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==INDEX==
 
CHAPTER XV.
THE DEVELOPMENT OF THE NERVOUS SYSTEM.
 
The nervous system of the adult, including the cerebrospinal axis and nerves, and the sympathetic system of ganglia
and nerves, is made up of the essential neural elements, the
neurons, together with the supporting framework or stroma.^
 
The neurons and a part of the stroma result from the
specialization of the ectodermic layer of the embryo. The
ecto<lermic origin of the nervous system acquires certain
interest in view of the conditions that obtain in some of the
lowest and simplest organisms. For example, in the ameba,
the single protoplasmic cell which constitutes the entire individual possesses the several fundamental vital properties of
protoplasm, such as resi)iration, metabolism, contractility,
motility, etc, in ecpial degree, no single property being more
highly developed than the others, and no particular part of
the cell exliibiting greater s])ecialization than the other parts.
In other words, the j)n)t()plasmic substance of the animal is
at once a respiratory mechanism, a nervous apparatus, and
an organ for the execution of the various other vital functions.
 
In somewhat more highly developed creatures, as the
infusoria, although there is no differentiation into separate
tissues and probably not even into separate cells, there is seen
some progress toward the sj)ecialization of certain parts of
the orgimism for the performance respectively (»f the different
functions of life. For example, the central part of the ani
' The neuronti are the units of which tlic nervous Bvstem is made up.
Each neuron consists of a nerve-cell with everything belonging to it -that
is, with its various processes, including the nxis-rylinder process or iieuritf
which becomes the axis-cvlindcr of a nerve-ri))er.
278
 
 
 
THE DEVELOPMENT OF THE NERVOUS SYSTEM. 279
 
mal has digestive functions, while it is by the superficial
portion alone that the creature is brought into relation with
the outside world, the sensitiveness or irritability of the
surface, by which the animal is made responsive to external
impressions, being the nearest approach to the function of a
nervous system that it possesses.
 
This primitive function of the surface of the organism
is suggestive as to the origin of the nervous system of
higher type creatures. It will be seen, indeed, that not only
is the nervous system proper derived from the ectoderm ic
cells of the embryo but that the peripheral parts of the
organs of special sense, as the olfactory epithelium, the organ
of Oorti, and the retina, have the same origin.
 
The alteration of those cells of the ectodermic stratum
that are to specialize into nervous elements begins prior to
the fourteenth day in the human embryo, in the stage of
the blastodermic vesicle. The change consists in a gradual
modification of the form of the cells, the cells common
to the general surface of the germ assuming the columnar type. The process affects the cells of the median
line of the embryonic area in advance of the primitive streak,
resulting in the production of a thickened longitudinal median
zone. This thickened area is the medullary plate (Fig. 41,
p. 70). On each side of the plate — which is apparent at the
fourteenth day — the adjoining ectodermic cells become heaped
up to form the medullary folds, which latter therefore bound
the medullary plate laterally. The medullary plate becomes
concave on the surface, forming the medullary groove (Fig.
137). By the deepening of the groove, the lateral edges of
the plate approach each other (Fig. 138), and finally they meet
and unite, thus producing a tube, the neural tube or canal.
 
Since the medullary folds similarly meet and unite with
each other — their union slightly preceding that of the edges
of the plate — the neural tube comes to lie entirely beneath
the surface-ectoderm and soon loses all connection with it.
The closing of the tube and the union of the medullary folds
occur first near the anterior end of the embryonic area, in a
position that corresponds with the region of the future neck,
 
 
 
2S*I TEXT-ROOK fiF EMUnYOLOGY.
 
flnil fnim this jwint it proceeds Iwlh cephalad and caudad.
Sinw the nie»!ullar>' fnUis at their caiidal extremity embrace
 
 
 
 
ITctKkarJ, Scmitr. Cnl /•laJrrm.
 
Fid. is;.— TrBDirene lecllon of > ■Ixteen-Rnd'a-l
 
 
 
mbryo poaMctlOK
 
 
 
the primitive streak {Y\g. -11, p. 70), the latter structure i»
inchidefl within the cjuidiil end of the neural tube by the \
 
 
 
 
isHTie BBcUiiii of II fitl<;i;n-mnl a-hslf-dnj sheap embryo
Kvca lomltci (Bonnet).
 
coming together of the folds, and thus the blastopore, which
was previously the external aperture of the archonteron.
 
 
 
THE DEVELOPMENT OF THE SPINAL CORD. 281
 
comes to constitute the neurenteric canal, or an avenue of
communication between the neural canal and the primitive
intestine.
 
The neural canal then is a tube composed of columnar
cells, which is formed by the folding in of the ectoderm and
which occupies the median longitudinal axis of the embryonic
area and consequently of the future embryonic body. From
this simple epithelial canal the entire adult nervous system is
evolved.
 
The evolution of the highly complex cerebrospinal axis
from such a simple structure as the neural canal is referable
both to the principle of unequal growth — the walls of the
tube becoming thickened by the multiplication of the cells —
and to the formation of folds.
 
The portion of the neural canal — approximately one-half —
that is devoted to the formation of the brain is delimited
from the part that produces the spinal cord by the dilatation
of the anterior or head-end of the tube, and the subsequent
division of this dilated sac-like portion into three communicating sacs called respectively the fore-brain, mid-brain, and
hind-brain vesicles (Fig. 142). These three vesicles give
rise to the brain, while the remaining part of the neural canal
forms the spinal cord.
 
THE DEVELOPMENT OF THE SPINAL CORD.
 
In the growth of the spinal cord from the spinal portion
of the neural canal we have to consider the evolution of a
cylindrical mass of nerve-cells and nerve-fibers with the
supporting stroma from a simple epithelial tube.
 
The wall of the neural tube, although consisting at first
of a single layer of epithelial cells, is not of uniform thickness throughout its circumference. While the external outline is oval, the lumen of the tube is a narrow dorsoventral
fissure (Fig. 45, p. 73). The cavity is therefore bounded on
the sides by thickened lateral columns, while the dorsal and
ventral walls, which connect the lateral columns with each
other, are thinner and are called respectively the roof-plate
and the floor-plate.
 
 
 
 
282 TEXT-BUOK UF EMBRYOLOGY.
 
After a short time, the walls of the tube having thickened
by the multiplication of the cells, the shape of the lumen
alters, two laterally projecting
angles being addetl (Fig. 139).
The effect of tiiis change is to
partiallydivide each lateral half
into a dorsal and a ventral
region. The neural canal at
tlii,-i stage may be said to consist of six columnri of cells, the
two dorsal zones connected with
each other liy the roof-plate, and
the two ventral zones united by
the floor-plate. These regions
are also distinguishable, with .
certain characteristic modifi<a- I
tions, in the head-region of tlw I
tnl>e. They are important in
their bearing upon the further
development of the atrnctnre,
since the dorsal and ventral
zones are related respectively to the dorsal or sensory and
the ventral or motor roots of the spinal nerves.
 
The differentiation of the cells of the neural tube into two ^
kinds of elements, one of which gives rise to susteutative
tissue or neuroglia wliile the other produces the nerve-cells, is (
observed at about the end of the third week. The single ,
layer of columnar cells which at first comjMises the wall of
the tube, the long axes of the cells being radially arranged,
soon exhibits near the lumen a row of roond cells, jirobably
the first offspring of the columnar cells. The round cella
are the genn-cells or Kerminating cells, from which are develoiied ihe neuroblasts or young nerve-colls as well as the
neuroglia cells. All the other cells, known as the spongioblfista or ependymal cells, are concerned in producing susten
 
 
36<rWim Kaillkcr): c.ccDtraJ
t, its eplthdUI nntng: Me'
ortyl, Iho original placu of
ot (he cuul ; a, the wblte lU
of the anterior coluouu : a. g
atance of BnlerolUiiral born
terlar column ; or. miterlui
pr, iMatarior roola.
 
 
 
tati\
 
 
 
ssue.
 
 
 
Tiie stroma of the central nervous system includes two
constituents — a connective-tiBsue element, and a part, the
neuroglia, which is of epithelial origin, and which is not to
 
 
 
THE DEVELOPMENT OF THE SPINAL CORD. 283
 
be regarded, therefore, as connective tissue. The connectivetissue portion of the stroma is produced by the ingrowth of
the pial processes from the pia mater, and is hence of mesodermic origin.
 
The neuroglia is derived from the spongiobhtsts, which
result from the specialization of the large colnmnar cells of
which the wall of the neural canal is composed. These cells,
whose length comprises the entire thickness of the wall of
the tube in the earliest stages, undergo partial absorption and
disintegration, each cell being transformed into an elongated
system of slender processes or trabecule, and each such system
 
 
 
 
no. IW.-CrOBB-Bectlon through the ipinal cord of ■ Tertebnite embryo {after
His}: a. outer nmltlng membrane; b, outer DeurogUa layer. re«lon of future white
matter; e, germ^cells ; d, central canal: e. Inner UmitlnE membrane or ependymal
t>rer 1 /, spongioblaiU : a, neuroblaata (mantle tajer) : A, anterior root-flben.
 
being a completed spongieblaet or ependymal cell (Fig. 140).
Thp inner ends of the spongioblasts coalesce with each other,
forming thus the Inteinal limiting membrane, while the peri])heral extremities interlace with each other to form a close
network, the marginal velum. As the walls of the neural
tube increase in thickness, the spongioblasts become mors
and more broken up to form the delicate neurogliar networf
 
 
 
284 TEXT-BOOK OF EMBRYOLOGY.
 
with interspersed nucleated glia cells, which latter are derived
from some of the round cells noted above as lying near the
hmien of the neural tube and which have taken a position in
the marginal velum. Such of the spongioblasts as border
the cavity of the neuml tube become the cells of the later
ependyma of the central canal of the spinal cord and of the
ventricles of the brain. The cells of the ejwndyma become
ciliated in the human fetus in the fifth week.
 
The nerve-cells of the spinal cord — as also of the brain —
are the specialized descendants of the germ-cells referred to
above. The proliferation of the germ-cells produces the
neuroblasts, or young nerve-cells (Fig. 140). The latter elements move away from the primitive position of the germcells near the lumen of the tube and, taking up a position
between the bodies of the ei)endymal cells and the periphery
of the neural tube, develop into the nerve-cells. The transition is effected by the accumulation of the cell's protoplasm on
the distal side of the nucleus and its elongation into a process.
This ])rocess is a neurit or axon or axis-cylinder process and
is the beginning of a nerve-fiber. The dendrites or protoplasmic processes apj>ear considerably later. Some of the
fibers thus prcHluced grow out from the neural tube to constitute the eiferent filxTs of the jxTiphcral nerves, that is, the
ventral roots of the spinal nerves, while others contribute to
the formation of the fiber-tracts of the cord.
 
After the appearance of the neuroblasts and developing
nerve-cells, the wall (►f the neural tube is divisible into three
layers (Fig. 140): an inner or ependsrmal layer, next the
lumen of the tube; adjoining this, the mantle layer, made
up of neuroblasts ; and a peripherally situated neuroglia layer
or marginal velum, which occupies the position of the future
tracts of white fibers (►f the cord.
 
The alterations in the form and size of the sj)inal cord go
hand in hand with the histological changes noted above.
While th(»se areas that have been mentioned a> the dorsal and
ventral zones in{.Teas<» greatly in thi<'kness, the floor-plate and
the roof-plate — the ventral and dorsal walls of the neural
tube — remain thin i Fig. 141). They are n<'ver invaded by
the nerve-cells, but consist of thin layers of neuroglia which
 
 
 
TUE DEVELOPMENT OF THE SPINAL CoRD.
 
 
 
285
 
 
 
later become penetrated by nerve-fibt-ra thatpro"' fnim oiif
side to the other. They thus represent the anterior and posterior white commiBsmes of the cord. Thetie plates remain
relatively fixed in position because of their failure to expand,
while the liitcral walls iif the tulie undergo great expansion,
in both the ventral and dorsal directions, as well as laterally.
In this way a median longitudinal cleft is produced on the
ventral wall of the spinal c<>r<l and a similar one on the
dorsal wall. These are the anterior and posterior median
fissures. Since the so-called jKisterlor median tissiiit is not a
true fissure but merely a eeptiim, it differs from the anterior
fissure, and It is held by some authorities that this septum is
 
 
 
 
 
 
 
formed by the gmwing togetiier of the walla of the dorsal
part of the central canal.
 
The flber-tracts or white matter of the spinal cord develop
in the outer or neuroglia layer, each filjer being the elongated
neurit of a nerve-cell. Some of the fibers originate from the
nerve-cells of the cord while others grow into the c«rd from
other sources. As examples of the former method may be
cited the direct cerebellar tract, composed of the axons of
the cells of the vesicular column of Clark, and the tract of ,*
 
 
 
286
 
 
 
TEXT-BOOK OF KMBRYOLOVY.
 
 
 
GrowtT, made up of the axons oi' cells t>f the dorsal gray
horn ; while the direct und crosat-d |iyniniiilul tracts ai-e the
axons of cells in the cortex of the ct-rebrum. and the tracta
of Goll and of Biinlach are composed largely of the axons >]
of the cells of the Hpinal ganglia (see p. 318), The devcl- 1
opmont of these filwr-tracts is not complete until the tilwrs
ap(|iiirc their niyeltn-sheaths (see p. 414), The myelination
of the tracts of Biirdach and of Goll occurs In the latter
part of the fourth month and in the tit\h month ; of the :
direct cerehellar tract, in the seventh month ; of ihe jiyramidal tracts, at or soon after birth.
 
As the walls of the neural canal thicken through the mul-^
tipliciition of the cells, the cavity of the tube is gradually]
encroached upon almost to obliteration. Whea development f
is complete, all that remains of the cavity is the sm&U ceatral f
canal of the spinal cord.
 
The lengtb of the spinal cord in the fourth fetal month \
corresponds with that of the spinal column, Its lower termi- J
nation being opposite the last coccygeal vertebra. From this 1
time forward, however, the cord grows less rapidly than does J
the spinal column, so that at birth, the cord terminates at the4
last lumbar vertebra, and in adult life at the second lumbar 1
vertebra. This gradually acquired disproportion in thai
length of the two structures explains the more oblique 1
direction of the lower spinal nerves as comi«ired with those g
 
E higher up. In the early condition of the cord, each pair of '
Uerves passes almost horizontally outward to the cprrespoading intervertebral foramina, but us the spinal column gradn- ]
ally outstrips the cord in growth, the lower nerves necessarily ^
pursue a successively more oblique course to reach their j
foramina, the lower nerves being almost vertical in direction
and constituting, collectively, the canda equina.
-J
(lev
dih
ves
nei
 
 
 
THE DEVELOPMENT OF THE BRAIN.
 
The encephalic portion of the neuml iuIk, — that part
devoted to the production of the bniin — after undci^ing
dilatation, becomes marked "ff into ihe thnc ])riman' brainvesicle«, the fore-brain or prosencephalon, the mid-brain or
mesencephalon, and llie hind-brain or rhombencephalon, by constrictions in the lateral walls of the tube (Fig. 142).
the constricted part of the hind-brain that adjoins the midbrain is the isthmns. This division occurs at an early stage,
before the closure of the tube is everywhere complete. The
vesicles communicate with each other by rather wide openings. As in the spinal part of the neural canal, the walls of
the primary brain- vesicles consist of epithelial cells, and it
is by the. muliiplicalion of these cells in unequal degree in different regions^ and by the fo)*r)iation of folds in certain localities,
that the various parts of the adult brain are developed from
these simple epithelial sacs.
 
The stage of three vesicles is soon succeeded by the fivevesicle stage, the primary fore-brain vesicle undergoing division into two, the secondary fore-brain (telencephalon) and the
inter-brain (thalamencepalon) or diencephalon, and the primary
hind-brain vesicle likewise dividing, a little later, into the secondary hind-brain (meteneephalon) and the after-brain (myelencephalon).
 
The division of the primary
fore-brain is preceded by the
appearance upon each of its
lateral walls of a small bulgedout area which soon assumes the
form of a distinct diverticulum.
This is the optic vesicle, the earliest indication of the development of the eye (Fig. 142). In
the further process of growth
the base of attachment of the
optic vesicle becomes lengthened
out into a relatively slender pedicle, which remains in connection with the lower }>art of the
hiteral wall of the brain- vesicle.
Following the appearance of the optic vesicle, the anterior
wall of the primary fore-brain vesicle projects as a small
ovagination, which latter is then distinctly marked off from
 
 
 
 
 
Anterwr hrtUn-wtieU.
 
Middle brain-vesicle.
P0Uerior breun-vesicle.
 
Fare-brain.
 
Primary optic vesicle.
 
Simik ^ optic vesicle.
Inter-brain.
Mid-brain.
Hind-brain.
 
After-brain.
Fore-brain.
 
Primary optic vesicle.
 
Jnier-brain.
Mid-breun.
 
Hind-breun.
 
J^er-brain.
 
Fio. 142.~Diagrram8 illustrating
the primary and secondary segmentation of the brain-tube (Bonnet).
 
 
 
 
the parent vesicle by a groove on either side. This anterior
divertieuhim is the secondary fore-brain vesicle or the vesicle
of the cerebrum, and the original or ]>riniary fore-brain vesicle is now the vesicle of the inter-brain.
 
The division of the primary hind-brain is eflTected by the
development of a constriction of its lateral wall, this resulting
in the production of the secondary hind-brain or the vesicle
of the cerebellum, and the after-brain or the vesicle of the
medulla oblongata.
 
While the three primary vesicles at first lie in the same
straight line, they begin to alter their relative positions
shortly before division. The change of position is coincident
with the flexures of the body of the embryo that occur at this
time. Three well-marked flexures appear, the result being
 
 
 
InUr-brain.
 
 
 
Fore-brain.
 
 
 
Cephalic flexure.
 
 
 
Mid-brain.
 
 
 
Olfactory lobe
 
 
 
Optic stalk.
 
 
 
 
I I I
 
Cerebral portion of Pontine
pituitary body. fltxure.
 
Fig. 143.— Diagram shuwin^ relations t)f l)raiii-vesiclvs ami flexures (Bonnet\
 
that the fore-brain is bent over ventrad to a marked degree.
The most anterior of these flexures, and the first t4) develop,
is the so-called cephalic flexure (Fig. 143), the primary forebrain, in the advanced stiite of the curvature, being bent
around the termination of the chorda dorsiUis so as to form a
right angle, and later, after its division, an acute angle with the
floor of the mid-brain. This curvature makes th(» mid-brain
very prominent as regards the surface of the embryonic body,
producing the parietal elevation or the prominence of mid-brain.
In the region of the future pons Varolii, on the floor or ventral wall of the secondary liind-brain, is a second wullmarked an^^iilarity. This is the pontal flexure. Its convexity projects forward.
 
A third bond, the nuchal flexure, is a less pronounced
cnrvature at the jniicture of the after-brain with the spinal
part (if the neural tube.
 
The Metamorphosis of the Fifth Brain-vesicle.—
The fifth brain-vesicle, the caudal division of the primary
hind-brain, (lifferentiates into the strnctnres whioh surnmnd
the lower half of the fourth ventricle, these stnictures con
 
 
 
F\a. 144.— Diagram of > sinrlttal section or the brain of ■ mammal, sbowlng
tbe trpc of stroctiirc and tlic pailB that develnji from the Beveral bialn-TeBlclei
(modified rrnm Edltim^r).
 
stituting the mTelenceplialoii (Pig. 144). The Ustological
clianges eorresjjond essentially with those that occur in the
spinal segment of the neural tube, the aerve-cells and fibers
and the neuroglia resulting from the diilbreiitiation of the
original ectoderniic epithelium of which the wall of the tube
is composed, and the coimective-tiflaue stroma growing into
these from the surrounding mesoderm.
 
There is a marked disproportion between the rate of growth
of the tube in different parts of its circumference. The
great thickening of the ventral and lateral walls produces the
several parts uf the mednlla oblongata. In the dorsal wall growth occurs to such slight extent that the wall in this
region remains a thin layer of epithelium. As a consequence,
the cavity of the neural tube is not encroached upon on its
dorsal side and the central canal of the spinal cord therefore
expands in the myelencephalon into a much larger space, the
lower half of the future fourth ventricle. This relative expansion of the central canal begins to be apparent in the third
week in the human embryo, from which period it continues
to increase. A cross-section through the lower part of the
developing medulla shows a cavity which is narrow laterally
but which has a considerable anteroposterior extent. A section at a higher level disclosc^s a triangular space, the base of
the triangle being the dorsal wall of the cavity.
 
At the time when the cavity of the after-brain acquires a
distinctly triangular shape — about the third week — each thickened lateral half of the tube is divisible into a ventral and &
dorsal segment, these being known respectively as the basal
lamina and the alar lamina (Fig. 145).
 
The first indication of the longitudinal fiber-tracts of the
medulla is presented by two bands of fibers which appear upon
 
the surface of the alar lamina and which
constitute the ascending root of the
fifth nerve and the ascending root (funiculus solitarius) of the vagus and glossopharyngeal nerves. These are later covorcil in by the folding over of the dorsal part of the alar lamina (Fiff. 146) and
thnaighupiHT part core- tlius coiiic to ()crn)>y tlicir permanent
bviiar rtri<m. of tho po^jticm ill tlic interior of the medulla.
 
fourlh venlrielo <.f an * /» i i i •
 
omhryo (His): r, roof of 1 hc parts of the alar lamina^ that are
 
mv.rai ;a""i : "/. "i"r f^^i^j^.^j ^,^,^.,. j^^ ^\^^ manner referred
 
Iniiiina ; W, basal luiiiina ;
 
r. ventral ixjrdiT. to diiliTeiitiatc for the most part
 
into the restiform bodies or inferior
peduncles. These are distinguishable in the third month.
The anterior pyramidal tracts develop from the ventral parts
of the basal lamiiue and are recognizjible in the fifth month.
CoiiK*i<leiitally with the f\)rmation of the fibers, the gray
matter of th(* medulla assumes its prrmancnt form and arrangement. This gray matter, although in part ])eculiar to the
 
 
 
nie<1iilla, is in great measure hut the continuation of the gray
matter of the spinal cor*l rearranged and differently related
because of the motor and sensory decussations and of the dorsal expansion of the central canal. A notable feature of this
 
 
 
 
■t/ {Ills) : V, ventml border : (, tenls : ot, otic vesicle ;
 
 
 
rearrangement is the presence of masses of gray matter immediately beneath the floor or ventral wall of the now expanded cavity or fourth ventricle.
 
As stated above, the dorsal \vall of the aftcr-brain vesicle
remains an extremely thin epithelial lamina, and the cavity
in consequence expands toward the dorsal surface. Owing
to the excessive delicacy of this dorsal wall of the cavity, it
ia easily destroyed in dissection, with the effect of disclosing
a triangular fossa (Fig. 151) on the dorsal surface of the
medulla, which in connection with a similar depression on
the dorsal surface of the pons, constitutes the rbomboidal fossa,
or the foortli Tentride of the brain.
 
It is often stated in descriptions of the medulla and fourth
ventricle that the latter is produced by the opening out of
the central canal of the cord to the dorsal surface. It should
be borne in mind, however, that the central canal does not,
in reality, open out to the surface, although it may appear to
do so l)ecause of the attenuated condition of its dorsal boundary. The thin epithelial roof or dorsal wall of the aflerbrain l>ecomes adherent to the investing layer of pia mater,
thus forming the tela choroidsa inferior, which roofs over
the lower half of the fourth ventricle (Fig. 144). The piamater invnj^inUcs ilu» epithelial layer to form the choroid
plexuses of the fourth ventricle. Although apparently
within the cavity of the ventricle, the choroid plexuses
are excluded from it hy the layer of epithelium, the morphological roof of the after-hrain, which they have pushed
before them.
 
AVhile, for the most part, the roof of the after-brain consists of the thin epithelial layer referred to above, there are
slight linear thickenings, the ligulsB, along its latei'al margins,
and at its lower angle, the obex. At the up|)er margin of the
roof, at the place of junction with the hind-brain, there is
also a thicken<>d area, the inferior medullary velum. These
regions of thick<T tissue serve to eife<'t the transition from the
thin epithelial layer that helps to form the inferior choroidal
tela to the more massive boundaries of the rhomboidal fossa.
 
The Hind-brain Vesicle or Metencephalon. — The
 
metencephalon <*onsists of the pons, the cerebellum with its
suj)erior and mid<lle jx^duncles, and the valve (valve of
A"ieuss(Mis). the>e structures are j)r()duced by the thickening of the walls of the fourth or hind-brain vesicle.
 
Th(» pons is forni<Ml by the thickening of the ventnd wall
of the vesi(rlc. Its tnmsverse libers become recognizable
durintr the fourth mouth.
 
The cerebellum grows iVom tin? posterior part of the nK)f
or dorsid wall of the vesicle (Fig. \A\). The lirst indication
of its development is seen as a thick transverse ridge or
fold on th(; po>terior extremity of the <lorsid wall (Fig^*.
147, 1 4S). In tli<* tliinl mouth the <M'ntral j)art of this
ridge, now grown larger, )>r('S('uts iour deep transverse
grooves with the n'sult oi* dividing the originid eminence
into five transverse ridges. The grooved ))art of the ridge
is the ]>ortion that subse<|ueutly becomes the vermiform
process or median lobe of the cerebelluui, while the smooth
lateral ])ortious becom<' the lateral hemispheres. As the
vermiform process increases in bulk, two of the ridges come
to lie ujM)n its upper surface and three? on the inferior aspi»ct.
These ridg(»s and furrows j)ersist throughout lifi^ as the
principal convolutions and fissures of the vermiform process (Figs. 14J», 1 :»<)).
 
 
 
 
The lateral parts of the primary ridge inercaso in size and
eventually, in the hiiin&n bmin, outstrip the niwlian lobe in
pritwlh. They acquire their chief transverse fifisures in the
fourth or fifth iiKinth, and the smaller sulci later.
 
The thickened cerebellar ridge nn the roof of the hindbrain vesicle being continuous with the lateml walls, the
continuity of the cerebellar hemispheres with the jHins
through the middle and superior cereliellnr |>eiliincles and
with the medulla by means of the inferior pcdnnrles. is easily
 
 
 
 
thickens and dcvelojxs into the cerebellum, all the remaining
part of this roof remains relatively thin and becomes the
anterior medullary velum or the valve of Vieussens (Fig. 144).
The relations of this structure in the mature brain, stretching across, as it does, from one sujHjrior cerebellar })eduncle
to the other and l)eing continuous posteriorly with .the white
matter of the cerebellum, ixw. easily explained in the light of
the fact that all these parts are but the specialized dorsal and
lateral walls of the hind-brain vesicle. Since the roof of the
hind-brain vesicle is continuous with that of the after-brain
or fifth vesicle, it will be seen that the cerebellum must be in
continuity with the roof of the medullary part of the fourth
ventricle. The transition from the cerebellum to the epithelium of the tela choroidea inferior is eifected by a pair of
thin crescent-shaped bands of white nerve-matter which i>ass
downward from the central white-matter of the cerebellum,
and which are collectively known as the inferior or posterior
medullary velum. Thus, as the result of unecpial growth,
there are ])ro<luced from the continuous dorsid walls of the
fourth and fifth vesicles the thin laminar medullary velum
or valve, the massive cerebellar lob(»s, the thin bands known
as the infi^rior niedullarv velum, and the single layer of epithelimn which, with a layer of pia mater, constitutes the
inferior choroidal tela.
 
Although the fourth and fifth brain-vesicles are at first
delimited from each other by a constriction, this constriction,
as development goes on, <Iisappears, the cavity of the fourth
vesicle and that of the iifth together constituting the fourth
ventricle of the brain.
 
The walls of th<» fourth or hind-brain vesicle then give
rise vent rally to the j)ons, latendly to the superior and middle cerebellar peduncles, and dorsally to the valve* and the
cerebellum, while its cavitv beciunes the anterior half of the
fourth ventricle.
 
The Mid-brain Vesicle. — The third brain-vesieh? or
the vesicle of the mi<l-l)rain or mesencephalon gives rise to
the structures surroun<ling the acpieduct of Sylvius, the ])ersistent part of the cavity constituting the aqueduct itself.
 
The thickeninir of the ventral wall of the vesicle results in the formation of the crura cerebri and the poaterior peribrated
lamina nr space included between them. The crura first
become apparent in the third month as a j»air of rounded
longitudinal ridges on the ventral siirface of the vesicle.
These remain relatively small until the fifth month, when
the longitudinal fibers of the pons begin to grow into them.
After this occurrence their increase in size is comparatively
rapid, their ventral parts or cmsta becoming separated from
each other ami iiidndiiig between tiieni the posterior perforated lamina.
 
The roof or dorsal wall of the mid-brain vesicle undergoes considerable thickening (Fig, 147), especially in the
Sauropsida (birds, reptiles, fishes). In the fifth week a longitudinal ridge appears upon the dorsal wall, which in the third
month is replaced by a furrow. The expansion of the wall
on each side of the furrow produces a pair of rounded eminences (Figs. 148-151), which, in birds, attain to a much
 
 
 
 
Fig. UK.— Brain
aire; f 6, foro-brafu ; lb,
brain: P, ruliln uf pLa
 
greater development than in mammals and constitute the
corpora bigemina or optic lobes. In the human emhr}'o, each
of these elevations is divided into two by an oblique groove,
and thus arc formed the coipora qtiadiigemina, which are
peculiar to man and other mammals.
 
The jiart of the dorsal wall of the vesicle that underlies
the corpora quadrigemina is the lamina qnadrigemina.
 
The thickening which the walls of the vesicle undergo to produce the several parts of the micl-bmin encroaches so
miicli iiiK)n its cavity thatan I'.xceedinj^'ly small cjiual, the
aqueduct of Sylvius, remains. It is scarcely necessary to
piHiit out llijtt llii>; canal is a part of the ventricular system
of the iiniii), ost:ihli'-hiu<;a ctminiunicatiou l>etweeu the fourth
ventricle ami the thini ventricle or tyivity of the intcr-braiu.
The Metamorphosis of the Inter -brain "Vesicle. —
The inter-limiii vesicle results fn.m the division of the primary fore-brain vehicle, comprising what in lell of the latter
after the outgrowth from it uf the diverticulum that l>ecome8
the secondary fore-brain. The thickening of the walls of
the inter-brain vesicle produces the sirueture-s which surround
the third ventricle in the mature eoudition, and which constitute collectively the thalomencephalon or inter-brain, the cavity
of tJie vesicle persisting as the adult third ventricle. These
Btructures are the optic thalajoi, which iirc iorincd from the
lateral walls; the velum interpoBitum and the pineal bod7>
which develop from the roof; and the lamina cinerea, the
 
 
 
Fig. ]4S,~A. mualsl icrtiDii IhroURh bniiii <i[ a huiunii rclUB of two-Bud-a-bfttf
months (Hla): cA. cerebral lii'mlnphi-'ru ; o. ituWc UinliiiDue:/Hi. ri>niin«n of Monro;
o{f, olOwtory tobc: p, pllultar; body ; no, minlulU (iblongaU: eq, corpora quadrlpmlDai tb, eerebetlum, B, brain of human Celui of (hrve montbi (HI*): olf,
olftntory Inlie; rM, rnrpUB sltUlum; eq, corpora quadriffemlna ; eft, eerebBllnrnj
inn, mcdiinn obloiiKOU.
 
tuber cinereum, the infiindibuluin, the posterior lobe of the
pituitary body and the corpora albicantia, which are differentiatoil fiiuii the floor of the vesicle.
 
The lateral walls of the vesicle undergo the most marked thickening. The cell-multiplication here is so r.ipid that
each lateral wall is converted into a large ovoiil inaris of
cells with iiiterminglctl bands of fibers, the optic thalamus.
 
The roof of the inter-brain vesicle, in nutahle coiitra.st
with the lateral walls, remains extremely thin throiighnnt
the greater part of its extent (Fig. 144). Fmm ihe Ijack
part of the roof, at a point immediately in front of the
lamina iiiiadrigeiuina of the mid-brain, a diverticulum grows
otit and becomes metamorphosed into the pineal tody. With
this e.\ception, the roof of the vesicle reinains a single layer
of epithelium, just aa in the ease of the roof of the afterbrain. This epithelial layer adheres closely to the pia mater,
which covers it in common with the other parts of the hrain.
As the fore-brain expands, it covers the inter-brain, the
under surface of the cerebral hemispheres of the former
l>eing closely applied to the roof of the latter. As a consetjucnce, the pia mater on the under surface of the fore
 
 
 
Fin. i:iO.—itra<n of A^tm of ihreemnnihs, enlarged. Tbc outer wall of the light
 
hcmlipheru hu been lemoveil ; LH, left bemlHpliere ; Ca, part of corpiu ■Irialum;
FS, site of fossa of Kylviiis; I', vascular fold of pia mater which has been InvagInalcd ihniuRh the mesial wall of the hemisphere: Mb, miil-broln; C.ceiebellum;
Jf , medulla oblongala,
 
brain is brought into contact with and adheres to the pia
covering the roof of the inter-brain. Thus the thin epithelial
roof of the inter-brain becomes closely united with the two
layers of the pia luater to form the velnm iutarpodtam or
tela choroidea anterior or superior of adult anatomy. Obviously, the edges of the velum interpositum rest upon the
optic thalami, and its piamatral layers are continued into the cavities of the lateral ventricles (Fig. 150). The space occupied by the velum is designated the transyerse fissure of the
brain, and it is often stated that the pia mater is pushed in
from behind, between the optic thalami and the cerebral hemispheres. As will be seen from the above description^ however, its development really begins in front.
 
The pineal gland or conarium develops from the back part
of the roof of the inter-brain at its point of junction with
the mid-brain (Fig. 144). This body is found in all vertebrate animals except the amphioxus, but its form varies
greatly in difierent groups. In all cases it begins as a small
pouch-like evagination from the roof of the inter-brain, the
diverticulum being directed forward. In the human brain
alone the structure is subsequently directed backward, so
that it conies to occupy a position just over the corpora
quadrigemina. This peculiarity of location is due probably
to the greater development of the human corpus callosum,
by whicli the conarium is crowded backward.
 
In selachians (sharks and dog-fish), the enlarged vesicular
end of the diverticulum, which is lined with ciliated columnar
cells, lies outside the cranial capsule and is connected with
the inter-bruin by the lonij: hollow stalk which perforates
the roof of the (M-aiiiuni. In many reptiles, the conarium is
more liighly specialized. In the chameleon, for example, the
peripli(;ral extremity has the form of a small closed vesicle
which lies outside the roof of the cranium and which is
covered by a trans|)aroiit pat(*li of skin. The stalk in this
case is ])artly a solid cord and ]>artly a hollow canal, which
latter oj)ens into the cavity of the inter-brain. The solid
portion lies within a foramen in the pjirietal bone, the parietal
foramen. A farther modification of the conarium is jiresented
in lizards, blind- worms, and some other reptiles. In these
the vesicle underji^oes a marked specializjition, its peripheral
wall being so nicxlilled as to become trans))arent and to resemble the crystalline lens of the eye, while the opposite
deeper wall comes to consist of several layers of cells — some
of which become piirmente<l — ainl ac(juire«* a striking resemblance to the retina. The stalk of the body, which perforates
the roof of the skull and is attacheil to the roof of the interbrain, bears a certain likeness to the optic nerve, being solid
and composed of fibers and elongated cells. The presence
of the transparent epidermal plate which covers the vesicle
serves to complete the similarity of this particular type of
pineal body to the eye of vertebrate animals. It is for this
reason that it is often designated the pineal or parietal eye
and that it has been looked upon as a third or unpaired
organ of vision.
 
In man and other mammals and in birds the pineal diverticulum does not reach the degree of development that is
attained in certain of the Reptilia. The evagination from
the roof of the inter-brain begins in the sixth week in the
human embryo. The peripheral end of the process enlarges
somewhat and small masses of cells project from it into the
surrounding mesodermic tissue. These cellular outgrowths,
giving off secondary projections, become converted into small
closed follicles lined with columnar ciliated cells. The follicles in the case of mammals very soon become solid or nearly
so by the accumulation of cells in their interior. Solid concretions of calcareous matter, the so-called brain-sand (acervulus cerebri) are found in the follicles in the adult. By
these alterations the pineal body of birds and mammals
acquires a structure resembling that of a glandular organ.
Since it is onlv the end of the diverticulum that becomes
thus altered, the remaining part constitutes the relatively
slender stalk of the pineal body, the stalk being solid at
maturity except at its point of attachment to the inter-brain,
where a portion of the cavity persists as the pineal recess of
the third ventricle.
 
The pineal body of man and the higher vertebrates is therefore a rudimentary structure and is the representative of an
organ that is much more highly developed in some of the
lower members of the same series. Its true significance is
still a matter of conjecture. Although resembling the eye in
its structure, and although regarded by some on that account
as primitively an organ of vision, it is considered probable by
others that in its most highly developed condition it is an
organ of heat perception.
 
The floor of the inter-brain vesicle presents several interesting
 
 
iiu'tainorphosos. Tlui anterior ])art of the floor n?mains quite
thill an<l Ix'coines the lamina cinerea of th<^ niatinv hraiu (Fig.
111). Iiuiiiediately posterior to this region, the floor of the
vesiele poiK^hes out, this evagination developing into a slender
IhIm', (he inftmdibuluin. Behind the [XHut of origin of the
ini'undilMihiin a sc>eond protnheranee indicrates the beginning
nf* (lie tuber cinereum. By subse(|uent altenitions, the tuber
eiurreiini enlarmnir in ('ircuniferenee so as to include the
point ol'ori<::in of the infundibnlnm, the base of attachment
III" the infnn(lil)nluin eonies to be the center of the tuber
riniTruni, so that the cavity of the former is a continuation of
thr eavilv of the latter. the end of the infundibulum
limiMies tiie posterior lobe of tiie pituitary body or hsrpoMliynlH ( Vi\r>, 144 and 140). Posterior to the tulxjr cinertiiiiii a small evagination of the floor of the vesicle
.ippiMi'i an<l berimes divide<l in the early part of the fourth
iiiniilh iiiln two lateral halves bv a median furrow. The
Iwii bllh' bodies thus forme<l become, after further developiiH lit. ihr corpora albicantia.
 
I hr hypophysis or pituitary body briefly referred to above
iii|iiiir: iimrr <'.\tende(l consideration because of its morlihuiti^firid liiipnriancc. The posterior lobe of this body is the
• iil.ii^fiil nid of the infiindihulnm, which is an evagination of
I hi iImiii 111' I he inlcr-brain. The cells in the lower end of
I hi iiitiiiiihbiihnu specialize into nerve-cells, and ncrvelilii I < .il:ii drv<'h»p. In some lower vertebrates these eleiiii 111 < .111* i-i'iiiiiird throu(rhout lii'<\ but in man and the
hi;'. hi I l\pi- niiiiiial> the distinctively nervous character of
ihi II- 111 • I- -ooii lost, and the cavity of this part of the
ihiiiiiilihiihiiii iill'ris oblitenition. The bmnched ])igmentiilh •iiiir(iiiir-i nM'oiTiii/jihh' in the j)osterior K)b(» of the
hiiiiiaii piiiiitiir\ body an* the only remnant of tiie early
III I \ »• ii'lU.
 
I hi' Hiitttiior lobn of the hypophysis is essentially different
III Hiii'.iii ii^ wi'll MM in structure from tin* ]>ost<»rior h»be. It
i- piodiii-nl b> nil cviiixination from the pcisterior wall of the
piiiiiili\c phar\n\,l»ut from that region of the ])harvnx which
i- anterior to the |»haryngcal membrane and which therefore
bcloii^> to the primitiv(^ mouth-cavity (Fig. GO, p. l;ilj. The out-pocketing of the pharyngeal wall begins in the fourth
week, shortly after the rupture of the pharyngeal membrane.
The little pouch is the pocket of Bathke. The pouch grows
upward and backward toward the floor of the inter-brain and
meets the end of the infundibulum. As the pliaryngeal
diverticulum lengthens, its stalk becomes a slender duct,
which for some time retains its connection with the pharynx.
As the membranous base of the skull becomes cartilaginous,
the duct begins to atrophy, and finally entirely disappears.
In selachians, however, it is retained permanently, establishing thus a connection between the hypophysis and the pharyngeal cavity. AVith the disappearance of the duct the enlarged
extremity of the diverticulum becomes a closed vesicle lying
now within the cavity of the brain-case, in contact with the
end of the infundibulum. From the wall of the vesicle numerous little tubular projections grow out into the enveloping
mesodermic tissue, and these, by detachment from the parent
vesicle, become closed tubes or follicles. The entire structure
becomes converted in this manner into a mass of closed follicles held together by connective tissue, after which event
this mass acquires intimate union with the infundibular lobe.
 
Thus the pituitary body consists of two genetically distinct
parts, the anterior lobe being derived from the ectoderm of
the primitive pharyngeal or buccal cavity, and the posterior
lobe from the ectoderm of the central nervous svstem. The
posterior lobe, developing as it does as an evagination from
the floor of the inter-brain, is to be regarded as a small outlying lobe of the brain.
 
AVHiat remains of the cavity of the inter-brain, after its
walls have thus developed into the several structures described, is the third ventricle of the adult brain, and the
aperture of communication with the secondary fore-brain
vesicles becomes the foramen of Monro. Since the lateral
walls become the massive optic thalami, while the dorsal and
ventral walls give rise to much thinner structures, the cavity
of the vesicle is encroached up(m to a greater extent on the
sides than from above and below, and hence the form of the
third ventricle in the mature condition is that of a narrow
vertical fissure between the thalami.
 
