Book - A text-book of histology arranged upon an embryological basis (1913) 1-3

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A personal message from Dr Mark Hill (May 2020)  
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I have decided to take early retirement in September 2020. During the many years online I have received wonderful feedback from many readers, researchers and students interested in human embryology. I especially thank my research collaborators and contributors to the site. The good news is Embryology will remain online and I will continue my association with UNSW Australia. I look forward to updating and including the many exciting new discoveries in Embryology!

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

   Histology with Embryological Basis (1913):   Part I. 1.1. Cytology | 1.2. General Histology | 1.3. Special Histology
Part II. 2.1. The Preparation of Microscopical Specimens | 2.2. The Examination of Microscopical Specimens
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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Part I. Microscopic Anatomy

III. Special Histology

Blood Forming And Blood Destroying Organs

Bone Marrow

Bone marrow is the soft tissue found within the central cavities of bones. Its source in the embryo is the vascular mesenchyma invading a cartilage which is being replaced by bone. Early in its development it contains osteoblasts and osteoclasts, and these cells may be found in adult marrow, where it is in contact with the bone. The greater part of the mesenchyma becomes reticular tissue with fat cells intermingled. The meshes of the reticular tissue are occupied by an extraordinary variety of cells, most of which are called myelocytes (marrow cells). In ordinary sections the tissue of the marrow appears to be riddled with large round holes. Under high magnification the holes are seen to be fat cells, the nuclei of which are here and there included in the section (Fig. 190.) The reticular tissue framework of the marrow consists of flattened cells, generally seen cut across; their nuclei then appear slender and elongated. The abundant meshwork of fibrils associated with these cells is not apparent in

FIG. 190. HUMAN BONE MARROW. ordinary sections. In the meshes are

e., Eosinophilic myelocyte; e-b., erythro- .

wast; e-c., erythrocyte; f. c., part of found giant cells; premyelocytes; myelocytes

the protoplasmic rim of a fat cell; g. c., J J J


r.. reticular tissueceii.

Wm ' Ch arC UtrOphtiic, bdSOpktiic Or eOSlHO phUi C ; erythrocytes ; lymphocytes; and mature corpuscles both red and white.

The giant cells of the marrow have a single polymorphous nucleus. They have therefore been named "megakaryocytes," in distinction from the multinucleate osteoclasts or "polykaryocytes." The nucleus is so large that it may be cut into several slices, and by combining these it has been found that the entire nucleus is a hollow sphere with perforated walls; the nuclei, however, are very irregular, and some may be of other forms. With Wright's stain the protoplasm clearly shows an outer hyaline exoplasm and an inner granular endoplasm. It has been said that the latter is divisible into two concentric zones, which differ from the protoplasm within the nuclear sphere. In ordinary preparations these details are not evident (Fig. 191). A large number of centrosome granules (over one hundred) have been found, and pluripolar mitoses have been observed. A phagocytic function has been ascribed to these giant cells, but it has also been denied. Their origin is unknown, but is said to be from the leucocyte series of cells. Their important function of producing blood plates has already been described (p. 200).

Premyelocytes are cells with large round vesicular nuclei containing one or two coarse chromatin masses, and surrounded by basic protoplasm free from specific granules (Figs. 190 and 191). These cells are parents of myelocytes, and are sometimes called "myeloblasts" a poor term, since they do not produce marrow. Stohr refers to those in Fig. 191 as "plasma cells"; others describe them as primitive wandering cells. Apparently they are set free from the reticular tissue and they may produce not only myelocytes but also erythroblasts.


A, From the femur at 10 years; B, from a cervical vertebra at 19 years; C, from the femur at 77 years;

D, from a rib at 59 years.

Myelocytes are cells larger than polymorphonuclear leucocytes, having round or crescentic nuclei and protoplasm containing a varying quantity of specific granules, either neutrophilic, basophilic, or eosinophilic. The young cells have round nuclei and few granules. The oldest become the granular leucocytes ready to enter the blood vessels. Several generations, derived by mitosis, intervene between the young myelocytes and the mature leucocytes. Most of the myelocytes are finely granular and neutrophilic. Some are coarsely granular and eosinophilic; others contain the basophilic mast cell granules, but these are not well preserved in ordinary specimens. In certain diseases myelocytes enter the circulating blood, and they appear in smears as shown in Fig. 187, p. 198.

Erythroblasts are generally found in clusters, some being young with vesicular nuclei, others being normoblasts with dense irregular nuclei, such as have already been described. Rarely a nucleus may be found which apparently is partly extruded. Cup-shaped corpuscles are seen in the tissue meshes.

Lymphocytes are not a conspicuous element of the marrow, yet they are present and sometimes in disease become abundant.

The relations of the blood vessels to the reticular tissue are of great interest. It has been thought that the endothelium blends with the reticulum so that no sharp distinction can be made between the two. It seems more probable that the endothelium is merely more permeable than usual, by a freer separation of its cells. The same problem is presented by the blood vessels and reticular tissue of the lymph glands and spleen.

The functions of the marrow are the production and dissolution of bone, the storing of fat, the formation of granular leucocytes (neutrophiles, eosinophiles, and mast cells), of red corpuscles, and to a less extent of lymphocytes; to these some would add the destruction of red corpuscles, as indicated by ingested fragments and intercellular granules.


The lymph glands arise as nodules of dense tissue in close relation with an artery, a vein and a lymphatic vessel, as seen in the photographs, Figs. 192 and 193. The first distinct lymph glands in the body are a pair in the axillary region, a pair in the iliac region, and a pair or two in the maxillary region. They are found in rabbit embryos of about 30 mm., and in human embryos of about 40 mm. These first glands are soon followed by others in their vicinity, producing axillary, inguinal and cervical groups, respectively; and scattered glands more peripherally situated along the vessels develop later. At the same time, the tissue around the jugular and mesenteric lymph sacs becomes transformed into dense lymphoid tissue, which is resolved into the chains of deep lymphatic glands. These acquire a structure similar to that of the superficial glands. There is no satisfactory evidence that the dense lymphoid tissue of which the glands are composed is produced by the emigration of cells from either the arteries, veins or lymphatics associated with them.

In further development the lymph glands become organized as shown in the diagrams, Figs. 194 and 195. The left half of each diagram represents a younger stage than the right half. These instructive figures were prepared by Stohr on the basis of Kling's studies (Arch. f. mikr. Anat., 1904, vol. 63, pp. 575-610). In the youngest stage (Fig. 194) it is seen that the blood vessels enter and leave the gland on one side, at a place called the hilus (Lat. hilum, a small thing, applied to the eye of a bean, and to similar hollows in bean-shaped organs). The lymphatic vessel, as a plexiform peripheral sinus, encircles the entire structure. After the gland has enlarged, lymphatic vessels extend into the mass of lymphoid tissue, as shown on the right of Fig. 194, and eventually they pass clear through it in a system of anastomosing sinuses. The lymph then flows into the gland from the periphery, and out at the hilus; both the afferent and efferent vessels are shown in Fig. 195. Finally a connective tissue capsule develops around the larger glands, and in some of them it extends into the interior, producing a system of supporting trabecula, either round or lamellar. These may unite with one another as shown on the right of Fig. 195. When present within the gland they are always found in the central axes of the lymph sinuses.


a, Artery; g, lymph gland; I, lymphatic vessel; v, vein.

By the production of the internal lymph sinuses, the substance of the gland is subdivided into rounded nodules and elongated cords of lymphoid tissue. The nodules are found at the periphery of the gland and collectively they form its cortex; the cords constitute the medulla. Several other organs, e.g., the kidney and suprarenal glands, are divided into an outer cortex (bark) and an inner medulla (pith). In the center of each cortical nodule there is often a light spot, seen with low power, which constitutes the germinal center. These general features of a lymph gland are shown in Fig. 196. It is evident that certain of the secondary



nodules in the cortex are imperfectly separated from one another, and that they are continuous below with the anastomosing medullary cords.

The lymph glands of the adult (lymp ho glandules, also called lymph nodes) are round or reniform structures varying in length from a few millimeters

Afferent lymphatic vessels.

Peripheral lymph sinus.


Lymphoid tissue

Lymph sinus.

/ Blood vessels.

Lymphatic vessel. Lymphatic vessel.

FIG. 194.

Afferent lymphatic vessels.

Lymph sinus.




to a few centimeters. The largest of them show trabeculae and are subdivided into cortex and medulla as above described; the small ones remain permanently in the various developmental stages shown in Figs. 194 and 195. The smallest structures consist of but a single nodule, with or without a germinal center; it contains a simple capillary network in its interior,



and a lymphatic plexus over its surface. Such solitary nodules occur in the mucous membranes of various organs. By contact with one another laterally they constitute the noduli aggregati, or "Peyer's patches" of the small intestine, which are macroscopic structures 1-5 cm. long. Lymphoid nodules irregularly massed about epithelial pits become the essential tissue of the tonsils. Wherever it occurs, lymphoid tissue has essentially the same structure as that observed in the lymph glands.




Lymphoid tissue (formerly called adenoid tissue) consists of a framework of reticular tissue (see Fig. 50, p. 61, and the accompanying description), together with detached cells, chiefly lymphocytes, which fill its meshes. Eosinophiles and the various forms of blood corpuscles brought in by the blood vessels, are present in small numbers. The lymphocytes are like those of the blood, and the lymph glands are centers for their production. Stained with haematoxylin, lympboid tissue, because



Lymph sinuses.

of the preponderance of nuclear material, is very dark, and its appearance even under low magnification is quite characteristic; it is shown in the medullary cords in Fig. 197, which illustrates also its relation to the lymph sinuses.

The lymph sinuses are not well-defined endothelial tubes, but appear rather as washed-out portions of the reticular tissue. If the endothelial tubes which line the lymphatic vessels enter the lymph gland to form the sinuses, it must be considered that their cells separate and that strands of reticular tissue pass across them. Some authorities consider that the endothelial tissue blends freely with the reticular tissue, so that any distinction is here arbitrary. The reticular tissue cells, or endothelial cells, lining the sinuses are highly phagocytic, and ingested fragments may be seen within them in sections. Certain of these cells become detached, and there is reason to believe that they are the source of the large mononuclear leucocytes. Lymphocytes from the adjacent cords and nodules also enter the lymph as it passes through the sinuses, and thus they are added to the circulation. Within the cords and nodules they are enclosed in a closer meshed reticulum than that of the sinuses, which may prevent them from escaping too freely. The germinal centers contain cells with larger and paler nuclei than those of lymphocytes. These central cells resemble premyelocytes, and they are supposed to give rise to lymphocytes. Mitotic figures are abundant. The germinal centers, however, are not found in certain nodules, and they are absent from the medullary cords. This has been explained as due to the slower and more scattered multiplication of cells in those places, but the germinal centers are absent also from the early stages of embryonic glands. Presumably they are not adequately explained by stating that they are centers for lymphocyte production.

The capsules of the lymph glands consist of fibrous connective tissue, containing elastic elements which increase in abundance with age. Smooth muscle fibers are present as scattered cells or as slender bundles. The trabeculae, which are extensions of the capsule, are composed of the



same tissues. They are completely surrounded by the lymph sinuses as shown in Fig. 197. The flat cells over their surfaces may be regarded as endothelial cells.

The blood vessels of a lymph gland enter chiefly at the hilus, but in the larger glands some of them come in from the periphery and run in the trabeculae; others however pass out through the trabeculae into the capsule. The principal artery enters at the hilus and divides at once into several branches, which travel in the trabeculae for a short distance, and then pass over into the medullary cords. They extend through the axes of the cords into the 'nodules, giving off small branches which form a venous network at the periphery of these structures. The veins which drain this network soon cross the sinuses and enter the trabeculae, in which they travel toward the hilus alongside the arteries (Calvert, Anat. Anz., 1897, vol. 13, pp. 174-180). A central artery surrounded by lymphoid tissue and drained by peripheral veins is found not only in lymph glands, but also in the spleen.

Nerves to the lymph glands are not abundant. They consist of medullated and non-medullated fibers, which form plexuses about the blood vessels, and supply the muscle cells in the capsule and trabeculae. They have not been found in the nodules and cords.

The function of the lymph glands is not only to produce lymphocytes which enter the lymphatic vessels and are conveyed through the thoracic duct into the blood, but also to "filter the lymph." If certain poisonous substances, inert particles, or bacteria are brought to the gland in the lymph, they may be removed by the phagocytic endothelial or reticular tissue cells. The gland at the same time may become enlarged by congestion, and by multiplication of its cells.


Haemolymph glands resemble small lymph glands, ranging in size from a "pin-head to an almond." They occur especially in the retroperitoneal tissue near the origin of the superior mesenteric and renal arteries, but are found elsewhere, and it has been said that their distribution coincides with that of ordinary lymph glands. They are darker than the lymph glands, and on section yield blood in place of lymph. No lymphatic vessels are associated with them, when typically developed, and instead of a lymph sinus they possess a similar structure filled with blood, the blood sinus. The lymphoid tissue with its blood supply, together with the capsule and trabeculae, are like the corresponding structures in lymph glands. The capillary blood vessels, however, are readily permeable, so that their contents, both plasma and corpuscles, escape into the blood sinus. The haemolymph gland is therefore a "blood filter." Many



blood corpuscles fragment in passing through it, and are removed from the circulation by phagocytic cells, which in consequence become pigmented. The eosinophilic cells which are found in haemolymph glands have been explained as due to the ingestion of haemoglobin products, but it has been questioned whether these cells are more abundant than in ordinary lymph glands. A second function of the haemolymph glands, depending upon the lymphoid tissue around their arteries, is the production of lymphocytes which may enter the blood vessels directly.

According to von Schumacher (Arch. f. mikr. Anat., 1912, vol. 81, pp. 92-150) the haemolymph glands begin their development like ordinary lymph glands, but after the formation of the peripheral sinus, the connections with afferent and efferent lymphatic vessels are lost. He finds various intermediate forms between the lymph and haemolymph glands, depending upon the extent of atrophy of the lymphatic connections, and the extent to which blood escapes from the intraglandular vessels. After accidents accompanied by extravasations of blood, the sinuses of ordinary lymph glands may become filled with red corpuscles, conveyed to them by the afferent lymphatic vessels. Such glands differ obviously from the true haemolymph glands, which structurally and functionally are intermediate between lymph glands and the spleen.


The spleen, being five or six inches long and four inches wide, is much the largest organ of the lymph gland series. It is the first of them to develop, appearing in rabbits of 14 days (10 mm.) as a condensation of the mesenchyma in the dorsal mesentery of the stomach. At this stage the


FIG. 198. DIAGRAM OF A HAMOLYMPH GLAND. A; AND OF A PART OF THE SPLEEN, B. The arteries are shown as slender lines (art.) and the veins as heavy ones (v.); c., capsule; b. s., blood sinus, corresponding with the splenic pulp, p.; s . n., secondary nodule; sp. n., splenic nodule; tr., trabecula.

only lymphatic vessels in the embryo are those near the jugular vein. Lymph glands are not indicated until six days later. The blood vessels enter the spleen at its hilus and branch freely. In early stages they form an ordinary capillary plexus, but subsequently their walls become so pervious that most of the blood escapes into the reticular tissue in passing



from the artery to the vein. Surrounding the arterial branches there is a zone of lymphoid tissue, which arises rather late in embryonic life. In reptilian spleens it is so abundantly developed that the organs resemble mammalian haemolymph glands. In the guinea-pig the lymphoid sheath of the arteries is continuous, though narrow; in man it is so interrupted as to form a succession of spindle-shaped or spherical masses, called splenic nodules (Malpighian corpuscles). An arterial branch passes through each nodule. Thus, as compared with the haemolymph

Terminal vein

[Sheathed artery. Pulpartery.

Pulp vein.

Beginning of a

trabecular vein.

Capillaries of a nodule.



, , Splenic V 1 /obule.

Hilus. Reticulum. Splenic noudle.



At x is shown the direct connection of terminal arteries with terminal veins (the existence of such a connection has been questioned). At xx and xxx are the free endings of the terminal veins in the pulp and near the nodules respectively.

gland, the spleen is deficient in lymphoid tissue (Fig. 198). The bulk of the spleen is composed of splenic pulp, which corresponds with the blood sinus of the haemolymph glands. Its framework of reticular tissue is continuous with that of the nodules, and it contains blood corpuscles of all sorts, special phagocytic cells known as splenic cells, and the terminal branches of both arteries and veins. There are no lymphatic vessels within the spleen. The capsule and trabecular framework are highly developed as in the largest lymph glands. The following features of the spleen may be described in turn the blood vessels, the pulp, the nodules, the capsule and trabeculae, and finally the nerves.

As shown in the diagram, Fig. 199, the splenic artery enters at the hilus and, accompanied by veins, its branches are found in the largest trabeculae. When about 0.2 mm. in diameter the arteries leave the trabec


ulae, in which the veins continue further. The arteries, however, are still surrounded by a considerable connective tissue layer, the outer portion of which becomes reticular and is filled with the lymphocytes of the nodules. The nodules occur near where the artery branches. Small arterial twigs ramify in the nodules, in the periphery of which they anastomose before passing into the pulp. When the main stems are about 15 n in diameter, they lose their surrounding lymphoid layer and pass into the pulp, where they form brush-like groups of branches (penicilli). These branches do not anastomose. For a short distance before their termination the walls of the branches possess ellipsoid thickenings, due to a longitudinal arrangement of closely applied fibers of reticular tissue. These "sheathed

arteries" are 6-8 M in diameter, and have been supposed to regulate the amount of blood which enters the terminal portion of the artery, beyond them. Some authorities state that this distal part connects with the terminal veins, meeting them


TERMINAL VEINS FROM THE HUMAN SPLEEN a |- an acilt e angle. AcCOrd e., Rod shaped endothelial cells, with projecting nuclei, n ; I., .

encircling reticular tissue; L, leucocytes passing between jngr tO Others SUCh COnnCC the endothelial cells. (After Weidenreich.)

tions are infrequent, and still

others believe that the arteries empty only into the reticular tissue. Numerous careful injections have shown the readiness with which the arterial blood mingles with the pulp cells.

The terminal veins or splenic sinuses begin as dilated structures (sometimes unfortunately called "ampullae," the latter term being applied also to the terminal arteries). Their endothelial cells are so long and slender as to suggest smooth muscle fibers, and like certain other endothelial cells they are contractile. Their edges are not closely approximated, so that corpuscles may pass between them freely (Fig. 200) . Around them are encircling reticular tissue fibers, and a continuous basement membrane has been described as stretching across the intervals between the endothelial cells. The existence of such a membrane has recently been denied. The endothelial cells project into the lumen of the vessel, and their nuclei are at the summits of the elevations. Frequently the nuclei show one or two longitudinal rod-like markings, said to be due to folds in the nuclear membrane (Fig. 200, B) Several terminal veins unite to form a pulp vein, which enters a trabecula in which it passes toward the hilus. The trabecular veins join to form the splenic vein.

For further details regarding the circulation see Weidenreich (Arch. f. mikr. Anat., 1901, vol. 58, pp. 247-376) and Mall (Amer. Journ. Anat., 1903, vol. 2, pp. 315-332).


The splenic pulp consists of a reticular tissue framework (Fig. 50, p. 61). It supports the terminal arteries and veins, and in its meshes are the white and red corpuscles passing between them.

The pulp appears as a diffuse mass of cells infiltrated with red corpuscles, and since the vessels within it are thin-walled and hard to follow, likewise containing corpuscles, it is often impossible in ordinary sections to determine which cells are inside and which are outside of the vessels (Fig. 201). The nodules are not sharply separated from the pulp, so that lymphocytes are abundant in their vicinity. These lymphocytes enter the terminal veins and thus are removed from the spleen. In the splenic vein the proportion of lymphocytes to red corpuscles is said to be seventy times as great as in the splenic artery. One for every four red


Pulp.~| |f7 Trabeculae.

Spindle-shaped nodule.

Sheathed artery.

Central arteries in splenic nodules.

FIG. 201. PART OF A SECTION OF THE SPLEEN FROM AN ADULT MAN. X 15 corpuscles has been reported by two investigators, but later estimates are lower. It seems evident that lymphocyte production is an important function of the spleen. Another is the filtration of the blood passing through the pulp. As in haemolymph glands, granular debris is found, and there are phagocytic, pigmented, and eosinophilic cells. The phagocytes are cells with large round nuclei and considerable protoplasm. They vary in size, but the small forms are most numerous; these are called splenic cells. Some are described as multinucleate. Erythroblasts are not found in the normal adult human spleen; they occur, however, in certain blood diseases, and are normal in some adult mammals, as in the skunk. They are abundant in the spleens of human embryos. Giant cells are numerous in the spleens of young animals but are seldom found in the human adult.



They are described as megakaryocytes, and are like those in bone marrow. The formation of granular leucocytes, which has been asserted, presumably does not occur.

The splenic nodules are quite like the secondary nodules of lymph glands. They consist of a reticular tissue framework continuous with that of the pulp, but having coarser meshes. Fine elastic fibers are associated with it. It contains lymphocytes, and near the central arteries

._ Surface blackened by precipitate of silver.

Nerve branches

tor the arterial


~ -- Nerves of the pulp.


Small nerve . . bundle.

Branches for the , , arterial wall..'-' ' Fie. 202. GOLGI PREPARATION OF THE!SPLEEN OF A MOUSE. X 85. The boundary between the splenic pulp and the lymphoid tissue is indicated by a dotted line. The nerves are chiefly in the wall of an artery.

germinal centers are sometimes distinct. The nodules have been regarded as varying in shape from time to time, being but transient accumulations of lymphocytes.

The capsule of the spleen is divided into two layers. The outer is the tunica serosa and the inner, the tunica albuginea. The serosa consists of the peritoneal mesothelium, which covers the spleen except at the hilus, and of the underlying connective tissue. The albuginea is a dense layer of connective tissue, containing elastic networks and smooth muscle fibers. Similar tissue is found in the trabeculae. The muscle


elements are less numerous in the human spleen than in those of many animals. By contraction they force blood from the pulp and cause the circulation to follow more definite channels. When they are paralyzed, the pulp becomes filled with the blood corpuscles.

The nerves of the spleen, from the right vagus and the cceliac sympathetic plexus, are medullated and non-medullated fibers, chiefly the latter. They form plexuses around the blood vessels (Fig. 202) and send fibers into the pulp. Besides supplying the muscles of the vessels and trabeculae, ( some of them are thought to have free sensory endings. Lymphatic! vessels are said to occur in the capsule and trabeculae, but not in the pulp or nodules of the spleen.

The spleen is a large organ, without obvious subdivisions. On its surface, when fresh, there is a mottled effect due to areas bounded more or less definitely by trabeculae. Such areas, about i mm. in diameter, have been described by Mall as ' 'lobules," and he states that they "can easily be seen on the surface of the organ or in sections." A lobule, as he describes it, has a central artery, and its base is where the lymphoid sheath of the artery terminates. It has peripheral veins, often three, enclosed in the trabeculae. A lobule is composed of some ten structural (or histological) units, imperfectly separated from one another by branches of the trabeculae. Each unit contains a central terminal artery (branches of the lobular artery) and has peripheral veins (branches of those about the lobule). Apparently, therefore, the lobules shown in the diagram, Fig. 199, except along its lower border, represent groups or pairs of Mall's lobules. Stohr notes that "a division into lobules in the interior of the spleen is impossible." The arrangement of lobules at the periphery suggests an ill-defined cortex. Lobes have also been described, corresponding with the main branches of the splenic artery, but the lobes are not generally recognized. The spleen may present inconstant subdivisions, which sometimes produce detached portions called accessory spleens.

The Entodermal Tract

Development of the Mouth and Pharynx

In a previous section the early development of the fore-gut or pharyngeal pocket of entoderm has been described and illustrated (Figs. 27 and 28). This fore-gut of the young embryo is to produce the pharynx, oesophagus, and stomach of the adult. Its anterior extremity encounters the ectoderm at the bottom of a depression. The ectoderm and entoderm there fuse to make the oral plate (Fig. 203) , which becomes thin, ruptures, and disappears. Just anterior to the plate, in the median line, the ectoderm sends a gland-like projection toward the brain. It branches and becomes detached from the oral ectoderm, lying in the sella turcica of the adult. It is known as the anterior lobe of the hypophysis, and will be described with the brain, from which the posterior lobe develops. The ectoderm in front of the oral plate forms also the epithelium of the lips and of the peripheral part of the mouth, including the enamel organs, as has already been described. The salivary glands are also considered



ectodermal, but before they develop the oral plate has disappeared and

the boundary between ectoderm and entoderm cannot be sharply drawn. The entoderm of the mouth and pharynx is a layer of epithelium lining

a broad, dorso-ventrally flattened cavity. From this cavity, a succession of paired outpocketings grow out laterally to meet the ectoderm on the side of the neck; these are the pharyngeal pouches. They reach the ectoderm at the bottom of furrows or clefts, corresponding in number with the pharyngeal pouches, and there the two germ layers fuse. The plates thus formed

FIG. 203. DIAGRAM SHOWING are comparable with the oral plate, and in fishes


ECTODERM AND ENTO- they rupture producing the branchial clefts (gill


MAMMALIAN EMBRYO. clef ts) a. 1., and p. 1., Anterior and

posterior lobes of the . , . , .

hypophysis; m. t., medui- Their arrangement in a young dog-fish is shown m Fig. o* r p., t oraipte; x. and^ 204. The mouth, m, leads into a cavity, the pharynx, which ?hefip^nd enam p e?o d u the opens freely on the outer surface of the fish through five && defts S' c - Xt also P ens to the surface through the spiracle, sp., a structure similar to the gill clefts, but anterior to them and having a more dorsal aperture. In respiration water is taken in through the mouth and spiracle, and passes out through the gill clefts; but sometimes water is ejected through the spiracle. In mammals the corresponding structure is counted as the first gill cleft.

In mammalian embryos there are four well-defined pharyngeal pouches on either side, which reach the ectoderm at the bottom of corresponding grooves; but if their closing plates ever rupture they are soon restored, and permanent openings from the pharynx on the side of the

m gc

FIG. 204. HEAD OF A YOUNG DOG-FISH. g. c., Gill cleft; m., mouth; n., nasal pit; sp., spiracle.



c. s., cervical sinus; g. c. 2., second branchial groove; h., hyoid arch; m., mouth; md., mandibular process; n., nasal pit; sp., auditory (spiracular) groove.

neck are not found. The first pouch, corresponding with the spiracle, connects with the auditory groove (Fig. 205, sp}. Around it the external ear develops, so that its position is always evident. The ectodermal depression which connects with the second pouch disappears, except in rare cases, where it forms a cervical fistula. This is a pit, or slender tube, in the skin of the neck, situated primarily between the hyoid bone and thyreoid cartilage. The third and fourth pouches connect with the




oesophagus; p. b., postbranchial body; t., thyreoid; th., thymus; tr., trachea; i> 2, 3, 4, the pharyngeal pouches.


ectoderm at the bottom of a single funnel-shaped depression known as the cervical sinus (Fig. 205, c.s.). This also wholly disappears normally, but it may remain as a cervical fistula low down on the neck, and its deeper parts may give rise to branchial cysts. Thus all the ectodermal branchial grooves except the first normally disappear before birth.

The pharyngeal pouches, or entodermal portions of the gill clefts, as they occur in a mammalian embryo are shown in Fig. 206. The pharynx opens to the exterior at the mouth, m, and divides posteriorly

into the trachea, tr, and CeSOphagUS, Oe.

In the median dorsal line it gives rise to the anterior lobe of the hypophysis, cut off at a. /., and in the median ventral line to the thyreoid gland, t. This gland is a median structure, entirely separate from the pharyngeal pouches. It grows down through the hind part of the tongue, acquiring a position in front of the trachea. Its branching terminal part becomes separated from its outlet by the obliteration of its duct (called the thyreoglossal duct). A blind pit, the foramen cacum, permanently retained at the back of the tongue, marks the former outlet of the duct (Fig. 207,7. c.}. Thus the thyreoid gland is a detached clump of entodermal tubules in front of the trachea.

The entodermal portions of the gill clefts are four paired lateral outpocketings. The first (Fig. 206, i) extends to the auditory groove in the ectoderm, and becomes the auditory tube (Eustachian tube). The pharyngeal orifice of this tube in the

FIG. 207. A MEDIAN SB TION THROUGH THE . . . , . _,. , , ^

PHARYNX OF AN ADULT. (After Corning.) adult IS SnOWTl in rig. 2C>7 (0. pn.)\

P leal archre^^iglottisTfJ'cf.'fo'rame'n csecmn the OUter end of the tube expands tO

i. s-t., supratonsillar fossa; o. ph., pharyngeal r . , . j_ r , i

orifice of the auditory tube; pal., soft palate; r. f Orm the tympanic Cavity Of the Car, ph., pharyngeal recess; s.t., sellaturcica (which i i i i

contains the hypophysis); t. 1., lingual tonsil; and Will be further Considered With tons., palatine tonsil; t. ph., pharyngeal tonsil.

the sense organs.

The second pharyngeal pouch (Fig. 206, 2) loses its connection with the ectoderm and becomes a relatively shallow depression on the side of the pharynx. At a certain stage it is in close relation with the orifice of the


auditory tube, and it has been thought to give rise to the pharyngeal recess (fossa of Rosenmiiller), but according to Hammar such is not the case. Instead, it produces only the sinus tonsillaris, into which a mound of lymphoid tissue, the palatine tonsil, later projects (Fig. 207, tons.}. Above the tonsil the supratonsillar fossa, which may readily be seen on looking into the mouth, is to be regarded as a remnant of the original second pouch (Hammar, Arch. f. mikr. Anat., 1903, vol. 61, pp. 404-458).

The lingual and pharyngeal tonsils, which are similar in structure to the palatine tonsils, develop as median structures with no relation to the pharyngeal pouches. Therefore the second pouches are to be regarded as the site rather than the source of the palatine tonsils; there are no tonsils in the second pouches of the rat (Hammar).

The third pouch (Fig. 206, 3) near its junction with the ectoderm, sends a tubular diverticulum (tti) down the neck behind the thyreoid gland; it continues into the thorax, lying ventral to the arch of the aorta (as seen in front view in Fig. 208). This diverticulum loses its lumen, becomes detached from the pharynx, and unites with its fellow on the opposite side to form the thymus. Besides this elongated structure, each third pouch produces _th an epithelial body, or nodulus thymicus, which is a ^ round clump of cells detached from the pouch at the

FIG. 208. upper end of the thymic diverticulum. Each epithe The reouu"t, f ^f ' 29 d m^." ^ body becomes attached to the posterior surface pa u r?h a y io e id br ^ikn p d of the thyreoid gland, forming the inferior pair of pou"h> e ; p. grparath 3 y d parathyreoid glands (Fig. 208, p.}.

reoid gland (derived ,, , . .., ., ,_,. - N

from the 4th pouch)-. The fourth pouch on either side (Fig. 206, 4) gives

p. L, pyramidal lobe of .

the thyreoid; ao. rise to an epithelial body similar to the nodulus thy aorta; v., vena cava x *

superior. (After Ver- micus. These likewise become detached as parathyreoid glands, and they constitute the superior pair (Fig. 208, p. #.). Sometimes a parathyreoid gland degenerates and disappears, and in other cases one of them may become subdivided, but typically there are four in the adult.

Behind the fourth pouch, on either side, there is a tubular prolongation of the pharynx variously known as the postbranchial, ultimobranchial or telobranchial body. As the fourth pouch becomes well formed, the postbranchial body is so closely associated with it that together they form a Y-shaped structure, attached to the pharynx by a common stalk (Fig. 206). The postbranchial bodies then grow toward one another across the front of the neck, after the manner of the thymic diverticula. Their ventral ends become detached and imbedded in the thyreoid gland, to the substance of which they were formerly believed to contribute. There is, however, no satisfactory evidence that they produce thyreoid tissue, and they are generally supposed to disintegrate.


The first recognition of the significance of the mammalian gill clefts is credited to Rathke, in 1832, who published the following significant conclusions in his "Untersuchungen iiber den Kiemenapparat der Wirbelthiere."

"In all vertebrates without exception, in the earliest period of development, there are formed the beginnings of a branchial apparatus. Its elements vary in number in the different vertebrates, yet in tissue, form, position and connections they are very similar to one another, and are built upon the same plan. Their development, however, proceeds along different lines in the various animals. In some it is partly regressive, bringing about the most manifold and divergent modifications of these structures, not merely in form but also in tissue, type, and significance. Yet there always remains an analogy between them; and through easy transitions, the forms and types pass into one another from the bony fishes even to man. The branchial apparatus is most highly developed in fishes; in the other vertebrates its development is the less complete, the further, in general, these vertebrates are removed from the fishes."

The mammalian gill clefts, although rudimentary as branchial organs, are of the utmost anatomical importance. A single large artery passes from the ventral aorta to the dorsal aorta between the successive pouches, and also in front of the first and behind the last. These aortic arches therefore number one more than the series of pouches; from them, portions of the aorta, carotid and subclavian arteries are produced, as described in works on embryology. The nerves send trunks down between the pouches, the facial nerve being between the first and second, the glossopharyngeus between the second and third, and the superior laryngeal branch of the vagus between the third and fourth. Thus these structures determine the arrangement of the vessels and nerves.

On the basis of comparative studies the presence of a fifth pouch in mammals was predicted, and the posterior arm of the Y-shaped outgrowth, including the postbranchial body, is often described as such. A branch of the superior laryngeal nerve is said to pass between the arms of the Y, but a typical branchial relation between the nerves and the fifth pouch has not as yet been established. A "fifth aortic arch" is often represented as passing between the fourth pouch and the postbranchial body, but it has been shown that this arch differs from all the others in its order of development (forming only after the "sixth" is complete). Whereas the third, fourth, and last aortic arches all produce very important vessels, the questionable "fifth arch" is an insignificant plexiform anastomosis, which disappears rapidly. Small vessels, however, are always to be found near the postbranchial body in rabbit, pig and human embryos measuring 5-10 mm. The most convincing evidence of the presence of a fifth pouch is an actual contact with the ectoderm, posterior to the fourth pouch; this was recorded by Hammar in a s-mm. embryo, but the contact on either side took place in only one 12 fi section. Grosser states that a closing membrane "is perhaps not always formed, and is at all events very transitory" (Human Embryology, ed. by Keibel and Mall, 1912, vol. 2). There are as yet very few observations to show that it ever occurs in mammalian embryos. The existence of a sixth pouch has been asserted on the basis of slight elevations which are perhaps inconstant.


The palatine tonsils are two rounded masses of lymphoid tissue, one on either side of the throat, between the arches of the palate (Fig. 207.)


Frequently they have been called amygdala (almonds), but the older Latin term for them is tonsilla (a stake to which boats are tied). They are covered by the mucous membrane or tunica mucosa, which throughout the digestive tract consists of several layers. The soft moist entodermal epithelium rests on a connective or reticular tissue layer, the tunica propria. A structureless basement membrane, the membrana propria, is often present immediately beneath the epithelium. The epithelium, membrana propria, and tunica propria together form the mucous membrane, which in dissection would be stripped off as a single structure. Beneath it, and sometimes not clearly separable from the tunica propria, is the submucous


a, Stratified epithelium; b, basement membrane; c, tunica propria; d, trabeculae; e, diffuse lymphoid tissue; f, nodules; h, capsule; i, mucous glands; k, striated muscle; 1, blood vessel; q, pits. (Prom Radasch.)

layer, or tela submucosa. It is a vascular connective tissue, by which the mucous membrane is attached to underlying muscles or bones. All the layers named are involved in the tonsils which, however, are essentially lymphoid accumulations in the tunica propria.

The epithelium of the palatine tonsils is a stratified epithelium of many layers, with flattened cells on its smooth free surface, and columnar cells beneath. Its attached surface is invaded by connective tissue elevations or papillae, so that it appears wavy in sections (Fig. 209). The stratified epithelium lines from ten to twenty almost macroscopic depressions, called tonsillar pits or fossula (crypts). These are irregularly tubular and sometimes branched. Many lymphocytes penetrate between the epithelial cells and escape from the free surface into the saliva, becoming "salivary corpuscles." In places the tonsillar epithelium is so full of lymphocytes as to appear disintegrated, a condition which was



first described by Stohr (Biol. Centrabl., 1882, vol. 2). It occurs also in the epithelium of the lingual tonsil as seen in Fig. 211. In the reticular


Fibrous sheath.

Germinal center. Epithelium and pit containing lymphocytes.


Emigrating lymphocytes. Fragments of epithelium.

Emigrated lymphocytes..

Stratified / >ithelium.


ra$ it



$ . .

> i.

    • f

. 1:' *

> *

Lymphoid tissue I


FIG. 211. FROM A THIN SECTION OF A LINGUAL TONSIL OF AN ADULT MAN. X 420. On the left the epithelium is free from lymphocytes, on the right many lymphocytes, are wandering through.

tissue of the tunica propria, especially around the pits, there are many lymph nodules, some of which are well defined, with germinative centers,


but many others are fused in indefinite masses. The lymphoid tissue constitutes the bulk of the tonsil.

The submucous layer forms a capsule for the organ, into which it sends trabecular prolongations. It contains many blood and lymphatic vessels, together with branches of the glossopharyngeal nerve and spheno-palatine ganglion which supply the tonsil. It contains also the secreting portions of small mucous glands, some of which empty into the pits, but most of their ducts terminate in the mucous membrane surrounding the tonsil. They resemble other mucous glands of the mouth which will be described presently. Beyond the submucosa is striated muscle, belonging to the arches of the palate and to the superior constrictor of the pharynx; striated muscle fibers are therefore readily included in sections of the tonsil.

The pharyngeal tonsil is an accumulation of lymphoid tissue on the median dorsal wall of the pharynx, between the openings of the auditory tubes (Fig. 207). In childhood it is liable to become irregularly enlarged so as to obstruct the inner nasal openings, thus forming the "adenoids" of clinicians. It is covered with stratified epithelium, which is ciliated in embryonic life; and in the adult, cilia may be found upon the epithelium within the pits. The pits and lymphoid tissue are quite like those of the palatine tonsils.

The lingual tonsil is an aggregation of pits surrounded by lymphoid tissue (Fig. 210). It is found in the back part of the tongue (Figs. 207 and 220), the surface of which is very different in texture from the front part, presenting low mounds with central depressions. Each depression is the outlet of a pit. Lymphocytes pass through the epithelium (Fig. 211) and become salivary corpuscles, which are said to produce substances protecting the tissue from bacterial invasion.


The thymus (Gr. 6vtw>, thymus) arises from the two tubular prolongations of the third pharyngeal pouches, which meet in the median line as shown in Fig. 208, and become bound together by their connective tissue coverings. The lumen is lost, and the cells proliferate. They form a broad, flat, bilobed mass with a tapering prolongation up either side of the neck. The bulk of the organ is in the thorax, beneath the upper part of the sternum. At birth it weighs generally between 5 and 15 grams (about half an ounce), and is relatively a large organ. Haller (1761) described it in older embryos as "a huge gland, scarcely smaller than the kidney; but in the adult it is diminished, and having become constricted, dried up and much harder, it is almost buried in the surrounding fat." Meckel found ordinarily no trace of it at twelve years, and according to



Hewson it gradually wastes until the child has reached between its tenth and twelfth year, when ordinarily it is perfectly effaced, leaving only ligamentous remains. These older observations have been generally accepted, and the persistence of the thymus in the adult is regarded as of

Thymic corpuscles.

J Connective tissue.

Transverse section of blood vessel.

~- Medullary cord.



Blood vessel.

Thymic corpuscle. >*,


considerable pathological importance. According to Waldeyer and Hammar, however, it persists for a much longer time. It increases in size and weight for some years after birth, probably until puberty, and then slowly atrophies. At fifteen years it is said to weigh 40-50 grams. It is considered an active organ even to the fortieth year, losing its functions with beginning old age (50-60 years). The duration of the thymus has apparently been underestimated. (See Hammar. Arch. f. Anat. u. Entw., 1906, Suppl.-Bd. pp., 91-182; Anat. Anz., 1905, vol. 27, pp. 23-89; and for development, Anat. Hefte, Abth. i, 1911, vol. 43, pp. 203-242).

The thymus is subdivided by connective tissue

layers into lobes from 4 tO FlG - 213. PART OF A SECTION OF THE THYMUS FROM A HUMAN

EMBRYO OF FIVE MONTHS. X 50. ii mm. in diameter, and

these are similarly subdivided into lobules of about i cu. mm. each. All the lobules in the right and left halves of the thymus, respectively, are attached to a cord of medullary substance, 1-3 mm. in diameter, as may be seen if the gland is pulled apart. This axial structure suggests the

Tangential sections of lobules.



original diver ticulum. Each lobule consists of a pale medulla, extending from the cord, and a darker peripheral cortex (Figs. 212 and 213). The entire structure somewhat resembles a lymph gland, from which, however, germinal centers are absent. It might be inferred that lymphoid tissue had developed in the mesenchyma surrounding the diverticulum, in the same way that such tissue forms about the tonsillar pits, but careful study has shown that the thymus is largely of entodermal origin. Whether the cells of its cortex, which closely resemble lymphocytes, are true lymphocytes or "deceptively similar epithelial cells" has not been determined.


Connective tissue

Thymic corpuscie

Entering Medullary leucocytes. substance.


X 50.

According to Bell (Amer. Journ. Anat., 1905, vol. 5, pp. 29-62) the thymus is at first a compact mass of entodermal cells. By vacuolization the cells form a reticulum, and certain of them become lymphocytes. The lymphocytes pass into the cortex where they are most abundant, and enter the vessels. The lymphoid transformation of the thymus "is noticeable in pigs of 3.5 cm. and is well advanced at 4.5 cm.' Thus lymphocytes appear in the thymus at about the time that lymph glands develop. The first indication of lymph glands was found by Miss Sabin in pig embryos of 3 cm.

That the thymus cells are lymphocytes, however, is denied by Stohr, who regards the cortex as composed of a network of stellate epithelial cells, containing in its meshes



small round epithelial cells deceptively similar to lymphocytes. Of true leucocytes in the thymus he says, "In the places where the medulla is directly in contact with the surrounding connective tissue and such places become constantly larger and more numerous as the organ grows many leucocytes wander into the medulla; they lie in the connective tissue surrounding the medulla but not in that around the cortex (Fig. 214)." He considers that the cortex with its many mitotic figures represents a zone of production, and the medulla, a zone of growth and degeneration (Anat. Hefte, Abth. i, 1906, vol. 31, pp. 409-457). Hammar (1905, loc. cit.) is unable to determine the source of the "thymus lymphocytes," but is confident that the reticulum is of epithelial origin. He finds that in birds this reticulum produces cells resembling striated muscle fibers, and these "myoid cells" he considers to be entodermal. In his later work (1911, loc. cit.} he states that the lymphocytes enter the thymus chiefly from the thymic blood vessels.

Not only lymphocytes, but other leucocytes, eosinophilic cells, and multinuclear giant cells have been found in the medulla. Erythroblasts are said to occur in its outer portion and in the cortex. The thymus

Degenerated epithelial cells.

Flat epithelial cells. Degenerated nucleus.


therefore is sometimes considered a blood-forming organ. Sometimes the medulla contains cysts, which may be lined in part with typical ciliated cells. The most characteristic structures in the thymus are the thymic corpuscles (Hassall's corpuscles) which are found exclusively in the medulla. They are rounded bodies, at first few in number and small (i 2-20 ju in diameter), but they increase rapidly in size (to a diameter of 1 8o/0 and new ones are constantly forming. They are said to be present at about the fifth month, and at birth they are numerous, varying in size as shown in Fig. 215. To produce them, the nucleus and protoplasm of an entodermal reticular tissue cell enlarge, and the nucleus loses its staining capacity by changes in its chromatin. A layer of deeply staining hyaline substance develops in the protoplasm. This increases until it fills the entire cell, often being arranged in concentric layers, and the nucleus becomes obliterated. Neighboring cells are concentrically compressed by the enlargement of this structure, and by hyaline is


transformation they may become a part of the corpuscle. The larger corpuscles are due to a fusion of smaller ones, or to hyaline changes occurring simultaneously in a group of cells. The central portion of a corpuscle may become calcified. Sometimes it is vacuolated, containing fat. The hyaline substance may respond to mucous stains, but generally it does not; it has been considered similar to the 'colloid' of the thyreoid gland. Leucocytes are said to become imbedded in the corpuscles, or to enter them and assist in their disintegration. Thymic corpuscles have been regarded not only as degenerative products of the entodermal epithelium but also as concentric connective tissue masses, and as blood vessels with thickened walls and obliterated cavities. Injections show that they are not connected with the blood vessels. Although they have recently been described as active constituents of the thymus, they are generally regarded as degenerations.

The arteries of the thymus enter it along the medullary strand, and extend between the cortex and medulla, sending branches into both but chiefly into the cortex. The cortical branches empty into veins between the lobules; the others into veins within the medulla. There are many interlobular lymphatic vessels, beginning close to the surface of the gland substance, and accompanying the blood vessels. There is nothing in the thymus to correspond with a lymph sinus. The nerves, chiefly sympathetic fibers, with some from the vagus, terminate along the vessels; a very few have free endings in the medulla.


The thyreoid (i.e., shield-shaped) gland is a median, entodermal downgrowth from the tongue; its thyreoglossal duct becomes obliterated, leaving the foramen caecum to mark its former outlet. The downgrowth is joined by cells from the postbranchial bodies, which fuse with it. This entire structure comes to lie beside and in front of the upper part of the trachea. It consists of two lateral lobes, each about two inches long and an inch wide, connected by an isthmus, about half an inch wide, which crosses the median line ventral to the second and third tracheal rings. An unpaired pyramidal lobe extends from the isthmus or adjacent part of the lateral lobe toward the tongue (Fig. 208). Irregular detached portions of the gland, such as occur especially along the course of the thyreoglossal duct, are called accessory thyreoid glands.

The proliferating mass of entodermal cells forms at first a network of solid cords. This becomes separated into small masses, within each of which a lumen may appear. The lumen enlarges and becomes spheroidal; the entodermal cells which surround it form a simple epithelium, either coleumnar, cuboidal, or flat. Flat cells are said to occur especially in old



age; usually the cells are low columnar or cuboidal. The mature thyreoid gland consists, therefore, of rounded, closed spaces, or follicles, bounded by a simple entodermal epithelium (Fig. 216). The follicles vary greatly in diameter. Generally they are rounded, but sometimes they are elongated, and occasionally they branch or communicate with one another. Among them are cords or clumps of cells which have not acquired a lumen.

Within the follicles, and forming the most conspicuous feature of the thyreoid gland in ordinary sections, is a hyaline material which stains

Flat epithelium.

Blood Connective tissue. vessels.

Artery with two thickenings.

Colloid with drops of mucus.

Oblique section of a follicle.


deeply with cosine and is named 'colloid.' The hyaline material in the thymic corpuscles, the hypophysis, and in the coagulum in the cervical blood and lymphatic vessels, has also been designated colloid. In sections of the thyreoid gland it usually does not fill the follicle but has contracted, producing a spiny border. Granules, vacuoles and droplets of mucus, detached cells, leucocytes, and crystalloid bodies may be found in it. It is a product of the epithelial cells, in the protoplasm of which similar material has been detected. It has been said that it is transferred to the blood and lymphatic vessels, passing out between the epithelial cells.

As has been learned by experiment, the thyreoid gland produces an internal secretion which is essential for the normal growth and development of the body. It is, however, not known whether this secretion leaves the basal or free surface of the thyreoid epithelium, and its relation to the



colloid material is not clear. The finding of two sorts of thyreoid cells, one of which produces colloid, and the other does not, lacks confirmation. The cells may exhibit refractive, secretory granules, which are larger and coarser toward the free surface. Eosinophilic granules have been reported, and in certain animals other granules of fatty nature have been found, especially near the basal surface. Since the terminal bars are said to be deficient at the angles where the epithelial cells meet, an opportunity is afforded for the contents of the follicles to pass out between the epithelial cells to the vascular tunica propria.

The thyreoid follicles are surrounded by loose elastic connective tissue, said to be reticular near the follicles, which contains very many blood and lymphatic vessels in close relation with the epithelium. Denser connective tissue forms a capsule and lobular partitions. It contains small arteries, the media and intima of which are said normally to present local thickenings (Fig. 216). The nerves from the cervical sympathetic ganglia form perivascular plexuses, and pass to the follicles.


It is generally stated that there are four parathyreoid glands in man, the anterior or upper pair being derived from the fourth pharyngeal



pouches, and the posterior or lower pair from the third (Fig. 208). They are therefore entodermal structures. In the adult they are round or oval bodies, said to measure from 3 to 13 mm., found on the dorsal or tracheal surface of the thyreoid gland. They may be imbedded in its capsule or attached to it by pedicles. Sometimes they (the lower pair?) are found in the thymus. The parathyreoid glands may be lacking on one side, where in other cases as many as four have been recorded; they may atrophy




and disappear, or increase in number by subdivision. Both pairs possess a similar structure unlike that of either the thyreoid gland or the thymus, but resembling the corresponding epithelial bodies of the lower vetebrates. They consist of masses and cords of polygonal, entodermal cells containing round nuclei with networks of chromatin. The protoplasm is pale, almost homogeneous" or "slightly granular," sometimes containing vacuoles. Cell membranes are not prominent. Between these cells and the large thin- walled blood vessels which pass among them (Fig. 217), there is only a very small amount of connective tissue. A capsule surrounds the entire structure. The blood vessels are branches of those which supply the thyreoid gland. Little is known of the lymphatics or nerves.


The glomus caroticum (carotid gland) is largely a knot of blood vessels at the bifurcation of the common carotid artery. It is a reddish body "5-7 mm. long, 2.5-4 mm. broad, and 1.5 mm. thick." Between its thin-walled, dilated capillaries there are strands of polygonal chromaffine cells, which are prone to disintegrate (Fig. 218). Many nerve fibers, both medullated and non-medullated, enter the glomus, and a few multipolar ganglion

cells are associated with them. Since the nature of the glomus caroticum is undetermined, the three views regarding it MAN - < After

i i j b.v., Blood vessels; e.v., efferent vein; tr., trabecula; c.t., connective

may be mentioned. tissue septum.

First, it has been considered as derived from the third pharyngeal pouch. Since it has recently been asserted that the "carotid gland" of Echidna comes from the second pouch, the non-entodermal origin of the human glomus is perhaps not beyond question. Second, it has been considered ganglionic or paraganglionic in nature, so that it is classed with nervous structures, and this opinion is probably correct. Third, it is considered essentially a vascular formation, containing strands of modified mesenchymal cells.






I-IV. Branchial arches; t 1 , anterior part of the tongue! t*, second arch, joining the posterior part of the tongue toward the median line. The thyreoid gland is dotted. The epigk>ttis extends over the fourth arch. (From McMurrich, after His.)


The tongue consists of two parts, an anterior and a posterior, which

differ in origin and adult structure. Separating the branchial clefts from

one another there are columns of tissue known as branchial arches. They come together in the median ventral line to form the floor of the mouth (Fig. 219). In this figure the upper jaw and roof of the pharynx have been cut away; the branchial clefts are seen as dark depressions bounded laterally by thin plates. The first branchial arch (i) is between the oral and auditory clefts. In the median ventral line an elevation (tuberculum impar) arises between this arch and the second; it becomes continuous

with a larger elevated portion of the mandibular arch to form the anterior

part of the tongue (t 1 ). The second and third arches unite toward the

median ventral line and there produce the

posterior part of the tongue (t 2 ). Between

the anterior and posterior parts is the opening

of the thyreoglossal duct, later the foramen

caecum. . The epiglottis is an elevated part

of the third arch separated from the posterior part of the tongue by a curved groove. In the adult (Fig. 220) the dor sum of the

anterior part of the tongue is roughened with

elevations or papilla. These are chiefly the

slender filiform papilla and conical papilla;

but knob-like forms, the fungiform papilla,

are scattered among them over the entire

surface, and in life they can be easily distinguished owing to their red color. Near the

junction of the anterior and posterior parts of

the tongue there is a V-shaped row of larger

papillae, generally six to twelve in number,

called vallate papilla. Their name refers to the deep narrow depression

which encircles them. Behind the apex of the V, which is directed




c., Conical papillae; ep., epiglottis; f., foliate papillae; f. c., foramen caecum; f.f., position of the filiform and fungiform papillae; 1., lenticular papilla; 1. t., lingual tonsil; p. t., palatine tonsil; v., vallate papillas.



Primary papilla.

Secondary papillae.

Filiform process.

toward the throat, is the foramen oecum. On either side of the tongue, as indicated in the figure, there are from three to eight parallel vertical folds (2-5 mm. long) occurring close together; these are the foliate papillcs. In the foliate and vallate papillae the organs of taste are most numerous. The under surface of the tongue is free from epithelial papillae; its mucosa resembles that which lines the mouth. The posterior part of the tongue has a nodular surface covered with soft epithelium and contains the lingual tonsil, which has already been described. Laterally it presents fold-h'ke elevations called lenticular papillce.

Filiform papillae (Fig. 221) are slender cornified epithelial projections, composed of pointed cells which are described as stacked like superimposed hollow cones. The cells have undergone a horny hyaline degeneration. These projections are arranged in clumps which rest upon a group of from five to twenty connective tissue elevations, or secondary papillae ; and these in turn are at the summit of a cylindrical or conical primary papilla, composed of vascular connective tissue with numerous elastic fibers. These primary papillae form the basal portions of the filiform papillae. They are well shown in Fig. 222, along with the secondary papillae, but the cornified processes of the thick epithelium above them have undergone post-mortem disintegration. Most of the papillae of the tongue are of the filiform type.

