If this hypothesis is correct, when a nucleus prepares for division the same chromosomes which entered it will reappear. Sometimes in the prophase the bands of chromatin are arranged hi a polar field such as is seen in the telophase (Fig. 12, H). This arrangement has been observed by Farmer and Shove in the prophase of Tradescantia, and by others in various plants and animals. It is regarded as evidence that the chromosomes are "independent and continuously perpetuated organs of the cell." Nevertheless it is generally true that in resting nuclei no trace of individual chromosomes can be made out. The great importance of accurate knowledge of the chromosomes is shown by the following considerations.

As a result of mitotic cell division, it is evident that every new cell regularly receives one-half of each chromosome found in the parent cell, and thus the number of chromosomes remains constant. But in the germ cells the number is invariably reduced, and hi some animals it becomes exactly one-half of the number found elsewhere in the body. In such a case, when the male sexual cell, or spermatozoon, unites with the female sexual cell, or mature ovum, in the process of fertilization, the original number is restored. Each parent thus contributes one-half of the chromosomes found in the cell which gives rise to a new individual; and since each of these divides with every subsequent cell division, it is evident that one-half of the chromatin in every cell of the adult body is of maternal origin and one-half of paternal origin. The process by which the sexual cells acquire the reduced number of chromosomes and become ready for fertilization is known as maturation. The production of the sexual cells in the male is called spermatogenesis and in the female oogenesis.



FIG. 16. DORSAL (a) AND LATERAL (b) VIEWS OF THE ABDOMEN OF GRASSHOPPERS. (After Hyatt and Scudder.)


Spermatogenesis

In its essential features, the process of spermatogenesis in insects corresponds with that in mammals, and very favorable material can be obtained in abundance from grasshoppers of various genera.

The males may be distinguished from the females by the shape of the abdomen. In males it is more rounded (Fig. 16) with various appendages directed dorsally. The abdomen of the female is pointed, terminating in the ovipositor, the parts of which as seen from the side may be together, or widely separated dorso-ventrally. The genital glands can be readily removed by dissecting as follows: Male grasshoppers, which have been chloroformed, are opened by a mid-ventral incision. The abdominal walls are pinned out on a wax plate under normal salt solution (0.6 per cent.). The intestinal tube, which is usually black or green, is taken out with forceps, and the yellow or orange testes are seen close together at the upper end of the abdomen, attached to the back. Each testis consists of a number of separate cylindrical lobes, and it should be worked loose from the surrounding tissue with forceps in such a way that these lobes remain together. The tissue may be preserved in Flemming's strong solution or in Hermann's fluid, and stained with iron haematoxylin.


Among the many publications upon spermatogenesis in the grasshoppers, the following may be cited: McClung, C. E., The accessory chromosome sex determinant? Biol. Bull., 1902, vol. 3, pp. 43-84; Sutton, W. S., On the morphology of the chromosome group in Brachystola magna, Biol. Bull., 1902, vol. 4, pp. 24-39; McClung, C. E., The chromosome complex of orthopteran spermatocytes, Biol. Bull., 1905, vol. 9, pp. 304-340; Robertson, W. R. B., The chromosome'complex of Syrbula admirabilis, Kansas Univ. Sci. Bull., 1908, vol. 4, pp. 273-305; Davis, H. S., Spermatogenesis in Acrididae and Locustidae, Bull. Mus. Comp. Zool., 1908, vol. 53> PP- 57-IS7; Wilson, E. B., The sex chromosomes, Arch, fur mikr. Anat., 1911, vol. 77, pp. 249-371.