 
 
The Metamorphosis of the Fore-brain Vesicle. —
 
The secondary lore-brain vesicle gives rise to the telencephalon, which includes the cerebral hemispheres and the
structures belonging directly to them. As above indicated,
this vesicle grows from the anterior wall of the primary forebntin vesicle as a diverticulum which is at first single, but
which sfK)n becomes divided into two lateral halves by the
formation of a cleft in the median plane (Fig. 147, /6). This
cleft or interpallial fissure is the early representative of the
longitudinal fissure of the adult cerebrum. The two vesicles
remain attached at their bases or stalks with the parent vesicle
and communicate by a common orifice with its cavity. The
vesicles of tiie secondarj' fore-brain grow in an upward and
backward direction as well as laterally, and their develo])ment is so much more raj)id than that of the other vesicles
that they soon spread over them and partially hide them
from view. It is for this reason that the mass resulting
from the fore-bniin vesicles, except their basal ganglia, is
known in comparative anatomy as tiie pallium or mantle
(Fig. 144).
 
The relative rate of growth of the cerebral hemispheres is
such that in the third month th(»y completely overlie the
inter-bniin and bv the sixth month thev have extended so
far back as to hide the corpora rjuadrigcniina.
 
The mesodermic tissue surrounding the developing brain
becomes ditlerentiated into the three brain-membranes, which
penetrate into the fissure and thereforc invest the vesicles
on their mesial surfaces as well as elsewhere. The invaginating layers of the dura mater constitute the ])rimitive
falx cerebri.
 
The metamorphosis of this pair of sacs into the cerebral
hemis])heres is broujrht about by three important processes :
first, the multiplication of the cells whicli compose its walls
to form the masses <»f nerve-cells and fibers of the hemispheres ; second, the formation of folds in the wall whereby
are pr<Khiced the fissures which divide the hemispheres into
lobes and convolutions ; and third, the development of adhesions within certain areas between the mesial walls of the two vesicles, by which the system of commissures of the
hemispheres is produced.
 
The walls of the cerebral vesicles are at first very thin,
consisting merely of several layers of spindle-shaped cells.
By the rapid multiplication of these cells, the walls are thick•ened and the cavity of the vesicle is gradually encroached
upon until the mature condition of the brain is attained,
when the cavity is relatively very much smaller than in the
fetus and constitutes the ventricle of the hemisphere or the
lateral ventricle. The nerve-cells develop processes or polar
prolongations, of which the most conspicuous, the axis-cylinder processes, lengthen out to form the axis cylinders of
nerve-fibers. The fibers thus formed are directed away from
the surface and make up the white medullary matter of the
hemispheres, while the more superficially placed layers of
cells constitute the gray matter of the cortex of the brain.
 
In addition to the cortical or superficial gray matter there
are masses of gray matter within the hemisphere, the basal
ganglia, which are likewise collections of nerve-cells. Witliin
a limited area on the lateral wall of each cerebral vesicle,
near the lower margin, the cells undergo excessive proliferation resulting in the production of a large ganglionic mass,
the corpus striatum, and of two smaller aggregations of cells,
the claustrum and the nucleus amygdala. These basal ganglia
are in reality an infolded part of the cortex.
 
Inasmuch as the cortical matter develops more rapidly, as
regards superficial extent, than does the medullary substance,
the cortex becomes thrown into folds, forming thus the convolutions and fissures of the hemispheres.
 
Some of the fissures of the brain are produced by an infolding of the entire thickness of the vesicle-wall so that
their presence is indicated by corresponding projections in
the walls of the ventricles. Such fissures are distinguished
as total fissures. Included in this category are the fissure
of Sylvius, which is represented in the wall of the lateral
ventricle by the corpus striatum; the calcarine fissure, the
dentate fissure, and the collateral fissure, which are responsible
respectively for the calcar avis, the hippocampus major, and the collateral eminence of the lateral ventricle ; and the gnst
transrerse flseure of the brain, the infolded wall in thia case
being very thin and consisting merely of the layer of epithelium which covers the choroid plexus.
 
The flssnre of Sylvins is the earliest fissure formed and one
of the most imjmrlant. At an early period in the history
of the secondary fore-brain, there is a region in the lower
part of the lateral wall of the vesicle where expansion is
loss rapid than elsewhere, this area, as it were, remaining
fixed. As the vesicle-wall innnediatoly surrounding thb
 
 
 
 
8))ot etmtiniies to expaml, n dimpling of the wall is produced,
(he depression bcinfj (li'sii;imtc(l the fossa of Sylvius (Fig. 152,
S'V The jKirt of the vcsiclo-wall In-hind the fos.'ia advances
forwanl and downward to form the future temporal lobe, and
thus till- fiwHJi ronu's ti» Ih' siirroiindwl hy a convolution
having the form of an incfiniplctt' rinfr, i>|M'n in front — the
ring lobe. The llcmr of tlii^ fossi undcinocs very eonsidorable thiekeninj; to form ilie basal ganglia — that is, the corpus
striatum, the amygdaloid niielens, and lln" ehuistnim. These
structures, most conspicuously tli<' i-orpns striafmn, cniToiioh
ujKin the cjivily of iho vesiili', the nucleus caudatus of the
 
 
 
 
corpus Btriatum bulging intu the floor anJ outer wall of the
adult lateral ventricle.
 
 
 
 
The cortical matter of the floor of the fossa of Sylvius,
beiug circumscribed by a groove or sulcus, constitutes tbe
 
 
 
 
bi. wllh right half of fore-braJn.
Dved: lb, CBvilf of inter-brHln; hy, stle of b^p. : Mbr. mid-brain roof; mv, nilil-braliL cuvily ; C wrvbi-lliiin : M. medulla
 
 
 
central lobe or island of Reil, which is subsequently brokea
up, by seiMjndnry fissures, into from five to seven email convolutions.
 
 
 
By the extension of the fossa of Sylvius backward, and by
the increased gi^owth of the vesicle-wall above and below it,
the fossil is converted into the flssnre of Sylvius (Fig. 156, B)y
and the island of Iteil is hidden from view. Subsequently
the ascending and anterior limbs are added to the chief or
horizontal part of the Kssure.
 
The anterior part of the ring lol>e corresponds with the
future frontal lobe, the ]K>sterior part represents the parietal
lobe while the lower part of the ring becomes the temporal
lobe. A backward extension of the ring lobe produces the
occipital lobe.
 
The cavity of the vesicle is mcKlifiiMl in form and extent eoincidentally with the formation of the corpus striatum and
the alterations in the ring lobe. Just as the ring lobe partially encircles the fossa of Sylvius, so does the cavity of
i\w. ventricle partially encircle the corpus striatum. An
anterior prolongation of the cavity extends into the com|>l<»tc(I frontal lobe as the anterior comu of the ventricle, and
iin (»xteusion downward and forward into the apex of the
temporal lobe constitutes the descending comu, while the
posterior horn is ^nulually protruded into the occipital lobe as
ihr latter dcvc](>])s. From the earliest stage, therefore, until
I he eoiii])lete(l condition is attained, the cavity of the ventrieli' eoiilonns in a general way to the shape of the henii■'?ph«Te. The a])ertiires of (Mummiuieation between the vesirli-j (>r thi* cerebrum and the eavitv of the inter-brain are the
lithr Y shaped foramen commune anterius or the foramen of
M«»iito.
 
The numial surfaces of the hemispheres are much modified
ht ehintieirr by the (levelopnicnt here of two total fissures,
(hr tiiruato flKHure and the choroid fissure. TIksc ap|)ear in
Hie lillh week while the ve^i<*les are >till separate fnun each
iiilnr ilnNMi III (heir Ntall\< of attaehnient to the inter-brain,
pii«ii it» the development, th(M*efor(', of tile eorpiis callosuni
.Old the Cnriiiv. The two lis<ni*e^ lie ejox' to<rother, pandlel
Willi iiiili othri* an«l with the niarLrin of the riuir lobe, their
r»»iir-.i' ront'nnniiiL: lo ihiit ot'ihe eaviiv t»t'the ventricle. Ik»'jiiiiiMiv ne;ir the anlrrioi* evtreniitv of the brain, ahove the
 
level of the corpus striatum, they pass backward and then
downward and afterward forward to terminate near the anterior extremity of the temporal lobe, thus incompletely encircling the striate body.
 
The arcuate flssnre is the more peripherally placed of the
two. Its anterior portion lies just above the region throughout which adhesions subsequently develop between the two
hemispheres, or in other words, above the position of the
future corpus callosum (Fig. 154, a./.). This part of the arcu
 
 
 
Fig. 154.— Mesial surface of left fore-brain vesicle of brain shown in Fig. 148 (F6) :
/.3/, foramen of Monro, or opening into inter-brain ; o/, arcuate fissure : chj, choroid fissure ; r," randbogen," corresponding to future corpus callosum and fornix;
o^f, olfactory lobe.
 
ate fissure is the sulcuB of the corpus callosum of the mature
brain. The posterior segment, that which belongs to the
temporal lobe (not present at this stage), is the future bippocampal or dentate Assure. The hippocampal fissure is represented ujion the mesial wall of the descending horn of the
lateral ventricle by the prominence known as the hippocampus
major.
 
The choroid fissure or fissure of the choroid plexus, forming
an incompI<?te ring within, and parallel with, that described
by the arcuate fissure, encircles the corpus striatum more
closely (Figs. 154, 155). It begins at the foramen of Monro,
and its anterior part lies under the position of the body of
the future fornix. It then sweeps around into the tem])oral
lobe and terminates near the anterior part of the latter. The
fissure of the chon)id plexus, like other total fissures, is an
infoMing of the wall of the cerebral vesicle. It presents the
l>eculiarity, however, that the infolded part of the wall is
extremely thin, consisting of but a single layer of epithelial cells. The pia mater, which everywhere closely invests the
surface of the bniin, is infolded with the vesicle-wall, the infolded part becoming very vascular and constituting the
choroid plexus of the lateral ventricle. The choroid plexus,
although within the limits of the ventricle, is excluded,
strictly sjKjaking, from its cavity by the layer of epithelium
which still covers it and which has been simply pushed before
it into that cavity. Since the epithelial layer is very thin
and easily ruptured, the choroid fissure is apjiarently an
opening into the cavity of the ventricle through which the
pia enters ; in the adult it is called the great transverse fissnre
of the brain.
 
The calcarine Assure, another of the total fissures, develops
in the latter part of the third month as a branch of the
arcuate fissure. It bulges into the mesial wall of the posterior horn of the ventricle, i)r()ducing the elevation known as
the calcar avis or hippocampus minor. Since the posterior
horn of the ventricle is developed as an extension of the cavity into the backward prolongation of the ring lobe which
forms the occipital lobe, the calcarine fissure necessarily is
later in appearing than the fissures above described.
 
The parieto-occipital fissure is added in the fourth month
as a branch of the calcarine, ellecting the definite demarcation between the parietal and occi])ital lobes.
 
The fissure of Rolando develo]is in the latter part of the
fifth month in two ])arts. The two furrows are at first
entirely se])arat('(l from each other by an intervening area of
cortex. Subsecjuently this part of the cortex sinks l>eneath the surface, as it were, sinec it expands less rajndly
than the adjacent regions, and in this way the upi>er and
lower limbs of the fissure become continuous. The sunken
cortical area is to Ix* found even in the adult brain as a deep
anneetant gyrus embedded in the Kolandic fissure at the position of its superior genu. TIk^ development of the fissure
of Kolando effects the division betw(HMi the fnmtal and jwirietal lobes.
 
The collateral fissure appears in the sixth month as a
longitudinal infolding of the mesial wall of the hemisphere below and parallel with the hippocarapal fissure. Being a
total fissure, its presence affects the wall of the cavity of the
vesicle, producing the eminentia collateralis. At about the
same time the calloso-marginal Assure . makes its appearance,
and this is morphologically continuous, through the medium
of the post-limbic sulcus, with the collateral fissure (Fig. 157).
These three fissures constitute the peripheral boundary of a
region of the mesial wall which is known in morphology
as the falciform or limbic lobe.
 
The longitudinal Assure in the early stage of the growth of
the cerebrum separates the two vesicles from each other except at the place where they are attached to the inter-brain ;
here the two sacs are united by that part of their common
anterior wall which is immediately in front of the apertures
of communication with the inter-brain and which is called
the lamina terminalis.
 
The development of adhesions between the mesial surfaces
of the hemisphere vesicles throughout certain definite areas
marks the beginning of the corpus callosnm and the fornix.
The fusion of these areas begins in the third month in the
region corresponding to the anterior pillars of the fornix, the
septum lucidum and the genu of the corpus callosum ; in
the fifth and sixth months adhesion occurs in the position of
the body of the fornix and of the body and splenium of the
corpus callosum.
 
Although the central white medullary matter of the cerebral hemisphere is covered almost universally by the cortical
gray matter, there is a limited area of the mesial surface from
which the gray matter is absent, leaving the white matter
ex]>osed. The area of uncovered white matter has the form
of a narrow band, which begins at the base of the hemisphere,
in front of the opening into the inter-brain, extends upward
along the anterior wall of the inter-brain, then passes backward along its roof and curves downward and outward behind,
and then forward under it, to terminate at the front part of
the temporal lobe. Thus this white band, which is known as
the fimbria, and which represents the lower mesial edge of
the hemisphere, almost encircles the inter-brain. The fimbria runs between the arcuate fit^sureand the fissure of the choroid
plexus (Fig. 155, /). It holds such a close relation to the lat
 
 
 
F[o. IM.— Mu'sl«l Bnrftcc of left hcmlsphcp;, hmln of fi'tim of three months
(ciilurRiid) : /.. fiirnii: r.r.. beginning of u>rpus cHlluiam; c.rl., partot vuriiui atrtstiiiii iirelilnB uruiind fmra of Sylvius ; a/., unttrinr. mill nj./i.. |HiitvrU'r parti of
urrnauj Huiin? ; rhj., i-l]i>ri>id Usrun.', the coni'ui li.v lH-tH'i'i.'ti u'liii'li und the corpni
klrlatnin ucComniudatLii the luUT-linin, which hna Ik'vii ivmiirtil. The Itiaare U
■icL'Upiiil by the pla luutuc.
 
tor fissure, Iwiiig placet! on its ]>erii)licr.il side, that it constitutes the e<lge of the apjtaroiit o|Htniiig into the cavity of the
vesicle thi-ough wliiuh the piji niiiter, iKiiriiig bloofl- vessels, is
reflctrted iuto the interior, and which, as pointed out above,
is the tniiisverM' fissure of the Itniin. The <i|ieuing is only
a])piir(^nt, however, since tlif wall is still iinlirokfii, although
reduced to ii single layer of ejntltflinin. The pia mater, forming, with its blood-vessels, the cliDniiil plexus of tlic lateral
ventri.-Ie, pushes the layer "f cpirhellnin before it, and althoM^di the |)lexns is s;ii«l to be within the eiivily of the ventricle, it is still covtirod by the layer of epitliclinm, the ependrma, whifh lines that oiivity.
 
Thi' part of (he (iuibria that ini mediately overlies the roof
of the inter-hrnin Iieeonii's iiitinmtely uniteil, as noted alx)ve,
with the eorri'sponiling jiarl of the titiibria of the other heniis|ihen>, these fused portions of the two limhri;e forming a flat
tnangular sheet, the body of the fornix. the anterior and
IKisti'i'ior portions of the fimbria, wich diverge from the
moilian plane, represent res|Mitiv<'ly the anterior ami posterior limbs of the fiiniix.
 
Noting the relation ->r tlu^ anterior part of the fimbria to
till- a]>or(ur(Mif eonniinnii'ation between the inler-brain and
the cereijnil vesicles, it becomes apparent that the anterior
pillar of the fornix forms the anterior and n|»per Iwuiidarj' of
 
 
the foranifQ of Munro. When, further, one considers the
relatiou uf the fimliria to the apparent oi>ening into the ventricle, through which the pia mater i« invaginated (the transverse fissure), it is explained why the edge of the fornix
appears as a narrcjw white band, not only as viewed from
within tile ventricular cavity, Imt tiho in a nit-Mal section of
the brain (Fig. 156, C).
 
 
 
 
t^o. liiK.—Fctal tiriiln at thr \ieg\xm\ng of Ihe tiiihlh moiitli <MlliaIkovlm> :
A.iiiperloi, B. Inderal, C, rnvslnl aiirfaru: K. tliaiiK of KoUndo: prf, ■■reecnlral
fi«Buw; S|f, HylirlanHwure: inip, lntBrp»rletiil llHiiin.'; jhw, parfet(ww«lpil«l(i»aurB!
pU, psratlel tiiuurv: eailra, calloiomirglnnl Buure: u'T, unoui : cale. culCBTine
 
Another important region of fusion of the opposed mesial
surfaces of the hemispheres is that corresponding lo the
future corpus calloBum Throughout this area the liemispheres
closely unite with each otiier The line of fusion begins
at the Imses of the vesicles, some little distance in front
of the anterior parts of the fimbriie (Fig. 155, f-c). and after
passing upward and luruird, curves horizontally backward kward I
 
 
 
in close relation with the fused portions of the fimbria?, now
the body of the fornix. The atUiesiou begins at the anterior
part in the third month, and atfects the regit)n of the body
and gplenlum of the future corpus callosum in the fifth and
sixth mouths. Fibers penetrate from one hemisphere to the ,
other throughout this zone of contact, intimately uniting the
cerebral hemispheres. The corpus callosum is therefore &
great commissure lietween the two halves of the cerebrum,
and is necessarily composed of fibers having a transverse 1
direction.
 
While the back part of the corpus callosum lies over the
body of the fornix and is in close contact with it, the front
part of the body of the corpus collusum, as also its genu or |
curve and its rostrum or ascending part are at some distance (
from the front parts of the fimbrite. In other words, while the j
great longitudinal fissure extends at first to the bases of the
cerebral vesicles, this fissure is made relatively loss deep by
the adhesions which occur between the mesial walls and which
result in the development of the corpus callosum ; and the
space below the anterior part of the corpus callosum, between
it and the anterior parts of the fimhriie (Fig. 1.56, C), is an
isobtied part of the great lonrjitudinal fissure. This space is
bounded on either side by thatjtart nf the wall of the corres- ,
ponding cerebral vesicle or hemisphere which is limited above |
and in front liy the corpus collusum, and behind by the anterior part of the fimbria or anterior limb of the fornix. The '
space is the so-cniled fifth ventricle of the adult brain. The |
circumscribed parts of the mesial walls of the hemisphei
which form the lateral walls of the space, together constitute '
the Beptum lucidum. The jtarts of the hemisphere walls that '
become the septum lucidum do not participate iu the procesa '
of fusion mentioned above. Their surfaces are iu contact) [
however, and do not develop the typical gray cortical matter, I
such OS appears elsewhere ujKin the surface of the cerebrum. J
Cortical gray matter is produced here, but only iu radi- 1
mentary form.
 
From what has been said, it will be seen that the t
layers of the septum lucidum are circumscritKid and opposed.
 
 
 
parts of the mesial walla of the hemispheres ; that the fifth
ventricle is not a tnie ventricle but an isolated part of the
longitudinal tiggure having no connection whatever witli the
system of ventricular cavities ; and that this .so-called ventricle is not, like the true ventricles of the brain, lined with
ependyma, but with atrophic gray cortical matter.
 
The limbic lobe has been referred to as that part of the
mesial aurf'ace of the hemisphere which is circumscribed by
the calloso- marginal fissure, the post-limbic sulcus, and the
collateral fissure. It is limite<l centrally by the fissure of the
corpus callosum and the hippocampal fissure, which are
represented in the fetal brain by the single uninterrupted
arcuate fissure. Hence the limbic lobe would include the
gyrus fornicatus, the isthmus, and the gyrus uncinatua, which
constitute morphologically a single ring-like convolution.
Schwalbe, however, includes with this so-called limbic lobe
all the surface of the mesial wall of the hemisphere included
between the arcuate fissure and the fissure of the choroid
plexua (Fig. 154), designating it the folciform lobe (Fig. 157).
 
 
 
 
 
 
 
The falciform lobo therefore consists of two ring-like convolutions, one within the other, the two l)eing separated from
each other by the arcuate fissure (the adult callosal and dentaf« fissures) and being limited centrally by the fissure of the
choruiii plexus (the great transverse fitisurc of the adult brain). While the outer of these concentric convolutions —
the limbic lobe of Broca — develops into the fornicate or callosal, the isthmian, and the uncinate gyri, the inner ring
differentiates but slightly, its cortical matter remaining
atrophic. The atrophic condition of the cortex here is associated with those adhesions between the mesial walls of the
hemispheres that result in the formation of the corpus callosum and the septum lucidum. By these adhesions the
continuity of the inner concentric convolution is broken, and
it is therefore represented, after the development of the corpus
callosum, by the atrophic gray matter of the septum lucidum,
by the gyrus dentatus, and by the lateral longitudinal striae
on the free surface of the cor])us callosum, the latter being an
atrophic or rudimentary convolution. Since the transverse
fissure of the brain is the centric boundary of the ring, the
fornix is also a part of the falciform lobe. To sum up, the
falciform lobe includes the gyrus fornicatus, the isthmus, the
gyrus uncinatus, the lateral longitudinal striae or taenia tectae
of the corpus callosum, the gyrus dentatus, the lamina* of the
septum lucidum and the fornix.
 
The olfactory lobe or rhinencephalon is an outgrowth from
the vesicle of the cerebral hemisphere. Its development begins in the fifth week by the pouehing-out of the wall of the
vesicle near the anterior part of its fioor (Figs. l-t7 and 149).
This diverticulum, which contains a cavity contiinious with
that (»f the vesicle, grows forward and so<ni l)ecomes somewhat clul)-sha]>ed. In the selachian.'^ (.sharks and dog-fish)
the projection attains a great relative size, the olfactory lobes
in these animals being one of the most eonsj)icuous ]uirts of
the brain. In all mammals except man it is well developed,
and in the horse its cavity persists throughout life. In man
the cavitv soon becomes obliterated and the lobe itself in
part aborts. The ])rotru(le(l })ortion, becoming more distinctly club-shaped, differentiates into the olfactory bulb and
the olfactory tract, the ))osition of the original cavity being
indicated by a more or less central mass of neuroglia conspicuous in cross-sections of those structures. The proximal
portion (»f the olfactory lobe is represented in the adult human Urain by the gray matter of the anterior perforated
lamina {or space), ami by the trigonuin ol facto riiiiii and the
area of Broca, as well aa by the inner and outer roots of the
olfactory tract (note olfactorj- lobe of dog's brain, t"ig. 156).
 
 
 
 
Fig.— Buc of dogabialn: al., o\lar\otj lube: a.ji,»., rcxiin corrcipandiiiK M
r perforaled spa<-c. which la Incluiled in the ulllictorv l<ibc ; /.S.. IliiBurt nT
i; a.h., hlppooampal ((>tus. deTelopvd to a Rn-iitor rtpRree than In human
1., leellonal siirCice of oltactoty lobe: m.. olltictury sulcus.
 
 
 
Because of the relation of the place of evagination of the
olfactory lobe to the fossa of Sylvius, it happens that a |>art
of this lobe, the anterior perforated lamina, is i^ititatcd at the
commeneeincnt of the fissure of Sylvius and that it is in eoiithmity with Iroth extremities of the ring lobe; hence, the
olfactory lobe Is connected with both extremities of the falciform lobe. To express it in the language of human anatomy,
the outer or lateral root of the olfactory tract is connected
with the gyiuB uncinatUB, while the inner or mesial root'may
be traced to the fore part of the gyrus fomicatns.
 
After what has been said, the reader need scarcely be reminded that the olfactory bulb and tract, often erroneously
referred to as the olfactory nerAe, are parts of a lobe of the
 
 
 
 
 
iirain, a lobe which in man is rudimentary but which in all
o(h(M* mammals is well developed.
 
Tahulaied Rijnim^ of ihe Derivatives of the Brain-Besides,
 
 
 
Hkain
VKtiK'l.KM.
AfliT
tiralu
 
Ililidbrnln
vuhIcIo.
 
 
 
Mill tiruin
VVblcie.
 
 
 
IlltlT
 
hritiii
 
 
 
Floor.
 
 
 
Medulla
oblongata.
 
 
 
PonH Varolii.
 
 
 
Roof.
 
 
 
Tela choroidea
inferior.
 
 
 
Lateral walls.
 
 
 
Inferior peduncles of cerebellum.
 
 
 
Hiiroiidary
fi in*
bruin
Vfnirlt*.
 
 
 
Peduncles of cerebrum. Posterior perforated
space.
 
(N)rpora albicantlii. Tuber cintTcum, infundlbulum, and
imrt of hypojihvHis. Uptic
ehiiiHm.
 
AntiTlor |K'rfonittMl lamina.
CorpUM 8triiitum. Island of
\W\\. Olfai'tory
lolic.
 
 
 
Po8terit)r medullary velum.
Cerebellum.
Anteri(»r medullary velum.
 
Posterior part of
toKnientum.
 
Corpora quadriuemina. I^amina quadrigemina.
 
Pineal l>ody. Posterior commissure. Epithelium of velum
interpositum.
 
 
 
Middle and superior peduncles
of cerebellum.
 
 
 
Brachia. Internal geniculate
bodies.
 
 
 
Optic thalami.
 
 
 
Convolutions of cerebral hemisplieres. Corpus rallosum. Fornix. iSeptum lucidum.
 
 
 
Cavitt.
 
 
 
Fourth
ventricle.
 
 
 
Aqueduct
of Sylvius.
 
 
 
Third
ventricle.
 
 
 
Lateral
ventricles.
 
 
 
TIII2 DEVELOPMENT OF THE PERIPHERAL NERVOUS SYSTEM.
 
'I'lif (l<»v<*l<>|)m<'nt of the pcri])heral nervous system is still
iiiV(»lvr<l ill some ilejxroe of obscurity. In general terms it
nmy Iw .slal4*(l that tin* ])eri])lieral nervous apparatus is derive<l as an e\t<>nsion of the central cerehro-spinal axis.
 
In the ease of the spinal nerves, e^'ich nerve-trunk is com*
poseil of hoth motor aiul sen.<orv fihers, the former being in
continuity with the spinal cord tlirough the medium of the
anterior (»r motor roots, and the latter through the posterior
or sensory roots, ea<'h sensory root ))o.^.sessing a ganglion.
The cranial nerves exhibit a less n^giilar composition. While
the trigeminal nerve, for e.xam|>le, arises by two roots, after
the manner of a spinal nerve, some others correspcmd in relative {N)sition and in mod(» of d<»velopinent to the ventral or
motor r(M>ts of th(» spinal nerves, and still others are equivalent to the Hi»nsory spinal roots.
 
 
The deTelopment of the sensoir nerve-Sbers ib d^jH-udent
upon and is preceiicd liy tliiit of the ganglia of the posterior
roots of the tspinal nervi'.=, and nf several ganglia of the head
rt'giiin which are related tu the development of certain of the
cranial nerves. Hence the consideration of the genesis of
 
 
 
Firi. IM.
 
 
 
oritr Rnbl). The
 
 
 
primitive ergments vn atlLt conneL'Icd wilh Ihe reiaiiluluK porllnn
genn-ltiTer. At the regiuu of tmnnltlon lliere ie to be ii
 
muHclc-plato ut the primiLlvc x
gemi-Uyer ; pmi. i»ri«Ul, imi6, vlwx-ral mldillB Ujrpr. 8, crotMCClion through a
Ilunl emhryo (nflvr Sagvuii'hl) : m, spinal cori] : tpa. lower thickened part of (he
nuurul ridge: ipp'. Its Dpptr allcnuated part, which Ii conllnuoiu with the Toorof
the Deural tube : ui. primitive aegmunt,
 
the ganglia niiist precede the account of the growth of the
sensory nerve-fibers.
 
The oriKin of the ganglia is connected with the early
history of the evoliilion nf the neural tuhe. Just after the sides of the medullary plate {vide p. 279) have united with
each other to form the neural tube, there appear two ridges
of cells between the tube and the epidermis, one on each side
of the raphe or line of union of the sides of the tube. Thene
ridges are the neural crests (Fig. 159). They first appear in
the region of the hind-brain and advance from this point
both headward and tailward. The ganglia develop from
these neural crests. The cells of the neural crest are usually
described as growing out from the neural tube, though according to His it is probable that they originate singly from
the ectoderm.
 
Tlic mass of cells composing the neural crest grows out\vard and then ventrad along the Avail of the neural tube, and
very soon undergoes segmentation into a number of cellmasses which are the rudimentary ganglia. In the spinal
region the number of segments corresponds to the number of future spinal nerves. In the head region there are
four segments. These latter, the cephalic ganglia, will be
referred to sul)se(iuently.
 
The segmentation of the neural crest corresponds in the
main witli the sogmcntation of the ])araxial plate of the
inc.-odcrm, whereby the myotomes are ))ro(luced, and ench
segment lies upon the inner side of :i myotome. The connection of the segments with the neural tnl)e beccmies reduced in each case to a slender strand, the point of continuity
of which with the tube is shifted farther awav from the
median line, as (level(>])nient proi^resses, to eorresptmd with
tl)e dorsolateral position of tiie sensory nerve-roots in the
mature condition.
 
theeells of the tranirlia ae<|nir(» eaeli an axis-evlinder process and a (h-ndrite or |)n>toj)la>iiiie process, beeoniing thus
bijKjIar e<'lls. Tli«' axons or axis-eylinder |>i'oeesses make
their way into the spinal e<)nl — in the ease of* the spinal
gauiilia — eunstitnting ilui> the dorsal nerve-roots, and ])nrsne
their eour>e within tiie eord as the e(>lnnins of (loll and Bnr<la('h. The (hndrites, eon>titntini:' llie distal ]>ortions of the
dorsal roots, join tlir ventral r(K)i< (»ii the distal si<le of the
iranulia and lM'<'onie liie sensory nerve-fibers of* the spinal
nerves. Allhonirji these two proe«.*sses grow out from opposite sides of the cell, the further growth of the cell is such
that both processes are connected with it by a common stalk,
thus producing the cell with T-sha|)ed process so characteristic of the spinal ganglia. Thus the ganglia are made up
of cells which are interpolated in the course of the sensory
nerve-fibers, and these cells may be regarded as having mignited from the developing cerebrospinal axis, or, if the
view of His be acc^ptwl, from the region of the e^ctoderm
from which the tube originates, their connection with the
axis l)eing maintained by the gradually lengthening out
axis-cylinder process.
 
The development of the motor nerve-fibers differs from
that of the sensory. These fibers, or at least, the axis cylinders of the fibers, are the elongated neurits of nerve-cells of
the spinal cord and brain. The neuroblasts of the thickened
neural tube, as they become fully differentiated nerve-cells,
migrate from their central position into the mantel layer, or
superficial stratum (Fig. 140). On the distal side of the nucleus of the cell, the protoplasm first becomes massed and
then lengthens out to form an axis-cylinder processor neurit,
which in all vertebrate animals grows out from the cerebrospinal axis to form the axis-cylinder of a motor nerve-fiber.
 
Although, in the wise of the spinal nerves, the motor and
sensory fibers are separated from each other at their origin
from the cord, they soon intermingle to constitute a spinal
nerve-trunk. In certain lower types, as cyclostomes and
amphioxus, the motor and the sensory fibers permanently
pursue separate routes to their ]>eriphenil distribution.
 
The envelopes of the nerve-fiber are acquired at a relatively late period. The appearance of the neurilemma precedes that of the white substance of Schwann. The
neurilemma is derived from the mesmlerm. The cells of
the latter apply themselves to the nerve and, penetrating
between the fibers, become arranged as an enveloping layer
upon each axis cylinder, ultimately forming a complete
sheath, the neurilemma. The persistent nuclei of these cells,
scantily surrounded with protoplasm, constitute the nervecorpuscles of the neurilemma. The medulla, or white substance of Schwann, is formed at a considerably later period
within the neurilemma. The ileixwit of the medullary sheath 1
varies as to time lor (Jifferent groups of SIhts — although the ]
time is constant for each groti|j — unJ proceeds always in a
direction away from the cell from which the fiber originates,
or, differently expressed, in the direction in which the fiber
eonveys impulses. Thus, in the spinal cord, groups of afferent
fibers may be distinguished from those that are efferent hf
observing the direction in wliich the medullary sheath develops — that is, whetiier tlio sheath appears first at the upper end |
of the fiber or at the lower end.
 
The cranial nerve-flbers in their development follow in the \
main the same general principles that govern the growth of
the iipina! nerves. That is to say, the motor fibers grow out 1
as extensions of the axis-cylinder processes of nerve-cells of i
the cephalic jmrt of the neural tul)e and the sensory fibers \
proceed from the cells of outlying ganglia, or in the case of
at least one nerve, the olfactory, from infolded and highly I
specialized ceils of the ectoderm.
 
The cephalic ganglia, four in number, have been referred I
to as resulting from tiie segmentation of the head-region of ]
the neural crest. As previously stated, the neural crest I
begins to grow first in the region of the hind-brain and j
extends from this point both forward and backward, occupying a iwsition upon the roof or dorsal wall of the hind-brain. \
The part of the neural crest belonging to the head-region i
then divides into the four masses or head-giinglia which are I
designated respectively the first or tiigemmaJ, the second or ]
acosticofadal, the third or glosBophanrngeal, and the fourth ;
or vagal, ganglia.
 
The trigeminal ganglion, which is very large, becomes di- i
vidwl into a smaller anterior (M>rtiou, the ophthalmic or ciliaiT
ganglion, and a. larger posterior segment, the tiigemlsal j
ganglion proper. These two become widely so|mrated durin^J
llic pnigres.'i of development, since they constitute respeo-J
lively the later ciliary and aasserian ganglia, the ciliary I
ganglion belonging to the ophthalmic division of the fifth I
nerve, while the trigeminal belongs to the sui^-rior maxillary J
division and the sensory part of the inferior maxillary divi- J
 
<n of the fifth. Their nerve-cells give rW. to the sensory fibers of these trunks in the same manner that the cells of the
spinal ganglia produce the sensory fibers of the spinal nerves.
 
The acusticofacial ganglion, afler its migration from its
original position on the dorsum of the hind-brain, lies just
in front of the otic vesicle. This ganglion subsequently
divides into the facial and the acoustic ganglia. The facial
ganglion, the geniculate ganglion or intumescentia ganglioform is of the facial nerve, situated in the facial canal of
the temporal bone, although described as a ganglion upon
a motor nerve, the facial, is, in reality, connected mainly
with the pars intermedia, a bundle of sensory fibers issuing
from the nucleus of origin of the glossopharyngeal nerve.
It is equivalent therefore to a spinal ganglion.
 
The acoustic portion of the acusticofacial ganglion divides
still further to become the ganglion on the vestibular part of
the auditory nerve, and the ganglion spirale of the cochlear
division of the auditory, which latter is situated in the spiral
canal of the modiolus. It is considered probable that the
lateral accessory auditory nucleus, which is connected with
the cochlear fibers of the auditory nerve and lies on the outer
side of the restiform body, is also a part of the acoustic
ganglion. From the cells of the vestibular ganglion, which
is situated in the internal meatus, centrifugal fibers develop
to form the vestibular nerve, while other centripetally growing
fibers become the ventral or mesial (vestibular) root of the
auditory nerve. The cochlear ganglion in the same way gives
rise to the cochlear branch of the nerve and to its dorsal or
lateral root. Thus the auditory nerve and its ganglia correspond respectively to the sensory root of a spinal nerve and
to a spinal ganglion.
 
The third cephalic ganglion becomes the ganglion of the
glossopharyngeal nerve, undergoing segmentation to form
the upper or jugular and the lower or petrous ganglia of this
nerve, while the axis-cylinder processes of its cells lengthen
out to become the sensory fibers.
 
The fourth cephalic ganglion similarly becomes the two
ganglia of the pneumogastric nerve and gives rise to its
sensory fibers.
 
 
From what has been said, it will be apparent that the
cranial nerves develop in a far less regular manner than the
spinal nerves, and that consequently their trunks consist in
some cases of only sensory fibers, in other cases of only
motor fibers, and in still others, of both varieties. Typically,
each cranial nerve would have a dorsal sensory root with a
ganglion, and two motor roots, one lateral and the other ventral. But by the suppression of one or two of these typical
roots there will be produced a nerve, for example, representing only the ventral root, as the sixth and twelfth nerves, or
a trunk containing sensory and lateral motor fibers, as the
vagus, or a nerve consisting solely of sensory fibers, as the
auditory.
 
By way of recapitulation the cranial nerves may be briefly
considered seriatim :
 
First Pair. — The olfactory nerve-filaments grow centripetal ly from the olfactory epithelium of the nasal mucous
membrane.
 
Second Pair. — The optic nerve is not a true nerve (see
Chapter XVI.).
 
Third Pair. — The oculomotor nerve represents a persistent
lateral motor root of the first head-segment (the ophthalmic
division of the fifth nerve being the sensory root of the same
segment).
 
Fourth Pair. — The trochlear nerve represents a lateral
motor root and belongs to the second head-segment.
 
Fifth Pair. — The trifacial or trigeminal nerve, containing
sensory and motor fibers, represents a persistent lateral motor
root and a dorsal sensory root. The ophthalmic portion of
the sensory root belongs to the first head-segment, while all
the remaining fibers, with the fourth nerve, are assigned to
the second segment.
 
Sixth Pair. — The abducens develops as a ventral motor
root and belongs to the third and possibly to the fourth
segments.
 
Seventh and Eighth Pairs. — The acusticofacialis nerve, or
the facial and auditory nerves, develop as a single nerve with
^veral roots. The auditory nerve and the sensory fibers ^^
 
 
 
the facial — that is, the pars intermedia — correspond to a dorsal sensory root, the division of the acusticofacial ganglion into
the several ganglia of the auditory nerve and the geniculate
ganglion of the facial accounting for the division of the root
into the auditory trunk and the ]xirs intermedia. (The sen*
sory tibors of the facial pass off through the chorda tympani
to go to the tongue as special-sense fibers.) The motor fibers
of the facial develop as a lateral motor root^ originating
from c-ells in the ventral zone. These two nerves, with the
sixth, belong to the third and possibly to the fourth head*
segments.
 
Xinth Pair. — The glossopharyngeal nerve, made up largely
of s(»ns()rv fibers?, re[)resents a dorsal sensory root and a lateral
motor root, the fib(»rs of which latter grow out from cells in
the dorsal ])art of the v(»ntral zone of His, the later micleus
ambigmis. It belongs to the fifth head-segment.
 
Tenth Pair. — The vagus develops in the same manner as
the glossoj)harvngeal.
 
Eleventh Pair. — The sj)inal acoessorv represents in part
motor spinal roots and in [)art probably the lateral motor and
dorsal scnsorv roots of the cranial nerves.
 
Twelfth l^iir. — The liy[)ogIossal develops as the ventral
motor roots of several segments, being identical in mode of
origin with the anterior roots of the sjnnal nerves. This
nerve and the vati^us b(*long to the head-sc»gments from the
sixth to the tenth inclusive.
 
THE DEVELOPMENT OF THE SYMPATHETIC SYSTEM.
 
There are two views as to the origin of the sympathetio
system. One theory, based uj)on the investigjitions of Paterson, is that the gangliated cord of the sym[)athetic is differentiated from mesodermie cells, the eell-eord thus formed
acquiring, secondarily, coinieetions with the spinal nerves,
and ])res<Miting still hiter the enlargements which C4mstitute
the ganglia.
 
The more genendly accept(»d vi(?w, based upon the researches of Rilfour and the later work of Onodi and His, is
that the Bsrmpathetie ganglia develop as offshoots from the ventral extremities of the spinal ganglia. Each little mass,
which has budded off from a spinal ganglion, moves somewhat toward the ventral surface of the body, its bond of
union with the parent spinal ganglion being drawn out to a
slender cord, the representative of the future ramus communicans. Each primitive sympathetic ganglion sends out
two small processes, one growing tail ward from its lower extremity, and one in the opposite direction from its upper end,
the approaching processes from each two adjacent ganglia
meeting and uniting and thus secondarily establishing the
connection between the different ganglia of one side of the
body and forming the gangliated cord of the ssrmpathetic.
From these ganglia migrating cells probably pass out to
develop into the secondary ganglia of certain viscera, as His
has shown to be the mode of origin of the ganglia of the
heart.
 
THE CAROTID BODY, THE COCCYGEAL BODY, THE ORGANS OF ZUCKERKANDL.
 
In connection with the sympathetic system may be mentioned the " carotid gland," or glomus caroticus, or intercarotid
ganglion, found at the bifurcation of the common carotid
artery ; the coccygeal body or " gland," Luschka's ganglion,
found at the lower extremity of the coccyx in relation with
branches of the middle sacral artery; and the organs of
Zuckerkandl, found in later fetal life and for a short time
after birth at the origin of the inferior mesenteric artery.
 
These structures present features in common with each
other in that they are made up of knots of blood-vessels
intermingled with collections of cells, among which are
numerous chroviaffine cells such as are found in the medulla
of the adrenal body and in the sympathetic ganglia ; and in
the furthor fact that they are penetrated by sympathetic
nerve-fibers.
 
That the cells of the carotid body and of the organs of Zuckerkandl are derived from the adjacent sympathetic ganglia has been established, but whether these bodies are for that reason to be classed as nervous structures is as yet uncertain.
 