Fungiform papillae (Fig. 222) are rounded elevations with a somewhat constricted base, varying in height from 0.5 to 1.5 mm. In life they are red, since their epithelium is not cornified and transmits the color of the blood beneath. They contain a primary connective tissue papilla, with but few elastic fibers, beset on all sides with secondary papillae.

The vallate papillae resemble broad fungiform papillae. They are from i to 3 mm. broad and i to 1.5 mm. tall, each being surrounded by a deep groove (Fig. 223). Their connective tissue often contains longitudinal, oblique, or encircling smooth muscle fibers, the last named being found near the lateral walls. Secondary papillae are confined to the upper wall. Occasionally the epithelium sends branched prolongations into the underlying tissue. These may become detached from the surface and appear as concentric bulb-like bodies such as are generally known as "epithelial pearls." There are also branched serous glands which grow

. Fat cells. FIG. 221. FROM

Fascia linguae.





down from the epithelium, having ducts which open into the deep grooves (Fig. 223). The foliate papillae are parallel folds of mucous membrane, in the epithelium of which there are many taste buds. These structures, which occur also in the lateral walls of the vallate papillae (Fig. 223), will be described with the nerves of the tongue.

The tunica propria of the mucous membrane is a loose connective tissue layer containing fat. It is not sharply separated from the denser submucosa. At the tip of the tongue, or apex lingua, and over the dorsum, the submucosa is particularly firm and thick, forming the fascia lingua. Three sorts of glands branch in the submucosa and

Cornified epithelium.


papillae of a



Primary papilla.

Oblique section

of a filiform



papillae of a filiform' papilla.

Primary papillae.


m - -- ;^ : W$fc :


FIG. 222. FROM A LONGITUDINAL SECTION OF THE HUMAN TONGUE. x, Epithelium showing post-mortem disintegration.

Fascia linguae.

.... Striated muscle fibers.

X 25.

may extend into the superficial part of the muscle layer. These are the serous glands found near the vallate and foliate papillae; mucous glands occurring at the root of the tongue, along its borders, and in an area in front of the median vallate papilla; and the two mixed anterior lingual glands, from half an inch to an inch long, each of which empties by five or six ducts on the under surface of the apex. The structure of these types of glands will be described in the section on oral glands.

The muscular layer consists of interwoven bundles of striated fibers which are inserted into the submucosa or into the intermuscular connect



ive tissue. Some of these striated fibers are branched. The musculature of the tongue is partly divided into right and left halves by a dense median connective tissue partition, the septum lingua, which begins low on the hyoid bone, attains its greatest height in the middle of the tongue, and becomes lower anteriorly until it disappears. It does not extend clear through the tongue since it ends 3 mm. beneath the dorsum. The muscles of the tongue are partly vertical (Mm. genioglossus, hyoglossus, and verticalis lingua}, partly longitudinal (Mm. styloglossus, chondroglossus, superior and inferior longitudinalis lingua} and partly

Tuica propna.

Secondary papillae. Taste bud. Vallate papilla. \



of a Small

serous papilla, gland.


Tunica propria

Striated muscle.

Muscle fibers in cross Nerye with Fascia Mucous Vein.

and longitudinal section. ganglion cells, linguae. gland.


transverse (M. transversus lingua). The glossopalatine muscle of the palatine group also enters the tongue. Some of the muscle fibers are oblique but many of the bundles cross at right angles. In the connective tissue between them, medullated nerves are abundant. Some are sensory nerves to the mucosa, but many of them are the lingual branches of the hypoglossal nerve which supply all the tongue muscles except the inferior longitudinal; the latter is supplied by fibers from the chorda tympani Sensory spindles have been found in the lingual muscles.

Blood vessels are numerous in the submucosa and form extensive capillary networks in the tunica propria of both primary and secondary



papillae. Small lymphatic vessels also form a network in the tunica propria, and this is continuous with a coarser net in the submucosa.

The sensory nerves are the terminations of the lingual branches of the mandibular nerve anteriorly, and of the lingual branches of the glossopharyngeus posteriorly. In the submucous connective, tissue they form a plexus of medullated and non-medullated fibers, and in some places, notably beneath the vallate papillae, nerve cells are found, grouped in small ganglia (Fig. 223). The terminal branches of these nerves probably end in part in bulbous corpuscles, but most of them, as non-medullated

Taste bud.

Fibers between the buds

Fibers overlying a bud.

Connective tissue. Epithelium.

Fibers within the buds.

Connective tissue.




fibers, enter the epithelium and extend to the outer epithelial cells, generally without branching (as on the left of Fig. 224) . Others enter the groups of specialized epithelial cells, known as taste buds, which are believed to be the special organs of taste. Within the buds the nerves divide into coarse varicose branches which end freely, without uniting with the cells or anastomosing with one another (Fig. 224).

Taste buds are round or oval groups of elongated epithelial cells, most of which extend from the basal to the free surface of the epithelium. In embryos of from five to seven months they are more numerous than in the adult, occurring in many filiform papillae, in all the fungiform, vallate and foliate papillae, and also upon both sides of the epiglottis. Subsequently they are destroyed with an infiltration of leucocytes except on the lateral walls of the vallate and foliate papillae, on the laryngeal surface of the


epiglottis, and a small portion of those on the anterior and lateral fungiform papillae. These remain in the adult. In the outer half of each bud the cells converge like the segments of a melon, so that their ends are brought together in a small area. This area is at the bottom of a little pore or short canal found among the outermost flat cells of the epithelium. The taste pore opens freely to the surface, but in oblique sections it may appear bridged as in Fig. 225. Within the bud two sorts of elongated cells may be distinguished, namely, supporting cells which are chiefly peripheral, Taste pore. and taste cells which are central. There are also certain cells which lie wholly in the basal part of the bud, and lymphocytes which P ceSu" 18 **.! have entered the bud from below are frequently seen among

it AI 11 mi Taste cells. "*

the other cells. The supporting cells are paler than the Stratified gustatory cells, and may be uni- e p ;thelium - '

form in diameter Or tapering FIG. 225. FROM A VERTICAL SECTION OF A HUMAN


toward their ends; they are

sometimes forked or branched below. The taste cells are darker and more slender, being thickened to accommodate the narrow nucleus which is usually near the middle of the cell. At the taste pore these cells end in a stiff refractive process which is a cuticular formation. The processes extend into the deeper part of the pore but do not reach its outlet. These cells are believed to transmit the gustatory stimuli to the nerves which branch about them. To a less extent the nerves are said to ramify around the supporting cells, which perhaps have other functions than their name implies.


The lining of the mouth, like the covering of the tongue, consists of epithelium, tunica propria, and submucosa. At the lips, toward the line of transition from skin to mucous membrane, hairs disappear from the skin. The epithelium becomes thicker but more transparent as it crosses the line (Fig. 226). Its outer cells are still cornified, but they are not so flat and compactly placed as in the skin. The deeper cells appear vesicular. Within the mouth, except on the tongue, cornified cells are absent, but granules of the refractive horny substance, keratohyalin, are said to occur in the outer cells, even in the oesophagus. The free surface of the epithelium



is generally smooth, but its under surface is indented by many connective tissue papillae, which are particularly long and slender in the gums and lips (Fig. 226). At the inner border of the lips at birth, there are free papillary projections described as " true villi," but these later disappear. Cilia are found on the oral, pharyngeal and cesophageal epithelia in the embryo, but in the adult cilia persist only in certain parts of the pharynx.

The tunica propria in the mouth, as is generally the case in the digestive tract, has few elastic fibers. Some of its tissue is reticular, and in it, lymphoid accumulations are frequent; they may extend into the submucosa. On the oral surface of the soft palate there is a layer of elastic

Sebaceous gland Tall papillae

Oblique sections of papillae.

Hair shafts and sebaceous glands.

Sebaceous s gland.

Hair shaft.



Bulb of a hair.

Corium. Epidermis.

\ I

Epithelium. Tunica Submucosa. Orbicular Mimetic

propria. muscle. muscle.

FIG. 226. VERTICAL SECTION THROUGH THE LOWER LIP OF A MAN OF NINETEEN YEARS. X 10. Epidermis and corium constitute the skin; epithelium, t. propria, and submucosa form the oral

mucous membrane.

tissue between the propria and submucosa. A similar layer is found in the oesophageal end of the pharynx. It increases in thickness upward, at the expense of the submucosa, so that it forms a thick layer in the back of the pharynx in. contact with the muscles, among the fibers of which it sends prolongations. This elastic layer, as the /asa# />^aryw#0&<m7am, is attached to the base of the skull.

In most of the oral region there is no sharp line of separation between the propria and the submucosa. The latter may be a loose layer containing fat, and allowing considerable movement of the mucosa, or, as in the gums and hard palate, it may be a dense layer binding the membrane closely to the periosteum. In the submucosa are the branches of various glands. On the inner border of the lips and the inner surface of the cheek,


there are sebaceous glands without hairs, which first develop during puberty. This type is described with the skin. The other oral glands are considered in the following section.


In the general account of glands (page 54) it has been stated that serous gland cells which produce a watery albuminoid secretion should be distinguished from the mucous gland cells which elaborate thick mucus. When examined fresh, serous cells are seen to contain many highly refractive granules. In fixed preparations they may appear dark and granular (empty of secretion) or enlarged and somewhat clearer (full of secretion), as shown in Fig. 44, p. 54. The round nucleus is generally in the basal half of the cell, not far from its center (Fig. 227). Mucous cells when

Man. Rabbit. Man.

Mucous glands. Serous glands.


Mucous AND SEROUS GLAND CELLS. b, Empty mucous cells; c, mucous cells full of secretion; d, lumen of the tubule. X 240.



fresh are much less refractive than serous cells. In fixed preparations they are typically clear, since the large area occupied by mucous secretion stains faintly. Fully elaborated mucus, however, may be colored intensely with certain aniline dyes, such as mucicarmine and Delafield's haematoxylin. In certain types of mucous cells the pale secretion area is large in all stages of activity. When full of mucus, the nucleus is flattened against the base of the cell, and when empty, the nucleus becomes more oval without essentially changing its position (Fig. 227). This differs from the type of mucous cell found in the gastric epithelium, in which the secretion area varies considerably with the elaboration and discharge of secretion (Fig.

45, P- 55) Glands may consist entirely of serous or of mucous cells, but frequently

they include cells of both sorts and are called mixed glands. The mixed glands contain some purely serous tubules or alveoli; the rest consist of both mucous and serous cells, so arranged that the latter appear more or less crowded away from the lumen. Often they form a layer outside of the mucous cells, partly encircling the tubule or alveolus and constituting a crescent (demilune), as shown in Fig. 237. The serous cells of the cres

~ ~/ Axial lumen.


cent are connected with the lumen by means of secretory capillaries (p. 57) which pass out to them between the mucous cells and branch around the serous cells, ending blindly (Fig. 228). Sometimes the cells of the crescent are directly in contact with the lumen. Since the serous crescents are always associated intimately and somewhat irregularly with mucous cells,

they were naturally interpreted as a functional phase of the latter. It is probably true that some crescents represent empty mucous cells which have been crowded from the lumen by those full of secretion. No secretory capillaries lead to such mucous crescents, which moreover are not abundant. Another sort of crescentic figure is made by the basal protoplasm

Crescent. *

FIG. 228. FROM A SECTION OF THE SUBMAX- in mucous cells otherwise full of secre ILLARY GLAND OF A DOG. X 320.

tion. Finally, in oblique sections, stellate cells associated with the basement membrane may resemble true crescents.

The oral glands include serous glands, mucous glands, and mixed glands to be described in turn.

Intercellular secretory capillary.

Serous Glands.

The serous oral glands are the parotid glands and the serous glands of the tongue (v. Ebner's glands). The latter are branched tubular glands limited to the vicinity of the vallate and foliate papillae. Generally they open into the grooves which bound these papillae. Their ducts are lined with simple or with stratified epithelium, which is occasionally ciliated. Their small tubules consist of a delicate membrana propria or basement membrane, which surrounds the low columnar or conical serous cells. In this simple epithelium, cell walls are lacking. With special stains and high magnification, a dark granular zone toward the lumen has been distinguished from the clear basal portion of the cell which contains the nucleus (Fig 229). The lumen of the tubules is very narrow and receives the still narrower intercellular secretory capillaries (Fig. 230).

The parotid glands are the largest oral glands. Each is situated in front of the ear and is folded around the ramus of the mandible; its duct, the parotid duct (Stenson's), empties into the mouth opposite the second


Secretory granules toward the lumen are finer than those further out. The light intercellular lines represent the secretory capillaries.



molar tooth of the upper jaw. The parotid gland is an organic, branched serous gland, subdivided into lobes and lobules. The accessory parotid gland appears as a lobe separated from the others. The parotid duct is

Intercellular secretory capillaries. ;


Prepared by Golgi's method, a precipitate has formed in the ducts. The right lower part of the figure has been completed by adding the cell outlines.


The basal rods (mitochondria) toward the lumen break apart into secretorygranules.

Fat cells.

End piece. .

End piecees.



X 252. The very narrow lumen of the alveolo-tubular end pieces is not


characterized by a thick membrana propria, and consists of a two-layered columnar epithelium with occasional goblet cells. As the duct branches repeatedly, the epithelium becomes a simple columnar epithelium, after



being pseudostratified, with two rows of nuclei (cf. Fig. 39, p. 49)- Possibly the epithelium near the outlet of the duct is also pseudostratified. This excretory portion of the duct is followed by the secretory part, formed of simple columnar cells with basal striations, perhaps indicative of secretory activity (Fig. 231). As shown in the diagram (Fig. 232) and in the section (Fig. 233) the secretory ducts become slender, forming the intercalated ducts. These are lined with flat spindle-shaped cells which are continuous with the large cuboidal serous cells of the terminal alveoli. The gland cells when empty of secretion are small and darkly granular,



Portions of three lobules are shown, which have drawn apart from one another in the process of preparation.

Note the abundance of secretory ducts.

and when full are larger and clearer. They rest upon a basement membrane containing stellate cells. Intercellular secretory capillaries end blindly before reaching the basement membrane.

Between the alveoli, which are somewhat elongated and branched, there is vascular connective tissue containing fat cells. In denser form it surrounds the lobules and lobes of the gland, and the larger ducts. The ducts which are found in the connective tissue septa are called interlobular ducts, in distinction from those which are surrounded by the alveoli in which they and their branches terminate. The latter are intralobular ducts. They are smaller and have less connective tissue around them than the interlobular ducts, of which, however, they are continuations.


Vessels and Nerves. The arteries generally follow the ducts from the connective tissue septa into the lobules, where they produce abundant capillary networks close to the basement membranes. The veins derived from these soon enter the interlobular tissue, and may then accompany the arteries. The lymphatic vessels follow the ducts, and branch in the interlobular connective tissue, in which they terminate. Only tissue spaces have been found within the lobules. The nerve supply is from several sources. Sympathetic nerves from the plexus around the carotid artery accompany the blood vessels into the parotid gland, and by controlling the blood supply have an Important bearing upon secretion. The nerves which reach the gland cells are in connection with the tympanic branch of the glossopharyngeal nerve. This branch extends to the otic ganglion, from which fibers pass to the parotid gland by way of an anastomosis with the auriculo-temporal branch of the mandibular nerve. Within the gland the nerves pass along the ducts, where they are associated with microscopic ganglia, and form plexuses beneath the basement membranes of the alveoli. From these plexuses, fibers penetrate the basement membranes and form simple or branched varicose endings in contact with the gland cells. Other nerves enter the substance of the gland, either to pass through it or to contribute to its nerve supply; these include branches of the trigeminal, facial and great auricular nerves, the last coming from the second and third cervical nerves. Free sensory endings of medullated fibers are said to occur in the epithelium of the ducts

Mucous Glands.

The purely mucous glands of the mouth are simple branched alveolotubular glands found on the anterior surface of the soft palate and on the hard palate (palatine glands), along the borders of the tongue (lingual glands), and in greater numbers in the root of the tongue. There they may open into the tonsillar pits through ducts lined with columnar epithelium, sometimes ciliated. The wall of the tubules consists of a structureless basement membrane and of columnar mucous cells, varying according to their functional condition as shown in Fig. 227, I-II. The empty cells are narrower than the others, and the nuclei, though at the base of the cell and transversely oval, are not as flat as in cells full of secretion. Seldom can cells be found completely occupied by unaltered protoplasm. A single gland, or even a single alveolus, may contain cells in different phases of secretion, as is clearly seen when special mucin stains are used. Secretory capillaries are not found in the purely mucous glands.

Mixed Glands.

The mixed oral glands are the sublingual, submaxillary, anterior lingual, labial, buccal, and molar glands. They all possess crescents of




serous cells such as are to be described in the largest glands of this group the sublingual and submaxillary.

The sublingual glands are two groups of glands, one on either side of the median line, under the mucous membrane in the front of the mouth. The largest component is an alveolo-tubular structure emptying by the

ductus sublingualis major on the side of the frenulum lingua. The main stem and the principal branches of the large sublingual duct are lined by a two-layered or pseudostratified columnar epithelium, as in the parotid duct. They are surrounded by connective tissue containing many elastic fibers. Ducts less than .05 mm. in diameter have a simple columnar epithelium, which in a few places becomes low and basally striated to form the secretory ducts. As shown in the diagram, Fig. 235, the secretory ducts are very short, and they are id pieces, accordingly infrequent in sections; the slender intercalated ducts are absent. The terminal FIG. 235. DIAGRAM OF THE HUMAN secreting portions of the gland are somewhat

tortuous structures, often presenting outpock etings. They consist of mucous and serous cells quite evenly mixed, so that the gland has a characteristic appearance under low magnification (Fig. 236). The serous cells sometimes border upon the lumen, but often they are separated from it by the mucous cells so that they form crescents (Fig. 237). Only the serous cells are provided with the branched intercellular secretory capillaries. Around the tubules there is a basement membrane including certain stellate cells. The interlobular connective tissue contains many lymphocytes.

Near the gland just described, but apparently quite distinct from it, there is a group of 5 to 20 alveolo-tubular glands which open by separate ducts, the ductus sublinguales minores. These glands consist almost exclusively of mucous cells.

The sublingual gland as a whole receives fibers from the submaxillary ganglion, and so from the chorda tympani, which passes to this ganglion by way of an anastomosis with the lingual branch of the mandibular nerve. Its ducts are said to have sensory fibers, probably derived from the lingual nerve. Sympathetic fibers from the superior cervical ganglion, which have ascended the neck as perivascular plexuses, extend to the sublingual gland around its arteries.

The submaxillary glands are a pair of branched alveolar glands, in part tubulo-alveolar, found in the floor of the mouth, each being drained by a submaxillary duct (Wharton's) which opens on the sides of the frenulum




YEARS. X 100.

A crescent consisting of eight serous cells.

Part of an excretory duct.



section of serous


Mucous cells and

thick mernbrana





Excretory duct.

linguae near its front margin. Sometimes this duct is joined by the ductus sublingualis major so that the two have a common outlet. Its orifice may be lined by stratified epithelium, but this soon gives place to the two layered form. Secretory ducts are well developed (Fig. 238) and their basally

striated cells contain a yellow pigment. The intercalated ducts, which are lined with simple cuboidal epithelium, lead to terminations of two sorts. Most of these consist entirely of serous cells. The others are mixed, but the crescents are small, composed of only a few or even of single serous cells (Figs. 239 and 240). Secretory capillaries such as have already been described, are related only to the serous cells. Elastic tissue surrounding intercalated the alveoli has been thought to aid in expelling the secretion through the ducts. The nerves have the same origin as those of the sublingual gland.

In the oral glands, not infrequently degenerating lobules occur, characterized by abundant connective tissue between tubules with wide lumens and low gland cells. Sometimes they are surrounded by leucocytes.

Secretory duct.


End pieces.



Serous gland cells.'

Intercalated duct.

Mucous gland cells.

Secretory duct.




Serous Intercalated Blood cells. duct. vessels.

Secretory duct

Connective tissu

Mucous cells.

w. Fat cells.

PIG. 240. SECTION OF THE SUBMAXILLARY GLAND FROM A MAN OF TWENTY-THREE YEARS. X 100. Note that the serous cells predominate, and that secretory ducts are abundant. (A characteristic

crescent is shown at z.)


The digestive tube of mammals arises as two outgrowths from the yolk-sac the fore-gut and hind-gut respectively. They are shown in Fig. 241, A, which represents a young rabbit embryo placed in a vertical position. Most of the spherical yolk-sac has been cut away. Anteriorly the fore-gut (pti) is seen extending from the yolk-sac to the oral plate; posteriorly the sac has given rise to a short hind-gut from which a tubular ventral outgrowth, the allantois, has begun to develop. The allantois will be described with the membranes which surround the embryo. In an older stage (Fig. 241, B) the fore-gut and hind-gut have elongated, and the connection of the tube, which they form, with the yolk-sac is becoming reduced to a slender stalk. The entodermal tube within the stalk is called the mtelline duct. Posteriorly the intestine and allantois unite and form the cloaca, which is closed to the exterior by the cloacal membrane.. (The marked bend in the intestinal tube shown in Fig. 241, B, which is often seen in human embryos, is exaggerated, if not produced altogether, by a post-mortem sagging of the yolk-sac.)



In the later stage (Fig. 241, C) both the fore-gut and hind-gut have greatly elongated; together they form a loop of intestine extending out into the cavity of the umbilical cord. Near the bend in this loop the yolk-sac is still attached to the intestine by a stalk; the sac itself has been cut away in the figure. In addition to the pharynx already described, the



2.15 mm. (after His). C. Pig. 12 mm. D. Man, 17.8 mm. (after Thyng). E. Man, about five months. a., Anus; al., allantois; bl., bladder; cae., bulb of the colon; cl., cloaca; du., duodenum; 1. i., large intestine;

oe., oesophagus; p., penis; pe., perineum; ph., fore-gut; r., rectum; s. i., small intestine; St., stomach;

u. c., umbilical cord; ur., urethra; ura., urachus; u. s., urogenital sinus; v. p., vermiform process:

y. s., yolk-sac; y. St., vitelline duct within the yolk-stalk.

fore-gut has given rise to an expanded portion or stomach. Between the stomach and pharynx it remains tubular and becomes the oesophagus; posterior to the stomach it is likewise tubular and there it forms a part of the small intestine. The first portion of the small intestine is called the duodenum, and is followed by the jejunum which passes without demarcation into the ileum. The ileum includes the portion to which the yolkstalk is attached, and terminates at a bulbous enlargement (Fig. 241, C, cae) which gives rise to the cacum and -vermiform process. This bulbus coli (Johnson) marks the beginning of the large intestine or colon, and the caecum and vermiform process are parts of the large intestine. Toward the cloaca the colon becomes the rectum, and near its termination it forms an elongated bulbous enlargement, the bulbus analis. As shown by F. P.


Johnson (in a paper about to be published) this bulb forms essentially the zona columnaris in the anal part of the rectum. The anus is produced after the cloaca has separated into dorsal and ventral portions. The ventral division, which carries with it the allantois, becomes expanded to form the bladder, but its outlet remains relatively narrow and becomes the urethra. The outlet of the rectum is the anus, which is at first closed by the anal membrane; this membrane ruptures in embryos measuring from 20 to 30 mm., except in the occasional cases of imperforate anus. The tissue which subdivides the cloaca reaches the surface and constitutes the perineum.

In human embryos of about 10 mm. the intestinal loop becomes twisted on itself (Fig. 241, D), and the large intestine is carried across the small intestine in the duodenal region. The vermiform process thus comes to lie on the right side of the body, and the colon, after it is withdrawn from the umbilical cord into the body, is so bent as to form ascending, transverse, and descending portions, below which, as the convoluted sigmoid colon, it connects with the rectum. The disposition of the adult intestines depends chiefly upon this primary torsion of the intestinal loop, and upon the subsequent elongation of tne small intestine, which forms many loops and coils.

Meanwhile the yolk-sac has become detached, and its stalk has disappeared, usually leaving no indication of its former position. The stalk does not become the vermiform process, as was once supposed, but occasionally it produces a blind pouch of the ileum, 3-9 cm. long, situated about three feet above the beginning of the colon. This is the diverticulum ilei, described and correctly interpreted by Meckel in 1812.

The division of the intestine into six parts is a heritage from the Arabians. Duodenum, jejunum, ileum, caecum, colon and rectum were well recognized in the fifteenth century, when, following Hippocrates, they were counted from below upward. The various names which have been applied to them are discussed by Hyrtl (Das arabische und hebraische in der Anatomie, Wien, 1879). Those which are now adopted have the following significance. The rectum is the straight terminal portion. "Colon is the K<!>\OV of Aristotle, which according to Pliny is a great source of pain (colic)." The caecum, or blind intestine, was so named by Galen, who did not practice human dissection and so referred to the more elongated pouch in lower animals. The name has generally been considered inappropriate for the human caecum. The Greek synonym rv<f>\bv (blind) is used in the medical term typhlitis (inflammation of the caecum). The ileum (from eiAe'w) is the coiled portion, and is arbitrarily defined as the lower three-fifths of the small intestine. The jejunum (Lat., fasting) is the portion generally found void and empty (Avicenna), since food passes through it rapidly. The duodenum, which has no free mesentery, was originally considered a part of the stomach; its name indicates that its length is twelve finger-breadths. Hyrtl notes that the same term has sometimes been applied to the rectum.

Layers of the Digestive Tube. The wall of the digestive tube is com


posed of four layers (i) tunica mucosa, (2) tela submucosa, (3) tunica muscularis, and (4) tunica adventitia or tunica serosa. The parts which are covered with peritoneum have a serous coat for their outer layer; the parts imbedded in connective tissue have the adventitious coat instead.

The tunica mucosa consists of epithelium, tunica propria, and the lamina muscularis mucosce. The epithelium is the entodermal lining of the tube, and is folded and inpocketed so as to form innumerable pits and glands, varying in their nature in different parts of the tube. The tunica propria consists of reticular tissue, which in places becomes characteristic lymphoid tissue. It is set apart early in development as a layer with abundant nuclei, thus differing from the underlying mesenchyma. At a later stage the lamina muscularis mucosce, or muscle layer of the mucous membrane, develops beneath it, separating it from the submucosa. The muscularis mucosae is a thin layer of smooth muscle fibers.

The tela submucosa (tela, tissue) is a connective tissue layer which contains many blood and lymphatic vessels, and the ganglionated plexus submucosus.

The tunica muscularis usually consists of an inner circular and an outer longitudinal layer of smooth muscle fibers, separated by a thin layer of connective tissue which contains the ganglionated plexus myentericus.

The tunica serosa is a connective tissue layer, covered by the peritoneal epithelium.

The layers enumerated are to be examined in the oesophagus, stomach and intestine, which differ from one another histologically, since these layers are variously modified.


The oesophagus is a tube about nine inches long, the several layers of which are continuous anteriorly with those of the pharynx, and posteriorly with those of the stomach. The mucous membrane is thrown into folds, except when the tube is distended by the passage of food; but the muscularis merely thickens on contraction, so that it always forms a smooth round layer (Fig. 242).

The epithelium is thick and stratified like that of the pharynx. Its outer cells are flattened in the adult, but in the embryo they include numerous islands of tall ciliated cells, some of which are found at birth. The basal surface of the epithelium rests upon connective tissue papillae or ridges.

The glands of the oesophagus are of two sorts, superficial and deep. The deep glands (glandules msophagece produndci) develop as scattered tubular downgrowths which pass through the tunica propria and muscu



laris mucosae into the submucosa, where their blind ends expand and branch, producing a cluster of tubulo-alveolar end pieces. The terminal portions at birth are still poorly developed. The tubules are composed wholly of mucous cells, although the basal protoplasm sometimes simulates crescents. The ducts are slender tubes generally lined with simple epithelium. They tend to slant toward the stomach, and they enter the epithelium where it dips down between the connective tissue papillae. The cells of the ducts become continuous with the basal layer of the epithelium. Large ducts are sometimes lined with stratified epithelium, often ciliated, and they may present cyst-like dilatations. Lymphocytes tend to accumu

Stratified epithelium.

Tunica propria. / /Muscularis / mucosae. Submucosae.

Mucous membrane.


Group of fat cells.

Circular muscles, f Longitudina mus- VMuscularis. cles. J

Mucous gland.

v Tunica adventitia.


late around the ducts and occasionally they form nodules in the tunica propria. The glands may show signs of infiltration and degeneration. The number of deep glands varies greatly in different individuals. They are usually more numerous in the upper half of the oesophagus.

The superficial glands (glandules cesophagea superficiales) are limited to two rather narrow zones near the ends of the oesophagus. They are always found at the entrance of the stomach, extending from i to 4 mm. up the oesophagus; and generally (in 70% of the cases examined by S chaffer) they occur between the level of the cricoid cartilage and fifth tracheal ring. They develop in the embryo much earlier than the deep glands, and appear as small areas of tall mucous cells which pass clear through the stratified epithelium. These islands of simple epithelium become depressed into shallow pockets from which a cluster of tubules grows


out, but they never pass through the muscularis mucosae into the submucosa. In the adult the upper group may be seen with the naked eye as an "erosion" of the mucous membrane. The glands produce a form of mucus which stains less readily with the mucus-stains than that of the deep glands. No special function has been assigned to this secretion. Glands of the lower group are shown in Fig. 243. They are freely branching mucous glands, the ducts of which open at the tops of connective tissue papillae. They very frequently show cystic enlargements.

d e f g



a, Duct of a superficial cesophageal gland; b, oesophageal epithelium; c, gastric epithelium; d, tubule of the gland a; e, lymphoid nodule; f, lymphatic vessel; g, lamina muscularis mucosae.

The tunica propria in the oesophagus has fewer cells in its meshes than that of the lower parts of the digestive tube. In places it includes solitary lymph nodules. The muscularis mucosae is very wide in the oesophagus. It is a layer of longitudinal smooth muscle fibers, which is thrown into longitudinal folds when the oesophagus is contracted. It begins anteriorly at the level of the cricoid cartilage, arising as scattered bundles inside the elastic layer of the pharynx. As the muscles increase to form a distinct layer, the elastic lamina terminates. The submucosa is a loose connective tissue layer, containing many vessels and nerves, groups of fat cells, and the bodies of the deep mucous glands. The muscularis consists of an inner circular and an outer longitudinal layer, as elsewhere in the digestive tube, but in the upper part of the oesophagus the layers are composed of striated


muscle fibers. These fibers are not a downward extension of the striated pharyngeal constrictors, but apparently develop from exactly such mesenchymal cells as produce smooth muscle further down. The striated muscles in man are limited to the upper half of the oesophagus ; in the rabbit they extend its whole length.

The adventitia is loose connective tissue, containing many vessels and the plexiform branches of the vagus nerves. From these nerves, medullated and non-medullated fibers enter the oesophagus and form a ganglionated myenteric plexus between the muscle layers, and the plexus submucosus in the submucosa. Medullated fibers proceed from the vagus trunks to the motor end plates of the striated muscles, which are thus stimulated reflexly from the central nervous system. Other fibers pass from the myenteric plexus to the plexus submucosus and thence to the epithelium, in which free nerve endings have been found. Such fibers, together with those to the smooth muscles, provide for local reflex action, whereby the contents of the oesophagus causes contraction above, and relaxation below, the place of stimulation. This takes place independently of the central system, and is the form of innervation characteristic of the intestine.


Form and Subdivisions. The opening through which the oesophagus connects with the stomach is the cardia (Gr. KapSui, heart), and the opening from the stomach to the intestine is the pylorus (Gr. 7rvA.o>pds, gate-keeper). The pylorus received its appropriate name from Galen (in the second century), who recognized that through its sphincter muscle it controlled the exit of food. The significance of cardia was discussed by Fabricius (1618) who cites Galen as stating that the upper orifice of the stomach is called the heart because the symptoms to which it gives rise are similar to those which sometimes affect the heart, sometimes even the brain; but for Fabricius, cardia, as applied to this orifice, merely indicates a chief part of the body. The stomach as a whole is termed gaster, from the Greek, but the Latin ventriculus was generally used by the early anatomists. Although flaccid and shapeless when seen in the dissecting room, the stomach has a very characteristic form. Its epithelium, from an embryo of 44.3 mm., is shown in Fig. 244, and an adult stomach is seen in Fig. 250. It is a tube which is greatly distended toward the left, where its border forms the greater curvature; its right border is the lesser curvature. As a whole the stomach is divided into two parts, the cardiac portion (pars cardiaca) and pyloric portion (pars pylorica). This fundamental subdivision occurs in many animals, as was recognized by Sir Everard Home in 1814. The pyloric part is relatively long in the embryo. It becomes subdivided into the pyloric vestibule and the pyloric antrum. The latter is its smaller part extending to the pylorus; between the two, on the greater curvature, is the sulcus intermedius, well shown in Fig. 250. (The term pyloric antrum has been variously employed, since in its original description by Willis (1674) the vestibule is not recognized; Cowper (1698) applies antrum to the terminal subdivision as above defined.) The cardiac part of the stomach is divided into a main portion, or body of the stomach (corpus gastri], and a blind pouch, formerly called the saccus caecus, but now less



appropriately known as the fundus gastri (the bottom of the stomach). Recently the gastric canal (canalis gastri) has been recognized along the lesser curvature of the human stomach. It is a channel, highly developed in ruminants, which conveys liquids from the cardia to the pars pylorica, when the stomach is filled with more solid contents. Ordinarily open toward the interior like a groove, it may become closed as a tube during its physiological activity. Beyond the cardia there is a conical expansion of the oesophagus, not always well defined, known as the cardiac anlrum, and beyond the pylorus is the first part of the duodenum, or duodenal antrum. (A further account of the development of these subdivisiods will be found in the Amer. Journ. Anat., 1912, vol. 13, pp. 477-503.)


Gastric canal. Angular incisure.


Duodenal antrum.



The inner surface of the stomach presents macroscopic longitudinal folds, which become coarse and prominent as the organ contracts. They are sinuous, and anastomose in an irregular network. As finer markings, there are rounded or polygonal areas, 2-4 mm. in diameter, which may appear as elevations or depressions. They have been ascribed to the contraction of muscle fibers in the mucous membrane, to varying amounts of lymphoid tissue, and to the varying height of the glands. Toward the pylorus there are small leaf-like elevations, the plica villosce, which may connect with one another in a network. The epithelium of the stomach is thin enough to transmit the color of the underlying tissue, and appears pinkish gray; whereas the color of the oesophagus, with a thicker epithelium, is white.

The gastric epithelium, like that of the entire intestine, is a single layer of columnar cells. In the stomach the cells are tall and contain mucus, but they do not ordinarily acquire the bulging goblet shape, since the adjacent cells likewise contain mucus. This simple layer of mucous cells is continuous at the cardia with the basal layer of the stratified epithelium of the


oesophagus, and the transition is abrupt. The outer strata of the cesophageal epithelium may form an overhanging wall (Fig. 243), or the number of layers may have become reduced so that such a wall is absent. Sometimes an island of stratified epithelium occurs just beyond the line of transition. The gastric epithelium forms three types of glands, known as cardiac, gastric, and pyloric glands respectively, none of which extend into the submucosa.

The cardiac glands are like the superficial glands at the lower end of the oesophagus, of which they may be regarded as a continuation. They extend only from 5 to 40 mm. into the stomach, and in the narrow zone which they occupy, they present a gradual transition to the gastric glands. Their branches, instead of continuing divergent, become groups of perpendicular tubes descending from epithelial pits; and deeply staining eosinophilic cells and the granular chief cells become included in their epithelium.

The cells characteristic of the cardiac glands contain a mucus which does not respond readily to mucin stains. Like the superficial glands of the oesophagus, the cardiac glands develop early, and they are found widely distributed among mammals.

The gastric glands (sometimes inappropriately called fundus glands) occur over the entire surface of the stomach, except near the cardia and pylorus. Each gastric gland is divided into an outer portion, or gastric pit (foveola gastrica] , and a group of slender cylindrical tubules which empty into the bottom of the pit. During development, as the lining of the stomach expands greatly, the number of pits increases. Toldt estimated that there were 129,912 in the stomach at three months; 268,770 at birth and 2,828,560 at ten years. The increase is accomplished by division of the pits from below upward. In spite of the fact that many new branches develop, the average number of tubules emptying into each pit becomes reduced as the pits become subdivided; and the average of seven per pit observed at birth becomes three in the adult (Toldt, Sitz.-ber. Akad. d. Wiss. Wien, 1881, vol. 82, pp. 57-128).

The pits are often described as if they were epithelial depressions separate from the glands, since the same sort of epithelium which lines them is found on the free surface. Developmentally, however, they are to be regarded as parts of the glands, comparable with ducts. The epithelial cells of the pits (Fig. 245) consist of a basal protoplasmic portion containing elongated, round, or sometimes flattened nuclei, and an outer portion containing the centrosome and secretion. The mass of mucus may cause the thin top plate to bulge, and in preserved tissue to rupture, but this may be due to reagents. The mucus first appears in granular form.

The gastric tubules are straight or somewhat tortuous slender structures, with narrow lumens. The portion which joins the pit constitutes



the neck of the gland, and the slightly expanded basal end is the fundus. Apparently the neck is the zone of growth, since it is the place where


' -* ^ ' '-

Gastric pit.

Neck. ,

Smooth muscle fibers.

Parietal cell.

%^ i..-;* \ i]ir Fundus Hi 'ii-'^ H'^-'Ml,

Ill; mm \'wd >'

Tubules of the gastric glands.

i : v


mitotic figures are found. Each tubule is composed of cells of two sorts, chief cells and parietal cells.

The chief cells usually form the greater part of the tubules. They are



Gland lumen


Axial lumen.

Parietal cells with intracellular se- \ cretory capillar- \

wedge-shaped cells, having a narrow contact with the lumen. In general

they have the aspect of serous cells, containing round nuclei and granular

protoplasm. The granules, which are coarser toward the lumen, do not

respond to mucin stains. They accumulate, and the chief cells enlarge, in

the absence of food from the stomach; but

during gastric digestion, the cells become .JftsSk Chief ceil.

small and the granules disappear. They Parietal ceii.

apparently give rise to the pepsin of the

gastric juice, and are called zymogen

granules. After death the chief cells

rapidly disintegrate, and the granules are seldom well preserved except in

special preparations.

The parietal cells, even in fresh tissue, may be readily distinguished from the chief cells; the latter are dark and contain refractive granules,

whereas the parietal cells are clear. They are large cells, containing one or occasionally two round nuclei, and are crowded away from the lumen like the cells in the serous crescents (Figs. 245 and 246). They discharge their secretion through secretory capillaries which produce basket-like networks within the protoplasm; thus they differ from the chief cells which have only intercellular secretory capillaries. The secretory capillaries of the parietal cells may be demonstrated by the Golgi method, which produces a precipitate wherever secretion is encountered (Fig. 247). After fasting, the parietal cells are small and their intracellular capillaries have disappeared. Following abundant meals, these cells enlarge and may contain vacuoles due to the rapid formation of secretion. They produce the hydrochloric acid which is found in the gastric juice.

In ordinary preparations they are better preserved than the chief cells, and exhibit a finely granular structure, being deeply stained with the anilin protoplasmic dyes. They differ so markedly from the chief cells that they have been erroneously believed to develop from the surrounding tunica propria. As seen in Fig. 245 they occur chiefly along the body of the tubule, being infrequent at its fundus.

Intercellular secretory capillaries.

Chief cells./



The pyloric glands are found near the pylorus, but the area which they occupy is not sharply set off; they pass over into gastric glands through a "transition zone." Pyloric glands have very deep pits, from which short, winding, branched tubules grow out. Their form in the adult is shown in Fig. 248. The cells in the pits are mucous cells, and those in

Simple epithelium cut obliquely, so that it appears to be stratified.

Tunica propria.

Pyloric gland.

Sections of pyloric glands.

Solitary nodule.

Muscularis mucosae.


the tubules are also regarded as mucous cells. The latter are columnar, with rounded nuclei in their basal part, and protoplasm which may closely resemble that of the chief cells. Parietal cells are occasionally found, and such cells have been reported in the duodenal glands and in the superficial glands of the oesophagus. Slender dark cells, apparently due to com





pression, are found in the pyloric glands of the dog. In certain respects the pyloric glands are transitional between gastric and duodenal glands.

The tunica propria consists of the small amount of reticular and connective tissue which is found between the closely packed glands and immediately beneath them (Fig. 249). It is sufficient to support the numerous capillaries branching about the glands, the terminal lymphatic vessels and nerves, numerous wandering cells and a few vertical smooth muscle fibers prolonged from the muscularis mucosae (Fig. 245). The lymphatic vessels begin blindly near the superficial epithelium and pass between the glands into the submucosa where they spread out and are easily seen; they continue across the muscularis and pass through the mesentery to join the large lymphatic trunks. Solitary nodules occur in the gastric mucosa, especially in the cardiac and pyloric regions (Figs. 243 and 248) ; they may extend through the muscularis mucosae into the submucosa. The muscularis mucosae may be divided into two or three layers of fibers having different directions. The submucosa contains its plexus of nerves and many vessels, together with groups of fat cells. Its elastic fibers are said to be abundant toward the pylorus.

The muscular coat of the stomach consists of three layers of smooth muscle, an outer longitudinal, middle circular, and inner oblique layer respectively. These layers can be recognized by dissection more readily than by microscopic examination, and were found by Willis in 1674. The middle layer is the one most highly developed. It not only surrounds the body of the stomach, but as the fundus pushes outward, muscle fibers of this layer encircle its apex concentrically. Toward the pylorus, along the antrum, the circular layer gradually thickens, thus forming the sphincter pylori; it becomes abruptly thin in the duodenum. There is no sphincter at the cardia, where the circular layer is continuous 17


Tunica propria.

M uscularis mucosse. ^


Smooth muscle cut lengthwise.

Connective tissue.

Smooth muscle cut transversely.


FIG. 249. VERTICAL SECTION OF THE WALL OF A HUMAN STOMACH. The tunica propria contains glands standing so close together that

its tissue is visible only at the base of the glands toward the

muscularis mucosae.


with that of the oesophagus, but elastic tissue in the muscularis is said to be specially abundant and to "contribute to the tonus of the cardiac musculature." The outer longitudinal layer, continuous with the outer layer in the oesophagus and duodenum, is an incomplete layer, being deficient toward the greater curvature. As the body of the stomach bulges outward to form this curvature, the longitudinal fibers apparently become separated into scattered bundles. In the pars pylorica, however, there is a continuous longitudinal layer, and some of its fibers, which become intermingled with those of the sphincter pylori, serve to dilate the pylorus. The innermost layer, composed of oblique fibers, is not represented

in the oesophagus and duodenum, and is said to be absent from the pars pylorica. The peculiar arrangement of its fibers is shown in Fig. 250, in which the outer longitudinal layer has been almost entirely removed, and windows have been cut through the circular layer; the oblique fibers are seen against the submucosa.

& L/ TiiSl'l/^M Al^UMF* The y form a longitudinal strand par allel with the lesser curvature, and they pass from one side of the ' OF THE S !TOMACH. (Spaitehohs!" * stomach to the other across the notch

a X;^ n o&^aytr7 r ^/pyior^sfs^ between the oesophagus and fundus.

These fibers are important in the

activity of the gastric canal, but they do not produce the canal as some have supposed. From these longitudinal bundles, fibers curve obliquely toward the greater curvature, where, as transverse fibers they cross to the opposite side. Thus the musculature of the stomach is so arranged that it is very difficult to determine the plane of section in a small piece of gastric mucous membrane, which is usually cut obliquely; but the section shown in Fig. 249, with inner and outer layers cut lengthwise and a middle layer cut across, is consistent with a longitudinal section of the corpus gastri.

The tunica serosa consists of connective tissue with well-developed elastic nets, and a covering of peritoneal epithelium interrupted only along the curvatures, at the mesenteric attachments. It contains the nerves and vessels which supply the stomach. The right and left vagus trunks descend beside the oesophagus as the main stems in a plexiform network, and then come together along the lesser curvature. From there they send plexiform branches over both sides of the stomach, and the main stems continue into the small intestine. Sympathetic nerves from the coeliac plexus pass to the pyloric end of the stomach and join the vagus



plexus. The further distribution of the nerves in myenteric and submucous plexuses is similar to that in the small intestine.


The duodenum contains branched mucous glands, the bodies of which are found in the submucosa. These are called duodenal glands (B runner's

Intestinal gland Epithelium. Villi.

Duodenal gland. /'Plica, circularis.

Fat. Duodenal glands in the submucosa.

Tunica propria


mucosas. ' Submucosa. r

Stratum of \ '

circular muscle. t-.

Stratum of longi- g tudinal muscle. ~.

Connective tissue .

Intestinal glands

Longitudinal section.


glands) and they occur nowhere else in the small intestine (Fig. 251). Their cells produce a mucus which stains with difficulty, thus contrasting with the mucus of the goblet cells in the tubular glands above them. The nature of their epithelium is shown in Fig. 252, which shows also that a portion of their tubules may lie above the muscularis mucosae, in the tunica propria. As in the pyloric glands, occasional parietal cells have been found, and also the dark cells, due to compression. Secretory capillaries extend out from the lumen between the cells, and the tubules are provided with a structureless basement membrane. The ducts of the duodenal glands may open on the free surface of the epithelium, or into the lower ends of the tubular pits situated in the mucous membrane and known as intestinal glands. The duodenal glands are so numerous toward the stomach that the submucosa may be filled with their tubules. They are also abundant near the duodenal papilla where the

Transverse section.

Longitudinal section.

of the tubules of a duodenal gland.


DUODENUM. X 240. Only the lower half of the mucosa and

upper half of the submucosa are




bile and pancreatic ducts enter the descending portion of the duodenum. Beyond this point they become fewer, and disappear before the end of the duodenum is reached. Except for these glands the duodenum is essentially like the remainder of the small intestine, described in the following section.


The lining of the small intestine, including the duodenum, has a velvety appearance, due to the presence of innumerable cylindrical, club-shaped or foliate elevations, known as mill (hairs or nap). True villi are found in the large intestine of the embryo but they disappear before birth; they are said to occur also in the pyloric end of the stomach, but it is questionable whether these are typical villi or merely irregular folds. Elsewhere in the digestive tube, villi are absent. At the bases of the villi there are simple tubular pits of glandular epithelium, which extend to the muscularis mucosse but do not penetrate it; these are the intestinal glands (glandul(B intestinales, formerly known as crypts of Lieberkiihn). An enlarged

^OV^ry* V :

$%i$m : to

FIG. 253.

A, Surface view of the hardened mucosa of the small intestine (after Koelliker). B, Side view of a wax reconstruction of the epithelium in the human duodenum (Huber). i. g., Intestinal gland; v., villus.

surface view of the hardened mucous membrane is shown in Fig. 253, A The orifices of the glands appear as round holes; the villi, which are from 0.2-1.0 mm. in height, have fallen over in various directions. Within the duodenum the villi are low leaf-like folds, 0.2-0.5 mm. high, seen in side view in the reconstruction, Fig. 253, B. Their shape cannot be determined from inspecting single sections (cf. Fig. 251).

It will be seen that villi are essentially circumscribed folds, and they have been said to arise through the subdivision of longitudinal ridges (Berry, Anat. Anz., 1900, vol. 17, pp. 242-249). According to Johnson (Amer. Journ. Anat., 1910, vol. 10, pp. 521-561) they develop as low knob-like elevations which increase in height. They may become subdivided, as indicated by bifid villi (Fig. 253).

The small intestine contains other elevations of its lining which are much larger than the villi. These are the circular folds (plica circulares,



formerly known as Kerkring's valvula conniventes), which are seen conspicuously on opening the intestine. They are thin leaf -like membranes, in places very close together, which, as their name implies, tend to encircle the tube. Sometimes they form short spirals, and they may branch and connect with one another. They begin in the duodenum, and beyond the duodenal papilla they are tall and close together. They are highly developed in the jejunum and form its most characteristic feature. In the ileum they are lower and further apart; and they may come to an end two feet above the colon. The villi correspondingly are taller and




Plica circularis.

Intestinal glands


Circular muscle.

FIG. 254. VERTICAL LONGITUDINAL SECTION OF THE JEJUNUM OF AN ADULT MAN. X 16. The plica circularis on the right supports two small solitary nodules, which do not extend into the submucosa; one of them exhibits a germinal center, x. The epithelium is slightly loosened from the connective tissue core of many of the villi, so that a clear space, xx, exists between the two. The isolated bodies lying near the villi (more numerous to the left of the plicaj circulares) are sections of villi that were bent, so that their ends were cut off in sectioning.

more numerous in the jejunum than in the ileum, in the distal part of which they are short and scattered, finally disappearing on the colic surface of the valve of the colon (ileo-csecal valve). Thus few and short villi and scattered plicae indicate that a section of the intestine is from the ileum. As seen in sections, the plica circulares are elevations of the submucosa (Fig. 254) covered on both sides by the entire mucous membrane villi, glands and the muscularis mucosae. A low plica of the duodenum is shown in Fig. 251.



The glands, villi, and plicae have usually been regarded as permanent structures, serving to increase the secreting and absorbing surfaces of the intestine. In mammals they apparently are not obliterated by the normal distention of the intestine, although the villi may become shorter, the glands shallower, and the plicae may be partially taken up like the folds of the oesophagus. In the guinea-pig, and to some extent in the rabbit and cat, Heitzmann found that the villi change their shape with the intes


(Johnson.) A, Strongly contracted; B, normally distended with food; C, distended with a pressure of 150 cm. of water.

tinal contractions and expansions associated with its physiological activity. Johnson (Amer. Journ. Anat., 1913, vol. 14, pp. 235-250) has shown that in guinea-pigs the villi and glands of the contracted intestine have the form seen in Fig. 255, A; with normal distention due to abundant food, they appear as in B ; and with extreme artificial distention, the glands and villi are nearly obliterated as in C. The tube expands to this limit, beyond which additional pressure has no effect until it ruptures. On releasing the pressure, glands and villi return to their normal size. Interesting questions are suggested, as to how the muscle fibers become rearranged in the thin layer when the intestine is distended, and what takes place in the blood and lymphatic vessels. These problems are under investigation.

Finer Structure of the Glands and Villi. At the blind lower end or fundus of the glands, there occur certain cells containing many coarse granules in that part of their protoplasm which is toward the lumen (Fig. 256). These cells were first described by Paneth (Arch. f. mikr. Anat., 1888, vol. 31, pp. 113-191) and are known as Paneth's cells. They are found in the glands of the duodenum, jejunum and ileum, but not in those of the large intestine. Although they may be observed with ordinary stains, they are more strikingly demonstrated in iron-haematoxylin preparations. Apparently they produce a special secretion, which enters the lumen of the gland in the form of fine granules when the digestion of fat



is taking place, and may perhaps be concerned also with protein digestion but not with that of carbohydrates (Miram, Arch. f. mikr. Anat., 1912, vol. 79, pp. 105113). They do not contain mucinogen granules, although goblet cells occur in their immediate vicinity.

A short distance above the fundus, the epithelial cells of the glands exhibit mitotic figures. From this it is inferred that the outer cells, including those of the villi, are renewed from below. The cells near the bottom of the gland have terminal bars, but they are not as distinct as those of the villi.



a, Cell in mitosis; b, lymphocyte; c, Paneth's cell;

d, goblet cell.

During division, the cell seems to be

drawn up from the basement membrane, as if held in position by the


Tunica propria.

Portion of a capillary blood vessel.


Nucleus of a lymphocyte.

Tangential section of a goblet cell.

Mucus in a goblet cell.

Nucleus of a smooth muscle fiber. Central lymphatic vessel.

FIG. 257. LONGITUDINAL SECTION THROUGH THE APEX OF THE VILLUS OF A DOG. X 360. The goblet cells contain less mucus as they approach the summit of the villus.

terminal bars (Fig. 256, a). The plane of division is at right angles with the long axis of the gland (as shown on the right of Fig. 256), and after





A KITTEN SEVEN DAYS OLD. X 250. The epithelium on the left contains many wandering

leucocytes (lymphocytes). The epithelium on the right

contains but three.

mitosis the nuclei move back to the basal layer. Lymphocytes which have made their way between the epithelial cells (Fig. 256, b), are frequently seen, and when near the lumen and over-stained they may be mistaken for mitotic figures.

The sides of the glands and surfaces of the villi are covered with simple columnar epithelium, similar to that shown in Fig. 256. It contains goblet cells separated from one another by cells free from mucus. The

cells of the villi are taller than those in the glands, and the goblet cells are somewhat larger, but toward the tip of the villus they become slender and empty (Fig. 257). The top plates or cuticula become thicker from the fundus of the gland outward to the tips of the villi, and when well developed they exhibit vertical striations which are considered to be protoplasmic processes lodged in pores. The top-plate of the goblet cells is thin and apparently ruptures to allow the escape of the mucus. Lymphocytes may enter the epithelium in abundance as shown in Fig. 258.