As seen in sections, each lobe of the testis of the grasshopper contains a considerable number of closed sacs or cysts, which are filled with sexual cells; and all the cells within a cyst are in approximately the same stage of development. The cysts are shown in Fig. 17, which represents a longitudinal section of a single lobe. Usually the testis is sectioned as a whole, and the specimen consists of a group of lobes cut transversely or obliquely. Cross sections from the apical portion, furthest from the outlet, will contain younger stages than the sections lower down in the lobe, since the cysts form at the apex and gradually move downward. At the apex, according to Davis, there is an apical cell which is surrounded by young sexual cells known as spermatogonia (Fig. 17, a). The spermatogonia move away from the apical cell, and each becomes enclosed in a cyst-wall derived from the surrounding tissue. Within the cysts thus formed, the spermatogonia multiply, and the cysts in the upper part of the lobe are filled with spermatogonia (Fig. 17, b). After repeated divisions the spermatogonia pass through a period of growth, accompanied by a rearrangement of their nuclear contents. The large cells with characteristic nuclei which are thus produced, are known as primary spermatocytes. They fill the cysts further down in the lobe (Fig. 17, c). Each primary spermatocyte divides into two secondary spermatocytes, and each of these divides into two spermatids, after which no further cell division is possible until fertilization takes place. But each spermatid becomes transformed from a round cell into a linear body with a whip-like tail, and is then capable of independent motion. Since in this form these cells were once thought to be parasitic animals living in the spermatic fluid, they received the name spermatozoa, which they still retain. 1 Cysts containing spermatozoa occur near the outlet of the lobe, or if the grasshoppers are collected late in the season, they may be found throughout most of the testis. Specimens from young grasshoppers, in which the spermatocyte divisions are abundant, are more desirable, even though no spermatozoa have become fully developed.


FIG. 17. LOBE OF THE TESTIS OF A GRASSHOPPER. Xso. (After Davis.)

a, apical cell.

b, spermatogonia.

c, spermatocytes.

d, spermatocytes dividing.

e, spermatids.

f, spermatozoa.


Spermatogonia


The succession of cell divisions described in the preceding paragraph is shown in tabular form in Fig. 18. Except for the number of chromosomes within the various cells, this diagram is quite as applicable to man as to the grasshopper. In this figure, however, only two spermatogonial divisions have been included. The number of times which the spermatogonia may divide before becoming spermatocytes is considerable and

1 It has been proposed to substitute the term spermium for spermatozoon; and consequently spermiocyte, spermid, etc., for spermatocyte and spermatid. The secondary spermatocytes are sometimes called praespermatids or praespermids; but these changes in names are of questionable value



Secondary Spermatocytes


Spermatozoa


FIG. 1 8. DIAGRAM OF THE CELL DIVISIONS IN SPERMATOGENESIS. The figures indicate the number of chromosomes found in the cells of certain grasshoppers.


presumably indefinite. As seen in sections, the spermatogonia, spermatocytes, and spermatids may be described as follows, using for illustrations Davis's figures of a common grasshopper Dissosteira Carolina.

Spermatogonia. The nucleus of each spermatogonium contains the full number of chromosomes, which in most of the grasshoppers (Acrididee) is 23. With every spermatogonial division, each chromosome is split lengthwise. In this and other respects the mitotic figures are quite like those occurring elsewhere in the body. They are shown in Fig. 20, A, B, and C. When the twenty- three chromosomes have formed the equatorial plate, it is sometimes possible to see all of them in a single transverse section of the cell (Fig. 19, A). It then appears, as found by Montgomery (1901) in certain Hemiptera, and a year later by Button in grasshoppers, that the chromosomes vary in size, but the "gradations in volume are not between individual chromosomes but between pairs, the two members of which are of approximately equal size." In Fig. 19, A, twelve forms of chromosomes have been identified by Davis; and all of these are paired except the one numbered 4. The members of a pair are often, but by no means invariably, side by side. In some cases, owing to foreshortening, their resemblance in size is not apparent in the drawing. The behavior of the odd or accessory chromosome is of special interest, since according to McClung's hypothesis, now well established, this accessory chromosome is the bearer of those qualities which determine sex.

Primary spermatocytes. After the last spermatogonial division, the cells begin their "growth period." At this time the chromatin tends to collect on one side of the nucleus, in a condition known as synapsis (or more recently as synizesis). This distribution of the chromatin has been frequently observed, but it has not been shown to be of special significance. In the primary spermatocytes drawn in Fig. 20, D, E, and F, the chromatin is evenly distributed. All of the chromosomes, except the accessory chromosome, have become filamentous, but the accessory chromosome remains as a compact, darkly staining body close to the nuclear membrane. It resembles a nucleolus, for which in fact it has been mistaken. True nucleoli may occur in these cells, together with the accessory chromosome, but they stain differently.