 
 
+++++++++++++++++++++++++
 
CHAPTER XVI.
THE DEVELOPMENT OF THE SENSE ORGANS.
 
In the organs of the .senses we have to do with peripheral
nervous mechanisms of greater or less degrees of complexity,
the essential elements of which are elalK>ratelv modified or
specialized neiiro-epithelial cells. These neuro-epithelial
structures are specialized cells of the ectoderm, derive<l from
it either directly, hy the infolding of patches of ectodermic
epithelium, as in the case of the olfactory cells, or indirectly,
by growth outward from the central nervous system, as in
the case of the retina. The organs of the sense of touch, the
tactih' corpuscles of the skin and mucous membranes, are
distributed somewhat irregularly, while such highly Sjweialized struc^tures as the organs of the special senses of vision,
hearing, smell, and taste are provided with special protective
and accessory apparatuses.
 
THE DEVELOPMENT OF THE EYE.
 
It will perhaps larilitatc th<* eoinpreliension of the general
principles involved in the (Ifvclopincnt of the eye if its
function as the organ of vision is. krpt in mind, and if,
therefore, the retina and the o|)tic nerve are recognized as
the essential parts (»f the organ, and the other structures as
accessories. The retina an<l the optic nerve are an outgrowth from the brain, the rod- ai]<I eone-visual <'ells of the
former being epithelial cells so specialized as to serve as
j)ercipient eh-ments, while the (»j)tic nerve-fibers are the (-onducting medium. To alh)Wof tlu' penetration and refra<'ti<m
of the ravs of liirht, the (»verlving epich^rmis differentiates
into a transparent and refra<*tive medium, the crystalline
lens, and the necessary prote<»tion an<l means of nourishment are provided by the other constituent of the eyeball. Further protection is furnished by two folds of modified skin
and subcutaneous tissue, the esrelida, and lastly for the
lubrication and still further protection of the exposed part
of the eyeball, there is formed still another set of accessory
organs, the l&crimal appuratiu.
 
The first step in the development of the eye is the growth
of a diverticulum from the side of the primary fore-brain
vesicle (Fig, 160). These optic evaginations are qnite large
 
 
 
 
II of two-d«j- chlck-embryn ; B. bmln of huni»n embrro of
Oiowii the development of tbe opllc leslclva and bnlD-resl<
h. inter-brain; ob, optic veulcles.
 
 
 
as com{)arGd with the h rain- vesicle. They begin to be evident even before the neural tube is completely closed. As
the attached part of the diverticulum expands less rapidly
than the distal ]>ortioii, the evagination soon assumes the
form of a sac or vesicle, the optic vesicle, connected by a hollow stalk with the primarj' fore-brain. When the secondary
fore-brain vesicles gn)w out anteriorly from the primary vesicle, the region of the latter that Itecomes in consequence the
inter-brain is the part to which the stalk of the optic vesiele
in attachetl. Hence the optic vesicle is an appendage of the
inter-brain or thalamoncephalon and its point of attachment
to the latter Is at the lateral iwrt of the base, in front of the
region of the infundihiihim (Fig. 147, A and C).
 
The optic vesicle expands laterally and dorsally until.it
lies immediately lieneath the epidermis, forming a prorai
 
nenee on the side of the head (Fig. 62). The ectoderm
at the point of omtaet with the optic ver^iele becomes thick*
ened and depressed, the dilTerentiation of this lens-area being
the >tartin^ p«.»int of the erystaUine lem. The depressed
|iatoli of ectixlemi, sinking more <k»eply, is converted into a
sac, the lens-vesicle, the o >nnt*etion of which with the surfacecells is <mM\ l«»st. The ilistal wall of the optic vesicle, upon
coming into contact M'ith the lens-vesicle, undergoes invagination, this wall sinking in until the cavity of the vesicle is
ahno-t obliterated. Thus the vesicle is converted into the
druilile- walled optic cnp, the o|K'ning of which looks laterally
toward the surface of the head, and is occupied by the leusvesiele.
 
The invaginated wall of the vesicle — thatis, the layer
nearer the c<*nter of the cuji — becomes the retina, except its
pigment-layer, the latter resulting fn)m the outer layer of
the cup. The stalk <»f the cup l)ecomes the optic nerre.
The surroimding mesodermic tissue grows into the openings
nrferred to above, and gives rise to the vitreous hnmor,
while the mesodermic cells that closely envelop the optic cup
produce the uveal tract and the sclera and cornea.
 
Having tra<*ed briefly the develripment of the organ, its
sevcnil parts may now bo considered in detail.
 
The Retina and the Optic Nerve. — These two structures, as stated abov<*, an; directly derive<l from the optic
vesieh; and its stalk.
 
To rep4*at, for the sake of continuity, some points already
mentioned, tin; optic vesicle grows forth as a diverticidum
from the side; of tin? primary fore-brain vesicle, its appearance beting foreshadowed by a lateral bulging of this vesicle
even before the neural canal is com[)letely eloswl. When
the primary fore-bniin vesicle divides into the secondary
fore-brain vesicles and the vesicle of the inter-brain, the
regir>n of origin of the optic vesicle falls to the latter, the
jM»int (»f attachment being at the outer edge of the base of
the vesicle in front of the infundibular evagination. The
optic nerve is to be regar<le<l th(Tefore as springing from the
inter-brain or thahunen<*e|)halon.
 
 
 
 
Fig. 162.— Three BUMtmlvc sMgca of dsTelopnient of the eye, showinE fOmt.tlon or aeenndary nptic cup and crrilaltlne leiu in human embryos or 4 mm. < J),
fl mm. (B), &nd H mm,(C]. (Tonnieui): a, a, primltlTe optic veilclei; b, extern *l
Uyer of neciuidary opllc cup (fliture pfttmenl-Uyor of rellna) ; c. Inner layer of cnp
(re linn proper) ; il, lens-pit llhlekened and depreued eatudena):
 
 
 
diattfly under the epidermis, separated from it by nnly a thin
layer of embryonal connective tisBtie. This lateral position
of the optic vesicles is characteristic of the early stages of
development. After the end of the first month the eyes gradually move forward and downward toward their permanent position, which is approximately attained probably
early in the third month.
 
Shortly after the fourth week the distal or lateral wall and
the under surface of the optic vesicle l>ecome invaginated.
The invagination begins when the vesicle comes into contact
with the lens-vesicle (Fig. 162). When the infolding is
complete, the vesicle has l>econie the secondsiy optic cap,
which latter consists therefore of two layers, an inner and
an outer. The month of the cup, which faces away ffom
the niMliaii plane of the head, is occupied by the lens-vesicle.
Since the under surface of the vesicle ]>artici))a(es in the invnginating pniee.'is (Fig. 163) there is also in this wall of the
enp an ajwrture, which is known as the choroidal flsmre.
The invagination likcwi.sc affects the under siirfatre of the
tubular .-^lalk of the vesicle sii that it is c<mvertc<l into an inverted (loultle-Iaycred trough. These invaginations bear an
important relation not only to the
further nK'taniorphosis of the optic
vcsieli" lunl its stalk into the retina
and thf.iptic- nerve, but also to the
(levc4o|)nnTit iif the vitreous body
ami of llic r<-iiti-al artery of tlic n.'lina. TliMs, the vitreous binly is
PrmIiiwiI in yurX at !ca>t by the
Kie^iilcrmic tissue that finds access
to the cui) tlin.Li<rh llic ehoniidai
fiTisure, and tlic arloria wntralis
n-i'mw is di'v.IojK'd in the vascular
■ tissue iliat invagliiates
-uriaee of the stalk of
 
 
 
 
leM-lcl
 
 
 
the I
the '
 
 
 
si.-Ie.
 
gnuhially
1-oiitraeis after ilic enti-!in<-e of the
niescRlerni, and in the liist motitli of fetal life it entirely closes.
Tiie mouth of ilie o]Hie cup einlinices tite lens, its rim being
always on the ilislal side of, or su]>crlii-ial to, that .-itructure.
This iijieiiiiig repiTM-ntf* the pupil of later stagi-s.
 
 
The further metamorphosis of the optic cup includes alterations peculiar to each of the two layers and also to the
different regions of the cup. The mouth of the cup contracts
somewhat by increased growth of the wall, and thus there is
a zone bordering this orifice which is anterior to the lens,
holding the same relation to the latter body that the future
iris holds. A second zone corresponds with the periphery
of the lens, while a third region, the fundus of the cup,
includes all the remaining part of its wall.
 
The flmdas of the cup undergoes much greater specialization
than the other regions. The outer layer of the cup remains
thin, consisting of a single layer of cells which assume the
cuboidal form and become infiltrated with pigment-granules.
This forms the pigmenlrlayer of the retina. The inner
lamina of the cup thickens, by the multiplication of its cells,
and soon consists of numerous spindle-shaped cells. The
thickened fundus is marked off from the zone that surrounds
the periphery of the lens by a slight groove which corresponds in position with the future ora serrata. These early
spindlc-cells give rise to two kinds of elements, the stroma
of the retina, or Miiller's fibers, and the various nerve-cells,
including the highly specialized rod- and cone-visnal cells.
 
The principal sustentacular elements, or Miiller^s fibers,
like the spongioblasts of the neural tube, are radially
arranged and extend throughout the entire tliickness of the
retina. Their inner expanded extremities, in close contact
with each other, form the inner limiting membrane, while
their outer ends, in the same way, constitute the outer limiting membrane, which latter is in contact with the pigmentlaver. The stroma of the retina receives a small contribution from the mesodermic tissue, which grows into it through
the choroidal fissure to furnish the vascular supply.
 
Of the nerve-cells, those near the pigment-layer undergo
great alteration in form and become the sensory epithelium
— that is, the rod- and cone-visual cells. At first these lie
entirely internal to the external limiting membrane, which
separates them from the pigment-layer. After a time, liowever, processes grow out — that is, away from the center of the eyeljall — and ]>erforate the external limiting membrane
to i»eiietrale Iwtwecn the cells of the pigment-layer. These
 
jmxresscs are llu' rods and fonen, and colWtively constitute
the layer of rods and conea of (lit- iidult The bodies of the
 
 
 
 
KoMlsni pi. pi BtQ* 11 led I'plthcUiiTii iif Ihc eju I'luttr luniella uf the opUc enp,fl^
aceondary optic veiklcl ; r.rolinii(iniiprlain*llB of the optic cup); it, mKi^iUln
nf (he optic cup. which farma the pun clllarls el Iriills rellnie: g, Til
with blood-TCBKla : fv, tunlm tucuIou leiilla; U. blood-uurpuwlei : tA.ehon
tf, lens-flben: It, leiu-eplthcllum ; r.nnu of the leni-tlbcr nuclei; ft, ftindaB
at the iK>niea; he, exlernAl corneal epithelium.
 
 
 
rod- and cone-visual cells, situated on (he inner side of the i
membrana limitan.s externa, are elongated into narrow ele- j
ments, the position of tho nnctei being indicated by slight I
enlai^ments. They constitnte the outer nuclear layer of '
the matnre retina. The outer nuclear layer and the layer of rods and cones are to be regarded, therefore, as one layer of
highly specialized neuro-epithelium, made up of the rodvisual cells and the cone-visual cells, the inner segments or
bodies of the cells being only apparently isolated from the
outer segments, the rods and cones respectively, by the fact
that the latter proj(?ct through minute apertures in the
external limiting membrane. The axis-cylinder pnxjcssesof
these cells pass toward the center of the eyeball.
 
The neuro-epithelium of the retina is the last of its elements
to develop. In man and in many mammals, it is present at
birth. In the cat and the rabbit, the rod- and cone-visual
cells develop after birth, and hence the new-born of these
species are blind. The macula lutea is developed after birth.
 
The cells of the inner part of the retina differentiate into
the remaining nervous elements, some becoming the bipolar
and other cells of the inner nuclear layer — the ganglion
retinae — while others form the large ganglion cells of the
ganglion-cell layer. The axis-cylinder processes of the
ganglion cells are directed inward to form the nerve-flber
layer, the fibers of this layer converging from all parts of
the inner surface of the retina toward the optic disk or
papilla. Here they perforate the retina, as well as the choroid and sclera, to pass, as optic nerve-fibers, to the brain.
 
This part of the optic cup, the ftmdus, produces then, in
the manner descrilKjd above, the functionating portion of the
retina, or the pars optica retinae, the anterior termination of
which is indicated by the orra serrata.
 
The lenticular zone of the optic cup, which is in relation
with the peripherj' of the lens, undergoes comparatively
slight specialization. Its outer lamella is pigmented, as in
the fundus of the cup. Its inner layer remains very thin
and consists of cells which at first are cuboidal, but which
later become cylindrical. At the end of the second month,
or the beginning of the third, the two layers of the lenticular
zone become plicated, owing to excessive growth in superficial extent. The folds are nearly parallel and are arranged
radially with reference to the lens, the margin of which they
surround. These folds are the first indication of the ciliary processes. The mcstKlermic tissue immediately external to
th(» <)j)ti(! cup (lifforentiates into the uveal tract, the part corresponding with the lenticular zone of the cup furnishing the
ciliary hody. The young growing connective tissue penetnitos between the folds of the lenticular zone of the cup,
acupiiring intimate union with the pigment-layer, and thus
provides the connective-tissue basis of the ciliary processes.
This lenticular zone of the two hiyers of the optic cup,
therci'on*, <'onstitutes the lining, or internal covering, of the
ciliary ImmIv, and must necessarily be reganled as the continuation of the retina, it is known as the pars ciliaris retiiUB
of the fully developed (?ye.
 
TIkj marginal zone of the optic cup, or the region bordering its orilice, is also related in its further growth with the
uveal traet. Although in the earlier stages of development
the lens lies in the mouth of the cuj), as time goes on the
relation is so altered that the a[K»rture and the zone which
borders it occupy a position in front of the lens. In this
marginal zone both lamelhe of the cuj) become pigmented
and aecjuire union with the layer oi' mes(Klermic tissue which
is dillerentiatiiiir into the iris, and thev therefore contribute
to the formation of that structure, ecmstituting its pigmentlayer. Th(^ pigment-layer of the ]>osterier surface of the
iris is, therefore, an extended but rudimentary part of the
retina. It is called the pars iridica retinae.
 
FnMu what has been said, it will be ap])arent that the
retina f(>rms a (Mnnplete tunie with an anterior perforation,
the pupil, and that it consists of the funetionally active part,
or retina proper, th(» pars optica retinae; of the pars ciliaris
retinae, marked olV iVom the latter by the ora s(»rrata ; and
of the pars iridica retinae, which terminates at the margin
of the pupil.
 
The evolution of the optic cuj) or secondary oj)tic vesicle
mav b(» thus summarized :
 
I. Marginal or most anterior The thin atrof)liic pare iridica reregion of cup. tinir, or pi^rnicnt layer of the iris.
 
n. Ltnticular zone of cup. Pars ciliaris rt-tina*, covering inner
 
surface of ciliary body.
 
 
 
 
III. Fundus of cup. Functionating part of retina, or pars
 
optica retins, including :
A. Outer layer. A. Pigment-layer of retina.
 
£. Inner layer. B. 1. Neuro-epithelial layer, made
 
up of layer of rods and cones (the processes of the rod- and cone-visual cells) ;
membrana limitans externa; outer
nuclear layer (the bodies of the rodand cone-cells).
 
2. Cerebral layer (representing an
interpolated ganglion with connecting
fibers), consisting of :
 
Outer reticular layer ;
Inner nuclear layer ;
Inner reticular layer;
Ganglion-cell layer;
Nerve-fiber laver.
 
 
 
The optic nerve is the metamorphosed stalk of the optic vesicle. When the distal and under surfaces of the vesicle suffer
invagination, the stalk participates in the process, its under
surface being marked by a groove which is a prolongation of
the choroidal fissure of the optic cup (Fig. 163). By this infolding, the cavity of the stalk is obliterated and the stalk is
converted into a double-walled tube enclosing mesodermic
tissue which follows the invaginating ventral wall. In this
mesodermic tissue is developed the arteria centralis retina.
In mammals the invagination affects only the distal part of
the stalk, the segment included between the eyeball and the
point corresjionding in the adult to the place of entrance into
the nerve of the central artery. It must be apparent that
the outer layer of the tube thus formed is directly continuous
with the outer layer of the optic cup, while the invaginated
lamina is the prolongation of the inner wall of the cup or
of the part that becomes the retina proper, since not only
the distal wall of the optic, vesicle is invaginated, but its
under or ventral wall as well.
 
the primitive optic nerve at this stage consists of layers
of spindle-shaped cells, with a central core of vascular connective tissue.
 
The manner in which the nerve-fibers are developed is still a matter of controverjsy. According to His and K5lliker, the fibers gi*o\v out from the ganglion-cells of the optic
thahimi and the anterior corpora quadrigemina^ while Muller
and Froriep believe that they are the prolonged axis-cylinder
processes of the ganglion-cells of the retina. According to
Ramon y Cajal, growth occurs in both directions. In
either ease, the cells of the optic stalk would furnish only the
sustentative tissue of the nerve. There is also a contribution of sustentative tissue or stroma fnmi the mesoderm, as in
the case of the central nervous svstem.
 
The Crystalline I^ens. — The lens, exclusive of its capsule, is, like the retina, of ectodermic origin. The first step
in its <leveIopm(»nt is the formation of a thickened and deI)ressed patch of the ectoderm on the lateral surface of the
hea<l, this area being situated at the place where the optic
vesicle is nearest the surface (Fig. U)2, By d). The depression is the lens-pit. It soon becomes converted into a
closed sac, the lens-vesicle, by the gradual a])proximation and
union of its edges. The pit receding from the surface as its
lips come together, the completed vesicle lies under the surface ectoderm, witli wliieh it is for a time connected by the
slender stalk of tho invagination. Upon the disapi)earanc(» of the strand of cells constituting the stalk, the lensvesicle is completely isolated from the outer germ-layer
(Kig. 1(12, (\i'l
 
The lens-voside in birds is a hollow epithelial sac several
lavers thick, but in nuunmals the central cavitv contains a
mass of (jells, which latter disappear in the later stages of
development.
 
I^pon the invagination of th<» optic vesicle to form the
se(M>ndary optic cu|>, the lens-vesich? is embracwl by the lips
of the cup and still later Cannes to lie within the cup, near its
orifice (Fig. 1^)4).
 
The further alterations in the vesicle are de|KMident primarily upon changes in its deep and sujH^rficial walls resjH^ctively, each of wliich consists of several layers of cylindrical cells. the cells of the sui>erficial wall alter their
form, bcH?oming cul)oi<lal, while the posterior or decjwr cells
lengthen so as to become fibers. Thus the deeper wall of the vesicle thickens at the expense of the central cavity — the
central mass of cells at the same time disappearing — while
the superficial layer remains thin. The two strata are continuous with each other at the equator of the lens, one form
gradually merging into the other at this region, which is a
zone of transition (Fig. 164).
 
The lens at this stage is composed, therefore, of a thin
superficial or anterior stratum of cuboidal epithelial cells and
a much thicker posterior or deep layer of so-called fibers, the
latter being simply the greatly elongated cells of the posterior
wall of the vesicle. Between the two laminse is a small
remnant of the cavity of the vesicle. The epithelial layer
persists throughout life as the epithelium of the lens, while
the fibrous layer is the basis of the lens-fibers of the mature
condition. The cavity sometimes persists as a small space
containing a few drops of fluid, the liquor of Morgagni.
 
The next important stage in the development of the lens
is the formation of additional lens-fibers. These result from
the proliferation of the cells of the epithelial or anterior
layer. The lens-fibers are formed in successive layers, as
may be made evident by the maceration of a lens. Each
fiber extends from the anterior to the posterior surface of the
lens. The ends of the fibers meet each other along regular
lines, producing thus the characteristic three-rayed figures or
stars of the lens, one of which belongs to each surface.
Hence, while the lens-fibers first formed are the elongated
cells of the posterior layer of the lens-vesicle, the fibers of
later gro^vth originate from the cells of the anterior wall.
The epithelial character of the lens-fibers is evinced by the
presence of a nucleus in each fiber of a young lens.
 
The lens-capsule results from the differentiation of the
mesodermic tissue which surrounds the lens. It is from this
enveloping vascular lamina, the tunica vasculosa lentis, that
the growing lens derives its nutrition. The capsule is well
marked in the second month. Its blood-vessels are derived
from those of the vitreous body. At the end of the seventh
month this well-developed, highly vascular membrane begins
to undergo retrograde alterations, the final result of which is its transformation into the thin, non-vascular^ transparent
capsii le of tlio mature lens.* The most active growth of the lens
itself occurs prior to the degeneration of the tunica vasculosa
lentis, so that even before the end of fetal life the lens has
nearly attained its full size. Thus the weight of the lens of
the new-born child is 123 milligrammes, while that of the
adult lens is but 11)0 milligrammes (Huschke).
 
Hence the crystalline lens has a double origin, the lens-substance or lens proper being derived from the ectoderm, while
the capsule oriirj nates from the mesoderm.
 
The Vitreous Body, — The vitreous body has been regarded usually as a roniparatively slightly differentiated form
of connective tissue, and as being derived from the middle
germ-layer. Recent investigtitions show, however, that it
originates in ])art at least from ectodermal tissue. According to these observations, processes grow forth from those
stromal elements of the optic cup which afterwanl Ix^come
Midler's libers, and these processes, advancing toward the
lens-vesiele, interlace to form a network, the primitive
vitreous (Kolliker, Froriep). This ])roeess continues for a
longer time at the marginal zone or month of the cup than
elsewhere, the j>roto|)lasniie fil>ers which grow from this
future ciliary and iridal j)ortion of the cup contributing to
the lorniaiion of the zonule of Zinn. In mammals the cells
of the lens-vesicle, another ectodermal structure, also send
torth processes which, according to Lenhossek, bear a prominent part in the d<velo|)ment of the vitreons body. The
mesodermic tissue, already in the sta<x<* <>f cnd)rvonal connective tissue, now gains access to the optic (Mip through the
choroidal fissure (Fig. 1 <>•>), its ingrowth, in fact, accomjKinying the invagination of the un<l(M' >urta<M' of the opti(^ vesicle,
and constitutes what K(")lliker designates the mesodermal
vitreous. The intermingling of these two constituent ele
' It sonu'titiu's li:i|>|»«.Mis that parts of the fetal lons-capsulc persist. The
most ootnmon exani]>le of 8\i<h persistence is the so-called meiiihrana pupillaris soinetinK*s present at hirth, pnuhicinK rmnjnn'tal otrtAia of the pupil.
This results from the persistence (»f that part of the fetal capsule which is
situated on the anterior surface of the lens, hehind the pupil.
 
 
 
THE MIDDLE AND OUTER TUNICS OF THE EYE. 339
 
ments produces finally the definitive vitreous. Since the
inferior surface of the stalk of the vesicle — the future optic
nerve — participates in the invagination of the optic cup, the
mass of mesodermic tissue which helps to form the vitreous
is continuous with that which invaginates the primitive optic
nerve to produce the central artery of the retina. As a consequence, the blood-vessels which soon develop so plentifully
in the vitreous b(xly are extensions from the central artery
of the retina, the latter itself being continued forward as the
hyaloid artery. The terminal branches of the hyaloid artery
pass on through the vitreous body to terminate in the vascular capsule of the growing lens, constituting the blood-supply
of that structure.
 
The intercellular substance of the young tissue undergoes
but little differentiation, while the cells become gradually
reduced to a few stellate elements which ultimately entirely
disappear. The peripheral part of the tissue develops into
the hyaloid membrane, which anteriorly acquires union with
the capsule of the lens.
 
The blood-vessels of the vitreous disappear during the last
two or three months of fetal life. The hyaloid artery persists, although in reduced form, for a longer time than the
smaller vessels. Upon its final degeneration it is replaced
by a canal, the hyaloid canal, or canal of Stilling, which is
present in adult life.
 
The Middle or Vascular and the Outer or Fibrous
Tunics of the Bye. — The outer fibrous coat of the eye, including the sclera and the cornea, and the middle tunic or
uveal tract, comprising the choroid, the ciliary body, and the
iris, are structures of mesodermic origin, being directly produced by the mesodermic tissue surrounding the optic cup.
The richly cellular mesoderm applies itself to the exterior of
the cup and differentiates into the two layers in question, the
changes involving on the one hand the metamorphosis of the
mesodermic cells chiefly into muscular and vascular elements,
and on the other hand the evolution of a tissue essentially
fibrous in structure. These two tunics are distinguishable
in the sixth week.
 
 
the cornea is formed from the thin laver of mesoderm that
penetrates l)et\veen the lens-vesicle and the surface ectoderm.
The lens- vesicle lies very near the surface, and the thin
stRitum of mesoderm that is interposed between the two is
the anterior layer of the lens-capsule (Fig. 164). This anterior layer thick(»ns by the immigration of other cells and subsequently splits into two lamime, a superficial one which produces the cornea (Fig. 104, A), and a deeiK?r, which is now the
proper anterior wall of the lens-c^apsule. Thus a space filled
with fluid a pjwars between the primitive cornea and the lens,
which (H)rresponds with the future anterior and posterior
chambers of the eye, the <livisicm of the sj>ace into these two
chambers being eflectcd subsequently by the development of
the iris. The further (leveloi)meiit of the cornea consists
simply in the differentiation of the mesodermic cells and the
int(M'eellular substance into the several characteristic elements
of the adult structure.
 
The uveal tract closely corresponds in extent with the two
layers of the oi)tic cup. The choroid is differentiated from
that portion of this primitive uveal tract which envelops the
pars optica of the retina. In this region the enveloping
layer of nicso<lcrniic cells develops into the several elements
of the choroid, the most C()ns])icuous of which are an inner
layer of capillary vessels, tlu^ choriocapillaris, and an outer
lay(»r of larg(?r vessels, th(» stroma-layer of the choroid. the
development of the clioroi<l bears a certain relation to the
choroidal fissure of the optic cup. This tissure has been referred to as a gap in the iMi<lcr surface of the eup corresponding with the line of invagination through which the
mesodermic tissue, of which the developing choroid is a
part, grows into the cup to pnxluee the vitreous. Although
normally this fissure in the retina entirely disjippears, its site
be<*onies pigmented later than other regions of the pigment«laycr of the retina, and h<*nce there is, for a time, a clear
streak in this part of the retina which has the appeanince of
a fissure in that membrane. As the pigment-layer of the
r(»tina was formerly assigncnl to the choroid, this streak appeared to be a breach of continuity of the ch<»roid ; hence the term choroidal fissure. In some cases, however, the choroidal
fissure fails to close, and as the development of the choroid
is largely dependent upon or is governed by that of the
retina there remains a corresponding gap in the choroid.
This defect enables the sclera to be seen from the interior in
a line extending forward from the optic nerve entrance. It
is known as coloboma of the choroid.
 
The ciliary body is developed immediately in advance of
the choroid and from the same layer of mesodermic tissue.
The deeper parts of the tissue in this region correspond with
the plications of the ciliary part of the retina, sending processes into and between the radial folds of this part of the
two layers of the optic cup, with which latter the highly
vascular mesodermic tissue acquires firm union. This results
in the formation of the ciliary processes. Some of the cells
of the more peripheral part of this zone are converted into
unstriated muscular tissue, thus producing the ciliary muscle.
All the characteristic or important elements of the ciliary
body are, therefore, derived from the mesoderm, while the
thin layer of tissue on its inner surface, representing an
undeveloped part of the optic cup, the pars ciliaris, is of ectodermic origin.
 
The iris, the most anterior zone of the uveal tunic, is produced from the same mesodermic tract that gives rise to the
choroid and to the ciliary body. As stated above, soon after
the lens-vesicle becomes constricted off from the surface ectoderm, it is enveloped by a mass of mesodermic cells which
constitute its primitive capsule, and the layer of these cells
lying between the lens-vesicle and the surface ectoderm splits
into an anterior layer, which becomes the cornea, and a posterior stratum which is the anterior wall of the lens-capsule.
This produces a space between the lens and the cornea. The
lens now recedes farther from the surface, and the margins
of the optic cup advance, so that the lens now lies within
the cup, the marginal zone of the cup being in front of the
lens, between it and the cornea, while its equator is in close
relation with the ciliary regions of the cup and of the uveal
tract. Thus the space between the lens and the cornea is
 
 
 
 
H divided into an iiiitcrior cdnipartnicnl, tlic anterior chamber,
 
H ntid a jmsterior fl])ai-p, tin- posterior chamber, the orifice of
 
■ tlic cup being a mi-nns of comiTHinication l>plnceii the twd
 
H and representing ttie pupil of a iaier sla^, the niar^ual
 
H zone of the cuj) furnishes the gniding line for the develo]*
^1 ment of the iris, The niesf«lcniiie tissue in rclutioD with
 
 
 
tm. Iffi.— BtgltEal iiectlon IhroUBh ttif pyp nf an embryo i
 
lUyi X 30 (Kailkerl: o,o|illc nttrc; j.. licmgimBl Hmni-nl-U
 
CllUry pun nf the rellnt ; p'. fi>rppiirl "f llie i>l.ll« PUp (rudiment of thB il
 
the srterli twnlralls retlnit unler II: f. trls; mj,, nienibr«n» pupllUriii «,«
with Piilib*tluro (,- pp.pn. iMiliwbne; I. lem; V. Icnt-cpllhcHum ; /. MlwottBjJ
 
 
 
the outer surface of the marginal stone of the euji difTerc
tiatca into the vascular, muscular, and couneclive-tiasuc elM
tnenta of the iris pn)iwr, while ils pfisterior pignient-laycr fa
constituted by the slightly specialized layers of the mot
anterior part of the optic cup, the part that in known a* th<
para iridica retinre. Recent investigations (Xushhauni, Her^
zog, etc.) indicate that the circular and the radial muscular'J
 
 
 
fibers of the iris develop from the outer epithelial layer of
the optic cup or, iu other words, from the part of the optic
cup that becomes the pars iridica retina?. The circular fibers,
sphincter pupillee, are distinguishable in the fourth month,
the radial or dilator fibers, in the seventh mouth.
 
Since the anterior and posterior chambers of the eye are
spaces hollowed out of the mesoderm, they represent a
lymph-space and are, as such, lined with endothelial cells.
 
The cleft in the inferior wall of the optic cup referred to
above as the choroidal fissure necessarily affects the marginal zone of the cup as well as the region posterior to it.
If this part of the fissure persists, as it sometimes does, it
may be accompanied by a corresponding deficiency in the
tissues of the iris projjer. Such a congenital defect, appearing as a radial cleft in the lower half of the iris, is known as
coloboma of the iris.
 
The Eyelids and the Lacrimal Apparatus. — The eyelids are developed from folds of the primitive epidermis that form over the superficial part of the developing
eyeball (Fig. 165, pp and pa). After the separation of the
lens- vesicle from the surface ectoderm, the latter pouches
out into two little transverse folds for the upper and lower
lids respectively. Each fold includes a certain quantity of
mesodermic tissue, from which are produced the connectivetissue elem.ent^ of the lids, as the tarsal plates, etc. After
the folds attain to a certain degree of development their
eilges approach each other and become adherent, thus enclosing a space between the primitive lids and the front of the
eyeball. The infolded ectodermic layers lining this space
acquire the characteristic features of mucous membrane and
constitute the epithelium of the coAJnnctiva, the part of this
membrane that covers the cornea adhering closely to that
structure as its anterior epithelial layer. The union of the
edges of the lids begins in the third month and lasts until
near the close of fetal life. A short time before birth the
permanent palpebral fissure begins to form by the breaking
down of the adhesions.
 
A part of the mesodermic tissue of the lids undergoes conversion into fibmiis connective tissue, thus producing the
tarsal plates of the upper and lower lids^ with the iMJpebral
DasciflB and tarsal ligaments by which the plates are attached
to the margins of the orbit.
 
During the period when the edges of the lids are adherent,
the Meibomian glands and the eye-lashes are formed. The
ghmds develop from solid cords of epithelial cells that grow
from the deepest or Malpighian layer of the primitive epidermis into the tarsal plates. The conls become hollow
tubes by degeneration of their eentnd cells.
 
In addition to the two principal folds that produce the
lids, a third, vertical fold app(\ars at the inner, nasal side of
the conjunctival space, beneath the lids. This fold remains
quit(i small in man and forms the plica semilunaris, but in
most other vertebrates it attains much greater size as the
third eyelid or nictitating membrane. A small part of this
third fold develops sebacw)us glands and a few hair-follicles
and becomes the lacrimal caruncle.
 
The lacrimal gland is devel(>]KHl in the same manner as
tl)(i Meibomian glands, by the growth of solid epithelial
cords from the conjunctiva. The cords grow into the underlying inesodcnn at th(» outer part of the line of reflection of
the conjunctiva from the inner surface of the upj)er lid to the
front of the eyeball. The conls acquire lateral branches and
then become liollowcd out to form the secreting tubules and
efferent ducts of the gland, the connective-tissue stroma of
which is contribut<'d by the surrounding mesodermic tissue.
The orifices of the adult efferent ducts in the upj>er outer
j)art of the conjunctival sacj corr(»sjK)nd with the points from
which the primitive cell-cords first grow forth.
 
Th(> efferent lacrimal apparatus, consisting of the nasal
or lacrimal du<'t and the canaliculi, is related genetically
to the growth of the nose and the upjxT jaw. S(M)n after
the appearance of the nasofrontal ]>rocess, a lateral projection, the lateral nasal process, grows from its side near the
base and advances <lownward so as to form the outer boundary of the nasal j)it and consequently of the future nostril
(Fig. ()7, A, i>). This lateral nasal process is separated from the maxillary process of the first visceral arch by an oblique
furrow, the naso-optic groove, which extends from the inner
angle of the orbit to the outer side of the nostril, or, before
the separation of the nasal pit from the primitive mouth, to
the upper boundary of the latter orifice. The naso-optic
groove indicates the situation of the lacrimal duct. By
some authorities — Coste and Kolliker — it is believed that
the duct results from the union of the edges of the groove.
Later investigations seem to indicate, however, that the
duct is formed by the hollowing out of a solid cord of epithelial cells that appears at the bottom of the furrow. In
either case the epithelial lining of the duct is an ectodermic
involution. When the nostrils are separated from the oral
aperture by the union of the nasofrontal, the lateral nasal,
and the maxillary processes (p. 133), the lower end of the
furrow is obliterated, and the partially formed duct is made
to terminate in the nasal cavity.
 
The canaliculi, representing the bifurcated upper extremity of the duct, result, according to one view, from the
division of the upper end of the epithelial cord into two
limbs, one for each lid, and their subsequent hollowing-out ;
according to another, from the continuation of the cell-cord
into the upper lid and the later addition of a limb for the
canaliculus of the lower lid. The lacrimal sac is merely an
expanded part of the duct.
 
THE DeVELOPMENT OF THE ORGAN OF HEARING.
 
As in the case of the other sense-organs, the auditory
apparatus consists of highly specialized nenro-epiiheliiim,
connected by nerve-fibers and interpolated ganglia with the
central nervous system, and of protective and auxiliary
structures. The neuro-epithelial structures, including the
organ of Corti and the cells of the cristsB and maculae
acusticae, result from the specialization of certain of the
epithelial cells which line the membranous labyrinth. The
perilymphatic space, which is a lymph-space, together with
its bony walls, the osseous labyrinth, serve for the protection of the delicate neural elements, while the middle ear
 
 
 
ami tlio nxleriial oar act ;
siinuroiis vibratiims.
 
Till' iiitcrniil ciir Iwiiig the css^scntial [lart of the organ of
hciiriiif^ ami hriiij; alsd the part first formed may i»ro|wrIy
n-ccive tirst eoiisiih-nit^m.
 
The Internal Car. — The membriuioiis labyrintli uf the
 
 
 
 
Uc vesicle of
> ilors pit: B.
Ihtf ol[c Malcle;
rftirc Lrloderm,
 
 
 
iiilerMal ear is tlic <il<h'.-l part df the "i-jran i>f licanng. Its
DiiM-iii is from a lliiekeiie.l .-in-iilar |»ateh i.|' eetiHlerm on the
liorsulateral Mirl'ace i.l" the hea.l-rrjrimi of the eiiihryo near
the liiirsil lerriiiitalioii i.f the fir.-l oilier viseerjil furrow. The
tiiieketi.'.! arra M1ll^^ l>i-lo\v the Mirfar*', forniiiig thus the
auditory pit, whic-li is iin>eiit in itie ihinl week (Kig, lOd, .1 ).
 
the j.il 1 nrs .h-e|.rr, it^ e.lut- a|.|.rua.-h eaeh other and
 
tiiially iiieel ami niiitc lo form the otic vesicle or otocyst.
Tills "lilile e]>ithelial sae fira.lnally nve<les from the nirface
eeli«l.'i-m. At llii,- Ma.L'i- "f d-veloimieiit then- is no eraiiial
eapsule oiher than tlir imlilti'r<'nt itu-oileriaie tissue \v I lieli
surrounds ihe hi-ain-ve.-ieles ; heuee, the otic vcsiele, cmbedded ill tlii,- ti»m-, lies in elo^,- proxhnily to tlw aftor!>n»iti, and e.mies into r<>Iation with the neiisli«)(aeial ^Mtiglion (jt. ;t"21). Till' vcsiele, at lirst s|ilierii-al, soon heeoiiies
 
 
 
THE lyTEEXAL BAH.
 
 
 
347
 
 
 
pear-shaped owing to the protrusion of its tlorsal wall. Tliis
dursal projection, the recesaos vestibtiU or la.b3rrmthi (Pig.
16ii, C), lengthens out into a slender tube, the ductus endolymphaticns (Fig. ^^^), llie slightly dilated end of wlneli,
the aaccua endolymphaticuB, is found in the adult occupying
the aqucdiictus veslilndi of the tempoml bone. ,
 
 
 
 
Fig. 187.— Development or the membranniu labjrrlnth of tbo human ear (W.
Hta, Jr.l : A. left labyrinlli of embryo of about four U'ecke, outer ildc; i>E, velUbutar and cochlear poitlons : rl. recessua Jabyrliithl. B, left labrrlnlh with part*
nftoclalanaaudllfiry nerre* of embryo of about four ani a half weeka: rl. reoealus labyrintlii ; hc. ptc, ok, saperlar. poaterior. and external nemlelrciilar canalg ;
I. Haccule: <. cochlea: vn./R. vdlbular and facial aervea; rg- rfi SB- veatlbular.
cochlear, and genlciilatcganElin. C, left lahyrlnlh of embryo of about (Ivcwoclu,
bom wllhnlit and below: labelllni; as In preredlngflKure.
 
The opiHJsite, anterior or ventral extremity of the otic
vesicle tilsti bulges out into a small cvagiuatioD, wliieh gradually elongates until it is a tapering tube, slightly curved
inward toward the median piano. This lengthens still more
and becomes spirally coiled, forming the cochlear duct or
scala media of the future cochlea (Fig. 168). The venicle
it'^e If becomes constricted in such manner by an inward projection of its wall as to indicate its <livision into an upper
larger and a lower smaller sac, the terms upper and lower
referring respectively to the head-eud and the tail-end of the
embryonic body. Before the con.strietion occurs, the wall of that part of the vesicle which is to become the future
iipjKT or utricular division presents two {)ouched-out areas
(Fig. I(j7, B). One of these gives rise to the extenud semicircular canal, while from the other are formed the saperior
and posterior canals. The pouch that produces the external
 
 
 
 
Fig. 168.— niafrram to illustrate the ultimate condition of the membimnotu lAbjMnth (after Wnldeyer): i/, utrieulus: if, sacculus; cr, oanaliH rcuniens; r, ductus
endolymphaticus: r. cochlea; k, blind sac of the cupola; r, vestibular blind cae
of the du(?tu8 coehleuris.
 
canal is scmieirciilar in form and flat, lying in the horizontal plane, its upper and lower walls bring in contact with
each other. The oppo.sed walls fuse, except at the periphery
of the pocket, and hence all that remains of its cavity is a
small marginal tube or channel, corresponding with its border and opening at each end into tlu* cuvity of the vesicle.
Throughout the region of fusion of the walls, the latter become thin and finally disaj)i)oar, being replaced by connective tissue. Thus a semicircular epithelial tube is formed^
which is the horizontal or external semicircular canal. One
end of the tube being dilated, the ampulla of the canal is
produced.
 
the superior and posterior semicircular canals are formed
in a somewhat similar inanucr by the other evaginatcd
ponch or ])ockct, which is irregularly globular. To pro<bi<*e this result, the walls of the pocket contract adhesions throughout two regions, which (U)rresp<md with the
rcs])cctive sj)accs enclosed by <'acli of the* two future canals
in (jiiestion. The fusion of the walls takes place in such
manner as to leave two narrow channels or tubes, one of
which ahnost encircles the inner or mesial asjKHJt of the
pocket, while the other bears the same relation to its jH)stcrior wall, the inner limb of the latter semicircle coinciding
with the posterior limb of the former. The result of this
arrangement is that two vertical semicircular canals are
formed with their planes at right angles to each other, the
two communicating with the otic vesicle by three openings,
one of which is common to both canals. The other two
apertures, being dilated, are the ampullated individiial orifices of the posterior and superior canals.
 