Interest in the villi centers chiefly in their relation to the absorption of nutritive material from the intestinal contents (chyme). Fat, chemically changed so that it does not blacken with osmic acid, is conveyed through the cuticula. Within the epithelial cells it forms characteristic fat droplets, which appear in abundance also between the epithelial cells. Lymphocytes ingest the droplets, and may then enter the lymphatic vessel in the central axis of the villus (Fig. 257), but apparently fat is conveyed to the lacteals also through intercellular spaces, without the intervention of leucocytes. Within the lymphatic vessel it forms the milky lymph known as chyle.

In regard to the absorption of protein material, the observations of Pio Mingazzini, which have been confirmed by some and denied by others, are of considerable interest. As shown in Fig. 259, he found that the basal protoplasm of the resting epithelium presented an ordinary appearance (A), but that after absorption had progressed, hyal ine spherules appeared iu it (B). As these became numerous they were detached from


A and D, The states of repose preceding and following the process, s., Spherules.



the cells, forming a reticular mass between them and the tunica propria (C). After the spherules had broken down and had probably been transferred to the blood vessels, the tunica propria entered into its usual relation with the shortened epithelium (D). The basal protoplasm was then restored. According to this interpretation protein absorption is accomplished as a secretory process of the epithelium, the product being eliminated from its basal portion. The spherules accumulate at and near the tips of the villi, in spaces which many authorities describe as due to the artificial retraction of the tunica propria (Fig. 260, a). The spherules have been considered a coagulum of the fluid squeezed from the reticular tissue. In part they may be boundaries of the basal ends of epithelial cells on the distal wall of the villus.

Sections of villi.


Muscularis macosae.

~ Submucosa. Intestinal glands. Oblique sections of intestinal glands.

PIG. 260. VERTICAL SECTION OF THE Mucous MEMBRANE OF THE JEJUNUM OF AN ADULT MAN. X 80, The space, a, between the tunica propria and the epithelium of the villus is perhaps the result of the shrink.

ing action of the fixing fluid. At b the epithelium has been artificially ruptured. The goblet cells

have been drawn on one side of the villus on the right.

Outer layers of the small intestine. The tunica propria, which forms the cores of the villi and extends between the glands, is a reticular tissue, containing the usual types of free cells and also a large number of plasma cells (see p. 68). Slender strands of smooth muscle extend up and down the villi, being inserted into the reticulum, and by contraction they cause the villi to shorten. The muscularis mucosa consists of an inner circular and an outer longitudinal layer, thus duplicating on a small scale the tunica muscularis. The submucosa is a connective tissue layer, such as has been described in the stomach and oesophagus, and the muscularis is divided into a thick inner circular layer of smooth muscle and a thinner outer longitudinal layer, between which is a thin stratum of intermuscular connective



tissue. The intestine is covered externally by the tunica serosa. The distribution of the vessels and nerves in these layers is as follows.

Blood vessels. The arteries pass from the mesentery into the serosa, in which their main branches tend to encircle the intestine. Smaller branches from these pass across the muscle layers to the submucosa, in which they subdivide freely (Fig. 261, A). In crossing the muscle layers they send out branches in the intermuscular connective tissue. These and the arteries of the serosa and submucosa supply the capillary networks found among the muscle fibers. The capillaries are mostly parallel with the muscles. From the submucosa the arteries invade the mucosa, form


FIG. 261.

A f Diagram of the blood vessels of the small intestine; the arteries appear as coarse black lines; the capillaries as fine ones, and the veins are shaded (after Mall). B, Diagram of the lymphatic vessels (after Mall). C, Diagram of the nerves, based upon Golgi preparations (after Cajal). The layers of the intestine are m., mucosa; m. m., muscularis mucosae; s. m., submucosa; c. m., circular muscle; i. c., intermuscular connective tissue; 1. m., longitudinal muscle; s., serosa. c. 1., central lymphatic; n., nodule; s. pi., submucous plexus; m. pi., myenteric plexus.

ing an irregular capillary network about the glands, and sending larger terminal branches into the villi. There is usually a single artery for a villus, and it has been described as near the center, with the veins at the periphery (Fig. 261), or sometimes on one side of the villus with the vein on the other. The network of blood vessels in the villi is very abundant as shown in Fig. 262. The veins branch freely in the submucosa and pass out of the intestine beside the arteries. The muscularis mucosae has been described as forming a sphincter for the veins which penetrate it; thus it may control the amount of blood within the villi. No valves occur until the veins enter the tunica muscularis; there they appear, and continue into the collecting veins in the mesentery. They are absent from



the large branches of the portal vein which receive the blood from the intestines.

Lymphatic vessels. The intestinal lymphatics (lacteals) appear as


Tunica propria.

Muscularis mucosae. Submucosa.

FIG. 262. VERTICAL SECTION OF THE Mucous MEMBRANE OF THE HUMAN JEJUNUM. X so. The blood vessels are injected with Berlin blue. The vein of the first villus on the left is cut transversely.


Intestinal glands.


Muscularis mucosae. Lymph nodules.

Circular Longitudinal layer. layer.

of the muscularis.

FIG. 263. TRANSVERSE SECTION OF AGGREGATE NODULES OF THE SMALL INTESTINE OF A CAT. The crests of four nodules were not within the plane of the section. X 10.

central vessels within the villi (Fig. 261, B). Each villus usually contains a single lacteal ending in a blind dilatation; sometimes there are two or three which form terminal loops. In some stages of digestion the disten



tion of these lymphatics is very great and their endothelium is easily seen in sections. When collapsed they are hard to distinguish from the surrounding reticulum. Small lateral branches and a spiral prolongation of the central lymphatic have been found by injection, but these may be tissue spaces into which the injected fluid has been forced. The lymphatics branch freely in the submucosa and have numerous valves. They cross the muscle layers, spreading in the intermuscular tissue and the serosa, and pass through the mesentery to the thoracic duct.

Lymphoid tissue. The lymphoid tissue of the intestine occurs primarily in the tunica propria, and in three forms diffuse lymphoid tissue, solitary nodules, and aggregate nodules. Solitary nodules are seen in Fig. 254. The nodules are surrounded by small vessels, the lymphatics being


FIG. 264.

A, Surface view of the plexus myentericus of an infant. X 50. g. Groups of nerve cells; r, layer of circular muscle fibers recognized by their rod-shaped nuclei. B, Surface view of the plexus submucosus of the same infant. X So. g, Groups of nerve cells; b, blood vessel visible through the overlying tissue.

drawn in Fig. 261, B. Blood vessels may make a similar net, and penetrate the outer portion of the nodule. The germinative centers are similar to those in the lymph glands.

Aggregate nodules (Peyer's patches) are oval areas, usually from i to 4 cm. long but occasionally much larger, composed of from ten to sixty nodules in close contact (Fig. 263). The nodules may be distinct or blended in a single mass. They distort the intestinal glands with which they are in relation, and immediately above the nodules the villi are partly or wholly obliterated. Thus they appear as dull patches in the lining of the freshly opened intestine, and may be readily seen. There are from fifteen to thirty of them in the human intestine (rarely as many as fifty or sixty) and they occur chiefly in the lower part or the ileum on the side



opposite the mesentery. A few occur in the jejunum and the distal part of the duodenum. In the vermiform process, diffuse aggregate nodules are always present, but they do not occur elsewhere in the large intestine.

Nerves. The small intestine is supplied by prolongations of the vagus nerves, which are joined by branches of the superior mesenteric plexus of the sympathetic system. The latter are regarded as the principal supply. This plexus is ventral to the aorta, and sends branches through the mesentery into the serosa. The manner in which they penetrate the other layers, forming the myenteric plexus (Auerbach's plexus) between the circular and longitudinal muscle-layers, and the submucous plexus (Meissner's plexus) in the submucosa, is shown in Fig. 261, C. In surface view, obtained by stripping the layers apart, these plexuses are seen in Fig. 264. Their branches supply the smooth muscle fibers. From the submucous plexus the nerves extend into the villi, where nerve cells have been detected by the Golgi method (Fig. 261, C); it has been suspected, however, that some of these "nerve cells" are portions of the reticular tissue. The nerve fibers probably terminate in contact with epithelial cells and provide for local reflex action, whereby the muscles contract in response to stimulation of the epithelium. Most of the intestinal nerves are non-medullated, but they include a few large medullated fibers said to have free endings in the epithelium.


The serous membrane which surrounds the intestinal tube and certain other abdominal viscera is a part of the lining of the body cavity. Its general relations are shown in the diagram, Fig. 265. After covering the ventral surface and the sides of the intestinal tube, the two layers of serous membrane come together to form the mesentery and extend to the dorsal body wall; then, separating, they pass laterally as the lining of the abdominal walls and again come together in the midventral line. This serous membrane, or peritoneum, consequently forms a closed sac. It is divisible into the visceral peritoneum which covers the viscera, and parietal peritoneum which lines the body walls. In all cases its free surface is covered with a single layer of flat polygonal cells, resembling endothelium (Fig. 266, B). Although quite flat, the cells have a thin cuticular border which

is said to be striated, and the cuticulae of adjacent cells fit together closely. The lateral walls of these flat cells are connected with one another by proto


a., Aorta; c. p., cavity of the peritoneum; int., intestine; mes., mesentery; p. m. and v. m., parietal and visceral layers of mesothelium.



plasmic bridges; thus in passing through the epithelium along the intercellular boundaries, one or two intercellular vacuoles would be encountered (Fig. 266, A). Wandering cells pass readily across this epithelium, between the cells, and substances in the peritoneal cavity are taken up into the subserous lymphatics. It has long been thought that there are permanent orifices or "stomata" between the epithelial cells (Fig. 266, B), bounded either by modified protoplasm or by separate small cells, and that lymphatic vessels open directly into the serous cavity through such stomata. This is contrary to recent investigations of the nature of lymphatic

vessels, and the existence of stomata as permanent apertures has been denied. The stomata, so frequently found in a great variety of animals may be shrinkage effects caused by reagents, but their interpretation is not clear. In any case, the transfer of material through the epithelium takes place readily, and the substances or cells which pass through may be taken up freely by the closed lymphatic vessels in the underlying tissue.

In the mesentery, a thin layer of connective tissue with elastic networks and interwoven bundles of white fibers fills the interval between the two epithelial layers. In this connective tissue there are many lymphatic and blood vessels, and nerves to the various organs. Mast cells may be found along the vessels, especially in young animals (Fig. 55, p. 68) and various other forms of wandering cells occur. The connective tissue layer is denser in the parietal than in the visceral peritoneum. In places where the peritoneum is freely movable there is a subserous layer of loose fatty tissue, but there is no subserous layer in the intestine.


A, Vertical section of the epithelium (after Heidenhain); B, Surface view, showing two stomata (after Ludwig).


The vermiform process is a "worm-like" prolongation of the caecum. Although small in size, in structure it more closely resembles the large intestine, of which it is a part, than the small intestine. In embryos of three and one-half to five months it is lined with villi, but with further development the villi flatten out and disappear. Meanwhile the glands, which are of the same type in both small and large intestines, have developed and are increasing in number and in length. Sometimes they pene


trate the muscularis mucosae. In the adult (Fig. 267) they are simple tubes, occasionally forked, thus indicating the way in which they multiply in the embryo. As early as the fourth month, lymphoid tissue has been found in the vermiform process, and at birth the lymphoid nodules in the tunica propria are abundant and more or less confluent. The great development of lymphoid tissue is the most important histological feature of the vermiform process in the adult (Fig. 267). It may invade and partly


Note the absence of villi and the abundance of nodules. Clear spaces in the submucosa are fat cells. Only

a part of the circular layer of the muscularis has been drawn.

break up the muscularis mucosae, and extend into the submucosa. The latter, together with the inner circular and outer longitudinal muscle layers, and the serosa, are similar to the corresponding layers of the small intestine, already described.

During the fifth month of embryonic life, Stohr has found an interesting normal form of degeneration in the glands of the vermiform process (Arch. f. mikr. Anat., 1898 vol. 51, pp. 1-55). The tunica propria around them appears to thicken, and the goblet cells in the neck of the degenerating gland, after becoming flattened, produce a solid strand. The strand then ruptures and the detached fundus becomes cystic. Subsequently it shrinks to a small nodule surrounded by dense connective tissue, and ultimately disappears. This degeneration is said to be limited to the fifth and sixth months.

The lumen of the normal vermiform process in the adult, when empty, is thrown into folds, between which are deep pockets; but the normal con



dition is found in scarcely 50% of individuals over forty years of age (Stohr). Often the lumen is narrowed or even obliterated. The epithelium with its glands and the lymphoid nodules then disappear, and are replaced by an axial mass of fibrous tissue. This is surrounded by the unaltered submucosa and muscularis; the serosa may show the results of inflammatory conditions.


The human caecum and colon contain villi only in the embryo. These villi disappear at about the sixth month. The production of new cells does not keep pace with the expansion of the epithelial tube, and the villi




'. ':' :-':;:. >:* :.'.'


?/% ^Miijf

-Tunica propria.

Fat cells

Solitary nodule with germinal center.


MAN. x 80.

The fat has been blackened with osmic acid. Compare the length of the glands with those of the small intestine (Fig. 260), from the same individual and drawn under the same magnification.

therefore gradually flatten and disappear. In the parts of the embryonic intestine distended with secretions and desquamated cells (constituting the meconium), the villi disappear earlier than in the contracted portions (Johnson).

After the villi have gone, the mucosa contains only tubular pits or glands, lined with simple columnar epithelium (Fig. 268). These glands are similar to those in the small intestine but are longer sometimes twice as long (0.4-0.6 mm.). They contain more goblet cells, but cells



of Paneth are absent. Striated cuticular borders appear near the outlets of the glands, and are well developed upon the columnar cells lining the intestinal lumen. Solitary nodules are numerous, especially in the caecum. They may extend through the muscularis mucosae and expand in a flask-shaped manner in the submucosa (Fig. 268) ; in peripheral sections of such a nodule the stalk by which it joins the tunica propria may not be included, and the area of lymphoid tissue may seem to be wholly in the submucosa. The latter is a connective tissue layer like that of the small intestine.

The tunica muscularis of the colon and caecum has a characteristic arrangement not found in the vermiform process. The longitudinal smooth muscle fibers of the outer layer become gathered into three equidistant longitudinal bands or tanicz (Fig. 269); between them the longitudinal fibers form a thin layer which may be interrupted. The taeniae come together at the root of the vermiform process and are continuous with its outer muscle layer. Since the longitudinal muscle layer does not elongate as rapidly as the parts within it, the inner layer of circular smooth muscle, together with the mucosa and submucosa, become thrown into a succession of transverse crescentic folds or plica semi- ,


lunar es. The horns of the crescents are op- 2SSS" fiS^AM? cSSJiTcSf. posite the taeniae. Between the semilunar (After sobotta.)

h., Haustra; t., taema.

folds the wall of the large intestine bulges

outward, forming the haustra (Lat., buckets) as shown in Fig. 269. The valve of the colon (valvula coli] is a pair of folds or labia, which resemble the semilunar folds; that is, they include fibers of the circular muscle layer, but the layer of longitudinal fibers passes directly from the ileum to the colon without entering the valves. The serosa of the colon contains lobules of fat which form pendulous projections known as appendices epiploica.



The rectum is divided into two parts, an upper which extends from the third sacral vertebra to the pelvic diaphragm, and a lower which continues downward to the anus. The lining of the first part is thrown into several folds, the plica transfer sales recti (valves of Houston). These are large semilunar folds which usually extend only part way around the rectum, but they have been described in some cases as having a spiral arrangement. The second part of the rectum, the pars analis recti (anal canal),




presents on its inner wall a number of longitudinal folds, known as rectal columns (columns of Glisson or Morgagni). At their lower extremities the columns unite with one another, thus forming small transverse plicae

Rectal gland

Linea ano-rectalis

Zona columnaris

Linea'sinuosa analis

Zona intermedia.

Linea ano-cutanea.

Circular layer of smooth muscle.

Longitudinal layer of smooth muscle.

Levator ani.

Internal sphincter. Intramuscular gland.

External sphincter.

-Sheath of a hair. -Sebaceous gland.

Zona cutanea.

FIG. 270. LONGITUDINAL SECTION THROUGH THE PARS ANALIS RECTI. From a human embryo of 187 mm. (about four months). (F. P. Johnson.)

or anal valves. The grooves between the columns extend downward behind the valves, forming a series of blind pockets, the sinus rectales.

The mucous membrane of the first part of the rectum is similar to that of the colon, but its glands are somewhat longer (0.7 mm.). Soli


tary nodules are present. The muscularis mucosae, submucosa, and circular layer of smooth muscle also resemble those of the colon, but the three taeniae spread out and unite so as to form a continuous layer of longitudinal muscle. In the upper part of the rectum this layer is specially thickened dorsally and ventrally. As the rectum loses its mesentery, the tunica serosa is replaced by adventitious connective tissue.

The pars analis recti is the region of transition from mucous membrane to skin. This transition is not gradual but takes place in three steps, thus forming three distinct superimposed zones. From above downward these are the zona columnaris, zona intermedia, and zona cutanea (Fig. 270). The last, however, does not belong to the pars analis, properly speaking, but to the outside skin.

The zona columnaris is the region of the rectal columns, but these are not always limited to this zone. They may extend upward into the first part of the rectum for a short distance, and they may also be continuous downward with the so-called anal skin folds. In the upper part of the zona columnaris the simple columnar epithelium of the superior portion of the rectum becomes two- or three-layered. Its outer cells are columnar, with finely granular protoplasm. The transition takes place gradually at the linea ano-rectalis . In the upper part of the zone there are usually a few intestinal glands containing numerous goblet cells, and a few goblet cells are found also in the surface epithelium. In the lower part of the zona columnaris, arising from the rectal sinuses, there are a few branched tubular gland-like structures, the intra-muscular glands (Fig. 270). There are seldom more than six or eight in any one rectum. The main ducts of these glands extend outward, and usually downward, and penetrate the internal circular muscle (internal sphincter). Here a flaskshaped swelling is usually met with. Extending beyond this ampulla there are several tubular branches which continue through the internal sphincter and end blindly in the intra-muscular connective tissue. Occasionally a tubule is seen piercing the longitudinal muscle layer. Around the terminations of the tubules, which are sometimes swollen, there is a small amount of lymphoid tissue. The epithelium lining the main ducts of these glands consists of several layers of polygonal cells, but the ampullae and branches are lined with one or two layers of cuboidal cells. Secretory cells are present in the embryo and at birth, but are apparently wanting in the adult.

The transition between the zona columnaris and zona intermedia is marked by a rather abrupt change in the epithelium, which becomes many layered and squamous. This transition takes place at the level of the anal valves, but between the valves it extends upward on the rectal columns. Thus it follows a zig-zag line, the linea sinuosa analis (ano-cutaneous line of Hermann). Within the zona intermedia the epithelium, com


posed of several layers of polygonal cells, is thicker than the epidermis. Dermal papillae are present, but hairs and sweat glands are absent. In the lower part of this zone there are a few isolated sebaceous glands without hairs, and the epithelium is slightly cornined. Thus it gradually goes over into skin, forming a true linea ano-cutanea, but this line is not well marked. It has been denned as the place where the first sheaths of the hairs appear.

The skin immediately surrounding the anus forms the zona cutanea. Sweat glands are absent from the region bordering on the anus, but at a distance of 1.0-1.5 cm. there is an elliptical zone, 1.25-1.5 cm. wide, containing simple tubular coiled glands, the circum-anal glands of Gay. These are very similar to sweat glands but are considerably larger.

The outer layers of the pars analis recti include a very vascular tela submucosa, which contains numerous nerves and lamellar corpuscles. The muscularis mucosae terminates in slender longitudinal bundles which extend for varying distances into the rectal columns (forming the M. dilatator ani internus of Riidinger). The circular layer of the tunica muscularis becomes thickened at its termination, forming the M. sphincter ani internus; it extends a little below the the linea sinuosa analis. Beyond the internal sphincter, which is composed of smooth muscle, striated muscle fibers surround the anus forming the M. sphincter ani externus. The outer longitudinal layer of the tunica muscularis ends in relation with connective tissue strands which diverge as they pass downward through the external sphincter, to terminate in the subepithelial tissue of the zona cutanea.


The liver first appears in human embryos of about 2. 5 mm. as a diver ticulum of the ventral wall of the fore-gut, near its junction with the yolk-sac. If the embryo is placed in an upright position (Fig. 271, A) the liver is seen to be below the heart, and between the vitelline veins as they pass from the yolk-sac to their cardiac termination. The diverticulum projects into a mass of mesoderm, to which His gave the old anatomical term for diaphragm, namely septum transversum. The diaphragm develops in the anterior or upper part of this septum; the lower or posterior part constitutes the ventral mesentery, which extends from the fore-gut to the ventral body wall. The hepatic diverticulum is in the mesenteric part of the septum, although it is always connected with the overlying diaphragmatic shelf.

Very early the liver becomes divided into two parts, (i) the somewhat rounded diverticulum proper, lined with columnar cells with pale proto



plasm, and (2) a mass of anastomosing cords or trabeculae, composed of deeply staining cells with round nuclei and abundant granular protoplasm. These two parts are so unlike in appearance that they have been thought to proceed from different germ layers, the trabeculae being described as formed from mesenchyma in the septum transversum. This opinion is erroneous; the entire structure is entodermal, and the trabeculae grow out from the diver ticulum. They encounter the vitelline veins, which ramify around them, producing the lacunar vessels or sinusoids already described (Fig. 160, p. 167).

In an embryo of io-i2mm. (Fig. 271, B), the hepatic diverticulum has elongated and is connected with the mass of anastomosing trabeculae at several points. It shows also some detached ducts and round knob-like

FIG. 271. DIAGRAMS OF THE DEVELOPMENT OF THE LIVER. A, From a 4.o-mm. human embryo. B, From a 12-mm. pig. C, The ducts in the human adult.


Cystic duct; c. p., peritoneal cavity; d., duodenum; d. c., ductus choledochus; dia., diaphragm; div., distal end of the diverticulum; f. 1., falciform ligament; g. b., gall bladder; g. o., greater pmentum; h. d., hepatic duct; ht., heart; int., intestine; li., liver; 1. o., lesser omentum; m., mediastinum; oe., oesophagus; p. c., pericardial cavity; p. d., pancreatic duct; ph., pharynx; p. y., portal vein; s. t., septum transversum; St., stomach; tr., trabecula; v. c. i., vena cava inferior; v. v., vitelline vein;;y. s., yolk-sac

swellings. The vitelline veins have given rise to the portal vein, which enters the liver from below and breaks up into sinusoids among the trabeculae. These reunite, and leave the liver above as the hepatic vein, which was originally a part of the vitelline veins. In the lo-mm. embryo the circulation of the liver is wholly venous. The trabeculae consist of cells which are doubtless very active, taking up and transforming material received from the blood, but it may be questioned whether bile is secreted at this stage, since no complete system of ducts has been demonstrated.

In later stages the mass of anastomosing trabeculae is drained by a system of ducts lined with clear cuboidal or columnar epithelium. These all empty into a single hepatic duct, which represents one of the original connections between the trabeculae and the diverticulum. (In the otter there are said to be as many as seven persistent ducts.) The hepatic duct (Fig. 271, C) is joined by the cystic duct which comes from the tapering pyriform gall bladder (vesica fellea). The latter is perhaps to be re


garded as a special subdivision of the original diverticulum, rather than as its expanded terminal portion. In certain mammals, as in the horse and elephant, the gall bladder is lacking. After the hepatic duct has joined the cystic duct, the common bile duct (ductus choledochus) thus formed proceeds to the duodenum into which it opens, together with the pancreatic duct, at the duodenal papilla. The common bile duct is an elongated portion of the original hepatic diverticulum.

Ligaments of the Liver. At the time of its earliest formation the liver bulges laterally from the ventral mesentery, on both sides, thus forming right and left lobes. The lobes are covered with the peritoneal epithelium.

The mesenchyma beneath this epithelium produces loose connective tissue externally, and a dense fibrous tissue, immediately surrounding the trabeculae, internally; this latter becomes the capsula fibrosa (or capsule of Glisson). The part of the ventral mesentery extending from the intestine to the liver is known as the lesser omentum, and the part between the liver and the ventral body wall is the falciform ligaFIG 272.-THE LEFT SIDE OF AN ment. These lie in the median plane (Fig. 272). nfnfeSf ' Beneath the liver, the peritoneal cavity comes

d. c., Ductus choledochus; g. b., to extend across the median line so that the

gall bladder; I. L, falciform

bladder is covered with peritoneum, except

d p 'iigame P nt7 a v. V c? i r.: along its attachment to the under side of the liver. On the upper surface of the liver, the

original broad connection with the septum transversum becomes relatively narrow dorso-ventrally, and forms a pair of lateral ligaments which pass from the upper surface of the liver to the diaphragm. They extend across the liver at right angles with the falciform ligament and lesser omentum. The left lateral ligament retains these simple relations and is known as the left triangular ligament. The right lateral ligament, except at its tip (the right triangular ligament} , extends down over the posterior surface of the liver as an extensive area of fusion with the diaphragm; this is the coronary ligament (Fig. 275). The significance of this asymmetrical condition will be explained with the veins of the liver.

Development of the veins of the liver. The hepatic trabeculae are always in close relation with the veins which are conveying nutriment to the heart. These are (i) the vitelline veins conveying nutriment from the yolk-sac, (2) the umbilical veins conveying nutriment from the placenta, and (3) the portal vein conveying absorbed food from the intestine. The liver also has important relations with the vena cava inferior.

The portal vein, which is the principal afferent vessel of the adult liver, is derived from the vitelline veins. The latter; as they pass from the yolk-sac into the abdominal


cavity, fuse with one another so as to form a single trunk (Fig. 271, B, r..). On reaching the duodenum, the trunk separates into its components, and they pass into the liver as the right and left vitelline veins (Fig. 273, A). Before entering the liver they anastomose with one another dorsal to the duodenum, as shown in the figure. Thus with the connections between the right and left veins within the liver, two complete venous rings are formed around the intestine. Branches extend out from these rings, notably the superior mesenteric vein which receives blood from the primary loop of intestine, and the splenic vein which not only drains the spleen but receives the inferior mesenteric vein together with pancreatic and gastric branches. The superior mesenteric vein (Fig. 273, s.m.v.) is joined by the splenic (s.) to form the portal vein (p.v.), and the portal vein is a persistent portion of the peri-intestinal rings formed by the vitelline veins. Other parts of the rings atrophy, and as the yolk-sac degenerates and becomes detached, the main vitelline trunk disappears. The portal system of veins is therefore a derivative of the vitelline system; its blood flows through the liver in the vitelline sinusoids.

The formation of the rings as above described takes place with great constancy, and apparently the only variations observed in their atrophy are the two cases described by Begg (Amer. Journ. Anat., 1912, vol. 13, pp. 105-110).

The umbilical veins are at first a pair of vessels, but they early unite in the umbilical cord. The p

single vein thus formed brings the embryonic blood The formation of the portal vein, p. v., back to the body after its excursion to the placenta. 5g SVTSd i^du ]dSfc On reaching the body, the vein divides into right r^^Ma'SB* and left vessels, which are contained in the ventral

body wall, and at first pass directly to the heart; later they anastomose with the vitelline sinusoids in the liver, and the right umbilical vein then atrophies, leaving the left vein to convey the blood to the liver. In Fig. 274, the left vein is larger than the right, and is seen connecting with the hepatic sinusoids. Gradually it shifts from the left side to the median line. It then passes from the umbilical cord to the under surface of the liver along the free edge of the falciform ligament, where, after the umbilical cord has been severed, it degenerates to form the round ligament of the liver (Fig. 275). This extends to the porta or entrance to the liver, where the portal vein goes in and the hepatic duct comes out. Beyond this point the umbilical vein may be followed as the ductus venosus in the embryo, or the ligament of the ductus venosus in the adult, to the vena cava inferior. The ductus venosus may be defined as the channel made by the umbilical vein in passing to the vena cava inferior across the under surface of the liver. It is sometimes completely enfolded by the hepatic trabeculae, and it communicates with the hepatic sinusoids. It follows the line of attachment of the lesser omentum, and empties into the vena cava inferior.

The vena cava inferior apparently does not send much blood into the liver but passes along its dorsal surface. An essential part of this great vein is formed from the hepatic sinusoids. Before the vena cava inferior has developed, the blood in the dorsal body wall flows to the heart through the posterior cardinal veins, one on either side of the aorta. Each posterior cardinal vein shows a ventral subdivision, the right and left subcardinal veins respectively, which are seen in section in Fig. 274. As shown in the figure, the stomach prevents the liver from approaching the dorsal body wall (at the root of the mesentery) on the left, but on the right there is no such obstruction, and the



liver approaches and fuses with the body wall immediately in front of the right subcardinal vein. This fusion constitutes the coronary ligament (cf. Fig. 275) ; and across it, the subcardinal vein anastomoses with the hepatic sinusoids. By a rapid enlargement of this anastomosis, the trunk of the vena cava inferior is formed. It drains the posterior cardinal system of veins, and the outlet of the vitelline veins into the heart becomes the terminal portion of the inferior vena cava; the main vessel from the liver, the hepatic vein, is thereafter described as a branch of the vena cava inferior. The development of the posterior part of the vena cava inferior is described in connection with the Wolffian body (p. 309) ; for a fuller account, see the Amer. Journ. Anat., 1902, vol. i, pp. 229-244. Occasionally the trunk of the vena cava is entirely surrounded by a band of hepatic tissue, as in Fig. 275.

v.c.i. o.b.

f.l. v'um.


ao., Aorta; f. c., fibrous capsule and serosa; f. 1., falciform ligament; g. o., greater omentum; 1. o., lesser omentum; 1. s-c. v., left subcardinal vein; o. b.. omental bursa; r. s-c. v., right subcardinal vein; St., stomach; v. um., left umbilical vein.



c. 1., Coronary ligament; f. 1., falciform ligament; g. b., gall bladder; 1. o., lesser omentum; 1. 1. 1., left triangular ligament; o. b., caudate lobe bounding the omental bursa ventrally; p. v., portal vein; r. 1., round ligament; r. t. 1., right triangular ligament; v. c. i., vena cava inferior.

Lobes of the liver. The structures already described form the boundaries of the lobes of the liver, which in man are few and not sharply marked out. Right and left lobes have already been mentioned as the lateral halves of the liver; they are not separated from one another by any internal septum or indentation of the surface. The left lobe is relatively small, and has a thin margin. It terminates in the appendix fibrosa at the extremity of the left triangular ligament. This appendix represents a portion of the liver from which the hepatic cells have degenerated and disappeared, leaving chiefly the anastomosing ducts. It indicates that in earlier stages the left lobe was more extensively developed. Similar tissue containing aberrant ducts (vasa aberrantia) may be found around the vena cava and in some other parts of the liver. The quadrate lobe is marked out by the porta, the round ligament, and the fossa containing the gall bladder. The caudate lobe is bounded by coronary ligament, lesser omentum and porta. The caudate process of this lobe extends to the right lobe over the foramen epiploicum (of Winslow) between the vena cava and the porta.

The hepatic artery. The liver in an embryo of 10 mm. has no arteries, but at that stage the hepatic artery can be followed to the porta. Later it



extends through the connective tissue around the gall bladder, so that the cystic branch of the adult appears to be the main vessel in the young embryo. Still later, as the connective tissue which surrounds the structures at the porta gradually extends into the liver around the branches of the hepatic duct and portal vein, the hepatic artery sends branches in with it, and they form capillaries which empty into the adjacent portal sinusoids. Branches of the artery ramify also in the connective tissue capsule around the entire liver. The quantity of blood supplied to the liver by the artery always remains much smaller than that brought in by the portal vein, and it is distributed to the connective tissue. There are no vessels between the hepatic cells other than the "capilliform sinusoids" derived directly from the embryonic lacunae of the vitelline veins.


Lobules. A section of the embryonic liver, or of the liver at birth, shows great areas of anastomosing trabeculas, with intervening sinusoids and occasionally a larger vein. In the adult pig the hepatic tissue is arranged in lobules bounded by connective tissue (Fig. 276). These subdivisions were

FIG. 276. LIVER OF A PIG. (Radasch.)

The lobules have artificially shrunken from the interlobular tissue, a; b, bile duct; c, hepatic artery; d, interlobular vein (a branch of the portal); e, trabeculse; f, central vein.

first recognized in the liver of the pig (Wepfer, 1664), and in 1666 Malpighi made the general statement that the entire liver is composed of a multiplicity of lobules. In the dog Mall finds that the lobules are short cylinders



averaging 0.7 mm. high and 0.7 mm. in diameter, and that the entire liver (of 175 c.c.) contains 480,000 of them (Amer. Journ. Anat, 1906, vol. 5, pp. 227-308). There has been prolonged discussion as to whether the lobules should be regarded as centering about the terminal branches of the portal vein or around those of the hepatic vein, for, although it was frequently stated that they were arranged like a bunch of grapes, there was no unanimity as to what formed the stem. If the human liver is examined (Fig. 277) it is seen that the lobules are not definitely marked out as in the pig, but the liver retains to a greater extent its embryonic appearance. Scattered about through the section, but at quite uniform distances from one another, there are islands of connective tissue containing branches of the portal vein, hepatic artery, and bile duct. The strands of connective

Branch of portal vein.

Large interlobular bile duct.

Interlobular connective tissue.

Central veins.

Central vein.


The three central veins in cross section mark the centers of three lobules, which are not sharply separated, at the periphery, from their neighbors. Below and at the right the lobules are cut obliquely and their boundaries are not seen.

tissue which conduct the portal branches were named portal canals by Kiernan (Trans. Roy. Soc. London, 1833, pp. 711-770). If the connective tissue should spread from one canal to another, connecting those nearest together, it would mark out lobules like those in the pig's liver, and this sometimes takes place pathologically in man. Normally the portal canals stand as isolated "boundary stones."

Within each lobule thus marked out there is a central vein or enlarged sinusoid, toward which the capilliform sinusoids between the hepatic trabeculae converge. Occasionally there are two veins, side by side. These central veins empty at right angles into sublobular veins (Fig. 278), which come together to form the main branches of the hepatic vein. All these veins, in contrast with the portal branches, have very little connective tissue around them, and they are not associated with bile ducts or arteries;



thus the hepatic veins are readily distinguished from the portal veins. The flow of the blood (Fig. 279) is from the portal veins (in the portal canals) through the capilliform sinusoids to the central veins, thence

Hepatic lobules.

Interlobular connective tissue.

Central (intralobular) veins.

Sublobular vein.

FIG. 278. FROM A VERTICAL SECTION OF A CAT'S LIVER. INJECTED THROUGH THE VENA CAVA INFERIOR The central veins and the sublobular vein into which they empty are cut longitudinally. X 1 5.

Two bile ducts in cross section.

Capilliform sinusoids.

Central vein.

Interlobular vein (branch of portal).


through the sublobular veins into the hepatic vein, which empties into the vena cava inferior. The arteries empty through capillaries into the capilliform sinusoids adjacent to the portal canals, and there is some


evidence that the hepatic cells at the periphery of the lobule are better nourished than those in its interior.

The recognition of the lobules above described, as the essential basis of hepatic structure, would have been unquestioned except that, as Kiernan stated, " the essential part of the gland is undoubtedly its duct; vessels it possesses in common with every other organ; and it may be thought that in the above description too much importance is attached to the hepatic veins." If the liver were divided into lobules comparable with those of other glands, the portal canals with their ducts and adjacent afferent vessels would be the axial structures, and the efferent central veins would be peripheral. By connecting the five central veins around the portal canal in Fig. 277 (two of the central veins are not labelled and the one at the lower edge of the figure is indistinct) , such a structural unit or secretory unit would be marked out. It has been proposed to call it a portal lobule (from its axial structure), in contrast with the hepatic lobules, which surround the branches of the hepatic vein. In the seal it is said that the portal lobules, or units, are bounded by connective tissue, but this must be regarded as very exceptional. However, in attempting to picture the complex relations of the lobules in the liver, the morphologist must regard the portal canals as axial, even though the term lobule is used for areas surrounding the central veins. The bile flows from parts of several hepatic lobules into a single portal canal.

Parenchyma. The parenchyma or essential tissue of the liver is found in the anastomosing trabeculae of the lobules. The general arrangement of the cells in these trabeculae is shown in Fig. 280, in which, however, the slender lumens are rendered conspicuous by special treatment. These lumens, or bile capillaries, are ordinarily inconspicuous, and the trabeculae appear on superficial examination as solid cords of cuboidal cells, with abundant granular protoplasm and large round central nuclei. Often the hepatic cells contain two nuclei, and large cells with several nuclei, produced by amitosis, have been reported. The general characteristics of hepatic cells are shown in Fig. 281. They are arranged chiefly in double rows which in certain positions appear single.

The hepatic cells have very delicate cell membranes, which are sometimes |aid to be absent. Their protoplasm often contains brown pigment, especially toward the central vein. Near the periphery of the lobule the cells may contain fat vacuoles of varying size, found normally in wellnourished individuals. Pathologically the vacuoles may be large and widely distributed. Glycogen (p. 78) occurs in granules and larger masses, especially after abundant meals. In the fasting condition, the cells are relatively small, dark, and obscurely outlined, but during digestion they become larger with a clearer central part and a peripheral zone of coarse granules. In man both conditions may be found in one liver.



The bile, secreted by the hepatic cells, probably through granule formation, frequently contains granules and fat droplets such as are found within the cells. It is eliminated through the bile capillaries.

The bile capillaries are minute tubes with continuous cuticular walls, presumably formed by the local modification of the cell membranes of two adjacent hepatic cells. The completed capillary, however, shows no

True meshes.

Lateral branches of bile capillaries.

Nuclei of

Sinusoids. Portion of a central vein.


Golgi preparation. The boundaries of the hepatic cells could not be seen. The black dots are precipitates of the silver.

indication of being formed of lateral halves which have fused. Cross sections of the large bile capillaries in the liver of Necturus are shown in Fig. 281, and their arrangement in the human liver is indicated in Fig. 280. They extend through the axis of the two-rowed trabeculae of cells, giving off short intercellular branches at right angles. Thus the bile capillaries shown in Fig. 281 between the two sinusoids, may be separate axial



capillaries, or they may be intercellular branches of an axial capillary which is in the plane of the printed page. In some places the bile capillaries completely encircle an hepatic cell, forming " true meshes" (Fig. 280). They may form larger meshes due to the anastomosis of trabeculae. Occasionally a bile capillary is in relation with three surrounding hepatic cells, or even more, thus resembling the lumen of an ordinary gland-tubule.

In addition to intercellular capillaries there are said to be intracellular branches, several of which may penetrate the protoplasm of a single cell and end in knobs, as shown by the Golgi method. Since neighboring capillaries may be free from these branches, they are regarded as tempo

FIG. 281. SECTION OF THE LIVER OF A SALAMANDER (Necturus). X 380. a, Endothelial cell; b, endothelial reticulum; c, blood vessel; d, bile capillary; e, red corpuscle; f, hepatic cell.

rary phases of functional activity, accompanying the discharge of secretion. They have been reported as forming baskets within the protoplasm, similar to those found in parietal cells of the stomach.

The bile capillaries and their branches are generally separated from the lining of the blood vessels by an appreciable portion of the hepatic cells (cf. Figs. 280 and 281). Pathologically they may extend nearer the vessels and may rupture, so that the bile escapes into the perivascular tissue and is distributed over the body, causing jaundice.

Endothelium and Perivascular Tissue. The endothelium of the capilliform sinusoids which border upon the hepatic trabeculas is specially modified; it is well shown in the coarse-grained liver of Nectunis (Fig. 281), but the same form occurs in the human liver. The endothelial cells, which are phagocytic, produce a network of reticular fibers toward the hepatic cells (Fig. 282). The reticulum contains no elastic elements, and the only cell bodies associated with it are those of the endothelium. In the reticular meshworkin the embryo, erythroblasts multiply in great numbers.



Hepatic trabecula. Blood corpuscles. Reticulum.

and to some extent leucocytes are formed, but in the adult the recticulum is free from cells. The endothelial cells, moreover, do not fit closely together, and are known as the stellate cells of Kupffer. It is probable that, whereas the blood flows through the capilliform sinusoids toward the central vein, there is a current of tissue fluid in the reticulum taking the reverse direction and passing toward the portal canal. This fluid is the source of the great quantity of lymph which flows from the liver.

According to Schafer (Quain's Anatomy, 1912, vol. 2) the blood flowing through the sinusoids comes into direct contact with the liver cells. He states that blood corpuscles may occasionally be found normally within the hepatic cells, into which they are readily forced by injections at low pressure; and he describes canaliculi within the protoplasm of the hepatic cells, which communicate with the sinusoidal blood vessels. These canaliculi are presumably secretory channels or canals of the trophospongium, which have been artificially invaded by the injection. At the same time, the reticulum has been compressed and its significance obscured.

Portal canals. The portal canals are strands of connective tissue extending into the liver from the transverse fissure or porta (which is essentially a hilus). They constitute the interlobular tissue of the liver, and the ducts, arteries, and veins which they contain are often called interlobular. In addition to the structures already considered, the portal canals contain lymphatics and nerves; these and certain features of the ducts require further consideration.

The lymphatic vessels are abundant, forming plexuses around the ducts and blood vessels, and receiving fluid from the perivascular reticulum within the lobules; but no lymphatic vessels enter the lobules. They pass out of the liver at the porta, where lymph glands are found. Certain of the lymphatics in the capsule of the liver drain toward the porta; others enter the diaphragm.

The nerves are chiefly non-medullated fibers from the sympathetic system, but the liver also receives branches from the vagus. These nerves are principally distributed to the blood vessels, but some are said to penetrate the lobules and end in contact with the pehatic cells.




The interlobular ducts are lined with simple columnar or cuboidal epithelium. They anastomose with one another, and have'blind pockets; in the larger ducts, there are branched mucous glands. The connection between the ducts and the hepatic trabeculae is difficult to observe, and it was once thought that the ducts with their ramifications produced the bile, leaving the parenchyma for the function of internal secretion. Through injections, however, or by using the Golgi method, the connections between the bile capillaries and the bile ducts can be readily demonstrated (Fig. 283). They are found at the periphery of the portal canals, and were


Branch of portal vein.

Small interlobular bile-duct, continuing in bile capillaries.

Large interlobular bile-duct.

Branch of hepatic artery.

Bile capillaries. '


Wall of the central vein. FIG. 283. GOLGI PREPARATION OF THE LIVER OF A DOG. X24O.

described histologically by Hering (Strieker's Handbuch, Leipzig, 1871). On the side toward the connective tissue these "canals of Hering," or periportal ducts, exhibit a flat or cuboidal epithelium, like that of ordinary ducts; but toward the lobule they are bounded by hepatic cells, or by flat cells interrupted by hepatic cells (Fig. 284). Thus the hepatic trabeculae are directly inserted into the walls of the ducts, and the bile capillaries connect with the lumen.

The hepatic, cystic and common bile ducts all have a simple columnar epithelium, with occasional goblet cells and branching mucous glands. Around the hepatic duct there is a wide zone formed by the ramifying ducts of these mucous glands, as they extend into the surrounding connective tissue. The connective tissue layer is said to contain many elastic



fibers. It is followed by a tunica muscularis consisting chiefly of circular fibers. These form a sphincter around the common bile duct, at the duodenal papilla. In the cystic duct there are folds of mucous membrane, containing muscle fibers, and forming the "spiral valve."

The gall bladder is lined with a folded mucous membrane covered with tall epithelial cells similar to those of the intestine (Fig. 285). They have elongated basal nuclei and secretory granules (mucin) in the outer part of their protoplasm. The free surface is covered with a distinct cuticular border, and terminal bars have been observed.

Goblet cells are absent and glands are infrequent. The muscularis consists of obliquely circular fibers arranged in a plexiform layer. Among them are groups of sympathetic nerve cells which supply the muscle, and medullated fibers which end in the epithelium. The subserous tissue is highly developed and contains large lymphatic vessels.


b. c., Bile capillary; h. c., hepatic cell; p. d., periportal duct.



Tunica propria


X S6o.

X too. B, the portion x of A


Development and General Features. Although the pancreas in the adult is a single gland, it arises in the embryo as two entirely distinct entodermal outgrowths, known as the dorsal and ventral pancreases respectively. The dorsal pancreas grows out from the dorsal wall of the intestinal tube, a little below the level of the common bile duct in most mammals, but a little above it in man. The ventral pancreas grows down from the common bile duct at its junction with the intestinal tube. As seen in Fig. 286, A and B, the ventral pancreas may be more or less bi-lobed. Usually it grows to the right of the intestine and there meets the dorsal pancreas, which approaches it in close relation with the portal vein. 19

2 go


The left lobe of the ventral pancreas sometimes grows around the left side of the intestine and joins the dorsal pancreas, so that the intestine is encircled by pancreatic tissue (annular pancreas); sometimes it grows out beneath the gall bladder where it ends in a cystic enlargement, as has been observed in adult cats (cf. Amer. Journ. Anat., 191 2, vol. 1 2, pp. 380-400). Usually the left lobe is scarcely indicated. As a rather frequent abnormality, accessory pancreases of small size, but sometimes of very typical

D. ch.

p. d.

L. d.



P d.

L. d.

Pr. v.

D. ch.

L. s.




A, 5.1 mm., B, 6.0 mm. D. ch., ductus choledochus; Int., intestine; L. d., right lobe, and L. s., left lobe of the ventral pancreas; P. d., dorsal pancreas; Pr. v., ventral process of the dorsal pancreas.

structure, are found along the intestine, or even in the wall of the stomach, especially at the constriction between its cardiac and pyloric portions. Such glands may or may not extend through the tunica muscularis.

After the dorsal and ventral pancreases have come in contact, they are related to one another as shown in Fig. 287, A. The dorsal pancreas is

much larger than the ventral pancreas, and it grows across the body toward the left until it reaches the spleen. Thus it gives rise to the body and tail of the pancreas of the adult; and it forms also the ventral part of the head of

. p. d., Accessory pancreatic duct; c. d., cystic duct: d., duodenum; d. c., .-> i j i i /-ii

ductus choledochus; d. p., dorsal pancreas; h. d., hepatic duct; p., duo- trie gland. WlllCn nils denal papilla; p. d., pancreatic duct; St., stomach; v. p., ventral pan- .

crea - the concavity in the

duodenal loop. In

the adult its duct opens into the duodenum 1-3 cm. above the orifice of the common bile duct, but it has been tapped by an anastomosis with the ventral pancreas. Its outlet persists as the accessory pancreatic duct, discovered by Santorini (1775). It is shown in the dissection, Fig. 287, B, but a large




branch ordinarily found descending from it in front of the pancreatic duct, p. d., is not included. In some cases the accessory duct becomes impervious, but it is generally functional, and if the outlet of the main duct were blocked by gall-stones or otherwise, the presence of this accessory duct would be of considerable importance. In some mammals, as in the pig, it is normally the chief duct.

The duct of the ventral pancreas either opens into the duodenum close beside the common bile duct (Fig. 287, B), or it retains its embryonic relation (Fig. 287, A) and opens into the common bile duct near its duodenal orifice. The duct of the ventral pancreas, by an anastomosis with the duct of the dorsal pancreas, becomes the outlet of the main pancreatic duct, which was first figured by Wirsung (1642). It will be noted that a large part of the dorsal pancreatic duct, extending through the body and tail, becomes incorporated in this main duct of Wirsung; the ventral pancreas supplies only its outlet.

In the adult no histological distinction has ever been found between the two pancreases, but although alike in structure and close together, there is no general anastomosis between them. Rarely they remain entirely separate. Usually, on injecting the ducts, only one connection is found between the dorsal and ventral pancreases, but in an abnormal case two connections have been observed. Moreover, anastomoses between the smaller ducts and tubules in the separate glands have not been found in human adults. Rings of pancreatic tissue occur in the embryo, and in adult guinea-pigs Bensley has demonstrated a free anastomosis of the ducts (Amer. Journ. Anat., 1911, vol. 12, pp. 297-388); such a condition has not yet been found in man.

Microscopic structure. As a whole the pancreas somewhat resembles the parotid gland. It is divided into lobes and lobules by connective tissue septa containing blood and lymphatic vessels, nerves, and interlobular ducts (Fig. 288). The lobules are composed chiefly of short tubules, or alveoli, which in models appear pear-shaped; in sections they are cut at all possible angles. Instead of exhibiting a well-defined lumen, the


a, Interlobular connective tissue containing an interlobular duct, c; b, capillary; d, interlobular duct; e, alveoli; f, pancreatic island.



alveoli appear to be clogged with cells, known as centro-alveolar cells (or centro-acinal cells). Irregularly distributed among the alveoli there are round areas of paler cells, peculiar to the pancreas (Fig. 288). ^ They may be at the center or periphery of the lobule, or occasionally in the interlobular connective tissue. These important structures were first described in Langerhans' thesis in 1869 (Inaug. Diss., Berlin), and are known as the pancreatic islands (islands of Langerhans).

The alveoli are composed chiefly of the secreting pancreatic cells (Fig. 289). Toward the lumen their protoplasm contains coarse granules of zymogen, which accumulate while the cell is inactive and are eliminated during secretion. Apparently they are transformed into fluid as they

Blood capillary.

Cells oi the al- ^"W Centro-aveolar eel'

Zymogen granules. A B


In ction A the granules are wanting, the centro-alveolar cells are flat and dark; in section B the granules are distinct, the centro-alveolar cells are cuboidal and clear.

are discharged, for they are not found free in the intestine. In fresh specimens the granules are refractive and easily seen, but in preserved tissue they are readily destroyed, so that the granular zone appears reticuiar. The granules are soluble in water, and are darkened by osmic acid. The basal protoplasm of the pancreatic cells is vertically striated. It contains the round nucleus which has coarse masses of chromatin. Within the pancreatic cells there have been found "paranuclei" of unknown nature, thought to be functionally important. After the discharge of secretion the cells become smaller and their boundaries more distinct. The pancreatic cells rest upon basement membranes containing "basket cells."

The centro-alveolar cells may be darker or lighter than the pancreatic cells (Fig. 289), but they are always smaller, and may be readily identified from their central position. They do not contain zymogen granules. The intralobular intercalated ducts, which connect with the alveoli, are very slender, and their walls are formed of flat cells (Fig. 289, A). They



terminate in clusters of alveoli, which often present clover-leaf forms. The centro-alveolar cells have been interpreted as due to the invagination of these ducts into the alveoli, but apparently they do not develop in this way; they are formed as an inner stratum of a two-layered epithelium. The secretory capillaries of the alveoli are shown in Fig. 290. They extend between the centro-alveolar cells to the pancreatic cells, and may be prolonged between the latter, but they do not reach the basement membrane.

The intercalated ducts pass into excretory ducts lined with cuboidal epithelium, without the intervention of secretory ducts such as are found in the salivary glands. The plan of the pancreatic ducts is shown in Fig. 291. The main pancreatic and accessory pancreatic ducts are composed of simple columnar epithelium surrounded by a connective tissue

Centro-alveolar cells.

Cells of the alveolus.

Intercellular secretory . ' capillary.



layer, outside of which is a zone of circular smooth muscle fibers. The latter are gathered into sphincters at the major and minor duodenal papillae, where the ducts open. Occasional goblet cells and small glands resembling mucous glands have been found in the mucosa of the large ducts.

The blood and lymphatic vessels and nerves of the pancreas resemble those of the salivary glands. . The capillaries have notably wide meshes so that considerable portions of the alveoli are not in contact with them. The nerves end around the blood vessels, ducts and pancreatic cells. They are chiefly non-medullated sympathetic fibers from the cceliac plexus, associated with scattered nerve cells found within the pancreas. Lamellar corpuscles occur in the connective tissue.

The pancreatic islands are usually not to be found in human embryos under 50 mm. in length. Thus they develop only after the pancreatic glands have come together and attained considerable size. They arise as outgrowths from the smaller ducts, with which they may retain a solid stalk-like connection, or they may become wholly detached. According to Bensley, detached islands in the guinea-pig are infrequent. In the



embryo, as in the adult (Fig. 292), they consist of coiled anastomosing cords of cells, or irregular masses, which are in close relation with the endothelium of dilated capillary blood vessels. The islands are composed of pale cells with very delicate cell walls, and they contain finer granules than those in the pancreatic cells. In fresh preparations Bensley observed that these granules exhibit the Brownian movement, and that colorless spaces occur among them, representing the canals of Holmgren's trophospongium. When preserved by special methods, two forms of island-cells may be distinguished by the staining reactions of their gran



ules. In one type of cell the nucleus is oval, with finely granular chromatin; and in the other it is round, with large chromatin granules. Having neither ducts nor lumen, the islands produce an internal secretion, which is received by the blood vessels. There is evidence that this secretion plays an important part in carbohydrate metabolism. If the pancreas is removed, sugar appears in the urine; but if the ducts of the pancreas are tied, the pancreatic alveoli degenerate, leaving the islands functional, and sugar is not found in the urine. Thus the islands are regarded as physiologically distinct from the remainder of the pancreas.

Morphologically the islands are likewise distinct, and Bensley finds that the possibility of the transformation of alveolar tissue into island tissue, or conversely of island tissue into alveolar tissue, "has not a single well-established fact to support it" (Amer. Journ. Anat., 1911, vol. 12, pp. 297-388). The number of islands, however, is subject to great variation,



there being from 13,00010 56,000 in the entire pancreas of guinea-pigs (Bensley), the average being twenty-two islands per cubic millimeter. In all stages, both in the guinea-pig and in man, they are usually most numerous in the tail of the pancreas, and least numerous in its head (Opie, Johns Hopkins Hosp. Bull., 1900, vol. n, pp. 205-209).