As the primary spermatocytes prepare for the next division, the spireme becomes resolved into eleven loops, each of which represents the two members of a pair of chromosomes joined end to end. The granules imbedded in the linin thread divide as usual, so that each loop contains a double row of granules (Fig. 20, F). These loops contract to form eleven chromosomes, which, because of their four parts, are known as tetrads. The structure of the tetrads is shown in Fig. 19, B-G. The filaments seen in the upper row of drawings contract into corresponding solid forms of chromosomes seen in the lower row, in which the place of attachment to the spindle fibers has been indicated.

Each tetrad represents two chromosomes joined end to end and split lengthwise. The simplest forms are shown in Fig. 19, B and C, which illustrate respectively two ways in which the tetrad may later divide. The two component chromosomes may simply be pulled apart, as indicated in Fig, 19, B, in which the spindle fibers are attached to the ends of the rod. If this takes place, each secondary spermatocyte will receive one member of every pair of chromosomes which occurred in the spermatogonium, but no part of the other member. Such a division, which eliminates one-half of the chromosomes from the daughter cell, is known as a reductional division. The other form of chromosome division is known as equational. When it takes place, every chromosome divides lengthwise, and the daughter cells receive one-half of every chromosome in the parent cell. This occurs in ordinary cell division, and also in the division of the tetrads provided that the spindle fibers are attached to the place where the two component chromosomes come together (Fig. 19, C).



FIG. 19. A, POLAR VIEW OF THE METAPHASE OF A SPERMATOGENIAL DIVISION IN Dissosteira Carolina. X 1450 (After Davis.) The pairs of chromosomes have been numbered. B-G, various forms of tetrads, rom primary spermatocytes. (After Davis and Robertson.)

As a stage in the separation of the two halves of a rod-shaped tetrad, crossshaped forms are produced (Fig. 19, D). If the separation is almost complete, such shapes are seen as in Fig. 19, E. The arms of the tetrad which are not attached to the spindle fibers may bend toward one another and unite, so as to form rings (F), or they may twist about like a figure 8, as shown in G. If the spindle fibers are attached to the points xx in the upper figure in G, the division will be equational; if as shown in the lower figure it will be reductional.

Usually it is considered that the division of the tetrads into double bodies or dyads, is equational, and that the division of the dyads, which takes place when the secondary spermatocytes divide, is reductional. According to Davis, however, the first division of the tetrads is reductional and the second division is equational. In either case the end-result is the same. Each spermatid will contain one of the four parts of each tetrad, and thus one member of every pair of chromosomes will be eliminated from any given spermatid.


Since in the testis tetrads occur only in the primary spermatocytes, the cells shown in Fig. 20, G-J, are easily identified. These are success ive stages in the division of the primary spermatocyte. In G the accessory chromosome is seen as a rod-shaped body above and to the right; in H it is below and to the right. In J it is obliquely placed just above the equatorial plate and in K it is passing to the upper pole of the spindle. In the spermatogonial divisions the accessory chromosome always divides with the others; but in the division of the primary spermatocyte it passes undivided into one of the daughter cells. Thus one secondary spermatocyte will receive eleven chromosomes (dyads) and the other will receive twelve (eleven dyads and the accessory chromosome) . In the late anaphase shown in Fig. 20, L, the accessory chromosome cannot be recognized.


FIG. 30. SPERMATOGENESIS IN Dissosleira Carolina A-FXI4SO; G-LX966. (Davis.)

A, B, C, prophase. metaphase. and telophase of a spennatogonial division. D-L, successive stages in the

.division of a primary spermatocyte into secondary spermatocytes.