The constriction in the otic vesicle referred to above increases until this sac is divided into two parts, a larger,
which includes the region from which the semicircular
canals have developed and which is now the utricle, and a
smaller vesicle, the sacculOi comprising the part from which
the cochlear duct was evaginated (Fig. 168). The line of
division coincides with the middle of the orifice of the ductus
endolymphaticus, the proximal end of which participates in
the division. Thus the ductus endolymphaticus becomes a
Y-shaped tube, and affords the only bond of connection between the saccule and the utricle (Fig. 168).
 
The beginning of the cochlear duct, failing to keep pace
in growth with the other parts, api)ears as a smaller tube relatively, and is known as the canalis reuniens (Fig. 168, cr).
 
The structures so far considered — the utricle, the saccule,
the semicircular canals, and the cochlear duct — being the product of the ectodermic otic vesicle, represent simply the adult
epithelial linings of those cavities. The fibrous layer of the
membranous labyrinth, in common with the walls of the bony
labyrinth, is a product of the enveloping mesodermic tissue.
While the cells of the otic vesicle thus for the most part constitute the walls of the several sacs and canals of the primitive internal ear, some of the cells specialize into neuro-epithelium. The most marked specialization of this sort occurs
in the cochlear duct, where most of the cells on that wall of
the duct which may be called its floor — the part corresponding to the future membrana basilaris — undergo such profound
modification in form as to produce the most highly specialized neuro-epitheliul cells anywhere to be found, the elements
that constitute the organ of Corti.
 
 
 
In the utricle and the saccule, as well as in the ampulIsB
of tlio semicircular C4inals, there is a similar but less marked
sp(»cializati(m of epithelial cells to produce in the former case
the maculae acusticse, and in the latter, the crista acusticfle of
the ampnlhT. While, therefore, the cells of the otic vesicle
which are to s(Tve as the lining mucous membrane of the
membranous labyrinth become flattene<l polyhe<lral cells
arranpMl as a sintrle lay(T, those cells which are to functionat(» as the periphenil part of th(» acoustic mechanism l>ecoine
the specially modified C(>lumnar cells, many of them with
cilium-like appendages, of the maculie, the cristae, and of the
organ of C(>rti.
 
From the first the otic vesicle lies in close relation with
the aeustieofacial ganglion (Fig. 167, />). As pointed out
in a preceding chapter (p. 321), this ganglion subsequently
divides into two parts, corresponding with the two divisions
of the auditory nerve. This division (jf the ganglion and of
th(* nerve is correlated with the separation of the otic vesicle
into a coelilear part, the cochlear duct, and the two vestibular
vesicles, the saccule and the utricle. While the cochlear duct
IS still a short, slightly curved tube, the cochlear ])art of the
ganglion lies in close proximity to the tube, in the concavity
on its inner side. As the duct lengthens and becomes more
coiled, tiie ganglion likewise lengthens into a band which
follows the spiral course of the duct, lying parallel with the
latter and on the side toward the axis about which it is
coiled. Ai'ier the formation of the bony parts of the cochlea,
this ganglion octMipies the sjnral canal of the modiolus and
is known as the gangUon spirale. It helongs to the cochlear
division of the auditorv nerve, which is distributed to the
cochleii.
 
The remaining part of the acoustic ganglion becomes rather
widely separated from the spiral ganglion, coining to occupy
a position in tluMuternal auditorv meatus, and the part of the
auditory nerve with which it is conne<*ted acquires relation
with the macular regions of the utricle and saccule as well as
with the crista' of the ampulhe of the semicircular canals.
These nerve-fibers constitute the vestibular division of the auditory nerve, M'hile the ganglion is the vestibular ganglion
or intumescentia ganglioformis of Scar])a.
 
The development of the bony labsrrinth of the internal ear^
as well as of the connective-tissue parts of the membranous
labyrinth, is effected solely by the differentiation of the mesodermic tissue which surrounds the epithelial structures above
considered. As previously stated, at the time when the otic
vesicle is first formed there is no indication of a cranial capsule, the brain-vesicles being surrounded and separated from
the ectoderm by indifferent mesodermic cells. During the
progress of the alterations in the otic vesicle, this tissue
undergoes condensation and alteration to form the membranous primordial cranium, and shortly thereafter the
petrous portion of the temporal bone is outlined in cartilage
by the further specialization of a portion of this primitive
connective tissue. The formation of cartilage does not affect
all of the tissue which is afterward represented by the
petrosa, the region that borders the semicircular canals,
the cochlear duct, the saccule, and the utricle remaining soft
embryonal connective tissue. There is thus a cartilaginous
ear-capsule produced which is more than large enough to
contain the primitive epithelial labyrinth, and the walls of
which are separated from the latter by embryonal connective
tissue.
 
The bony semicircular canals are almost exact reproductions, on a larger scale, of the epithelial canals^ and they are
formed by the ossification of the cartilaginous petrosa. Even
before this ossification occurs the soft connective tissue
between the cartilage and the epithelial semicircular canals
differentiates into three layers. The inner layer, becoming
more condensed, is converted into fibrous tissue, and, adhering to the epithelial walls of the canals, furnishes the connective-tissue component of the completed membranous
canals. Its blood-vessels serve for the nutrition of the
canals. The outer layer also undergoes condensation and
forms a fibrovascular membrane, the perichondrium, which
later becomes the internal periosteum of the bony canals.
The middle layer, on the contrary, becomes softer — by the liquefaction nf tin* intercellular siul>stance ami the degeneration of the cells — so that gradually increasing, fluid-filled
cavities make their ap|H*a ranee, and these latter becoming
lar^(T and many of them coalescing, a ^pace is formed
around the niemhranous canals which is filled with fluid, the
perilymph. This perilymphatic space is bridged across at
intervals by con nectivt»-t issue processes that serve for the
convevanct* of l)hMMl-vess<*ls to the membranous canals.
 
The vestibule of the internal ear is formed in practically
the same manner as the Imny semicircular canals, the epithelial saccule and utricle at'quirinj;^ their cimnective-t issue
constituents in the same way. TheR* is the difference, however, that the bony v<»stibule dm^s not conform to the shape
of the vestibular |mrts of the membranous labyrinth, since
it is a sinjrle unilividinl cavitv enclosinjr the two little vesiclcs, the siiccule and the utricle.
 
The bony cochlea, while develo|K»d u|M)n the same general
plan as the other parts of the bony labyrinth, presents certain cons|)iouous uKNlitications. The epithelial cochlear duct,
as stati'd above, in its early sta^ is a short, tapering, and
sli«rhtly curved tube. While it is still in this condition,
chondritication of tlir petrous bone (K»curs, whereby the
duct ac(|uircs its cartilaginous capsule (Ki^. 109, hk). This
capsule is i)\\v\\ at the prnximal end of the duct and
thronjrh this o|K*iiinLr lh<' cochlear bninches of the audittuy ncrvi' gain access U) the capsule, beinj; connected with
the c(K'hlcar <livi>inn of the auditory «ran«rlion, which, owing
to its prcviou>ly having a>>iuncd a jK>sitiou beside the duct,
Ciuucs to be enclosed by th<* <'apsulc as the latter is formed
(Kig. 1^>9, m\ <j^it). It is only after the chondrification
that the ci>chlear duct lengthens out and becomes t>pirally
coiled. The coiling is in such n)ann(>r that the cochlear
nerve is surrounded bv the duct — thatis, it lies in the axis
about which the duct is spindly wound. Within the cartilaginous capsidc, filling all the space not occupied by the
spirally coiled duct and the ccH^hlear nerve with its lengthened-out ganglion, is the end)ryoni<* conn<»ctive tissue of
which f(»nnerly the entin* cartilaginous {x^trosa ccmsisted.
 
 
 
Thu cochlea consists now of a spirally coiled epithelial tu!>e
Ijiug within an eloiigati'd cavity in the cartilaginous petrosa,
a cavity, the walls of which arc, therefore, cartilaginous.
The peripheral wuU of the coiled tube is in contact with the
inner surface of the wall of the iTnrtilaginous cnpfule (Fig.
169, x), a fact which has an imiHtrtaiit bearing upon ihc
further stages of growth.
 
 
 
 
I
 
 
 
Tio, Ue,— Pan of ■ Bectloii Itiniugh the cocblea or an embryo Ml. II cm. (3.l> in.)
long litter Boellchcrl: bt. cartl]*gln(nis capadte. In vhLch llie cocblmr duel
describe* aseending »|"lral turns i ilf. duetUB t'orhletrin ; c. iirgun of Cortl; It,
lamina vialibul«rt«: i, ouler wall of the membranous duclu«cochl«arl» nilh Ugamenlum aplrale; SV. acila vfitninll : ST*. S7'. acila tympanl; g. Kelallnoun tiaae,
which itni Hlls the tcala Tvatlbulf inO In Ita I'M tniat: a\ remnant of the gelatinous tliiue, which I> not yet llquvllcd : M. Arm connective tluua summnillne
the cochlear nerve lac); gip. ganglion splcnle; .v. nerrc which rum to Cortl'a
or>n>n in (he nilure lamina aplrallanuea: r, c<im|<Hct cnnnL-dlre-tiune UfCT.wblch
liepomo onined and shares In boundlBK ""o bone cochlear duct : P, pcrichon
 
 
The embryonal connective tissue within the capsule now
undergoes important modifications, which vary greatly in different regions. That portion of this tissue which immediately
envclo])ri the cochlear nerve becomes first dense connective tissue, which is afterward <lire<*tly coiiverte^l into bone, constituting the modiolus, or axis, of the cochlea. The processes of eondensation and subsequent ossification extend
outward from the nuKliolus in a spiral line, which corresponds
with the intervals bi^tween the successive turns of the cochlear duet, until they meet the wall of the original capsule,
thus produein^ the bony cochlea. That is, by the development of this spiral plate and its connection internally with
tlu» uKHliolus and externally with the wall of the capsule, a
tub(* at first partly membranous and jwrtly cartilaginous,
and at a later sta^e osseous, is produced, which encloses the
mueli smaller cochlear duct, and like it is wound spirally
arc>und the modiohis. To repcnit, the original sample eavity
of the cartilaginous capsule is subdivided by the growth of
the modiolus and of th(» spiral shelf in such manner as to
become a t<jjinil/t/ coiUd tube.
 
The cochlear nerve, enclosed within the coil of the cochlear duct, semis branches (Fiji:. 1^^, -V) i»^ a continuous
spiral line to th<* duct, and the soft tissue surrounding and
supporting these branches condenses to form a connectiyetissue plate whicli extends outward from the modiolus tc
the cochlear duct and which, therefore, has a spiral course
about the modiobis, its entire inniM* edge being attached
to that central axis, while; its outer border is, throughout
its entire extent, in continuity with the inner wall of the
duct. At a later sta^e tliis s])iral plate undergoes direct ossification to form tlic two lainclhe of the bony lamina spiralis.
Thus it is that the ganglion s])ir:de and the successive terminal branches of the cochlear nerve come to be enclosed
within th(; s[)iral lamina. Recalling the condition of the
cochh»a before the growth of the spiral lamina, it will be seen
that the latter, in connection with the epithelial cochlear duct,
divides the tube into two parts (Fig. IfJO, aST, .ST). It
will be evident, to(), that the epithelial cochlear duct now
holds a relation to the* larger tube of the future bony cochlea
which is similar in principh* to the relation of the membranous semicircular canals to th(» bony canals, but with
the dilVcrcnce that the outer wall of the epithelial duct is in close contact with the outer wall of the future bonv canal
at Xf and that the inner walls of the two are connected by a
spiral plate, the lamina spiralis.
 
The cochlear duct, then, is surrounded by undifferentiated
mesodermic tissue, except on the side farthest from the
modiolus, where its wall is in contact with and finally
adheres to the wall of the cartilaginous capsule. The lamina
spiralis divides this tissue into two parts which respectively
occupy the positions of the future scala vestibuli and scala
tympani. Tliis soft embryonal tissue, as in the case of the
corresponding tissue of the semicircular canals, develops differently in different regions. The innermost stratum, which is
in relation with the epithelial cochlear duct, becomes fibrous
connective tissue and constitutes the flbrons layer of the adult
cochlear duct ; that is, on the side of the duct toward the
scala tympani, it becomes the connective- tissue layer of the
membrana basilaris, while on the side toward the scala vestibuli it forms the fibrous stratum of the membrane of Beissner (Fig. 169). The peripheral zone of indifferent tissue,
that in contact with the now cartilaginous wall of the future
bony cochlea, as well as that which lies against the lamina
spiralis, also undergoes condensation and forms a fibrous,
or fibrovascular, membrane, the internal perichondrinm or
future periostenm. The tissue intervening between these
two layers retrogrades, the cells degenerating and the intercellular substance liquefying, until finally the spaces known as
the scala vestibuli and the scala tympani are hollowed out.
These channels are lymph-spaces and the fluid they contain
is the perilymph. This perilymphatic space is in communication with that of the vestibule. Therefore, while the cochlear duct or scala media encloses an epithelinm-lined space,
as do the saccule, the utricle, and the membranous semicircular canals, and in common with those structures contains the so-called endolymph, the scala vestibuli and the
scala tympani are in the same category with the perilymphatic spaces of the other parts of the internal ear.
 
The Middle and the External Ear.— The middle ear,
consisting of the tympanic cavity and the Eustachian tube, is devclojx}il from the back part or dorsal ond of the first
inner visceral fUrrow. The external ear, comprising the external auditory meatus and the auricle, comes from the dorsal extremity of the first outer furrow and the tissue about
its margins, the tympanic membrane representing in part the
closing membrane which sepanites the inner furrow from the
outer.
 
The first inner viscend furrow, in common with the
other inner furrows, is an evagination of the lateral wall
of the primitive pharvngcal ciivity, or head-end of the guttract. The ventral end of this groove suffers obliteration,
but the dorsal s(»gment, designate<l the tnbotympanic sulcus, becomes converted into a tube by the growing together
of its edges. The tube is composed therefore of entodermic
epithelial cells. It elongjites in the dorsal and outward
dire(?tion, and its dorsal extremity becomes enlarged to produce the cavity of the tympanum, the remaining part of the
canal becoming the epithelial lining of the Eustachian tabe.
The canal being formed before the development of the
cranium, and approximat(?ly its posterior half being surrounded bv the mesodermic embryonal connective tissue
that al'terward becomes the petrosa of the temporal bone, the
tympanic, cavity and a part of th(» Eustachian tube come to
be enclosed within that bone, while the connective tissue
enchasing the anterior part of the tube differentiates into the
curved plate of ciirtilage that forms the cartilaginous part of
the Eustachian tube.
 
Since the posterior end of the primitive epithelial tube
insinuates itself between the otic vesicle and the surface,
the tympanum comes to o(*cupy its normal position on the
outer side of the internal ear. The tympanum, being derived from the back part of the first vis(»eral cleft, is in
close relation with the first and second visceral arches, and
the ossicles of the mid<ll(» ear ar(» derived from the dorsal
extremities of the cartilaginous bars (jf these arches in the
manner described in C-hapter XVIII. Necessarily the
primitive ossicles are exterior to the primitive epithelial
tympanic sac, as is also the chorda tympani nerve, which jKisses ulcDg its outer nide. After the ossification of the
temporal hone, these structures are emhedded within the
abundant soft connective tissue which is between the epithelial sac, now the mucous membrane, and the bony walls
of the tympanum. This mass of soft tissue undergoes verj'
considerable diminution, owing to which the mucous membrane comes into contact with the bony walls, and as a result
the ossicles and the chorda tympani are enclosed in folds of
the mucous membrane and seem to lie within the tympanic
cavity,' They are excluded, however, from the true cavity
of the tympjinum, since they are exterior to the epithelial or
mucous- membrane layer.
 
 
 
 
Fin. 170. -Showing the gracjua] cluvi;lii|iiiii.-iil uf tlio fiartiinf the external car
ftom promlncncss upon the mandibular and hyoldean Tistcral archus lHI»),varlnmly magnlHeil : 1. 2. prunilnenceB on mandibular arcb : 3. prominence between
the two archea, prolooged postertorly in leconil fixture tu Sr; t.b. and 0, pniminencea on hyoidcau nr lecond riaceral arph; ^.lowerjaw. Prominence I forms
the li^rua; 2, 3. ^, the helix; 4. Ihu anllbelli ; G, Ihc antitragug: S, the lubule.
 
The external auditory meatus is simply the persistent posterior part of the fir«t outer visceral furrow or hyomandibular cleft (sec pp. \\2, 1 Hil, this cleft doffing completely
everywhere but in this region. The closing plate of the firrt
cleft becomes the tympanic membrane. Hiiice the outer
layer of this membrane is of wt.xlennie orijjin, while the
inner layer is entodermic, being continuous with the epithelial tympanic lining, and the middle fibrous layer is
derived from the mesoderm. The relation of the malleus to
the membrane and of the latter to the bony tyinjKinie plate which forms part of the wall of the meatus is dealt with in
the chapter on the development of the skeleton.
 
The auricle is ilerived from the tissue around the margin
of the uucIoscmI hack part of the first outer cleft (Fig. 171, C).
Six little elevations make their appearance here, the projections
being mesodermic tissue covered with ectoderm. The mesodermic component of the elevations diiferentiates into the
<':irtilaginous and other connective-tissue jKirts of the auricle.
The nodules marked 2 and 3 in Fig. 170 bi^coming a continuous ridge, produce* the helix, while nodule 4 becomes the antihelix. The tragus and antitragus develop resijcctively from
the projections 1 and 5. At the end of the second month,
these parts are so far a<lvan(?ed as to be easily distinguishable, and the connective-tissue basis of the ridges and projections and the continuous plate-like mass to which they
all are attache<l be^in to undergo chcmdrification. From
the third month onward, this primitive auricle, by continued
growth and greater sepanition from the si<le of the head,
assumes more? and more the charactei>> of the fullv formed
member. The lobule, however, which results from the
growth of the little elevation marked G, lags behind the
other parts in development and is rather indistinct until the
fifth month, after which time it increases in size and gradually acquires its normal i)rop(>rtioiis.
 
 
 
THE DEVELOPMENT OF THE NOSE.
 
The nose is primarily a special sense-organ, although a
pjirt of its cavity serves, in air-br(»athing vertebrates, as an
adjunct to the respiratory system. The evolution of the
mature organ of smell may be epitomized by the statement
that the olfactory epithelium, the ess(>ntial part of this senseorgan, is a patch of dei)ressed or infolded ecjtoderm, the cells
of which are highly specialized and ar(» brought into relation
with the central nervous system by means of the outgrowth
from the latter of a part of its mass, the olfactory lobe.
 
Verv wirly in intra-uterine life — before the end of the
thinl week — the olfactory plates apjn^ar as loealizt^l thicken
 
 
rut:
 
 
iiigs of the ectoderm situated just in front of or above the
on»l fossH. Tbese nasal areas are the forerunners of the
future olfactory epithelium. It is worthy of note that the
olfiictory [ilatf.* un.- \n very close relation with thf iirimiiry
 
 
 
 
 
— Devetupment or (he flu» nr ihc bum&neiDbryn (TUB) - A, embryo of
it twenty-nluv dayi. The nuuCroalal plate dlOeren Hating into proccuui
gioDularea, toward whlph the mailllnry proccsies of llret vlucral arcb are eitendiag. B. embryo of about Ihlrty-fourdajra: Ibe global ar, lateral frontal, and nuiillnry prufetwea are In apposition ; the prlmlllTe openlnt; la now better deflned, C,
embryo of about the eighth week - Immediate Imiinilsrle* of moulh are more deflntte and the nasal oriHces are partly (brmed, external ear appearing. D. embryo
at end of oecond munth.
 
fore-brain vesicle, being, in reality, on the outer surfiiee of
the ectodermic covering of its ventral wall.
 
Owing to the rapid outgrowth of the surrounding tissue,
the olfactory plates l)ecome relatively depressed, constituting now the nasal pits, which arc distinguishable at about the
twenty-eighth day. The pits are separated from each other by
a broad mass of tissue, the nasal or iia8ofh>ntal process (Fig.
 
171), which is, as it were, a localized thickening of the inesoderniic tissue on the ventral wall of the primary fore-brain vesicle ; and this process makes its appearance in the third week.
During the fifth week the nasofrontal process thickens greatly
along its lateral margins, the thick edges being known as the
globular processes (Fig. 171, A, B). At the same time the
lateral nasal processes bud out from the nasofrontal process,
one on each side, above the nasal pits, and, growing downward, form the external boundaries of the pits, each of which
depressions is bounded on its inner side by the corresponding
globular process. The nasal pits, therefore, have well-marked
walls on every side except below, where they are directly
contimious with the oral fossa.
 
In the latter end of the sixth week the nasofrontal process, which, it will be remembered, constitutes the upper
limit of the oral fossa, is joined on each side bv the united
maxillary, and lateral nasal, processes. This effects a division between the oral fossa and the nasal pits, and forms,
though as yet crudely, th(* external nose, and the upper lip
as well. The detinite formation of the external nose may be
said to be indicated about the cH/htli rack. The orifices of
the na>al pit.-^ are now the anterior nares, while the pits themselves have bectnne short canals, opening by their deep
orifices, the posterior nares, into the primitive mouth-cavity
ahove the palatal shelves. The nanvs are separated from
i'ach other by the still broad nasofrontal j)roeess. That
portion of the nasofrontal process that separates the nares
gradually becomes thinner and produces the septum of the
nos(\ while its external or superticial })art gives rise to the
bridge and tip of the organ.
 
The growth of the palate- shelves (Fig. 172) toward the
median line, resulting in their union with each other and
with the recently-fornn^d septum, definitely divides the nasal
chambers from the cavity of the mouth, th(» posterior nares
now opening into the pharynx. This separation is completed
toward the end of the third month.
 
 
 
The complexity of the ndiilt nasal cavities js proclticetl W
the formation of ridges ami jKHielies on (he lateral walls of
the original nasal pits. Three inwardly projecting horizontal
folds of the eeio<lernii(i lining of the cavity, tho superior, middle, and inferior tnrbi&al folds, appear njion the outer wall nf
each nasal fossa (Fig. 173). Each fold contains a stratnni
of mesodermie tissue which develops into cartilage and nubseqnenily into bone, forming respectively the three turbinated
bones. The cartilaginons character of these folds becomes
apparent at the end of the second, or the early part of the
third, month. An cvagination on the lateral wall of each
 
 
 
 
Fin. ITS.— Roof or Ihe oral rnvlly ofa human emhirj-o with the niiidiunenta of (he
poUlal prouenes (alter HIa), < 10.
 
nasal fossa, between the middle and the inferior tnrbinnl processes, becomes the antnun of Higbmore ; this is fnrmcil in
the sixth month. Other cvafrinalions jirtxiiiec the ethmoidal,
the frontal, and the sphenoidal sinuses, the last two of which
are not completed, however, until after birth. Very early in
the development of the nose a small invagination appears on
the mesial wall of the nasal pit. In the fourth month of
gestation this invagination has become n canal in the iipptum
(Fig. 173, /), running from before backward and ending in
a blind extremity. It is the so-called organ of Jacobson,
which, in man, is merely a rudimentary strnctnre, but which,
in most other mammals, is more highly developed, Iwing
surrounded by a cartilaginons capsule and receiving a special
nerve-supply from the olfactory nerve.
 
 
 
The olfactory plates lictKime sejiaratod from the fore-brain
vesicle and ajii.-cijiiontly from the later brain and its oulgrowtli, the olfactwry bulb, by the development of au intervening bony plate, the cribriform lamina of the etlimoid
bone. The ectoderraic cells of the olfactory plates differentiate into the highly specialized nenro-epithelial elements of
the olfactory mncoiis monibranc, the olfactory epithelliun, and I
their asiioeiuted supporting cells. The axon*; of the neuro- I
epithelial cells piws upwuiil (liroiifrli die tribrlform ]»late of
the ethmoid bone as the olfactory nerve-fibers, and, entering
the ventral surface of the olfac'lory Ijnlb, arliorize with the
proces-ses of the mitnd cells of ilie bull), whereby they acquire J
functional relationship with the olfactory centers in the brain, 1
 
 
 
 
Fm. 173.— Cr SB a i i tl t tl tl hmd f ai en I rj i
orown-rump meuureuient The iiuml cavlllw Bre (leen lot
wlih thcoralisTilyal IhepUieii dealsniileiJ by a* K cBrUlHseaf tbeoual Mp-fl
tuni;iH,turblnal»rlIl&ge J argan i f Jucnbson J the place when ItopaulBttfl
Ihe naiial dbtUs' ; gf, paUul proceis; of. mftxlUary proceM; il. dBntal rld|».|
|Hert»[g).
 
The esternal nose, as previously stated, first acquires defi- 1
nite form about the eighth week by the union of the distal J
ends of the lateral nasal processes with the nasofrontal proo* 1
C8.S, the former proilucing the ala and the latter thebiid|«|
and the tip of the nose. In the third month the organ is j
tmduly flat and broad, but from this time on it gradually j
a-ssume-s the familiar characteristic form. From the third i
month to the fifth each external naris is closed by a gelat- J
inous plug of epithelial cells.
 
 
 
+++++++++++++++++++++++++
 
CHAPTER XVII.
 
THE DEVELOPMENT OF THE MUSCULAR SYSTEM.
 
THE STRIATED OR VOLUNTARY MUSCLES.
 
The voluntary muscular system, genetically considered, is
divisible into (1) the muscles of the trunk and (2) those of the
extremities. The muscles of the trunk include two distinct
sets : (a) the muscles of the trunk proper, or the skeletal
muscles, and (b) the muscles of the visceral arches or the
branchial muscles.
 
To arrive at a proper comprehension of the evolution of
the muscular system it is necessary to revert to an important
fundamental emhryological process, the segmentation of the
body of the embryo, or, as it is sometimes expressed, the segmentation of the coelom, or body-cavity. As pointed out in
Chapter IV., this process of segmentation occurs in all vertebrate animals and in some invertebrates.
 
The Muscles of the Trunk Proper.— At a very early
stage of development the tracts of mesodermic tissue situated
one on each side of the median longitudinal axis of the future
embryonic body, the paraxial mesodermic tracts, undergo
division or segmentation, in lines transverse to the long
axis, into 'a series of pairs of irregularly cubical masses
of mesodermic cells. These masses are the mesoblastic
somites or primitive segments, often inappropriately called
the protovertebrse. The somite first formed corresponds
with the future occipital region, the second one lies immediately in front of the first, while two others, situated still
more anteriorly, that is, near the cephalic end of the embryonic area, and seven more, behind the first, are added almost
simultaneously. The formation of the primitive segments
 
 
 
 
tlicn pr(^>cociU tiiilwiinl until a considerable number have
l)e(.'ii luldi'd. Tliorie in front of tlic one first formed are
di'iiomiimted tlip head-segments, while the others are known
iis the tnmk-segments. Kacli mniito ii> at first triangular in
iToss-stt;fion, the haso of the triangle looking toward the
chunla dor.sidi:*. Siibsc<iiiently they assume a more ciiboidal
whiiiM'. In the lower vertebrates — amphibians and fishee —
the somite is hollow, its cavity being in these cases a constricted-iift" portion of the IxKly-cavity (hence the term " 8^
 
 
 
 
 
 
el'iRciiuui tlMie
iiiiting tnirt rb!
roin wbo«e wall.
 
 
 
mentation of the eieloni" to exjin.-is this iirtHrsf). In the
higher vert eh rates, liowevcr. the eavitv is obliterated by the
eneroiielinieiit of the eells of the wall> of the soniit<-.
 
The eells of the somites soon iin(h'r<;o ditfereiitiation nnd
rearm iifreii lent. It is nsiiiilly stated that, preparatory to the segmentation of the paraxial mesodermic tract, this tract has
become separated from the remaining lateral plate of the
mesoderm. The separation is not complete, however, and
therefore, after the appearance of the primitive segments,
each segment is connected with the more laterally placed
lateral plate — by the separation of which latter into two
lamellae the coelom is formed — by a smaller mass of tissue,
the iieplirotome, also called the middle plate, or intermediate
cell-mass (Fig. 17-!, vb). As development progresses the distinction between the primitive segment proper and the nephrotome becomes more sharply expressed, and the former is
designated the myotome. The primitive segment on its mesial surface, near the point of union with the nephrotome,
sends forth cells which form a mass called the sclerotome
(Fig. 174, sk). The sclerotomes spread out and blend with
each other, forming a continuous mass of tissue which envelops the chorda and the neural canal, and which also extends
laterally between the myotomes, separating them from each
other and constituting the ligamenta intermuscnlaria {vide p.
375) ; this tissue, being concerned in the production of the
permanent vertebrse, has no further interest in this connection.
 
What remains of the primitive segment after the forma-*
tion of the nephrotome and of the sclerotome is the myotome
proper or the muscle-plate. Although, as previously stated,
the primitive segments of the higher vertebrates contain no
cavity, the myotome and the nephrotome each enclose a
space, that belonging to the former being known as the
myoccel. The myotomes or muscle-plates are so called because they give rise to the voluntary musculature of the
trunk. But not all of the cells of the muscle-plate undergo
transformation into muscular tissue. While the cells on the
mesial or chordal side of the myoccel are going through
certain alterations preparatory to their metamorphosis, the
cells nearer the body-wall become rearranged to form a
characteristic layer which is known as the cutis-plate from
the fact that it contributes to the formation of the corium of
the skin (Fig. 174, cp). The cutis-plate and the remaining part of the miisde-plate are conliniiniis around the myoctpl,
the trunsition from one to the other being more or less gradMal. To suminarize, the primitive segment is differentiated
into the nephrotome, the sclerotome, the myotome or mnaclaplate, and the cutis-plate.
 
The Metamorphosis of the Muscle-plate. — By the
terra inujKle-ftlate. ia meant here the thickened layer of cells
on the chordal or mesial side of the myotome proper, which
layer condtitiites what remains of the myotome after the
differentiation of the eutis-plate. These cells having pro- |
liferated and increased in size, and having encroached '
thereby upon the cavity of the myotome, next undergo
alteration in shape, becoming cylindrical, with their long axes
parallel with that of the body of the embryo. The length
of each cylindrical cell equals the thickness of the primitive segment, at least in the Amphibia and probably also |
in the chick. The next step in the transformation is the '
acquisition of the transrerse Btriation characteristic of ver- j
tebratc voluntary mupclc. Soon after this the protoplaf
of tlic cell uudergoc];) longitudinal division iVito minute]
fibrillie — which latter do not necessarily correspond, how- J
ever, with the primitive fibrillfe of mature muscle — and tlw I
cell-nucleus likewise divides. The metamorphosis of the f
now tibrillated protoplasm into muscular tissue is first com— 1
pleted at the periphery of the fiber, so that a young musole- 1
fiber contains a central core of undifferentiated material, |
including the daughter-nuclei resulting from the divis
of the original nucleus. Soon after the appearance of stria- 1
tion and the fibrillation of the fil>er, the fibers begin to sepa^l
rate from each other, and developing connective tissue witJtT
young blood-vessels penetrates between them, the fibers now 1
showing aggregation into bundles. For some time longer i
the fibers are naked, since the earcolemma is not acquired
until considerably later. The differentiation into muscular .
tissue gradually extends from the periphery of the fiber to 'J
its core, the process being complete in the human embryo at J
about the end of the fifth month for the muscles of the upper.jj
extremities and in the seventh month for tho.-ie of the lower.
 
 
 
THE STRIATED OR VOLUNTARY MUSCLES. 367
 
The embryonic muscle-fibers are smaller than the mature
elements and increase in size until the third month.
 
It is considered highly probable by most embryologists
that muscle-fibers undergo multiplicatioii during embryonic
life. There are several theories as to the method of this
multiplication. The most generally accepted view is that
put forth by Weismann, the essential feature of which is
that the fibers multiply by longitudinal division or fission.
Reference was made above to the repeated division of the
nucleus of the cell as one of the initiatory steps in the formation of the muscle-fiber. According to the fission theory,
there is one class of fibers in which the nuclei are arranged
in a single row, and the fibers of this class do not undergo
fission ; while there is another class, the fibers of which have
their nuclei arranged in several rows. Fibers of the latter
type divide longitudinally into as many daughter-fibers as
there are rows of nuclei.
 
Although many of the details of the development of the
muscular system are still involved in obscurity, it is a generally accepted fact that each fiber is derived from a single
cell, the protoplasm of which develops the function of contractility to the subordination of the remaining vital properties of protoplasm. With this specialization of function
there is nefcessarily a concomitant alteration of structure.
 
The muscular mass resulting from the transformation of
each myotome grows in the ventral direction between the ectoderm and the parietal leaf of the mesoderm, or in other words
into the somatopleure, to produce the muscular structures of
the ventrolateral body-wall. The off-shoots of the myotomes
which thus jKjnetrate the body-wall in the fourth week produce, in the fifth week, a muscle-mass which, for the most
part, is non-segmental, and which gives rise to a dorsal and
a ventrolateral division ; the dorsal division, derived from all
the spinal myotomes, l)eing destined for the musculature of
the back, while the ventrolateral division, springing from the
thoracic myotomes alone, gives rise during the fifth, sixth, and
seventh weeks to the muscles of the thoracic and abdominal walls (Banleen and Lewis *). The dorsal division extends
in the dorsal direction, covering and acquiring points of attachment to the vert<}bral column, which has meanwhile
Ix'cn forming. In addition to the ventnd and dorsal extension of the muscl(?-|)lates, each one grows both forward and
backward — cephahid and caudad — in such manner that overhipping and intermingling result. During the diflTerentiation of the various muscular mass(\s from the myotomes, ventnd and dorsal l)ranches of the corresi)onding spinal nerves
grow forth, their final distribution being to muscles developed from the particular myotcmies with which the respective
nerv(»s correspond. According to Bardeen and Lewis the
struc'turcs of the l)v)dv-wall are well differentiated by the end
of the sixth week, although their extension to the mid-line is
not completed until near the end of the third month.
 
What has been said above concerning the evolution of
the trunk-musculature from the primitive s(»gments refers to
those muscles that are develope<l from the segments of the
trunk. As to the evolution of the head-segments comparatively little is definitely known. It is generally accepted
thatin ela^niobranchs — a group including sharks and rays —
there an? nine primitive segments in the region of the future
head. The number present in mammalian embryos has not
been clearly worked out. Three? oeeiptal and thirty-five
spinal myotomes have been seen in human embryos of the
fourth week, at which time the formation of myotomes is
said to cease. In th(» l(>wer vertebrates each segment contains
a eavitv lined with flattened e(»lls, the mesothelium, the metamorphosis of which into muscular tissue may be inferred to be
essentially as alreadv outlined ai)ove. The first head-segment,
which lies in contact with and partially envelops the optic
vesicle, gives rise to the su|)erior rectus, the inferior rectus,
antl the inferior t>bli(jue muscles of the eye-ball (innervated
by the thinl cranial nerve) : the second segment produces the
superior obli(jue (iiniervated by the fourth nerve); and the
third, the external rectus (iiniervated by the sixth nerve).
The fourth, fifth, and sixth segments al)ort and hence pnxluce
 
* Amerirnn Jtntrnal nj AniitomUj vol. L, No. 1.
 
 
 
no adult structures ; while the seventh, the eighth, and the
ninth segments become metamorphosed into the muscles that
connect the skull with the shoulder-girdle.
 
From recent studies^ it would appear that individual muscles undergo peculiar and significant migrations during their
development, and that the origin of the nerve-supply of a
muscle indicates the location of the particular myotome or
myotomes from which it originated, since the segmental
nerves are connected with their respective myotomes and
supply the muscles derived from such myotomes. For example, the serratus magnus, being innervated by branches
of the cervical nerves, develops from myotomes in the neck
region, and subsequently moves down to become attached to
the scapula and the ribs.
 
The Branchial Muscles. — This term embraces the
muscles of mastication and the various muscles connected
with the hyoid bone, with the jaws, and with the ossicles of
the middle ear. They result from the metamorphosis of the
mesothelimn of the visceral arches and acquire connections
with structures that have arisen from the so-called mesenchymal cells of these arches or, in other words, from the
embryonal connective tissue which makes up the chief part
of their bulk. For an account of the growth of the visceral
arches the reader is referred to Chapter VII. From this
account and from that found in Chapter IV., it will be seen
that the formation of the visceral arches and clefts is in
reality the segmentation of the ventral mesoderm of the headregion of the embryo, or to express it in another way, it is
the segmentation of the ventral coelom of that region. It is
interesting to note that whereas in the trunk the segmentation of the mesoderm is restricted to the dorsal part of the
body, in the head-region the ventral mesoderm also participates in the process. Hence the visceral arches, as might be
exj)ected, consist of so many masses of mesodermic tissue,
each arch containing a small («ivity lined with mesothelium,
which cavity is a constricted-off part of the body-cavity or
 
* See '* Development of the Ventral Abdominal Walls in Man/* Franklin P. Mall, Johns Hopkins Papers, vol. iii., 1898.
 
Od'loiii. It is those mesotholial ct4l8 that produce, by their
diift'rentiation, the niusoles under consideration. While so
nuieh ooneerninjj: the origin of tliis group of muscles is practically assured bv ol)servations upon the embryos of the
lower vertebrates, the details are still obscure. His assumes
the origin of the palatoglossus, the styloglossus, and the levator
palati from the second or hyoid arch ; of the stylopluayngeus,
perhaps the palatopharsrngeus, the hyoglossus and the superior
constrictor of the pharsrnx from the third arch ; and of the
middle and inferior pharyngeal constrictors from the fourth
arch. Further, it is held bv Rabl that the muscles of the
i\u\\ including those of the scalp and the platysma — the
muscles of expression — originate* from the mesothelium of
the hyoid anrh in the form of a thin superficial sheet, which,
gratlnally spreading out from the place of origin, breaks up
into the intlividual muscles.
 
The Muscles of the Extremities. — The relation of
i\\o (It'vclopnirnt ot'tlic inn>cles of tiie limbs to the myotomes
i> >till a (li<j)uttMl point. Sdihc authorities hold that the
linil)-mu-cK> of inaiiinials orii^inate from the mvotomes, as
\\{\< >li()\vn l)v l)«)lirn U) l)c the ca>e with the fin-musculaturc of Selachian-. A tact adduced as a strong argument in
t'avnr «>f tht'ir niyotomic oriiiiii is that the ncrve-su|)[)ly of
each liiiil) ('(U'rc-jjonds with the nerves of the number of myotoniie x'unients in relr.tion with which the limb-bud <lcvelops
(rid, |). loi)). ( )n th(* other iiand, it is stated* that the myotome- do n(»t extend into th(» developini:- limb-buds, but that
the inn.-ch'< ol" the liinl).- are diil'ci-entiated from the mesenchvinal core ol' the liinh-bud, thi- procos following the entrance of the motor nerve-fihi'rs into the member. The
mn>chs of the npper lind) are so well advanced in their
devel<»|)ment by th(*' >ixth week a< to be individually distingni-hablc. th(»-^e (if the lower limb reaciiing a corresponding
stau'e in the >eventh week.
 
^ r»anU*oii Mini Lc\vi>, /-*(•. cit.
 
 
THE INVOLUNTARY OR UNSTRIATED MUSCULAR TISSUE.
 
This variety of muscular tissue, like that considered
above, is of mesodermic origin. But while the voluntary
muscles arise from the flattened or mesothelial cells of the
primitive segments, involuntary muscle results from the
transformation of the embryonal connective-tissue elements,
the mesencliymal cells, of the mesoderm. It is for this
reason that some authors speak of the voluntary muscles as
the mesothelial muscles and designate the involuntary muscular tissue as mesenchymal muscle.
 
While it is a generally accepted fact that each of the fibercells which make up nnstriated muscle is a metamorphosed
mesenchymal or connective-tissue cell, the details of the
process have not been accurately worked out. One may
assume that necessarily the young connective-tissue cell
elongates and that its protoplasm must undergo such differentiation as will fit it for the exercise of its future function,
contractility.
 
THE CARDIAC MUSCLE.
 
The account of the development of the heart-muscle will be found in Chapter X.
 
 
 
+++++++++++++++++++++++++
 
CHAPTER XVIII. THE DEVELOPMENT OF THE SKELETON AND OF THE LIMBS.
 
Although the skeleton is the framework of the body in
the aiiatoiuieal or meehanieal sense, it is not so embryologically, since its deveh)pnient is not l)egun, at least not to any
important extent, until nearly all the prineijial organs are
well differentiated, and its growth is largely subsidiary to
that of the structures which, in the mature state, it supports
and })r(>tects. M4»rph()logists s])eak of the exoskeleton and
the endoskeleton, the former having reference to the hard
structures found superficial to the soft parts, for whose protection they serve, such as the canipace of the lobster, and
the hartl scales of certain fishes ; while the latter term
si<rnifies the cartilairinous or bonv structures found within
the bodies of most vertebrate animals. Kven in the highest
vertebrates, certain bones, such as those (►f the vault of the
cranium, are usually cousi(l(»r(»d by morphologists as l>eing
thi' representatives <>f p;irt of the cx<>skeleton of lower ty])es.
 
The skeleton, using the W(>rd in its 4>rdinary sense, consists of the axial skeleton and the appendicular skeleton, or
skeleton of the limbs, '^fhe former, inclu<liug the head and
the trunk, is connnon to all vert(»brates ; the latter is not
found in the lowest members of tliis class and hence is to \ye
regarded as a later ac(|uisition in the evolution of the skeleton.
 
In studying the development of t]w skeleton, as in considering thatof otht'r systems an<l organs, cleaner conceptions
of the y:rowth of the individual mav be obtained bv comparing it witli the evolution of the ty[)e. For example, the
simplest form of skeletal apparatus is that of the amphioxns.
 