Development. The respiratory apparatus, consisting of the larynx, trachea, bronchi, and lungs, arises as a median ventral outgrowth of the fore-gut, immediately behind the last pharyngeal pouches. It apparently is in no way related to the branchial pouches, but it may correspond with the air-bladder of the bony fishes. At the stage when the lung-bud develops, the fore-gut is laterally flattened, so that its lumen is a dorso- ventral cleft. The lung-bud develops as a pear-shaped swelling, directed downward, on the ventral border of the fore-gut; and this swelling becomes split off, from below upward, to form the trachea, which is at first short but which rapidly elongates. The upper end of the trachea, with the cartilages which develop around it, constitutes the larynx. At the lower end of the trachea, the pyriform dilatation spreads out on either side to form the primary bronchi (Fig. 293, A).

The tracheal and bronchial tubes are lodged in a mass of connective tissue, situated above and behind the pericardial cavity, and since this tissue stands in the middle of the thorax it is known as the mediastinum. It is comparable with a broad mesentery. As the bronchi push out laterally they occupy right and left folds bulging from the mediastinum, called by Ravn the pulmonary wings (ala pulmonales}. Into these the bronchi extend and produce branches after the manner of a gland (Fig. 293, B). The pulmonary wings consist of mesenchyma, covered by the epithelium which lines the body cavity. At first they project into the part of the body cavity which connects the peritoneal with the pericardial cavity; later, by the development of the pleuro-pericardial and pleuro-peritoneal membranes respectively (the latter being a part of the diaphragm) the chamber into which the pulmonary wings project is entirely cut off from the rest of the body cavity. On either side, it forms a pleural cavity (see Fig. 169, p. 175). The epithelium and underlying connective tissue covering the pul


A, A younger stage than B; ep, apical bronchus; I, II, primary bronchi.


monary wings, constitute the visceral pleura; and the similar layers toward the thoracic wall form the parietal pleura. These layers are comparable in development and structure with the corresponding layers of the peritoneum. Other subdivisions of the pleura are the mediastinal, pericardial, and diaphragmatic pleurae. The lung is connected with the mediastinum by a short and broad stem of connective tissue, across which the bronchi, vessels and nerves extend. This is the root of the lung, and the vessels enter at the hilus.

The branches which are given off by the stem-bronchus within the pulmonary wings, are formed with great regularity, and they have been carefully studied in many mammals. Very early in development, the human lungs become asymmetrical, and at the stage shown in Fig. 293, B, the three lobes of the right lung and the two lobes of the left lung are already indicated. In the pig the asymmetry is greater, since on the right an unpaired lobe proceeds directly from the trachea; in certain animals, as in the seal, the right and left lungs have symmetrical bronchi. Whether the symmetrical condition is the primary one, and how the bronchi of one lung should be homologized with those of the other, are questions which have been much discussed. For the comparative anatomy of the bronchi, see Huntington, Ann. N. Y. Acad. Sci., 1898, vol. n, pp. 127-148; for their development, especially in the pig, see Flint, Amer. Journ. Anat., 1906, vol. 6, pp. 1-137.

The blood vessels of the lungs are derived from several sources. They include the large pulmonary arteries and veins, which are the principal vessels of the lung, and the small but important bronchial arteries and veins. The pulmonary vessels are shown in Fig. 294, which represents the trachea and right lung of a human embryo, seen from the left side; the left lung has been cut away at /. br.

The pulmonary arteries develop in connection with the pulmonary arches, which are two vessels, one on either side, passing from the ventral aorta to the dorsal aorta. Approximately midway in its course, each of these arches sends a branch to the lung of the corresponding side. Subsequently the trunk of the ventral aorta becomes spirally subdivided by a septum, so that the portion leading to the pulmonary, arches is split off from the rest; the way in which its root becomes connected with the right ventricle only, has been described with the development of the heart. As a result of this subdivision, the pulmonary artery leaves the heart and divides into right and left arches, each of which sends a branch to the lung on the same side and then passes on to the dorsal aorta. The connection between the right arch and the right dorsal aorta is soon lost, however, so that the vessel to the right lung (Fig. 294, r. r.} appears to be given off from the main pulmonary artery. The left pulmonary arch enlarges, and until birth it forms a great vessel, known as the ductus arteriosus, which conveys most of the blood from the pulmonary artery into the aorta. The amount of blood which goes to the inactive lungs may be inferred from the relative size of the vessels shown in the figure. Soon after birth, when respiration



has begun, the ductus arteriosus closes, becoming a fibrous cord, and then the volume of blood going through the pulmonary artery equals that in the aorta. (For further details regarding the development of the pulmonary arteries, see Bremer, Amer. Journ. Anat., 1902, vol. i, pp. 137-144).

The pulmonary veins are at first represented by a capillary plexus around the lung-bud, which receives its blood in part from the pulmonary arteries already described, and in part from branches of the dorsal aorta, some of which persist as the bronchial arteries. The capillary plexus is drained partly by branches of the posterior cardinal or azygos veins, representing the future bronchial veins, and partly by a minute vein which has grown out from the left atrium and is destined to become the great pulmonary veins. At a certain stage these veins, two from each lung, have a common orifice in the left atrium; but in later stages, as the heart enlarges, their short common stem is taken up into the wall of the atrium, so that the four pulmonary veins acquire separate openings. The early stages in the development of the pulmonary veins in the cat have recently been studied by Brown (Anat. Rec., 1913, vol. 7, pp. 299-330).

The small bronchial arteries, one or two on each side, are branches of the upper part of the thoracic aorta (Fig. 294); sometimes one of them proceeds from an intercostal artery. The bronchial arteries enter the hilus of the lung and pass into the fibrous tissue in the walls of the bronchi. The main stems branch with the bronchi. They produce capillary networks in the bronchial mucous membrane, and send branches to the peribronchial connective tissue, supplying it with capillaries and becoming the vasa vasorum of the main branches of the pulmonary artery (Miller, Anat. Anz., 1906, vol. 28, pp. 432-436). In some animals Miller finds that the bronchial arteries pass on into the pleura, as in the horse; in others, like the dog, terminal branches of the pulmonary arteries supply the pleura; and in the human lung the pleura receives both pulmonary and bronchial vessels (Amer. Journ. Anat., 1907, vol. 7, pp. 389-407).

The bronchial veins are small branches of the azygos vein. They do not receive all the blood from the bronchial arteries, since some capillaries from the latter are drained by the pulmonary veins.



ao., Aorta; d.a., ductus arteriosus; 1., entodermal part of the lung; 1. at., left atrium; 1. br., left bronchus; 1. r., left ramus of pulmonary artery, p. a.; r. r., its right ramus; oe., oesophagus; p. c., pericardial cavity; p. v., pulmonary vein; s. t., septum transversum; th. ao.^ thoracic aorta; tr., trachea.



The mucous membrane of the larynx is a continuation of that of the pharynx, and accordingly consists of epithelium and tunica propria. A submucosa connects it with the underlying parts. In most places the epithelium appears to be stratified and columnar, but it is said to be pseudo-stratified, with nuclei at several levels (Fig. 38, p. 49). It is difficult to determine whether or not all the cells are in contact with the basement membrane. This type of epithelium, which occurs also in the trachea, is ciliated. The stroke of the cilia is toward the pharynx. A stratified epithelium with squamous, non-ciliated outer cells is found on the vocal folds (true vocal cords) , on the anterior surface of the arytaenoid cartilages and on the laryngeal surface of the epiglottis. The distribution of the two sorts of epithelium above the vocal folds is subject to individual variation. The squamous epithelium often occurs in islands. The tunica propria is composed of fibrous connective tissue with many elastic fibers, and beneath the epithelium it forms a basement membrane (membrana propria). It includes reticular tissue containing a variable number of lymphocytes, which are gathered in solitary nodules in the wall of the laryngeal ventricle (sinus of Morgagni). Connective tissue papillae are found chiefly beneath the squamous epithelium. At the free border of the vocal folds and on their under surface, the papillae unite to form longitudinal ridges. On the laryngeal surface of the epiglottis there are only isolated papillae, against which rest the short taste buds.

The submucosa contains mixed, branched, tubulo-alveolar glands, measuring from 0.2 to i.o mm.; they are abundant in the ventricular folds but are absent from the middle part of the vocal folds. The ventricular folds (false vocal cords) consist of a loose vascular fatty tissue, often containing small bits of elastic cartilage about i mm. long, and similar cartilages measuring 2-3.5 mm. are sometimes found in the anterior ends of the vocal folds.

The cartilages of the larynx are mostly of the hyaline variety, resembling those of the ribs. To this class belong the thyreoid, cricoid, the greater part of the arytaenoid, and often the small triticeous cartilages. Elastic cartilage is found in the epiglottis, the cuneiform and corniculate cartilages, the apex and vocal process of the arytaenoids, and generally the median part of the thyreoid. In women this portion is not involved in the ossification (chiefly endochondral) which begins hi the thyreoid and cricoid cartilages between the twentieth and thirtieth years. The triticeous cartilages (nodules in the lateral hyothyreoid ligaments, named from their resemblance to grains of wheat) are sometimes composed of fibro-cartilage. The blood vessels form two or three networks parallel with the surface.


followed by a capillary plexus just beneath the epithelium. The lymphatic vessels similarly form two communicating networks, of which the more superficial consists of smaller vessels and is situated beneath the capillary plexus. The nerves form a deep and a superficial plexus which are associated with microscopic ganglia. Non-medullated fibers end either beneath the epithelium in bulbs and free endings with terminal knobs, or within the epithelium in free ramifications and in taste buds. Below the vocal folds, subepithelial nerve endings and buds are absent, but many intraepithelial fibers occur, which surround individual taste cells. The nerves and vessels of the larynx are numerous, except in the dense elastic tissue of the vocal folds.


The trachea consists of a mucosa, submucosa, and a fibrous outer layer containing the tracheal cartilages. The general arrangement of the layers is the same as that found in the large bronchi (Fig. 295).

The mucosa consists of pseudo-stratified columnar epithelium with cilia proceeding from distinct basal bodies (Fig. 38, p. 49). Exceptionally, the lining of the trachea, toward the oesophagus, has been found to consist of stratified squamous epithelium resting on connective tissue papillae. Beneath the epithelium there is a broad basement membrane, followed by a layer of reticular tissue containing many lymphocytes, forming a tunica propria. Beneath the reticular tissue there is a layer of coarse longitudinal elastic fibers, which may readily be seen in haematoxylin and eosin preparations. This layer may be compared with the muscularis mucosae of the intestine.

The submucosa is a layer of loose fatty connective tissue extending to the perichondrium of the tracheal cartilages. It contains the bodies of the tracheal glands, which include both serous and mucous cells, and are beautiful objects for the study of serous crescents.

The outer layer of the trachea is continuous with the tissue of the mediastinum. It contains abundant blood and lymphatic vessels, and nerves, both medullated and non-medullated. Internally it forms the perichondrium around the succession of C-shaped hyaline cartilages, the free ends of which are toward the oesophagus. In the intervals between these ends there is a layer of transverse smooth muscle fibers, usually accompanied by outer longitudinal fibers. As in the intestine, elastic fibers are abundant among the. muscle cells. In old age, the hyaline cartilages show fibrous degenerative changes, and may become partly calcified.

The primary bronchi have the same structure as the trachea, but in their subdivisions changes occur, and the C-shaped rings of cartilage are



replaced by irregular plates found on all sides of the tube (Fig. 295). These diminish in size as the bronchi become smaller, and disappear in those about i mm. in diameter. Usually the cartilages are hyaline, but elastic cartilage is said to occur in places. The circular muscle fibers form a layer completely surrounding the tube internal to the cartilages. Branched tubulo-alveolar bronchial glands extend further down the tubes than the cartilages. In the larger bronchi they are present in great numbers,

Tunica Epithelium. propria. m

Connective tissue. Bronchial gland. Duct of gland.


and their bodies lie outside of the muscular layer and project into the spaces between the cartilages. The mucosa is thrown into longitudinal folds; it is covered with ciliated epithelium containing goblet cells and resembling that of the trachea. Lymphocytes are numerous in the tunica propria, sometimes collecting in solitary nodules and wandering into the epithelium. The small bronchi, 0.5-1.0 mm. in diameter, are known as bronchioles. They are free from cartilage and glands, and are lined throughout with ciliated columnar epithelium.




The arrangement of the ultimate branches of a bronchiole is shown in the diagram, Fig. 296. The respiratory bronchioles, 0.5 mm. or less in diameter, at their beginning contain a simple columnar ciliated epithelium. Further in their course the goblet cells disappear, cilia are lost, the cells become cuboidal, and among them are found thin, non-nucleated plates of different sizes. These plates constitute the respiratory epithelium. The transition from the cuboidal to the respiratory epithelium occurs irregularly, so that a bronchiole may have cuboidal epithelium on one side and

Bronchial artery.

Pulmonary vein.

Pulmonary artery.

~ Respiratory'bronchiole.

Pleural capillaries




respiratory epithelium on the other; or one sort of epithelium may form an island in the midst of the other. Hence the respiratory bronchioles contain a mixed epithelium (Fig. 297, A). The respiratory epithelium steadily gains in extent until the cuboidal epithelium has disappeared.

At irregular intervals along the bronchioles the respiratory epithelium forms hemispherical outpocketings or alveoli. The alveolar ducts, from i to 2 mm. long, differ from the respiratory bronchioles in that they contain only the respiratory epithelium and are thickly beset with alveoli. The layer of smooth muscle fibers may be traced to the end of the alveolar ducts, where it terminates. Since the muscles do not extend over the



alveoli, but merely surround the main shaft of the duct, the layer is greatly interrupted, and some consider that it ends in the course of the duct. The respiratory bronchiole may be continued as a single alveolar duct or may divide into two or more. The alveolar ducts branch to produce ahe

Pores. Cuboidal epithelial cells. Non-nucleated \ ,__JbtfMUb Plates.


epithelial Non-nucleated cells. plates.

Border of an alveolus. B Fundus of an alveolus. FIG. 297. FROM SECTIONS OF THE HUMAN LUNG. X 240.

A, Mixed epithelium of a respiratory bronchiole; B, an alveolus sketched with change of focus; the border of the alveolus is shaded; it is covered by the same epithelium as that of the (clear) fundus of the alveolus; the nuclei of the cells are invisible. (Silver nitrate preparation.)

olar sacs (infundibula) which are cavities in the center of clusters of alveoli. The sacs resemble the ducts as shown in Fig. 296.

According to Miller (Arch. f. Anat. u. Entw., 1900, pp. 197-228) who has made

careful reconstructions of the terminal branches in the human lung, an atrium, or round cavity, should be recognized between the alveolar duct and the alveolar sac. The alveolar duct is said to terminate by opening into 3 to 6 atria, the entrances to which are surrounded by smooth muscle fibers forming "a sort of sphincter"; the atria possess no muscle fibers. Each atrium is connected with two or more alveolar sacs, and is moreover beset with alveoli (Fig. 298). Stohr states that the recognition of an atrium between the alveolar duct and alveolar sac seems to him superfluous; "in good casts of the human lung it is not to be distinguished, and in other animals it is inconstant."


The stippling indicates smooth muscle and cuboidal epithelium ; the lines, respiratory epithelium. B. R., Respiratory bronchiole; D. A., alveolar duct; A., atrium; A. S., alveolar sac.

In sections, without resort to reconstructions, very little can be found out concerning the relations of the alveoli to the bronchial ramifications. The following structures are all which can easily be identified: (i) alveoli;



(2) spaces bounded by alveoli (alveolar sacs, atria and alveolar ducts, the ducts having muscle fibers in their walls); (3) small bronchioles having scattered alveoli along their walls, and therefore presenting a mixed epithelium (respiratory bronchioles); and (4) bronchioles with no respiratory epithelium.

The study of sections of the adult lung is facilitated by comparison with those from an embryonic lung. Comparable sections, including the pleura, and drawn at the same scale of magnification, are shown in Figs. 300 and 301. In the lung of the embryo of four months, the terminal branches of the bronchioles are found in the centers of lobules, one of which is shown in Fig. 300 (bounded by b. v. and lym.). The axial bronchioles break up into ramifying tubules lined with cuboidal cells, and at birth the alveoli which are found at the end of these structures are also lined with cuboidal epithelium. The main arteries run with the axial bronchioles in the centers of lobules; and the large veins and lymphatic vessels are at their periphery. This arrangement is retained in the adult (Fig. 296). Deep in the lung, the small bronchi are surrounded by considerable connective tissue, containing arteries, veins and large lymphatic vessels.

After respiration has been established, the alveoli become greatly distended, so that the connective tissue containing the capillary vessels is flattened out in very thin layers. These layers are bounded on either side by the respiratory epithelium of adjacent alveoli (Fig. 301). In producing this epithelium, the cells not only become flattened but they are transformed into thin structureless plates, and those from several cells may fuse to form large plates. In amphibia, nuclei in small amounts of protoplasm are found attached to the basal or connective tissue side of the plates, often associated in groups. In addition to these cells, the alveolar walls contain the endothelial cells of the capillaries, connective tissue cells, wandering cells, and many elastic fibers. These fibers surround the alveoli and encircle their outlets; the alveolar walls are so elastic that in inspiration they may expand to three times the diameter to which they return during expiration (o.i to 0.3 mm.). Pores have been described leading from one alveolus to another (Fig. 297, B).

The richness of the capillary network in the alveolar walls is seen in injected specimens (Fig. 299). Respiration takes place by the transfer of gases between the blood in these vessels and the air in the alveoli, therefore through the endothelial cells and alveolar plates, together with the trivial amount of connective tissue which may intervene.

The pulmonary and bronchial blood vessels have already been described, and their relations to the lobule of the lung are shown in Fig. 296. The pulmonary arteries are axial vessels within the lobules, breaking up into terminal branches at the atria, and these branches become axial along the alveolar sacs. Each terminal branch has been described as the center of


an ultimate lobule or structural unit. The veins are peripheral both in the units and larger lobules; between the latter they run through connective tissue septa.

The abundant lymphatic vessels are arranged in a superficial set draining into the pleura by way of the interlobular septa; and a deep set draining toward the hilus along the bronchi, accompanying the large vessels. Lymphatics of the deep set do not extend into the lobules; they terminate along the alveolar ducts. Around the larger bronchi and at the root of the lung, lymph glands are numerous. A conspicuous feature of the sections of the lung is the presence of black soot in the tissue around the lymphatic

vein vessels. It penetrates the pulmo nary epithelium in the smallest bronchioles, apparently passing Artery. between the epithelial cells. Some of it is taken up by phagocytes. Having entered the lymphatic FIG. 299. FROM A SECTION OF THE LUNG OF A vessels it is distributed along their


ARTERY, x so. courses. On the surface of the

Of the five alveoli drawn^thejhree upper ones are l ung J t [ 5 seen j n tne interlobular

septa, marking out the boundaries

of the lobules. Because of the steady increase in this deposit, the color of the lungs changes from birth until old age.

The nerves of the lung include the pulmonary plexus derived from the sympathetic system. Its fibers enter at the root of the lung and spread around the bronchi and vessels, to which they are chiefly distributed. Small ganglia are found within these plexuses. The vagus also sends branches to the lungs, including medullated and non-medullated fibers, which join the sympathetic plexuses.


The visceral pleura is a thinner layer than the parietal pleura, and is closely attached to the lung. It is covered with a single layer of flat mesothelial cells, which in the collapsed lung become thicker and shorter. In specimens which have been handled, this layer is often lacking. It rests upon a thin layer of fine-meshed fibrous tissue, beneath which is the coarse subserous layer continuous with the interlobular septa of the lung (Fig. 301). This tissue is highly elastic. In the subserous layer, blood vessels, derived from both pulmonary and bronchial arteries, form an abundant capillary plexus. The superficial lymphatic vessels are very evident, and in relation with them lymphoid tissue is found, and occasionally lymph nodules. Stomata, which have been described, are presumably artificial apertures in the epithelium and are not connected with the lymphatic vessels.


FIG. 300.

ep c.t. s.s.

al.s. al.

FIG. 301.


al., Alveolus; al. s., alveolar sac; br., bronchiole; b. v., blood vessel; c. t., outer layer of pleural connective tissue; ep., pleural epithelium; lym., lymphatic vessel; pi., pleura; s. s., subserous connective tissue; t. b.. terminal branch of the bronchiole.



. i-W.b.

The parietal pleura is a thicker and less elastic layer. Ventrally and below, toward the pleuro-pericardial membrane, it exhibits folds containing fat (plica adiposce); and sometimes it forms vascular elevations resembling synovial villi the pleural mlli. Fat may be found in the pleura elsewhere.

The nerves of the pleura are derived from the phrenic, sympathetic and vagus nerves. In the parietal pleura typical lamellar corpuscles may be found, together with the smaller variety, known as the Golgi-Mazzoni corpuscles.

Urinary Organs

Wolffian Bodies And Wolffian Ducts

On the twenty-eighth of November, 1759, Caspar Friedrich Wolff, then in his twenty-sixth year, defended a thesis entitled "Theoriagenerationis" and obtained the degree of doctor of medicine at Halle. In addition to the fundamental principles which this renowned thesis set forth, it included

an account of the development of the kidneys in chick embryos. From the diffuse substantia cellulosa along the ventral side of the spinal column, beginning on the third day of incubation, Wolff saw two elongated bodies gradually take form, and become the kidneys, each being connected with the cloaca by a ureter. These structures, however, are not the kidneys of the adult, and they are generally known as Wolffian bodies', their ureters are the Wolffian ducts. They are large and important organs in human embryos, as shown in Fig. 302. The true or permanent kidneys of mammals arise later, and the Wolffian bodies degenerate, becoming vestigial in the female; in the male, however, they acquire new functions, and are retained as a portion of the genital ducts (namely the duct of the epididymis). In the embryo they are renal organs built upon the same plan as the permanent kidneys, and moreover in the fishes and amphibia they are the kidneys of the adult.

Still another renal organ develops in embryos, anterior to the Wolffian body, and it has been found that the Wolffian duct is primarily the due of this anterior kidney or pronephros; consequently the Wolffian duct is sometimes called the pronephric duct. The pronephros is the functional kidney in only the lowest of vertebrates (myxinoids). Singularly it has been found that " the human pronephros is by far the best developed within the groups of mammals" (Felix, in Keibel and Mall's Human Embry


al., Bladder; 1., lung; St., stomach; s. tr., septum transversum; u. c., umbilical cord; W. b., Wolffian body; W. d., Wolffian duct.



ology, Vol. 2). Except for its duct, it entirely disappears in very young embryos (5 mm.). All the renal organs pronephros, Wolffian body (or mesonephros) , and kidney (or metanephros) are developed from the nephrotomes. They are all composed of mesodermal tubules, each of which is in close relation with a knot of capillary blood vessels derived from branches of the aorta. Such a knot of vessels is a glomerulus, and certain products are eliminated from the glomerulus into the tubules to form the urine.

Development of the Wolffian Body and Wolffian Duct. The general relations of the neplirotome to the mesodermic somites and to the coelomic







C, HUMAN EMBRYO. 10 MM. mo, Aorta; c., posterior cardinal vein; coe., ccelom; gl., glomerulus; g. r., genital ridge; int., intestine; mes.

mesentery; mes. seg., mesodermic somite; my., myotome; nch., notochord; neph., nephrotome; s-c. v.

subcardinal vein; si., sinusoid; sy., sympathetic nerves; u. v., umbilical vein; W. d., Wolffian duct

W. t., Wolffian tubule.

epithelium have already been briefly discussed (p. 41). A nephrotome from a young rabbit embryo is seen in section in Fig. 303, A, together with its elevation which contributes to the formation of the Wolffian duct. The nephrotome here shown is from one of the anterior segments and belongs with the pronephros.

In human embryos, according to Felix, pronephric tubules are formed from the seventh to the fourteenth segments, and perhaps from those further forward. The elevations to which these nephrotomes give rise turn posteriorly and unite with one another to form the Wolffian duct. This is at first a solid cord of cells which grows posteriorly in the trough

3 o8


between the somites and somatic mesoderm. It lies near the ectoderm, but it is now generally agreed that the ectoderm takes no part in its formation. Finally its growing extremity reaches the ventral portion of the cloaca and fuses with it. Later this ventral part of the cloaca becomes cut off to form the bladder, and the Wolffian duqts then empty into the neck of the bladder. The pronephric tubules meanwhile become detached from the ccelomic epithelium, but they remain rudimentary and degenerate without having any glomeruli formed in connection with them.

The mesonephric tubules develop from the more posterior nephrotomes, after the Wolffian duct has formed. They acquire openings into the Wolffian duct, but do not contribute to its development. In producing mesonephric tubules, the nephrotomic tissue becomes detached and separates into masses which form vesicles (Fig. 303, B). Each vesicle elongates and becomes an S-shaped tubule, one end of which fuses with the Wolffian duct and opens into it; the other end remains blind. A knot of capillaries, derived from a branch of the aorta, develops in the distal concavity of the S and becomes a glomerulus ; a glomerulus is formed in connection with every Wolffian tubule. The tubules then elongate and become coiled, and together they produce the rounded swellings on either side of the root of the mesentery, which are the Wolffian bodies (Fig. 303, C). The genital glands arise as mesodermal thickenings on the ventro-medial surface of these bodies.

A single Wolffian tubule is shown in Fig. 304, and the way in which its distal end envelops the glomerulus is clearly indicated. It is said to form the capsule of the glomerulus. By passing through the inner layer of this capsule, fluid from the blood vessels enters the tubule and is conveyed through the Wolffian duct to the bladder. The tubules are generally unbranched, and are lined with simple epithelium. The epithelium is in part glandular, and contributes to the formation of the urine. Finally it may be noted that a nephrotome may divide into several vesicles (sometimes perhaps as many as four), and therefore the number of Wolffian tubules is greater than the number of corresponding segments. In man the maximum number is 83 (Felix). The mesonephric tubules also extend forward, so that some segments contain both mesonephric and pronephric tubules.


FROM A HUMAN EMBRYO OF 10.2 MM. (Except the

glomerulus, after Kollman.) c., Inner layer, and c. a., outer layer of the capsule of

the glomerulus; div., diverticulum ; gl., glomerulus;

W. d., Wolffian duct.



It is generally believed that the Wolffian bodies of mammalian embryos are active renal organs, producing a form of urine which collects in the allantoic sac. In pig embryos this sac and the Wolffian bodies are both unusually large. MacCallum (Amer. Journ. Anat., 1902, vol. i, pp. 245^59) notes that the tubules of the Wolffian body in the pig "show a very distinct division into a secretory and a conducting part." In the human embryo, however, the allantois is very small and the Wolffian bodies degenerate early, before the kidney can become functional. Therefore Felix (Keibel and Mall's Human Embryology, vol. 2) regards the question as settled. The Wolffian body " does not function as an excretory organ"; but he adds, "This does not, of course, imply that it may not have been active in another manner unknown to us."

Veins of the Wolffian Body. In determining the arrangement of the large veins of the abdomen, the Wolffian bodies are of fundamental importance. They are supplied by the posterior cardinal veins which pass from the tail of the embryo, on either side of the aorta, to the heart.

I/, c. c.

h. a 2.


i I.'



a. c., Anterior cardinal; as. L, ascending lumbar; az., azygos; c., caudal; c. s., coronary sinus; h., hepatic; h. a. z., hemiazygos; h. az. a., accessory hemiazygos; il., common iliac; in., innominate; j., jugular; K., kidney; I.e. c., left common cardinal; m. s., median sacral; p. c., posterior cardinal; r. c. c., right common cardinal; s. c., subcardinal; scl., subclavian; sp., spermatic; sr., suprarenal; sup., supra cardinal; T., testis; v. c. i., vena cava inferior; v. c. s., vena cava superior; W. B., Wolffian body.

Before entering the right atrium of the heart, they are joined by the anterior cardinal veins from the head, thus forming the right and left common cardinal veins, or "ducts of Cuvier." As each posterior cardinal vein extends along the dorsal side of the Wolffian body, it sends branches in among the tubules, and these unite ventrally on either side in the subcardinal vein (Fig. 305, A). Thus each Wolffian body is lodged in a venous loop formed by the posterior cardinal and subcardinal veins, and

3 io


such a loop is found in all classes of vertebrates. Venous blood entering the Wolffian body posteriorly flows out from it anteriorly, and circulates among the tubules in lacunar vessels, closely resembling the hepatic sinusoids. This is the "renal portal system." It should be noted, however, that the renal sinusoidal vessels are poorly developed in mammalian embryos.

In sections these veins are readily recognized. The mesonephric arteries pass from the aorta to the glomeruli of the Wolffian body, between the subcardinal vein in front and posterior cardinal vein behind (Fig. 303, C). In places, the subcardinal veins form large anastomoses across the mid-ventral line; the posterior cardinal veins are further apart, and receive intersegmental branches from the dorsal musculature.

As the kidneys grow upward behind the Wolffian bodies, their ureters become encircled by a branch from the posterior cardinal vein (Fig. 305, A). The venous loop around the ureter was described by Hochstetter (Morph. Jahrb., 1893, vol. 20, pp. 543-648), and its dorsal limb, together with secondary anastomoses, has been named the supracardinal vein (Huntington and McClure, Anat. Rec., 1907, vol. i, pp. 29-30). The transformation of these veins into the branches of the inferior vena cava is represented somewhat diagrammatically in Fig. 305, B, and may be briefly described as follows:

The anastomosis between the subcardinal veins becomes a part of the left renal vein. Above this anastomosis the right subcardinal vein connects with the veins of the liver and forms a portion of the vena cava inferior. The left subcardinal vein, above the renal anastomosis, becomes reduced to the left suprarenal vein (Hochstetter). The subcardinal veins below the renal anastomosis are associated with lymphatic vessels to which they apparently give rise; otherwise they disappear.

The posterior cardinal veins above the renal anastomosis, after they have been tapped by the formation of the vena cava inferior, are known as the azygos and hemiazgos veins, and the outlet of the left common cardinal becomes cut off as the coronary sinus (Fig. 305, B, which shows also the formation of the superior vena cava). Below the renal anastomosis the posterior cardinal veins give rise to the genital veins (spermatic or ovarian), and the Wolffian body becomes reduced to an appendage of the genital glands. As the genital glands descend into the pelvis, their veins become elongated; and the corresponding arteries, derived from the mesonephric arteries, are likewise elongated. The supracardinal vein on the right side becomes a part of the vena cava inferior; on the left it is probably represented by the ascending lumbar vein.

The kidneys are supplied by vessels which enter them after they have attained their permanent position. Their arteries and veins consequently pursue a straight course from the aorta and vena cava, respectively, to the hilus of the kidney.


Development. The kidney develops after the Wolffian body has been formed. It arises in two parts, one of which is an outgrowth of the Wolffian duct; the other is a mass of dense mesenchyma surrounding this outgrowth, and said to be derived from the posterior nephrotomes. Both


parts are mesodermal. The part derived from the Wolffian duct may be considered first.

Each Wolffian duct, near the place where it enters the cloaca, forms a knob-like outpocketing which elongates rapidly, becoming the ureter. The distal end of the outpocketing expands and becomes lobular, thus producing the pelvis of the kidney. After the ventral part of the cloaca

W.d. M.d. Md



A, Human embryo of n.s mm. (4i weeks); B, 25 mm. (8J-9 weeks), a., Anus; al. d., allantoic. duct bL, bladder; cl., cloaca; M. d., Mullerian duct; p., pelvis of the kidney; r. t rectum; ur., ureter; u. s., urogenital sinus; W. d. Wolffiian duct.

has been split off to form the bladder, the ureter and Wolffian duct, on either side, open into it by a common outlet (Fig. 306, A). Later, the terminal portion of each Wolffian duct is taken up into the wall of the expanding bladder, so that the ureters acquire openings separate from



a., Primary collecting tubule, with dilated extremity; b,b'., inner layer, and c. ( outer layer of dense mesenchyma; d., loose mesenchyma; e., vesicle, the beginning of a renal tubule.

those of the ducts. With further growth the orifices of the Wolffian ducts are carried toward the median line and downward toward the outlet of the bladder (Fig. 306, B), and this position is permanently retained. Meanwhile the lobes of the renal pelvis have become deeper and formed

3 I2


pouches known as the major and minor calyces. In the adult there are usually two major calyces, one at either end of the pelvis, and from these most of the minor calyces grow out; the others spring directly from the main pelvic cavity. There are about eight in all. From the minor calyces the collecting tubules grow out. Each tubule has an enlarged extremity

FIG. 309.


(Fig. 307) which divides into two branches with a U-shaped crotch, like a tuning-fork. The branches subdivide repeatedly in the same manner, so as to make pyramidal masses of straight tubules radiating from the calyces. Thus the renal outgrowth from the Wolffian duct produces the





epithelial lining of the ureter, pelvis, calyces and collecting tubules, including all of their branches.

The second part of the kidney, which consists of dense mesenchyma, becomes subdivided into masses enveloping the enlarged tips of the branching collecting tubules. Some of its cells become arranged so as to form vesicles (Fig. 308), one of which is shown in the reconstruction, Fig. 309, A. The vesicles are at first entirely separate from the collecting tubules. Each vesicle becomes elongated, making an S-shaped tubule (Fig. 309, B, C), and its outer or upper end unites with the collecting tubule (Fig. 309, D). A glomerulus develops in the lower curve of the S, and is gradually enveloped in the terminal part of the tubule, which thus forms its capsule. Between the capsule and the collecting tubule, the renal tubules become greatly convoluted. One of the loops in the coils thus formed slongates downward, lying close beside and parallel with the collecting tubule; this is the loop of Henle (Fig. 309, J).

Three tubules of the adult kidney are shown diagrammatically in Fig. 310. Each capsule connects with a proximal convoluted tubule, which, after extending outward toward the surface of the kidney, turns downward as the descending limb of Henle's loop. The descending limb is a straight tubule, the lower portion of which is of small diameter owing to the flatness of the cells in its walls; its lumen is not reduced. This "thin segment," as shown in the diagram, does not form the entire descending limb, but only its lower part. Frequently it passes around the bend into the ascending limb. The tubule, after turning the bend, forms ihe ascending limb of Henle's loop. It returns to the vicinity of the capsule from which it arose, and makes a few coils, thus constituting the distal convoluted tubule (intercalated or intermediate tubule). By means of the functional tubule it joins the arched collecting tubule and this passes into




a. 1., Ascending limb of Henle's loop; c., capsule; c. t., collecting tubule; d. c., distal convoluted tubule; d. 1., descending limb; j,, junctiona) tubule; p. c,, proximal convoluted tubule; p. d. .papillary duct.

A, cortex; B-D, medulla, subdivided into an inner zone (D) and an outer zone (B-C) ; the latter includes an inns' band or stripe (C), and an outer band (I).


the straight descending collecting tubule. From the capsule to the collecting tubule no branches occur; and this extent of the tubule represents the part derived from mesenchyma. The collecting tubules receive many branches. Traced toward their outlet in the pelvis they become larger, finally forming the papillary ducts.

In the diagram (Fig. 310) the tubules are represented as much coarser than is actually the case. Their true proportions in the rabbit's kidney have been shown by Huber, who, with extraordinary success, has isolated individual tubules, keeping them intact from the capsule to the collecting tubule (Anat. Rec., 1911, vol. 5, pp. 187-194). They are 20-30 mm. in length and less than o.i mm. in diameter. Huber's account of the development of the kidney, from which Figs. 307-309 have been taken, is in the supplement to the Amer. Journ. Anat., 1905, vol. 4.

Surface Markings. Before studying sections of the kidney microscopically, the small subdivisions of the organ which may be seen upon its cut surface should be examined. They are shown in transverse section in Fig. 311, but appear equally well when the kidney is divided lengthcortex.

Pars convoluta. Pars radiata.


Pelvis.- "MMfcT :^i ^ ^%^ A (Medulla).

Renal anery. Ureter.

Renal vein.

Renal column.

^'< \ Calyx.


wise. The ureter opens into the pelvis, which is prolonged into the cuplike calyces, two of which are shown in Fig. 311. Each calyx receives a nipple-like projection of the substance of the kidney, known as a renal papilla. Sometimes two of them project into one calyx. They are soft, dark red structures, and it does not appear on inspection that the grayish lining of the calyx is reflected over their outer surface; this is seen in sections. Toward the apex of each papilla there are from 15 to 20 foramina, which are the orifices of as many papillary ducts; through them the urine enters tte calyx. The foramina are barely visible without magnification. Each papilla forms the apex of a renal (or Malpighian) pyramid, described by Malpighi (1666) in his treatise " on the structure of the viscera," which gave the first account of various almost microscopic "corpus



cles" and surface markings. The base of the pyramid is toward the periphery of the kidney, and may be lobular as in the figure. From two to nine embryonic or primary pyramids are said to fuse to form a pyramid of the adult kidney. In favorable specimens the pyramid is seen to be divided into an inner and an outer zone, and the latter is composed of two concentric bands. The significance of these markings will be considered later. The pyramids collectively constitute the medulla of the kidney, a term more fittingly applied to the kidneys of many animals which have but a single pyramid. The base of each pyramid is surrounded by a lighter zone, the cortex, which shows radial striations. With low magnification the striations are seen to taper outward. They constitute the processes or pyramids of Ferrein and are known collectively as the radiate part of the cortex (pars radiata). They consist of straight radial tubules which are continuous with those in the medulla. Consequently they are often called "medullary rays," but being in the cortex they may more properly be designated "cortical rays." Between these rays is the convoluted part of the cortex (pars convoluta}\ it may be recognized by the presence of many renal corpuscles (Malpighian corpuscles), which are bodies consisting of a glomerulus and its surrounding capsule. They are barely visible without magnification.

Over the outer surface of the kidney, there is a fibrous capsule (tunica fibrosa) which may be readily stripped off when normal; and outside of this there is a fatty layer (capsula adiposa) . The fat surrounds the pelvis and extends into a hollow of the kidney known as the renal sinus; this is the excavation which contains the pelvis and its calyces. In this fatty tissue the large blood vessels enter the kidney, passing chiefly over the anterior or ventral surface of the pelvis; having reached the boundary zone between cortex and medulla they enter it, and pursue an arched course, giving off both cortical and medullary branches. In certain places, the cortex dips down to the renal sinus; this occurs between the Malpighian pyramids, and constitutes the renal columns (of Bertini); one of them is shown in Fig. 311.

The arrangement of the renal tubules in relation to the cortex and medulla is as follows. The convoluted part of the cortex contains the capsules, and both proximal and distal convoluted tubules. The rays contain the collecting tubules, together with the outer portions of Henle's loops. The medulla contains the larger collecting tubules and the deeper portions of Henle's loops; since these are all straight tubules, the medulla resembles the radiate part of the cortex. Tubules which are connected with capsules deep in the cortex, near the boundary zone, send their Henle's loops much further into the medulla than those from the outer capsules; and in the deeply placed tubules the thin segment of Henle's loop is not limited to the descending limb but extends well up into the



ascending limb. Thus it happens that a broad inner zone of the medulla (i.e., toward the papilla) contains only thin segments of renal tubules in addition to the large collecting tubules (Fig. 310, D); and the zone so characterized may be distinguished macroscopically. The papilla contains only collecting tubules, but the loops of Henle turn back at different levels, and therefore the papillary zone entirely free from loops is not well defined. The outer zone of the medulla contains both thick and thin seg Renal corpuscle. Convoluted tubules. Cortical ray.

Interlobular vein.

Rente's loop. Arciform vein. Arciform artery.

FIG. 312. PART OF A RADIAL SECTION OF A HUMAN KIDNEY. X 5. At x a renal corpuscle has dropped out.

ments of Henle's loops, in addition to the collecting tubules. In the descending limbs the change to thin segments occurs at a more or less definite level within this outer zone, thus subdividing it into a narrow outer band, with few thin segments, and an inner band containing many of both sorts. These zones have only recently been recognized (Peter, Untersuchungen iiber Ban und Entwickelung der Niere, Jena, 1909).

The renal tubules which have their capsules close to the medulla are the first to develop; the others are formed successively outward, the young



est being immediately beneath the capsule. Thus a single section of an embryonic kidney shows various stages in the development of the tubules. Sections of the Kidney. Since a radial section of the kidney shows both cortex and medulla, it is the form usually made for pathological examinations (Fig. 312). The tubules may be studied to better advantage, however, in tangential sections, one through the cortex and the other through the medulla. The tubules are then seen in cross section. The

Capsule of the glomerulus (outer layer.)

Thick segment of the descending limb of Henle's loop.

Proximal convoluted tubule.

\ I

Capillary. Ascending limb of Henles' loop ;

Collecting tubule.

FIG. 313. TANGENTIAL SECTION OF THE CORTEX OF A HUMAN KIDNEY. X 200. (Schaper.) The pars radiata is seen in the lower left corner. The line from "capsule of the glomerulus" passes between

two distal convoluted tubules.

rays of the cortex appear as islands of circular sections surrounded by the irregular convoluted tubules, among which are the scattered renal corpuscles. The greater part of such an island is shown in the lower portion of Fig. 313. The renal tubules are lined throughout with simple epithelium and their characteristic features will now be considered, beginning with the glomerular capsule.

The glomerular capsule (of Bowman) consists of two layers. Its inner


layer is a flat syncytium blending with the perivascular tissue of the glomerulus, and following its lobulations. The outer layer of the capsule is smooth, and is composed of flat polygonal cells. Terminal bars, which have been found in all other divisions of the renal tubules, have not been demonstrated in the capsule. The flat epithelium of the outer layer changes at the "neck" of the capsule to the low columnar epithelium of the proximal convoluted tubule. The neck may occur in various positions, generally being opposite the aperture through which the vessels enter and leave. The space between the layers of the capsule is continuous with the lumen of the convoluted tubule.

The proximal convoluted tubules are large (40-60 /* in diameter) , with irregular lumens and indistinct cell walls. In some animals the walls are folded so as to be vertically plaited. The cells show signs of secretory activity and are believed to excrete urea and pigments; the fluid part of the urine comes chiefly from the glomeruli. The nuclei are toward the base

Collecting Thin Thick

tubules. segment. segments.



of the cells, and the protoplasm contains granules arranged in vertical rows which form basal rods (Fig. 314). Toward the lumen there is a "brush border" suggestive of short non-motile cilia. It is uncertain whether this is normal or due to post-mortem disintegration. Clear spaces are sometimes seen in the outer part of the cells. The lumen is wide and the cells are low after copious urine production; and the reverse is true when the urine is scanty.

The upper segment of the descending limb of Henle's loop is similar in structure to the proximal convoluted tubules. It is a straight tubule, however, and is found in the radiate part of the cortex (Fig. 313).

The upper segments of the ascending limbs are also found in the pars radiata. Their protoplasm is less granular than that of the descending limbs, but closely resembles that of the distal convoluted tubules. The latter are typically shown in Fig. 313 (there being one on either side of the label line to the " capsule of the glomerulus"). Huber (loc. tit.) describes these tubules as showing "an outer dark zone which is finely striated,



and an inner zone which is lighter, the nuclei being placed at the junction of the two zones." It is probable, from their position, that the distal convoluted tubules in Fig. 313 are parts of the tubule which connects with the glomerulus shown in the figure.

The arched collecting tubules, into which the distal convoluted tubules empty, pass into the collecting tubules of the rays, which are readily identified. They have round and clear-cut lumens; cell walls are distinct (in all but the smallest), and the nuclei are regularly arranged. Thus the collecting tubule resembles an excretory duct.

The structures seen in the radiate part of the cortex are therefore the ascending and descending limbs of Henle's loops, and the collecting tubules;

Large collecting tubule.

Thick segments

of Henle's loop


Thin segments

of Henle's loop



they are shown in longitudinal section in Fig. 315. The convoluted part of the cortex contains proximal and distal convoluted tubules and glomerular capsules.

The medulla (Fig. 316) contains the same elements as the rays. The collecting tubules are larger, and their walls are more distinct. Among their columnar cells a few are decidedly darker than the others. The thick segments of Henle's loops are easily distinguished from the thin segments. The latter are slender (9-16 A* in diameter) but have large lumens. Cell walls are absent, and the cells are so flat that their nuclei cause elevations. The thin segments are generally descending, but they may also ascend, as seen in the inner zone of the medulla ; Fig. 3 1 5 is from the outer zone, in which most, if not all, of the thin segments are descending. (In comparing Fig. 316 with Fig. 313, it should be noted that the former is more highly magnified, and the thick ascending limbs appear more granular than those tubules of the cortex with which they are continuous.)



Connective tissue. Between the renal tubules there is a small amount of interstitial connective tissue. It is more abundant toward the papillae and around the vessels and glomeruli than elsewhere. Beneath the



Arched colecting tubule.

Papillary du

>.-' Tunica fibrosa.

Stellate vein.

_ Interlobular

artery. - Interlobular vein.

, Arciforw artery

Arcif or vein.

Interlobar artery.

Interlobar vein.


epithelium of the tubules it forms basement membranes, apparently homogeneous, but actually composed of fine fibrils. The normal amount of interstitial tissue should be carefully studied, since its increase is indicative of an important pathological condition. This tissue is continuous



with that of the fibrous capsule. The latter contains elastic fibers, which increase in abundance with age, and also smooth muscle fibers.

Lobes and lobules. In embryonic life the kidney is divided into lobes, bounded by the renal columns, and indicated by grooves upon the outer surface (Fig. 318). The grooves become obliterated during the first year. In the ox similar grooves are permanent; in many mammals as in the cat and rabbit, they never exist, since the kidney has but one lobe, papilla and pyramid. The lobules or structural units of the kidney are the areas centering around each radiate division of the cortex, by which they are drained

(Fig. 317)septa.

Blood vessels. The kidney has a capillary circulation. The renal artery passes from the aorta to the hilus, or notch on the medial border of the kidney. It divides into several branches, most of which pass over the


They are not bounded by connective tissue * CHILD AT BIRTH.

J J ( After Hertwig.)

Partly injected glomerulL

Jwterlobular artery. ""Interlobular vein.


ventral surface of the pelvis into the fat around the calyces (Fig. 311). Thence, as interlobar arteries, they extend to the boundary layer between the cortex and medulla where they are known as arciform arteries (Fig. 317). These send interlobular arteries through the convoluted part of the cortex and their terminal branches enter the fibrous capsule. It will be noted that the kidney is exceptional in having its arteries at the periphery of its lobules. From the interlobular arteries small stems pass to the glomeruli, each of which receives a single twig (Fig. 319). This is resolved into a knot of capillary loops, the endothelium of which seems to blend with the surrounding syncytium and indirectly with the inner layer of the capsule.



The glomerulus often appears lobed, due to the arrangement of its vascular loops. The capillaries unite to form a single efferent vessel which is smaller in diameter than the afferent vessel; thus the pressure within the glomerulus is increased. The entire glomerulus is regarded as arterial. On leaving it, the efferent vessel divides into small branches. These spread among the convoluted and straight tubules of the cortex, and some continue into the medulla. The latter is supplied also by straight branches (arteriola recta) from the interlobular, efferent and arcif orm arteries, as shown

in Fig. 317. The veins of the medulla begin around the papillae, and as venula recta empty

. , . ,

into the arciform veins The cortical veins are 9 the interlobular vessels which are beside the corresponding arteries. They arise from converging veins in the renal capsule, which on surface view form stellate figures (vena stellata). The interlobular veins drain the capillaries of the cortex, but have no direct relation with the glomeruli. Interlobar "veins follow the arteries, passing out from the hilus of the kidney over the ventral surface of the renal

Uriniferous tubules. PIG. 320. FROM THE KIDNEY OF A P

MOUSE. GOLGI PREPARATION. Lymphatic vessels are said to occur within the cortex and to follow the blood vessels out at

the hilus. The cortical lymphatics also pass through the tunica fibrosa to connect with a network in the adipose capsule. They proceed to neighboring lymph glands.

The nerves are medullated and non-medullated. There is a sympathetic plexus at the hilus associated with small ganglia, and from it interlacing nerves extend into the kidney around the vessels (Fig. 320). Fine branches supply the epithelial cells, especially those of the convoluted tubules. They form plexuses beneath and above the basement membrane, and have free intercellular endings.


The renal pelvis and ureter both consist of a mucosa (and submucosa), muscularis and adventitia (Fig. 321). The mucosa includes the epithelium and tunica propria, the latter blending with the submucosa. In sections the epithelium resembles that of the moderately contracted bladder (Fig. 322), and its cells when found detached in urine are not distinguishable from bladder cells. The epithelium is stratified but consists of few layers. The basal cells are rounded, those of the middle layer are club shaped or conical with rounded ends, and the outer cells are columnar, cuboidal,



or somewhat flattened. Their lower surface may be indented by the rounded ends of several underlying cells, as is particularly the case in the contracted bladder (Fig. 323). Two nuclei are often found in a superficial cell, and

Tunica adventitia.

Tunica mucosa.


e., Epithelium; t., tunica propria; 1, inner longitudinal muscle bundles; r, circular layer of muscle bundles

li, outer longitudinal muscle bundles.

in some animals they are known to arise by amitosis. Leucocytes frequently enter the epithelium. In some animals mucous glands have been found extending into the tunica propria, and there are gland-like pockets in man. Some of these have no lumen and it is said that none



a, Columnar cell with cuticular border; b, lymphocyte; c, tunica propria.


are true glands. Capillary blood vessels, which are abundant in the mucosa, are found directly beneath the epithelium and present the deceptive appearance of becoming intra-epithelial. The tunica propria consists of fine connective or reticular tissue, with few elastic fibers. It contains


many cellular elements and some lymphocytes, and passes without a definite boundary into the loose connective tissue of the submucosa.

The tunica muscularis has considerable connective tissue among its smooth muscle bundles. The latter form an inner longitudinal and an outer circular layer. In the lower half of the ureter there is a third, outer longitudinal layer, specially thickened along the last 5 cm. Around the papillae of the kidney the circular fibers form a "sphincter." The part of the ureter which passes obliquely through the wall of the bladder has only longitudinal fibers, ending in the tunica propria of the bladder. By contracting they open the outlet of the ureter. The adventitia consists of loose fibro-elastic connective tissue.

Lymphatics and blood vessels are numerous. There are sympathetic nerves to the muscles, and free sensory endings in the tunica propria and epithelium.


The development of the bladder from the ventral part of the cloaca has been described on page 245. Its epithelium is entodermal whereas that of the ureters opening into it is mesodermal. There is however no demarcation between the layers in the adult, since both produce the same sort of "transitional epithelium." (This term, introduced by Henle (Allg. Anat., 1841) as a designation for epithelia which are intermediate between stratified squamous and simple columnar, such as occur at the cardia and elsewhere, is now generally restricted to the peculiar epithelium of the bladder, ureter and renal pelvis.)

The bladder consists of a mucosa, submucosa, muscularis and serosa. The epithelium has been described as two-layered in the distended bladder, the outer cells having terminal bars; in the contracted condition it becomes several-layered and the bars form a net extending into the epithelium. Thus it is not believed that during distention the layers shown in Fig. 322 merely flatten; they are thought to "slip by each other." The columnar cells may, however, become extremely flat. The appearances of the epithelium in the bladder and ureter of the dog under various conditions of distention and contraction have been figured by Harvey (Anat. Record, 1909, vol. 3, pp. 296-307). The superficial cells have a cuticular border; they often contain two nuclei, and their darkly granular protoplasm has been considered suggestive of secretory activity. Round or oval pockets extend into the tunica propria (Fig. 324). Some of them have no lumen, or are detached from the epithelium, but others are pits containing a colloid substance. The pits are rudimentary glands. In the adult, branched tubules lined with cylindrical epithelium may sprout from the bottom of the pits, thus forming true glands. Their occurrence is limited to the



fundus, which is the dorsally bulging lowest part of the bladder, and to the neighborhood of the urethral outlet. In the latter position they have been regarded as rudimentary prostatic glands.

The tunica propria sometimes contains solitary nodules. It blends with the submucosa, as in the ureter, and contains lymphatic and blood vessels, the latter extending very close to the epithelium.

The muscularis consists of smooth muscle fibers arranged in three interwoven layers, which are seldom separable in sections. They are an inner longitudinal, middle circular and outer longitudinal layer. The

Tangential sections of pits.



Tunica propria. Smooth muscles.


circular fibers are strengthened at the beginning of the urethra to form the "internal sphincter" of the bladder, a muscle not always distinct.

The serosa is a connective tissue layer covered with mesothelium. In the non-peritoneal part of the bladder it is replaced by an adventitia or fibrous layer.

Non-medullated nerves, with scattered groups of ganglion cells, are found outside the muscles and also among them. Medullated fibers terminate around the ganglion cells; others pass through the ganglia to intra-epithelial sensory endings.


The male urethra will be described with the genital organs; only its upper portion is homologous with the urethra of the female. The latter


is exclusively the outlet of the urinary tract. The epithelium has been variously described as stratified, with outer squamous cells, or as pseudostratified, and columnar. It may be of different forms in different individuals. The lumen is irregularly crescentic, with longitudinal folds (Fig- 325). Branched tubular urethral glands are found only in small numbers, except near the outlet. Their secretion is mucoid, but is not typical mucus. In the submucosa there are many thin-walled veins con


d., Gland-like diverticulum; e., epithelium; L., lumen of the urethra; m., striated muscle; s., corpus spongiosum, containing venous spaces (v) and smooth muscle.

stituting the corpus spongiosum. This is comparable with the upper part of the more highly developed corpus cavernosum urethras of the male. (Compare with Fig. 349, p. 347.) The muscularis is a thick layer, consisting of inner longitudinal and outer circular smooth muscle fibers, among which the veins extend, and connective tissue with many elastic fibers is abundant. The striated constrictor urethra is outside of the smooth muscle layer, as shown in the figure.