Secondary spermatocytes. The secondary spermatocytes pass rapidly from the condition shown in Fig. 20, L, to that of Fig. 21, A. A nuclear membrane has developed, and the dyads have become somewhat filamentous. Without passing through a complete resting stage they proceed to divide as shown in Fig. 21, B-F. The dyads separate into their component halves. In those secondary spermatocytes which received the accessory chromosome, that body will be seen dividing with the dyads, and each spermatid will receive one-half of it. It has been questioned whether the division of the accessory chromosome is longitudinal and therefore equational, or transverse and reductional. Many cytologists consider that if a chromosome splits lengthwise, all of its parts will be represented in the resulting halves, but if it divides transversely, essential elements will be lost. This conception lends importance to the question of transverse or longitudinal division of the accessory chromosome. By the division of this chromosome it comes about that one-half of the spermatids contain twelve chromosomes, and one-half contain eleven, as indicated in the diagram, Fig. 18. The spermatids shown in Fig. 21, F, contain the accessory chromosome.

Spermatids and Spermatozoa. In forming spermatozoa, the spermatids become elongated, passing from the condition shown in Fig. 21, F, to that of Fig. 21, G. The chromatin within the nucleus is distributed in fine granules throughout the linin reticulum. Close to the nuclear membrane a small dark body has appeared, from which a slender filament has grown out. This body is usually described as the centrosome. A condensation within the cytoplasm, seen also in F, is known as the paranucleus. It is of uncertain origin, but may proceed from the cytoplasmic structure called mitochondrium. The paranucleus forms a sheath about the axial filament.

Successively later stages are shown in Fig. 21, H, I, and J. The chromatin within the nucleus becomes homogeneous and very dense; at the same time the nucleus elongates and forms the head of the spermatozoon. This is enveloped by the cell membrane, but there is no appreciable layer of protoplasm around it. The centrosome elongates and forms the middle piece of the spermatozoon; and the axial filament, with a covering derived from the paranucleus and cytoplasm, constitutes the tail. Only a portion of the tail is included in the figure. The human spermatozoon likewise consists of a head, which is essentially the nucleus, a middle piece containing the centrosome, and a tail; but the form of the head is very different from



FIG. 21. SPERMATOGENESIS IN Dissosteira Carolina. Xi4So. (Davis.)

A-F, successive stages in the division of a secondary spermatocyte into spermatids. G-J, successive stages in the transformation of spermatids into spermatozoa.

that in the grasshopper. It will be described in a later chapter

Although the spermatozoa of the grasshopper appear alike, it has been shown that one-half of them contain eleven chromosomes, and one-half contain twelve. The mature ova all contain twelve chromosomes. If a spermatozoon with eleven chromosomes unites with an ovum with twelve, a male animal will be produced, in every cell of which there will be twentythree chromosomes. But if the spermatozoon contains twelve chromosomes, a female animal is formed, containing twenty-four chromosomes in every cell. Thus sex appears to be determined by the presence or absence of a chromosome within the spermatozoon.

In some cases, as in several Hemiptera described by Wilson, the accessory chromosome is paired, but its mate is of small size. Thus the spermatozoa all have the same number of chromosomes; but half of them contain the large member of the pair and will produce females, and the other half contain the small member and will produce males. The mature ova all contain the large member. In tfyese insects, therefore, both sexes contain the same number of chromosomes, but the cells of the male contain a small chromosome, whereas the corresponding one in the female is large. From these observations it is reasonable to conclude that sex may be determined by a difference in the nature of certain chromosomes in those animals in which there are no appreciable differences in size or number.

In man, a difference in the number of chromosomes in the sexes has been reported, but the observations have not been confirmed. It is supposed that the spermatogonia contain twenty-four chromosomes, but it has not been shown that they exist as pairs. The spermatocytes, spermatids and spermatozoa apparently contain twelve. As the principal constituents of the spermatozoon, the chromosomes are believed to be the essential agents in the transmission of all qualities inherited from the male parent, and certain of them may determine sex.