In this animal the only representative of the skeleton is the notochord, a cyliiulrical rod composed of cellular or gelatinous
tissue in which neither cliondrification nor ossification ever
takes place. Such an animal furnishes an example of the
notochordal stage of the skeleton. the surrounding of the
chorda with a sheath of embryonal connective tissue, by
which it is strengthened and thereby better fitted to serve as
the body-axis, furnishes the membranous type of skeleton, a
stage a little farther advanced than the preceding. The next
higher type of skeleton is the cartilaginous form. In tliiscase
the eml)ryonal (ronnective tissue has undergone transformation
into cartilage, at which p%int development is arrested, the
stage of ossification never being attained. The cartilaginous
type of skeleton is illustrated by that of the selachian (sharks
and dog-fish).
 
The third and highest type of skeleton is the osseous. This
results from the replacement of the cartilaginous tissue l)y bone.
The process of ossification does not, however, affect every
part of the cartilaginous skeleton, there being some portions
of the latter which remain permanently unossified. As there
are, throughout the vertebrate series of animals, various gra(hitions in the degree of differentiation of the skeleton, so in
the course of development does the osseous system of every
higher vertebrate pass througli these stages from the simplest
condition, that of the notochordal skeleton, to the highest
form of the almost completely ossified skeletal apparatus.
 
THE AXIAL SKELETON.
 
The axial skeleton, as stated above, includes the bones of
the trunk and those of the head. Logically the development
of the former will first claim attention.
 
The Development of the Trunk.
The Stage of the Chorda.— The formation of the
(jhorda dorsalis or notochord is the earliest indication of the
axis of the embryonic body and it will be recalled that it is
also one of the earliest embryological processes. The mode
of development of the chorda from the entodermal epithelium
has been described at p. 73. The chorda serves the purpose, as it were, of an axis about which the permanent vertebral column and a part of the skull are, at a much later
date, built up. The anterior or headward termination of the
chorda corresponds to the position of the later hypophyais, or
pituitary body, and thus the chorda is coextensive, not only
with the vertebral column, but also with a portion of the
cranium. The cells of the chorda enlarge and become distended with fluid, the protoplasm of each cell being reduced
to a thin layer. The peripheral cells, however, constituting
a distinct layer, the chordal epithelinm, remain small, and it
is by their proliferation that the •horda increases in size. In
the amphioxus the chorda is the only " skeleton *' that is ever
acquired, and in this animal it is a permanent structure. In
all other vertebrates it becomes surrounded by embryonal
connective tissue, mesenchyme, which latter undergoes chondrification, and in the higher types ossification also. While
in some of the lower vertebrates, as in certain classes of
fishes, the chonla persists as a structure of more or less importance, in the higher members of the series, birds and
mammals, it retrogrades as the processes of chondrification
and ossification go on, until finally it is represented only by
the pulpy centers of the intervertebral disks.
 
The Membranous Stage. — The notochordal stage of
 
the development of the vertebral column is succeeded by the
menihninous staire. The transformation is effected bv the
appearance of an ensheathing mass composed of embryonal
connective-tissue cells which surround not onlv the chorda
but also the neural canal or fundament of the nervous svstem
(Fig. 17-1, nh). The source of this embryonal connective tissue
or mesenchyme bears an imi)ortant relation to the primitive
segments. As the develo[)nient of the primitive segments
was described in the last chapter, and also in Chapter IV.,
it will suffice to remind the reader that each primitive segment undergo(»s differentiation into the myotome or muscleplate, the cutis-plate, the nephrotome, and the sclerotome (Fig.
174), the sclerotouK? occupying the mesial surface of the segment and lying in close proximity to the chorda.
 
AVhile the myotome originates from the flattened or mesothelial cells of the primitive segment, the sclerotome is made
up of cells of the type characteristic of young-growing connective tissue — that is, of the mesenchymal part of the primitive segments as distinguished from their mesothelium. Owing
to the rapid multiplication of its cells, each sclerotome spreads
out headward and caudalward, and dorsad and ventrad, surrounding both the chorda and the neural canal, until both these
structures become enclosed in a common, continuous sheath of
embryonal connective tissue. That part of this tissue which
surrounds the chorda is often designated the skeletogenous
sheath of the chorda and also the membranous primordial vertebral column. The cells of the sclerotomes not only surround the chorda and the neural canal, but they also spread
out laterally into the intervals between the muscle-segments
to constitute the ligamenta intermuscnlaria or the bands
or strips of connective tissue which separate adjacent muscle
 
 
MuscU'Segments
 
 
 
Intersegmental
arteries
 
 
 
 
ist spinal nerve
 
Ligamentum
intermuscularium
 
2d spinal nerve
 
Ligamentum
intermuscularium
 
jd spinal nerve
 
 
 
Skeletogenous sheath of chorda^ \^horda
 
Fig. 175.— Frontal projection fVom a series of sections through a cow embryo of
 
8.8 mm.(0.35 in.). (From Bonnet, after Froriep.)
 
 
 
segments from each other (Fig. 1 75). It is worthy of note
that while this skeletogenous sheath of the chorda originates
from segmented structures, the somites or primitive segments,
and is to that extent related to the segmentation of the body,
it now presents no trace of segmentation.
 
 
 
Very soon, however, this ensheathing membranous tissue
exhibits areas of condensation alternating regularly with less
dense areas. Eaeh such condensed area has the form of u
somewhat obli(|uely i)lace<l bow or half-arch. This halfareli of condensed mesenchymal tissue is called the primitive
vertebral bow by Froriep (^Figs. 175 and 176), whose investigations upon chick and cow embryos established most of
the facts known concerning the development of the vertebra?.^
The median portion of the lx>w is on the ventral side of the
cliorda and is known as the hypochordal brace. The lateral
extremities of the bow abut against the eorresj>onding myotomes, eadi extremity becoming bifurcated. The dorsal
limb of the bifurcation, the neural process, extends gradually
over the dorsal surface of the primitive spinal cord, forming
the neural arch; while the ventral limb advances ventrad,
foreshadowing the hemal arch or costal process of the vertebra, or, as regards the thoracic region of the body, the future
rib.' Both dorsal and ventral processes grow into the intervals between iidjaoeiit myotomes and hence are intersegmental, that i>, they, n> well as the vertebral \m)\\ from which
they >j>rintr, (•orre>pon(l to the intervals between the primitive segments of the body. Sub>e(|iieiitly these j)roeesses of
the bow uive rise to the variou«- processes of the ecmipleted
vertebra. The median part ol' the bow, the hypochordal
brace, subseqncMitly becomes cartilaginous and assists in
forniiii<r th(» bodv of the vertebra in l)inl>, but in mammals
it reuiaius unchondrified and becomes an inconspicuous and
transitory part of the intervertebral ligament — the future
intervertebral disk — exce]>t in the case of the first cervical
vertebra, the atlas, the ventral arch of which it furnishes.
The nieinbran(nis anlage of x\w cartilaginous body of the
vertebra is fcnind in a special condensation of the mesen
' M<»re rcH'ently the ]>r(>cess lias K'cii stiKlicd m tlu* human embryo by
IJiinh-eii. Afurrn'tin Jourunl nf Annfonv/. vol iv , No. 'J, \W\.
 
' Mnr]»h(>l()irir:illy, each vertehni is po^n^ssod of a ii'iirnl nrrh^ f(f>r tlio
protection of the spnial cord ; and a bnnni nrrh^ for the protection of the
ortnms of emulation, re<y)i I'll turn, and diirc^tion, the ril)> of man and the
higher Vi'rtebnites U'lny the ])er»^istent hemal arches in tfie rei^ion of the
tfiorax.
 
 
 
 
eliymatous slieath of tlio i-lionla on tin; caudal aide of the
liyi>ocliortlal brace. The intarrertebral ligament is develowd
fnmi the (erichonlal tissue on the dorsal side of the liyi>ocliordal bniee. Banleen's piimilivc rlluk iiiultides this aniiigc
nf the iiitorvtTtebnil ligament phis the hyiwK'lionial brace of
Froriep, which latter Bardceii regards as a transitory thicken of e n r 1 gi. Ilk
 
 
 
 
The Cartilaginous Stage.— This stage of the devcloi>niciit (if titc spine is bi-onght abont by the metamorphosis of
IKirts of the niembrannus vertebral column into the cartilaKinons vertebra. Other and alternating \arts of the same
strncttiro furnish the Intervertebral disks and the ligaments
that hind together the individual elements of the spine. 'J'he
hisliilogic:il changes neeessiry to effect the transformation of
the embryonal connective tissue into cartilage are, briefly,
the moving ajwrt of the cells and the modification of both
the cells and the intercellular substance, the latter acquiring
the characteristic qualities of the matrix of cartilage.
 
For each vertebral bodr there are two centers of chontlrifieation, one on each ^ide of the chorda within the mass of
tissue referred to above (Fig, 176), The formation of cartilage l>egins in the second month. The two centers are soon
connected with each other by a third, which lies on the ventral
side of the chorda, the three forming now a cartilaginons half
 
 
crliuder wliicli js later coniplete<l by the development of c
tilage on tLe dorsal side of the chorda (Fig. 177). Accord- '
ing to Bardeen, the tartilage of llie body grows at the exjwhse of the primitive disk anterior to (above) it. At the
time when the chorda is completely encased in cartilage the
Bi>inal cord is still ensheathed by merely membranous tig
Before the end of the second month the neural uches of the J
vertebrte are indicated by small isolated niassi's of cartila)
which develop in the connective tissue BUrrounding the spinal I
cord, the lateral parts of the membranous vertebral bows. 1
 
 
 
Parachordal carlilagt
 
 
 
 
In the eighth week these fuse with the bodies and appear I
llieri as projections from them. By the end of the third I
month the processes, or neural arches, have grown snfficiently ]
to" meet with their fellows on the dorsal side of the spinal J
conl, and in the fourth month the corresponding arches of ,
the two sides become niii ted, thuscompletingthecartilariiuma I
sbeath of tbe cord.
 
The masses nf connective tissue occupying the intervalsj
 
 
between the vertebral bodies, originating, as stated above, in
condensations of the mesenchymal sheath of the chorda on
the dorsal aspect of the hypochordal braces, become the
intervertebral ligaments (Bardeen's primitive disks) upon
their fusion, in mammals, with the hypochordal braces.
Subsequently they become the intervertebral disks. The
tissue between the cartilaginous arches becomes differentiated into the ligamenta subflava.
 
While the unsegmented skeletogenous sheath of the
chorda is gradually differentiating into the separate elements
of the cartilaginous vertebral column, the chorda itself begins
to retrograde. Within the bodies of the vertebrae its development is completely arrested, while those portions of it contained within the intervertebral disks continue to grow. The
chorda at this stage consequently shows alternating enlargements and constrictions. In certain fishes it persists as a
structure of more or less importance. In vertebrates above
cartilaginous fishes, all traces of the parts of the chorda
within the vertebral bodies are lost as soon as ossification
occurs, while in the intervertebral disks parts of it jxjrsist
as the soft pulpy cores of the latter.
 
Thus the cartilaginous vertebral bodies or centra originate
in masses of mesenchyme situated between the primitive
vertebral bows and are, according to Froriep, segmental, that
is, they correspond in position with the muscle-segments,
each centrum being developed within the limits of a single
segment ; while the processes develop from the lateral parts
of the vertebral bow and later unite with the body. Bardeen, on the other hand, refers the origin of each vertebral
body to two segments, since, according to his observations, the
body grows at the expense of the next anterior primitive
disk.
 
The cartilaginous trunk is completed by the chondrifieation
of the ligamenta intermuscularia to form the cartilaginous
thorax.
 
The Osseous Stag^e. — The process of ossification begins
in certain parts of the trunk at the end of the second month.
 
 
W\\\vv \\w work (»f olioiulrifiration is entirely completed. As
i)ii« liiisioloirical details of Ixuio-foriiintion are to be found in
I 111' h'\l-lMM)ks of lii>toloirv, it will not 1)C necessary to enter
into till' suhjcct lu»ix'. The place's in any individual cartilage
wlu-rr tissificMtion houiiis aix» called the cHintcrs of ossification.
I'lh' proiM'ss is oni' of siii)stitutk>n, the cartilage becoming
broki'ii down an<l absorlunl as the fonnati«)n of bone goes on.
riit' oBHification of each vertebra is l)ogun at three conliT-n, onr in the bodv and one in cjieh arch. The centers
l»»r the arches appear in the x'venth week. The centers
ti>r ilic bodies a)){M'ar a little later and are found first in the
d»n-;d MTtehiie, ap|)«'arinLr sueeessively later in the vei"t<»bi.i- I'iiiihrr lip and farther down. The ossified arches unite
wtili liu'ji other diiriiiii: the lirst vear of life, but their nnicin
Willi ihr ImmIv of the vertebra takes place betwtHjn the thinl
.Old rii.ditli vciirs. At a much later periinl five accessory canU-.ia ul' H-.-i Ural ion are added to each vertebra. Two of these
til I.. Oil to the body an<l jrive rise to two annular ]>lates of
i.i.iu . I In- epiphyses, one for tlu' upper or cephalic surface and
.•III lix ihr opposite t)r eautlal snrt:u*e. The remaining three
I . (Oi I ■ briMiin rr>p<M*tively t<) the spinous process and the two
ii.iii.»viii:ii» pi'orusses. The i*piphyses do not acrpi ire osseous
Mil. Ill wiih ilu' vrrtrbra projuT until al)out the twenty-fifth
 
1 1 1. ... 1 .1 1 hi I transverse process of a e<'rvieal vertebra, enI III. I i.'i.oniii, and <'onsistini: of an aiiteriorand a j>osterior
I III III liuh • iimrr than th«' tran>vcr>r process ])roper, since
I. ml iiii iM \ i-iiir:i I port i(>iM> t hi' rudiment of a cervical rib.
I '.II 1.1 ihi (Mitr (it' the fii>ion ot' this nidimentarv rib with
I J, I. Ill ^.1 . pitMi-s, the vertebral artery, which passes
i , . ,1 (lu III i' -iMTtimidctl by tlu» two processes, and thus
.1, . h.ii I \ ii .d luiii-vcrM* processes di tier from those of
,1, I t, III III. f III the possession of a foramen.'
I I. aUi- III. I ill*' rtxlH, bein^ strikintrly modified eervic^al
 
I I 111. 1 .1.1.1. li '.uiir iiiitlii>rili«'s. :iN Miiiot, that tht* >k)ium]()08
I ti.. ..«., I » . tiul iliiii thi' arti'iy j^rows through the* ossifying
 
 
 
I ii
 
 
 
vertebne, require special mention. The atlas contains less
and the axis more than an ordinary vertebra, since that which
corresponds to the body of the atlas never unites with it but
fuses witli the body of the axis to constitute its odontoid
process.
 
The atlas presents two centers of ossification for its neural
arches — the so-called posterior arch — just as other vertebra?
do. Unlike other vertebrae, these centers do not unite with
the body but become joined to each other on the ventral side
of the position of the cliorda by a piece of cartilage which
results from the chondrification of the hypochordal brace,
referred to on page 376. This forms the cartilaginous ventral or anterior arch of the atlas, which, in the first year of
life, develops a center of ossification. The arch acquires
bony union with the lateral parts between the fifth and
sixth years.
 
The axis or epistropheus develops from the usual centers of
ossification and from an additional one for its odontoid process. Bony union of the odontoid process with the proper
bodv of the axis occurs in the seventh year. The odontoid
process, in common with every other vertebral body, is
traversed in the cartilaginous stage by the notochord.
 
The transverse processes of the lumbar vertebrse, like those
in the cervical region, include not only the transverse process proi)er but also the rudiment of a rib.
 
The sacral vertebra each present the usual ossific centers.
Inasmuch as they become articulated firmly with the pelvic
bones and undergo fusion to form a single adult bone, the
sacrum, their form is much modified during the course of
development. The transverse processes of each side coalesce to
form the lateral mass of the sacrum. Each transverse process
consists, as in the cervical and the lumbar vertebrae, of the
transverse process proper and a rudimentary rib, the center of
ossification for the latter being quite distinct during early
stages of development. The intervertebral disks of the sacral
vertebrae begin to ossify in the eighteenth year, the process
being completed in the twenty-fifth year.
 
 
 
 
Tlio coccygeal vertebrse are quite rudimentary. Each one
is ossified from a single piece of cartilage, and usually from
hut a single (H'uter of ossification. Occasionally the first
piece of tli<.' coccyx <levelops from two ossific centers, the pnx?ess l)eginning at hiiili. Ossification begins in the second
vert<*l)ra lu'twcen the lifth and the tenth years ; in the third,
shortly l)elon' puberty ; in the fourth, scnm after puberty.
The lower three pieces fuse into one before middle life, and
this unites with the first, and the latter with the sacrum, at
variable periods thereafter.
 
The Development of the Ribs and Stemtun. —
 
Ivcference has been made in the preceding pages to the liganieiita iulcriiuiscularia as strips or l)ands of embryonal conn<'('live tissue lying between adjacent muscle segments, which
have ()rigiii;i(<'d, in conuunn with the sheath of the chorda,
iVoiii tlx' c(>lls of the sclerotomes. TIh^ ligamenta intermnscularia become invaded by the costal j)roeesses of the primitiv<' v<'rt<'bral bows, th(^ costal process, which is the ventral
division ol* the tip of tlw i)ow, growing ventnul and j)enetratiiig th(^ substance of the ligament to constitute a curved
rod of connective tissue, the forerunner of the future rib.
Thus (lu'rc are form<'d coiniective-tissne rej)resentatives of
the ril)s, ench of which is enibedded in the looser connective
tissue nl' th<' corresponding intei'nniscular ligiunent. It is
b\ the development of* cartilage within these curved rods of
conden-ed moenchvme, the membranous ribs, that the cartiliiginous ribs are jn'odnced. 'Vhv pi*ocess of chondrification
commences in the >econd month, but does not involve the
proximal <'n(U of the ribs, the tissue heie becoming ligamentous and servinir to bind to<reth<'r the ribs and the vertebi.e. Ivibs are formed throughout the entire extent of the
Vertebral <M>lumn, except in the coccygeal region, but while in
tlu' lower vertebrates the entire series goes on to mature de\.lt»|>meut, in mammals, including man, their growth is
arrc-ted in the cervical, hunbar, and sacral regions. In the
case ot' man and mammals only the thoracic ribs persist and
iM'conh' adult structures.
 
 
 
As the distal (ventral) extremities of the ribs advance
toward the ventral median line, the tips of the first five, six,
or seven each exhibit an enlargement. These broadened ends
soon coalesce, thus forming on either side of the median line
a continuous strip of cartilage, the anlages of the sternum.
The other ribs remain free at their ends. The sternum is
therefore produced from two lateral halves, a circumstance
that explains some of its anomalies, as for example, cleft
sternum, which is a condition due to arrested development
or deficiency of union.
 
The ossification of the ribs begins in the second month of
fetal life and from a single center for each. The process does
not involve the entire rib, a portion near the distal extremity
remaining cartilaginous and becoming the adult costal cartilage. Accessory centers of ossification for the head and
tubercle appear between the eighth and fourteenth years
of life.
 
The ossification of the sternum proceeds from numerous
centers. There is one for the manubrium and from six to
tw^elve for the gladiolus. The ensiform acquires a center of
ossification in the early years of life, but for the most part
remains cartilaginous.
 
Although, as stated above, the ribs of adult human anatomy are limited to the thoracic region, their rudimentary
representatives are found throughout the other regions of
the vertebral column. In the cervical, lumbar, and vsacral
regions each rudimentary rib becomes blended with the
transverse proceas of the corresponding vertebra to form the
transverse process of human anatomy. It is from the persistence of the seventh rudimentary cervical rib and its failure to fuse with the corresponding transverse process that
the anomaly of a free cervipal rib results ; while the presence
of a thirteenth or lumbar rib, as occasionally met with, is due
to the unusual development of the first lumbar rudimentary
rib.
 
 
 
The Development of the Head Skeleton.
 
Just as the wkck'tdii of tlif trunk consists of a doraally
sihiatcd bony casir tor tlit- protection of the spinal con! aud a
wrics of vcntrul nr licnisil arclics (or the protection of the
orgiins of circiilutioii iiiiil respiration; so does the head
skeleton comprise n Iiony eiise for the accommodation of the
lirain with smaller ac(.^;ssory osseous coni[>artment3 for the
orgiuis of special sense, as the orbits and the nasal chamlwrs;
and also a ventrally situated ai>i>aratus which constitutex
both a receptacle for the oral anil the pbarii'ngeal jxirts of
the digestive system and a nicebanlsm for the mastication of
 
 
 
 
r.
 
<l..velop,.
 
rounding' th<> h<'adsimilar l.> that i.l" ■
till' ventral parts, -.v
stni<'tTircs,<-oiistiliit
from the nie.-ioilcnn
 
 
 
iirt. the cranial capsule, or brain-case, is
exli'iit IVnm the c-i.iinective tis-^iic sai-id of tlic cliorila. its .>riL'in thus being
r spiml chmm. On the other hand,
he jaws and the liyojd bone and related
^' till- <(>-ealle<l viBceral skeleton, develop
tissue of the visceial aicbes. As in the
of ih.' trnrik skeleton, the eraniimi is first outlined in
branous tis^n<' re>nliiii,n I'rom the dilliTcnliatioii of the embryonal connective tissue which ensheaths the head-enJ
of the chorda, and also of the connective tissue of the visceral
arches, this differentiation producing the membranotis primordial cranium. The metamorphosis of the membranous cranium
into cartilage brings about the cartilaginous stage of the
cranium, while the replacement of the cartilage by bone is
the final step in the process.
 
Bones that develop from centers of ossification in previously formed masses of cartilage are styled primordial
bones, while those that are pixxiuced independently of cartilage,, either in the skin covering the membranous cranium,
or in the mucous membrane lining indentations in its walls,
arc known as coveiing or dermal bones. The development
of bone is therefore said to be either endochondral or membranous. For the most part, the bones of the base of the
skull are of endochondral formation, while those of the vault
are develoj)ed in membrane. The membranous or dermal
bones are similar in point of origin to the exoskeleton —
placoid and ganoid scales — of certain fishes.
 
The Membranous Cranium. — The membranous braincase is differentiated from the mesenchymal tissue which
ensheaths the anterior or head-end of the chorda. As previously stated, the anterior end of the chonla is at a point
ventrad to the mid-brain vesicle, in the angle formed by the
latter with the fore-brain, at a position corresponding with
that of the i)ituitary body (Fig. 178). The skeletogenous
sheath of the chorda, in this situation as elsewhere, results
from the multiplication of the cells of the sclerotomes, since
this region of the body undergoes segmentation in common
with the trunk. The number of bead-segments is uncertain.
According to recent investigations upon shark embryos, there
are at least nine primitive segments formed in the headregion.
 
The skeletogenous sheath of the chorda spreads out dorsad
to cover the brain- vesicles. From the terminal point of the
chorda, beneath the inter-brain, the sheath advances anteriorly to invest the fore-brain, which latter at this stage is bent over vcntrcul. From the part investing the fore-brain,
a protiibcnint mass, the nasofrontal process, extends toward
the j)riinitivo inoutli-cavity, constituting the anterior or upper
h()iin<lary of the hitter. Meanwhile the mesencbsrmatic tissue of the visceral arches — that is, that part of the mesodcrnii(^ tissue of these structures wliich does not form
muscular tissue — is un<lergoing similar transformation into
menibninous tissue. The first visceral arch divides into an
anterior or upp(T part, the maxillary process, and a posterior
or hnvvv mass, the mandibular arch, these being the membninous jaw arches. The four jaw arches, with the nasofrontal process, form the boundaries of the primitive mouthcavity, the mandibular arches of the two sides having united
in the median line to form its lower border, and the maxillary
arches having fused with the lateral nasal and the nasofrontal
j)n>cesses to c( institute its upper boundar}'.
 
"^rhe membranous primordial cranium, then, consists of a
cnniplete connective-tissue investment for the brain-vesicles,
of tlui nieiiibi-anous jaw arches, and of the hyoid and the
branchial a relics, and presents in its walls the indications of
the cavities for speeial-sens<» organs in the shape of the surface iuvauinations which constitute rtv-^pectivcly the otic vesicle, the Icns-vesieh', and the nasid pits.
 
The Cartilag^inous Cranium. — By the further differentiation of the memi)ranous cranium the cartilaginous stage
is attainecl. The development of cartilage begins in the
second month. \Vhih» thc^ membmnous cranium furnishes a
coni]>let(* <*a])snle for the brain, tJK* cartilaginous brain-case is
deficient, sinc(^ the process of ehondrification <loes not affect
the i-egions of the future parietal and frontal bones. This is
true at lea>t <>f man and the high<*r vertebrates. In those
case^ where the >i<elet«)n rcMuains })ermanently cartilaginous,
as in selachians (sliarks, <log-tish, ct<\), the entire brain-case
partici])ates in the chondritying ])rocess. As the skull ext<'n(ls verv nnich farther forward than the end of the chorda
— wliich latter terminat<*s at the j)osition of the future sella
turcica — the regions of the |)rimitive skidl are designated respectively chordai and prechordal (Kolliker), or vertebral
and everid>ral (Gegenbauer), according as they fall behind or
in front of the end of the chorda.
 
The formation of cartilage begins in the region corresponding to the base of the future skull. On each side of the end
of the chorda a mass or bar of cartilage is formed, extending forwand and backward, this pair of parallel bars being designated the parachordal GartUagflB(Fig. 179,1). Farther forward,
 
 
 
 
 
Fig. 1T9.— Kirtt tundameat of the cartilaginous primordial c
Wlcderebelm) ; 1. J^ntStage! Cchordai jpE, parachordftl cartilage TV. Batbke'a
trabecule cranll ; PR, passage for the bypophysla S, A, O naeal pit apcic veiiiclc, otncysl. 2. Second Stage: C, chorda; B, basilar plate TV trabeculie cranll,
which have become united In front to eonrtilute the nagal Beptum (S) and the ethmoid plat«; a.AF, proccsBusnf the ethmoid plate enclosing the naial otsan ; (H,
foramina olfactorla for (he passage of the olfactory ni^nca FF poilorhltal process; SK. nasal pit; A, 0. optic and lahyrtnihlne veskle*
 
in the prechordal r^Ion, another pair of cartilaginous masnea
is producetl, known as the trabetmla cituiU. The latter are
not straight bars, but have somewhat the form of a pair of
calipers. In a short time the cranial tni)>eculfe unite with
each other, but not throughout their entire extent, an aperture
being left at the position of the pituitary body. It is through
this aperture that the oropharyngeal diverticuluni, which
forms the anterior lobe of the pituitary body, projects to
come into rolatioa %vith the diverticulum from the inter-brain,
which pnxliices the posterior lol)e. At a later period ossification fM-curs lien*, as elsewhere in the Ikiso of the skull, thus
eoniph'tely isolating the pituitary IkkIv from the wall of the
j)harynx. The jKiraehonhil eartilages also fuse with each
other an<l with the eraniti! tral)ecula?, the four pieces now
foniiinji: <»»H^ mass. The j»n)oess of ehomlrification extends to
iither parts of the memhranous enuiiuni so as to produce a
eartilagiiKnis hrain-ease, just as, in the case of the vertebral
column, the «lorsal extrusion of cartilage-formation gives rise
to a case (»r canal for the sj»inal cord. As before stated, however, the chondrifyiug j)nx'ess does not affect the entire
niembnuious cranium in the higher vertebnites, chondrificatiou oceurring around the ]»osition of the foramen magnum
and in the lateral walls of the cranial capside, while parts of
tli(? vault remain membranous. The anterior extremities of
\\n\ unite(l cranial trahecuhe become so modified in form as to
constitute the plate of the ethmoid and the nasal capsule for
the lo<lg('ment of th<» olfactory epithelium. In each lateral
n'<rion the cartilaginous ear capsule is differentiated.
 
Meanwhile the cartilaginous visceral skeleton is developing
from the memhranous .-truetures of the visceral arches. As
in the ease of the hniin-eapsule, the ehondrifying process does
not involve all \yav{< of the membranous visceral skeleton,
]>arts of the latter heing replaced later by dermal or c<^vering
l)one — that i<, bones that develop in membrane without
having been ])revion>ly mapped out in cartilage.
 
In tlu; first visceral arch, the formation of cartilage occurs
only in the mandil)nlar jMU'tion, the maxillary pnx'ess contiimintr memhranou*^. The eartilairc <>f the mandibular arch
a])p<'ars in the form of a eurved bar running ventrodorsally.
'^rhi> bar divides into a smaller j)roximal or dorstd piece, the
palato(juadratum of comparative luiatomy, and a longer distal
or ventral s(^gment, Meckel's cartilage, "^rhe ])alato-quadratum
sul)s<'(|nently divides into two ])arts, the cartilaginous anlages
resj)ectiv<'ly ol* tin- ])aIato-j)teryg<)id plate and the incus.
MeckeFs cartilage like\vi>e undergoes division, there being
se])arate<l from the chief mass a small ju'oximal st»gment
called the arti<*ulare, which is the forerunner of the future
malleus. Thus th(» cartilaginous bar of the mandibular arch has to do with the formation of certain of the ossicles of the
middle ear as well as, to a limited extent, with the development of the mandible.
 
In the second visceral or anterior hyoid arch, chondrification also occurs, but not throughout its entire extent. A bar
of cartilage, the hyoid bar or Beichert's cartilage, is produced
in this arch and undergoes division into three segments, of
which the proximal or dorsal is the forerunner of the future
stapes of the middle ear, while the other two pieces represent
respectively the styloid process and the lesser horn of the
hyoid bone. The tissue intervening between the position of
the styloid process and the lesser hyoid cornu does not chondrify in man but remains membranous and becomes the stylohyoid ligament (see Fig. 185).
 
In the third visceral arch, or the posterior hyoid arch, a rod
of cartilage develops which represents the greater cornu of
the future hyoid bone. Ventral to this, there is formed a
median unpaired piece of cartilage, the copula, belonging to
the arches of the two sides, which later develops into the
body of the os hyoides.
 
To summarize, the head skeleton in the cartilaginous stage
of development presents an imperfect cartilaginous brain-case,
capsules for the organs of smell, sight, and hearing, and a
cartilaginous visceral skeleton, the several parts of which map
out the lower jaw, the hyoid bone, the styloid process, and
the ossicles of the middle ear.
 
The Osseous Stage. — The bony condition of the head
skeleton is brought about in part by the development of bone
from centers of ossification in the cartilages described above, and
in part by the growth of covering or dermal bones in the integument covering those areas which are deficient in cartilage ; in
other words, by both endochondral and membranous ossification. It may be stated in general terms that the bones of the
baseand of the sidesof the skull, including the auditory ossicles,
the ethmoid, and the inferior turbinated bone, are produced by
ossification in cartilage and are hence called primoi'diaJ bones;
and that the bones of the vault of the cranium, and for the
most part of the face, result from the membranous method of
 
 
 
nssi Rent ion, unci :irp tlicrcforo stylod (Icrmal or covering bones.
Smio of tii(! iiulivkliial bones, however, are partly of carlila}rinouij and partly of nicnibraiioiia origin, the several {Mrtioiis reniuining iwrnmncntly distinct in certain lower vertebrates,
but in niEin nuiting so iutiniately
witli (iich otlitT a» to present no
trai-e tif their previously separate
comlition.
 
The occipital bone consists of
 
two genetically distinct jKirts,
 
the )iUiK>rior or intetpatietal -por
tion, which \n a dermal bone,
 
and the occipital bone proper,
 
rijiin. The OBsiflcation of the latter
 
(Hie on each side of the fommen
 
mitions, one in frtmt of the foramen
 
ihi' liii^ilar process, and one (losterior to that fli)Ort«re for
 
III' lalmliir imnioii nfilic !»iiie ni>t belonging to the iuter
>ial Minium. ( Wiliciiiinn lnjiiii^ in these centers early
 
K' ihird li'ial ni«n[h and pr«<-<rds at ^yv]\ rate that at tlic
 
ol' bii'ih ihi- lioin' n>n-^i-.ts of fmir bony jwns which are
 
tliii) hiyers of cartilage.
 
tiiaiii separate tlntingboiit
 
ijrir-ls, iTsjteetively, the ex
supra-occipital (Fig. 181).
 
ihennion with it of the in
riial bone that ossifies from
 
! wifli the .-iUpra-oiK^iintal
 
iilli ot'fi'tal life. Consisting at panilcil liv caililagc, the oceip
1 be<-onii-s a ,-ii,._de l...nc by the end i.f the thin! or fourth
 
ir by thi- buiiy uiiiuii c.fllie sciKinitt' se-rmeiits.'
 
J'lic temporal bone is made up of thi-ee genetically distinct
 
' In s.iiiir- m^,-' \}\i- union of llir iii(i'rii:irifl;il willi llii' sii]irH-<>n-ipitnl is
iiniili'iiv Ihi' ailiill Ihiiu' tluii iiiTwiitin;! Iwii iniusvcrau linsures which
s, iiui: rriilii LMi'li laliTjl :iii^lv, tiiwunl tlio lULiliun line.
 
 
 
 
parts, the smuunoBal or BqaamosTgomatic, the petrosal or petrom&stoid or periotic, and the tympanic. At the time of birth
these three elements of the bone are still separate from eacli
other, the tympanic being an incomplete ring, and the petro
 
 
 
mastoid being still without a mastoid process. The petromastoid is the only part of the temporal bone that is outlined
in cartilage, the squamozygomatic and the tympanic being
represented in the eartilaginous stage of the cranium by
mem bran OU3 tissue.
 
The. sqnunosygomatic (Fig. 182) is ossified in previously
 
 
 
 
 
Fig. l^J.— S«iuamozy«<>mjitic 1^7) and tyin|»aiiic ('), of t«'iu|M>ral IxtUL' at birth.
 
 
 
formed niemhrano from a single center of ossification, which
ai>j)oar.s in the lower part of this segment at about the seventh
\v<»('k. The proeess of bone-formation extends in all directions from this center, but especially
upward into the squamosa and outward and forwaril into the zygoma.
The periotic or petromastoid results
from the ossification of the cartilaginous ear-oaj>sule, which latter constitutes a part of the cartilaginous portion of the early cranium. It should
be remembered that the essential part
of the orgiui of hearing, the internal
ear, is differentiated from a small
pouch of epithelium, the otic vesicle,
wliieh is produced by an infolding or
iiivairination of the surface* ectoderm, and that it is the cartilat^inniis tissue cuclosiuir the otic vesicle and its outgrowths,
the semicircular canals and the cochlea, that constitutes the
cartilai^iuous car-capsulc.
 
The (Ksitication of (he ])criotic is usually descrilx?d as pro(•('(Mlinir iVom three ceuter<. The first of these, the opisthotic,
inake< its apix-araiiec iu the hitter part of the fifth month on
the outer wall ot' th<' ea]>sule, at a poiut corresj)onding to the
])o>i{iou of the |U"ouiout<»ry, \\ heuee the formati<m of bone
-preads iu such uiauuer a> to ]>ro(lu(M' that part of the petros:i
which is below the iuterual auditory eanal. A secoml center,
the pro-otic, aj>]>eais a little later over the superior seniiein'ular eaual aud <::ive< rise to that ])art of the ])etrosa above
the iutiMMial auditory uieatus, aud also to the inner and up|)er
part of the uia>toidea. The third nucleus, the epiotic, arises
iu the ueiiihborhood «»f the ])o-iterior semicircular eanal.
( )>sifieatiou proeee(N rapidly, the three parts speedily uniting
to t'orui oue boue, the [xriotic or petroiuastoi<l. The |)etrous
portion of the ju'riotic is the ruon' important and the more
constant. I'he luastoid is of variable size in different animals, and in the human sp<'cies, at birth, it is fiat and devoid
of the ma>toi<l pnx'ess w hich is so conspicuous in the mature condition of the skull. The mastoid process develops during
the first two years of life, but its air-cells do not appear until
near the age of puberty.
 
The pars tyznpamctis, or the tsrmpanic (Fig. 182), whicli constitutes the bony part of the wall of the external auditory meatus, is ossified in membrane from a single center of ossification.
This center appears in the third fetal month in the lower part
of the membranous wall of the external canal, from which
point the pnxiess of bone-formation extends upward on either
side so as to form an incomplete bony ring, open above.
This tympanic ring is situated external to both the ear capsule and the ossicles of the middle ear and gives attachment
to the periphery of the tympanic membrane. The further
growth of the tympanic ring being in the outwanl direction,
it becomes a curved plate or imperfect cylinder of bone
which constitutes the bony wall of the external auditory
canal. At birth, the pars tympanicus still has the form of
the incomplete ring, its further development taking place
during the first few years of life. The extremities of the
ring unite with the squamozygomatic before birth. The
tympanic unites also with the petrosa except in a region
adjacent to the proximal end of Meckel's cartilage, where
an aperture is left which is the petrotympanic or Glaserian
fissure. Since upon the part of Meckel's cartilage which is
thus enclosed bv the union of the two bones is formed the
long process of the malleus, the presence of this process in
the Glaserian fissure is accounted for.
 
The stybid process of the temporal bone belongs to the
visceral-arch skeleton. It ossifies in two parts in small
masses of cartilage that belong to the anterior hyoid arch.
One, the tympanohyal, gives rise to the base of the process
(Fig. 186); it begins to ossify before birth and soon unites
with the temporal. The other segment, the stylohyal, undergoes ossification later and joins with the tympanohyal only
after adult age is reached. Sometimes it remains separate
throughout life.
 
The sphenoid bone is for the most part ossified in cartilage.
The body of the bone is represented in the fetus by two
sej>arate parts, the posterior body, or basisphenoid, or post
 
 
 
.spli<'noi<l (Fijr. 1S.3, hs\, wliicli incliulos all that part of the
IhmIv ni* th<» niatiiro bono whicli is jx)stcrior to the olivary
rinlnrnco and to whirh belong the greater wings (alisphenoitls): and an anterior body or presphenoid (;>^), situattnl in
front of the olivary ominonoe, to which belong th|? lesser
wings (orbitos|)h('noids). The ossification of the basisphenoid {)roeeeds from two centers placed side by side, which
 
 
 
 
%^
 
 
 
/^
 
 
 
 
 
Ki«.. l.s:;. - Splu'noiil Imhu\ fiflh or sixth A tnl month: seen from above: p«. pre>I))h-iiui«1 itr iiiiti-riur Ixuiy, \\ itti h'sscr wiii}:-^; (i{(, greater wings ; 6<, baffisphenoid
 
or |»i»'ti'i lur I'luly.
 
 
 
;i|»jMar in the eii::htli w(M'k. 1\vo months later two secniid.-irv (M-nh rs M])pear for the lateral parts of the b(Kly.
'rih- presphenoid likewise develops fnjni two centers, which
nre appMniit in ilie ninth week. The union of the
|)ir-plu'n<)id with tlir basisplieiioid occurs in the seventh or
eiL'lith nionili. Maeli greater wing develops from a sinprle
eenlci* of o»ili<:ition, uliieli is ]>resent in the eighth week.
The pmrrssof o>sifieaiion >j)read> from this center to produce
imi (nilv the LTi'eatrr wini: but also the external pterygoid
j)lal('. i'lie ureatrr win^s remain separate from the body
until <nnie tiinedurinsr tli(» lir>t vear after birth. P^aeh lesser
wing <»s>ities from a center that appears about the ninth
wrcL. 11ie les<er >vinij:s unite with the j)rcsj)henoid in the
>i\ih fetal montii.
 
The internal pterygoid plate dilfers from the other parts of
the .vpheiioi<I in that it does not os>ify in cartilage but in
membrane. It is stated. howev(»r, that its hamular process
tir-^t hreomes eartilaiLrinons bet'ore it ossities. It is, therefore,
a run rin(/ hum'. \\< center or <'enters of os>itication aj>pear
in the fourth month in the connective tissue in the lateral
walls of the oro])harvnireal t-avitv. In manv animals this
plat<' ac(|uires n«» connecti(»n with the external pterygoid plate, but remains throughout life a distinct l>one, the ptervgoiil.
In man it fuses witli the external plate in the fifth month.
 
The presphenoid with its attaclied leaser wings, and the
basisphenoid, to which are united the greater wings and the
pterygoid plates, remain permanently separate bones in some
animals. In man, as noted above, the two parts of the body
of the bone unite shortly before birth, although the greater
wings remain separate until some ranutha after that event.
 
The etlunoid bone and the Inferior turbinate are formed in
cartilage, resulting from the ossification of the jKiHterior portion of the cartilaginous nasal capsule (Fig. 184, m). The
 
 
 
 
vtlli Ih^ nntl ruTltj at (hcplMea dmtEnntri by n 't K. »rt1lnge»t ibe iiikwI Hptum ; n, (urbliml cartilage ; J. omnn nf Jacobsnn : J', the iilaoe wbeie II apclW into
tlia nasal raviij- ; gf, palatal proeeia; of, maiillarj pruccu; iJ. dental rtdgc
 
IHuTlWlB).
 
latter represents the anterior extension of the cartilaginous
trabecule cranii so mmlified as to constitute a rc<»ptacle for
the olfactory epithelium. The anterior part of this capsule
remains cartilaginous throughout life as the septal and lateral
cartilages of the nose. By the ossification of the posterior
part of the nasal capsule the ethmoid and the inferior turbinate bones are produced. Ossification, beginning in the
fifth month, involves the lower and the middle turbinals and
a psirt of the lateral masses. The ixwificatlon of the superior
turbinal, of the vertical plate, of the crista galli, and of the
 
 
 
rcintiiiiinjL^ jxirts <»f tho lateral masse? is efiected after birth.
Tlic Imiiiv iiiiinii <»f till' lateral masses with the median plate
i> <'oinplct(*<l U'twii'ii the fifth ami seventh years.
 