Male Genital Organs

Development and General Features

The discovery that the Wolffian bodies become a part of the genital system was made by Oken, through dissections and injections of dog embryos (Beitrage, Heft II, 1807). Rathke studied these "Oken's bodies" further, and found more accurately their relation to the epididymis and ductus deferens. Miiller (Bildungsgeschichte der Genitalien, 1830) wrongly declared that they do not form the epididymis; but he discovered that "at the time when the Wolffian bodies are most highly developed, the germ of the ovary or testis lies on their inner side; and on their outer side, extending even to their upper end, there is a duct which does not connect with the Wolffian bodies it appears to have arisen from their short and much stouter excretory duct." He saw that this second duct, now known as the Mullerian duct, formed a part of the uterine tubes. In fact it forms the entire tubes together with the uterus and vagina; in the male it produces interesting vestigial structures which are constantly present in the adult.

The Mullerian duct arises as an outpocketing of the ccelomic epithelium near the anterior end of the Wolffian body. The orifice into the peritoneal cavity becomes surrounded by irregular folds known as fimbrice. As the Mullerian duct grows posteriorly by the elongation of its blind end, it lies in contact with the Wolffian duct as seen in Fig. 326, but the Wolffian duct does not contribute toward its formation. The two Mullerian ducts reach the neck of the bladder side by side, and acquire openings into it between those of the Wolffian ducts. Near the bladder the two Mullerian ducts fuse with one another so that their distal part is represented by a single median tube, on either side of which is a Wolffian duct (Fig. 306, B, page 311). In the female the united portion becomes the -vagina and uterus, and the separate parts are the uterine (or Fallopian) tubes. In the male the united portion becomes a small blind pocket, the prostatic utricle, opening into the prostatic urethra. Each fimbriated extremity becomes transformed into the appendix testis, and the remaining portion of the ducts, except for occasional fragments, becomes obliterated. Thus only the two extremities of the Mullerian ducts are ordinarily permanent in the male (Fig. 328).

The genital glands in either sex begin as a thickening on the ventromedial border of each Wolffian body (Fig. 326). A section of this genital ridge is shown in Fig. 303, C, page 307. The ridge is a dense mass of mesoderm covered by the peritoneal epithelium, which here consists of a syncytium very closely connected with the underlying tissue. According to Felix (Keibel and Mall's Human Embryology, vol. 2) everything that is later developed within the genital ridge has a common origin from the peritoneal epithelium. The ridge becomes filled with an epithelial mass which then separates from the peritoneal layer. Beneath the peritoneum this mass produces the dense connective tissue capsule which surrounds the testis, called, from its whiteness, the tunica albuginea; within the genital ridge it is "quite suddenly" resolved into anastomosing cords with looser tissue between them, and the cords become the tubules of the testis. Allen, in an earlier account (Amer. Journ. Anat., 1904, vol. 3, pp. 89-155), likewise finds that the cells of the peritoneum and the underlying mesenchyma appear to form a continuous protoplasmic network, and "the stroma cells are practically identical with the peritoneal cells from which they are originating." But Allen concludes that " the tubules of the testis are formed as solid imaginations of the peritoneum, which later become separated from it, and grow by the activity of their component cells." There is, then, a difference of opinion as to whether the tubules of the testis are formed directly from the stroma within the genital ridge (Felix), or as invaginations from the peritoneal epithelium (Allen). A figure of an n-mm. human embryo published by Felix appears to accord with Allen's interpretation, and such a condition is shown diagrammatically in Fig. 327.

FIG. 326. FROM A RECONSTRUCTION OF A 13.6. MM. HUMAN EMBRYO. (F. W. Thyng.) bl., Bladder; f., fimbriae; g. g., genital ridge; g. p.

FIG. 327. DIAGRAM OF THE DEVELOPMENT OF THE TESTIS, BASED UPON FIGURES BY MACCALLUM AND B. M. ALLEN. genital papilla; M. d., foullerian duct; p. c., Glomerular capsule; i. c. f inner or sex cords; M. d., Mullerian duct; o. c., outer or rete cords; W. d., W. t., Wolffian duct and tubule. renal pelvis; r., rectum; ur., ureter; u a., urogenital sinus; W. d., Wolffian duct.

As the cords become detached from the peritoneum, they form arching anastomoses, convex toward the periphery of the ridge; and with further growth they become greatly convoluted. They acquire lumens, and become the tubuli contorti, in the walls of which spermatogenesis takes place. The shapes presented by these tubules in the embryo have been carefully modelled by Bremer (Amer. Journ. Anat., 1911, vol. n, pp. 393-416).

Toward the interior of the genital ridge the cords become more slender and converge toward the Wolffian body. There they are imbedded in a considerable mass of tissue, which in the adult becomes the mediastinum testis. The inner ends of the contorted tubules, toward the mediastinum, remain straight, forming the tubuli recti; and these, further inward, become thin-walled and anastomose freely, thus constituting the rete testis (Fig. 328).

All the tubules thus far considered are produced by the genital ridge. Their inner ends, which form the rete, acquire openings into the capsules of the degenerating Wolffian glomeruli, or sometimes directly into a Wolffian tubule. From ten to fifteen Wolffian tubules thus become connected with the rete testis, and serve to convey the genital products to the Wolffian duct; these tubules are known as the ductuli efferentes. In the adult each of them is a greatly convoluted tube which if straightened measures 8 inches (20 cm.). When coiled, it forms a conical mass or lobule of the epididymis, with its apex toward the rete, and its base toward the Wolffian duct which it enters (Fig. 328). The Wolffian duct, which passes along the dorsal surface of the testis, is also greatly convoluted so that it measures about 20 feet when straight (6-7 meters). Together with the efferent ducts this coiled mass constitutes the epididymis (Gr. eVt, upon; S6'Su/w>?> testis). Along the testis the Wolffian duct is called the ductus epididymidis, and from the testis toward the urogenital sinus it is named the ductus deferens. Near its termination a saccular outgrowth, like a distended gland, develops from each Wolffian duct. It is called the seminal vesicle, and that portion of the Wolffian duct between the duct of the vesicle and the urethra is named the ejaculatory duct. Thus the Wolffian duct is arbitrarily divided in the adult into three parts, the ductus epididymidis, ductus deferens, and ductus ejaculatorius.

FIG. 328. DIAGRAM OF THE MALE SEXUAL ORGANS. (Modified from Eberth, after Waldeyer.) (The course of the Mullerian duct is indicated by dashes.)

It has been noted that only 10-15 f the Wolffian tubules persist as efferent ducts; in man, according to Felix, these are the fifty-eighth to seventieth out of a series of eighty-three which develop. Thus a great many degenerate, and certain appendages of the epididymis are explained as persistent remnants. The appendix epididymidis may represent a part of the Wolffian duct or an anterior tubule (Fig. 328); its history is still obscure. Other anterior tubules may be retained as appendages of the rete. The paradidymis is "a functionless remnant of the Wolffian body," situated behind the head or upper end of the epididymis and in front of the cord of veins which accompany the ductus deferens.

Giraldes first described it (Bull. Soc. Anat. Paris, 1857) and Koelliker named it the "organ of Giraldes"; Henle called it the paraepididymis (i.e., the organ beside the epididymis), and Waldeyer later shortened the term and changed its meaning. Felix (loc. cit., 1912) contrary to the earlier descriptions, places the paradidymis "between the epididymis and the testis, slightly below the head of the epididymis." Toldt (Verb. Anat. Gesellsch, 1892, pp. 241-242) recognized two forms of paradidymis, but both are behind the epididymis and in front of the veins of the spermatic cord. The precise origin of these tubules from the Wolffian body has not been determined.

Other remains of the Wolffian body, apparently derived from the tubules below those which become efferent ducts, are known as aberrant ducts (ductuli aberr antes}. There may be two or three of them; usually there is said to be but one. It proceeds from the duct of the epididymis, or rarely from the ductus deferens at its junction with the duct of the epididymis, and terminates in a coiled mass, sometimes having branches. The length of the aberrant duct is "4-36 cm., generally 5-8 cm." (Henle).

The External Genital Organs. After the cloaca has been divided into ventral and dorsal portions by the downward growth of the perineal septum, the ventral portion below the outlets of the Wolffian ducts is called the urogenital sinus. It receives both urinary and genital products, and in the male it forms all of the urethra below the orifices of the ejaculatory ducts. In the young embryo, the distal part of the urogenital sinus becomes laterally compressed so that it forms an epithelial plate. This plate reaches the external surface of the body along the mid-ventral line of an elevation known as the genital papilla (or tubercle). The genital papilla (Fig. 326) becomes very prominent in embryos of both sexes. In the male it continues its development and forms the penis, along the under side of which the urogenital sinus acquires a cleft-like opening (Fig. 329, A). This elongated aperture closes from behind forward, along the line permanently marked by a rap he (or seam). In the abnormal cases of hypospadias, the urogenital sinus retains a more or less extensive opening on the under side of the penis. A rounded terminal glans is early differen



tiated at the apex of the genital papilla. The epidermis is adherent to it, but later becomes separated by the formation and splitting of an epithelial plate, thus producing the reflection of skin called the prepuce. The urogenital sinus becomes secondarily prolonged through the glans so as to form the terminal part and external orifice of the urethra. The entire urethra is divided into three parts: (i) the prostatic portion (pars prostatica), which includes the outlet of the bladder together with the upper end of the urogenital sinus, and receives the ejaculatory and prostatic ducts; (2) the membranous part (pars membranacea) , which is the short dilatable portion traversing the "pelvic diaphragm"; and (3) the long cavernous portion (pars cavernosa), which is surrounded by the cavernous vascular tissue.

The scrotum develops as a median pouch at the dorsal end of the urogenital raphe. It is continuous above with the pair of large genital folds which tend to encircle the base of the genital papilla, being deficient only below (Fig. 329, A). At the stage when the testis and Wolffian body are


a., Anus; ep., epididymis; g., glans penis; g. f., lesser genital folds; g. g. f., greater genital folds; p. c., peritoneal cavity; p. v., processus vaginalis; r., raphe; t., testis; p. 1., parietal layer of the tunica vagmalis; u. s., urogenital sinus; v. 1., visceral layer of the tunica vaginalis.

still within the abdomen, lying behind the peritoneum, the peritoneal cavity sends a prolongation, the processus vaginalis, over the pubic bone into each half of the scrotum (Fig. 329, B). A large retroperitoneal column of connective tissue, the gubernaculum testis, extends from the posterior end of each testis into the depth of the scrotum. For reasons still obscure, such as unequal growth or the shortening of this cord, the testes pass down in front of the pubic bones, into the scrotum (Fig. 329, C). The Wolffian duct becomes bent over the ureter as shown in Fig. 328, and this important relation is found in the adult. Except on its dorsal border, the testis is closely invested by the peritoneum of the processus vaginalis. Later the distal part of the processus becomes separated from the abdominal cavity by the obliteration of its stalk. The part remaining about the testis is the tunica vaginalis, having a parietal and a visceral layer as shown in Fig. 329, D. The descent of the testes is completed shortly before birth, except in the occasional cases of "undescended testis."




Septa, Vessels, and Nerves. The general arrangement of the parts of the testis, as they appear in cross section, is shown in Fig. 330. From the tunica albuginea, small connective tissue septa (septula testis} pass to the mediastinum, dividing the testis into "100-200" pyramidal lobules with their apices toward the rete. The tunica albuginea is a dense connective tissue layer, containing numerous elastic fibers which increase in abundance with age. Its outer surface is covered with the visceral layer of the tunica vaginalis. The inner portion of the albuginea is very vascular, forming a distinct layer at birth (the tunica vasculosa) .

Ductus deferens. Blood vessels.



containing the

rete testis.

Straight tubules.

Tunica vasculosa.

Tunica albuginea.


Connective tissue extends from the septula among the convoluted tubules. Immediately surrounding them there is a delicate basement membrane, followed by a layer of closely interwoven elastic fibers and flat cells. In the looser connective tissue between the tubules, there are clumps of interstitial cells (Figs. 331 and 335), which arise from mesenchymal cells of the genital ridge. Sometimes they retain protoplasmic processes, but more often they are rounded or polygonal structures in close contact, and without distinct cell boundaries. In their abundant protoplasm there are pigment and other granules, fat droplets, and rod-shaped



crystalloids, the significance of which is unknown. The nature of the granules is discussed by Whitehead (Amer. Journ. Anat, 1912, vol. 14 pp. 63-71).

The interstitial cells, although not intimately related with the vessels, are thought to produce an internal secretion, and certain observations suggest that the sexual instinct is dependent on these cells rather than upon the spermatozoa (cf. Whitehead, Anat. Rec., 1908, vol. 2, pp. 177182). During senile atrophy of the testis, the interstitial cells at first increase; later they are destroyed. At the same time the basement membrane becomes thickened and hyaline, fat droplets accumulate, and the sexual cells disappear from the tubules, leaving the sustentacular cells.

The arteries of the testis are branches of the internal spermatic artery, which descends through the spermatic cord, beside the ductus deferens. The branches enter the testis in part through the mediastinum, and in

Interstitial cells.

Connective tissue


part through the tunica vasculosa. They extend through the septula, and form capillary plexuses around the convoluted tubules. The veins accompany the arteries. Lymphatic vessels are numerous in the tunica albuginea and extend among the tubules. Nerves from the spermatic plexus surround the blood vessels; the presence of intraepithelial endings has not been established with certainty.

Convoluted Tubules. The shape of the tubules of the testis has been repeatedly investigated, but whether blind ends occur has not been established; generally the tubules end in loops. Anastomoses have been recorded, not only between the tubules in a single lobule, but also between adjacent lobules. The extent of the anastomoses among the closely coiled tubules is difficult to determine.

For more than seventy years eminent anatomists have recorded their success or failure in finding blind ends Krause, Kolliker, Sappey and LaValette St. George state that they exist; Hyrtl, Henle, Mihalkovics and Eberth fail to find them. Two



recent papers have dealt with the subject. Bremer (191 1) concludes that the tubules may end blindly; Huber and Curtis (1913) state that the seminiferous ubules in the rabbit present no blind ends.

The convoluted tubules are lined with a highly specialized stratified epithelium (Fig. 332). The cells divide and differentiate as they pass from the basal layer outward. Finally each outer cell produces a single


Sustentacular cell.


Blood vessel with blood corpuscles.

Fat granules


Sustentacular cell. Spermatogonia, beneath Sustentacular cells, large spermatocytes.

large cilium, or flagellum, projecting from the free surface, and becomes detached as a spermatozoon. The process of transformation of the basal cells, or spermatogonia, into spermatozoa is known as spermatogenesis. Its cytological features, as observed in the testis of the grasshopper, have already been described (p. 21). Ordinary sections of the human testis present the following characteristics:

Each tubule is composed of cells of two sorts sexual cells and sustentacular cells. At birth the cords and developing tubules contain relatively few sexual cells (Fig. 333). These are characterized by their large size, clear protoplasm, and round vesicular nuclei. It is said that they retain a primitive granular arrangement of their mitochondria. These cells multiply by ordinary mitosis, producing the spermatogonia. Thus the sexual cells in various forms eventually far outnumber the Sustentacular cells.



The sexual or genital cells are apparently produced from the cords in the testis, relatively late in embryonic development. It was suggested by Nussbaum, however, that the sexual cells are set apart much earlier "they do not come from any cells that have given up their embryonic character or gone into building any part of the body." In accordance with this idea, it is considered by some authorities that in the segmentation stages, a line of undifferentiated cells is set apart to become the sexual cells, add that from the beginning they are distinct from the somatic cells which form the rest of the body. As stated by Allen (Journ. Morph., 1911, vol. 22, pp. 1-36), the sexual cells do not belong to any one germ layer; they are free to follow their own path in their travels from the place of origin to the genital glands where they finally come to rest. Thus the sexual cells have been reported as distributed somewhat diffusely in the entoderm and mesoderm. (For papers on this subject, see Allen, Anat. Anz., 1906, vol. 29, pp. 217-236.) In a human embryo of 2.6 mm. Felix found seven of these large clear cells, all in the immediate vicinity of the cloaca. Another embryo of 2.5 mm., showed twelve "primary genital cells." But he adds that they all disappear in later stages, and when the genital glands are formed there are no genital cells. At present it has by no means been demonstrated that the mammalian sexual cells are not differentiated products of the testis or ovary, adapted for the special purpose of reproduction.



a., Isolated (Koelliker); b., Gplgi preparations. (B6hm and von Davidoff.)

The sustentacular or supporting cells, often called Sertoli's cells, are at first indifferent cells forming a syncy tium (Fig. 333) . With the increase in the number of spermatogonia, their protoplasm is resolved into a network of strands, molded by the surrounding cells (Fig. 334). Their nuclei are radially compressed into ovoid shapes, and lie in columns of protoplasm extending from the periphery of the tubule toward its lumen. Each nucleus has a distinct nucleolus, apart from which its chromatic material is very scanty. Usually the nuclei are in the lower half of the branching protoplasmic columns, the polygonal bases of which are in contact with one another beneath the spermatogonia. Within the protoplasm fat droplets occur, together with brown granules; crystalloid bodies in pairs may also be found. As seen in Fig. 334, a, the heads of the spermatozoa appear attached to, or imbedded in, the protoplasm of the sustentacular cells, which are supposed to nourish them. The



spermatozoa may be gathered in characteristic clumps at their upper ends (Fig. 332).

In ordinary sections of the testis, the sustentacular cells may be recognized by their distinctive nuclei (Fig. 335). The sexual cells in the basal row are presumably spermatogonia. Above them are the spermatocytes, which are larger; their nuclei usually show spiremes or other indications of cell division. Secondary spermatocytes are further out than the primary spermatocytes; and above them are the spermatids in various stages of transformation into spermatozoa. Since spermatogenesis occurs in "waves," the outer cells in a tubule cut lengthwise form a succession of zones, each of which shows gradations from young spermatids to mature spermatozoa; a single zone is included in Fig. 335. In transverse sections all the superficial cells may be of one stage, which differs from that in the adjoining tubule (Fig. 332).

Heads of _ spermatozoa.


Crystalloid, in


Nuclei of sustentacular cells.


Inter stitial con necti ve tissue.


Stages in the transformation of a spermatid into a spermatozoon are shown in the diagram Fig. 336. The chromosomes of the spermatid disappear in a dense chromatic network which becomes apparently homogeneous. This deeply staining nucleus passes to one end of the protoplasm of the spermatid. It becomes the essential part of the head of the spermatozoon, which in man is a flattened structure, oval on surface view, and pyriform with its apex forward when seen on edge (Fig. 337). The head is at the anterior end of the spermatozoon, which during its development is directed toward the basal layers of the convoluted tubule. The anterior end of the head is probably covered by a thin layer of protoplasm, known as the galea capitis. The archoplasm of the spermatid (known as the idiozome) is said to leave the centrosome and to enter the protoplasm of the galea capitis, where it forms the perforatorium. If this exists in man, it is in the form of a cutting edge following the outline of the front of the head; in other animals the perforatorium may be a slender spiral or barbed projection, which enables the spermatozoon to penetrate the ovum.



The protoplasm of the spermatid forms an elongated mass at the posterior end of the nucleus. It contains the centrosome which soon divides in two. Of these the anterior forms a disc which becomes adherent to the nuclear membrane. The posterior centrosome also becomes a disc after giving rise to a motile axial filament, which grows out from it like a cilium. The disc-like centrosome attached to the anterior end of the filament becomes thin in such a way that its peripheral portion is detached, and as a ring surrounding the filament it passes to the posterior limit of the protoplasm. The protoplasm between the two parts of the posterior centrosome is reduced to a thin layer in which a spiral filament develops, winding about the axial filament. Distal to the centrosome ring, the axial filament, which consists of fine fibrils, is surrounded by a thin membrane, which terminates or becomes very thin near the extremity of the filament. This membrane, which in salamanders forms a conspicuous undulating frill, is thought to be a product of the filament and not an extension of the protoplasm. In man it is inconspicuous, and many of the details here described can be made out only under most favorable conditions. The preceding account is based on studies of the guinea-pig (Meves, Arch. f. mikr. Anat., 1909, vol. 73, pp. 751-792).

Mature spermatozoa are divided into three parts the head, neck, and tail. The head (3-5 /* long and 2-3 /* wide) includes the nucleus, galea capitis and perforatorium. The neck consists of the anterior centrosome and the substance, not traversed by the axial filament, between it and the posterior centrosome. The neck in man is not constricted as in some forms, yet it is a place where the head may become detached. The tail includes three parts, the connecting piece, chief piece

and end piece. The connecting piece (6 ft long and scarcely i /* wide) consists of protoplasm, axial and spiral filaments, and the two parts of the posterior centrosome. The chief piece (40-60 /* long) is the axial


a. c., anterior centrosome; a. f., axial filament; c. p., connecting piece; ch. p., chief piece; g. c., galea capitis; n., nucleus; nk., neck; p., protoplasm; p. c., posterior centrosome.



a, Head; b, connecting piece,

and c, chief piece of the

tail, i, 3, and 4, Surface

views; 2, side view. X 360


filament with its surrounding membrane; and the end piece (10 /*) is a prolongation of the filament. When the spermatozoa become free they float in the albuminous fluid secreted in small quantity by the tubules of the testis. They pass through the straight tubules and rete to the epididymis, in which they accumulate, and where they first become motile. Their motility is greater, however, in the seminal fluid, which is a mixture of the products of the epididymis, seminal vesicles, prostate and bulbourethral glands. By an undulating movement of the tail, the head is propelled forward, always being directed against such a current as is made by cilia, at a rate of | of an inch in a minute. Water inhibits the motion, which is favored by alkaline fluids; it occurs also in those faintly acid. For three days after death spermatozoa may retain their activity in the seminal passages; in the female urogenital tract they may live a week or more. In addition to normal spermatozoa, giant forms, and some with two heads or two tails occur, but these are probably functionless abnormalities. The production of spermatozoa, beginning at puberty, continues throughout life, but with advancing age the rate diminishes. Since about 60,000 spermatozoa occur in a cubic millimeter of seminal fluid, it has been estimated that 340 billions are produced in a lifetime.

The discovery of spermatozoa was reported to the Royal Society of London, in 1677 by Leeuwenhoek. They were first seen by Dr. Ham, "a man of singular modesty," to whom Leeuwenhoek gives full credit for the discovery in his letters to the Royal Society. He wrote as follows:

"This discerning youth visited me and brought with him, in a small glass vial, seminal fluid from a man who had cohabited with a diseased woman; and he stated that after some minutes when the fluid had become so attenuate that it could be put in a slender glass tube, he had seen living animalcules in it, which he thought were produced by some putrefaction. He added that those animalcules seemed to him to be provided with tails, and that they did not survive the space of twenty-four hours. Moreover he declared that when terebinth had been given to the patient internally, the animalcules appeared to be dead.

"I poured this material in a glass tube and examined it in the presence of Dr. Ham, and saw some live animalcules in it. But when after two or three hours, I examined the material more carefully, by myself, I saw that all the animalcules were dead."

Leeuwenhoek diligently pursued the study of these animalcules, and found them in enormous numbers in the semen of insects, fishes, birds and quadrupeds. He estimated that there were 150,000,000,000 in the milt of one fish, or more than ten times the number of men then living (13,385,000,000 homines in orbe terrarum). Leeuwenhoek believed that the animalcules were of two sexes, and that the egg consisted of a fluid in which they swam about and developed. To some it seemed not unreasonable that new individuals should be enclosed in the spermatozoa, like an insect in its chrysalis, and Dalenpatius (1699) thought that he could observe them. As quoted by Vallisneri, he wrote as follows, illustrating his account with the figure here reproduced (Fig. 338).

"We have seen some animalcules having just the form of tadpoles such as are found in brooks and muddy bogs in the month of May. The tail is four or five times as long, as the body. They move with wonderful rapidity and by the strokes of their tails pro



duce little waves in the substance in which they swim. But who would believe that in these a human body was hidden? Yet we have seen such with our own eyes. For while we were observing them attentively, a large one threw off its surrounding membrane and appeared naked, showing distinctly two legs, thighs, breasts and arms. The cast-off skin, drawn upward, covered the head like a cap, and it was a delightful and incredible sight. Because of the minuteness of the object, the sex could not be distinguished. After the little creature had lost its membrane it soon died."

This is a gross presentation of the preformation theory, according to which the various parts of the adult are represented in the very young embryo. It was held by many who could not verify such observations. An alternative theory is that of epigenesis, according to which the body FlG 8 and its parts arise out of formless substance. Descartes (1664) wrote that the source of a new individual "seems to be only a confused mixture of liquors, which, serving to leaven one another, become heated; some of their agitated particles dilate, and press upon the others, gradually disposing them in the way necessary to form organs." Such physico-chemical speculations however, are quite as imaginative as any views of the preformationists and Descartes's epigenesis was early characterized as " a very lame account of the forming of an animal." Nevertheless, the doctrine of epigenesis, as advocated by Harvey (1651) and Wolff (1759), prevailed over the cruder ideas of preformation. If, however, the spermatozoon can contribute to the production of only one of the myriad forms of animals, even the sex of which is apparently predetermined, it is evident that the spermatozoon must possess a very definite chemical composition, and perhaps a corresponding ultra-microscopic structure. Doubtless there is a preformation no less remarkable than that expressed through the active imagination of Dalenpatius.

Tubuli Recti and the Rete. The large convoluted tubules are 140 ft in diameter. As they pass toward the epididymis they decrease in size; they receive branches at acute angles and their windings diminish. Sexual cells disappear, leaving only the sustentacular cells in the form of a simple columnar epithelium. This flattens abruptly to form the lining of the straight tubules. Both the straight tubules and the rete are lined with a simple epithelium of low cells. In some places these are very flat, suggesting endothelium; in others they are columnar. The characteristic dilatations of the rete tubules are shown in Fig. 339. They contain spermatozoa and immature sexual cells together with pigment granules and broken down cells.


The efferent ducts, which pass from the rete to the duct of the epididymis, are lined with an epithelium in which groups of columnar cells alternate with those which are cuboidal (Figs. 340 and 341). Thus the inner surface of the epithelium has depressions suggesting glands, but the basal surface is free from outpocketings. The epithelium is generally simple, although in the tall parts it may appear two or three layered. The cells contain fat, pigment, and other granules, and produce a secretion which



P*^. ' /> ?' .? : - f . ' ji?s^>' -/^'

fe^^'/f*?'*:*^ ' -%P+ *



A, -Artery; C, rete tubules; L, lymphatic vessels; s, connective tissue partly surrounded by 'rete tubules Sk, part of a convoluted tubule, to the left of which are sections, probably of straight tubules; V, vein.

.Tangential section

of a ductulus


Connective tissue.

Blood vessel. Epithelium Circular muscles Transverse section of a

ductulus efferens. of the ductus epididymidis.




Cubical cells. Columnar cells.

may appear in vesicular masses on the surface of the cells. Often the tall cells, and occasionally the short ones, are ciliated. The cilia vibrate so as to produce a current toward the ductus epididymidis. The epithelium rests on a striated basement membrane which is surrounded by a layer of circular smooth muscle fibers, several cells thick. The muscle layer is thickest toward the ductus epididymidis. Among the muscle cells there are elastic fibers, which, like those of the ductus epididymidis and ductus deferens, first appear at puberty. There are no glands in the efferent ducts, but the irregularities in the epithelium are thought to be due to glandular activity. Before puberty and in old age these irregularities are slight.

Smooth muscle fibers. Connective tissue.


FROM THE TESTIS OF AN ADULT MAN. The right-band end of the illustration is schematic. No cilia

could be seen, although those of the epithelium of the epi didymis were well preserved. X 360.

The ductus epididymidis consists of a two-rowed epithelium with rounded basal cells and tall outer columnar cells. The latter contain secretory granules and sometimes pigment, and have in the middle of their upper surfaces long non-motile hairs, which in sections are usually matted in conical processes (Fig. 41, &, p. 51). The epithelium may contain round cavities opening into the lumen or forming closed cysts. The delicate membrana propria and thick circular muscle layer complete the wall of the ductus, the convolutions of which occur in a loose connective tissue. Toward the ductus deferens the muscle layer thickens.

There are no glands in the

ductus epididymidis, but its cells produce considerable secretion in which the spermatozoa become active.

The blood vessels of the epididymis, which are few in comparison with those of the testis, lie in part so close to the efferent ducts as to cause the membrana propria to bulge toward the epithelium. The nerves, besides perivascular nets, form a thick plexus myospermaticus provided with sympathetic ganglia. It is found in the muscle layer, which it supplies, sending fibers also into the mucosa. In the ductus deferens and seminal vesicles this plexus is said to be more highly developed than in the epididymis.

" Epithelium.

Membrana propria.

Circular layer of muscle fibers.

Loose connective tissue.





.Tunica propria.

Inner longitu.-'dinal muscles.

Circular 'muscles.

Outer longitu'dinal muscles.


The ductus deferens begins as a convoluted tube continuous with the ductus epididymidis; it becomes straight and passes to its termination in the ductus ejaculatorius. Shortly before reaching the prostate it exhibits a spindle-shaped enlargement or ampulla about | inch long and f inch wide (Fig. 344). The ductus deferens consists of a mucosa, muscularis and adventitia. The epithelium is generally in two rows, the tall inner cells producing round masses of secretion. Toward the epididymis it may also have non-motile cilia. Toward the ampulla it may be

several rowed, resembling the epithelium of the bladder. It rests on a connective tissue tunica propria, which is surrounded by the three layers of the muscularis. The inner and outer layers are longitudinal and generally less developed than the middle circular layer. The adventitia is a loose elastic connective tissue, blending with that which forms the spermatic The latter contains arteries, veins,

and nerVCS, tO gether with the striated mus cle fibers of the cremaster muscle, and the rudiment of the processus vaginalis. The veins are very numerous and constitute the pampiniform plexus (i.e., tendril-like). Their walls are usually provided with a very thick musculature including both circular and longitudinal fibers.

In the ampulla the longitudinal folds, which are low in the ductus deferens, become tall and branched, so that they partly enclose irregular spaces ( dive rticula) . Similar folds occur in the seminal vesicles. It is doubtful whether in either place any of the spaces should be considered glands. Around the ampulla the musculature is irregularly arranged; the longitudinal layers separate into strands which terminate toward the ejaculatory ducts.


The seminal vesicles grow out from the ductus def erentes at the prostatic ends of their ampullae. Each consists of a number of saccular expansions arranged along the main outgrowth, which is irregularly coiled. The

cord. numerous




lining of the sacs is honeycombed with folds as shown in Figs. 344 and 345. The epithelium is generally simple or two-layered, the height of the cells varying with the distention of the vesicles by secretion. Granules occur in the cells, which produce a clear gelatinous secretion in sago-like masses. Spermatozoa are generally found in the human vesicles, but except during sexual excitement they are absent from the vesicles of rodents; this and other facts indicate that the function of the organ is primarily glandular. The lumens of the various sexual glands are generally of very large caliber, associated with the storing of secretions. Pigment granules in varying


FIG. 344. SEMINAL VESICLE AND DUCTUS DEFERENS. (This is natural size.) (After Eberth.)

ad., Adventitia; am., ampulla; d., diverticulum; d. d., ductus deferens; d. e., ductus ejaculatorius; m., muscularis; s. v., seminal vesicle; t. p., tunica propria.


SEMINAL VESICLE. (After KSlliker.) ep.,. Simple epithelium; g., gland-like depression; m., muscularis; t. p., tunica propria.

quantity occur in the epithelial cells and in the underlying connective tissue. They may impart a brownish color to the secretion.

The ductus ejaculatorii, along their dorso-median sides, are beset with a series of appendages, which do not project externally but are wholly enclosed in the connective tissue wall of the duct. Some of these appendages show the same structure as the seminal vesicles and therefore might be described as accessory seminal vesicles; others are simply convolutions of alveolo-tubular glands which may be compared with prostate glands. The mucous membrane of the ductus ejaculatorii is like that of the seminal vesicles, except that its folds are not so complicated. Muscle


fibers occur only around the appendages. The wall of the duct itself consists of an inner dense layer of connective tissue with circular strands, and an outer loose layer (adventitia).


The appendices are frequently called hydatids, which is a general term for watery cysts. The appendix testis is a small lobule of connective tissue projecting from the groove between the head of the epididymis and the testis (Fig. 346). It is quite constant, having been reported in 90% of the testes examined. The projection is covered with the peritoneum of the tunica vaginalis, which may be thickened around it, or corrugated, suggesting the fimbriated orifice of the uterine tube. The appendix consists of vascular connective tissue and encloses a canal, or fragments of canals, lined with simple columnar epithelium which is sometimes ciliated. It is generally not cystic, and it may be pedunculated, so that the terms "hydatid of Morgagni" and "sessile hydatid," formerly applied to it, are inappropriate. Although its canal has been reported as connecting with the seminal ducts, this is not now believed to be the case; the structure is regarded as the degenerated end of the Miillerian duct.

The appendix epididymidis (stalked hydatid) is not always present. Among 105 cases examined by Toldt it was found twenty-nine times. It consists of loose vascular connective tissue covered by the vaginalis, and contains a dilated canal lined with columnar epithelium, sometimes ciliated. The canal generally has no connection with the tubules of the

. , r epididymis. It is regarded as a persistence of detached


A TESTIS, somewhat re- degenerating WolfEan tubules, or possibly of the terminal

duced. (AfterEberth.) , ...

a. e., Appendix epididymi- portion of the Wolffian duct.

iif* c t "e a , P capu n t ( epf" The P^adidymis, according to Toldt (Verh. Anat. didymidis; 't, testis; t- Gesellsch., 1892, pp. 241-242), occurs in two forms. The

v., tunica vaginalis.

first is found frequently, but by no means regularly, in

older embryos and in children. It is a round or elongated structure, conspicuous because of its white color, found on the ventral side of the spermatic cord, either behind the head of the epididymis or higher up. Microscopically it is seen to be a thin, coiled, blind canal, expanded in places, and lined with a simple columnar epithelium. Occasionally there are two to four such structures at varying distances from one another. In later years they all disappear. They never contain spermatozoa.

The second form of paradidymis was found by Toldt in late childhood and in adults, but it does not occur regularly. It is always immediately behind the head of the epididymis and in front of the pampiniform plexus. It consists of a canal, sometimes with saccular dilatations, which is easily followed with the naked eye. The tubule may be closed at both ends, or one end may connect with the epididymis or testis; sometimes one end connects with the testis and the other with the epididymis. These tubules may contain spermatozoa, and they have been said to resemble the efferent ducts in structure. They may be ciliated.

Toldt regards the first form of paradidymis as due to persistent WolfEan tubules, and the second as a late separation of an efferent duct from its connection with the epididymis. He notes that the second form may give rise to cysts of varying size. Other cysts in the vicinity of the epididymis are said to arise from inpocketings of the tunica vaginalis.




The prostate is a group of branched tubulo-alveolar glands, imbedded in a mass of muscular tissue, which stands before the outlet of the bladder. The smooth muscle of the adult prostate forms a quarter of the bulk of the organ, and together with an elastic connective tissue, it unites the numerous glands in a compact mass. The development of these glands up to the time of birth, has been studied by Lowsley (Amer. Journ. Anat., 1912, vol. 13, pp. 299-349). He finds that the prostate includes from fifty-three to seventy-four separate glands (the average number being sixty-three) which are grouped in five lobes. The middle lobe consists of nine to ten large glands growing out from the dorsal side of the urethra, between the bladder and the openings of the ejaculatory ducts. The glands of the posterior lobe grow out from the dorsal wall of the urethra below the ejaculatory ducts; those of the right and left lobes develop from the sides of the prostatic urethra; and those of the anterior lobe proceed from its ventral surface. The anterior lobe is well developed in young embryos, but it "shrinks into insignificance at the twenty-second week." It may persist in the adult, but the great mass of the prostatic glands is at the sides and back of the prostatic urethra. The number of glands apparently becomes reduced. In the adult it is said to be from thirty to fifty.

The glandular epithelium is simple and either cuboidal or columnar. It may appear stratified as it passes over the folds in the walls of the tubules. Near the outlet of the larger ducts the epithelium is like that of the bladder and prostatic urethra. In the prostatic alveoli, of older persons especially, round or oval colloid masses from 0.3 to i.o mm. in diameter occur; as seen in sections (Fig. 348) they exhibit concentric layers. Their reactions on treatment with iodine solutions suggest amyloid. These concretions are probably deposited around fragments of cells. Octahedral crystals also occur in the prostatic secretion, which is a thin milky emulsion, faintly acid; it has a characteristic odor, whereas the other constituents of the seminal fluid are said to be odorless.




The smooth muscle fibers are found everywhere between the prostatic lobules; toward the urethra they thicken to form the internal sphincter of the bladder. Smooth muscle is also abundant on the surface of the prostate, and it borders upon the striated fibers of the sphincter of the membranous urethra. The prostate is abundantly supplied with blood and lymphatic vessels. The numerous nerves form ganglionated plexuses from which non-medullated fibers pass to the smooth muscles; others of the nerves have free endings; still others, both in the outer and inner

Red corpuscles in a blood vessel.

Connective tissue


Smooth muscle fibers.

FIG. 348. FROM A SECTION OF THB PROSTATE OF A MAN TWENTY-THREE YEARS OLD. X 360. The epithelium is cut obliquely at x, and has artificially separated from the connective tissue at xx.

parts of the gland in dogs and cats, end in cylindrical lamellar corpuscles. The utriculus prostaticus (uterus masculinus, vagina masculina) is a small pocket lined with stratified epithelium, opening into the dorsal wall of the urethra midway between the orifices of the ejaculatory ducts, or a little below them. It is sometimes absent, and is occasionally quite deep. Lowsley failed to find any small prostatic tubules opening into it, such as have been reported as occasionally present. The utriculus prostaticus is the lower end of the Mullerian ducts, which have fused, and it corresponds with the vagina in the female.


The form of epithelium found in the bladder extends through the prostatic to the membranous part of the urethra. Its outer cells gradu


ally become elongated and it changes to the simple or few-layered columnar epithelium of the cavernous portion. In the dilatation of the urethra near its distal end, the fossa navicularis, the epithelium becomes stratified with its outer cells squamous; the underlying papillae of the tunica propria become prominent, and the whole is the beginning of the gradual transition from mucous membrane to skin.

Glands. Small groups of mucous cells are scattered along the urethra, and in the cavernous part, especially on the upper wall, they form pockets called urethral glands (of Littre). Often these pockets are on the sides of epithelial pits so that the glands are branched. Non-glandular pits

Mucous membrane of the urethra.

Epithelium. Tunica propria. Urethral glands. Submucosa.



Arteries. Connective tissue Bundle of smooth Venous spaces, trabeculae. muscle.


also occur, known as urethral lacuna, and the "paraurethral ducts" near the external orifice are large lacunas of various sorts.

Two glands of considerable importance empty by irregularly dilated ducts, i^ in. long, into the beginning of the cavernous urethra. The bodies of these bulbo-urethral glands (Cowper's glands) are found one on either side of the membranous urethra, in close relation with striated and smooth muscle fibers. The end pieces, which are partly alveolar and partly tubular, anastomose. They consist of mucous cells, with intercellular secretory capillaries, and produce a clear, glairy mucus, discharged during sexual excitement. The ducts, surrounded by thin rings of smooth


muscles, consist of simple low epithelium. They may connect directly with the end pieces, or a secretory duct may intervene.

The muscularis of the prostatic part of the urethra consist of an inner longitudinal and an outer circular layer of smooth muscles. Both layers continue throughout the membranous part; the circular layer ends in the beginning of the cavernous urethra leaving only oblique and longitudinal bundles in its distal part.

Corpus cavernosum urethra. In the submucosa of the cavernous urethra there are many veins (Fig. 349) which become larger and more numerous in and beyond the muscularis. This vascular tissue which surrounds the urethra is limited by a dense elastic connective tissue layer, the tunica albuginea, and the structure which is thus bounded is the corpus cavernosum urethra. Toward the perineum it ends in a round enlargement, the bulbus urethra, and distally it terminates in the glans penis. The urethra enters the upper surface of this corpus cavernosum near the bulbus. Branches of the internal pudendal (pudic) artery, namely, the arteriae bulbi and the urethral arteries, penetrate the albuginea, and the former pass the length of the cavernous body and

PIG. 35o.-CRosi SECTION end in the g lans - These arteries have particularly thick walls of circular muscle, and in cross sections the initma may be seen to form coarse rounded proEla8ti (AfterEberth S .) ain ' jections into the lumen. These projections contain longitudinal muscles and subdivisions of the inner circular elastic membrane (Fig. 350). The arteries in the corpus cavernosum produce capillaries found chiefly toward the albuginea. The capillaries empty into thin-walled venous spaces which appear as endothelium-lined clefts in a connective tissue containing many smooth muscle fibers. The cavernous body is permeated with these spaces which, at times of sexual excitement, become distended with blood, reducing the tissue between them to thin trabeculae. Such distensible vascular tissue is known as erectile tissue. Some arteries connect directly with the venous spaces, and such as appear coiled or C-shaped in a collapsed condition are called arteries helicina. The vena cavernosa have such very thick walls that they resemble arteries. They contain an abundance of inner longitudinal muscle fibers, and since these are not evenly distributed but occur in columns, the lumen of the veins is usually crescentic or stellate in cross section. Emissary veins pass out through the albuginea and empty into the median dorsal vein of the penis.

The corpora cavernosa penis are a pair of structures similar to the cavernous body of the urethra, and are found side by side above it (Fig. 351). The septum between them is perforated distally so that they


communicate with one another. Each is surrounded by a very dense albuginea, i mm. thick, divisible into an outer longitudinal and an inner circular layer of fibrous tissue. The septum is formed by the median fusion of these layers. The cavernous or erectile tissue of which these corpora are composed is essentially like that around the urethra.

All three cavernous bodies are surrounded by fascia and subcutaneous tissue containing blood vessels, lymphatics and nerves. The lymphatic vessels form a superficial and a deep set, the latter receiving branches from the urethra. The principal sensory nerves are the medullated dorsal nerves

of the penis. They terminate in many ^^^B^Mark Hill (talk) 16:16, 8 September 2017 (AEST)^~~ g

tactile corpuscles in the papillae beneath FIG. 351. CROSS SECTION OF A

the Skin, in bulboUS and genital COrpUS- 3 bug{nea; S d., C dorsatve S inri" e TO C rporTcave^ i . .-, , . . .. i nosa penis;!., urethra; g., corpus caver

cles in the deeper connective tissue, and nosum urethra. (Baiiey.) in lamellar corpuscles found near or in

the cavernous bodies. Free endings also occur. The sympathetic nerves are from a continuation of the prostatic plexus. They constitute the cavernous plexus, which includes the major cavernous nerves accompanying the dorsal nerves of the penis, and the minor cavernous nerves which enter the roots of the corpora cavernosa penis. The sympathetic nerves supply the numerous smooth muscles of the trabeculae and cavernous blood vessels. They are said to be joined by fibers from the lower spinal nerves, the nervi erigentes.

Female Genital Organs

Development and General Features

Although it is probable that sex is determined at the time of the fertilization of the ovum, and that it cannot be modified by subsequent conditions of any sort, the sex of young embryos cannot be recognized. All human embryos of 13 mm. possess a prominent genital papilla; they have both Wolffian and Miillerian ducts, in so far as the latter have developed; and they contain genital ridges which are still in an "indifferent stage" it cannot be said whether they will become ovaries or testes (cj. Fig. 326, p. 328). In the female the Miillerian ducts become highly developed, the Wolffian ducts degenerate, and the genital ridges produce ovaries.

The Mṻllerian Ducts. Before reaching the urogenital sinus, the lower ends of the Miillerian ducts are in contact, being situated between the Wolffian ducts (Fig. 352). The figure here reproduced represents a portion of the genital apparatus shown in Fig. 306, B, p. 311, both being sketched from the beautiful lithographs accompanying Keibel's fundamental account of the development of the human urogenital tract, which students should consult in its original form (Arch. f. Anat. u. Entw., 1896, pp. 55-156). A fusion of the Miillerian ducts begins just above their lower termination and extends downward to the urogenital sinus. Thus the entire ducts form a Y-shaped structure, and the lower part of the stem becomes the vagina. It is at first a solid cord of cells, but those in the center break down and a lumen appears, "first in embryos of 150-200 mm." The lower end of the vagina remains closed by epithelium for some time longer, and as the vagina enlarges, a transverse fold, the hymen, is formed at this point. With the breaking down of the central cells, the hymen becomes perforate; it then usually forms a crescentic fold on the dorsal side of the entrance to the vagina (Fig. 353). Its remains permanently mark the orifice of the Miillerian ducts.

Above the vagina the Mullerian ducts form the lining of the uterus, which develops from the upper part of the stem of the Y, and from the inner ends of its arms. This region of junction becomes surrounded by a very thick layer of smooth muscle. The occasional occurrence of a median septum in the uterus or vagina, dividing them into right and left halves, is due to imperfect fusion of the Mullerian ducts.

The outer portions of the Mullerian ducts retain relatively thin walls and become the uterine (or Fallopian) tubes. Each opens freely through its fimbriated extremity into the abdominal cavity.

The Wolffian Bodies and Wolffian Ducts. In the female these structures become functionless and degenerate. Their principal derivative is a group of blind tubules, which may readily be seen in the translucent mesentery-like membrane extending between the ovary and tube. These tubules were named the "organ of Rosenmuller" after their discoverer, who described them in 1802, and were called the "parovarium" (later corrected to paroophoron) because of their position beside the ovary; but when it was shown that these tubules were homologous with the epididymis, they were given a corresponding name, and are now known as the epoophoron ( Vt, upon; <Lo<6pos, ovary). The epoophoron consists of "8 to 20" transverse ducts, which begin with blind ends in or near the upper end of the ovary and follow a more or less convoluted course to the longitudinal duct, into which they empty (Fig. 353). They are lined with simple cuboidal or columnar epithelium, sometimes ciliated, and are surrounded with muscle fibers. Occasionally there are detached solid cords in their vicinity, and sometimes the tubes become cystic. Obviously they correspond with the efferent ducts of the testis, and the longitudinal duct, into which they empty, represents the duct of the epididymis. Some of the transverse tubules, or the main duct itself, may extend into soft round nodules of tissue projecting from the mesentery, to which they may be attached by slender pedicles. These appendices vesiculosce correspond with the appendix of the epididymis. Frequently there is a vesicular appendix entirely separate from the epoophoron, situated near the fimbriated orifice of the uterine tube, and said by Felix to develop around an accessory Miillerian duct. Although accessory ducts have not been found in the male, the relations of this structure to the Miillerian duct suggest a comparison with the appendix testis. Both in the female and the male the appendages have been described as of two sorts, connected with the Miilleria n and Wolffian ducts respectively.


bl., Bladder; M.d., Mulleriae duct; u., ureter; ur., uruthra; u.s., urogenital sinus; W.d., Wolffian duct.


The paroophoron is a remnant of the Wolffian tubules corresponding with the paradidymis. It was first described as nearer the uterus than the epoophoron, and situated as in the diagram, Fig. 353. The tubules there shown, however, are presumably a part of the epoophoron; the paroophoron is now said to be on the opposite side of the ovary (toward the right of the diagram), in relation with the ovarian vessels. It disappears by the fifth year.

The lower end of the Wolffian duct, which corresponds with the ductus deferens, may remain as the canal of Gartner. This canal terminates near the hymen. It may extend upward beside the vagina, and be enclosed in the musculature of the lower part of the uterus; usually it is entirely obliterated.

Development ol the Chary. Like the testis, the ovary is formed from the middle portion of the genital ridge. The peritoneum which covers it gives rise to the mass of cells in its interior, and deep within, the cells become arranged in medullary cords and a rete ovarii. These are rudimentary structures. The rete cords do not connect with the Wolffian tubules. They are said to acquire lumens toward birth, so that they are bounded by simple epithelium; they remain in the adult and may become cystic. Sexual cells disappear from the cords in the central part of the ovary, which becomes filled with vascular connective tissue and forms the medulla in the adult. The peripheral part of the ovary, or cortex, contains great numbers of sexual cells, which instead of being lodged in tubules as in the testis, are arranged in small groups surrounded by indifferent cells. The entire structures are primary follicles, and they are imbedded in a stroma likewise derived from the peritoneum. Felix considers that the

follicles develop, for the most part at least, directly from the tissue of the genital ridge, and states that tubes or cords growing in from the peritoneal epithelium, as described by Pfliiger, do not exist in the human ovary. Generally it has been said that the primary follicles arise by the subdivision of such cords (Fig. 354).

Ligaments. As the Miillerian ducts come together below, they occupy ridges covered with peritoneum. These ridges .. Epithelium; STSSSw cord; c, sexual coalesce so as to form a partition which crosses the pelvis from side to side and rises upward from its floor. Ventral to the partition is the bladder, separated from it by the vesico-uterine pouch; dorsal to it is the rectum, separated by the deeper recto-uterine pouch; and within it are the uterus and tubes. In the adult these folds of peritoneum extending laterally from the uterus constitute its broad ligaments. The Wolffian bodies and ovaries, which at first occupy vertical ridges on either side of the root of the mesentery, appear to slip down or descend into the interior of the broad ligaments, from the dorsal surfaces of which the ovaries later project.


Above each ovary there is a band of fibrous tissue which extends to the orifice of the tube, and running along this band there is a fimbria known as the fimbria ovarica; this arrangement apparently serves to keep the orifice of the tube in close relation with the ovary. Below the ovary, between the laminae of the broad ligament, a cord of fibrous tissue passes from it to the musculature of the uterus, lying just below the uterine tubes; this is the ovarian ligament. The round ligaments start from the uterine musculature not far from the ends of the ovarian ligaments. They pass downward, one on either side within the broad ligament, and terminate in the folds which correspond with those of the scrotum. The ovarian and round ligaments are believed to be subdivisions of a single structure equivalent to the gubernaculum testis.

The External Genital Organs. The urogenital sinus, which receives the urethra and vagina, becomes a shallow space called the vestibule (Fig. 353). The genital papilla, with the glans at its apex, becomes relatively shorter as the female embryo develops. It forms the clitoris, analogous with the penis, and is covered by the lesser genital folds, the labia minor a. (Compare Fig. 355 with Fig. 329, A, page 331.) The labia form a prepuce for the clitoris but do not unite beneath it to make a raphe; they remain separate, as parts of the lateral boundaries of the vestibule. The larger genital folds, labia majora, likewise remain separate. They receive the ends of the round ligaments of the uterus which pass into them over the pubic bones, sometimes accompanied by a prolongation of the peritoneal cavity formmg a processus vagmalis. In late stages of development the labia majora become large enough to foids to conceal the clitoris and labia minora, which previously sinus project between them.


. . . a., anus; g., glans clitori dis: g . f lesser genital

folds (labia mmora) ;


The ovary is an oval body about an inch and a half long, covered by a modified portion of the peritoneum. Along its hilus it is attached to a mesentery, the mesovarium, which is a subdivision of the broad ligament of the uterus. The epithelium of the mesovarium is continuous with that of the ovary, and its connective tissue joins the mass which forms the ovarian medulla. This tissue, rich in elastic fibers and containing strands of smooth muscle, surrounds the vessels and nerves. The blood vessels are abundant, and they pursue a very tortuous course both in the mesovarium and within the ovary. This is strikingly shown in Clark's injections (Johns Hopkins Hosp. Rep., 1900, vol. 9). They are derived in part from branches of the uterine vessels, but are chiefly the terminations of the ovarian artery and vein. Large stems traverse the medulla and form capillary plexuses around the follicles in the cortex. Thin-walled lymphatic vessels arise in the cortex below the rather dense sub-peritoneal layer (or tunica albuginea} and pass out at the hilus. The nerves are chiefly non-medullated sympathetic fibers, derived from the plexus which accompanies the ovarian artery, and distributed to the blood vessels. Ganglion cells have been found near the hilus, and a few medullated fibers occur. It is said that certain fibers end in contact with the cells of the follicles.

The relation of the cortical stroma to the looser tissue of the medulla is so characteristic that sections of the human ovary containing few ova and no active follicles may be readily identified. Usually a section of the ovary may be recognized as such without magnification, owing to the presence of the large cysts or follicles in which the maturing ova are contained. These extend from the cortex into the medulla, and are numerous even in childhood (Fig. 356).

Growth of the Follicles. It is probable that all the sexual cells which are to be produced in a life-time are present in the ovaries at birth. At that stage, at least, many of those previously formed have already degenerated; and the ovaries contain a great excess of ova, all but a few hundred of which are destined to atrophy within the limits of the genital glands. In so far as the sexual cells have ceased to multiply and have entered upon the growth period, they represent the last generation of oogonia, and are being transformed into primary oocytes. During this transformation they increase greatly in size, finally becoming about 0.3 mm. in diameter. These egg cells have already been described in detail (p. 29). They are conspicuous in sections as large, round, deeply staining cells, with round or oval vesicular nuclei, each containing a prominent uucleolus. The cells become so large that frequently they are cut into several sections, and portions of protoplasm without nuclei are to be expected. The larger oocytes are surrounded by the clear, radially striated zona pellucida (Fig. 22, p. 30); their protoplasm may contain the viteUine bodies previously described.