Oogenesis

Mature ova result from a succession of cell divisions closely comparable with those which produce spermatozoa. The primitive female sexual cells correspond with the spermatogonia, and are called oogonia. They are provided with the full number of chromosomes, and divide an indefinite number of times. After a period of growth they become primary oocytes, in which the number of chromosomes is reduced one-half. The primary oocytes divide to form secondary oocytes; and these again divide to produce the mature ova, which are incapable of further division unless fertilization takes place. (The term ovum is ordinarily loosely applied, so that it includes not only the mature cells, but also oocytes, and the clusters of cells resulting from the division of the fertilized ovum.)


Although the mature ovum and the spermatozoon are closely similar in their nuclear constitution, they differ radically as to form, size, and cytoplasmic structure. The ova are very large cells, stored with nutriment for the embryo which each one may later produce. In the higher vertebrates they are formed in relatively small numbers. According to Hensen's estimate, about two hundred, ready for fertilization, are produced by the human female in a life-time. But the male, according to Lode, produces 340 billion spermatozoa, or, as stated by Waldeyer, nearly 850 million per ovum. A large number must be produced, since many will fail to traverse the uterus and tube so as to find the ovum at the time of its discharge from the ovary. The ova of lower vertebrates, which are fertilized and develop outside of the body, are discharged in great numbers; in certain fishes from three to four million are produced annually.


The multiplication of oogonia in the human ovary takes place before birth, and about fifty thousand are produced. At birth, or shortly thereafter, all the oogonia have become primary ob'cytes (Keibel). At first the oocytes are small, but they enlarge at varying rates, and the largest are indistinguishable from mature ova except by their nuclear contents. Since some grow more rapidly than others, the ovary in childhood contains primary oocytes of many sizes. Each oocyte becomes enclosed in a cyst or follicle. The way in which these follicles develop, and the. manner in which the oocyte escapes into the uterine tube by the rupture of these follicles, will be described in connection with the ovary. Between the cells of the follicle and the oocyte, there is a broad, radially striated membrane, known as the zona pellucida or zona radiata (Fig. 2 2) . This zona has sometimes been regarded as a cell membrane, but the oocyte divides within it as if enclosed in a capsule. It does not invest the daughter cells like a membrane. The radial striations have been interpreted as slender canals containing processes of the f ollicular cells, and the zona has been considered as a product of these cells. In certain cases a perivitelline space has been described as encircling the oocyte and thus separating it from the zona, but this space has been considered as artificial, or as a refractive line wrongly interpreted as a space.

The cytoplasm of the oocyte becomes charged with yolk granules or spherules. They constitute the deuteroplasm (or deutoplasm), but this term is equally applicable to fat droplets and other secondary products of the protoplasm. In the human oocyte the granules are centrally placed (Fig. 22), and they are so transparent, when fresh, as to cause only a slight opacity. In the eggs of many animals the yolk is more highly developed, and it may be evenly distributed or gathered at one pole. Within the cytoplasm of the developing oocyte, a large dark body of radiate structure is sometimes conspicuous. It is inappropriately known as the yolk nucleus, and is probably a derivative of the centrosome and surrounding archoplasm. Other "vitelline bodies," of uncertain origin and significance, have been described. Some have been considered as nuclear extrusions.

The nucleus of the oocyte is very large and vesicular. The chromatin occurs chiefly along the nuclear membrane and about the nucleolus. The nucleous is also very large, and Nagel stated that in the fresh condition it exhibits amoeboid movements, but this observation has not been verified. The nuclei of the oocytes ordinarily show no signs of mitosis, and they may remain in the resting condition for thirty years or more and then divide. Many of them, however, will degenerate without division.


FIG. 22. THE OVUM AS DISCHARGED FROM A VESICULAR FOLLICLE OF AN EXCISED OVARY OF A WOMAN THIRTY YEARS OF AGE. Examined fresh in the liquor folliculi. (Nagel.)

c. i., Corona radiata composed of cells of the follicle; n., nucleus; p., granular protoplasm; p. s., perivitelline space; y., yolk; z. p., zona pellucida. (From McMurrich's "Embryology.")


The cell divisions which give rise to the secondary oocyte and the mature ovum respectively, have never been observed in man. Some of the cells within the ovary may be secondary oocytes and the cell shown in Fig. 22 may be of this sort, or possibly a mature ovum, but this cannot be determined. From what is known of other mammals, however, it may confidently be assumed that the cell divisions take place as shown in the diagram, Fig. 23.