Tlif frontal bone is a ot evening or dermal l)one, being ossitird ill iiK'inhran*.' tVom two centers of ossificatioUy one for
earh iatt-nil half. These centers are situateil above the
<)rl>ital arrhrs and are tir>t a]»|mrent in the seventh week. At
hirtli, tin* two halves of the bone are still se|)aratey their
niiiuii not nr<Mirriiig until during the first year of life. Sonieiinic> till* union fails to take platv, the ciimlitiou of the per>i>t<Mit frontal or metopic suture being known lus metopism.
M<toj»i-in is ronsid(*ral)ly more common in European skulls
than in tlm.-t' «»f lower tyjH'.
 
TIk- parietal bone i< also o.-sificnl in membrane. It develops
from tuo mhlci wliirli soon c<»alesce. Their position eonx?-jMnid- to that of tile future parietal eminence.
 
TIh.' bones of the face are for the most jxirt dermal l>ones.
< )f tin-**, tin* nji|K'r and the lower maxilhe and the palate
Imiiu- IxJoiiLT to the vi-ccral-arch skeleton. The others devrlnp ill tin- iiHinhranous wall <>f the cranial eapside.
 
Tlw nasal jithI lacrimal bones ossify each from a single
(•(•lit*!-, uhirli a|»|M':ir- in the ci^dith week.
 
The malar i- o--ifn d in nicniKrane from three nuclei, the
ppKM-- iMMiniiiiiir in tlic eighth week.
 
riic palate bone is iorinr<l in ninrous membrane fn>m a
-iiw^lc cciitf r whirli i- .-itnal<_'(l at the jniiction of the vertical
and tlic horizontal pl.itc^.
 
riic vomer d<'V('l(»|>s from two center- of ossification which
a|>|H':ir at the hack |)art of the cartilaLrinotis nasid septum.
Ivi'h <'eiiirr Liive- rise to a laniina ol' hone, the two laminae
;:rMdually iinitiiiLr with each other from behind forwanl, and
eiiil>r:uinL'" Ix'tween them anteriorly the septal cartilage.
 
The vomer and the palate bone are examples of the formation of JM)iie in iniieoiH in<'inl>rane. The centers of ossification lir-t aj)peMr in the eighth week in each ca-e.
 
'i'he skeleton of the visceral arches includes the upi>er and
lower niaxilhe, the liyoi<l Ixnu* with a ])art of the styloid
|)rocess, tlu? ear ossicles, and the j)alate bones. The ]>alate
hone- have heeii referred to above. These hones of the visceral-arch skeleton are partly primordial and partly membranous.
 
The snperior maxilla comprises two parts, the superior
maxilla proper and the intermaxillary bone. While these
intimately unite in man, in some animals, as the dog, they
are permanently distinct, the intermaxillary lK)ne constituting
the important and conspicuous premaxilla of the dog. The
superior maxilla ossifies in membrane — within the membranous maxillary process of the first visceral arch — from
an uncertain number of centers. It seems probable that
there are five nuclei of origin, one for the palate process,
one for the malar or external part of the bone, one for the
portion internal to the infra-orbital foramen and a part of
the nasal wall (orbitonasal center), one for the part of the
bone between the frontal process and the canine tooth, and
one for the premaxilla. The formation of the antmm begins
in the fourth month by the development of a recess or fossa
on the inner or nasal wall of the bone.
 
The palate process is formed by the growth, on the inner
aspect of the bone, of a shelf-like projection which advances
toward the median line until it meets and unites with its
fellow of the opposite side (Fig. 172j. The horizontal plate
of the palate bone develops similarly and very shortly after,
and thus is produced the hard palate, which separates the
nasal chambers from the mouth. The two halves of the
hard palate unite first in fnmt, their union being completed
by the twelfth week. If union is incomplete, the anomaly
of deft-palate results. The intermaxillary segment begins
its development in the seventh or eighth week upon that
part of the nasofrontal process which lies between the nasal
apertures. In the fifth month the intermaxillaries fuse with
the maxillse, the line of union being indicated by a suture
which is apparent upon the oral surface of the palate processes. The intermaxillaries contain the germs of the four
incisor teeth. As previously mentioned, deficiency of union
between the maxilla and the intermaxillarv results in the
deformity of hare-lip. Obviously, the hiatus in hare-lip
will be found to be not me<lian, but lateral, corresponding to
the position of the line of normal union.
 
 
 
Tliti lower jaw or mandible is intimatcl}' associated ia its
(K^vclopriiuiit witli that of the malleus and incus of the middle
(•(ir. Inasmuch as thoric thrt-e Inmes are dilfcrentiatcd from
ihi! it:irti[:i;riiioiis anil inumliniiioiis visceral skeleton of the
first vi.-wrrtil arch it is di'sir.ibk< to consider their develop
IIHlll toj^tiuT.
 
As dfscrilied aliove, the inenihmiiotis jaw-arches form the
hiliral and Idwit Iiniiiidtirics of the month-cavity, the first
vis<i'ral arch dividing into the nuixillary process and the
iiuiiidihiihir :m-li. Thenr apjH'iirs in the mandibular arch a
bar of cartilage which abuts liy its jmiximal extremity upon
th Iter wall of the au<litory labyrinth. This cartilaginous
 
 
 
 
" r'ii;)il<'t'ii n-i-Lka nld with the
1til. 1'lii- liiniT Jaw lomewhU
'li i.'iiti'iiil:> III till' niHlkiii. The
lyuitaiiicu!! iKvikible: Aa.nwlfiirlilnip-. .Vt: uk, biiiijr lower
 
 
 
>. iia*
 
 
,,:,h,l,..iu.dm.u,
 
 
 
orliim, Meckel's cartilage (Fifr.
 
•.iNii.ial pi.ve. whifh is called,
 
- |>al:ito<|ii:iiIi-:itLnti. From the
 
Hie {i:<[iili>|ilery[ruiil process.
 
 
 
 
grows toward the roof of the mouth-cavity and becomes a
separate segment. The piece of cartilage remaining, which
represents the proximal end of the original bar, undergoes
ossification, becoming the incus (Fig. 185, am). The posterior or proximal extremity of Meckel's cartilage, becoming
a partly sej)arated cartilage, the articulare, ossifies to produce the mallens (Fig. 185, ha). Though the form of the
malleus is recognizable, it is still in direct continuity with
Meckel's cartilage. In the opposite direction it is articulated
with the incus. As the tympanic ring develops, and the interval below, between tliis ring and the petrosa, is gradually
narrowed to the petrotympanic or Glaserian fissure, the malleus comes to He within the tympanic cavity, being continuous,
through the fissure, with Meckel's cartilage. Upon the separation of the malleus from the cartilage of Meckel, the long
process of the malleus represents the former bond of union
and therefore occupies, in the mature state, the Glaserian
fiasure. The joint between the malleus and the incus represents the primitive vertebrate jaw articulation. In the shark,
for example, the mandibular joint is between the two pieces
into which the cartilaginous bar of the first visceral arch
divides — that is, between the palatoquadratum and the representative of Meckel's cartilage, the mandibulare. In mammals, however, the malleus, as we have seen, loses its connection with the mandible, the joint between the latter and
the skull, the temporomaxillary articulation, being secondarily acquired in a manner to be pointed out hereafter.
While the malleus develops for the most part by ossification
in cartilage, its long process develops in membrane as a small
covering or dermal bone, the angulare.
 
The membranous lower jaw with its enclosed bar of cartilage becomes osseous, not by the ossification of the cartilage, but by the development of a casing of bone within the
surrounding membrane. In other words, the lower jaw
develops chiefly by the intramembranous method of boneformation. Several centers of ossification appear, and from
these the process of bone production extends rapidly, forming, by the fourth month, a covering or dermal bone, the dentale (Fig. 185, uh)^ which is situated mainly on the outer
side of Meckel's cartilage. A smaller plate appears on the
inner side. Thus the cartilage comes to be surrounded by an
irregular cylinder of bone. The cartilage of Meckel plays
a comparatively unimportant part in the ossification of the
lower jaw-bone and begins to degenerate in the sixth fetal
month. Its distal extremity, however, undergoes ossification, thus aiding in the formation of a small part of the
mandible near the symphysis; while a posterior segment,
with the fibrous tissue encasing it, which extends from the
temjx>ral bone to the inferior dental foramen, persists as the
internal lateral ligament of the lower jaw. With these
exceptions, Meckel's cartilage entirely disappears. The
angle of the mandible and a small part of the ramus are
also ossified in cartilage, which latter is developed independently of Meckel's cartilage. From the posterior part of the
dentale the condyloid process develops and becomes articulated with the glenoid fossa of the temporal bone, thus establishing the temporomaxillary articulation. This joint, as previously stated, is a secondary one and replaces in mammals
the primitive articulation between the mandibulare and the
palatoquadratum of the lower vertebrates.
 
At birth, the two lateral halves of the inferior maxilla
are united at the symphysis by fibrous tissue ; bony union
occurs during the first or second year after birth.
 
To summarize, the inferior maxilla develops as a part of
the visceral -arch skeleton and is chiefly a covering bone, since,
with the exception of the angle, a portion of the ramus, and
a small part near the symphysis, which are of cartilaginous
origin, it is formed by the membnmous method of ossification. The two other products of the mandibular arch, the
malleus and the incus, are ossified from cartilage, with the
excej)tion of the processus gracilis of the malleus, which is
of membranous origin.
 
The development of the hyoid bone, of the styloid process of
the temporal bone, and of the stapes was referred to in considering the c4irtilaginous visceral-arch skeleton, but for the
sake of clearness and completeness it may not be amiss to  repeat, in this connection, Bome points previously mentione<!.
 
The membranons ulterior hyoid or second Tisceral arch, at a
certain stage of development, presents, in ita interior, the
dorsoventral cartilaginoiiii bar known as Keichert's cartilage. This is parallel with Meckel's cartilage, and, like it, is
in contact by its dorsal or cranial end with the outer wall of
the auditory labyrinth. A shorter bar of cari:ilage appears
in the third visceral arch, which latter is known also as the
posterior hsroid arch. Together, these two cartilaginous elements furnish the stapes of the middle ear and the hyoidean
apparatBS, the latter consisting of tlic hyoid bone, the stylohyoid ligaments, and the styloid processes. In man the
 
 
 
 
llfOldeaii apparatuB and Inryni at dog.
 
 
 
hyoidean apparatus is somewhat nidlmontary, but in the dog
and many other mammals it is present in its typical form
(FIr. 18G). In such animals the stylohyoid ligament of human anatomy is roprescntrd by a b()ne, the epihyal, the hyoid
bone being, therefore, connected with the skull by a series
of small bones artieulatod with earh other. AH the elements
of the hyoidean apparatus, ^ave the IwmIv and the greater
cornna of the livotd liono, are produced bv Reichort's cartilage ; the hyoid Iwdy, known in comiKirutive anatomy as the
basiliyal, .tihI the greater eornua, or the thyrohyals, ossify from the cartilage of the third arch, the cartilage for the
body being a median unpaired segment known as the copula.
Beicliert's cartilage undergoes division into five s^ments.
The segment at the cranial end, upon ossification, becomes
the stapes/ This ossicle, by the closing of the walls of the
tympanic cavity, is isolated from the other segments. The
second piece, the tsrmpanohyal, ossifies to form the base of the
styloid process and ankyloses firmly with the temporal bone
at the point of junction of the periotic portion of that bone
with its tympanic plate. The third portion, the stylohyal,
forms the lower part of the styloid process. It undergoes
ossification later than the tympanohyal and does not acquire
osseous union with it until the time of adult age. It sometimes remains separate throughout life. The fourth segment, the epihyal, does not even become cartilaginous in
man, but remains fibrous, constituting the stylohyoid ligament. In most mammals it ossifies, to form a distinct bone,
the epihyal. The ventral extremity of the cartilage of
Roichert, the ceratohyal, produces the lesser cornu of the
hvoid bone.
 
THE DEVELOPMENT OF THE APPENDICULAR SKELETON.
 
The upper and lower limbs articulate with the trunk
through the modium respootivcly of the pectoral and pelvic
jj^irdlcs, tho fornicr being constituted bv the scapula and the
elaviele, and the latter by the ossa iiinoniiiiata. As in the
ease of the axial skeleton, the hones of the limbs in their
develo]nu(jnt ])ass sueeessively through a nienibnmous and a
cartilaginous stage.
 
The general (h^velopnient of the upper and lower extremities is deseribed in a later section. As stated in that account,
each linii)-bud is to be regarded as an outgrowth from, or as
eorresj>ou(ling in posili(ni to, several primitive segments, the
tissue composing the little bu(l-lik(» ])rocess subsequently differentiating into the muscular, cartilaginous, and connectivetissue elements (►f the member. The origin of each limb
from more than one primitive segment has been established
 
^ Si'u ft)ot-n()te, i>:igt.' J 15).
 
 
 
chiefly by eml)ryological investigations upon the lower vertebrates, and is borne out by the fact that each extremity
receives- its nerve-supply from a series of spinal nerves instead of from the nerve-trunk of any one segment.
 
The Development of the Pectoral and the Pelvic
Girdles. — The pectoral or shoulder girdle consists in its
earliest stage of a pair of curved bars of cartilage, each of
which is made up of a dorsal limb occupying approximately
the position of the future spine of the scapula and approaching but not touching the spinal column, and a ventral segment lying near the ventral surface of the trunk. At the
angle of union of the dorsal and ventral parts is a shallow
depression, an articular surface, which represents the point
of articulation with the future humerus.
 
The scapula is developed, except its coracoid process,
from the dorsal part of the primitive shoulder-girdle. This
soon acquires a form resembling that of the adult scapula
with the infraspinous portion of the bone very much shortened. Ossification begins at the neck of the scapula about
the eighth week, and in the third month extends into the
spine. The ventral part of the cartilaginous shoulder-girdle
extends almost to the median line of the chest-wall. It
divides into two diverging bars, the lower one of which
undergcx^s ossification in birds and in some other vertebrates
to form the conspicuous coracoid bone. In mammals, however, it aborts and gives rise to a smaller element, the
coracoid process of the scapula. At birth the human scapula
is but partially ossified, the coracoid process, the acromion,
the edges of the spine, the base, the inferior angle and
margins of the glenoid cavity being cartilaginous. The
coracoid process ossifies from a single center and acquires
osseous union with the body c>f the bone at about the age of
puberty. The acromion ossifi(s from two or three nuclei
and joins the sj)ine between the twenty-second and twentyfifth years. Still other centers of ossification ai)pear from
time; to time. Thus there is an accessory center for the base
of the cf)racoid and the* adjacent part of the glenoid cavity, and one at the inferior angle of the hone, from which latter
ossification extends along the vertebral border.
 
The clavicle does not develop from the primitive shouldergirdle, but is formed in membrane, for the most part, as a
dermal bone. 1\^ ossification begins in the sixth or seventh
week, before that of any other bone in the body. Subsequently, cartilaginous epiphyses are added, one at each end.
It is by means of the epiphyses that the bone grows in
length.
 
The cartilaginous pelvic girdle consists of a pair of cartilages, which are united with each other by their ventral
extremities, and each of which, by its dorsal end, is articulated with the sacral region of the cartilaginous spinal
column. At about the middle of each cartilage, on its outer
surface, is a depression representing the future acetabular
fossa. Anterior to the depression is a large ajjerture, the
thyroid foramen, the upper and lower boundaries of which
are respectively the pubic and ischiatic rwls or bars, which
make up the ventral portion of the cartilage, while posterior
to the fossa is the iliac segment, which has a somewhat
irregular jilatc-likc form. Ossification Ix'^ins in the third
month, ]>roccc(Iiii<2; from three centers, one for each of the
tlire(? divisions of the innominate l)one. At the time of
birth a large pro|)oriion of the orijxinal cartilage is still
present, the os pnl>is, the ischium, and the ilium being separated from each other np to the age of puberty by strips of
cartilag(». The ischium and the pubes unite first, and later
acquire osseous union with the ilium. In addition to the
three ])rimarv centers of ossification, other and secondary
nuclei apj)ear at a later date in the crest of the ilium, the
tuberosity of the ischium, and in i\w various spines and
tubercles.
 
The skeleton of the free portions of each extremity, consisting at first of a continuous mass or rod of partially metamorphosed mesenchymal tissue, undergo(»s division into segments
which represent the skeleton of the arm or of the thigh, of
the forearm or of the leg, and of the hand or of the foot.
This segmentation corresponds with that of the entire mass of the limb, both as to extent and order of appearance (see
page 406). Nuclei of chondrification now appear, one in
the center of each skeleton-piece, from which cartilage formation extends toward either end. The several cartilaginous
elements thus pnKluced present approximately the respective
forms of the future bones. The larger cartilages are present
in the upj)er extremity in a six weeks' embryo, but not until
somewhat later in the lower limb. All the bones of the
extremities are of endochondral origin.
 
The long bones develop in a fairly uniform manner. The
shaft or diaphysis ossifies from a single center, while the two
epiphyses each present several centers. The centers for the
diaphyses appear at about the eighth week, ossification proceeding at such rate that at birth only the ends of the long
bones are cartilaginous. The centers for the epiphyses appear
at various times after birth. Osseous union between the
diaphysis and the epiphyses does not occur until the growth
in length of the bone is completed. As the details conceniing the time of appearance and the number of these centers
are to be found in the text-books of anatomy, they are omitted
here.
 
Each bone of the carpus and of the tarsus ossifies from a
single center, except the os calcis, which has two ossific
nuclei. The bones of the carpus are entirely cartilaginous at
birth, their ossification beginning in the first year with the
appearance of a center in the scaphoid. The pisiform bone
is the last of the series to ossify, its ossification beginning in
the twelfth year.
 
The bones of the tarsus begin to ossify earlier than those of
the carpus. The os calcis and the astragalus present osseous
nuclei in the sixth or seventh ft^tal month, and the cuboid
shortly before birth. With thos(^ excej^tions the tarsal bones
undergo ossification between the first and the fourth years.
 
The metacarpal and the metatarsal bones and the phalanges
present each a center of ossific^ation for the shaft and one
epiphyseal center. In the case of the phalanges and of the
metacarj)al bone of the thumb and of the great toe, the epiphyseal center is at the proximal extremity, while in the
 
 
n'inaininj: nu'tatarsiil an<I inetacarjKil l)onos it is at the distal
vuiV The ossification of tlio >haft begins in the eighth or
ninth wwk of fetal life; <»f the epiphyses, n(»t until scvenil
y«irs after hirtli. The development of the ungual or distal
phalanges — of the hand, at least — is jH-euliar in that the
ossification l)egins at the distal extnMuity, instead uf in the
middle uf the shaft.
 
 
 
THE DEVELOPMENT OF THE LIMBS.
 
The linihs of vertehnites develop fi-om little bud-like
i)r(KH'sses (Fig. iVl) liiat spring from two lateral longitudinal
ridges, situattnl one on eaeli side of the body. Thes(» ridges
are not exactly i^anillel with the nuMhau ]>hine of the body,
but converge somewhat toward that jilane as they api)roaeh
the «uidal end of the embryo. It results from this circumhiancc that the jMistcrior limbs are jilaeed chaser together
than the anterior. In man, the limb-buds ai)pear soon after
the thin! wivk. Kju'Ii bud contains a basis (»f j)rimitive eonnivtivc tissue oontributt»<l by several somites, as >vell as muscubr <tnietnrt\ whioh is th(? ofi«hoot from the muscle-plates
0-' J 1.^ numlKT of primitive si^gments.
* Thr assumption of the origiu (►f each limb-bud from more
i «r^mitivc socmont is borne out by the nerve-sup])lv
Th' fu]lv-l>"^*^' l"»l^' ^*"*''* extremity being innervated by
' ' ' ^ ■ •' <nir.-il nerves (compare page ;}GS). The eon.>a<m of the limM)ud i)ro(iuces the bony structures
while the i»utgn)Wths from the mustrle-plates
\r.y mnsriilntun*. Previous reference has been
-.. t.^ ihe work of Banleeu and Lewis on
v^ ihr limbs. Aci'ordiug to their findings,
 
 
 
T^y fVTcnd into the limb-buds, i)ut the limb
.^ f«rtni the mestMichymal core of the bud.
 
""""'* -,,*; thf bnJ for the arm is at first ojiposite
 
 
i.\'/*!ol' the third ('crvical >cgm«'nt,
^. N.-^v* 10 its adult position ; and that the hud for the leg, at first attached at the region of the lower
four lumhar and first sacral myotomes, extends to include the
first lumbar and the second and third sacral segments,
assuming later a more caudal position.
 
In the fifth week each limb-hud becomes divided, by a
transverse groove, into two segments (Fig. 59, 12, 13), of
which the distal part becomes the hand or foot, while the
proximal j>ortion very soon afterward divides into the
forearm and arm or leg and thigh. Even as early as the
thirty-second day, the digitation of the limb-buds — in the
case of the up]>er extremities — is indicated by four longitudinal parallel lines or grooves on the distal extremity
of each (Fig. 59, 14). By the conversion of these grooves
into clefls, the fingers appear, in the sixth week, as separate
outgrowths. The development of the upper extremities precedes that of the lower by twelve or fourteen days, so that,
when the fingers are present as distinct projections, the toes are
just being marked off in the manner noted above for the
fingers. The toes begin to separate, by the deepening of the
intervening clefts, from the fiftieth to the fifty-third day. By
the end of the eighth week, the fingers are perfectly formed,
with the exception of the nails. The nails have their beginning
in the seventh or eighth week, in little claw-like masses of epidermal cells, which are attached to the tips of the digits
instead of to the dorsal surfaces. Subsequent transformations
result in bringing the nail into its normal position on the
dorsal surface of the distal phalanx. The nails are well
formed by the fifth month, at which time the covering of
modified e])idermal cells begins to disapi>ear. The extremity
of the nail, however, does not break through so as to project
beyond the finger-tip until the seventh month. A more
complete account of the development of the nails will be
found in connection with the origin of the skin (page 270).
 
The Position of the Wmbs.— The paddle-like limbbuds at first project laterally almost at right angles with the
axis of the trunk. At this time the future dorsal surface of
eacrh limb looks toward the back of the fetal Ixxly (dorsad),
the future flexor surface toward its anterior aspect (ventrad), while the first digits — the future thumb and great toe — and
consequently the radius and tibia, occupy the side of the
member that is directed headvvard or cephalad, the future
little finger and fifth toe with the ulna and fibula looking
caudad. As the limbs enlarge and differentiate into their
respective segments, they apply themselves to the ventral
surface of the body, this change in position being facilitated
by the occurrence of the future elbow- and knee-flexions,
which cause the flexor surfaces of the forearm and leg, respectively, to approach the corresponding surfaces of the
upper arm and thigh. At abf)ut the same time, the distal
segments, the hand and foot, become bent in the opposite
direction, producing the condition of the limbs that is permanent in the Amphibia — that is, the condition in which the
dorsal surface of the proximal segment of the limb faces in
the same direction as the dorsal surface of the trunk, while
the middle segment is flexed and the distal is extended. To
establish the permanent condition of the human limbs, there
occur an outward rotation of the arms and an inward rotation
of the lower extremities, on their long axes. The thumb and
nulius, therefore, instead of looking cephalad, are now directeil dorsad — with the forearm in the supine jxjsition and
the arm outstretched — or laterad, away from the median
plane of the body, if the arm hangs by the side in the anatomical j)osition. By the inward rotation of the lower limb,
the great toe and the tibia come to lie toward the median
plane of the body, cjiusing the extensor surface to look ventrad, the flexor surface, dorsad.
 
 
 
TABULATED CHRONOUXIY OF DEVELOPMENT.
 
 
 
PertillMtioS!"*
 
 
STAGE OF THE OVH«.
FiiCT Weik. Second Week.
 
 
Obanctwi.
 
 
SegmenUtlon of fartlllied
while pauine along ovl
Great Increase In tl«.
Cella of loner cell-maw rearranged lo form cnto
cell* of Rauber.
 
eytlum (.--3 dar).
 
 
Ovnm Id ntema, embedded
 
C°ortSn'and il> Tilll (FlgJ
49). YapeularitalloQ of
Fliorlau and ila villi.
 
Yalk-uc partly formed.
 
 
ViLtenUr
 
 
 
 
rf"yolk-wc. '''""
 
 
"Vy'SS,.
 
 
 
 
Oral pit (12thMl«h day).
«m-lmrt partly «.i-.n.ted
from yHlk-BBc.
 
 
 
 
•*?SS!i.«"'
 
 
 
 
 
 
STAGE OF THE EMBKYO.
TuiRD WEEK, Fotiavn W««.
 
 
Ouiarml
Cbuutan.
 
 
Body of embryo indicated.
 
VUcural arche. and clefia
 
NMo^fciltkr"™™
AllBiitoiCBtnlklFlgisTt.
lHjllnctton betweuii chorion
Jevc and rhorion fWn
 
Marked flexion of body (21irt
to lOd day): sradual uncoiling after aSd day.
 
Vlacerararchea and jolk-aac
 
to flatten.
CephBllo Bcxurea.
 
 
"nsas..
 
 
Heart wllb einitle caTlty
present, aooo dlvldltig Into
 
VlaCBral-areh -veueli beglu
to appear.
 
 
IMviilon of atrium begini.
 
 
DlceiUTa
TlyaMm.
 
 
(5ut-lracl a strBlght lube connetted wlih yolk-«ae by a
 
Anal Plate.
 
 
Alimentary canal ptcocnla
pharvni. euipbapu, atomach. and intwliue.
 
Fancrcaa bvguii.
 
l.lver-dlTPrileulum divides,
 
Blle-diicta acquire himlna.
 
brealiB down,
 
 
-C£'
 
 
tral wflU of esophagus,
arterward becoming a
 
 
Pulmonary anlage blftircall's, the two poaches
being eonne-ted by « pedicle,^he prlraltlTC irwhea.
with the pharynx.
 
 
"XS^""
 
 
Wolffian bodies reeoBnlsuble.
 
 
 
 
BUn.
 
 
SeKmenlation of paraxial
mesoikTm,
 
HeurBl canal : Its cells sho*
 
El obi oats and Kerm-cellH
Fourth ventricle Indlcaled.
Fore-bralQ mtd-brBln, and
hind-brain -reniples. s-kiu
illvidlns Into five vealelcs.
 
Auditory pll followed by ollc
 
nlfaclory plalM.
Optic vL-hlclcflbi-frtn,
 
 
I'utL-plite.
 
 
Vettmu
87item.
 
 
thicken.
Yenlral roots of aplual
 
Anterior' lobe of hypopbrila
 
begins.
 
 
Smel&l Bsnae
 
OTgMM.
 
 
Ollc vealcle with recenu
 
NHMlplu dl'itlnd.
lJl4lc veaicic atalked and
tmuBformetl IntooptlccDp,
 
 
MOBCular
ByaCem.
 
Bkflleton Mid
 
Limbi.
 
 
iiicsodcrm.
ft-Km.nlallon of paraxial
 
 
Somites or primitive acf
 
Hyiilomea.
 
S.iniltcs or prlmlllre aeg
Sk"ci(.'t.ien.ina ahcath of
 
phorrla.
Llmb-hiidii Biijarent (aboot
 
2lst day).
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
411
 
 
 
Tabuulted Chronoloot of Development (Oontinued),
 
 
 
STAGE OF THE FETUS.
 
 
 
Fifth Week.
 
 
 
Sixth Week.
 
 
 
Body shows dorsal coDcavity in neck- NasofVontal, lateral nasal, and maxil
 
 
 
region
 
Globular and lateral nasal processes.
Lacrimal groove.
Third and fourth gill-clefts disappear in
 
sinus prscervicalis.
Umbilical cord longer and more spiral.
Umbilical vesicle begins to shrink.
Length of fetus 1 cm. ({ inch).
Larynx indicated.
 
 
 
lary procesnes unite.
I'mbilical vesicle shrunken.
Amnion Larger.
 
 
 
Primitive aorta divides into aorta and
I)ulmonary artery.
 
The only corpuscular elements of the
blood during the first month are the
primitive nucleated red blood-cells.
 
 
 
Vitelline circulation atrophic and replaced by allantoic circulation.
 
 
 
Intestine shows flexures, notably the
U-loop, inaugurating the distinction
between large and small bowel.
 
Anal pit.
 
 
 
Right and left bronchi divide into three
and two tubes respectively (5th to 7th
week).
 
 
 
First indication of teeth in the form of
the dental shelf.
 
Submaxillary gland indicated by epithelial outgrowth.
 
Duodenum well formed; caecum; rectum (end of week).
 
 
 
Larynx indicated as dilatation of proximal end of trachea.
 
Arytenoid oartiliiges indicated (though
not cartilagiiiouM).
 
Thyroid and thymus bodies begun.
 
 
 
Genital ridges appear on wall of b<»dycavity and soon t>ecome the indifferent genital srlands.
 
Ducts of Mtiller apfiear.
 
 
 
(ienital tubercle, genital folds, and gen- '
ital ridge (external genitals). '
 
 
 
Epidermis present as two layers of
cells.
 
 
 
CoIIh of oiiti>)-t>late proliferate and gradually hpreau out beneath epidermis.
 
 
 
Olfactory lobe begins.
 
Arcuate and choroidal Assures on mesial surfaces of fore-brain vesicles.
: Cells of central canal of cord ciliated.
i Ridge-like thickening of roof of mid! brain.
 
 
 
Membranes of brain and cord indicated.
I*inenl body U;gins.
Dorsal roots of spinal ner\'es.
Home tracts of spinal cr>nl indicated,
and its lumen alters (Fig. 139).
 
 
 
Semicircular canals indicatcnl. Semicircular canals.
 
Eyes begin to move forward from side Concha of external ear.
of head. ' Outer flbrous and middle vascular tu
1 nicR of eye.
Eyelids
 
 
 
I Mandibles unite (a')th day).
 
, Meckel's cartilage.
 
I Limb-buds segment.
 
I Digitation indicated (32d day, for hand.
 
 
 
I^wer jaw begins to ossify.
 
Clavicle iHJgins to ossify.
 
Rilw l>egin to chondrify.
 
Bodies of vertebra: are cartilaginous.
 
Fingers as sefMirate outgrowths.
 
 
 
412
 
 
 
TEXT-BOOK OF EMBRYOLOGY.
 
 
 
Tabulated Chronology of Development {Contiuued).
 
 
 
•
 
 
STAGE OF THE FETUS.
Seventh Week. Eighth Week.
 
 
General
Characten.
 
 
Fetal body and limbs well
defined (Fig. 64).
 
Head less flexed.
 
No longer any trace of syncytium on dccidua vera.
 
 
Head more elevated (Fig. 65).
Free tail begins to disappear.
Subcutaneous lymph-vessels
 
present.
Oils lining the coelom are
 
true endothelium.
 
 
VaBcular
Bystem.
 
 
Interventricular septum of
heart completed, tne heart
now having four chambers.
 
Other corpuscular elements
added to blood during second month.
 
 
 
 
Dlgeetlve
System.
 
 
Transverse colon and descending colon indicated.
 
 
Parotid gland begins.
 
True endothelium lines the
body-cavity.
 
Gall-bladder present (2d
month).
 
Anlage of spleen recognisable (2d month).
 
 
Respiratory
System.
 
 
Median and lateral lobes of
thyroid unite.
 
 
lArynx begins to ehondrify.
Formation of follicles of
thymus.
 
 
Oenlto-nrlnary
System.
 
 
Maximum development of
Wolffian body.
 
 
Mailerian ducts unite with
each other. Genital groove.
 
Bladder present as spindleshaped dilatation or allantois.
 
Suprarenal bodies recognizable.
 
 
SUn.
 
 
Nails indicated by claw-lIkc
masses of epithelium on
dorsal surfaces of digits.
 
 
Corium indicated as a layer
of spindle-cells beneath
epidermis. Development
of mammary glands began.
 
 
Nervous
System.
 
 
Fore-brain vesicles increase
in size disproportionately.
Cerebellum indicated.
 
 
Sympathetic nerves discernible.
 
 
Special Sense
Organs.
 
 
 
 
External nose definitely
formed (Fig. 171).
 
Lens-capsule.
 
Palpebral conjunctiya separates from cornea.
 
 
Muscular
System.
 
 
Muscles begin to be recognizable, though not having
as yet the characters of
muscular tissue.
 
Ossific centers for vertebral
arches and for vertebral
l»odies; ossiflc renters for
frontal bone and for sciuamosa.
 
Membranous primordial cranium begins to ehondrify.
 
Claw-like anlages of nails.
 
 
 
 
Skeleton and Limbs.
 
 
Ribs begin to ehondrify. Centers of ossification of bestsphenoid, of greater wings.
of nasal and lacrimal
bones, of malar, vomer, palate, neck of scapula, diaphysos of long bones and of
metacar)>al bones. Fingers
perfectly formed. Toes begin to seiMirate (53d day).
 
 
 
Tabulated Chronology of Development (Continued),
 
 
 
STAGE OP THE FETl'8.
Ninth Week. Third Month.
 
 
 
Weight, 15 to 20 frrams ; length, 25 to 30 Weight (end of month), 4 ounces: length,
 
mm. (1 to Ij inchefi). 2) Inches.
 
Hard palate completed. - At first chorion Icve and chorion fron
• Free tail has disapfteared. dosiim prt'oent : later, formation of
 
Differentiation or lymph-nodes begins placenta (see second frontispiece).
(O. Schultze). (Uoaea divided. I
 
 
 
Pericardium indicated.
 
 
 
Placental system of vessels.
Blood-vessels fienetrate spleen.
 
 
 
Anal canal formed by division of cloaca. Mouth-cavity divided fh)m nose (end of
 
 
 
(Anus opens at end of '2d month, ac-,
cordiuK to Tourncux.)
 
 
 
month). Soft (Mlate completed dlth
week). Papillie of tongue. Evagination for tonsil. Intestine begins to rect»de within abdomen (10th week). Rotation of stomach. Vermiform api>endix as a slender tube. Omental bursa,
(tastric glands and glands and villi of
intestine fairly well formed (lOth
week I. Liver verj* large. Peritoneum
has its adult histological characters.
 
 
 
Epiglottis.
 
 
 
External genitals begin to show distinctions of sex.
 
Ovary and testis distinguishable fVom
each other.
 
Kidney has its characteristic features.
 
Urogenital sinus ac<iuires its own aperture by division of cloaca.
 
 
 
Union of ti-stis with canals of Wolftian
 
lM)dy conn>lete.
Testes in false jx-lvis.
Ovaries descenM.
Prostate K*gun d'ith week).
 
 
 
r«>riuin projK»r present as distinct layer. |
Nails not (juite iH»rfeetly fi>rmed.
Hepinning Jif <ievelopiiient of hair as '
solid ingrowths of epithelium.
 
 
 
Corpus striatum in«licated.
j Corpora quadrigemina represented by
two elevations on mid-brain roof.
 
 
 
Cerebrum covers inter-brain. Fornix '
and corpus callosum iK'gun. Fissure '
of Sylvius. Calcarine fissure. Crura
cerebri. Kestiftirm bodies. Pon.s.
 
 
 
! External ear indicated (Fig.
Ciliary processes intlicated.
 
 
 
170).
 
 
 
Eves nearly in normal iK>8ition.
Eyelids begin to adhere to each other.
 
 
 
\
 
 
Centers of ossillcation of presphenoid. Ik'jrinningossiticationof occipital bone. "
of les.«ier wings of sphenoid, and t)f of tympanic, of spine of scapula, of
shafts of metatarsal bones. ossu innominata. '
 
Cnrtilacinovis arches of vertebrse close.
Limbs have definite shape ; nails almost
|)erfeetly formed.
 
 
 
Tabulated Chronology op Development (Coniinwd).
 
 
 
General
CliaracterB.
 
 
 
I
 
 
 
Vasciilax
System.
 
 
 
Digestive
System.
 
 
 
Respiratory
System.
 
 
 
Oenito-urlnary
System.
 
 
 
SUn.
 
 
 
Nervous
System.
 
 
 
Special SenBo
Organs.
 
 
 
STAGE OF THE FETUS.
Fourth Month. Fifth Month.
 
 
 
Weight, 7j ounces: length, 5
 
inches.
Head constitutes about oneI quarter of entire body.
 
 
 
Enamel and dentine of milkteeth. Germs of permanent
teeth tl7th wk) : (for 1st molar, 16th wk). Muscularis
(longitudinal and cirt>ular)
of stomach and esophagus.
Intestine entirely within
abilomen. Acid cells of
peptic glands. Malpighian
I bodies of spleen. Anal
' membrane disappears.
 
! Cells of tracheal and bron' chial mucous membrane
ciliated.
 
 
 
Weight. 1 lb. : length, 8 in.
Active fetal movements begin. Two layers of decidua
(Mialesce, obliterating the
space between vera and reflexa. Lymphatic glands
begin to appear.
 
Heart very large.
 
 
 
Salivary glands acquire lamina.
 
Villi of large intestine begin
 
to disai>piear.
i Liver very large.
 
Meconium shows traces of
bile (sometimes early in
fourth month).
 
 
 
Sexual distinctions of external organs well marked.
Closure of genital farrow.
Scrotum. Prepuce. I*rostate well formed.
 
 
 
Pftpilln? of corium. Subcutaneous fat first appears. I^iiiugo or embryonal down
on scalp and some other
parts.
 
rnrieto-orcipital fissure.
 
 
 
Distinction between uterus
 
and vagina.
Hymen begins.
 
 
 
j CorfHtra alhicantia.
Tmnsvei
 
 
 
»rse fibers of p<m«.
 
Middle i>eduncle8 and chief
fissures of cerebellum.
 
Spinal cord ends at end of
riM'cyx.
 
Deposit of myelin on fihrrs
of |Mist«Tior* roots, extending to liurdaeh and (ioll.
 
 
 
I
 
 
 
Panniculus adiposus.
lanugo more abundant.
Sel>aceous and sweat-glands
iH'gin.
 
 
 
Fissure of Rolando. Body of
fornix and corp. caliosum.
I>on>:itudinal fibers in crura cerebri. Superior peduncles. A nterior pyramids of
medulla. Chief transverse
fissures of lateral lobes of
cerelwllum. Deposit of myelin completed for tract of
(roll and later of Burdach.
and for short commissural
fibers (Tourneux).
 
 
 
Kyolids and iiostriN closed.
("urtiljiKcof Kustnchiun tnlK?.
 
 
 
Orj^m of Corti indicated.
 
 
 
Muscular
 
 
 
 
Difiirentiation of muscular
 
 
System.
 
 
 
 
tissue of arms.
 
 
Skeleton and
 
 
Os»i<'(ms crntcr for inteninl
 
 
(>s*!ifl<«tion of stapes and petriisji. Opisthotic and prootic apjH-ar. Ossificati<m
 
 
Limbs.
 
 
ptrrvjroid |»ljite.
 
 
 
 
Antnnn of lliirhmore b^'cins.
 
 
 
 
<Ksin«'Mtioii of malleus and
 
 
iM'irins in middle and infe
 
 
 
incus.
 
 
rior turt)inals and lateral
 
 
 
Tnav»jr«i of t>thmoid. Internal pt<'ryiroi<l plate Aisea
witlj •'Xternal. Intermaxillarifs fuse with maxilla.
I^'us longer than arms.
 
 
 
Tabulated Chronology of Development (OonHwued),
 
 
 
STAGE OF THE FETUS.
Sixth Month. Seventh Month.
 
 
 
Weight, 2 poands ; length, 12 inches.
Vemix ca«eo«a begins to appear.
Amnion reaches maximum size ; amniotic fluid of maximum quantity.
 
 
 
Weight, 3 pounds ; length, 14 inches.
Sur»ce less wrinkled owing to increase
of fiit.
 
 
 
Peyer*s patches. I Meconium in large intestine.
 
Trypsin in pancreatic secretion (fifth ■ Ascending colon partly formed,
or sixth month). Csecum below right kidney.
 
 
 
Air-vesicles of lungs begin to appear.
 
 
 
Walls of uterus thicken.
 
 
 
Vernix caseosa begins to appear.
Eyebrows and eyelashes begin.
 
 
 
Testes at internal rings or in inguinal
canals.
 
 
 
Epithelial buds for sebaceous glands acquire lumina. Branching of cords of
milk-glands. Eponychium of nails
lost; nails said to break through,
lanugo over entire body.
 
 
 
Collateral and calloso-marginal Assures.
 
Body of fornix and corpus callosum
complete.
 
Hemispheres of cerebrum cover midbrain.
 
 
 
Cerebral convolutions more apparent.
 
Cori>ora nuadrigeraina.
 
Myelination of fibers of direct cerebellar
 
tracts. (Crossed pyramidal tracts not
 
until after birth.)
 
 
 
Lobule of ear more characteristic.
 
 
 
Lens-capsule begins to acquire trans- ,
mrcnry. Kyelids permanently open. ,
rupilliiry membrane atrophies.
 
DiflTorontintion of muscular tissue of ,
lower extremities.
 