Germinal epithelium; 2, tunica albuginea; 3, peripheral zone with primary follicles: 4, vesicular follicle; 5, stroma ovarii; 6, medulla; 7, 8, peripheral section of vesicular follicles; 9, hilus, containing large veins.

The follicles are composed of the cells which surround the oocytes. After the groups of egg cells and indifferent cells become subdivided, each oocyte is typically surrounded by a single layer of flat follicular cells, and this primary follicle lies isolated in the stroma of the cortex, beneath the tunica albuginea (Figs. 357 and 358). As the follicle enlarges, the follicular cells become columnar and then stratified (Fig. 358). A crescentic cleft filled with fluid appears in the midst of the stratified epithelium on one side of the follicle, and by the accumulation of fluid, or liquor folliculi, this cleft becomes a spherical cavity (Fig. 359). The fluid is regarded by some as a transudate from the blood vessels, which are abundant in the stroma outside of the follicle. Others consider that it is actively secreted by the cells of the follicle, certain of which undergo liquefaction. Spaces containing a stainable fluid, differing from that in the main cavity, may appear in the epithelium (CallExner bodies), around which the cells are radially arranged. By the development of the main cavity, the stratified epithelium becomes a relatively thin layer, the stratum granulosum, which decreases in width as the follicle enlarges. The oocyte is on one side of the follicle, contained in a heap of cells known as cumulus oophorus (formerly the discus proligerus) . This is connected with the wall of the follicle, but in certain sections it may appear completely detached (e.g., in a section at right angles with the plane of the page, near the top of the cumulus in Fig. 359).



Surrounding the follicle, even in early stages, there is a connective tissue sheath, the theca folliculi (Fig. 358). This later becomes differentiated into a vascular tunica interna, and a fibrous tunica externa (Fig. 359). The tunica interna contains many cells with abundant protoplasm. It is separated from the epithelium of the follicle by a delicate membrana propria.

In distinction from the solid primary follicles, those with cavities are known as vesicular follicles (Graafian follicles). They increase in diameter from 0.5 to 12.0 mm., and are then ready to discharge the contained oocyte. Occasionally a single follicle has two oocytes, and rarely more. Arnold (Anat. Rec., 1912, vol. 6, pp. 413-422) describes the ovaries of a negress, in which he found forty-three follicles containing four oocytes or more, including one which contained eleven. It cannot be stated whether the additional oocytes develop by division of the oogonium within a primary follicle, or by the failure of a group of primitive sexual cells to become separated from one another.

FIG. 359- SECTION OF A LARGE VESICULAR FOLLICLE OF A CHILD EIGHT YEARS OLD. X 90. The clear space within the follicle contains the liquor folliculi.

Ovulation and the Corpus Luteum. Around the mature vesicular follicle, the tunica interna becomes very thick and cellular, forming elevations toward the stratum granulosum. At this stage the follicle is large, being about half an inch in diameter, and one surface of it is so close to the ovarial epithelium as to cause it to bulge and then to rupture. Through the opening thus made the liquor folliculi escapes, together with the oocyte. The latter is said to become detached by the formation of fluid-filled spaces between the cells of the cumulus; it generally carries with it more or less of the innermost layer of the cumulus, and these cells, because of their radial arrangement, are termed the corona radiata. As the oocyte leaves the follicle there is apparently a chance for it to become lost in the abdominal cavity, but the fimbriated orifice of the tube is near at hand, and the stroke of its cilia produces a current toward its entrance. In a guinea-pig Hensen observed that the fimbriae were in very active motion, sweeping here and there over the surface of the ovary so powerfully that the effect of ciliary action must have been trivial. The ova, surrounded by the mucoid cells of the follicles, adhered more closely to the fimbriae than to the smooth surface of the ovary. Except toward the time of ovulation, Hensen found that the fimbrias were relatively inactive (Zeitschr. f. Anat. u. Entw., 1875, pp. 213-270). The discharge of the ovum from the follicle is known as ovulation.

It may be noted that in approaching the peritoneal epithelium, through which the rupture occurs, the follicle must push aside or distend the connective tissue of the tunica albuginea. This is ordinarily a rather weak layer, but it has been suggested (by Reynolds) that in some cases it is more highly developed and acts as an obstruction to ovulation.

After ovulation, blood escapes from the capillaries of the tunica interna and forms a clot within the empty follicle (Fig. 360). This clot is sometimes called the corpus hcsmorrhagicum. On all sides it is surrounded by the cells of the stratum granulosum, which enlarge and produce a yellow fatty pigment. They form a yellow convoluted zone which may easily be seen without magnification; the entire structure is then known as the corpus luteum. Vascular strands of connective tissue extend between the lutein cells (Fig. 361) and enter the central clot. The extravasated blood breaks down into granules and haematoidin crystals, and is gradually absorbed. It is replaced by gelatinous connective tissue which finally contracts into a dense white fibrous nodule, and this scar is known as the corpus albicans. Meanwhile the lutein cells undergo hyaline degeneration and become resorbed. The surface of the ovary, which is smooth in childhood, becomes pitted and irregular with the increasing formation of these corpora albicantia.


i., Aperture through which the ovum escaped; c. a., corpus albicans; cl., blood clot in a corpus luteum of ovulation; th., theca folliculi; v. f., vesicular follicle. (After Rieffel.)

FIG. 361. A, PORTION OF A CORPUS LUTEUM OF A RABBIT. B, PORTION OF A CORPUS LUTEUM OF A CAT. X 260. r In B the lutein. 'cells have become fatty and contain large and small vacuoles.

Provided that pregnancy does not take place, the corpus luteum reaches its maximum development in about two weeks after ovulation, and it becomes reduced to a scar in about two months. If pregnancy occurs, it enlarges further and persists at the height of its development until the fifth or sixth month. Its diameter is then 1.5-3.0 cm., and at the end of pregnancy it is still quite large and yellow. If the corpus luteum is removed, the ovum fails to become attached to the wall of the uterus. There is both experimental and histoiogical evidence that it produces an internal secretion which is probably received by the blood vessels invading it from the theca. In order to distinguish between the corpus luteum of pregnancy and that of unproductive ovulation, the former is called the true corpus luteum; the latter is the corpus luteum spurium.

Many follicles degenerate at various stages in their evolution without discharging their ova. Leucocytes and cells from the stratum granulosum are said to invade the protoplasm of the oocytes, in which they disintegrate. The zona pellucida, which surrounds the oocyte, may become conspicuously folded and persist for some time (Fig. 358). The basement membrane of the stratum granulosum may also thicken and become convoluted. These degenerating or atretic follicles are finally reduced to inconspicuous scars. After the menopause the degeneration of the oocytes becomes general.

Within the stroma of the cortex, interstitial cells are found, which resemble lutein cells but are smaller. They have been compared with the interstitial cells of the testis, and are said to contain secretory granules. Apparently they are derived from the thecae of atretic follicles (Cohn, Arch. f. mikr. Anat, 1903, vol. 62, pp. 745-772; Allen, Amer. Journ. Anat., 1904, vol. 3, pp. 89-153).

Uterine Tubes

Each uterine tube is about 5 inches long and extends from its orifice in the abdominal cavity to its outlet in the uterus. It is divided into the fimbriated funnel or infundibulum; the ampulla or distensible outer twothirds, the lumen of which is about a quarter of an inch in diameter; the isthmus or narrow inner third, not sharply separated from the ampulla; and the uterine portion which extends across the musculature of the uterus to the uterine orifice. The wall of the tube is composed of three layers, a mucosa, muscularis. and serosa (in addition to which a tela submucosa is enumerated in the Basle nomenclature). The mucosa is thrown into thin longitudinal folds, which are low in the isthmus, but tall and branched in the ampulla (Fig. 362). Occasionally the branches anastomose, enclosing a pocket, but glands are absent. The epithelium is chiefly simple columnar, and ciliated, the stroke of the cilia being toward the uterus; but there are areas of nonciliated cells which are said to produce a mucoid fluid. The two types of cells are connected by intermediate forms. Mucous cells are absent.

The folds of the mucous membrane are occasionally indented or overhanging, so that in transverse sections detached fragments may appear, suggestive of villi (Fig. 363) ; but the fact that almost all of the many projections connect with the submucous layers indicates that they are elongated folds. Each of them contains a thin layer of cellular connective tissue, in which there are small arteries and veins running chiefly lengthwise of the tube. Lymphocytes occur in the meshes of the tissue and lymphatic vessels have been reported. Occasionally strands of smooth muscle fibers are found within the folds.



The mucous membrane rests directly upon the tunica muscularis, and Schafer considers that " the larger part of the muscular layer must probably be regarded as a much thickened muscularis mucosae." The muscle coat consists of a thick inner circular layer and a thin outer longitudinal layer of smooth muscle fibers, but both layers are resolved into coarse bundles by the abundance of intermuscular connective tissue.

Since the uterine tubes are imbedded in the broad ligaments, they are not closely invested by the peritoneum. There is a considerable layer of loose vascular connective tissue outside of the muscularis, and toward the ovary this tissue may include sections of the tubules of the epoophoron. It contains the branches of the ovarian and uterine blood vessels which supply the tube. These are accompanied by lymphatic vessels and nerves. The latter innervate the tubal musculature and the mucous membrane.


The uterus is a pyriform, muscular organ, flattened dorso-ventrally. It is about two and a half inches long, receiving the uterine tubes at its upper end or fundus, and ending below in the vagina. It is divided into fundus, corpus and cervix. The corpus and fundus together have a triangular cavity, which opens into the canal of the cervix through the internal orifice; the canal communicates with the vagina through the external orifice of the uterus. The lining of the cervix presents a feather-like arrangement of folds on its dorsal and ventral surfaces; these are the plica palmatcs. The walls of the uterus consist of a mucosa, muscularis and serosa (constituting the endometrium, myometrium, and perimetrium, respectively).

The uterus is lined with simple columnar epithelium, some areas of which are ciliated. The cilia have been described as diflicult to preserve, and their absence from certain cells has been attributed to faulty fixation. According to Gage the uterine cilia are as readily preserved as those which occur elsewhere, and he finds that only one cell among fifteen or twenty is actually ciliated. Mucous cells are absent. The epithelium forms slender tubular pits, the uterine glands, but these produce no definite secretion. They are branched tortuous tubes extending through the broad mucosa (which is i mm. thick) , and invading to a slight extent the muscular tissue beneath. They have been carefully modelled by Hedblom, whose studies are not yet published; he finds that occasionally they anastomose with one another, and that in their deeper portion they have long horizontal branches, at right angles with the main tube. Sometimes a small group of glands opens into a single depression of the surface epithelium (Fig. 365). In older persons the glands degenerate, losing their connections with the surface and becoming cystic. Each gland is surrounded by a delicate basement membrane, and between them there is an abundant tunica propria, containing many blood vessels. These form capillary networks around the glands and especially beneath the free surface. The propria contains also many lymphocytes, and its lymphatic vessels form a widemeshed plexus with blind extensions. These structures are supported by a reticular tissue framework containing many nuclei.

FIG. 364. THB DORSAL HALF OF A VIRGIN UTERUS. Twothirds natural size. (After Rieffel).

FIG. 365. Mucous MEMBRANE OF THE RESTING UTERUS OF A YOUNG WOMAN. (After B6hm and von Davidoff.) X 35

The upper and larger part of the cervix of the uterus is likewise lined with simple columnar ciliated epithelium, but its cells are taller than those of the corpus (60 n as compared with 20 /*) Mucous cells occur, especially in the outpocketings of epithelial pits which constitute the branched cervical glands. They discharge a secretion which occludes the canal of the cervix during pregnancy. Often they produce macroscopic retention cysts, named "ovules of Naboth," after the Leipzig anatomist who first described them. Toward the external orifice of the uterus the epithelium becomes stratified and squamous, and rests on connective tissue papillae. Thus it resembles the lining of the vagina of which it is a continuation, and after the first child-birth it extends further up into the cervix than before.

The musculature of the uterus is a thick investment of interwoven bundles which cannot be subdivided into well-defined layers (Fig. 366). It begins immediately outside the tunica propria, and its inner portion has been regarded as "an immensely hypertrophied muscularis mucosae."

Further out there is a zone containing many blood vessels, which according to this interpretation marks the position of the submucosa (Schafer). Accord' ing to Henle and Stohr, these vessels belong with the middle of three muscle layers, which is named, therefore, the "stratum vasculare." It is the thickest of the layers and its fibers are chiefly circular. The innermost layer or "stratum submucosum" (Stohr) consists principally of longitudinal fibers. The outermost layer or "stratum supravasculare" contains circular fibers internally and longitudinal fibers externally. Some of the latter are continuous with the longitudinal fibers of the uterine tubes; others are said to enter the round ligaments, which contain also some striated fibers; and still others spread into the broad ligaments.

In the cervix the three strata of muscle fibers are found to be very distinct inner and outer longitudinal, and middle circular. Although the uterus generally contains few elastic fibers, found only in its peripheral layers and running perpendicular to the plane of contraction of the muscles, elastic fibers are abundant in this position in the lower segment of the corpus and vaginal portion of the uterus. During the first half of pregnancy both elastic and muscular fibers increase in size and number; in the second half, the elastic fibers decrease in the musculature, but increase in the perimetrium (Stohr). The way in which the thick layer of muscles in the resting uterus becomes arranged in the thin layer of late pregnancy is an unsolved problem, similar to that presented by the musculature of the bladder and intestine during distention.


a, Epithelium; b, tunica propria; c, glands; i, inner muscular layer; 2. middle muscular layer; 3, outer muscular layer.

The serosa covering the dorsal and ventral surfaces of the uterus is in part a well-defined layer, but it blends with the connective tissue of the broad ligaments laterally and below; and this tissue, from its position beside the uterus, is known as the "parametrium." Imbedded in the parametrium the main trunks of the uterine vessels run along the lateral margins of cervix and corpus, both artery and vein showing many kinks and convolutions. The vessels are thus apparently adapted to the future expansion of the uterus, but when it retracts after pregnancy they are said to show more pronounced bendings, as if they had been permanently elongated. The parametrium contains also numerous lymphatic vessels, together with the ganglionated sympathetic utero-vaginal plexus. Nerves from this plexus and from the third and fourth sacral nerves supply the uterus.


Menstruation is the periodic degeneration and removal of the superficial part of the mucosa of the uterus, accompanied by haemorrhage from the vessels of the tunica propria. Three successive stages may be distinguished, namely (i) the stage of congestion, lasting four to five days; (2) the stage of desquamation and hamorrhage, four days; and (3) the stage of regeneration and repair, seven days. Thus the entire process requires about sixteen days, and after an interval of twelve days the cycle begins anew.

For four or five days before the discharge occurs, the thickness of the mucosa increases greatly, due to the congestion of its vessels and the proliferation of the reticular tissue. The glands become wider, longer, and more tortuous, opening between irregular swellings of the superficial epithelium. Red corpuscles pass out between the endothelial cells of the distended veins and capillaries, and form subepithelial masses. This stage of congestion and tumefaction is followed by one of haemorrhage and desquamation. The epithelium of the surface and outermost parts of the glands becomes reduced to granular debris, or it may be detached in shreds. The underlying vessels rupture and add to the blood which had escaped by diapedesis. In the stage of regeneration, the epithelium spreads from the glands over the exposed reticular tissue, the congestion diminishes, and the mucosa returns to its resting condition. The cervix takes no part in menstruation except that the secretion of its glands may increase during the stage of congestion.

Beginning at puberty (13-15 years) menstruation takes place normally once in 28 days for 33 years, more or less. During pregnancy it is interrupted, although the time when it should occur may be indicated by slight uterine contractions and finally by those which cause the delivery of the child. Thus the duration of pregnancy is described as ten menstrual cycles. The significance of menstruation is suggested by conditions in those mammals in which sexual seasons are annual or infrequent. In them a period of congestion, accompanied by uterine changes which are sometimes closely comparable with those of menstruation, precedes sexual intercourse and ovulation. Thus in the bitch ovulation takes place when the external bleeding "is almost or quite over," and this is the time of coitus. Domestication in various animals causes an increased frequency of the congestive cycles, sometimes unaccompanied by ovulation. It is generally accepted that human menstruation may take place without ovulation, and that ovulation may occur between menstrual periods, and also during pregnancy. It may even occur in children before menstruation has begun. Nevertheless ovulation probably occurs usually and normally at the close of menstruation. Coitus is not considered to be a factor in inducing ovulation, but it is said that in the rabbit and ferret, and in pigeons, ovulation may fail to occur in the absence of the male.


The following considerations are also important in establishing the age of young embryos. The time required for spermatozoa to travel to the upper end of the tube, where fertilization takes place, is probably about twenty-four hours. There they may fertilize the ovum at once if ovulation has just occurred. They retain their vitality and are capable of fertilizing the ovum during a period of ten days in the rabbit, and this may be true also of man. Thus it is probable that if coitus has occurred shortly before menstruation, the spermatozoa may remain active in the tube, and fertilize the ovum discharged at the close of the following menstruation.

The Discovery of Mammalian Ova

During the seventeenth century the ovary was called the testis muliebris, or testis foemineus. It was believed to produce the mucoid secretion which escapes from the genital orifice, and this was regarded as seminal fluid. The uterine tubes were accordingly the vasa deferentia mulierum, serving to convey this fluid to the uterus, where, through a mixture and interaction of the male and female semina, an embryo was produced. Aristotle had argued to the contrary, but his opinion was summarily disposed of by Bartholin, who discussed the ovaries as follows (Anatomia, 1666):

      "Their function is to produce semen in their own way, which Aristotle, against all reason and observation, has dared to deny to women, contrary to the express teaching of Hippocrates. "

The ancient doctrine of Aristotle, expounded in his treatise on the generation of animals, was based upon the familiar facts that menstruation marks the beginning, and ceases at the end, of the child-bearing period; and moreover menstruation is interrupted while the embryo is being formed. Therefore he concluded that the menstruum supplies the substance and material for the new body, which arises like the curd in milk, through the agency of the semen. The semen engenders; the menstruum nourishes. The theory had already been advanced that the semen comes from all parts of the body, and that its particles reproduce the structures from which they are derived. This enticing speculation, revived by Darwin in his theory of pangenesis, was discussed at length and rejected by Aristotle.

Generation, therefore, was considered to result from the mixing of two fluids, and would have remained a barren physico-chemical problem until recent times, if further morphological observations had not been made. The view of Bartholin had at least the merit of definitely associating the ovary with the reproductive function. Vesalius and Fallopius had seen the follicles and corpora lutea; Fallopius described them as "vesicles filled with water or aqueous humor, some limpid and others yellow (Observationes, 1588). Many others had observed them, and from their resemblance to the ova of birds they had even been called "ova," when in 1672 a young Dutch physician, Regnerus de Graaf , made his thorough study of the female genital tract.

De Graaf concluded that the "semen muliebre" is not produced by the "testes muliebres," but that the general function of the latter is "to produce and nourish ova, and bring them to maturity." Consequently he proposed to substitute the name ovary, and to call the tubes oviducts. He declared that the ova escaped from the follicles through minute apertures (in the rabbit admitting a bristle) and made their way through the tubes to the uterus, in which they developed. The abnormal formation of a human embryo within the tube was figured and, to a certain extent, explained. De Graaf studied many mammals, and especially rabbits* He found minute ova in the oviducts and observed the follicles from which they had escaped. In older stages he recorded a general agreement between the number of corpora lutea and embryos.

Since, however, he frequently referred to the entire follicles as ova, his results were not promptly accepted; the diameter of the isthmus of the tube is so small that the entrance of the follicles into the uterus was considered impossible. It was a matter of easy observation to determine more precisely the relation of the ova to the follicles. After many years this was done by Von Baer, an eminent embryologist, whose studies of the chick are regarded as " the most profound, exhaustive and original contribution to embryology which has ever been made" (Minot). This work bears the famous subtitle "Beobachtung und Reflexion" the German expression of Haller's "Observations suivies de Reflexion" and De Graaf's " Cog itationes atque observationes." After describing the condition of the ova in the tubes of the bitch, Von Baer writes:

"It remained for me to ascertain the condition of ova in the ovary, for it seemed clearer than day that ova so small as those found in the tubes did not represent Graafian follicles expelled from the ovary; and I did not consider it probable that such solid bodies had been coagulated from the fluid of the vesicles. Now, contemplating the ovaries before making an incision, I clearly distinguished in almost every vesicle, a yellowishwhite point unattached to the walls, which swam about freely in the fluid when the vesicle was pressed upon with a probe. Led on by a certain curiosity, rather than moved by hope that with the naked eye I had seen ovules in the ovaries through all the coats of the Graafian follicle, I opened a vesicle, and taking out a point in question on the blade of a knife, I placed it under the microscope. I was overcome with amazement when I saw the ovule, now recognized outside of the tubes, so clearly that a blind man could hardly doubt it. Surely it is strange and unexpected that an object so persistently sought for, and endlessly described as inextricable, in every physiological compendium, could so easily be placed before the eyes" (De ovi genesi, Lipsiae, 1827).

Thus the ova in mammalian ovaries, which had long been believed to exist, were first definitely seen within the follicles one hundred and fifty years after the discovery of the microscopic spermatozoa, the existence of which had never been suspected.

The Decidual Membranes of the Uterus and Embryo

Development and General Features

Before describing the mucous membrane of the uterus during pregnancy, it is necessary to consider the membranes which envelop the embryo. Although these are in contact with the lining of the uterus and in part intimately blended with it. they are portions of the embryo itself. The external membrane, toward the uterus, is known as the chorion; the inner membrane, toward the embryo, is the amnion. Since the embryo receives its nutriment from the wall of the uterus through blood vessels in the chorion, these membranes develop very early and thus provide for rapid growth. They are already present in the youngest human embryos which have yet been obtained.

Of the fertilization and segmentation of the human ovum, which doubtless take place in the upper part of the uterine tube, nothing is known except by inference from lower animals. The four-celled stage has been observed once in a monkey, but the youngest known human embryo is already provided with ectoderm, mesoderm and entoderm, and has entered the uterus. As a purely hypothetical figure, we venture to present the diagram Fig. 368, A, followed by the diagrams B and C which include many features actually observed.

In Fig. 368, A, a mass of cells (ect.} represents the ectoderm which will later cover the body and line the inner membrane or amnion. This ectoderm probably arises in connection with the layer (tr.} which covers the entire vesicle and becomes the epithelium of the outer membrane or chorion. The layer in question has been named the trophoblast (or trophoderm).


al., Allantois; am. c., amniotic cavity; cho., chorion; coe., ccelom; ect., ectoderm; m, mesodertn; tr. trophoderm (trophoblast); z, entodermal cyst; y. s., yolk-sac.

The term trophoblast (i.e., nutritive layer) was introduced by Hubrecht to correspond with the terms epiblast, mesoblast and hypoblast, which he used for the other germ layers. Since these are now generally called ectoderm, mesoderm and entoderm, the outer layer should be trophoderm, and the substitution of this name is therefore recommended. Trophoderm has, however, been used by Minot for the proliferating part of Hubrecht's trophoblast. It may be noted that a similar difficulty is encountered in His's angioblast which, as a germ layer, should be angioderm. Schafer applies angioblast logically to the individual cells which become the endothelial lining of vessels. Consistency requires the use of "-derm" for germ layers, leaving " -blast" for formative cells.

In addition to the trophoderm and ectoderm, the hypothetical stage shown in Fig. 368, A, exhibits a yolk-sac completely lined with entoderm. Between the trophoderm and entoderm, the mesoderm has appeared and is separating into somatic and splanchnic layers, with the body cavity between them. The somatic mesoderm is closely applied to the trophoderm, and together they form the chorion; the splanchnic mesoderm is against the entoderm of the yolk-sac, and forms the outer layer of its wall. The early and rapid development of the mesoderm is characteristic of human embryos, as may be inferred from the later stages.

In the diagram Fig. 368, B, the amniolic cavity has appeared in the ectoderm. It is believed to arise as a cleft in a solid mass of cells, and not by the coalescence of ectodermal folds as in the chick; however, in the youngest human embryos observed, it is completely formed. The entoderm shows an outpocketing extending into the mesoderm at the future caudal end of the embryo; this is the allantois, which soon becomes a slender tube (Fig. 368, C). The mesoderm in which it is lodged later produces the "body stalk."

The allantois develops very early in human embryos, being present in most if not in all of the specimens thus far obtained. Possibly there is no allantois in the very imperfect embryo described by Bryce and Teacher (Contributions, etc, Glasgow, 1908), and there is uncertainty as to its presence in Peters's embryo (Ueber die Einbettung des menschlichen Eies, Leipzig, 1899); but in other very young specimens it is well defined. According to Keibel, the allantois first appears in chicks of about twenty segments; in rabbits of eleven segments; in pigs of four to five segments; and in the apes and man, before any segments have formed. Its very early appearance in human embryos is probably correlated with the rapid establishment of the placental circulation, for the umbilical vessels are primarily the vessels of the allantois.

In Fig. 368, B and C, the entoderm of the yolk-sac is represented as giving rise to a detached cyst (x). There is a cyst of this sort within the chorionic cavity of the somewhat damaged Herzog embryo in the Harvard Collection, and a smaller detached cyst in the very perfect Minot embryo. (These will be further described by the writer in a subsequent publication.) It is possible that such cysts are of regular occurrence, although destined to atrophy. They may be lodged in a strand of mesoderm extending from the lower pole of the yolk-sac downward to the chorion (Grosser, Anat. Hefte, 1913, Abt. I, vol. 47, pp. 653-686), and they may arise as indicated in the diagrams (Fig. 368).

As the body cavity develops between the somatic and splanchnic layers of mesoderm, it is at first bridged by strands of mesenchymal tissue, forming the "magma reticulare." These strands become attenuate and break down, so that the yolk-sac is then suspended in a well-defined "extra-embryonic ccelom." This part of the ccelom, although within the embryonic membranes, is outside of the body proper of the embryo, as will appear in the following diagrams.

The arrangement of the membranes surrounding human embryos of about 2 mm. is shown in Fig. 369, A. The chorion has become covered with branching elevations or villi, which contain a vascular core of chorionic mesoderm. not shown in the diagram. The body of the embryo is connected with the chorion by the mesodermic body stalk containing the allantois. This has become relatively slender. On one side it is covered by the ectoderm of the amnion. The ectoderm, as in preceding stages, may be divided into two parts. Toward the yolk-sac it is thickened and there it forms the axial medullary tube and gives rise ultimately to the epidermis covering the body. Continuous with this epidermal ectoderm is the thinner portion which lines the amnion, as shown in the figure. The amnion forms a membranous sac attached to the ventral side of the embryo, leaving an aperture through which the yolk-sac projects downward into the extra-embryonic ccelom. The ccelom now extends between the amnion and chorion, except at the narrow body stalk. The yolk-sac has given rise to the fore-gut and hind-gut, and the allantois now appears as an appendage of the latter.

In Fig. 369, B, the embryo is represented as rotated so that its head is downward and its ventral side toward the left. It is now connected with the membranes by an umbilical cord, the composition of which may be seen by comparing A and B. Its principal constituent is the elongated body stalk, containing the allantois and covered above and on the sides with adherent amnion. Below, the amnion also forms the covering of the cord, but here it is separated from the body stalk by an extension of the body cavity. The yolk stalk passes from the primary loop of intestine through the cavity of the umbilical cord to the yolk-sac, in which it terminates.

FIG. 369. DIAGRAMS ILLUSTRATING THE DEVELOPMENT OF THE EMBRYONIC MEMBRANES AND THE FORMATION OF THE UMBILICAL CORD.) al., Allantois; am., amnion; am. c., amniotic cavity; cho., chorion; coe., coelom; y. s., yolk-sac.

This sac is now lodged in its permanent position between the amnion and chorion. Ultimately the parts of the allantois, yolk stalk and body cavity within the cord are obliterated.

The appearance of a human embryo at a stage intermediate between those shown in Fig. 369 is reproduced in Fig. 370. An irregular piece cut out from the chorionic vesicle forms the background of the picture. Around the cut edges of this piece the shaggy chorionic villi are seen, directed toward the wall of the uterus. At the top of the figure is the spherical yolk-sac lodged between chorion and amnion, between which the yolk stalk passes to the distal end of the umbilical cord, which it enters. The amnion is a membranous sac completely enclosing the embryo; in the figure, half of it has been cut away to show the embryo within. The skin of the embryo is continuous with the covering of the umbilical cord, and distally this covering is reflected and becomes continuous with the amnion.

In later stages the umbilical cord is greatly elongated. It contains the umbilical vessels which pass between the embryo and the chorion, through the persistent body stalk. The amniotic cavity greatly enlarges to accommodate the growing embryo, and the mesoderm of the amnion comes in contact with that of the chorion, to which it adheres more or less firmly. The embryo is bathed in the amniotic fluid (liquor amnii) of uncertain derivation, once thought to be sweat from the embryo, and later considered to contain the products of the Wolffian body, and urine from the permanent kidneys. Occasionally toward birth the meconium from the intestine mingles with it and discolors it. It is now generally believed to be secreted by the amniotic epithelium.


Relation between the Embryonic Membranes and the Uterus. When the embryo within its chorionic vesicle passes from the tube into the uterus, it is probably in a stage comparable with that shown in Fig. 368 (B or C). By the activity of the proliferating trophoderm, the uterine mucosa is partially destroyed and the chorionic vesicle becomes imbedded in its substance. This process is known as the implantation of the ovum. The walls of the vessels in the tunica propria of the uterus are broken down, and the maternal blood flows over and around the chorionic villi, in contact with which it does not clot. Elsewhere in the body, except in reticular tissue, blood clots on escaping from the endothelial tubes. Toward the uterine cavity, however, there is a clot which completes the encapsulation of the chorionic vesicle in the mucosa. The mucous membrane itself later passes entirely around the vesicle as shown in Fig. 371, A. The greater part of the mucosa of the uterus becomes cast off at the end of pregnancy; thus it forms a membrana decidua, which may be subdivided into three parts (i) the decidua basalis (or serotina) on which the implanted chorionic vesicle rests, and which forms the maternal part of the placenta; (2) the decidua capsularis (or reflexa) which spreads over the part of the vesicle which is toward the uterine cavity; and (3) the decidua vera, which lines the remainder of the uterus. These subdivisions of the decidua are indicated in Fig. 371, A.

FIG. 371. THE UTERUS AND DECIDUAL MEMBRANES IN EARLY PREGNANCY. A, AND IN LATE PREGNANCY B. THE CORD HAS BEEN CUT AND THE EMBRYO REMOVED FROM B. am., Amnion; am. c., amniotic cavity; c., cervix; ch., chorion; c. u., cavity of the uterus; d. b., decidua

basalis; d. c., decidua capsularis; d. v., decidua vera; m., amnion and chorion laeve drawn as one line; pi., placenta; u. c., umbilical cord; y. s., yolk-sac.

Soon after the ovum becomes implanted, the chorion ceases to be uniformly covered with villi. The villi toward the decidua basalis elongate and branch freely, producing the shaggy chorion frondosum; this is the embryonic portion of the placenta. As the chorionic vesicle enlarges, the villi directed away from the wall of the uterus, toward the decidua capsularis, become shorter and disappear, so that a large portion of the chorion becomes smooth the chorion Iceve. Usually the umbilical cord passes to a nearly central position in the chorion frondosum; rarely it has a "marginal attachment" at the periphery of the frondosum, and it may have a "velamentous insertion" in the adjacent part of the chorion laeve, through which the umbilical vessels then extend to the frondosum.

With the growth of the embryo, which fills and distends the uterine cavity, the decidua capsularis becomes thin, degenerates, and is resorbed, so that in the last half of pregnancy the chorion laeve rests directly against the decidua vera (Fig. 371, B).

The placenta at birth is a discoid mass of spongy vascular tissue, about 7 in. in diameter and i in. thick, weighing a pound. It is composed of two parts, the placenta uterina and placenta fetalis, which in certain lower mammals can be readily separated, but in others, and in man, they cannot, be disengaged. The uterine portion, as already stated, is the decidua basalis, and the embryonic or fetal portion is the chorion frondosum. At the margin of the placenta, the chorion frondosum is continuous with the chorion laeve, which is adherent to the decidua vera. Lining the chorionic cavity and spreading from the distal end of the umbilical cord, the amnion forms a complete sac, with a smooth and glistening surface toward the embryo. It is lightly adherent to the chorion laeve and to that surface of the placenta which is toward the embryo. From the way in which the chorion Iseve and chorion frondosum become differentiated, the fact that small accessory placentas sometimes occur near the main mass may be readily understood; detached groups of chorionic villi continue their growth, and their vessels communicate with those of the adjacent placenta. Such small accessory structures are known as succenturiate (i.e., recruited) placentas.

Fate of the Membranes at Birth. Shortly before birth, the cervix of the uterus dilates and the sac of membranes containing the liquor amnii bulges into it. The membranes thus exposed are ruptured, and the amniotic fluid escapes. The birth of the child follows, and the umbilical cord then extends from the navel through the vagina to the placenta. The cord is so short in some mammals that it ruptures with the expulsion of the embryo; in other forms it is bitten off or otherwise severed, setting free the embryo. Occasionally the membranes rupture in such a way that the head of the infant remains more or less covered with a cap of amnion and chorion laeve, formerly known as the "caul." After the birth of the child the uterine musculature contracts quite rapidly, and in about half an hour the after-birth is expelled, the sac of membranes being turned inside out in this process. The part from the fundus of the uterus is forced out first, and that from the lower segment of the uterus follows. Thus the amnion and the amniotic surface of the placenta are on the outside of the afterbirth. The denuded uterine mucosa is gradually restored to its normal condition. As after menstruation, the epithelium spreads from the glands over the tunica propria.

The entire after-birth, since its delivery follows that of the child, was called the secunda or secundina by the ancient anatomists. The round flat mass which is its principal part was named the placenta by Fallopius, from its fancied resemblance to a pan-cake. Long before this, the membranes enveloping the embryo were known as the chorion, allantois. and amnion, and were described as the outer, middle and inner layers respectively. These ancient terms are of obscure derivation. Chorion (Gr., x6pu>v) is the same as the Latin corium, which is applied to the vascular layers of the skin. In its Greek form it is used to designate the vascular chorioid coat of the eye, and the chorioid plexuses of the brain, but it refers particularly to the vascular embryonic membrane. Amnion is derived indirectly from d/wos (a sheep) and Hyrtl reasonably asks "How came the sheep to have his name enrolled in anatomy?" Whether the amnion was first observed in the sheep, or was so named because of its softness, or for some very different reason, is discussed by the early commentators. The allantois was first observed in the lower mammals in which it attains great size. For example, in the sheep and pig it forms an elongated sac filled with fluid and attached like the arms of a "T" to the distal end of the allantoic duct. This duct, which corresponds with the entire human allantois, issues from the ventral abdominal wall and divides into its two branches, as seen indistinctly through the chorion in Fig. 372 (over the body of the embryo). The allantoic sac extends almost the entire length


of the chorion, and its ends break through the chorionic membrane, projecting freely as the allantoic appendages. In Fig. 372, the one at the right extends upward, and the one at the left, downward. Such an allantois was sought for in man, between the amnion and chorion, where a corresponding structure should be located. Hale (1701) was among those who thought that he found one, but he declared that "most of the ancients allow a human allantois not from their experience of it, but because they took it for granted that men and other animals were alike in their viscera." It was not until 1885 that it was clearly and finally stated that the human allantois was merely a blind tube in the body stalk, never being free or vesicular (His, Anatomic menschlicher Embryonen).

As to the appropriateness of the term allantois (sausage-like, from the Gr. dAAas there is difference of opinion. Fabricius (De formato fcetu, 1600) one of whose drawings is reproduced in Fig. 372, considers that the word really means "intestinal," or like a sausage skin.

Decidua Vera, Amnion, and Chorion Laeve

The three structures named above may readily be included in a single vertical section of the wall of the uterus, in the latter part of pregnancy. Care must be taken, however, not to detach the amnion. In Fig. 373 the amnion is seen on the upper surface of the section, having its simple cuboidal or flat epithelium toward the embryo, and its mesodermic conneclive tissue toward the chorion. Adhesions in the form of slender strands bind it to the connective tissue of the chorion. The chorionic epithelium


forms a layer over the surface of the vera; it presents slight irregularities but is without villi. The superficial uterine epithelium has degenerated; it disappeared in an earlier stage. The modified mucosa. or decidua vera, is divisible into a superficial compact layer and a deep cavernous layer. After the epitheb'um of the glands in the compact layer had degenerated and was resorbed, the connective tissue came together obliterating the gland cavities. The compact layer is therefore without glands. The cells of the tunica propria have enlarged, and become decidual cells (Fig. 374). These cells, which occur only in pregnancy, are flattened, round, oval or branched structures of large size (0.03 to o.i mm.). Usually they contain a single nucleus but often there are two or more, and in giant forms there may be thirty or forty. The cavernous layer of the mucosa contains slender clefts parallel with the muscularis. These are glands which have been stretched laterally; some of them retain areas of normal epithelium, but in many the epithelium has degenerated, and from some it has wholly disappeared. The connective tissue is but slightly modified. Throughout the decidua, but especially in the superficial portion, the vessels are greatly distended.

FIG. 374. DECIDUAL CELLS FROM THE Mucous MEMBRANE OF A HUMAN UTERUS ABOUT SEVEN MONTHS PREGNANT. One cell shows a mitotic figure. X 250 (Schaper.)


The chorionic villi, the interlacing branches of which form the fetal portion of the placenta, are shaped as in Fig. 375. The finding of such structures in a uterine discharge or curetting is diagnostic of pregnancy. The villi in the earliest stages are composed entirely of epithelium, but they soon acquire a core of the chorionic mesenchymal tissue, in which are the terminal branches of the umbilical vessels. The epithelium is very early divisible into two layers. The outer layer consists of densely staining protoplasm, said to contain fat granules and to present a brush border; it has dark, round or flattened nuclei. Since cell boundaries are lacking, this is called the syncytial layer (Fig. 376). Mitotic figures are seldom seen in it. Generally its nuclei are in a single layer at varying distances from one another, but they may accumulate in "knots" or "proliferation islands," especially in late stages (Fig. 377). The knots project from the surface of the villi, so that in certain planes of section they appear completely detached and suggest multinucleate giant cells. The syncytial layer perhaps completely invests the villi at first, but later it is interrupted in many places.


The deeper layer of the chorionic epithelium consists of distinct cells with round nuclei and clear protoplasm. Although this is a single layer at the base of young villi, it produces great masses of cells at their tips. These columns or caps of cells in which the villi terminate fuse with one another next the decidua, and the uterine tissue seems to be dissolved as this mass of epithelium proliferates. All the superficial epithelium of the decidua basalis degenerates and disappears, and the underlying parts of the blood vessels in the tunica propria are destroyed. The uterine blood escapes into the intervillous spaces, bounded by the syncytium, or where this is deficient, by the basal cells. The maternal blood circulates in the intervillous spaces as shown in the diagram Fig. 378, and does not clot. So extraordinary is this, that attempts have been made to detect an endothelial covering for the villi, but without success. (The syncytial layer has been considered endothelial or otherwise of maternal origin, but this view is not accepted.) It is said that the products of the disintegration of the maternal tissue, including haemoglobin and even entire red corpuscles, are taken up by the syncytium and used for the nutrition of the embryo.




The placenta at birth, being an inch thick, presents in cross section a vast number of the branches of villi cut in various planes. A small fragment is shown in Fig. 379. On the left, there is a section of a large villus, containing fibrous tissue of the loose embryonic type, in some cases forming a thin basement membrane beneath the epithelium. Each villus contains a branch of the umbilical artery which ends in capillaries of very large but varying caliber. They are situated close beneath the epithelial layer, through which nutriment is transferred from the maternal blood in the intervillous spaces to that of the embryo in the vessels of the villi. Maternal and fetal blood never mingle, as may readily be seen in early stages when the embryonic blood contains nucleated red corpuscles.

The two primary layers of the chorionic epithelium are difficult to recognize in many parts of the placenta at birth. Thus in the villi shown in Fig. 377 it is seen that the epithelium is in places hardly distinguishable from the connective tissue. This thin portion may represent the basal layer and the dark clumps of nuclei scattered over its surface may arise from the syncytium, but the reverse relation of the two types of epithelium to the original layers is sometimes stated. Frequently the villi are covered in part with very conspicuous masses of hyaline material, apparently derived from epithelial degeneration (Fig. 379). Deposits of a substance staining deeply with eosin and resembling the fibrin of blood clots may also be observed. This material is often in the form of layers, with intervals between them, and is known as "canalized fibrin." It is believed to be derived from the blood, but the origin of these deeply staining masses is "not yet fully understood" (Stohr).


The surface of the placenta toward the embryo is covered with amnion, which has remained in place in the section shown in Fig. 380. Sometimes it becomes detached in preparing the specimen. It consists of homogeneous connective tissue toward the chorion, and is covered on its free surface by simple low columnar epithelium, sometimes containing fat droplets and vacuoles. The chorionic membrane is a much thicker layer, consisting of vascular connective tissue, and covered with epithelium continuous with that of the villi. The root of a villus is cut tangentially in Fig. 380. The epithelium at term is often in relation with the hyaline material or " canalized fibrin" which partially replaces it. In Fig. 380, cells of the deeper layer of the chorionic epithelium may still be recognized, but these are often lacking.

Toward the uterine wall the placenta is formed by the decidua basalis, which, like the decidua vera, includes a superficial compact layer and a deeper cavernous layer. The compact layer, which is detached with the placenta at birth, consists of connective tissue, blood vessels, giant cells and decidual cells (Fig. 381). Some of the chorionic villi have free endings toward this layer; others are extensively fused with it, forming such masses as shown on the right of Fig. 381.


Chorionic villus.

The decidua basalis extends out among the villi in the form of septa, which subdivide the mass of villi into lobes or cotyledons. (In the ruminants, the cotyledons are widely separated by areas of smooth chorion, but in man they are closely adjacent, with septa between them.) The septa end before reaching the chorionic membrane, except at the placental margin, where they form an enclosing wall. As the uterine arteries approach the intervillous spaces of the chorion, they pursue a coiled course, so that they may be cut several times in one section (Fig. 378). They pass, without branching, into the septa of the placenta, and before they empty into the intervillous spaces, their walls are reduced to mere endothelium. The veins which drain the intervillous spaces are not found in the septa, except at the placental margin. They pursue an oblique course downward from the floor of the cotyledons, beginning as large thin-walled tubes, into which free ends of villi may project (Fig. 378).


Umbilical Cord

The umbilical cord is a translucent, glistening, white or pearly rope of tissue about 2 feet in length, extending from the umbilicus to the placenta. It consists of mucous tissue (p. 62) covered with epithelium, and contains at birth three large blood vessels, two umbilical arteries and one umbilical vein (Fig. 382, B). The parallel arteries generally wind around the vein making sometimes forty revolutions. The surface of the cord shows corresponding spiral markings and often irregular protuberances called false knots. (True knots, tied by the intrauterine movements of the embryo, are very rare.) There are no lymphatic vessels or capillaries in the cord, and the large blood vessels do not anastomose. The walls of the arteries contain many muscle fibers but very little elastic tissue, and they are usually found collapsed in sections; their contraction is of interest since nerves have been traced into the cord for only a very short distance. The vein generally remains open.

The umbilical arteries arise in young embryos as the main terminal branches into which the dorsal aorta bifurcates. These vessels curve ventrally on either side of the pelvis and pass out through the cord to the chorion; they are equidistant from the allantois which they accompany. In the adult the parts of these vessels near the aorta are known as the common iliac arteries, and the small offshoots from them which have grown down the limbs, have become the external iliac arteries. The distal course of the original vessels may still be followed through the hypogastric arteries (internal iliacs) up on either side of the median line to the navel; toward the navel the vessels have become reduced to slender cords. The umbilical vein, within the cord, represents the fusion of a pair. On entering the body it conveys the blood from the placenta, through the persistent left umbilical vein, directly to the under side of the liver, which it crosses as the ductus venosus, and then empties into the vena cava inferior. In the adult, its former course is marked by the round ligament of the liver and the ligament of the ductus venosus.


A, from an embryo of two months, X 20; B, at birth, X 3. aL, Allantois; art., artery; coe., coelom; ?.. vein; y. s. t yolk stalk.

The allantois, which the umbilical vessels accompany, at first extends the entire length of the cord as a slender epithelial tube. Its condition at three months is shown in Fig. 383. At birth, it has become reduced to a very slender, and generally interrupted, solid strand of epithelial cells. That it may retain its continuity is stated by Ahlfeld (Arch. f. Gynak., 1876, vol. 10, p. 81). This remnant may be sought for near the body of the embryo, and its tendency to retain its original position equidistant from the umbilical arteries is the best guide for locating it. By the use of Mallory's connective tissue stain, the epithelial cells may be stained red in contrast with surrounding blue fibrils. Within the body of the embryo the allantois is prolonged to the upper end of the bladder, with which it is continuous; this intra-abdominal part has long been called the urachus (i.e., vas urinarium). If it remains pervious at birth, which is abnormal, urine may escape at the umbilicus.


Ent., Entodermal epithelium; mes., mesenchyma.

The yolk stalk, surrounded by an extension of the body cavity, is found in young umbilical cords (Fig. 382, A). This stalk is a slender strand of mesoderm, containing the entodermal vitelline duct, and the vitelline vessels which accompany it to the yolk-sac. The loop of intestine from which the yolk stalk springs may also extend into the cavity of the cord, and if it has not been drawn into the abdomen at birth, umbilical hernia results. If the cavity of the vitelline duct remains pervious at birth, the intestinal contents may escape at the umbilicus. (Such a condition is known as a fecal fistula, whereas the pervious urachus constitutes a urinary fistula.) Ordinarily the yolk stalk and its vitelline vessels, together with the ccelom of the cord, have been obliterated before birth, so that no trace of them remains in sections of the cord.



Ep., Epitrichium; S. C., stratum corneum; S. g., stratum granulosum; S. G., stratum germinativum; M. B., homogeneous layer; F. T., fibrous tissue; . T., areolar tissue.

The yolk-sac may be found with almost every placenta, as a very small cyst adherent to the amnion in the placental area. If the distal end of the cord is gently stretched, a wing-like fold appears (Fig.. 384), differing from all others by containing no large vessels; the fold indicates the direction of the yolk-sac, which may be exposed by stripping the amnion from the chorion. It may be beyond the limits of the placenta. Further details will be found in Lonnberg's admirable Studien tiber das Nabelblaschen, Stockholm, 1901.

Amniotic villi are irregular, flat, opaque spots on the amnion near the Distal end of the cord. They are often present and may suggest a diseased condition. As seen in Fig. 385 they are areas of imperfectly developed skin, and as shown in this case (Lewis, Art. "Umbilical Cord," Buck's Hdb., 2nd ed.) they present all of its fundamental layers. Frequently these cornified areas are less fully developed. They have been compared with the pointed epithelial elevations which cover the surface of the umbilical cord in ruminants, but the latter do not appear as areas of imperfect skin, and probably are entirely different structures. They may appropriately be called villi, but the human "villi" scarcely rise above the surface. Their significance is unknown.

Vagina And External Genital Organs

The vagina consists of a mucosa, submucosa, muscularis and fibrosa. Its epithelium is thick and stratified, its outer cells being squamous and easily detached. It rests upon the papillae of the tunica propria, and is thrown into coarse folds or ruga. Glands are absent. The tunica propria is a delicate connective tissue with few elastic fibers, containing a variable number ^ of lymphocytes. Occasionally there are solitary nodules, above which numerous lymphocytes wander into the epithelium. The submucosa consists of loose connective tissue with coarse elastic fibers. The muscularis includes an inner circular and a small outer longitudinal layer of smooth muscle. The fibrosa is a firm connective tissue, well supplied with elastic elements. Blood and lymphatic vessels are found in the connective tissue layers, and wide veins form a close network between the muscle bundles. There is a ganglionated plexus of nerves in the fibrosa.

The mucous membrane of the vestibule differs from that of the vagina in possessing glands. The numerous lesser vestibular glands. 0.5-3 mm. in diameter, produce mucus; they occur chiefly near the clitoris and the outlet of the urethra. The pair of large vestibular glands (Bartholin's) also produce mucus; they correspond with the bulbo-urethral glands in the male and are of similar structure. The hymen consists of fine-fibered, vascular connective tissue covered with mucous membrane. The clitoris is an erectile body; resembling the penis. It includes two small corpora cavernosa. The glans clitoridis contains a thick net of veins. It is not, as in the male, at the tip of a corpus cavernosum urethrae which begins as a median bulb in the perineal region; the bulbus in the female exists as a pair of highly vascular bodies, one on either side of the vestibule. Each is called a bulbus vestibuli. The labia minora contain sebaceous glands, 0.2-2.0 mm. in size, which are not connected with hair follicles; they first become distinct between the third and sixth years. The labia majora have the structure of skin.


The skin (cutis) consists of an ectodermal epithelium, the epidermis, and a mesodermal connective tissue, the corium (Fig. 386). The ectoderm is at first a single layer but it soon becomes double, the outer cells staining more deeply, and being notably larger than the inner cells. Their

characteristic dome shape is seen in the figure. The outer layer has been named the epitrichium, since the hairs which grow up through the underlying epithelium do not penetrate it, but cause it to be cast off. The epitrichium has been found on the umbilical cord and in places on the amnion. It may possibly be related to the chorionic syncytium. The deeper layer of ectoderm becomes stratified, and gives rise to the hairs, nails, and enamel organs. It also produces two types of glands, the sebaceous glands which are usually connected with hairs, and the sweat glands. These are widely distributed; locally the ectoderm forms the mammary glands, ceruminous glands of the ear, ciliary glands of the eyelids, and other special forms. The greater part of the surface of the skin presents many little furrows, the sulci cutis, which intersect so that they bound rectangular spaces. On the palms and soles the furrows are parallel for considerable distances, being separated from one another by slender ridges, the cristce cutis, along the summits of which the sweat glands open. The ridges are most highly developed over the pads of tissue at the finger tips, where they present the familiar spiral and concentric patterns. These pads of connective tissue, the toruli tactiles, must not be confounded with elevations due to underlying muscles.

FIG. 386. SKIN FROM THE OCCIPUT OF AN EMBRYO OF Two AND ONE-HALF MONTHS. (After Bowen.) The outer layer of dark cells is the epitrichium.


In the pentadactylous mammals, each extremity typically presents five digital toruli, at the tips of the fingers or toes; four interdigital toruli, near the metacarpo- or metatarso-phalangeal joints; and two or three proximal cushions a tibial and an elongated fibular; or a radial and two ulnar, one behind the other. Often the interdigital cushions fuse, as in the paw of the cat and the ball of the human foot, and the one between the thumb and fingers may be suppressed. These toruli are very prominent in the embryo. According to Miss Whipple (Zeitschr. f. Morph. u. Anthr., 1904, vol. 7, pp. 261-368) they are primarily walking pads, witfi ridges at right angles to the slipping force. Usually they are considered primarily tactile. The extensive literature pertaining to them has been reviewed by Schlagenhaufen (Anat. Hefte, 1906, Abt. II, vol. 15, pp. 628-662).


The corium is a layer of densely interwoven bundles of connective tissue extending from the epidermis to the fatty, areolar subcutaneous tissue (Fig. 387). Toward the epidermis the corium forms papilla, which vary considerably in size and number in different parts of the body. They are tallest (even 0.2 mm. high) and most numerous, often being branched, in the palms and soles. Beneath the epidermal ridges they may occur quite regularly in double rows (Fig. 388), as long since observed by Malpighi. In the skin of the face the papillae are poorly developed, and in advanced age they may wholly disappear. The papillae are composed of cellular connective tissue, which forms a tunica propria; and each papilla contains a terminal knot of capillary blood vessels, or a tactile corpuscle (Fig. 152, p. 159). The corpuscles are most numerous in the sensitive finger tips, where they may be found in one papilla in every fur. 25


The entire corium is somewhat arbitrarily subdivided into an outer stratum papillare and an inner stratum reticulare (Fig. 387). These layers blend with one another, but the outer portion consist of finer bundles of connective tissue, more closely interwoven than those in the coarse network characteristic of the stratum reticulare. Beneath the skin, but inseparable from it, is the stratum subcutaneum, which is composed of areolar tissue with large areas of fat cells; where the fat forms a continuous layer, it is known as the panniculus adiposus. Finally the bundles of the stratum subcutaneum connect more or less intimately with the fascia around the muscles, or, in places, with the periosteum.

The elastic fibers of the corium form evenly distributed networks, which are finer in the stratum papillare and coarser in the stratum reticulare. There is said to be a subepithelial network, and a layer of

Depressions which were occupied by papillae.

Ridge corresponding

to a furrow of the


Portion of the duct of a sweat gland.


FROM THE LOWER SURFACE. Xi2o. The dark epithelial network between the papilla is the rete Malpighii.

numerous coarse fibers immediately above the general layer of fascia. In old age a notable decrease in the elastic fibers has been recorded. The muscle fibers of the corium are chiefly the small bundles of smooth muscle attached to the sheaths of the hairs, forming the arrectores pilorum. Smooth muscle is diffusely distributed in the nipple, and in the scrotum it forms a layer pervaded by elastic tissue, known as the tunica dartos. Striated muscle fibers derived from the muscles of expression terminate in the skin of the face. The vessels and nerves of the corium are described on page 399.