When the primary oocyte divides, the chromosomes, reduced in number, also divide and are equally distributed to the daughter cells; but the great mass of cytoplasm remains with one of these cells, namely, the secondary oocyte. The other cell, which is relatively very small, is known as the first polar body, or polar cell. It has the same nuclear contents as the secondary oocyte, and may divide into two other polar bodies, equivalent to mature ova. More often it degenerates without division. When the secondary oocyte divides, it likewise produces one large cell, the mature ovum, and one small cell, the second polar body. The latter is said to be capable of fertilization, but to what extent it may develop is unknown. Functionally the production of polar bodies serves to prevent the subdivision and distribution of the nutritive material elaborated within the primary oocyte. One mature ovum with abundant yolk is provided at the expense of three ova (polar bodies) which degenerate.

Although the maturation of the ovum has not been observed in man, nor even the presence of definite polar bodies, the entire process has been carefully studied in other mammals, notably in the mouse. 1 It has been shown that the maturation of the ovum of the mouse takes place rapidly, both of the oocyte divisions being accomplished within from four to fifteen hours. The first polar body usually forms before the oocyte is discharged from the ovarian follicle in other words, before ovulation takes place. The second polar body is usually formed in the uterine tube, after the spermatozoon has entered the oocyte. Long and Mark have found that the chromosomes of the primary oocyte are tetrads, or bodies showing transverse and longitudinal divisions; and that those of the secondary oocyte are dyads. They believe that the first division is transverse or reductional, and that the second is equational.



FIG. 23. DIAGRAM OF THE CELL DIVISIONS IN OOGENESIS. (Compare with Fig. 18.)


  • 1 Among the most important papers are: Sobotta, J., Die Befruchtung und Furchung des Eies der Maus. Arch. mikr. Anat., 1895, vol. 45, pp. 15-91. Long, J. A., and Mark, E. L. The maturation of the egg of the mouse. Carnegie Inst. Publ. No. 142, 1911, pp. 1-72.


The difficulty of counting chromosomes is apparent from the varying numbers which have been reported in the mouse After reduction the number has been placed at 8, 12, 16, 18 and 20 by different observers.

The polar bodies in the mouse are relatively large. In the upper part of Fig. 24, A, a polar body is about to be formed, and it is completely cut off from the oocyte in Fig. 24, C. In D and G, two polar bodies are shown.

Fertilization

In the mouse, from six to ten hours after coitus, spermatozoa have made their way to the distal end of the uterine tube, where fertilization takes place. According to Long and Mark, the maturation of ova usually occurs at some time during the period from "13! to 28^ hours " after the mouse has given birth to a litter; and during the process of their maturation, the oocytes are discharged from the ovary and enter the distal end of the tube. Here, if fertilization takes place, a single spermatozoon penetrates the zona pellucida. In a section obtained by Sobotta, the entrance of the spermatozoon has been partially accomplished (Fig. 24, B). Its tail lies outside of the zona, and appears to have become thickened. In another specimen Sobotta found the head, middle piece and a part of the tail within the cytoplasm of the oocyte. The tail had broken as it crossed the zona, and the portion remaining outside had drawn together and was disintegrating. In some animals it is said that the entire spermatozoon enters the ovum, but in others only the head and middle piece. In any case the tail appears to be a propelling apparatus which becomes functionless after the head and middle piece have passed through the zona. It has entirely disappeared in the stage shown in Fig. 24, A, in which the head of the spermatozoon is seen within the oocyte on the right side of the figure. Meanwhile the oocyte is becoming a mature ovum by undergoing divisions and producing the second polar body; and the anaphase of this division is shown in Fig. 24, A. Sobotta stated that no centrosomes occur in connection with the spindles of the maturation divisions, and Long and Mark have likewise failed to find any "typical centrosomes."