 
 
i I^esser winps unite with presphcnoid. i Basisphenoid and presphenoid unite
' Mockel'H cartilHffo h<»cin« to rctrogrn<lo. I (7th or blh month).
' Ossific nuclei of os calcis and astragalus.
 
 
 
 
Tabulated Ciironoloot of Development (Conduded),
 
 
 
 
 
STAGE OF THE FETUS.
Eighth Month. Ninth Month.
 
 
General
Charftcters.
 
 
Weight, 4 to 5 pounds ; length,
 
16 Inches.
Body more plump.
 
 
Weight, 6 to 7 pounds ; length,
 
20inches.
Umbilicus almost exactly in
 
middle of body.
 
 
Vascular
System.
 
 
 
 
9
 
 
Digestive
System.
 
 
Ascending colon longer.
Caicum below crest of Ilium.
 
 
Meconium dark greenish.
 
 
Respiratory
System.
 
 
 
 
 
 
Oenito-orinary
System.
 
 
Testes In inguinal canals.
 
 
Testes in scrotum.
T^bia m^Jora in contact.
 
 
SUn.
 
 
Vemix caseosa covers entire
 
body.
Skin briehter color.
Lunug(j begins to disappear.
Nails project bi'yond hnj^er
tii>a.
Increase of subcutaneous
 
fat.
 
 
Lanugo almost entirely absent.
 
Galaetopherous ducts of
milk-glands acquire lumina.
 
 
Nervous
System.
 
 
 
 
Spinal cord ends at last lumbar vertebra.
 
 
Special Sense
Organs.
 
 
 
 
Ossification of bony lamina
spiralis and of modiolus.
 
Neuro-epithelial layer of retina completed; macula
still absent.
 
Choroidal fissure closes.
 
 
Muscular
System.
 
 
 
 
 
 
Skeleton and
Limbs.
 
 
 
 
Ossification in lower epiphysis of femur, sometimes
also in ui>i>er ei>iphy8e8 of
tibia an(i humerus.
 
Tyinpanohyal begins to ossify.
 
Ossitlc nuclei for body and
great horn of hyoid bone.
1
 
 
----
 
 
INDEX.
 
 


Abdominal cavitv, development of,  
Abdominal cavitv, development of,  
Line 5,874: Line 68:


.skehjtal apparatus of, 372  
.skehjtal apparatus of, 372  
*>7


Ampull(e of semicircular canals, development of, 34rt  
Ampull(e of semicircular canals, development of, 34rt  
Line 5,905: Line 95:
pyramidal tmcts of medulla, development of, 2JK>  
pyramidal tmcts of medulla, development of, 2JK>  
Antitragus, formation <»f, 35*<  
Antitragus, formation <»f, 35*<  
Antrum of liighmore. di'velopment
Antrum of liighmore. development


of, 301  
of, 301  

Latest revision as of 22:03, 29 October 2012

INDEX

Abdominal cavitv, development of,

215 Accessory sii])nirenul oi*};ans, 242

thyroid. 227 Acetabiilur fossa, 404 Achoria, 1)4 Achroiuatin, 27 Acid colls, formation of, 200 Acoustic gaii)j:lioii, ;J21 Acusticofacial jraiijilioii, 321 Adaniantoblasts. I'.VJ Adenoid tissue. develo|>ineut of, 129 Adipose tissue, formation of, 12G After-birth, 104 After-brain, 2S7 Age of fetus, estimation of, 122 Air-chambtT of hen's e;;i;, 29 Air-sjK's, development of. 2"2o Ala* of n<)se. <h'volopment of, 3<)2 Aiiir lamina. 290 Alecithal ova, 2()

Alimentary c^inal, develoi)ment of, Ls5 * diflcrentiatiou into separate region^. 11>7 histolojiical alteratitms in. 20.")

tract, alteration in position of parts, 202 in«.'rease in length of. 201 Alisphenoids. .'{91 Allantoic arteries. 90, 104

circulation. 90 formation of, 103

stalk. .N'>

veins, im, 104 Allantois, ^9, 190. 2.")

function of. JM)

resi)iratory function of. 200 Alveoli, ]>ulmonarv, development of,

225 Ameloblasts. 1.39 Amnion, HI, t<'2

false, si

of man, >^^ Amnion-fold, 80. J^l. R3 A mil iota. H3 Amniotic cavity, 54, 84. 5i5

lluid. 85. SO functi<ui of, J^O

suture, S3 Amphibians, blastula of. 51 Amt)hioxus. blastula of. 50

.skehjtal apparatus of, 372

Ampull(e of semicircular canals, development of, 34rt

seminal, 240 Anal canal. 257

membrane, 195

l»late, 195 Anamnia, Si Angioblast. 147 Animal ]Mjle. 27 Animalculi^ts. 18 Anhige, 175

median, of thyroid body. 228 Annular sinus, 179 Annulus ovalis, 157 Anomalous arrangements of aortic

aivh, lOs Anterior chamber of eye, 342

nares. development of. 1 10, 3«U»

pyramidal tmcts of medulla, development of, 2JK> Antitragus, formation <»f, 35*< Antrum of liighmore. development

of, 301 Anus, development (»f. 195

imi)erforate, 1!»7 Aorta, caudal, KJO

develo]iment of. 159

l)rimitive, 151. 1(J5 Aortic arch, anomalous arrangements t.f, HW

arciies. 105

septum. 159 Api>enda«j:es of skin. 27n Appendicul.-ir skeleton. 372

deveIo])ment of. 4<>2 Aque«lu«t of Sylvius, development of,

29(> Arch, hyoid. 115

mandibular. 115

maxillary. 115

of aorta. d«*velo]»ment of, 107 Arched eolle(>ting tubule of kidney,

240 An'heiitenm. 52 Ar;lie>. aortic. 105

branehial. 114

mandibular. 1.35

visceral, 112 Arehibla«<t. Of! Arcuate fissure. 300. .307 Area, embryonal, 5S

glamlular, 275

opaca, 59

417


418


INDEX.


Area pellucida, 59

vasculosa, 59, 88, 150 Areas, iiasail, 145, 359 Areola, development of, 276 Areolar tissue, develo))tueiit of, 125 Arrectores piloruiii, 2b9 Arteria centralis retinae, development

of, ;«5 Arterial system, fetal, 165 Arteries, allantoic, 90, 164

umbilical, 103, 165

vitelline, 151 Artery, carotid, common, development of, 166 external, development of, 166 internal, development of, 166

innominate, development of, 167

middle siicral, development of, 166

pulmonary, development of, 168

subclavian, left, development of, 168 right, development of, 167

superior vesical, 182 Arytcno-cpiglottidean folds, 226 Arytcn»>id cartilages, development of, 22()

ridges, 22(> Ascending colon, formation of, 203

me.socolon, formation of, 203

root of lifth nerve, 224

root of vagus, 290 Aster. 45

Atlas, formation of, 381 Atresia of pupil, 3:tt. Atrial crescent, 157 Atrioventricular canal, 156

valves, ]56 Atrophic tubules of Wolffian bmlv,

23() Attract ion -sphere, 45 Auditory apparatus, development of. 3*15

meatus, external, formation of. 357

nerve, formation of. 321

nucleus, lateral accessorv. 321

]>it. 3I(» Auricle, (levelopuKMit of, 35R Auricles, division into right and left,

157 Auricular ap])eudages. 159

canal. 15<)

<;cptuni. 157 Auriculovcntricular a]»ertures. 161

valves. \i]'2 Axial tiher of spern«arozo<m. 20, 22

skeleton. 372

development of. 37'> Axis. (leveio]>ment of. 3>0 Axis-cylinder process. 2>1

IUui>kkn's primitive disk. 377, 379 Hartholiji. glands uf. '2til Basil ganuHia. .".(K]. ,3Hl

lamina. 2!>o Hasi-ocripital bojie. .390 Rasisphcnoid, 394


Belly-stalk, 85 Bifid uterus, 253

Bile-capillaries, formation of, 209 Bile-ducts, formation of, 209 Bladder, development of, 255 Blastema, Wolffian, 236 Blastodermic vesicle, mammalian, 50 stage of, 49

two- layered stage of, 52 Blastopore, 52 Blastula stage, 49 Blood, development of, 126, 147 Blood-islands. 148 Blood-lacunee, 97 Blood-platelets, 150 Blood-vessels, 150 "Blue baby," 158 Bodie^i, polar. 3.3, 34 Body of vertebra, formation of, 377 Body-cavity, 63, 6(>, 214 Body-wall, development of muscles of, 367

formation of, 79 Bony cochlea, development of, 352

labyrinth, development of, 351

semicircular canals, 351 Bowman, capsule of, 238, 240 Brain, development of, 286 Brain-case, 384 Brain-membranes, development ot,

302 Brain-vesicles, 287

derivatives of, 316 Bninchial arches, 114

development of, 369 Bran chiome res, 78 Bridge of nose, development of, 362 Broad ligament of uterus, 255 Brunner. glands of, 206 Bud, embryonic, 54 Bulbus arteriosus. 156

vestibuli. 259 Burdax'h. tract of. myelination of, 414 Bur>;i. omental. 20l,'21S

pharyngeal. 136 Bni-sal sacs, development of, 126

('ADrcors niembrjm<»s, 195 C'tecum, develo]mient of, 202, 203 Calcaravis. 308 Calcarine fissure, 303, 308 Callosoniarginal fissure, 309 ("'anal, anal, 257

atrioventricular, 156

auricular. 15()

hyaloid. 339

medullary. 70

neural. 70. 279. 281

jieurenteric. 74. 281

of anus. 197

«.f His. 145. 227

of Xuck. 255

of Stilling, 339 Canaliculi. hurrimal, development of, 345


INDEX.


419


Canalis reuniens. 349 Capsule of Bowman, 238, 240

of kidney, 241 Cardinal veins, 164 anterior, 189 posterior, 169 Carotid artery, common, development of, 166 external, development of, 166 internal, development of, 166

body, 325 Carpus, development of bones of, 405 Cartilage, formation of, 126

Meckel's, 115, 398

Reicbert's. 115 Cartilage-cells, 126 Cartilaginous capsule of cochlea, 352

cranium, 386

ear-capsule, 351

ribs, 382

sheath of spinal cord, 378

stiige of skeleton, 373 of trunk skeleton, 377

vertebral bodies, origin of, 379 processes, origin of, 379 Caudal aorta, 166 Cavity, amniotic, 54, 84

cleavage-, 50

pleuroperitoneal, ^

segmentation-, 50 Cell-cords, 150 Cell-mass, inner, 50

intermediate, 77, 232

outer, 50 Cells, sexual, 31

mesenchynjal, 66 Cementum of tooth, 137

development of, 141 Central canal of cord, formation of, 286

lobe, formation of, 305 Centrolecithal ova, 27 Centrosome, 45 Cephalic flexure, 112,288

ganglia, development of, 320 Ceratohyal, 402

Cerebellum, development of, 292 Cerebral fissures, development of, 302

vesicles, 287, 288 Ce numinous glands. 273 Cervical fistula, 116

flexure. 112

rib. 380. 383 Chalazsp, 29 Chambers of eye. 342 Chin ridge. 13.'> Chorda dorsjilis. 73 formation of. 373 stage of. 373 Chordte tendiiu'jp. 162 Chordul ei)ithelium, 374

plate, 74

region of primitive skull, 387 Choriata, 94 CIiorioc:;pillaris, 340


Chorion, 92

frondosum, 93

leve, 93

primitive, 92

true, 92 Choroid, coloboma of, 341

development of, 340

fissure, 306, 307

plexus, 308

plexuses of fourth ventricle, 291 Choroidal fissure, 330, 341 ChromafiSne cells, 325 Chromatin, 27

Chromosomes, reduction of, 23 Cicatricula, 28 Ciliary body, development of, 341

ganglion, 320

muscle, development of, 341

processes, development of. 333, 341 Circulation, allantoic, 90, 163

placental, 147

portal, 177

vitelline, formation of, 147 Claustrum, 303 Clavicle, development of, 404 Cleavage, kinds of, 47

of ovum, 45

partial discoidal, 48 peripheral, 48

total equal, 47 unequal. 47 Cleavage-cavity, 50 Cleavage-nucleus. 43 Cleavage-planes, 46 Cleft palate, formation of, 137

stem urn, 383 cause of, 82

uvula, fonnation of, 137 Clefts, visceral, 112 Climacteric, 38 Clitoris, development of, 259 Cloaca, 190, 196, 256 Cloacal depression. 197, 256 Closing membrane, 113, 117, 106 Coccygeal body, 325

curve, 112

vertebra?, ossification of, 382 Cochlea, bony, development of, .352 Cochlear duct, formation of, 347

ganglion. 321

nerve, .'154 Coplenteron, 52 Ccplom, 63. (>6, 214" Collateral fissure, 30,3. .308 Collecting tubules of kidney, 237 Coloboma of choroid, 341

of iris, 343 Colon, ascending, formation of, 203

descending, formaticm of, 201, 203

transverse, formation of, 203 Coluinnre carnete, 154 Commissures of brain, development of, 303

of cord, white, 285 Conarium, 298


420


INDEX.


Conariuiii. modificatious of, 298 Coue-visuul cells, 331 Congcuital iitresia of pupil. 33d <lia])brHKniatic heruia, 177 fecal fistula, 207 hernia, 249 umhilic4il hernia, 205 Colli vasoulosi, formation of, 246 Connective tissues, development of,

124 Constructive stage of menstrual cycle,

39 (.'oi>iila of hyoid hone, 3S9 Conicoid hone, 403

process of scapula, 40,3 Cord, spinal, <levelopment of, 2sl

umhiiical, 102 (^)^ds of cells. 147 Corium, development of. 2()8 Cornea, development of. 340 Cornicular tuberch'S. 2i<) Corona radiata. 2."). 31 Coronarv lipvment. 210 of liver. 221 sinus of heart, 172 valve. Kil Corpora alhicantia. 296 hijreniina. 29.1

cavernosa, formation of, 262 (juadrijjemina. '^9.") Corpu«; callosum, formation of, 309, 311 hemorrhaKicum. 37 luteum of prej^nancv. 37, 38 false, :W

of menstruation, 3S true. .38 spongiosum, formation of, 262 striatum, 303 Corpus.;le of Hassal. 230 Corti. or;ran of, 319 Costal process of vertebra, formation

of. 37«). 3'-2 Ci»tyl»Mlous of placejita. !»9 Coveriiii; l)ones, .3*»r> Cowper. ulaiuls <»f. 2<J3 Cranial capsulf, IJ*^!

nerve-fibers. dev«'lopmcnt of, 320 Cranium, cartilairiuous. 3s(> membranous, .385 ossL'ous. 3*^9 Crescent, atrial. l'>7 <'ricoid cartilai^e. 22»» ( 'rista' Mcusticu'. 350 (rossrd pvramidal tract. mvelinati<ui

of, 115 ('rum cerebri, develoi>ment of, 295 Crusta |)etrosa, 1 11 Cryptorcbism. 219

CrystjiUine lens. cl('veloi)ment of. .336 Cuueitorm tubi'rcle>. 22<» Cu-.]iioiis, endocardial. 15«) Cutis-plat'*. 77. 26S. :ij;.-> Cuvi«'r. duct of. Ml. 17(K 176 Cv-^tic dtirt. develo])ment of, 209 cVt.»bIast. 54


Daughter-cells, 22 Daughter-wreaths, 45 Decidua nienstrualis, 39, 95

of pregnancy, 96

reliexa, 9rocesses, 141

ridge, 1,37

shelf. 137 Den tale, 400 Dentate tissure, 30.3, 307 Dentinal fibers, 141

tubules, 141 IX'ntine, 137 Dermal bones, 385

navel, 82 Descending colon, formation of, 203 Descent of testicles. 218 Destructive stage of menstrual cycle,

39 Deutoplasm of hen's egg, 28

of ovum. 26 Develo])inent during eighth mouth,

during eighth week, 119

during fifth month. 121

during fifth week, 118

during ninth month, 122

during secoiul moiAh, 118

during seventh unrnth, 121

during sixth month, 121

during third month, 120

•luring tiiird week, 117

length of time necessary for, 18

tabulated chnmology of, 409

theories of. 17 Diaphragm. develo]>ment of, 177 Diaphragmatic hernia, congenital, 177

ligament. 248 Dieiicepbalon. 287 Digestive svstem. development of,

1.S5. 411-416 Diiritation of limb-buds, 407 Dipbyodont. 137 Dire<t cerebellar tra<*t. nivelination

of. 411 I)isc«>idal cleavage, partial, 48 Discus proiigerus, 31. 251 Disk, germinative. 28 Distal convoluted tubule of kidney,

210 Divrrticubi of primarv renal pelvis,

237 l)ors;il curve, 112

mesrnter.v. 1{H»

nerve-roots of s])inal ganglia, 318

pjincreas. 211 Double monster, origin of, .58

uterus, 2.53


Duct of Cnvier, 164, ITO, ITS luuouephric. £U of UutuKr. 251

ur Miuicr. 24a. air, 253, se&

i>f Batbkc. 246 <if SaotoriDi, 812 of Wirauug. as proDcphrlc, 233

8i'KllieuUl. 233

tlivrtiKlossal. 145. 227

tlivruld, B2T

vitelline, HO, fC, 1S»

Wuiffiau, 234 Ductus Arunlii. 180

arteriosus, 16H

commuDia chuledochus, formation of, 20!)

vndol Till p till lie us, 347

veiuhiUs. lli:i, l-u Duodi'Dum. furtuution of, 217

Ear. eitemiLl, development of, 355. 358 iiiliTiiiil. Ji'velopnient of, 316 iiiiildl, ilcvelopmonC of. 35G

l;.riin-iiU\ cartiUginomi, 351

Ectudemi, 'fi

Egg. iihiinate origin of, 31 E^Wuiuns. 31. iHQ E«K-^'>v«lapeB, 85 Egg-plasm. 26 EKg-lubi», priniary, 31 EigbCh mouth, ilovelopnicat during. 123.416

pair cranial nervea, developmeat of, 323

week 'development during, llil, 412 Ellaculiitarjr iluct. furmntlan of, 347 Elaatii: tissue, formatian of. ]^ Elevontb paircrnnial ii.TTes. 324 EmbpddiiigofDX'iini. M Embryo, diScreiitiuIion of. ffi>

of elffht and B half weclu, 121

of lin«ctitlj riBT, Its

c.frtli weeks, I'lP

of hirtwnthdnr. lOS

of three weekx. 112

of tirenlv-eight days, 118

sejtnii-iiUlion nf body of, 78

slSKi-of. in. 107 Eiubryolijay de fined, 17 Enibryouiil area, .>8

Embryonic bud. I>4 cn-KCcnl. -")!)


Eutl-kuob i.f lipunUBtozooii, 21. 22 EriducardiaU'nslii..iis lat* Eudoutrdiiim. 1>4 Endochoudral Uiuus, 3»5 Eudolyniph. aoA Cndoskeloton, 3T^ KiLili'ih'liuiii. I'liniintluu of. 66, 126 Eiid-piPd- i,r swrmaf


] I ' . i'.ii.. ; \.:\:r of loammaliau

l>l:if ttnlrtmjo vesicle, QO Ependyma. 310 Epcndymal celU, 392. £83


E|.ihyttl, 402

Epiotic center uf usdtlcation, 392 Epitbeliul lHHlie«. ^9 Epithelium, terminal. '29, 31, 244 Epitriebium, 2U0

Efn^'phoroiuVVl Erythruhlustii. 14^ Erythrocytes, 119 Elhniold bune. iwifiuition of, 305

cribriform plate of. 388 Ethmoidal sinuB. dDvelopmcnt of, 3^1 Euatnchian tube, development of, fJW fominli'iuof, 19-1

Vhtve


Exoccipitals. 300

Eioskclelon, 372

ExBtmphy iif lihiddor, cause of, fi3

Eitcrniil uaditory meatus, formation




of, 31.-),


jimiitiils, ^mu!.^ 250, 266

.uiiic. an. iW

Eyp. devoli.pment of. I'lH. 326 Evclashoi. duvclopment of, 344 Eyvlid, third. 344 Eyelids, development of. 343 primitive, 134

Fack, derclopmcnt of, 117, 130 Fai'ial KHKRllon, 321 Falciform ligament of liver, formation of. 210 lobe, 309. 313 Fnllopian tube«, development of, 253


1. SI I Fall cerebri, 303


422


INDEX.


Fecal fistula, congenital, 207 Female external genitals, 259, 266

internal genital organs, 249

pronucleus, 34

sexual system, 266 Fertilization, 41

artificial, 44

external, 42

internal, 42 Fetal arterial system, 165

membranes at birth, 104

vascular system, final stage of, 181

venous system, 169 Fetus, length of, at term, 122

stage of, 20, 118

weight of, at term, 122 Fiber-tracts of cord, development of, 285 myelination of. 414, 415 Fibrillee of muscle, formation of, 366 Fibrous tunic of eye, development of,

339 Fifth brain-vesicle, metamorphosis of, 289

month, development during, 121, 414

pair cranial nerves, development of, 323

ventricle. 312

week, development during, 411 Fimbria, 309

Fingers, development of, 407 First pair cmnial nerves, development of, 323

week, development during, 409 Fissure, arcuate, 306, 307

calcarine, 303, 30H

calloso-niarginal, 309

choroid. :W), 307

choroidal, 330

collateral, 303, 308

dentutc. 303, 307

great transverse. 304, 308

hippocampal. 307

of choroid ]>loxus, 3^)7

of Kohmdo. 30,S

of Sylvius, 303, 301

purieto-occii»ital. 308 Fissures, cerebral, development of, ,^02. 303

median, of cord. 285 Fistula, congenital fecal, 207

M Ml hi Heal urinary, 25<j Flexure, cephalic. 112. 288

nuchal, 289

pontal, 2.^9 Floor-i)late, 2.'-l. 282 Fold, ])h'uropericardial, 176 Folds, medullary. 72 Folliflo. (Jraafian. 2J»

of tooth, 1 11 ForauHMi cR'cuni. 115, 227

eomnunu' anterius. 30()

of Monro. 301. 3<M)

of V/inslow, 221


Foramen ovale, 157

thyroideum, 226 Fore-brain, 286, 302

secondary, 287

vesicle, 73

metamorphosis of, 302 Foregut, 81 Formative yolk, 26 . Fornix, formation of, 309, 310 Fossa of Svlvius, 304

oral, 192

ovalis, 158 Fourth month, development during, 120, 414

pair cranial nerves, development of, 323

ventricle, 291 development of, 290, 294

w^eek, development during, 410 Fretum Halleri. 156 Frontal bone, ossification of, 396

lobe, 306

sinuses, development of, 361 Funiculus solitarius, 290 Furcula, 225

Gall-bladder, development of, 209 Ganglia, cephalic, 320

spinal, 317 Gangliated cord of the sympathetic,

325 Ganglion, acoustic. 321 acusticofacial, .'^21 cephalic, fourth, 321

third, 321 ciliary. 320 cochlear. 321 facial. 321 Gasserian, 320 intercarotid, 325 Luschka's. 325 ophthalmic, 320 s]>irale, 350 trigeminal, 321 vestibular, 351 Ganglion -cell laver, development of,

333 Gartner, duct of, 254 Gasserian ganglion, 320 Gastrjil mesoderm, 63 Gastrohepatie omentum, 209, 220

formation of, 205 Gastrosplenic omentum, 214 (rastrula, 52

stage, 52 Generative organs, external, development of. 258 internal, development of, 243 Genital cord. 243 eminence. 259 in male, 261 folds. 259

in female. 259 in male. 262 gland, indifierent, 265


INDEX.


423


Genital groove, 258

ridge, 243, 258 in female, 259

ridges, 31 Genito-urinary system, development

of, 232, 409-416 Germ-cells, 224 Grcrm-disk, 27 Germ-layers, 52

derivatives of, 67 Germinal epithelium, 29, 31, 244

sj)ot, 25, 27

vesicle, 25, 27 Grerraiuative disk, 28 Giral<les, organ of. 247 Glands of alimentary tract, formation of, 206

of Bartholin, 261

of Brunner, development of, 206

of Cowper, development of, 263

of intestine, development of, 206

of Lieberkiibn, development of, 206

of Moll, 273

of stomach, development of, 206 Glandular area, 275

hy]K>spudias, 262 Glaus clitoridis. formation of. 259

penis, formation of, 259, 262 Glasorian fissure, 393, 399 Globular processes, 118, 132, 360 Glomerulus of kidney, 233, 238, 240 Glomus caroticus, 325 Goll, tract of, myelinatiou of, 414 Graafian follicle, 29 development of, 251 formation of new, 252 Gray matter of bniin, formation of, 303 of medulla, development of, 224 Great omentum, formation of, 204,

220 Groove, dental. 140

lacrimal, 119, 132

medullary, 71

naso-optic, 345

primitive, 60

pulmonary, 22.S

transverse, crescentic, 398 Gubernaculum testis, 248 Gum, development of, 136 Gut, postanal, 196 Gut-tract, 80. HI. 186, 188 Gyrus fornicatus, 315

uncinatus, 315

Hair, development of, 271 Hair-bulb. 271

development of, 272 Hair-follicle, 271.

development of. 272, 273 Hair-germs. 272

Hard palate, development of, 397 Hare-lip, 134, 397 Hassal, corpuscles of, 230


Head, muscles of, development of, 367

of epididymis, 246

of spermatozoon, 20, 22 Head-fold, 80

of amnion, 80, 83 Head -gut, 188 Head-kidney, 232 Head-process of primitive streak. 62,

70 Head-segments, 364 Head-skeleton, development of, 384 Heart, development of, 152

lymph-, 128 posterior, 128

metamorphosis of single into double, 156

valves, development of, 161 Helix, formation of, 358 Hemal arch, formation of, 376 Henle's loop, 240 Hen's egg, description of, 27 Hensen's node, 62 Hepatic cylinders, 209

vein, development of, 181 Hermaphroditism, 263, 2()7 Hernia, congenital, 249

umbilical, 205 Highmore, antrum of, development

of, 361 Hilum folliculi, 31 Hind-brain. 28(J. 292

secondary, 287

vesicle, 73, 292 Hindgut. 81, 1H8 HipiKicanipal fissure, 307 Hippocampus migor, 307

minor. 308 His, canal of. 145, 227 Holoblastic ova, 47 Homogeneous twins, origin of, 59 Homologies of the sexual system, 263 Hyaloid arterj*, formation of, 339

canal, 339

membrane, formation of, 339 Hydatid of Morgagni, 247

8i»s8ile, 247

stalked, 247

unstalked, 247 Hydramnios, 86 Hymen, formation of, 261 Hyoglossus. origin of. 370 Hyoid arch, anterior, 389 l)osterior, 3'^9

arches. 115

bar. 3H9

bono, development of. 389, 401 Hyoidean apparatus, 401 Hyonmndibular cleft, 115 Hypobhist. 5'^ Hypochordal brace, 376 Hypophysis, ,'^(X>

formation of. I'i5 Hypospailias, 262

glandular, 262


424


lyDEX.


Iliac segment of pelvic girdle, 404 vein, left cuiumun, development of,

I m perforata anus, 197 luipresisious, maternal, 120 Ini'us, development of, 388, 399 Indifferent genital gland, 265

si'xual gland, 244 Inferior medullary velum, 292

peduncles of brain, 290 Infundibula of lungs, development of,

225 Infundibulum of brain. 2i>6, 300 Inguinal ligament, 248

in female, 254 Inner cell-mass, 50 Innominate artery, development of,

liu Inter-brain, 287, 296

vesicle, metamorphosis of, 296 Intercarotid ganglion, 325 Intermaxillary bones, formation of,

l.iei, 397 Intermedial cell-mass, 77, 232, 365 Internal ear, development of, 346

fertilization, 42

lateral ligament of lower jaw, 400

limiting membrane of spinal cord, 3v'^3 Interpallial fissure, 302 Interrenal organ, 242 Intervertebral disks, 377, 379

ligament, development of, 377, 379 Intervillous spaces. 97, 102 Inte.stinal canal, formation of, 79

glands, development of, 20*>

mesentery, 216

mucosa, formation of. 189

villi, formation of, 206

]>ortals, 81. 186 Intestine, small, development of, 202,

205 Intestino-bodv cavitv, 52 Intumescentia ganglioformis. 351 Involuntarv muscle, development of,

371 Iris, coloboma of. 343

development of, 341 Ischiatic rod. 404 Island of Keil, :J05

.Tacobson's organ, development of.

3()1 Jaw. upper, development of, 134 Jaw-arch. 115 .lellv of Wharton, 103 Joint-cavities, development of, 128 Jugular vein, primitive. 1({4, 169 transverse. 172

Kidney, development of, 232

Labia majora, 260

minora, formation of, 259 Labyrinth, bony, development of, 351


Labyrinth, membranous, development of, 346 Lacrimal bones, ossification of, 396

canal iculi, 345

caruncle, 344

duct, development of, 344

gland, development of, 344

groove, 119, l.'i2

sac, development of, 345 Lamina cinerea, 296, 299

quadrigemina. 295

spinilis, bony, development of, 354

terminal is, 309 Langhans' laver, 97 , lanugo, 121, 273 I I^iryux, development of, 225 I Uitebra, 29 Lateral cartilage of nose, 395

folds of amnion, 80

frontal processes, 118, 132, 134 in formation of nose, 146

ligaments of liver, 210

nasal process, 344, 360

plate of mesoderm, 65

plate of somite, 63

ventricle, development of, 303 length of fetus at term, 122 Lens, crystalline, development of,

336 Lens-area, 328

Lens-capsule, development of, 337 Lens-pit, 336

Lens- vesicle, 110, 134, 328, 336 Lenticular zone of optic cup, 333 Lesser omentum. 220 formation of, 205 Leukocytes, 149 Levator palati, origin of, 370 Lids, union of edges of, 343 LieberkiJhn, glands of, 206 Ligament of ovary, 255 Ligamenta intermuscularia, 365, 375

subflava, 379 Ligaments of liver, formation of, 209 Ligamcntum venosum Arantii, 184 Liguhr. 2\yZ Limb-buds, 119. 406 Limbic lobe, 309, 313 Limb-muscles, development of. 370 Limbs, bones of. development of, 405

development of, 406, 409-416

position of, 407 Limiting membrane, intier, formation of, 3:ji outer, formation of, 331 Lin in, 27 Lip ridge. 135

upper, development of, 136 Liquor amnii, 85. 86 function of. 86

n.lliculi, 31. 251

of Morgagni. 337 Liver, development of, 207

first rudiment of, 198

ligaments of, formation of, 209


INDEX.


425


Liver-ridge, 175, 208 Iiobes of liver, 208 Lobule of ear, development of, 358 Longitudiual liber-tracts of medulla, 2«0

fissure of brain, 302 Loop of Henle, 240 Lower jaw, ossification of, 398 Lumbar rib, .'i83

vertebne, ossification of, 381 Lungs, development of, 223 Luschka's ganglion, 325 Lymph, formation of, 126 Lymph-clefts, development of, 128 Lymph-hearts, 128

posterior, 128 Lymph-sacs, development of, 127 Lymph-spaces, development of, 127 Lymphatic system, development of, 127

vessels, development of, 128 Lymphoid follicles of tonsil, 195

tissue, development of, 1'^

Macula lutea, formation of, 333 Maculse acustictc. development of, 350 Malar bone, ossification of, 396 Male external genitals, 261, 267

internal genital organs, 245

pronucleus, 42

sexual system, 245, 266 Malleus, development of, 388, 399 Malpighian corpuscle, development of, 213, 238 primitive, 236 Mammalia deciduata, 99

indeciduata, 99 Mammals, blastula of, 49 Mammary gland, development of, 274 Mandible, ossification of, 398 Mandibular arch, 115, 135, 386 Mantle layer, 284 Marginal sinus, 102

velum of spinal cord, 283, 284

zone of optic cup, 334 Marshall, vestigial fold of, 172 Maternal impressions, 120 Maturation of ovum. 32 Maxilla, superior, ossification of, 397 Maxillary arch, 115

process, 135, 386 Meatus, external auditory, 357

urinarius, male, 262 Meckel's cartilage, 115, 388, 398

diverticulum, formation of, 207 Meconium, 122 Median fissures of cord, 285

lobe of cerebellum, 292 Medulla oblongata, development of,

289 Medullarv canal, 70

cords, 246. 252

folds, 72. 279

farrow, 71

groove, 71


Medullary plate, 70, 279 tube, 279

velum, anterior, 294 inferior, 292, 294 Meibomian glands, development of,

344 Membrana adamantiua, 139 basilaris of cochlea, formation of,

eboris, 141

granulosa, 31 formation of, 251

prseformativa, 141 Membrane, anal, 193

closing, 113, 117, 194

nuclear, 27

of Xasmyth, 140

of Reissner, 355

pharyngeal, 117, 131, 188, 192

vitelline, 25, 26

tympanic, 194, 357 Membranes, caducous, 95

deciduous, 95 Membranous bones, 385

cranium, 385

labyrinth, development of, 346

ribs, 382

stage of skeleton, 373 of trunk, 374 Menopause, 38 Menstrual cycle, 39 Menstruation, 38

relation of, to ovulation and conception, 40 Meroblastif ova, 48 Mesencephalon, 286, 294 Mesenchymal cells, 66

muscle, 371 Mesenchyme, 66 Mesenteric artery, superior, 152

vein, superior, 181 Mesenteries, 190 Mesentery, intestinal, 216

ventnil, 204 development of, 220 Mesoblast, 62

Mesoblastic somites, 65, 75 Mesooardium anterius, 153, 174

posterius, 153, 174 Mesocolon, ascending, production of, 203

formation of, 203 Mesonephrogenic tissue, 239 Metanephrogenic tissue, 239 Mesoderm, 62

derivatives of, 68

gastral, 63

paraxial, 65

peristomal. 63

somatic, 66

splanchnic, 66

structures developed from, 12Aet8eq. Mesodermal vitreous, 338 Mesogastrium, 204, 216 Mesonephric duct, 235


426


JSDEX.


Mesonephros, 234, 264 Mesorchiuu, 24b, 255 Mcsothelium, (Hi, 126 McHovarium, 24>::^

Metacar|>al l>oneg, development, 405 MetamorphoHiH of single into double

heart, l.VJ MctanephroH, 237, 265 Metatarsal boneif, development, 405 Meteiiceplialon, 287, 292 Me topic suture, 396 Metopism, 396 Micropyle. 25, 42 Mid-brain, 2»6, 294

prominence of, 288

vesicle, 73. 294 Mid-gut, las Middle ear, development of, 194, 355

piece of Kpermatozoon, 20, 22

plate. 77, 232, 3<I5

sacral artery, development of, 166

tunic of eye, development of, 339 Milk-lines. 275 Mi Ik -ridges, 275

Mo<liolus of cochlea, development, 354 Moll, glands of. 273 Monorchism, 249 Monro, foramen of, .'$01, 306 Mons v<?neris, formation of, 259 Morgagni, hvdatid of, 247

liquor of, 337 Morula, 45 Mother-cells, 22

Motor nerve-fibers, development, 319 Mouth, development of, 134, 192 Mucous tissue, formation of, 125 Mulberry -mass, 45 Miiller, duct of, 243, 247, 253, 265 Miillcr's lib«;rs, :W1 Muscle, involuntary, development, 371

Vdluntary, development of, 363 Muscle-plate, 78, .365

metamorphosis of, .366 Muscles, bnmchial, development of, .3()9

of extremities, development of, 370

of trunk, development of, .363 Muscular coat of inti'stines, formation of, 205

system, development of, 363, 409416 Musculi papillares. 163

pectinati. 154 Myeleiucphalon. 287. 289 Myelin. <leposit of. 319 Mv<K-(el, .365 Myotonu", 77, 365

N'ml-rki). 271 Nail-phite. 270 Nails, development of, 270

of toes, 271 Nail-welt. 271 Nares, anterior, formation of, 146

develo])ment of, 3<)0


Nasal areas, 145, 359 bones, ossification of, 396 capsule, 388

cavities, development of, 361 pits, 118, 132, 145,360 process, 132. 360 lateral, 344, 360 Nasmych, membrane of, 140 Nasofrontal process, 115, 118, 132, 134, 360, 386 in development of nose, 145 Naso-optic furrow, 132, 134 in formation of nose, 146 groove, 345 Nephridial funnels, 233 Nephrogenic tissue, 236 Nephrostomata, 2«33 Nephrotome. 77, 234, 264, 365 Nerve-cells, formation of, 2c»2

of cord, formation of, 284 Nerve-corpuscles of neurilemma, 319 Nerve-fiber, envelopes of, formation of, 319 layer, development of, 333 Nerve-fil>ers, cranial, development of, 320 motor, development of, 319 sensory, development of, 317 Nerve-trunk, spinal, development of,

319 Nervous system, development of, 278, 409-416 peripheral, development of, 316 sympathetic, development of, 324 Neural canal, 70, 279, 281 crest, si'gmentation of, 318 crests, 318 process of vertebra, formation of,

376 tube, 279 Neurenteric canal, 74, 281 Neurilemma, formation of, 319 Neurit, 278, 284 Neuroblasts, 282, 284 Neuro-epithelium of retina, development of. .3.33 Neuroglia. 2S2.283

layer, 284 Neurons. 278

Nictitating membrane. 344 Ninth month, development during, 122. 416 pair cranial nerves, development of,

.324 week, development during, 413 Nipple, development of, 276 Node. Hensen's. (>2 Normoblasts. 149 Nose, development of. 145, 358 Nott>chord. 73

Notochordal stage of skeleton, 373 Nuchal flexure, 2S9 Nuck. canal of. 255 Nuclear.juice. 27 layer of retina, outer, 332


INDEX.


427


Nuclear membrane, 27

ttpiiidle, 45 Nucleus amygdalae, 303

cleavage-, 43

of uvuiii, 27

scgmeutation-, 43 Nutritive yolk, 26 Nymphffi, formation of, 259

Obex, 292

Occipital bone, ossification of, 390

lobe. 30<) Odontoblasts, 141 Odontoid process, development of,

3m Olfactory bulb, 314

epithelium, 359, 362

lobe, 314

nerve-fibers, 362

plates, 132, 145, 358

tract, 314 Omental bursa, 204, 218 Omentum, gastrohepatic, 209, 220 formation of, 205

gastrospleuic. 214

great, formation of, 204, 220

lesser, 220 formation of, 205

phrenicosplenic, 214 Omphalomesenteric veins, 151 Ontogeny, 17 Oocytes, 32 Oogenesis, 29 Oogouia. 32

Ophthalmic ganglion, 320 Opisthotic center of ossification, 392 Optic cup, 328

secondary, 330

lobes, formation of, 295

nerve, development of, 335

thalami, 29fj

vesicle, 287. 327 Ora serrata. 'Xil Oral cavity, development of, 192

f ( JSSft 1 ' f"^

pit. lOH. 117, 131. 135, 192

plate, l.SO, 134, 192 Orbitonasal center, 397 Orbitosphenoids, 394 Organ of Corti. 349

of Giraldts, 247

of .Tacobson, development of, 361

of Kosenniuller, 254 Ors^ans of Ziirkerkandl. 325 OsstMMis cnmiuni, 3H9

stag«» of trunk skeleton. 379

tissue, formation of, 126 r)ssirlcs of ear. d(*velopment of, 356 Ossification of ribs, 383

of skull. :}H9

of st<Tnuni. '.V<\

of vcrtcbne.'JrsO Ostium int(?rv«'ntrieulare, 158 Otir v<'sirlc. 109, :i46 Otocyst, 34()


Outer cell-mass, 50 Ova, alecithal, 26

centrolecithal, 27

classification of, 26

formation of, 29

holoblastic, 47

meroblastic, 48

primitive, 31, 245, 250

telolecithal, 26 Ovaries, change of position of, 254 Ovary, development of, 249 Oviducts, development of, 253 Ovists, 18 Ovulation, 36

relation of, to menstruation, 40 Ovum, 24, 251

embedding of, 95, 96

maturation of, 32

rii)ening of, 32

segmentation of, 45

stage of. 19, 106

Palate bone, ossification of, 396

formation of. 136

process, development of, 397 Palate-shelves, 3<)0 Palatoglossus, origin of, 370 Palatopharyngeus. origiu of, 370 Palpebral fasciae, 344

fissure, 343 Pancreas, development of, 211

dorsal, 211

first rudiment of, 199

ventral, 211 Pancreatic duct, development of, 211 Pander's nucleus, 28 Panniculus adiposus, 269 Papilla? of tongue, formation of, 145 Parablast, (Hi

Paraehoi-dal cartilages. 387 Paratlidymis, 247 Parathyroid bcnlies. 229 Paraxial nies<Mlerm. 65 Parietal bones, ossification of, 396

elevation, 288

eye, 299

fomnien, 2JW

layer of pleura. 177

lobe, 3(KJ

zone. 76 Parieto-oocipkal fissure, 308 Pan»oj)horon. 254 Parovariunj. 254 Pars ciliaris retime. 334

i n te rm ed i a 1 i s, 259

iridiea retina?. 334

menibranacea septi, 159

optica retinje. 333 Parthenogenetic <'ggs. 34 Patuh)us foramen ovale, 157 Pectoral girdle, development of, 403 Pelvic girdle. 404 Pelvis of kidney, primary, 237 Penis, development of, 259 Perforated lamina, anterior, 315


428


ISDEX.