Epidermis. If a piece of skin is boiled, the epidermis may be stripped off, carrying the tunica propria with it; and the epidermis itself may be separated into two layers. The outer layer is the stratum corneum; the inner is the stratum germinativum.

The stratum germinativum was formerly called the stratum mucosum or rete Malpighii. It was first described by Malpighi who recognized its soft or "mucous" nature, and referred to it as a rete since it forms a network between the papillae of the corium (Fig. 389). Malpighi considered that the color of the Ethiopian skin was confined to this layer.

The stratum germinativum and stratum corneum are subdivisions of a single thick stratified epithelium. The basal cells, which rest directly upon the papillae of the corium, constitute a single row of columnar cells, with elongated nuclei and no cell walls (Fig. 390). Through mitotic division these cells multiply and give rise to the outer polygonal cells, but it is noteworthy that mitotic figures are seldom seen. The polygonal cells which form the bulk of the stratum germinativum are connected with one another by slender intercellular bridges (Fig. 43, p. 53), through which fibrils pass from cell to cell. Because of this striking feature, the stratum germinativum was formerly called the stratum spinosum.


The transition from the stratum germinativum to the stratum corneum is abrupt. It may be marked by an incomplete layer of coarsely granular cells, such as are highly developed in the skin of the palms and soles, where they form the stratum granulosum (Fig. 390). In the stratum corneum the cells acquire a horny exoplasmic membrane; the bridges become short stiff spines; the protoplasm and nucleus are dry and shrunken; ane in the outermost cells the nucleus wholly disappears. The cells becomd flatter toward the surface, from which they are constantly being desquamated.

The process of cornification presents a more elaborate picture in sections of the palms and soles. Outward from the stratum germinativum there is a darkly staining, coarsely granular layer, one or two cells thick, which is followed by a clear, somewhat refractive band in which the cell outlines are indistinct. This layer seems saturated with a dense fluid formed by dissolution of the underlying granules. In haematoxylin and eosin specimens, the granular layer or stratum granulosum is followed by a pink and then by a bluish band, which are subdivisions of the clear stratum lucidum. These' are followed by a very thick stratum corneum. Except in the palms and soles, the granulosum is thin and the lucidum is absent. Chemically the coarse granules of the stratum granulosum resemble the horny substance keratin (from which they differ by dissolving in caustic potash) ; they are therefore called kerato-hyalin granules. Their diffuse product in the stratum lucidum is named eleidin. In the corneum it becomes pareleidin, which, like fat, blackens with osmic acid, but the reaction occurs more slowly. The pareleidin is not due to fat entering the skin from oily secretions on its outer surface. Further information regarding these substances is supplied by Pinkus (Keibel and Mall's Human Embryology, vol. i).

The color of the skin is due to fine pigment granules in and between the lowest layers of the epidermal cells. Underlying cells of the corium sometimes contain groups of finer pigment granules, but such cells are absent from the palms and soles and are infrequent elsewhere. They may be found in the deeply pigmented circum-anal tissue, and in the eyelids.


The nails are areas of modified skin consisting of corium and epithelium. The corium is composed of fibrous and elastic tissue, the bundles of which in part extend vertically between the periosteum of the phalanx and the epithelium, and in part run lengthwise of the finger. In place of papillae, the corium of the nail forms narrow longitudinal ridges, which are low near the root of the nail but increase in height toward its free distal border; there they abruptly give place to the papillae of the skin. The epithelium consists of a stratum germinativum and a stratum corneum. The latter, according to Bo wen (Anat. Anz., 1889, vol. 4, pp. 4 2 1-4 50) , represents a greatly thickened stratum lucidum, but this opinion requires confirmation. In the embryo the horny substance is entirely covered by a looser layer, the eponychium, and this name is applied in the adult to the skin-like tissue which overlaps the root and sides of the nail (Fig. 391). The eponychium is the stratum corneum of the adjoining skin.

FIG. 391. DORSAL HALF OF A CROSS SECTION OF THB THIRD PHALANX OF A CHILD. X 15. The ridges of the nail bed in cross section appear like papillae.

It is now generally considered that the cells of the stratum germinativum covering the greater part of the "nail bed" do not produce any of the overlying horny material. This function is reserved for the germinative cells at the root of the nail, beneath the crescentic white area, the lunula, and its extension backward under the nail fold. The latter is a fold of skin which is deep at the root of the nail, but becomes shallower as it extends forward on either side, bounded by the nail wall (Fig. 391). It is now stated that cornification in the nails takes place without the formation of kerato-hyalin granules, and a fibrillar arrangement of the keratin has been thought to account for the whiteness and opacity of the lunula. The cornified cells of the nail may be separated by placing a fragment in a strong solution of caustic potash and heating to boiling. The cells differ from those in the outer layers of the skin by retaininig their nuclei (Fig. 392).



The hairs arise as local thickenings of the epidermis. They soon become round columns of ectodermal cells extending obliquely downward into the corium (Fig. 393). As the columns elongate the terminal portion becomes enlarged, forming the bulb of the hair, and a mesodermal papilla occupies the center of the bulb. On that side of the epithelial column which from its obliquity may be called the lower surface, there are found two


swellings (Figs. 394-396). The upper MoNTHs K x F 2 A 3 " UMAN EMBRYO OF FlVE is to become a sebaceous gland, discharging its secretion into the epithelial column; the lower or deeper swelling is called the "epithelial bed," and



FIG. 396. VERTICAL SECTION OF THE SKIN OF THE FOREHEAD OF A HUMAN EMBRYO OF FIVE MONTHS. X23O. Differentiation of the sheaths of the hair.

its cells, which increase by mitosis, contribute to the growth of the column. (The lower swelling is often described as the place of insertion of the arrector pili muscle). Beginning near the bulb, the core of the column separates from the peripheral cells; the latter become the outer sheath of the hair. The core forms the inner sheath and the shaft of the hair. The cells of the shaft become cornified just above the bulb, and they are surrounded by the inner sheath as far as the sebaceous gland. Beyond this point the inner sheath degenerates, so that in later stages the distal part of the shaft is immediately surrounded by the outer sheath. As new cells are added to the hair from below, the shaft is pushed toward the surface. The central cells in the outer end of the column degenerate, thus producing a "hair canal" which is prolonged laterally in the epidermis (Fig. 397). The shaft enters the canal, breaks up the overlying epitrighium, and projects from the surface of the body. That portion of the hair which remains beneath the epidermis is sheaths, the root in all larger hairs possesses a connective tissue sheath, derived from the corium. This serves for the insertion of a bundle of smooth muscle fibers, the other end of which is connected with the elastic and fibrous elements in the superficial part of the corium. Since this muscle by contraction causes the hair to stand on end, it is called the arrector pili. Its insertion is always below the sebaceous gland and on the lower surface of the hair, as shown in Fig. 398. The hairs which cover the body of the embryo, persisting after birth to a variable extent, are soft and downy, and are known as lanugo. Arrector muscles are absent from the lanugo of the nose, cheeks and lips, and also from the eyelashes (cilia) and nasal hairs (vibrissae) .


The staining with iron hsematoxylin has made the horny parts so black that their details are invisible.

Adult Structure. The general appearance of hairs in sections of the adult skin is shown in Fig. 398, which includes also the sebaceous glands emptying into the sheaths of the hairs, and sweat glands which are usually entirely separate structures. Occasionally a sweat gland opens into the sheath of a hair near its outlet. Each hair consists of a papilla, bulb and shaft, together with sheaths around the root, namely an inner and outer epithelial sheath and, external to these, a connective tissue sheath. These structures, together with the arrector pili muscle which is inserted into the connective tissue sheath, are indicated in Fig. 398, but they are shown in detail in the longitudinal section, Fig. 399, and in the transverse sections, Figs. 401-405. They may be described as follows:

The connective tissue sheath, derived from the corium, is found around the roots of the coarser hairs, but is absent from the lanugo. It may be subdivided into three concentric layers. The outermost consists of loose connective tissue with longitudinal fibers, and contains elastic tissue and numerous vessels and nerves. The middle layer, which is thicker, consists of circular bundles of connective tissue without elastic fibers. The inner layer, also free from elastic tissue, is sometimes longitudinally fibrous, and sometimes homogeneous. It forms the outer stratum of the hyaline (or vitreous) membrane, and is continuous below with the thin but distinct


Cortical substance.

Shaft of the hair.

Longitudinal fiber


Circular fiber layer.

Outer layer of the hya' line membrane.

Inner layer of the hyaline membrane.

Outer epithelial sheath.

Henle's layer.

Huxley's layer

Cuticle of the inner 'sheath.



the human scalp.) X 200. The kerato-hyalin granules are colored red.





layer which covers the papilla (Fig. 399) . An inner stratum of the hyaline membrane is formed, according to Stohr, from the epithelial cells of the root sheath. This inner stratum is provided with fine pores, and is always clear and homogeneous. It may unite with the connective tissue stratum so that both may appear as a single membrane. The connective tissue sheath is found fully developed only around the lower half of the root. The outer epithelial sheath is an inpocketing of the epidermis. The stratum corneum extends to the sebaceous gland; the stratum granulosum continues somewhat deeper, but only a thinned stratum germinativum can be followed to the bulb. All of these are included in the outer epithelial sheath (Figs. 401-405, I, II, and 5).

The inner epithelial sheath extends from the sebaceous gland to the bulb. It begins as a layer of cornified cells below the termination of the stratum

granulosum, but it is not a continuation of that layer. Toward the bulb the inner sheath is divisible into two layers. The outer or Henle's layer consists of one or two rows of cells with occasional atrophic nuclei; for the most part they are non-nucleated. The inner or Huxley's layer is a row of nucleated cells. The inner surface of Huxley's layer is covered by a membrane, the cuticula of the sheath, composed of nonnucleated cornified scales. Traced downward, the elements of the inner epithelial sheath and its cuticula all become nucleated cells, but the layers may be distinguished almost to the neck of the papilla. There they lose their sharp boundaries, but may still be distinguished from the pigmented cells of the bulb. Traced upward, it is

found that kerato-hyalin granules appear in Henle's layer at the level of the papilla, and in Huxley's layer somewhat higher (Fig. 399) ; still higher these granules disappear and the cells of the inner sheath become cornified.

The shaft of the hair is entirely epithelial; it consists of cuticula, cortex and medulla (Fig. 400). The cuticula, which covers its surface, is a thin layer formed of transparent scales directed from the center of the shaft outward and upward, thus overlapping like inverted shingles. This arrangement is readily seen in wool and the hairs of various mammals, but is much less evident in human hair. The cuticula is composed of nonnucleated cornified cells.

The greater portion of the shaft is included in the cortex. Toward the bulb, the cortex consists of soft round cells; distally these cells become corni





/ ' / *)* " ' . N

- '/< S'* ^ ***^ .*

' "^ ^*Tz:^' i f V* ' v*


Bulbus pili. -^



FIG. 401.

FIG. 405.


A, Cuticula; B, cortex; C, medulla. I, Str. corneum; II, str. germinativum; HI, corium. 1-3, Connectivetissue sheath; i, longitudinal fiber layer; a, circular fiber layer; 3, conn, tiss hyaline membrane; 4; epithelial hyaline membrane; 5, outer epithelial sheath; 6, inner epithelial sheath; 6a, Henle's layer, 6b, Huxley's layer; 7, cuticula of the sheath; Muse., arrector pili; Seb., sebaceous gland.



fied, elongated and very closely joined together. Their nuclei are then linear. The cortex of colored hairs contains pigment both in solution

and in the form of granules. These granules are partly within the cells, and partly between them. Moreover every fully developed hair contains minute intercellular air-spaces, found within both cortex and medulla. But a medulla is lacking in many hairs, and when present, in the thicker hairs, it does not extend their whole length. It consists of cuboidal cells containing kerato-hyalin (Fig. 399), and generally arranged in a double row. Their nuclei are degenerating. Growth and Replacement of Hairs. The growth of the shaft, and of the inner epithelial sheath with its cuticula, takes place through continued

Remains of inner sheath.

Epithelial bed.


Parts of A and B are shown enlarged in Figs. 407 and 408.

Cornified bulb.

Remains of inner sheath.

Cornified bulb.

Epithelial cord.

Atrophic papilla. Connective tissue.

FIG. 407. LOWER PART OF FIG. 406, A. X230. FIG. 408. LOWER PART OF FIG. 406, B. X 230.

mitotic division of the epithelial matrix cells of the bulb of the hair. These become cornified, and are added from below to the cells previously cornified. Accordingly the oldest cells are at the tip of the hair and the young



est are immediately above the bulb. The outer epithelial sheath grows in a radial direction from the inner surface of the hyaline membrane toward the shaft.

Shortly before and after birth, there is a general shedding of hair, subsequent to which the loss and replacement of individual hairs is constantly taking place. A hair of the scalp is said to last 1600 days, but the duration of other hairs has not been definitely determined. The process of removal begins with a thickening of the hyaline membrane and circular fiber sheath. The matrix cells cease to produce, first the inner epithelial sheath, and then the cuticulae and shaft. The hollow bulb becomes a solid cornified "club." The matrix cells increase without differentiating into hair cells or sheath cells, and the clubbed hair, with its inner sheath, is forced outward to the level of the orifice of the sebaceous gland, where it may remain for some time (Fig. 406, D). The lower part of the outer epithelial sheath, which has become empty, forms an epithelial strand which shortens and draws the papilla upward; but the connective tissue sheath remains behind, forming the "hair stalk." After some time, the columnar cells of the epithelial bed proliferate, causing the epithelial cord to return to its former depth (Figs. 407 and 408), and a new hair develops in the old sheath upon the old papilla. The new hair in growing toward the surface completes the expulsion of its predecessor, which is dislodged together with cells of the adjacent epithelial bed.


The sebaceous glands are simple, branched or unbranched alveolar structures situated in the superficial layer of the corium and usually ap

Epidermis. J f, ' =


Cell with shrunken nucleus.

Cell with well-developed drops of secretion.

l{_ Cell with developing drops of secretion.

Cuboidal cell

Pig. 400. A, PROM A VERTICAL SECTION THROUGH THE ALA NASI OF A CHILD. X 40. C. Stratum corneum; M, stratum genninativum; t, sebaceous gland consisting of four sacs, a, duct of the same; w, lanugo hair, about to be shed; h, sheath of the same, at the base of which a new hair, z, is forming.

B, FROM A VERTICAL SECTION OF THE SKIN OF THE ALA NASI OF AN INFANT. X 240. Sac of a sebaceous gland containing gland cells in various stages of secretion.

pended to the sheath of a hair (Fig. 398). In connection with the lanugo, a large gland may be associated with a very small bair (Fig. 409), and in


exceptional cases as at the margin of the lip or on the labia minora, they occur independently of hairs. They vary in size from 0.2 to 2.2 mm., the largest being found in the skin of the nose where the ducts are macroscopic. None are found in the palms or soles, where hairs also are absent.

The short duct is a prolongation of the outer epithelial sheath of the hair and is formed of stratified epithelium, the number of layers of which decreases toward the alveoli. The alveoli consist of small cuboidal basal cells, and of large rounded inner cells in all stages of fatty metamorphosis. As the cell becomes full of vacuoles, the nucleus degenerates, and the cell is cast off with its contained secretion. In life the product of the glands is a semi-fluid material, composed of fat and broken-down cells.

Glandules prceputiales are sebaceous glands without hairs which are sometimes, but not always, found on the glans and praeputium penis. The designation "Tyson's glands" is not justified since Tyson described the epithelial pockets ^ to i cm. long which regularly occur near the frenulum praeputii. Praeputial glands and crypts are not found in the embryo. The praeputium is united to the outer surface of the glans by an epithelial mass, which often persists after birth and is broken up by the formation of concentric epithelial pearls. Glands and crypts are absent from the praeputium and glans of the clitoris.


The glandula sudoriparce are long unbranched tubes terminating in a simple coil (described by Oliver Wendell Holmes as resembling a fairy's intestine, Fig. 410). The coil is found in the deep part of the corium or in the subcutaneous tissue (Fig. 387). The duct pursues a straight or

somewhat tortuous course to the epidermis which it enters between the connective tissue papillae. Within the epidermis its spiral windings are pronounced (Fig. 387); it ends in a pore which may be detected macroscopically.

The epithelium of the ducts consists of two or three layers of cuboidal cells; it has an inner cuticula, and an outer basement membrane

FIG. 410. MODEL OF THE COILED j i_ i -j. j- i

PART OF A SWEAT GLAND covered by longitudinal connective tissue fibers.


(After Huber.) Within the epidermis its walls are made of cells

of the strata through which it passes. The

secretory portion of the gland (3.0 mm. long according to Huber) forms about three-fourths of the coil, the duct constituting the remainder. The secretory epithelium is a simple layer of cells, varying from low cuboidal to columnar, according to the amount of secretion which they contain. Those filled with secretion present granules, some of which are pigment and



fat. The product is eliminated through intra- and intercellular secretory capillaries. It is ordinarily a fatty fluid for oiling the skin, but it becomes the watery sweat under the influence of the nerves. The gland cells are not destroyed by either form of activity. The secretory tubule is surrounded by a distinct basement membrane, within which there is a row of small longitudinally elongated cells described as muscle fibers. They do not form a complete membrane, and they appear as a continuation of the basal layer of cells of the ducts.

Sweat glands are distributed over the entire skin, except that of the glans and the inner layer of the praeputium penis. They are most numerous in the palms and soles. In the axilla there are branched sweat glands and large forms with 30 mm. of coiled tube. They acquire their large size at puberty and have been considered as sexual "odoriferous" glands. In the vicinity of the anus there are also branched sweat glands, together with the large unbranched "circum-anal glands."

A. Duct in cross section.

Nuclei of Muscle gland cells, fibers.

Membrana propria. Cuticula.

Muscle fibers

B. Columnar epithelium from the coiled tubule.

C. Surface view of the coiled tubule.

D. Low epithelium from a coiled tubule.

Membrana propria.

Muscle fibers.

Muscle nucleus. Cuticula.

>.-^\l*/ Membrana propria Muscle fiber.

E. Cross section of coiled tubule.



The arteries proceed from a network above the fascia, and branch as they ascend toward the surface of the skin. Their branches anastomose, forming a cutaneous plexus in the lower portion of the corium. From this plexus branches extend to the lobules of fat and to the coils of the sweat glands, about which they form "baskets" of capillaries. Other branches pass to the superficial part of the corium where they again anastomose, forming a subpapillary plexus, before sending terminal arteries into the papillae. The subpapillary plexus sends branches also to the sebaceous glands and hair sheaths, but the papilla of a hair receives an independent artery. The veins which receive the blood from the superficial capillaries form a plexus immediately beneath the papillae, and sometimes another just below the first and connected with it. The veins from these plexuses accompany the arteries and the ducts of the sweat



glands to the deeper part of the corium, where they branch freely, receiving the veins from the fat lobules and sweat glands. Larger veins continue into the subcutaneous tissue where the main channels receive specific names.

The lymphatics form a fine- meshed plexus of narrow vessels beneath


Branches of the subpapillary arterial plexus.

Veins of the second superficial plexus.

Veins along the duct of a sweat eland.

Large vein. Vessel to the Vessel to the

fat tissue. sweat gland.

FIG. 412. PART OF A VERTICAL SECTION OF THE INJECTED SKIN OF THE SOLE OF THE FOOT. X 20. The veins are not completely filled by the injection.

the subpapillary network of blood vessels, receiving tributary loops from the papillae. This plexus empties into a wide-meshed subcutaneous plexus. There are lymphatic vessels around the hair sheaths, sebaceous glands, and sweat glands.

The nerves form a wide-meshed plexus in the deep subcutaneous tissue, and secondary plexuses as they ascend through the skin. The sympathetic,



non-medullated nerves supply the numerous vessels, the arrector pili muscles, and the sweat glands; an epilamellar plexus outside of the basement membrane sends branches through the membrane to terminate in contact with the gland cells. Medullated sensory nerves end in the various corpuscles already described, and in free terminations, some being intraepithelial. Medullated fibers to the hairs lose their myelin and form elongated free endings with terminal enlargements in contact with the hyaline membrane. (The nerves to the tactile hairs of some animals penetrate the hyaline membrane and terminate in tactile menisci among the cells of the outer epithelial sheath.) Small, round or discoid elevations of the epidermis, visible with the naked eye, occur close to the hairs as they emerge from the skin, being on the side toward which the hairs slope. These "hair discs" (Pinkus) are said to be abundantly supplied with nerves. The corium beneath the nails is rich in medullated nerves, the non-medullated endings of which enter the Golgi-Mazzoni type of lamellar corpuscle (having a large core and few lamellae), or they form knots which are without capsules. Elsewhere the skin contains tactile corpuscles in its papillae and lamellar corpuscles in the subcutaneous tissue, together with free endings in the corium and epidermis (as far out as the stratum granulosum) .


In young mammalian embryos generally, the mammary glands are first indicated by a thickened line of ectoderm extending from the axilla to the groin. Later much of the line disappears, leaving a succession of nodular thickenings corresponding with the nipples. In some mammals

FIG. 413. SECTION THROUGH THE MAMMARY GLAND OF AN EMBRYO OF 25 CM. i. Connective tissue of the gland. (After Basch, from McMurrich.)

this row of nipples remains, in others only the inguinal thickenings, and in still others only those toward the axilla. Thus in man there is normally only one nipple on each side, but structures interpreted as accessory nipples are frequent; they are not always situated along the mammary line. In an embryo of 25 cm. (Fig. 413) several solid cords have grown out from 26



the ectodermal proliferation. There are ultimately from 15 to 20 of these tn each breast, and they branch as they extend through the connective tissue. At birth the nipple has become everted, making an elevation, and at that time the glands in either sex may discharge a little milky secretion similar to the colostrum which precedes lactation. The glands grow in both sexes until puberty, when those in the male atrophy and only the main ducts persist. In the female enlarged terminal alveoli are scarcely evident until pregnancy. The glands until then are discoid masses of connective tissue and fat cells, showing in sections small scattered groups of^ductlike tubes.

Toward the end of pregnancy each of the fifteen or twenty branched glands forms a mammary lobe, and its alveolo-tubular end pieces are

Branch of an excretory duct. Connective tissue.


. ;.'!.',*.: *. ?:,/' vt '--- 'VV /-"Si r * v ' ~- > v .n >; ^OOiiMif^. ^i^SSff^'H ^i^;Sr

M^8^!l " - : - ' ^>



grouped in lobules. The secretory epithelium is a simple cuboidal or flattened layer, in which fat accumulates at the seventh or eight month of pregnancy. The fat first appears as small granules at the basal ends of the cells, where it is taken up from the surrounding tissue. It is not produced by the gland cells. Leucocytes, derived from the connective tissue, make their way between the epithelial cells of the alveoli and enter the gland lumen, where some of them degenerate; others receive fat from the gland cells, either in solution, or in drops which are devoured by phagocy tic action. These fatty leucocytes grow to considerable size and are called colostrum corpuscles. Beneath the alveolar epithelium there are basal or basket cells, which have been compared with the muscle fibers of sweat



glands. A basement membrane separates them from the connective tissue which contains many lymphocytes and eosinophilic cells.

After the birth of the child, the gland cells become larger and are filled with stainable secretory granules and fat droplets; the latter are near the lumen and are often larger than the nucleus (Fig. 415). After two days of lactation, some of the gland cells are flat and empty of secretion. Others are tall and columnar, with a rounded border toward the lumen; often they contain two nuclei. The fat within them is not the result of degeneration as in sebaceous glands, nor a secretion produced by the nucleus; it accumulates through protoplasmic activity, and the cell may be filled several times before it perishes. Transitions between low empty cells and columnar forms occur, but mitoses are absent from the lactating gland, sions are numerous during pregnancy.

Milk consists of fat droplets, 2-5 /x in diameter, floating in a clear fluid which contains nuclein derived from degenerating nuclei, and occasionally a leucocyte or colostrum corpuscle. Free nuclei may be found, and some cells which undoubtedly are to be interpreted as detached from the alveoli of the gland.

Gland cell. Membrana Oil drops, propria.


Mitotic divi




i, Cell containing uncolored fat globules; 2, cell containing minute colored fat globules; 3, leucocyte; 4, milk globules.


i, Large excretory duct; 2, small excretory duct; 3. gland lobules, separated from one another by connective tissue.

At the end of lactation, the connective tissue, which has become greatly reduced owing to the enlargement of the glands, increases in quantity and the leucocytes reappear; as during pregnancy, they form colostrum corpuscles. The lobules become smaller and the alveoli begin to degenerate.


In old persons all the end pieces and lobules have gone and only the ducts remain.

The ducts are lined with simple columnar epithelium, surrounded by a basement membrane and generally by circular connective tissue bundles. Toward the nipple each duct forms a considerable spindle-shaped dilatation, the sinus lactiferus. The epithelium near the outlet of the ducts is stratified and squamous.

The skin of the nipple, and of the areola at its base, contains abundant pigment in the deepest layers of its epidermis. The corium forms tall papillae and contains smooth muscle fibers, some of which extend vertically through the nipple and others are circularly arranged around the ducts. There are tactile corpuscles in the nipple, and lamellar corpuscles have been found beneath its areola. It is particularly sensitive, and upon irritation becomes rapidly elevated, due both to muscular and vascular activity. There are many sweat and sebaceous glands in the areola, and occasional rudimentary hairs. The areolar glands (of Montgomery) are branched tubular glands having a lactiferous sinus and otherwise resembling the constituent mammary glands. Their funnel-shaped outlets are surrounded by large sebaceous glands. The areolar glands are regarded as transitions between sweat glands and mammary glands.

Blood vessels enter the breast from several sources and form capillaries around the alveoli. Lymphatic vessels are found in the areola, around the sinuses, and in the interlobular tissue. The collecting lymphatics pass chiefly toward the axilla; a few penetrate the intercostal spaces toward the sternum. The nerves are mostly those which supply the blood vessels, but fibers are said to extend to the glandular epithelium.


Development and General Features. The suprarenal glands are two flattened masses of cells, without lumen or ducts, situated in the retroperitoneal tissue above the kidneys. They vary considerably in size and shape, but are usually about a quarter of an inch thick and between i and 2 inches tall, sometimes being wider and sometimes narrower than their height. The right suprarenal gland is generally described as triangular and the left as crescentic.

The gland resting upon the kidney (Glandula Rent incumbens) was first described by Eustachius (Tractatio de Renibus, 1564). It was apparent from the outset that the relation of the suprarenal glands to the kidneys was merely that of juxtaposition, nevertheless most anatomists still find it convenient to describe them with the urinary organs Certain early writers supposed that they were renal structures and named them "succenturiate kidneys." Bartholin (Anatomia, 1666) perceived the medulla, which he described as a cavity containing a black humor; and he published an extraordinary figure in which the gland resembles a cocoanut cut across "with the lid lifted.



In accordance with this conception he named the structures "atrobiliary capsules," and the name capsule is still often applied to them. Diemerbroeck (Anatome, 1672), following Wharton, states that "the glands are found at a place where there is a plexus of nerves, to which they are firmly united." In reviewing the various "conjectures" as to their function, he writes, "Wharton thinks that in these capsules a certain juice is removed from the plexus of nerves on which they lie, useless indeed to the nervous system, but which, flowing thence into the veins, may serve some useful purpose." The intimate relation of these glands to the nervous system, and the production of an internal secretion received by the veins, have since been demonstrated; in certain recent works the glands have even been described as parts of the nervous system. Diemerbroeck concludes by hoping that physicians, through many autopsies, may find out to what diseases these glands give rise. In 1855, Addison described the disease, usually fatal, which is thought to depend upon the loss of function of these glands. Their physiological importance has been amply demonstrated, but they still present fundamental problems, both as to function and structure.

A section through a fresh suprarenal gland reveals at once the division into cortex and medulla. The cortex is yellowish, owing to the presence of lipoid substance, and the medulla is dark brown, due in part to the large amount of blood which it contains. The color contrast is usually very striking, and it is shown also in unstained sections of tissue preserved in chromic acid solutions (Fig. 418), although the medulla may then be lighter than the cortex. Not only do the cortex and medulla differ in gross appearance, but they are radically different in embryonic origin, and in the sharks they exist as separate organs. In sharks the medulla is represented by groups of chromaffin cells associated with the sympathetic ganglia, and the cortex takes the form of an "interrenal gland," composed of cords of mesodermal cells with a sinu Cortex. Medulla.

soidal circulation. Inhuman embryos correspond- FIG. 418. SECTION OF THE


ing parts arise separately, but they come together to * CHILD, x is. form a single gland.

The cortex appears first, and is formed from cells which develop as buds of the ccelomic epithelium, growing into the mesenchyma on either side of the root of the mesentery, medial to the Wolffian bodies. In embryos of 8-12 mm., the buds or cords have become detached from the peritoneal epithelium (Zuckerkandl), and in cross sections they appear as round masses of cells penetrated by a network of slender veins. The cells of these masses rest directly against the vascular endothelium, so that the vessels are described as sinusoids.

Meanwhile cells from the sympathetic ganglia grow ventrally along the medial side of these masses, where they are conspicuous because of their dark stain (Fig. 419). These cells, which give rise to the medulla of



the suprarenal gland, do not appear like nerve cells and may be radically different from them, although always closely associated with the sympathetic ganglia. Because of their affinity for chromium they are known as chromaffin cells. They produce the important internal secretion, adrenalin, which on injection causes contraction of the musculature of the blood vessels, with consequent rise in blood pressure. The chromaffin cells are not confined to the suprarenal glands, as already stated (p. 152). In embryos of 15-20 mm., strands of chromaffin cells are seen penetrating the cortical portion of the gland, but it is not until much later that they gather in a central mass which constitutes the medulla; even at 190 mm.

the invasion is not complete (Zuckerkandl) . As a whole, the gland acquires a relatively very great size in embryos.

From this mode of development, it is seen that islands of medullary substance may occasionally occur in the cortex, and that outlying portions of the gland may not contain any medulla. Moreover portions of the gland frequently become detached, forming accessory suprarenal glands. These may remain near the main glands or may be carried down, with the descent of the adjacent sexual glands, into the broad ligament, or epididymis (cf . Wiesel, Sitzb. kais. Akad. Wiss., Wien, 1899, vol. 108, pp. 257280). Such glands usually consist entirely of cortex, but they may contain medullary substance. Isolated paraganglia, consisting entirely of medullary substance, are not regarded as suprarenal glands. There is no evidence that accessory suprarenal glands may arise from the ccelomic epithelium at a distance from the main glands (Zuckerkandl, Keibel and Mall's Human Embryology, vol. 2).

Adult Structure. The cortical substance may be divided into three layers or zones the zona glomerulosa, zona fasciculata, and zona reticularis (Fig. 420). The zona glomerulosa, found just beneath the capsule, is said to develop between the second and third years after birth, "reaching its characteristic structure only in the later years of childhood." It consists of round masses of cells which in man are much like those of the zona fasciculata; in some animals they are distinguished by their columnar shape. The zona fasciculata is composed of cords of rounded or cuboidal cells, containing secretory granules and an abundance of fat vacuoles (Fig. 421). There is no lumen within these cords and they are not surrounded by basement membranes. Thin-walled vessels pass between them, sometimes lodged in connective tissue strands proceeding from the capsule. The


A, Aorta; R, cortical portion; S, chromaffin tissue, penetrating to form the medulla at SB. (From McMurrich's Development of the Human Body.)



cords of the zona fasciculata are perpendicular to the surface; they end below in a network, the zona reticularis. In this deeper portion the cells


Zona glomerulosa.

\ Cortex


Zona fasciculata

Zona reticularis.

Cell cords of the medulla. Nerve in cross section Ganglion cells ....

Bundles of smooth muscle fibers in cross section.



become pigmented, so as to form a dark brown band visible without magnification. Fat vacuoles are here smaller or absent, as seen in Fig. 421, which shows also the close relation between the cells and the vascular endothelium. In portions of the suprarenal gland where the medulla is lacking, the zonae reticulares of the opposite sides come together, forming the core of the organ.

The medulla is composed of chromaffin cells arranged in strands and masses which unite to form a network, with lacunar veins filling the interstices (Fig. 420). The cells contain an abundant granular protoplasm, but they tend to shrink, even in well-preserved specimens, so


appear steiiaie ^r ig. OF AN ADULT, x 360.


Long meshed capillary net of the cortex.

421). These are the cells which are believed to produce adrenalin; the function of the cortical cells remains unknown.

The capsule of the suprarenal glands is a connective tissue layer, said to contain smooth muscle fibers and small ganglia, in addition to vessels and nerves. Around the blood vessels especially, it contains elastic tissue. The capsule sends slender prolongations into the gland, and elastic tissue occurs in the medulla. The cortex contains very few if any elastic fibers, and its framework appears to consist of reticular tissue.

The arteries supplying the suprarenal glands are from several sources. They divide into many small branches in the capsule, and these penetrate the cortex, forming a long- meshed capillary network (Fig. 422). In the

medulla the meshes become round and the

T^^^M^^H^^^^^I ^HH

Artery. sfl vessels collect to form

veins, the larger of which are accompanied by longitudinal strands of smooth muscle fibers. Some arteries are said to pass directly from the capsule to the medulla, without branching in the cortex. Within the medulla the veins unite to form the central veins, which are the main stems of the suprarenal veins (Fig. i68,p. 173). They emerge at the hilus; the right empties into the inferior vena cava and the left joins the left renal vein.

Lymphatic vessels have been found in the capsule, where they may drain the cortex, and also in the medulla, emerging at the hilus.

The numerous, mostly non-medullated nerves, of which a human suprarenal gland receives about thirty small bundles, proceed chiefly from the cceliac plexus and pass with the arteries from the capsule into the medulla. Within the capsule they form a plexus, from which branches descend into the zona glomerulosa and zona fasciculata; there they end on the surface of groups of epithelioid cells, without penetrating between the individual cells. The plexus in the zona reticularis is more abundant, and is formed from fibers which descend directl thryough the outer zones; its fibers likewise terminate on the outer surface of groups of cells. In the medulla, the nerves are extraordinarily abundant and each cell is surrounded by nerve fibers. Groups of sympathetic ganglion cells are found

Round meshed net of the medulla.


Vein of the




here and there in the medulla but only rarely in the cortex, nerves terminate in the walls of the vessels.

409 A part of the



Development and General Features. The formation of the medullary tube, which gives rise to the spinal cord and brain, has already been described (cf. Fig. 125, p. 133); in the following section, the differentiation which takes place in its wall will be considered, together with the general features of the spinal cord in the adult.

Very early in development, the cells of the medullary tube form a syncytium. Those nuclei of the syncytium which border upon the lumen


TUBE. (Schaper.)

The germinal cells are stippled, and the indifferent cells are empty circles. Circles with dots represent neuroglia cells, and the black cells are neuroblasts. Circles containing an z are germinal cells in mitosis.

of the tube, or central canal, divide repeatedly by mitosis, and many of them are forced outward laterally, so that the sides of the tube become greatly thickened. In the floor and roof of the tube a corresponding thickening fails to take place, as shown in Fig. 423.

The lateral walls of the tube very early become divisible into three layers (Fig. 423). The inner layer consists of germinal or prolif crating cells and is wide only in the embryo. In the adult it becomes reduced to a single layer of inactive cells, which surround the central canal like a simple epithelium and constitute the ependyma (Gr., r/8v/*a, a cloak). The middle layer is composed of cells derived from the germinal layer, and in the adult it constitutes the gray substance of the cord. Its cells early differentiate into two types the supporting cells, or neuroglia, and the



nerve cells. The processes of the nerve cells, in so far as they are within the limits of the gray substance, are non-medullated. The outer layer is at first entirely free from nuclei, and later it contains only a few cell bodies, belonging with the neuroglia and with the endothelium of vessels which penetrate the cord; it contains no nerve cells. This layer consists of a network of neuroglia fibers through which nerve fibers extend in various directions, but chiefly up and down the cord. As these fibers become medullated, the layer becomes white macroscopically, and it forms the white substance of the adult cord. In preparations in which myelin is

Dorsal Median 1 Portion of

Entrance median Dorsal [ dorsal

zone. septum, funiculus | Lateral j root. Dorsal root.

Dorsal column.

Groups of nerve cells.

Central canal.

Ventral root. Whjte Ventral Ventral funiculus.


deeply stained, the white substance appears darker than the gray substance (Fig. 424). From what has been said, it appears that the medullary tube early becomes divisible into inner, middle, and outer layers, which give rise to ependyma, gray substance and white substance respectively.

As the medullary tube enlarges, ventral swellings are formed on either side of the median line (Fig. 423). These later project so far ventrally that the flloor of the medullary tube is found at the dorsal end of a ventral



median fissure, which is bounded on either side by the bulgings just described. Into each of these two swellings the gray substance projects, forming the ventral "horns" or columns (columna anterior or ventralis}. The term "horn" refers to the appearance in sections, and "column" applies to their true form, taken as a whole. Corresponding with the ventral columns of gray substance, there are two dorsal columns, which arise somewhat later, and cause the gray substance, as seen in sections, to assume the form of a letter H. With many variations this appearance is characteristic of the entire spinal cord in mammals generally. As seen in Fig. 424, there are secondary swellings on the sides of the "H" which are called lateral columns; at certain levels they are ill-defined or absent.

Instead of forming a dorsal median fissure, the medullary tube produces a dorsal median septum. The lower or ventral part of the septum is apparently formed by the coalescence of the lateral walls of the medullary tube, thus leaving the ventral portion of the original lumen as the central canal of the adult. Occasionally this small cavity, 0.5-1.0 mm. wide, is entirely obliterated. The dorsal portion of the septum consists of neuroglia fibers extending from the roof of the central canal to the periphery of the cord. Thus in the adult the cord is divided into right and left halves, except for the transverse connections or commissures near the central canal. These include a dorsal commissure, a ventral gray commissure, and a ventral white commissure.

The white substance of each half of the cord is subdivided into three longitudinal juniculi, each of which includes several smaller bundles or fasciculi, otherwise known as "fiber tracts." The funiculi are dorsal, lateral, and ventral respectively, and their boundaries are seen without magnification. The dorsal or sensory roots enter the cord along a groove known as the dor so-lateral sulcus, and the ventral or motor roots emerge along the ventro-lateral sulcus. All the white substance between these two sulci is included in the lateral funiculus. The dorsal funiculus extends from the dorso-lateral sulcus to the median dorsal septum; and the ventral funiculus extends from the ventro-lateral sulcus to the midventral fissure.

The fasciculi of which each funiculus is composed cannot be studied profitably in normal specimens. They have been followed chiefly by observing the effects of local injury and disease, for if a group of nerve cells is destroyed, all the fibers proceeding from it will degenerate. In this way it has been shown that the fibers of the funiculi are not arranged indiscriminately, but occur in definite tracts, which in some respects are radically different in different animals. Thus the fibers of voluntary motion which descend from the cerebral hemispheres to the motor cells of the cord, forming the cerebro-spinal fasciculi, are found in the dorsal fun


iculi of rodents but in the lateral and ventral funiculi of the human cord. In man most of these fibers, in descending from the brain, cross to the opposite side in the medulla oblongata and complete their descent in the lateral funiculus of the cord, where they form the lateral cerebro-spinal fasciculus; they terminate in relation with motor cells on the same side of the cord. A smaller number of these fibers fail to cross in the medulla, and descend in the ventral funiculus as the ventral cerebro-spinal fasciculus; these fibers cross to the opposite side in the cord, passing through the ventral commissure, and then terminate in relation with the motor cells. Thus the cerebro-spinal fibers all cross, but the decussation may take place either in the medulla or in the cord.

The fibers which convey tactile stimuli to the brain enter by the dorsal roots and pass into the gray substance of the cord, where they terminate in relation with small cells dorsally placed. Fibers from these cells cross to the opposite side of the cord through the gray commissure, and then enter the white substance of the lateral funiculus in which they ascend to the brain. One of these fibers and a descending fiber of the lateral cerebro-spinal fasciculus are shown in the diagram, Fig. 123, p. 131.

In addition to fibers of the long tracts, such as pass between the spinal cord and the hemispheres, cerebellum and other parts of the brain, the ventral and lateral funiculi contain fibers which emerge from the gray substance of the cord at one level and re-enter it at another, thus placing the cells at different levels in communication. The fibers of these "ground bundles" or fasciculi proprii generally remain close to the gray substance. Their entrance and exit along the lateral concavity of the gray substance causes it to be broken up into a formatio reticularis (Fig. 424).

The dorsal funiculi in the upper part of the cord are each subdivided into a slender medial fasciculus gracilis (column of Goll) and a wider lateral fasciculus cuneatus (column of Burdach), which are partially separated from one another by a septum. These fasciculi are composed chiefly of the fibers of "muscle sense," which enter by the dorsal roots and divide into ascending and descending branches. Many of these pass into the gray substance of the cord after traveling varying distances in the dorsal funiculi. Some of the ascending fibers, however, are very long and extend to the medulla oblongata, gradually approaching the median septum in their ascent. The gracile fasciculi are composed of these long ascending fibers, and since they are not segregated in a distinct bundle in the lower portion of the cord, this fasciculus is absent from the lumbar region. In addition to the fibers of muscle sense, the dorsal funiculi contain some fibers of general sensation, a limited number of association fibers, and others.

The description of the fiber tracts in the spinal cord and brain is the subject of special text-books; they are briefly and clearly described by



Villiger (Brain and Spinal Cord, translated by Piersol, 1912). The form of the cord at different levels is considered in works on gross anatomy. In general, the white substance increases toward the brain, since the cervical cord contains the fibers to and from all the lower levels in addition to those for the cervical region itself. In levels which supply the nerves to the upper and lower limbs, there is a general increase in both gray and white substances, producing the cervical and lumbar enlargements, respectively. The lower end of the cord tapers into the rudimentary filum terminate.

Adult Structure. The spinal cord and brain are surrounded by two membranes or meninges, of which the outer is dense and fibrous, and is known as the dura mater; and the inner is thin and vascular, forming the pia mater.

Curiously they are not called membranes, and the term meninx (in the singular) is not employed in anatomy. They retain the ancient Arabic designation of "mother of the brain," following, according to Hyrtl, a general Arabian tendency to name things "mothers," "fathers," etc. (The vena cava was the mater venorum, and the pupil, the filia oculi.} Carrying the figure further, the adjectives of double meaning, dura and pia, were substituted for dense and thin. In the fifteenth century it was said that these membranes were called matres because they produce the membranes surrounding the nerves, the coats of the eye, and the periosteum of the skull, with which they are continuous; but Hyrtl denies that the term has any such significance.

The dura mater spinalis, or dura mater of the cord, consists of compact fibrous connective tissue with many elastic fibers, flat connective tissue cells and plasma cells. Its inner surface is covered by a layer of flat cells forming a mesenchymal epithelium. It has few nerves and blood vessels. Anteriorly it is continuous with the dura mater of the brain at the foramen magnum. It does not fill the vertebral canal, and is not continuous with the vertebral periosteum. Around it externally there is a layer of vascular fatty connective tissue; and internal to it there is a capillary cleft containing a very small amount of fluid. This subdural space connects with tissue spaces in the dura and with those which extend out in the perineurium of the peripheral nerves. It communicates freely, but probably indirectly, with the lymphatic vessels.

The pia mater spinalis, as described by Stohr, is a two-layered sac. The outer layer is covered on its free outer surface with a simple layer of flat cells, which is lightly connected with the dura, and forms the inner wall of the subdural space. The inner layer, or pia proper, is a delicate and very vascular connective tissue, closely connected with the spinal cord, into which it sends prolongations accompanying the blood vessels. The arteries of the spinal cord are primarily two pairs, situated as shown in Fig. 125, E (p. 133) and in Fig. 424. One pair is ventral to the dorsal roots, and the other is near the mid-ventral fissure; their branches supply both the white and gray substance, and the collecting veins branch freely



White External limiting substance. membrane.


Cross sections of medullated nerve fibers consisting of

"Axis cylinder

and ^Medullary sheath.

in the pia mater. Between the two layers of the pia, as described by Stohr, there is a wide space filled with cerebro-spinal fluid and traversed by many strands and membranes which pass from one layer of the pia to the other. These strands constitute the arachnoid membrane, so-called from its cobwebby texture. Often the name is restricted to the subdural membrane (following Henle), so that the spaces between the meshes of the arachnoid are described as subarachnoid. They are preferably termed arachnoid spaces and they are of great importance. The fluid which they contain has access to that within the central canal of the cord and the ventricles of the brain, through an aperture in the thin roof of the medulla

oblongata. Whether the arachnoid spaces open directly into lymphatic vessels may be questioned, but undoubtedly they are freely drained by the lymphatic system.

On either side .of the cord, between the successive spinal nerves, there is a frontally placed triangular plate of fibrous connective tissue, which passes from the pia to the dura and serves to support the cord. The succession of these pointed projections, with their bases attached to the pia, constitutes the denticulate ligament.

White Substance. The white substance of the cord consists essentially of medullated nerve fibers supported by a network of neu roglia. Toward the outer surface, the neuroglia fibers become felted together, forming an external limiting membrane just within the pia mater (Fig. 425); this is an ectodermal tissue, which must be distinguished from the adjacent connective tissue penetrating the cord with the blood vessels. Although in transverse sections the neuroglia fibers appear to be radially arranged (Fig. 425), longitudinal sections show that they extend also up and down the cord (Fig. 426), and in fact they form a diffuse syncytial network. The protoplasm of this network is characterized by the presence of stiff neuroglia fibrils, imbedded in the peripheral exoplasm, and passing freely from one cell territory to another. They are well shown in specimens stained with Mallory's phosphotungstic acid haematoxylin, and resemble the myoglia and fibroglia fibrils both in form and staining reaction.

7 Neuroglia cells.

,. i - \fr~~-"-\. Connective tissue.

Blood vessels.




As the nerve fibers which occupy the interstices of the neuroglia network increase in number and acquire myelin sheaths, thus becoming larger, the protoplasm of the neuroglia is compressed into stellate accumulations, often surrounding a nucleus (Fig. 428, A). In Golgi preparations they appear as in Fig. 427, and are described as long rayed, and short rayed or mossy cells. These forms represent clumps of neuroglia fibers, sometimes clogged with precipitate, in the center of which there may or may not be a nucleus.

The nerve fibers of the white substance vary in diameter, the coarsest being found in the ventral funicali and lateral parts of the dorsal funiculi;

FIG. 426. NEUROGLIA CELLS AND FIBERS FROM THE SPINAL CORD OF AN ELEPHANT. (Hardesty.) c-i, Successive stages in the transformation of neuroglia cells, ending with disintegrating nuclei (i) ; 1, a leucocyte. Benda's stain. X 940.

the finest are in the medial parts of the dorsal and lateral funiculi. Elsewhere coarse and fine fibers are intermingled. Their general direction is parallel with the long axis of the cord. Like other nerve fibers they consist of fibrillae imbedded in neuroplasm. Most of them are medullated, and in cross section the myelin often forms concentric rings. Although a few observers have described nodes, it is generally considered that there are no nodes in the central nervous system. During the development of the myelin, fibers have been found encircled by sheath cells, Fig. 428, B, as described by Hardesty (Amer. Journ. Anat., 1905, vol. 4, p. 329-354). In longitudinal view, these sheath cells are seen in depressions of the myelin, where they greatly resemble the neurolemma cells of peripheral



nerves. With the increase of myelin the sheaths become very slender and can seldom be detected in the adult. It is ordinarily stated that the medullated fibers of the central nervous system are without a neurolemma.

Gray Substance. The gray substance consists of neuroglia, nerve cells, and a confused mass of non-medullated nerve fibers running in all directions. The nerve

Bloodvessels. *. , , cells ^ Q f

types: (i) large motor cells with processes which enter the peripheral nerves; (2) cells with processes limited to the central nervous system and extending through its white substance from one part to another; and (3) small cells with processes confined to the gray substance. The neurax ons of cells of the third type branch freely, and they may cross to the

gray substance on the opposite side of the cord.

The motor cells occur in groups in the ventral columns (horns). In

the cervical and lumbar enlargements there are two groups, a ventro medial and a dorso-lateral (Fig.

424), which unite in the upper cervical and thoracic portions of the

cord; less well defined are the

dorso-medial and ventro-lateral

groups. In all of these groups

the motor cells are large (67-135 ju

Short rayed cells. Long rayed cells.



C ac

FIG. 428.

in diameter), with round or oval nuclei and prominent nucleoli (Figs. 429 and 430). Their protoplasm appears densely granular in ordinary preparations, but when specially treated it is seen to contain an abundance of neurofibrils;

if preserved in alcohol and stained with methylene blue, the groups of granules known as Nissl's bodies may be demonstrated. As already noted, these are abundant in vigorous cells but become reduced or disappear in various conditions of exhaustion. Granules of brownish

A, Neuroglia cells and nerve fibers from a crosa section of the spinal cord of an elephant. B, Neuroglia cells, nerve fibers and sheath cells, from the spinal cord of a pig, 2 weeks after birth. C, Isolated fiber from the cord of 21 cm. pig embryo, stained with osmic acid. (After Hardesty.) a. c., Axis cylinder; my., myelin; n., neuroglia nuclei; n. f., neuroglia fibrils; s. c., sheath cell.


pigment are sometimes conspicuous. All of these features may be observed in the smaller nerve cells, but they are most evident in the large motor cells. The dendrites of the motor cells extend far into the dorsal columns (horns), and they even pass out of the gray substance into the ventral and lateral funiculi. The neuraxon begins as a slender nonmedullated fiber at the tip of a clear "implantation cone" and acquires its myelin sheath as it crosses the white layer. Ordinarily it has no collaterals; when present they are very small. None of the neuraxons cross to the opposite side of the cord before entering the motor roots.

The nerve cells of the second type, usually smaller than the motor cells but more abundant, are distributed throughout the gray substance either singly or in groups. Definite groups of nerve cells in the spinal cord and brain are known as nuclei, and at the root of the dorsal column (horn) near its junction with the gray commissure, there is the important


dorsal nucleus (column of Clarke). It is composed of cells which send their neuraxons into the lateral funiculus, in which they ascend to the cerebellum. The dorsal nucleus is limited to the thoracic portion of the cord, and adjacent parts of the lumbar and cervical regions.

The fibers of the ground bundles are derived from scattered cells of the second type. Their dendrites are long but sparingly branched. The neuraxons give off collaterals in the gray substance, and enter the ventral and lateral funiculi (rarely the dorsal) of the same or opposite side. In the white substance most of them divide into ascending and descending fibers, which send collaterals back into the gray, either singly or in bundles, and the main branches finally terminate like the collaterals. After re-entering the gray substance they ramify freely around the motor cells.

In transverse sections the dorsal column appears capped by the zona spongiosa which covers the substantia gelatinosa (Fig. 424). The former contains spindle-shaped "marginal cells" which send fibers into the white substance. The substantia gelatinosa contains a limited number of very small nerve cells which send processes into the zona terminalis




(Fig. 424); it contains also stellate neuroglia cells, the processes of which are said to become transformed into a granular substance.

Ependyma. The ependyma is that part of the neuroglia which lines the central canal. It appears like a simple columnar epithelium, but its cell-like bodies are the ends of strands which primarily extend clear across the spinal cord to the external limiting membrane. A nucleus is generally found in the strand near the central canal, and there may be others further out (Fig. 431). Although in the embryo strands may readily be traced from the central canal to the periphery, in the adult they are generally broken up into stellate cells, or forms retaining a chief

From the substantia gelatinosa of a newborn rat.

Neuroglia cell.

Central canal.

Ependymal cells.

Neuroglia cell of the white substance from a cat 6 weeks old.

Concentric- neuroglia cell from a cat six weeks old.

Chief process.

Neuroglia cell of the gray substance of the base of the dorsal column of a human embryo.


process directed either toward the central canal or the periphery (Fig. 431). All these cells are parts of a general syncytium, as already described.

The ependymal cells at birth, and for sometime afterwards, possess cilia projecting into the central canal, but in the adult these disappear. It is questionable whether or not they are motile. Single bodies have been found at their bases, but not diplosomes.

Surrounding the central canal, outside of the ependymal layer, there is a zone of central gray substance, characterized by concentrically arranged neuroglia cells, one of which is shown in Fig. 431.


Development and General Features. If a human embryo of 4 mm. is placed in such a position that the spinal portion of the medullary tube is



approximately vertical, the anterior end of the tube, from which the brain develops, is bent as shown in Fig. 432, A. The first portion, beginning at the anterior extremity where the neuropore is still open, passes vertically upward. At the head-bend it turns backward and passes to the neck-bend, where it curves downward, becoming continuous with the part of the tube which forms the spinal cord. The anterior ascending portion is the fore-brain (prosencephalon) ; the part where the head-bend occurs is the mid-brain (mesencephalori) ; and the remainder is the hindbrain (rhombencephalon) . These three fundamental parts have become more distinct and exhibit subdivisions in the 10 mm. embryo shown in Fig. 432, B. Prosencephalon. The fore-brain becomes subdivided into the telencephalon anteriorly, and the diencephalon posteriorly; the latter connects with the mid-brain. In very early stages the forebrain produces two lateral outpocketings, one on either side, called the optic vesicles. Each expands distally to form the retina of an eye, and its connection with the fore-brain becomes reduced to a slender stalk. In later stages, the depression on the

irmpr wall of tViP brain which FlG - 432. A, THE BRAIN OF A 4.0 MM HUMAN EMBRYO

(after Bremer); B, THE BRAIN OF A 10.2 MM. EMBRYO

marks the pOSltlOn Of the Except the isthmus, is. the principal subdivisions of the brain


stalk is called the optic reIt is shown in the me


are indicated by prefixes of the term encephalon; sp. c., spinal cord; h., hemisphere; o. v., optic vesicle; r., rhinencephalon; v., roof of the fourth ventricle.

dian sagittal sections of the bran of an embryo of three months and of an adult, in Figs. 433 and 434 respectively.