In Fig. 24, C, the second polar body has become a separate cell. The chromosomes of the ovum, which is now mature, have formed a compact mass. They next become resolved into a chromatic reticulum, and a resting nucleus is produced, provided with a nuclear wall and distinct nucleoli (Fig. 24, D and E). This nucleus, which becomes large and moves toward the center of the cell, is known as the female pronucleus. Meanwhile the head of the spermatozoon has enlarged and formed the male pronucleus, as shown in Fig. 24, C, D and E.


The two pronuclei, which are very similar, develop rapidly, "probably within a few minutes after the entrance of the spermatozoon." Simultaneously they prepare for division, and the chromatic reticulum of each becomes resolved into the reduced number of chromosomes which it received during maturation (Fig. 24, F). A centrosome with astral radiations is now seen between the two groups. In Fig. 24, G, it has divided in two, and the spindle has developed. There has been much discussion as to the origin of these centrosomes. Since in this case they arise by the division of a single body, the possibility that one comes from the spermatozoon and one from the ovum has been eliminated. Moreover in the mouse they cannot be derived from the surviving centrosome of the last maturation division of the ovum, for that division takes place without centrosomes. Therefore the centrosome must either be brought in by the spermatozoon as a constituent of its middle piece, or it must be a new formation. Sobotta accepted the former alternative, and he observed a centrosome in connection with the head of the spermatozoon in certain stages (Fig. 24, C) but not in all. It is probable, according to Conklin, that "the source of the cleavage centrosomes may differ in different animals, or even in the same animal under different conditions."


FIG. 34. MATURATION AND FERTILIZATION OF THE OVUM OF THB MOUSE .A,C-J,Xsoo; 8X750.

(After Sobotta.)

A-C, entrance of the spermatozoon and formation of the second polar body. _ D-E, development of the pronuclei. F-J, successive stages in the first division of the fertilized ovum.


Later stages in the division or "cleavage" of the fertilized ovum into two cells are shown in Fig. 24, H-J. The two groups of chromosomes come together upon the spindle so that the full number, characteristic of the species, is restored. Each chromosome then divides lengthwise, and thus each daughter cell receives one-half of its chromosomes from the male parent and one-half from the female parent. This is strikingly evident when the eggs of the fish Fundulus, which have long rod-shaped chromosomes, are fertilized with the sperm of Menidia, which has shorter rods. Moenkhaus, who performed this experiment (Amer. Journ Anat., 1904, vol. 3, pp. 29-64), states that the two kinds of chromosomes remain grouped and bilaterally distributed on the spindles during the first and second divisions of the fertilized ovum, but that later they become gradually mingled.

Important information in regard to the nature of fertilization has been obtained by experiments upon unfertilized eggs. Changes in the concentration or composition of the sea water in which the eggs of marine animals have been placed, mechanical agitation, or, in the case of frogs' eggs, puncturing the outer layer with a needle, have led to repeated cell divisions. In this way embryos or larvae have been produced from unfertilized eggs, and, in a few instances, adult animals. Loeb, who has been a foremost investigator in this field, concludes that the spermatozoon causes the development of the egg by carrying a substance into it which liquefies the cortical layer of the egg, and thereby causes the formation of a membrane. "This membrane formation, or rather the modification of the surface of the egg which underlies the membrane formation, starts the development." At the same time there is an acceleration of the oxidations in the egg. "What remains unknown at present is the way in which the destruction of the cortical layer of the egg accelerates the oxidations."

For the physicist and chemist, the production of mitotic figures and the process of fertilization, have been subjects of great interest, and discussions of their significance will be found in various text-books of physiology and biological chemistry. For further morphological details the student is referred to "The Cell in Development and Inheritance," by E. B. Wilson (2nd ed., New York, 1900) and to the chapters on "Die Geschlechtszellen" and "Eireife, Befruchtung und Furchungsprozess," by W. Waldeyer and R. Hertwig respectively, in vol. i of Hertwig's "Handbuch der vergl. u. exp. Entwickelungslehre der Wirbeltiere," (Jena, 1906).



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Reference: Lewis FT. and Stöhr P. A Text-book of Histology Arranged upon an Embryological Basis. (1913) P. Blakiston’s Son and Co., 539 pp., 495 figs.

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