Perforated space, posterior, 295 Pericar<lial aivity, 175 Pericardium, development of, 174 Perilymph, 352, 355 Perilymphatic space, 352 Perineal bwly, 197 Perineum, formation of, 197 Perionyx, 271 Periotic bone, 392 Perijiheral cleavage, 48

nervous system, 316 Peristomal mesoderm, 63 Peritoneal cavity, 215 Peritoneum, development of, 214

visceral layer of, 189 Perivitelline space, 25 Permanent kidney, 237

teeth, development of, 141 eruption of, 143 Petromastoid bone, 392 Petrotympanic fissure, 399 Pfliiger's egg-tubes, 250 Phteochrome bodies, 242

cells, 242 Phalanges, development of, 405 Pharyngeal bursa, 136

constrictors, origin of, 370

membrane, 117, 131, 188, 192 in formation of mouth, 135

pouches, 113, 188, 193 Pharynx, 193

Phrenicosplenic omentum, 214 Phylogeny, 17 Pial processes, 283 Piiimcnt-lavcr of retina, 331 Pillars of Uskow, 177 Pineal body. 296

or gland, 2f)7, 298

eye, 299 Pit, auditory, 316

oral, 1 OS 117 Pits, nasi I, 360 Pituitary body, 300

formation of, 135 Placenta, 98

at term, 101

discoidoa, 99

pnevia. 102

zonaria, 99 Placental sinuses, 100

spaces. 102

system of blood-vessels, 164 PlaVentoblast. 51 Plaues of cleavage, 46 Plantar born, 270 Plasiuodobljist, 54 Plate, chordal. 71

medullary. 70

vertebral, 65 Pleura, parietal layer of, 177

visceral layer of, 177 Pleune. develoi)ment of, 174, 175. 226 Pleural sjics, formation of, 174, 175 P]euroi)eri('ardial fold, 176 Pleuroperitoneal cavity, 66, 215


Plica semilunaris, 344 Pocket of Rathke, 301 Polar bodies, 33, 34

striation, 45 Polarity of egg, 27 Pole-corpuscles, 3ii Polyphyo<lont, 137 Polyspermia, 42 Pontal flexure, 289 Pons, formation of, 292 Portal circulation, 170, 177

vein, development of, 181

venous system, 170, 177 Postanal gut, 196 Postbranchial bodies, 229 Posterior chamber of eye, 342, 343

nare«, development of, 360 Post-limbic sulcus, 309 Postsphenoid, 393 Preformation theory, 18 Prehepaticus, 175, 208 Prehyoid gland, 227 Premaxilla, 397 Prepuce, formation of, 262 Prespbenoid, 394

Primary collecting tubules of kidney^ 239

egg- tubes. 31

renal pelvis, 237 Primitive aorta, 151, 165

chorion, 92

disk, 377, 399

enamel -germ, 138

eyelids, 134

groove, 60

heart- valves. 161

jugular veins, 161, 169

Malpighian corpuscle, 236

nails, 270

ova. 31, 2-15, 250

segment i)late, 65

segments. 65. 75

sexual e«dls, 245

streak. 59

vertebral bow, 376

vitreous. 33S Primordial bones, ,*^5 Proamnion. i\\ Process, latenil frontal, 118, 132, 134

nasal. 132, 3()0

nasofrontal. 115, 118, 132, 134, 360 in formation (jf nose, 145 Processes, dental, 141

globular. 118, 132, 360 nasal, 360

maxillary, 135

of vertebra, development of, 376 Processus vnginalis, 249 Proeborion, 50, 92 Proctodeum. 196 Pronei)bric duet, 233 Pronephros, 232. 264 Pronucleus, female, 34

male, 42 Pro-otic center of ossification, 392^


INDEX.


429


Prosoncephalon, 28(5 Prostate kI&"^* formation of, 257 Prostatic urethra, formatiou of, 257 Protoplasmic processes, 264 Prottivertebru, 6.3

Proximal convoluted tubule of kidney, 240 Pterygoid plate, internal, development of, 394 Pubic rod, 404

Pulmonary alveoli, development, 225 artery, development of, 159, \G6 diverticulum, 223 grtK>ve, 223 Pulp of spleen, development of, 213

of teeth, 137 Pupil, :VM) congenital atresia of, 333 development of. ;W2 Pvnimiilul process of thyroid gland, 22H tracts, anterior develojmient of, 290 crossed, of cord, mvelination of, 415

Ramus communicans. 325 Riithke's piK-ket, 13<>, 193, 301 Kiiuber's layer, 50 Ki'ceptaciilu chyli. 12S Keceptive i)rominence, 42 Kecessus labyrinthi, kWI

vestibuli. .'M7 Kectuui, 197

Ri'current laryngeal nerves, 1(38 Ke<luctiou of chromosomes, 23 Reduction-division, 23 Reichert's cartilage, 115, 389, 401 Reil, island of, 305 Reissner, membrane of, 355 Renal vein, left, 173

vesicles, 210 RepHMliiction, theories of, 17 Re>pinitorv svstem. development of,

222,'40i>-41() Restiform bodies, develoimieut of, 290 Rete mucosum. 270

testis, formation of. 21() Retina, development of, \^Z6 Rhinencephalon, 314 Rhombencephalon, 2H() Rhomboidal fossji, 2i)l i:ib, cervical, :W0. 3^3

lumbar, 38.3

thirteenth, 383 Rib-j, development of, 382 Ri«lne, genital, 213

terminal, 58 Ring lobf, formation of, 304 Ri|M'Mlug of ovum. 32 Roil-and-cone laver, formation of, 332 Rod-visual cells,\'{;n R(»lando, fissure of. 30^ R(M)f-plate, 2'^1, 2>^2 Rotation of stomsu'h, 203, 217 Round ligament of liver, Irtl


Round ligament of liver, formation of, 210 of uterus, 248, 255

Saccule, development of, 349 Saccus endoh'mphaticus, 347 Sacral vertebrse, ossitication of, 381 Sacrum, formation of, 381 Salivary glands, development of, 143 Santorini, duct of. 212 Sauropsida, blastula of. 51 Scala media of cochlea, development of, 347

tympani, development of, 355

vestibuli. development of, 355 Scapula, development of, 403 Schwann, white substance of, 319

deposit of, upon libers of tract of conl, 411, 415 Si'lerotome, 77, 365, 375 Scrotum, development <tf, 263 Selniceous glands, development of, 274 Second month, development in. lis

IMiir cranial nerves, development of, 323

week, development during, 40i) Secondary hair, 273

optic cup, ;J30 Secreting tubules of kidney, 237, 239 Segmental duct, 233 Segmentation of body of embryo. 78

of ovum, 45 S(;gmen tat ion -cavity, 50 Segmentation-nucleus, 43 Semicircular canals, bony, 351

<levelopment of. ,'i4.S SiMuilunar valves, development of. KC Sc>minal ampulhe, 21<)

vesicle, format i(m of. 247 Seminiferous tubules, formation, 246 Sense (ngans, development of, 32t),

409-416 Sensory epithelium of retina, 331

nerve-libers, development of, .317. 31s Septa placenta*. 102 Si'ptal cartilage of nose. 395 Septum, aortic, 159

auricular. 157

intermedium. 1.56

luciduni. fornuition of. 312

primum. 157

secundum, 157

spurlum, 1.59

transvcrsum. 164, 175 SeroMi. si

Si'rous membranes, development, 126 Sertoli's columns. 21. 246 Sessile hydatl«l. 217 Seventh month, development during. 121, 415

pair cranial nerves, development of, 32,3

week, development during, 412 Si^xual cells. 31


430


INDEX.


Sexual cells, primitive, 245

cords, 31, 245 female, 250

gland, inditl'erent, 244

system, female. 24f», 2t>6 homologies of, 2t).'i inditferent type, 243 male, 245, 2m SJiell of lien's egg. 29 She 11 -membrane, 21) Shoulder girdle, development of, 403 Sinus, annular, 179

pcK'ularis, 247, 257

pneeervic^ lis, 110

reuuiens, 159

terminal is, 150

urogenital, UK), 256

venosus, 159, 169 Sixth month, development during, 121, 415

pair cranial nerves, develo])mentof, 32.3

week, development during, 119. 411 Skelelogenous sheath of chorda dorsal is, 375

tissues, 77 Skeleton, appendicular, 373

development of, 402, 409-416

axial. 373

development of, .372

of head, development of. 384

of trunk, cartilaginous stage, 377 chonlal stage of. 373 development <»f. 373 membranous stage of, 374

visceral. 3>4 Skin, ajipeiulujjes of, 270

development of. 2(»S. 409-116 Small intestine. develo]unent of, 205 Snu'gnia enibryonum. 270 Somatic mesmlcrni. 6(5 Somatopleure, 6»l, 1>6 Sornitrs. 6.3, 75

mesoblastic, 65. 75 Space, peri vitelline. 25 Spaces, intervillous. 97. 102 Spermatie cord, 249

veins. 173 Spermatids. 22 Spermatoblasts. 2*? Sperniatogenesi>. 'Jl Spermatoyenic eell>. 21 SpiTUiatocytes. primary, 22

S4'0niid:irv. 22 SjM'rmato<;(»Tiia. 22 Spernialozoon. tjo

power of loeoiMotion of. 21

vitJility of. -Jl Splu'uoid b«t!ie. os>iri(at ion <»f. 39."» S]>heTioi«lal sinus, development of. ."',61 Spinal eonl. <lev<]«ipmeiil of. •^•'1 Spinous p^oee«^> of vertebra, develop nn'Tit of. .'Jsn Splaiiebuie nn>.od<'rni, 66 Splanebnopleure. i\i\, INJ


Spleen, development of, 212 Spongioblasts, 282, 283 Spot, germinal, 27 Sprouts, vessel, 150 Squamozygomatic bone, 391 Stage of embryo, 19, 107

of fetus, 20, 118

of ovum, 19, 106

of quiescence of menstrual cycle, 40

of rei>air of menstrual cycle. 40 Stalked hydatid, 247 Stapes, development of, 389 Stem-zone, 75 Sternum, cleft, 383

development of, 383 Stigma. 31

Stilling, canal of, 3:i9 Stomach, development of, 203

first rudiment of, 198

glands of, development of, 206

rotation of, 203, 217 StomodsBum, 131, 192 Straight collecting tubules of kidney,

239 Stratum Malpighii, 270 Streak, j)rimitive, 59 Striated muscles, development of, 303 Stroma-laver of choroid, development

of, 340 Styloglossus, origin of, 370 Stylohyal, 402

cartilage, 393 Stylohyoid ligament, 389 Styloid process of hyoid. 389

temponil, development of, 393 Stylopharyngeus, origin of, 370 Subclavian arterv, left, development of, 1(38 right, development of, 167 Submucosaof intestines, formation of,

205 Substance-islands, 147 Subzonal layer of mammalian blasto <lerniic vesicle, 50 Sulcus interventricularis, 1.58

of corpus callosum. ;i07

terniinalis, 160 Suju'rior maxilla, ossification of, 397 Suprahyoid gland, 227 Supra-<M'(*ipital bone, 390 Su]M"aperi('ardial bmlies, 228 Suprarenal bo<lies, development of,

241. 265 Suspensorv ligament of liver, fonna tioil of. 210 Sustentacular cells of seminiferous

tubule. 21 Suture, amniotie, KJ Sweat-glands, development of, 273 Sylvius. a<iueduet of, 2fW>

' ti-sure of. ;}n:;, :ioj

l'oss;» <)f. 1504 Syujpathetie nervous sy.stem, 324 Syncytium. 93, 97 Synovial sacs>. dev<'lopment of, 126


/


INDEX.


431


Tail of spermatozoon, 20, 22

Tail-fold, «0

Tarsal ligameuts, 344

plates, 344 Tarsus, development of bones of, 405 Teeth, development of, 137

permanent, development of, 141 eruption of, 143

temporary, development of, 137 eruption of, 142 Tela choroidea, 297 Telencephalon. 287, 302 Telolecithal ova, 26 Temporal bone, ossification of, 390

lobe, formation of, 304 Temporary teeth, development of, 137

eruption of, 142 Temporomaxillary articulation, 400 Tendon, development of, 125 Tendon-sheaths, development of, 128 Tenth pair cranial nerves, development of, 324 Terminal filament of spermatozoon, 20,21

ridge, 58 Testicle, development of, 245

descent of, 248 Thalamencephalon, 287, 296 Thebesius, valve of, 161 Theca foUiculi, 29 Thecal sacs, development of, 126 Theory of evolution, 17

of unfolding, 17 Third eyelid, 344

month, development in, 120, 413

pair cranial nerves, development of, 32:i

ventricle, formation of, 296

week, development during, 410 Thirteenth rib, 383 Thoracic prolongations of abdominal

cavity, 175 Throat-pockets, 113, 188, 193 Thymus body, 194, 230 Thyroglossal duct, 145, 227 Thyroid body, accessory, 227 development of, 194, 226

cartilage, 226

duct. 227

foramen, 404 Thyroids, lateral. 226. 228 Tissue fungus, 97 Toes, development of, 407 Tongue, development of, 143, 194 Tonsil, development of, 194 Tonsillar pit, 195 Trabecula; cranii, ;J87 Trachea, develoiuuent of, 225 Tragus, formation of, 358 Transverse colon, formation of, 203

crescentic groove, 80

fissure of brain. 2})H

processes of vertebne, 380 Trigeminal ganglion, 320 Trophoblast, 92


True chorion, 92

Truncus arteriosus, 113, 151, 154, 165 Trunk, skeleton of, development of^ 373 osseous stage of, 379 Trunk-muscles, development of, 363 Trunk -segments, 364 Tuber cinereum, 296, 300 Tubercles, corn icular, 226

cuneiform, 226 Tuberculum impar, 144, 194 Tubotympanic sulcus, 356 Tunica albuginea of ovary, 250 of testicle, 246 fibrosa, 30 propria, 30 vaginalis testis, 249 vasculosa, 29 lentis, 31^7 Turbinal folds. 361 Turbinate bone, inferior, ossification

of, 395 Turbinated bones, development of,

361 Twelfth pair cranial nerves, development of, 324 Twins, origin of, 59 Tympanic cavity, formation of, 194 membrane, 194

development of. 357 portion of temporal bone, development of, 393 Tympanohyal, 402

cartilage, 393 Tympanum, development of, 356

Umbilical aperture, 87, 186

arteries, 103. 165

cord, 102

hernia, congenital. 206

urinary fistula, 256

vein, 103.165, 169

vesicle, 80, 87, 186 function of, 89 human, 89

vessels, 103 Uncinate gyrus, 315 Unstriated muscle, development, 371 Urachus. 91, 256 Ureter. 237

development of, 232 Urethra, female. 257

male, formation of. 262

prostatic, formation of. 257 Urinary fistula, umbilical, 256 Urogenital aperture, 257

sinus, 190,196,256 Uskow, pillars of, 177 ,

Uterus bicomis, 2.53

development of. 253

double. 25.3

masculinus, 247. 257 Utricle, development of, 349 Uveal tract, development of, 340 Uvula, formation of, 137


432


INDEX,


Vagina, development of, 253

median septum iu, :i^ri Valve, coronary, IGl

Eustachian, 160

ofThebesius, 161

of V'ieussens, 294 Valves, atrioventricular, 156

auriculoventricular, 162

of heart, development of, 161

semilunar, development of, 163 Van Beneden's embryonic bud, 54 Vas aberrans, 247

deferens, formation of, 246 Vasa eflerentia, 246

recta, formation of, 246 Vascular area, 88

system, development of, 147, 409-416 fetal, final stage of, 182

tunic of eye, development of, 339 Vegetative pole, 27 Vein, <'ardinal, 164

hepatic, 181

iliac, leftc(mimon. development, 172

portal, development of, 181

renal, left, 173

superior mesenteric, 181

umbili(tal, 103 Veins, allantoic, 90, 164

cardinal, 164 anterior, 169 pdsterior, 1(>9

omphalomesenteric, 151

primitive ju^iular, 169

spermatic. 173

umbilical, 165, 169

vitelline, 151, 169 Velum interposi turn, 296. 297 Vena azygos major, 173 minor. 174

cava, inferior, 171. 174 superior, 170 Veiue hepaticie advehentes, 179

reveheiites. 179 Venous segmeut of heart, 156

system of fetus, 1()9 ])ort:il, 170 V'^iitral nu'Sfiitery. 190. 204 developuient of, 220

pancreas, 211 Ventricles, separation of, 158 Vermiform appendix. dcvelopment,203

process of cerebellum. 292 Virnix caseosa. 87, 121, 270 Vertfbni'. ossification of, 3^0 Vertebral bow, primitive, 376

column, <levelopment of. 373-3S2 meuibranous primordial. 375

])lati'. 65

region of primitive skull, 3'^7 Vesicle, blastodcrmii'. stage of, 49 tWi)-layered stage of, 52

germinal. 25, 27

lens-, 110

otic, 1(K», 34(5

umbilical, 80, 87, 18<)


Vesicles, cerebral, 287, 288 Vessel sprouts, 150 Vestibular ganglion, 351

nerve. 321 Vestibule of ear, development of, 352

of vagina, 259

of vulva, 257 Vestigial fold of Marshall, 172 Vieussens, valve of, 294 Villi of chorion. 93

of intestine, formation of, 206 Visceral arch, first, function of, 115, 131

arches, 112

metamorphosis of, 115 moqdiological significance of, 113

clefts, 112

layer of peritoneum, 189 of i)leum, 177

skeleton. 384 Vis«'eral-arch vessels, 113, 151. 165 Vitelline arteries, 151

artery, right, 152

circulation, formation of, 147

duct, 80, 87, 186

membrane, 25, 26

veins, 151, 169 Vitellus, 25, 26 Vitreous body, development of, 338

mesodermal, 338

primitive, 3,'i8 Voluntary' muscles, development, 363 Vomer, ossification of, 396

Weight of fetus at different stages, 412-416 at term. 122 Wharton, jelly of, 103 White commissures of cord, 285 tibrous tissue, formation of, 125 matter of brain, formation of. 30.3

of ct»rd. development of, 285 of hen's egg. 29

substance of Schwann, development of, 31 J». .|(»9. 41 Winslow. fonimen ol. 221 Wirsung, duct of. 212 Witches' milk. 276 WoltUan blastema, 236 body, 2:M duct. 235

in female. 253 ridge. 232. 234 Wolff's doctrine of epigenesis, 18 Wreath, 45

VoLK of ovum. 25 Yt»lk-sac, M), v87, 186

ZiNX, zcmule of, 3:58 Zona pellucida, 25. 31

r.idiata, 31 Zone, parirtal. 7(>

stem-, 76 Zonule of Zinn. .338 Zuckerkandl. organs of, 325


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" It is an excellent book^-concise, comprehensive, thorough, and up-to-date. It is a credit to you ; but, more than that, it is a credit to the profession of Philadelphia — to us."

Wm. E. Quine. M. D.

Professor of Medicine and Clinical Medicine, College of Physicians and Surgeons, Chicago. " I consider Anders' Practice one of the best single-volume works before the profession at this time, and one of the best text-books for medical students."

Bulletin of the Johns Hopkins Hospital

'• The success of this work js well deserved. . . . The sections on treatment are excellent and add greatly to the value of this work. Dr. Anders is to be congratulated on the continued success of his text-book."


Pusey and Caldwell on X-Rays

in Therapeutics and Diagnosis


The Practical Application of the Rontgen Rays in Therapeutics and Diagnosis. By William Allen Pusev, A. M., M. U., Professor of Dermatology in the University of Illinois; and Eugene W. Caldwell, B. S.. UifL-ctor of the Kdward N. Gibbs X-Ray Memoiial Laboj ratory of the Univtrsity and Believue Hospital Medical College. New York. Handsnine octavo of 625 pages, with 200 illustrations, nearly all clinical. Cloth, SS-OO net; Sheep or Half Morocco, s6.oo net

RECENTLY ISSUED NEW i2il) EDITION. REVISED AND ENLARGED TWO LARGE EDITIONS IN ONE YEAR

Two large editions of lliis work wuhin a year testify to its practical value to both the specialist and general practitioner. Throughout the work it has beeo the aim of the authors to elucidate the practical aspects of the subject, and to this end the text has been beautifully illustrated with clinical pictures, showing the condition before the use of the X-rays, ac various stages of their application, and the final thera|ieutic result obtained. Details are also given regarding the use and management of the apparatus necessary for X-ray work, illuslratinE the descriptions with instnictive photographs and drawings. In making the reviri<m

listories of the cases cited have been brought down to the present time.


OPINIONS or THE MEDICAL PRESS

f BrilMi Joucnal of DennMology

' The most compleli; rind up-lo-dale conlrjbulion on llip subjecl of llie Iherapeu of Ihe Ranti;pn riv? wliicli h.i, been publiiheti iii English. '■ k.Boitoa Msdical and Sur^Ckl Journal

' It 15 indi;ptiis,i))lc to Ihose wlio use Ihc X-rays B&n ihi-rapeullc agFnl ; and lis illi ) numerniis . , . ttiBi ii bedoniM viiluable to every one."

Haw Yoi^ Medical Jounial

<We have noibini; but praise for lh< a aa one n better filled by Ironing oi


PRACTICE OF MEDICINE


Sahli's Diagnostic Methods

Editors: Francis P.Kinnicutt^ M.D.^ and Nath'I Bowditch Potter, M.D.


A Treatise on Dias^nostic Methods of Examination. By Prof. Dr. H. Sahli, of Bern. Edited, with additions, by Francis P. KinniCUTT, M. D., Professor of Clinical Medicine, Columbia University, N. Y. ; and Nath'l Bowditch Potter, M. D., Visiting Physician to the City and French Hospitals, N. Y. Octavo of 1008 pages, profusely illustrated. Cloth, j»6.50 net ; Half Morocco, J87.50 net.

JUST READY

Dr. Sahli's great work, upon its publication in German, was immediately recognized as the most important work in its field. Not only are all methods of examination for the purpose of diagnosis exhaustively considered, but the explanation of clinical phenomena is given and discussed from physiologic as well as pathologic points of view. In the chemical examination methods are described so exactiy that it is possible for the clinician to work according to these directions.

Lewellys F. Barker, M. D.

Professor of the Principles and Practice of Medic ine^ Johns Hopkins University " I am delighted with it, and it will be a pleasure to recommend it to our students in the Johns Hopkins Medical School."

Friedenwald and Ruhrah

on Diet

Diet in Health and Disease. By Julius Friedenwald, M. D., Clinical Professor of Diseases of the Stomach, and John Ruhrah, M. D., Clinical Professor of Diseases of Children, College of Physicians and Surgeons, Baltimore. Octavo of 728 pages. Cloth, II4.00 net.

JUST ISSUED— NEW(2nd)EDITION

This work contains a complete account of food-stuflfs, their uses, and chemical composition. Dietetic management in all diseases in which diet plays a part in treatment is carefully considered. The feeding of infants and children, of patients before and after anesthesia and surgical operations, and the latest methods of feeding after gastro-intestinal operations are all taken up in detail.

George Dock, M. D.

Professor of Theory and Practice and of Clinical Medicine, University of Mickigtm

" It seems to me that you have ost valuable work of the kind now

I am especially glad to see the Ion ^ different kinds of foods."


SAUNDF.RS' nOOfCS ON


I


Rolleston on the Liver


Diseases of the Liver. Gall-bladder, and Bile-ducts. By H.

D. Rolleston, M. D. (Cantab), F. R. C. P., Physician to St. George's Hospital, London, England. Octavu volume uf 794 pages, fully illustrated, including a number in colors. Cloth, g6.00 net.

ENTIRELY NEW-RECENTLY ISSUED

This work covers the enlirc field iif diseases of the liver, and i.'i die most voluminnus work on this subject in Enj;hsh, Dr. Rulleston has for many years past devoted his tiinc exclusively to diseases of the di^;estive organs, and anything from his pen, therefore, is authoritative a.nd practical. Special 3 given to pathology and treatment, the former being profusely illustrated.

Medical Record, New York


Boston's Clinical Diag>nosis

Clinical Diagnosis. By L. Napoleon Boston. M.D., Associate in

Medicine and Director of the Clinical Laboratories. Medico-Chirurgical College, Philadelphia. Octavo of 563 page.'^. with 330 illustrations, many in colors. CInth, £4,00 net.


Dr. Boston here presents a practical manual of ihe clinical and laboratory examinations which furnish a guide to correct diagnosis, giving only such methods. however, which can be carried out by Ihe busy practitioner in his office as well as by the student in Ihe laboratory. In this new second edition the entire work has been carefully and thoroughly revised, Incorporating all the newest advances. Borton Modic&l Mid Sur^nl Journal

" Up h:is produccil a book winch muy he regarded emineolly as a praelical luid servioable guide. . . . The iliuslralions are both namerous and good."


■w^ T»« 


MATERIA MEDICA.


GET A • THE NEW

THE BEST /\m6riC2kn standard


Illustrated Dictionary

Just Issued— New (4th) Edition


The American Illustrated Medical Dictionary. A new and complete dictionary of the terms used in Medicine, Surgery, Dentistry^ Pharmacy, Chemistry, and kindred branches; with over lOO new and elaborate tables and many handsome illustrations. By W. A. Newman Borland, M. D., Editor of " The American Pocket Medical Dictionary," Large octavo, over 800 pages, bound in full flexible leather. Price, $^^0 net ; with thumb index, J5.00 net.

WITH aooo NEW TERMS

The immediate success of this work is due to the special features that distinguish it from other books of its kind. It gives a maximum of matter in a minimum space and at the lowest possible cost. Though it is practically unabridged^ yet by the use of thin bible paper and flexible morocco binding it is only i % inches thick. In this new edition the book has been thoroughly revised, and upward of two thousand new terms have been added.

Howard A. Kelly, M« D.» Professor of Gynecology, Johns Hopkins University, Baltimore.

"Dr. Dorland's dictionary is admirable. It is so well gotten up and of such convenient size. No errors have been found in my use of it."


Goepp's State Board Questions

state Board Questions and Answers. By R. Max Goepp, M. D.,

Professor of Clinical Medicine, Philadelphia Polyclinic. Octavo of

800 pages.

READY SOON

Every graduate who desires to practice medicine must pass a State Hoard Examination, and to aid him in successfully passing such an examination this work will be of inestimable value. Dr. Goepp has taken great pains to collect the many questions asked in the past by Boards of the various States, and has arranged and classified them under subjects in such a manner that the prospective applicant can acquire the knowledge on any branch with the least difficulty.


THE PRACTICE OF MEDICINE, ii


Hatcher and SoUmann's Materia Medica

A Text-Book of Materia Medica : including Laboratory Exercises in the Histologic and Chemic Examination of Drugs. By Robert A. Hatcher, Ph. G., M. D., of Cornell University Medical School, New York City ; and Torald Sollmann, M.D., of the Western Reserve University, Cleveland, Ohio. i2mo of 41 1 pages. Flex, leather, $2.00 net.

RECENTLY ISSUED— A NEW WORK

This work is a practical text-book, treating the subject by actual experimental demonstrations.


Journal of the American Medical

"The book is well written, the classifications are good, and the book is to be recommended as a practical guide in the laboratory study of materia medica."


Eichhorst's Practice

A Text-Book of the Practice of Medicine. By Dr. Hermann EiCHHORST, University of Zurich. Translated and edited by Augustus A. EsHNER, M. D., Professor of Clinical Medicine, Philadelphia Polyclinic. Two octavos of 600 pages each, with over 150 illustrations. Per set : Cloth, g6.cx) net ; Sheep or Half Morocco, $7.50 net

Bulletin of Johnt Hopkint Hospital

    • This book is an excellent one of its kind. Its completeness, yet brevity, the clinical

methods, the excellent paragraphs on treatment and watering-places, will make it very desirable."

Bridge on Tuberculosis

Tuberculosis. By Norman Bridge, A. M., M. D., Emeritus Professor of Medicine in Rush Medical College, in aflfiliation with the University of Chicago. i2mo of 302 pages, illustrated. Cloth.

Ji.50 net.

Medical News, New York

" Thoroughly representative of our practical methods of diagnosis and treatment of the disease."


12 SAUNDERS' BOOKS ON


Thornton's Dose-Book

Dose-Book and Manual of Prescription-Writinsr. By E. Q. Thornton, M. D., Assistant Professor of Materia Medica, Jefferson Medical College, Phila. Post-octavo, 392 pages, illustrated. Flexible Leather, $2.QO net.

Recently Issued— New (3d) Edition

Dr. Thornton, in making this revision, has brought his book in accord with the new (1905) Pharmacopeia. Throughout the entire work numerous references have been introduced to the newer curative sera, organic extracts, synthetic compounds, and vegetable drugs. To the Appendix, chapters upon Synonyms and Poisons and their antidotes have been added, thus increasing its value as a book of reference.

C. H. Kfiller. M. D.,

Professor of Pharmacology, Northwestern University Medical School, Chicago,

" I will be able to make considerable use of that part of its contents relating to the correct terminology as used in prescription-writing, and it will afford me much pleasure to recommend the book to my classes, who often fail to find this information in their other text-books."


Lusk on Nutrition

Elements of the Science of Nutrition. By Graham Llsk, Ph.D., Professor of Physiology in the University and Bellevuc Hospital Medical College. Octavo of 325 pages. Cloth, $2.50 net.

JUST READY

This practical work deals with the subject of nutrition from a scientific standpoint, and will be useful to the dietitian as well as the clinical physician. There are special chapters on the metabolism of diabetes and fever, and on purin metabolism.

Lewellyt F. Barlcer. M.D..

Professor of the Principles and Practice of Medicine^ Johns Hopkins University. " I shall recommend it highly. It is a comfort to have such ;i discussion of the subject."


Mathews' How to Succeed in Practice

How to Succeed in the Practice of Medicine. By Joskpii M. Mathews, M.D., LL.D., President Amerir^*^ ^edic»il Association, 1898-99. i2mo of 215 pages, illustrated. "CO net.


THE PRACTICE OF MEDICINE, 13


Gould and Pyle's Curiosities of Medicine


Anomalies and Curiosities of Medicine. By George M. Gould, M. D., and Walter L. Pyle, M. D. An encyclopedic collection of rare and extraordinary cases and of the most striking instances of abnormality in all branches of Medicine and Surgery, derived from an exhaustive research of medical literature from its origin to the present day, abstracted, classified, annotated, and indexed. Handsome octavo volume of 968 pages, 295 engra^rfngs, and 12 full-page plates.


Popular Editioa : Cloth, I3.OO net ; Sheep or Half Morocco, I4*00 net

As a complete and authoritative Book of Reference this work will be of value not only to members of the medical profession, but to all persons interested in general scientific, sociologic, and medicolegal topics ; in fact, the absence of any complete work upon the subject makes this volume one of the most important literary innovations of the day. •

The Lancet, London

"The book is a monument of untiring energy, keen discrimination, and erudition. . . . We heartily recommend it to the profession."

Saunders' Pocket Formulary

Recently Issued— New («th) Edition— Adapted to the New (ljK)5) Phannacopeia

Saunders' Pocket Medical Formulary. By William M. Powell, M. D., author of "Essentials of Diseases of Children"; Member of Philadelphia Pathological Society. Containing 1 83 1 formulas from the best-known authorities. With an Appendix containing Posological Table, Formulas and Doses for Hypodermic Medication, Poisons and their Antidotes, Diameters of the Female Pelvis and Fetal Head, Obstetrical Table, Diet-list, Materials and Drugs used in Antiseptic Surgery, Treatment of Asphyxia from Drowning, Surgical Remembrancer, Tables of Incompatibles, Eruptive Fevers, etc., etc. In flexible morocco, with side index, wallet, and flap. $i-7S riet

Johns Hopkins Hospital Bulletin

" Arran.£jed in such a way as to make consultation of it as easy as possible. It is remarkable how much information the author has succeeded in gettmg into so small a book."


TfD'SMS' Hd'Ol^S ON


SoUmann's Pharmacolo^

Including Therapeutics, Materia MedicB: Pharmacy, Prescription -writing. Toxicology, etc.


A Text-Book of Pharmacology. By Torald Sollmann, M.D.

Proretisor of Pharmacol. icry and Materia Medica, Medical Deiiartment of Western Reserve University, Clevjiand, Ohio, Handsome octavo vohjme of 1070 pages, fully illustrated. Cloth. S4.00 net.

RECENTLY ISSUCD—NEW [2d| EDITION

Because of the radical alterations which have been made in the new (1905) Pharmacupeia, it was found necessary to reset this book cnlirelv. The aulbi bases Ihe study of therapeutics on a systematic knowledge of Ihc nature and properties of drugs, and thus brings out forcibly the intimate relation betivee pharmacology and practical medicine.

J. r. Foflwrintflum, M. D.

/Vfl/. n/ Tkir.ifti,tici ,iaJ Tkrary airJ Praaic^o/ Prtrcriiiig Trinilt Med. CnUige. Tanid. " TJie work teriaiiiiy r>couplES groimd not covered in so conciu. luctul. and scientific mannor hv :iny oihi^i I<:>1 [ tiuvc re.id on Ihe ^ubjccla embm^ed."

Butler's Materia Medica

Therapeutics, and PharinacoIo{^


A Text-Book of Materia Medica, Therapeutics, and Pharmacologyk

By Georgf. F. Buti.er, Ph. G., M. D., Associate Profes.sor of Therapeutics. College of Physicians and Surgeons, Chicago. Revised bjr Smith Elv Jelliffe, M. D., Profes-ior of Pharmacognosy, Columb^ University. Octavo of 694 pages, iilustratcd. Cloth, S4.00 net; Half Morocco, 1S5.00 net.

RECENTLY ISSUED~NEW 15th EDITION Adapted to the New ( 1905) Pharmacopaa For this fifth edition Ur. Butlers textbook has been entirely remodeled, written, and re.sel. All obsolete matter has been eliminated, ami special at. lion has been given to the toxicologic and therapeutic effects of the newer c«  pniinds, A classification has been adopted which groups tOKelher those di the predominant action of which is on one system of organs.

M«dicnl Record. New York

■ Niillimg hiis littn omuled by the author whicli, J" hi. ■ijdgmtnl. «i pletenesa of lb« lexl, anrf Ihp itmlenl or general rej Ibe l,en.

bearini; upon Ibe value of ilruRs and remedi« lonr



PRACTICE, MATERIA MEDIC A, Etc. 15

The American Pocket Medical Dictionary. 4ihCd. Recently issued

The American Pocket Medical Dictionary. Edited by W. A. Newman Dorland, M. D., Assistant Obstetrician to the Hospital of the University of Pennsylvania. Containing the pronunciation and definition of the principal words used in medicine and kindred sciences, with 64 extensive tables. Flexible leather, with gold edges, ^l.oo net ; with thumb index, j$i.25 net.

"I can recommend it to our students without reserve."— J. H. Holland. M. D., of tkt Jefferson Medical College, PhilaeUlphia.

Vierordt's Medical Diagnosis. Fourth Editioii, Revised

Mkdical Diagnosis. By Dr Oswald Vierordt, Professor of Medicine, University of Heidelberg. Translated from the fifth enlarged German edition by Francis H. Stuart, A. M., M. D. Octavo, 603 pages, 104 wood cuts. Cloth, $4.00 net; Sheep or Half Morocco, %^.oo net.

    • Has been recoKnized as a practical work of the highest value. It may be considered indispensable

both to students and practitioners."— F. Minot, M. D., late Professor o/ Theory and Practice in Harvard University.

Cohen and Eshner's Diagnosis. Second Revised Editioii

EssENTiAi^ of Diagnosis. By S. Solis-Cohen, M. D., Senior Assistant Professor in Clinical M^icine, Jefferson Medical College, Phila. ; and A. A. Eshner, M. D., Professor of Clinical Medicine, Philadelphia Polyclinic. Post-octavo, 382 pages; 55 illustrations. Cloth, $1.00 net. /// Saund^rs^ Question -ComJ^^nd Series.

"Concise in the treatment of subject, terse in expression of fact." — American Journal of the Medical Sciences.

Recently Issued

Morris' Materia Medica and Therapeutics. New (7th) Edition

ESSENTIAI^S OF MATERIA MeDICA, ThERAPEITICS, AND pRKSCKinioN-WRn ING.

By Henry Morris, M. D., late Demonstrator of Therapeutics, Jefferson Medical College, Phila. Revised by AV. A. Bastedo, M. D., Instructor in Materia Medica and Pharmacology iit Columbia Univer.sity. 1 2mo, 300 pages. Cloth, ^ 1. 00 net. In Saunders* Question- Compcnd Series.

    • Cannot fail to impress the mind and instinct in a lasting manner." — Buffalo Medical Journal.

Williams' Practice of Medicine Recently issued

• Essentials of tiik Practick ok MiniriNi. By W. R. Williams, M.D.. formerly Instructor in Medicine and Lecturer on Hygiene, Cornell Universiiv ; and Tutor in Therapeutics, ('<.lumbia University, X. V. l2mo of 450 pnge«-, illustrated. In Satmdrrs* QnestioH-ConiptnJ Series. Double nuniler, $1.75 net.

Stoney's Materia Medica for Nurses N^S^ i£n

Materia Medica ior Nirsivs. By Emily M. A. Stoney. Supvrintmdent of the Training School for Nurses ;it the Cnrney Hr>spital. South Boston, Mass. Handsome i2mo vohime of 300 pai^es. Cloth. 51.50 net.

"It contains about everything that a nurse ouKht to know in reg^ard to Atvl%^."— ^Journal of the American Medical Association.

Grafstrom's Mechano-therapy Seco^d*E&n!^ia,««l

A Text- Book of Mf( hano-therapy (Massai^e and .Medical GymnasticsL Bv Axel V. Grafstrom. B. Sc, M. D.. .\ttendini,' Physician to Augustus Adolphus Orphanage, Jamestown. N. Y. i2mo, 200 piijes, illustrattMl, 51-25 net.

"Certainly fulfills its mission in rcnderinR^ comprehensible the subjects of massage and medical gymnastics." — Xew Vork Medical Journal.


i6 SAUNDERS' BOOKS ON PRACTICE, Etc.

Jakob and Eshner's Internal Medicine and Diagnosis

Atlas and Epitome op Internal Medicine and Clinical Diagnosis. By Dr. Chr. Jakub, of Erlangen. Edited, with additions, by A. A. Eshner, M. D., Professor of Qinical Medicine, Philadelphia Polyclinic. With 182 colored figures on 68 plates, 64 text- illustrations, 259 pages of text. Cloth, j^j.oo net. In Sounder^ Hand- Atlas Series.

    • Can be recommended unhesitatingly to the practicing physician no less than to the student." —

Bulletin 0/ J^knx Hopkins Hospital.

Lockwood's Practice of Medicine. Re^^ia^ed

A Manual of the Practice of Medicine. By Geo. Roe Lockwood, M. D., Attending Physician to the Bellevue Hospital, New York City. Octavo, 847 pages, with 79 illustrations in the text and 22 full-page plates. Cloth, ^4.00 net.

A work of positive merit, and one which we gladly welcome."— AV«v York Medicml Joumai.

Keating's Life Insurance

How TO Examine for Life Insurance. By the late John M. Keating, M. D., Ex-President of the Association of Life Insurance Medical Directors. Royal octaro, 211 pages. With numerous illustrations. Cloth, j$2.oo net. *

" This is by far the most useful book which has yet appeared on insurance examiaation." — Medicml New*.

Corvrin's Physical Diagnosis. Thiid Cditioii. Revised

Essentials of Physical Diagnosis of the Thorax. By A. M. Corwin, A. M., M. D., Professor of Physical Diagnosis, College of Physicians and Surgeons, Chicago. 220 pages, illustrated. Cloth, flexible covers, j$i.25 net.

" A most excellent little work. It arranges orderly and in sequence the various objective pheaomena to logical solution of a careful diagnosis."— y<7vr»a/ 0/ Nervous and Mental Diseases.

Barton and Wells* Medical Thesatnrus

A Thi<:saurus of Medical Words and Phrases. Hy W. M. Barton, M. D., and W. A. Weli-S, M. D., of Georgetown University, Washinjjion, D. C. l2mo of 535 pages. Flexible leather, $2.50 net ; thumb indexed, $3.00 net.

Jelliffe's Pharmacognosy Recently issued

An Introduction to Pharmacognosy. By Smith Ely Jelliffe, Ph. D,, M. D., of Columbia University. Octavo, illustrated. Clolh, I2.50 net.

Stevens' Practice of Medicine. New (7th) Edition— Recently issued

A Manual of thk Practice or Medicinf. By A. A. Stevens. A. M., M. D.,

Professor of Pathology, Woman's Medical College, Phila. Specially intended for

students preparinp for j^raduaiion and hospital examinations. Post-octavo, 556 pages; illustrated. Flexible leather, $2.50 net.

"An excellent condensation of the essentials of medical practice f«ir the .student, and may be found also an excellent reminder for the bu»y physician." — Buffalo Mt'dical Journal.

Paul's Materia Medica for Nurses just Ready

Materia Mkhica for Nursf:s. By Georc.e P. Paui, M.D., Assistant Visiting Physician and Adjunct Radiographer to the Samaritan Hospital, Troy, N. Y. i2mo of 240 pages. Cloth, ^1.50 net.

In Dr. Paul's new work the physiologic actions of the drui^s an: arranged according to the action of the drug and not the organ acted upon. Another important section is that on pretoxic signs, giving the warnings of the full action or the bejjinninij toxic effects of the drug. If these signs be known many cases of drug poisoning may be prevented.

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