Telencephalon. The principal derivatives of the telencephalon are a pair of lateral outpocketings which arise somewhat later than the optic vesicles and are known as the cerebral hemispheres. Each contains a cavity, or lateral ventricle, which opens into the medullary tube through the interuentricular foramen (foramen of Monro). In later stages this foramen is relatively small, and it appears in Figs. 433 and 434 as a darkly shaded cleft in front of the thalamus (th.). As the hemispheres expand, they approach one another in the median line above the brain, being separated by a thin plate of connective tissue. They grow backward, cover



ing all the hind part of the brain. Their outer walls (constituting the pallium, or mantle) become convoluted, forming gyri, with intervening

FIG. 433. SAGITTAL SECTION OF THE BRAIN OF AN EMBRYO OF THREE MONTHS. (After His.) bl.JCerebellum; hem., hemisphere; hy., hypophysis (posterior lobe) ; isth., isthmus; med., medulla oblongata; mes., mesencephalon; ol. b., olfactory bulb; o. r., optic recess; p., pons; p. b., pineal body; p. s. pars subthalamica; th., thalamus.




cbl.. Cerebellum: c. c., corpus callosum; c. q., corpora quadrigemina; f., body of the fornix; hy., posterior lobe of the hypophysis; med., medulla oblongata; o. b., olfactory bulb; o. r., optic recess; p., pons; p. b., pineal body; p. 8., pars subthalamica; s. p., septum pellucidum; th., thalamus.

sulci, and each hemisphere as a whole is divided into frontal, parietal, occipital and temporal lobes, as described in works on gross anatomy. A


more independent subdivision of the hemisphere is the olfactory lobe, which terminates anteriorly in the olfactory bulb an expansion which receives the olfactory nerves. The entire olfactory portion of the brain is called the rhinencephalon.

Connecting the hemispheres with one another, there is a great transverse commissure known as the corpus callosum (Fig. 434, c.c.). Below this is the arched body of the fornix (f ) , representing a median fusion of two longitudinal bundles of commissural fibers, only small parts of which are included in a median section. Between the corpus callosum and the fornix, there is a thin septum pellucidum which consists of two vertical plates with a closed cleft-like cavity between them.

It is probable that the corpus callosum and body of the fornix develop in a thickening of the front wall of the telencephalon, where it crosses the median line. The cavity of the septum pellucidum is, accordingly, a secondary cleft in the thickened wall. A fusion between the adjacent medial walls of the hemispheres, to provide a path for the fibers of the corpus callosum and to account for the cavity in the septum, has been described, but not confirmed.

In addition to the hemispheres with their commissures and olfactory lobes, and the optic vesicles which are not counted as a part of the brain, the telencephalon produces the pars o plica hypothalami. This "optic portion of the region below the thalamus" includes the optic recess, and in the mid-ventral line it forms a funnel-shaped depression, the infundibulum, terminating below in the posterior lobe of the hypophysis. (The anterior lobe of the hypophysis is derived from the pharynx.) The median cavity of the telencephalon is a laterally compressed space which forms the front part of the third ventricle. The lateral ventricles, which open from it, are counted as the first two.

Diencephalon. In the mid-dorsal line the diencephalon produces a cone-like body, the corpus pineale. Laterally, in its thick walls, there is a mass of gray substance called the thalamus (bed). External to the thalamus are the great bundles of fibers passing from the hemispheres to the spinal cord. The sensory fibers ascending from the cord terminate in the thalami, where there is a relay of nerve cells to convey the impulses to the hemispheres. The thalami have other connections of equal importance. They come in contact with one another across the cleft-like cavity of the diencephalon (which is a part of the third ventricle) and may fuse, forming the massa intermedia. The ventricle surrounds this mass. Beneath the thalamus the diencephalon forms the pars mammillaria hypothalami, which is represented on the under surface of the brain by the pair of rounded mammillary bodies, one on either side of the median line (Fig. 435, B).

Mesencephalon. The mid-brain remains undivided, and its walls become very thick. Dorsally it forms four rounded elevations, the



corpora quadrigemina (Fig. 435, A). These are arranged in pairs, the anterior pair being known as the superior colliculi, and the posterior as the inferior colliculi; the former have important relations with the optic tracts, and the latter with the auditory tracts. On the under side of the mid-brain there are two great bundles of fibers, the cerebral peduncles (pedunculi cerebri) , which diverge as they pass forward from the hind-brain, and swing upward on the sides of the mid-brain to connect with the hemispheres (Fig. 435). Between the cerebral peduncles on the under side of the mid-brain, the oculomotor nerves emerge. They are derived from groups of motor cells situated just beneath the floor of the cavity of the mid-brain. This cavity remains a slender tube and is known as the cerebral aqueduct (aquaductus cerebri).

i : iS^3^ oc.


b. c., Brachium conjunctivum; b. p., brachium pontis; c. m., corpus mamillare; c. p., cerebral peduncle; c. q. a., and c. q. p., anterior and posterior corpora quadrigemina; inf., infundibulum; med., medulla; ol., olive; p., pons; p. b., pineal body; pyr., pyramid; r. b., restiform body; ven., floor of fourth ventricle. The nerves are oc., oculomotor; tr., troclear; tri., trigeminal; abd., abducens; int., intermedius, fa., its facial portion; ac., acoustic; glo., glossopharyngeal ; va., vagus, ace., its accessory portion; hy., hypoglossal.

Between the mid-brain and the hind-brain there is a marked constriction, known as the isthmus (Fig. 432, B). From the dorsal surface of the isthmus the trochlear nerves make their exit (Fig. 435, A); they are processes of nerve cells situated beneath the floor of the cavity, but they pass to the dorsal surface and cross to the opposite side before emerging.

Rhombencephalon. The rhombencephalon (or hind-brain) receives its name from the diamond shape which it presents when seen from above. This form is established in young embryos and persists in the adult (Fig. 435, A). The roof of the rhombic cavity becomes a thin membrane and is readily torn away, but the sides and especially the floor are greatly thickened. The form of the hind-brain may be imitated, as described by His, by cutting a short slit in the upper side of a piece of rubber tubing



and forcing the ends toward one another; the region with the weakened dorsal wall buckles downward and bulges toward either side. The most prominent part of the embryonic hind-brain, as it buckles downward, becomes the pons in the adult. From the dorsal part of the front end of the hind-brain, the cerebellum develops, overhanging the thin roof of the posterior portion. Pons and cerebellum are thus both derived from the anterior part of the rhombencephalon, which is set apart as the metencephalon; the remainder of the hind-brain is included in the myelencephalon (Fig. 432), which becomes the medulla oblongata and is continuous with the spinal cord.

Before considering the subdivisions of the hind-brain in further detail, the relation of the principal parts of the adult brain to the primary vesicles may be reviewed in the following table:

Fore-brain. .


Hemisphere :



Corpus callosum. Optic part of the hypothalamus.

Hypophysis (posterior lobe).

Pineal body. Thalamus.

Mammillary part of the hypothalamus.

Mid-brain. . . { Mesencephalon . . . { Corpora quadrigemina.

[ Cerebral peduncles.

Diencephalon ,


[ Isthmus Isthmus.

Metencephalon...( CerebeUlim . I Pons.

Myelencephalon. . Medulla oblongata.

Metencephalon. The pons, as seen from the under side of the brain (Fig. 435, B), appears as a broad bundle of transverse fibers interrupted for the passage of the motor and sensory roots of the trigeminal nerve. The superficial fibers of the pons pass dorsally around the wall of the brain-tube, forming a pair of arms, the brachia pontis, which enter the cerebellum. In addition to these large bundles, the cerebellum receives fibers through the brachia conjunctiva which extend into it from the isthmus, and also from the restiform bodies (i.e., rope-like) which ascend from the posterior part of the hind-brain (Fig. 435, A). Thus on either


side the cerebellum connects with three bundles of fibers, which come together to form its medulla (corpus medullare). The medulla is surrounded by the gray cortical substance, and the entire cerebellum is divided into many lobes and lobules.

The cavity of the hind-brain, which is continous posteriorly with the central canal of the cord, and anteriorly with the cerebral aqueduct, is known as the fourth ventricle. It extends upward toward the medulla of the cerebellum, forming a tent-like recess, the apex of which is the fastigium.

Myelencephalon. The myelencephalon becomes the medulla oblongata, continuous without demarcation with the medulla spinalis or spinal cord. The ventral median fissure becomes shallow, but it may be traced to the pons (Fig. 435, B). On either side of its upper portion, there is an elongated swelling, the pyramid, corresponding in position with the ventral funiculus of the cord. Each pyramid is bounded laterally by the ventro-lateral groove, from which the motor roots of the hypoglossal nerve emerge; this groove is continuous with the ventro-lateral groove of the cord, from which the motor roots of the spinal nerves proceed. Near the pons the abducent nerve comes out close beside this groove. The dorso-lateral groove of the cord likewise extends to the pons; and in line with the dorsal, roots of the cord, the sensory roots of the vagus, glossopharyngeal, acoustic and facial nerves enter this groove. The lateral roots of the accessory, glossopharyngeal and facial nerves emerge just below them. The space between the ventro-lateral and dorso-lateral grooves corresponds with the lateral funiculus of the cord. Toward the upper end of the medulla, it presents a rounded swelling known as the olive (Fig. 435, B).

The dorsal funiculus of the upper part of the cord is divided into the medial gracile and lateral cuneate fasciculi; these may be followed into the medulla where they become broader (Fig. 435, A). Some of their fibers enter the restiform body, and pass to the cerebellum; others pass downward on either side of the central canal and continue beneath the floor of the fourth ventricle to the hemispheres. Where the central canal expands to become the thin-roofed fourth ventricle, all nerve fibers either pass downward into its floor, or turn aside to enter the restiform body.


The study of the medulla oblongata requires full consideration of the fiber tracts of the cord and anterior portion of the brain, which cannot here be taken up; only a few of the most fundamental features of the medulla are to be mentioned. Sections through the lower end of the medulla resemble those of the cord, and the gray substance retains the form of



an H. The fibers from the hemispheres, which descend to the motor cells of the cord, run mostly in the lateral funiculi, as previously stated. They descend from the brain, however, in the ventral funiculi, in which they form the pyramids in the upper part of the medulla (Fig. 437). In the lower part of the medulla they decussate, crossing to the lateral funiculus of the opposite side, as shown in Fig. 436; they appear to cut off the ventral columns (horns) from the remainder of the gray H. Then they descend in the spinal cord as the lateral cerebro-spinal tract (also called crossed pyramidal). A few fibers, however, descend in the ventral funiculi of the cord without having crossed in the medulla. Such fibers of the ventral cerebro-spinal tract (direct pyramidal) cross to the op




The right half of the section shows the effect of Weigert's stain, the myelinated portions being dark; the left half shows the gray substance stippled; the white is blank, f. c., Fasciculus cu neat us; f. c. 1., fasciculus cerebro-spinalislateralis; f . c. v., fasciculus cerebro-spinalis ventralis; f. g., fasciculus gracilis; d. c., dorsal column; d. p., decussation of the pyramids; d. r., dorsal root of first cervical nerve; v. c., ventral column.

FIG. 437. SECTION OF THE MEDULLA. (After Dejerine.)

d. c., Dorsal column; d. L, decussation of the lemnisci; f. c., fasciculus cuneatus; n. ace., nucleus of the accessory nerve; n. c., cuneate nucleus; n. g., gracile nucleus; py. f pyramid; t. s. n. t., spinal tract of the trigeminal nerve; v. c., ventral column.

posite side in the cord before terminating in contact with the motor cells of the ventral columns.

The fibers in the cerebro-spinal tracts are the neuraxons of the pyramidal cells in the outer layers of the hemispheres, which will be described in a following section. They descend through the internal capsule (which in a layer of white substance lateral to the thalami), thence through the cerebral peduncles, pons, medulla oblongata and spinal cord, without interruption. This motor path from the hemispheres to the voluntary muscles includes, therefore, only two neurones or nerve cells, one from the cortex to the motor cells of the ventral column of the cord, and the other from the ventral column to the end plate on the muscle fiber. Other motor fibers from the hemispheres to the cord terminate in the red nucleus deep within the substance of the mid-brain; cells of the red nucleus send neuraxons to the opposite side, and these descend in the lateral funiculi of the cord as the rubro-spinal tract. They terminate in relation with


motor cells on the same side, and thus is formed a motor path composed of three neurones. Other tracts to the cord proceed from the cerebellum. The motor nerves of the medulla oblongata, pons, and mid-brain arise from groups of cells, or nuclei, which are typically near the median line and only a short distance below the floor of the ventricle or cavity. Fig. 438 includes the nucleus of the hypoglossal nerve, which is in this position. The lateral motor roots are further below the ventricle and are more lateral. The nucleus ambiguus, which is an elongated structure containing the motor cells of the accessory, vagus and glossopharyngeal nerves, is of

this sort (Fig. 438). These

ts. n.h. v motor nuclei correspond with

cell groups in the ventral columns of the cord, and they are similarly in connection with fibers from the pyramidal cells of the hemispheres. In so far as the latter pass to these cerebral nerves, they form the corticobulbar tract, "bulb" being a general term for the expanded part of the hindbrain. The cortico-bulbar fibers decussate at different levels.

Somewhat higher in the


FIG. 438. SECTION OF THK MEDULLA. (After Dejerine.)

c. i., Corpus restiforme; f. c. o., cerebello-olivary fibers; lem.' lemniscus or fillet; n. am., nucleus ambiguus; 'n. h., nucleus hypoglossi; ol., olive; py., pyramid; t. s., tractus solitarius; t. s. n. t., tractus spinalis nervi trigemini; v., fourth ventricle.

medulla than the decussation of the descending motor fibers or pyramids, the sensory fibers ascending in the gracile and cuneate fasciculi terminate in relation with groups of cells known as the gracile and cuneate nuclei respectively (Fig. 437). They appear as additional horns of gray substance. The neuraxons from the cells in these nuclei pass ventrally and decussate beneath the central canal, as shown in Fig. 437. The bundles to which they give rise are known as the medial lemnisci or fillets. In their course through the upper part of the medulla, they are vertically placed bands of longitudinal fibers, on either side of the median line (Fig. 438). The fillets not only receive fibers of muscle sense through the gracile and cuneate fasciculi, but they are joined by the spino-thalamic fasciculi of fibers of cutaneous sense, which pass up the cord in the lateral funiculi. Moreover, they receive accessions from the cerebral sensory nerves. The fibers of the latter enter the medulla and divide into ascending and descending branches, like the dorsal root fibers of the spinal nerves, but the descending fibers are relatively longer. The position of the descending fibers of the trigeminal nerve (tractus spinalis


nervi trigemini] is shown in Fig. 438, and the tractus solitarius, containing sensory fibers from the vagus and glossopharyngeus, is shown in the same figure. In connection with these bundles of sensory fibers, there are groups of nerve cells forming the nucleus of the tractus solitarius, and nucleus of the spinal tract of the trigeminal nerve. These correspond with the gracile and cuneate nuclei, and send fibers into the fillets. The fillets continue through the pons and cerebral peduncles to the thalami, in which they terminate. Nerve cells of the thalami convey the impulses received onward to the hemispheres. Thus the sensory tract is composed of three neurones, the first being in the ganglia of the sensory nerves, outside of the central nervous system; the second begins in the gracile and cuneate nuclei, or in the gray substance of the cord in case the impulse travels by the spino-thalamic tract, or in the nuclei associated with central tracts of the sensory cerebral nerves, and in all three cases extends to the thalamus; the third begins in the thalamus and extends to the cerebral cortex.


The medullated nerve fibers of the restiform bodies, brachia pontis, and brachia conjunctiva come together to form the medulla of the cerebellum, and place the cerebellum in connection with spinal and cerebral nerves and with the hemispheres. The medulla contains several paired nuclei, the largest being the dentate nuclei, which have convoluted gray capsules resembling those of the olivary nuclei (shown in Fig. 438).

The restiform bodies include the fibers derived from the dorsal nuclei or columns of Clarke in the spinal cord; these fibers ascend in the lateral funiculi, within which they form the dorsal spino-cerebellar tract (of Flechsig). The restiform bodies contain also fibers from certain cells in the gracile and cuneate nuclei, and many fibers from the olivary nuclei, mostly of the opposite side. The brachia pontis contain fibers passing to the cerebellum from the numerous nuclei pontis. The latter are in connection with fibers descending from the hemispheres, thus forming cerebro- or cortico-cerebellar tracts. Some fibers pass in the reverse direction. The brachia conjunctiva contain fibers of the ventral spino-cerebellar tracts (of Gowers), which arise from central or lateral cells in the gray substance of the cord, and pass through the lateral funiculi to the brachia conjunctiva, through which they turn back to enter the cerebellum. The main part of the brachia conjunctiva consists, however, of fibers passing outward from the cerebellum and its dentate nucleus, to end, after decussating, in the red nuclei of the mid-brain. Thence fibers pass on to the thalami and hemispheres, and also downward to the medulla and spinal cord.

The medulla of the cerebellum extends into the small peripheral lobules, where it is covered by the cortical substance (Fig. 439). The latter consists of three strata an inner granular stratum, which is rust-colored in the fresh condition; a middle ganglionic stratum, composed of a single row of large cell bodies ; and an outer gray stratum.



Gray stratum.

Ganglionic stratum .

The inner granular stratum consists of many layers of small cells which

by ordinary methods show relatively large nuclei and very little protoplasm. With the Golgi method it appears that besides neuroglia cells, two sorts of nerve cells are present, the small and large granule cells; the former (Fig. 440) are multipolar ganglion cells with short dendrites having claw-like terminations, and slender nonmedullated neuraxons which ascend perpendicularly to the gray layer and there divide in T-form into two branches. The branches run lengthwise of the transverse folds or convolutions of the cerebellum and have free unbranched endings. In sagittal sections (Fig. 442) the terminal branches of the neuraxons are cut across. The small granule cells form the bulk of the granular stratum. The less frequent large granule cells (Fig. 442) are more than twice the size of the small ones; their branched dendrites penetrate the gray stratum and their neuraxons, going in the opposite direction, are soon resolved into very numerous branches which ramify throughout the granular stratum.

The granular layer contains also a thick network of medullated fibers which enter it chiefly from the white substance. A part of these fibers end in the "eosin bodies" of the granular stratum, which are heaps of stainable particles found between the small cells (Fig. 441).



gr., Cells of the granular stratum; n. their neuraxons in the granular layer and n'., in the gray stratum; p., p'., Purkinje's cells. (From Bailey's "Histology.")



Eosin bodies. !\

Nuclei of small cells of the granular stratum.


Some of the fibers form bundles parallel with the surface, running between the granular and ganglionic strata in the sagittal direction; they send branches into the gray layer. A small portion of the granular stratum is formed by the medullated neuraxons of the cells in the ganglion layer.

The middle ganglionic stratum consists entirely of a single layer of very large multipolar ganglion cells, called Purkinje's cells. Their oval or pear-shaped bodies send two large dendrites into the gray stratum, where they form an extraordinary arborization (Fig. 442) Their many branches do not extend in all directions but are confined to the sagittal plane, that is, to a plane at right angles with the long axes of the convolutions. When the convolutions are cut lengthwise, Purkinje's cells appear as in Fig. 440. The neuraxons arise from the deep surface of the cell bodies, and as medullated fibers they pass through the granular stratum to the white substance. Within the granular layer they produce collateral fibers which branch and in part run back into the ganglionic layer, ending near the bodies of other Purkinje's cells (Fig. 442).

The outer gray stratum, of gray color, contains two sorts of nerve cells, the large and the small cortical cells. The large cortical or basket cells are multipolar ganglion cells, the dendrites of which pass chiefly toward the surface. Their long neuraxons, thin at first but later becoming thicker, run parallel with the surface in the sagittal plane. They send occasional collaterals toward the surface, and at intervals produce fine branches which descend and terminate in baskets around the bodies of Purkinje's cells (Fig. 442), often surrounding also the beginning of their neuraxons.

The small cortical cells, distinguishable from the basket cells since their neuraxons are not in relation with Purkinje's cells, may be divided into two types, connected by intermediate forms. The cell bodies of the first type are nearly or quite as large as those of the basket cells. Their two to five dendrites lie in the sagittal plane like those of Purkinje's cells; the slender neuraxons, i mm. long or more, sometimes form loops and are characterized by abundant branches in their proximal parts. The terminal branches are few. Cells of the second type are in general somewhat smaller; their shorter neuraxons branch in the immediate vicinity of the cell bodies. The elements of the first type form the bulk of the relatively numerous small cortical cells, and are found throughout the




gray stratum, though they are more abundant in its superficial part. The second type likewise appears throughout the gray stratum.

The medullated nerve fibers found in the gray layer are prolongations of those in the granular stratum. In part they proceed toward the surface, where, after losing their myelin, they end in branches among the

Purkinje's cell. Neuraxon of a basket cell.

Short-rayed cell Neuroglia cell.

Collaterals of a Purkinje cell.

Long-rayed cell.

w --*^ _ ^-ki ^-~>jf,e j v s ' \ *--~\ i . V flmr

Collaterals of a \. cortical cell. /

>v->_/ ^ } f n- '^Y~O"L ' ^ ^ " "* ?

Neuraxon of a large cell of the granular.

stratum. Fibers to the cortex

Small cells of the granular stratum.


Except the large granule cell, which is from a kitten, the cells are drawn from Golgi preparations from an adult man. K, large cortical or basket cell.

dendrites of Purkinje's cells; in part they run between the bodies of Purkinje's cells lengthwise of the convolutions.

The neuroglia of the cerebellum consists of short-rayed stellate cells found in all the layers; of long-rayed cells in the white substance; and of epculiar cells with small bodies at the outer boundary of the granular layer



(Fig. 442). These send only a few short processes inward, but many long processes straight out to the free surface, where they end in triangular expansions. In this way a thick peripheral neuroglia layer is produced. As long as the cerebellar cortex is not fully developed, it presents a series of peculiarities which are lacking in the adult. Thus in embryos and young animals the partly developed gray stratum is covered by a superficial granular layer, the cells of which later become more deeply placed.


The ascending sensory fibers from the thalamus and the parts below, and the descending motor fibers which pass out of the hemispheres are contained in the internal capsule, which is a layer of white substance between the thalamus medially and the basal nuclei of the hemispheres laterally. The path by which these fibers enter and leave the deep white substance of the hemispheres is indicated in Fig. 443. Surrounding the inner white substance is the peripheral layer of gray, which forms the cerebral cortex. The cortex is divided into four ill-defined layers an outer molecular or neuroglia layer; a layer of small pyramidal cells; a layer of large pyramidal cells ; and next the white substance, a layer of a .t polymorphous cells. From the pyramidal


cells the fibers of the descending motor THE BRAIN. About j natural


tract arise. The layers are shown in Figs. 444 The gray substance is stippled; the

white is blank, a. t., Ascending

and 44 v tract, including the fillet; c. c.,

corpus callosum ; d. t., descendin g

The molecular layer, which in ordinary sec- tract, leaving the hemisphere to

enter the cerebral peduncle; n. 1.,

tions appears finely punctate or reticular, con

tains besides many neuroglia cells, a network

of medullated tangential fibers, which are parallel with the surface. Other fibers, as shown by the Golgi method, are partly neuroglia, and partly dendrites of pyramidal cells. The "cells of Retzius" found in this layer have bodies of irregular shape, which send out processes parallel with the surface, and these processes send short branches outward; other processer descend into the deeper layer (Fig. 446). They are probably neuroglia cells.

The layer of small pyramidal cells contains a special form of nerve cells, with pyramidal bodies measuring 10-12 P.. Since they taper into a dendritic process, their length cannot be definitely determined. The chief dendrite, after producing small lateral branches, enters the molecular layer



Supra Iradial


Inter- i radial etwork

Radial bundles


or white



' i- ' iX- ifh\ *\ *~7 , i !-; l i '

^itU^w&? -^ l ^Vv


?5^1^^r^^U. b\_-*^^

j^^;.v:'^S^^ ti f ^ ^^r^i---^^-^"-^-^-!' i-v

?^S^^f ' : :

Molecular layer.

Layer of






Layer of large

pyramidal cells.

Layer of polymorphous nerve cells.

i. / .. r' s. (

'- ' ! f/ . ." : . '-\' /r ,". ../':,

^ t ~^ ^ ^LJ_I "^ ^ *

FIG. 445.

FIGS 444 and 445 are from vertical : tions of the cortex (central convoli tion) of an adult. Fig. 444 is a Weige preparation; Fig. 445 is from a sectip stained with hsematoxylin and X 45.

FlG. 444



Cell of Retzius. Short-rayed neuroglia cell. Blood vessel.

--.. Small

pyramidal cell.


/ pyramidal cell.


Xeuraxon of a polymophous nerve cell.

Long-rayed neuroglia cell.

FIG. 446. DIAGRAM OF THE CEREBRAL CORTEX. The cells on the right are drawn from Golgi preparations of an adult man. X 120. The left portion of the diagram is X 60.

where it arborizes freely; its terminal branches often show small irregular projections. Lesser dendrites proceed from the sides and basal surface of the pyramidal cell body. The neuraxon always arises from the basal surface, and after producing branched collaterals, it generally enters the white substance where it may divide in two (Fig. 446, 3). Sometimes the neuraxon turns toward the molecular layer, joining the tangential fibers; 28


infrequently an inverted pyramidal cell is found. The neuraxons and collaterals are medullated.

The layer oj large pyramidal cells contains those with bodies 20-30 /* long (the "giant pyramidal cells" of the anterior central convolution measure even 80 /*). The very large neuraxon always goes to the white substance, after sending out several collaterals in the gray.

The layer of polymorphous cells includes oval or polygonal cells which lack a chief dendrite directed toward the surface; their slender neuraxons produce collaterals, and enter the white substance where they may divide into two branches in T-form (Fig. 446, 4). Polymorphous cells with branched neuraxons limited to the vicinity of the cell body, are found in this layer and in the pyramidal layers also. The neuraxon may branch in the molecular layer (Fig. 446, 6).

Many medullated fibers are found in the deeper layers of pyramidal and polymorphous cells. They are grouped in tapering radial bundles which terminate toward the layer of small pyramidal cells, as seen in Fig. 444. The bundles include the descending medullated neuraxons of the pyramidal and polymorphous cells, and the ascending medullated sensory fibers from the white substance. The latter branch repeatedly, forming the supra-radial and tangential networks. The medullated collaterals of the pyramidal cells run at right angles with the radial bundles; they form an inter-radial network, the outer part of which is so thick in the region of the calcarine fissure that it can be seen without magnification, and is there known as the "stripe of Vicq d* Azyr." Similar bands may be detected elsewhere in thick sections (Baillarger's stripes).

In the gyrus hippocampi and gyrus uncinatus the tangential fibers are so abundant as to form a considerable layer, the substantia reticularis alba. The hippocampus (Ammon's horn), olfactory bulb, and some other areas of the cortex, differ in details from the central region which has been described; these pecularities are considered in the larger special works on the nervous system.

The neuroglia of the hemispheres, like that of the cord, is at first a syncytium with strands extending from the ventricle to the periphery. Later, the syncytium is divisible into short-rayed neuroglia cells found chiefly in the gray substance, long-rayed cells found chiefly in the white, and ependymal cells lining the ventricles. The ependymal layer is continuous through the aqueduct with that of the fourth ventricle and central canal. In early stages its cells have cilia-like processes which are in part retained in the adult. The short-rayed cells, which are characterized by knotted branching processes, are often in close relation with the blood vessels; they may serve to transfer the nutritive and myelin-forming material from the vessels to the nerve fibers. The outer surface of the cerebral cortex is covered with a feltwork of neuroglia fibers.




The hypophysis (i.e., a growth beneath the brain) is a rounded mass, about half an inch wide and a quarter of an inch thick, attached to the tip of the infundibulum, and lodged in the sella turcica of the sphenoid bone. Its stalk of attachment to the infundibulum extends through the fibrous membrane fastened to the four posts or corners of the sella, and in removing the brain, the hypophysis is therefore often torn from its stalk and left in the bony excavation. It is now known to be a most important organ of internal secretion, consisting of two parts which are as distinct from one another as the cortex and medulla of the suprarenal gland. The anterior lobe is formed from Rathke's pouch (Rathke, Arch. f. Anat.> Phys., u. wiss. Med., 1838, pp. 482-485) which grows upward from the oral ectoderm and encounters the knob-like posterior lobe which is a part of the brain (Fig. 203, p. 216). The anterior lobe then sends up a short process on either side of the posterior lobe, like the thumb and first finger of a hand, and in later stages Gushing ventures to describe the pos

FIG. 447. DIAGRAMS OF THE HYPOPHYSIS CEREBRI. (From Morris's Anatomy, after Testut.)

A, Posterior surface. B, Transverse section. C, Sagittal section, i, Anterior lobe; 2, posterior lobe; 3, infundibulum; 4, optic chiasma; 5, infundibular recess; 6, optic recess.

terior lobe as resting in the anterior lobe like a ball in a catcher's glove The anterior lobe becomes separated from the roof of the mouth by the obliteration of its duct, which is reduced to a slender solid epithelial strand and ruptures in embryos of about 20 mm. A depression marking its former outlet has sometimes been found in the vault of the pharynx, and there may be a canal through the sphenoid bone, the craniopharyngeal canal, which follows the course of the former duct. It is said that a small "pharyngeal hypophysis," having the structure of the anterior lobe, is constantly found near the pharyngeal end of this canal, on the under surface of the sphenoid bone.

The posterior surface of both lobes, as they appear in the adult, is shown in Fig. 447, A, and a sagittal section is shown in C; the orientation of the latter may readily be understood by comparing it with the region of the optic recess in Fig. 434.

The hypophysis can hardly be overlooked in examining the brain, and its existence is recorded by the earliest writers. The epiphysis, on the top of the brain, was called



the pineal body from its resemblance to a pine cone, and according to Hyrtl the hypophysis below, being a round structure attached to a stem, was named the "rose hip" by the Mohammedan physician Avicenna (ca. A. D. 1000). Vesalius introduced the name pituitary gland. The pituita or phlegm was believed to be excrementitious material, eliminated by the brain and received by the naso-pharynx, and its possible origin by way of the olfactory nerves had been discussed. Vesalius and his followers believed that it was collected by the infundibular funnel and eliminated by the pituitary gland. If the sella turcica of a prepared skull is examined, four grooves may be traced from it, two passing forward to the optic foramina, and two passing backward to the lacerated foramina. Vesalius pictured these four channels as outlets for the pituitary gland, the two latter (which in life are closed by cartilage) being in relation with the naso-pharynx. Bartholin recorded another function of the pituitary gland, namely, "to close the infundibulum lest vital spirits should escape," and finally V. C. Schneider showed conclusively that the pituitary gland is not the source of phlegm. According to Hyrtl this was accomplished in five classic but lengthy books, De catarrhis, 16401642, and he adds "No physician and no anatomist should leave this fundamental and learned work unread if he has time for it."

The anterior lobe consists of solid branched epithelial cords, of irregular caliber, connected with one another by frequent anastomoses. Between

Portion of the, anterior lobe.\

Epithelial cord.

Epithelial follicle.

Blood vessel containing blood corpuscles.

Portion of the posterior lobe.

Multipolar cell.

Connective tissue fibers.

FIG. 448. PORTION OF A HORIZONTAL SECTION OF A HUMAN HYPOPHYSIS, showing the boundary line between the anterior and the posterior lobes. Two gland follicles on the left each contain a dark epithelial cell. X 220.

the cords and in close relation with them, there are wide lacunar capillaries derived from several arterioles which descend along the stalk of the infundibuhim. The wide terminal vessels are arterio-venous connections having a sinusoidal structure. Along their margins, especially in the central part of the lobe, the cords are covered with eosinophilic cells, having round nuclei; the axial cells of the cords are neutrophilic and less granular. Although the nature of the marginal cells has not been fully determined, they are usually described as glandular, and their granules presumably represent an internal secretion which is discharged into the adjacent vessels. At the periphery of the anterior lobe, basophile cells occur.


Like the cortex of the suprarenal gland, the anterior lobe of the hypophysis is the larger part, and has a characteristic epithelial structure, whereas the portion associated with the nervous system is smaller, with less striking morphological characters. Nevertheless the latter, in both cases, produces the more active extracts, and its products are better understood. The anterior lobe of the hypophysis appears to "preside more intimately over skeletal growth;" and overgrowth, acromegaly and gigantism are attributed to its excessive activity. The administration of extracts of the posterior lobe causes a rise in blood pressure, owing to the contraction of the vascular musculature, thus resembling adrenalin in its action. Repeated injections cause emaciation; and deficient secretion, or the removal of the gland, leads to a high tolerance for sugars with the resultant accumulation of fat. "Thus normal activity of the posterior lobe is essential for effective carbohydrate metabolism" (Gushing, The Pituitary Gland and its Disorders, 1912).

The posterior lobe consists of a mass of neuroglia cells, the pars nervosa, and an epithelial investment, the pars intermedia. The latter is of special interest since its cells, sometimes ciliated, tend to become arranged in cysts containing a hyaline or colloid secretion. According to Stohr, these cysts belong with the anterior lobe, and since the two lobes are in contact near the anterior part of the infundibular stalk, it is possible that its elements have grown around and invested the pars nervosa, thus producing the pars intermedia. Except anteriorly, however, the two lobes of the hypophysis are generally separated by a cleft.

The pars nervosa contains ependymal and neuroglia cells, but no nerve cells and only a few nerve fibers. The ependyma lines the cavity which extends downward into the lobe from the inf undibulum. According to Tilney, "very often in the human hypophysis the lumen is not only seen to be distended by large masses of colloid, but its walls are evaginated so as to give rise to cysts of varying sizes, all containing colloid" (Mem. of the Wistar Inst., No. 2, 1911). The colloid material is believed to be evidence of a secretion which is eliminated into the third ventricle, and which finds its way into the cerebro-spinal fluid. Possibly it may be given off from the outer surface of the lobe, for the inconstant cavity or lumen is not a typical duct; but the secretion apparently does not enter the blood vessels, which in this lobe are neither abundant nor sinusoidal. Eosinophilic cells are generally absent.


The pineal body (sometimes called the epiphysis) is a median dorsal outpocketing of the diencephalon (Figs. 434 and 435), terminating in a small nodule composed of neuroglia and round or polygonal epithelial cells. The human pineal body contains no nerves (Kolliker) but below it there is the commissura habenularum. A connective tissue capsule sends prolongations into its interior and surrounds groups of epithelial cells and follicles.


It is generally considered that the pineal body is a function! ess rudiment. In lower vertebrates an eye-like structure develops just in front of it, sometimes being found beneath a transparent cornea, but the extent of the visual functions of this organ remains undetermined. The corpus pineale immediately behind this eye may take its place to some extent, and "often shows, as in certain lizards, traces of visual structure" (Kingsley). The unimportant position to which this organ has been relegated, contrasts with the familiar conjecture of Des Cartes that all ideas which proceed from the five senses are perceived in the pineal body as a center, and that from it all nervous impulses irradiate. In man not the slightest function is now assigned to it.

Within the pineal body, acervulus cerebri or "brain sand" is usually found, consisting of round or mulberry-like concretions, 5 n to i mm. in diameter (Fig. 449). In specimens preserved in glycerin or balsam these concretions show distinct concentric layers. They consist of an organic matrix containing calcium carbonate and magnesium phosphate, and are sometimes FIG 449 ACERVULUS surrounded by a thick connective tissue capsule.

BODY OF A WOMAN Not infrequently, especially in old age, the brain

OU> ENT X so YEARS substance contains round or elongated bodies, distinctly

stratified, which are colored violet by tincture of iodine

and sulphuric acid, and therefore are related to amyloid. These cor puscula amylacea are found almost always in the walls of the ventricles,

and also in many other places in both gray and white substance, and

in the optic nerve. They have a homogeneous capsule with occasional

processes, and are evidently neuroglia cells transformed by amyloid



The dura mater cerebralis or dura mater of the brain, includes the periosteum of the inner surface of the cranium and consists, therefore, of two lamellae. The inner is like the dura mater of the cord but contains more elastic fibers; the outer corresponds with the periosteum of the vertebral canal. It contains the same elements as the inner layer, but its fibers run in a different direction. In order that the dura of the brain and cord may be strictly comparable, some anatomists count the vertebral periosteum and the considerable layer of vascular fatty tissue beneath it, as a part of the dura of the cord. In relation with the brain, the dura forms reduplications extending between the cerebellum and the hemispheres, and between the right and left hemispheres. Its two layers separate to enclose large, thin-walled veins, the sinuses of the dura. These receive


veins from the substance of the brain, but the arteries of the dura, or meningeal arteries, supply the cranial periosteum. The dura has many nerves, some with free endings, and others supplying the musculature of the vessels.

The arachnoid membrane, as in the cord, is separated from the dura by a cleft-like sub-dural space. In certain places, especially along the sides of the superior sagittal sinus, there are found arachnoid villi (Pacchionian bodies or granulations), which project into the cavity of the venous sinus. They are elevations of the arachnoid covered with a thin portion of the dura and venous endothelium, and possibly facilitate the transfer of fluid between the arachnoid (or subarachnoid) spaces and the veins. These spaces contain the cerebro-spinal fluid, and are continuous with the corresponding spaces around the cord. Through apertures in the thin roof of the fourth ventricle, they communicate with the central cavity of the cord and brain.

The pia is a delicate and highly vascular layer, containing arteries which send branches into the cortex from all points on its surface. These cortical arteries arise from the anastomoses between the internal carotid and vertebral arteries at the base of the brain, which produce the arterial circle of Willis. Other branches from these vessels enter the substance of the base of the brain, supplying the basal nuclei, thalamus and internal capsule. Because of the effects of haemorrhage in relation with the motor and sensory tracts in this region, these small arteries are of very great importance. The vascular membranes which cover the thin portions of the roof of the third and fourth ventricles are in places invaginated into the ventricles, forming the chorioid plexuses. These networks of small vessels, covered only by thin membranes, are found in the lateral ventricles, as well as in the third and fourth; their position is described in text-books of gross anatomy. The simple layer of cuboidal epithelium, which covers the plexuses, contains pigment granules and fat droplets, and may perform secretory functions.


Development and General Anatomy. The eyes first appear as a pair of optic vesicles, which are lateral out-pocketings of the fore-brain (Fig. 451, A). They enlarge rapidly, but their connections with the wall of the brain remain relatively slender, forming the optic stalks. The epidermal ectoderm immediately overlying the vesicles thickens and becomes invaginated (Fig. 451, B and C). The invaginated portion is then detached in the form of a vesicle, the inner wall of which is distinctly thicker than the outer; this "lentic vesicle" becomes the lens of the eye. Meanwhile, as seen in B and C, that layer of the optic vesicle which is


toward the epidermis sinks in upon the deeper layer, transforming the vesicle into the optic cup. At first the cup is not complete, being deficient on its lower side (Fig. 450). The arteria centralis retina is seen passing through this indentation, which begins on the lower surface of the stalk and extends to the free margin of the cup; the cleft is sometimes called the "chorioid fissure." Distal to the point of entrance of the artery into the optic cup, the edges of the fissure fuse; the artery then appears to perforate the base of the cup, and it retains this relation in the adult. The artery is shown in section in Fig. 451, D.

In a remarkable series of experiments upon tadpoles, Warren Lewis has shown that "the lens is dependent for its origin on the contact influence or stimulus of the optic vesicle." If the optic vesicle is removed, the epithelium in the region of the normal lens does not become thickened or invaginated; but if an optic vesicle is transplanted by detaching it from its stalk and pushing it caudally through the mesenchyma, it will cause the formation of a lens from any portion of the epidermal epithelium which happens to be above it. Moreover, if an area of skin from the abdomen of a frog of one species is grafted over the optic vesicle of another species, a lens may be produced from the grafted epithelium. Thus there is no predetermined area for lens formation, and its development depends upon the presence of the vesicle beneath (Amer. Journ. Anat., 1904, vol. 3, pp. 505-536, 1907, vol. 7, pp. 145-169).

The two layers of the optic cup, the inner of which is toward the lens, are normally in contact with one another, although in sections they have often become more or less separated. They constitute the retina, which includes a thin outer pigmented layer, and a thick inner visual layer; the FIG. 450. OPTIC CUP AND latter is composed of several strata of nerve cells and


avfter Y Konmann ) ""' n t> ers - The stimulus of light is received by tapering projections extending from the outer surface of the visual layer toward the pigmented layer; to reach them the rays of light must traverse the strata of the visual layer. In explanation of the fact that the sensory processes are turned away from the light, it may be said that the outer surface of the skin ordinarily receives stimuli, and that through the infolding which makes the medullary tube and the outpocketing which makes the optic vesicle, the sensory surface of the retina is continuous with the outer surface of the skin. Since in mammals the optic vesicles begin to form before the related portion of the medullary groove has closed, they first appear as depressions in a thickened epidermal ectoderm.

Nerve fibers grow from the inner surface of the visual layer toward the central artery and vein of the retina, around which they pass out of the optic cup (Fig. 451, D). They grow beneath and among the cells of the optic stalk to the brain, which they enter. These fibers, which constitute the optic nerve, cause the obliteration of the optic stalk. It is



shown in the figure that the optic nerve at its origin interrupts the retinal layers, producing a "blind spot." The part of the nerve which forms the blind spot, with the vessels in its center, is called the papilla o1 the optic nerve.

The lens (Fig. 451, D) loses its central cavity by the elongation of the cells in its posterior layer. These become the fibers of the lens. The anterior layer remains throughout life as a simple epithelium, called the epithelium oj the lens. The lens becomes covered by an elastic capsula


FIG. 451. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OF THE EYE. A, 9} days. 3.0 mm. B, 10} days, 5.4 mm.; C, n days, 5.0 mm.; D, 14 days, 18 hours, 12.0 mm.; E, 20 days, 29 mm.

a c. r., Arteria centralis retinae; c., conjea; c. a., anterior chamber; conj., conjunctiva; c. p., posterior chamber; c. v., corpus vitreum; e. 1., eyelid; f. b., fore-brain; 1., lens; 1. e., lens epithelium; 1. f., lens fibers; o. c., optic cup; o. n., optic nerve; o. v., optic vesicle; r. p., pigmented layer of the retina; r. v., visual layer of the retina.

lentis, and in embryonic life it possesses a -vascular capsule (Fig. 451, E) containing branches of the central artery. The vascular layer covering the anterior surface of the lens is designated the pupillary membrane, and it disappears shortly before birth. Its occasional persistence interferes with vision.

Between the lens and the retina there is a peculiar tissue, mucoid in appearance and resembling mesenchyma in form. Since processes from


the retina and from the lens have been found extending into it, it is considered to be essentially ectodermal. Its blood vessels become obliterated and it forms the vitreous body of the adult, consisting of a stroma and a humor. Extending through it, from the papilla of the optic nerve toward the lens, is the hyaloid canal, which in the embryo lodged the hyaloid artery (a prolongation of the central artery). Sometimes this artery is represented in the adult by a strand of tissue. The vitreous body is surrounded by a fibrous layer called the hyaloid membrane.

A cavity forms in the tissue in front of the lens and becomes filled with a watery tissue fluid (aqueous humor). It is bounded by a mesenchymal epithelium. The portion of the cavity which is anterior to the retinal cup and lens is called the anterior chamber of the eye; the smaller part within the retinal cup but in front of the lens and the fibrous covering of the vitreous body, is the posterior chamber (Fig. 451, E, c.p.).

The retinal cup is surrounded by two layers of mesenchymal origin. The inner tunica vasculosa corresponds with the pia mater and forms the chorioid coat of the eye; the outer tunica fibrosa corresponds with the dura mater and forms the sclera, into which the muscles of the eye are inserted. The portion of the retinal cup which forms a curtain, circular in front view, between the anterior and posterior chambers, is called the iris. It consists of tunica vasculosa with a thin pigmented prolongation of the retina over its posterior surface (Figs. 451, E, and 452). This pars iridica retina is rudimentary and without visual function. The iris is covered by the mesenchymal epithelium of the chambers. At the attached border of the iris the vascular coat contains important muscle fibers, and is there thickened to form the ciliary body. This is also covered by a rudimentary pigmented layer on its inner surface, the pars ciliaris retina. At the ora serrata (Fig. 467) an abrupt thickening of the visual layer of the retina marks the boundary between its ciliary and optic portions. The pars optica retina extends from the ora to the optic nerve, covered externally by the chorioid and sclera.

As a relatively frequent congenital anomaly, the chorioid fissure fails to close normally and the resulting defect is known as coloboma. If the closure has been nearly complete, so that there is merely a notch at the free margin of the optic cup, it appears in the adult as a median ventral cleft in the iris, so that the pupil is shaped like an inverted pear. If the deeper parts of the chorioid fissure fail to unite, there will be a median ventral gap in the optic portion of the retina, which may seriously interfere with vision.

The cornea is the tissue in front of the anterior chamber, consisting of a non-vascular mesenchymal tissue, bounded posteriorly by mesenchymal epithelium and anteriorly by the epidermal ectoderm. It is extremely transparent. The epidermal ectoderm extends from the cornea and front of the eye over two folds which form the eyelids. They have met in



Fig. 451, D, and fused temporarily. Externally the lids are covered by skin, and internally by the conjunctiva palpebrarum, or conjunctiva of the lids. The latter is continuous with the conjunctiva bulbi which forms the opaque vascular "white of the eye." It surrounds the cornea, the epithelium of the two structures being continuous.

The parts of the eye to be examined histologically are therefore the retina, optic nerve, lens, and vitreous body, all of which are ectodermal;

pithelium -Anterior basal lamina Substantia propria .Posterior basal lamina

Mesenchymal epithelium

of the cornea.

Sphincter muscle Stroma

Pars iridica retinae Angle of the iris.

of the iris.

Sinus venosus sclerae


.. Epithelium "I o f the

L conjunctiva - Tunica f bulbL J



Zonula. Ciliary process muscle fibers.

Circular Meridional Pars ciliaris retinae.

FIG. 452. MERIDIONAL SECTION OF A PART OF THE EYE. X 15. The radial fibers of the ciliary muscle cannot be distinguished with this magnification.

then the tunica vasculosa, including the chorioid, the ciliary body, and iris; next the tunica fibrosa, including the sclera and cornea; and finally the accessory structures the lids, conjunctivas and glands.


The retina extends from the papilla of the optic nerve to the pupillary border of the iris, and is divisible into three parts; the pars optica retina includes all which is actually connected with the optic nerve and which therefore is sensitive to light. It covers the deeper portion of the optic



cup, ending near the ciliary body in a macroscopic, sharp, irregular line bounding the ora serrata. The pars ciliaris and the pars iridica retina are the rudimentary layers covering the ciliary body and iris respectively. The pars optica retinae in a fresh condition is a transparent layer colored reddish by the "visual purple." In sections it presents many layers arranged as seen in Fig. 453, the cells of which are related to one another as in the diagram, Fig. 454. The outer layer of the optic cup forms the pigmented epithelium of the retina, which consists of a simple layer of six-sided cells. Toward their outer surface (that next the chorioid, where the nucleus lies) they are poor in pigment, whereas in their inner portion they contain numerous rod-shaped (1-5 p, long) brown granules of


Chorioid. {

Pigmented _ epithelium. f

Layer of rods and cones.

Membrana^imitans _ externa.


Outer nuclear I layer.

Henle's fiber layer j

Outer reticular layer.

Inner nuclear layer.

Inner reticular ,' layer.

Ganglion cell layer Nerve fiber layer. Membrana limitans interna.

Vessels of the

S chqriocapil laris. Lamina basalis.

Rods 1 Outer.

- Cones J segment.


s Rods

Inner segment.


\ ^jy

\ \



1 ,. Base of a cone fiber.

Nucleus of a radial fiber.

Nucleus of an amakrine cell.

Pyramidal base of a radial fiber.

Blood vessels.


the pigment " fuscin." In albinos the pigment is lacking. From the inner surface of the pigmented epithelium, numerous processes extend between the rods and cones.

The visual cells, which are found along the outer surface of the inner retinal layer, are of two sorts, rod cells and cone cells. In both, the nucleus is found in the inner half of the cell, and the outer non-nucleated half projects through a membrane, the membrana limitans externa. This causes the visual cells to appear divided into layers, their nucleated parts beneath the limiting membrane constituting the outer nuclear layer (or outer granular layer), and the non-nucleated parts outside of the membrane forming the layer of rods and cones.

The rods are four times as numerous as the cones. They are regularly placed so that three or four rods are found between every two cones (Fig.



453). The rods are elongated cylinders (60 n long and 2 ju thick) consisting of a homogeneous outer segment, in which the visual purple is found exclusively, and a finely granular inner segment. In the outer third of the inner segment there is said to be an ellipsoidal, vertically striated structure (which in some lower vertebrates is very distinct). The portion of the rod cells below the limiting membrane is a slender thread, expanding to surround the nucleus which is characterized by from one to three transverse bands. Beneath the nucleus the protoplasm again becomes thread-like, and this basal prolongation of the cell terminates in a small club-shaped enlargement, without processes (Fig. 454).

Cone cell Rod cell.

Stellate ganglion cell. Bipolar cells.

Amakrine cells. Centrifugal nerve fiber.

Multipolar ganglion cell.

11 Layer of rods and cones.

Membrana limitans ' externa.

Outer nuclear layer.

Henle's fiber layer. 1 Outer reticular layer.

Dinner nuclear layer.

Inner reticular layer.

Ganglion cell layer. ==^-_ = Nerve fiber layer.

Collateral. Pyramidal bases of radial fibers. FIG. 454. DIAGRAM OF HUMAN RETINA. SUPPORTING SUBSTANCE RED.

The cones likewise consist of an outer and an inner segment. The conical outer segments are shorter than those of the rods. The inner segments are thick and somewhat dilated so that the entire cone is flaskshaped. Moreover, the inner segment contains a vertically striated "fiber apparatus." The nuclei of the cone cells are situated just beneath the limiting membrane; below the nuclei the protoplasm forms a fiber, ending in an expanded pyramidal base.

The entire visual cells therefore form three layers of the retina, namely, (i) the layer of rods and cones; (2) the outer nuclear layer, containing the nuclei of the rod and cone cells; and (3) Henle's fiber layer, composed of the basal processes of these cells. The three layers next beneath are formed essentially of superposed parts of the radially arranged bipolar nerve cells, which constitute the ganglion retina. Immediately beneath Henle's fiber layer, dendritic processes of these cells form an outer reticular layer, whereas their nuclei are situated in an inner nuclear layer, and their centripetal processes, or neuraxons, enter an inner reticular layer. There


they terminate in relation with dendrites and cell bodies of large ganglion cells which constitute the ganglion of the optic nerve. Cell bodies of this ganglion form the ganglion cell layer, and their neuraxons, traveling toward the papilla of the optic nerve, are the principal elements in the nerve fiber layer. The latter is separated from the vitreous body by an internal limiting membrane. Thus visual stimuli, received by the rods and cones, are transferred by means of the bipolar cells of the ganglion retinae, to the ganglion cells of the optic nerve, through the neuraxons of which they proceed to the brain. These layers may be described in further detail as follows:

Henle's fiber layer contains not only the fiber-like basal ends of the rod and cone cells, but also the slender unbranched dendritic processes of the bipolar cells of the ganglion retinae. Each bipolar cell sends one such process through Henle's layer to terminate in a little thickening near the membrana limitans externa. In the outer reticular layer, however, these dendrites of the bipolar cells send out branches which bifurcate repeatedly, becoming reduced to the finest fibrils; they form a close subepithelial felt- work (Fig. 454).

Occasionally nuclei are found in the outer reticular layer. Most of these belong with bipolar cells displaced outward (Fig. 454, x). Toward the inner nuclear layer, however, there are stellate ganglion cells with neuraxons which pursue a horizontal course and then turn inward to join the optic nerve fibers, as shown in Fig. 454. The existence of such fibers has been denied by some writers. The neuraxons of other stellate ganglion cells in this region end in relation with the bases of the visual cells (Fig. 454, +).

Toward the inner reticular layer, the inner nuclear layer contains the bodies of ganglion cells, which appear to lack a chief or large process, and are therefore called "amakrine" cells. They send branching fibers into the inner reticular layer, where they interlace with the fine varicose branches of the bipolar cells, and with the ramifications of the dendrites from the ganglion nervi optici.

The ganglion cell layer consists of a single row of large multipolar cells containing Nissl's bodies. Certain of these cells because of exceptional size are known as "giant ganglion cells," and they occur at quite regular intervals. "Twin cells" have been found, consisting of two cell bodies united by a short bridge; only one of the pair has a neuraxon.

The nerve fiber layer consists chiefly of the non-medullated neuraxons of the ganglion cells, arranged in plexiform bundles. Occasionally the neuraxons send collaterals back to the ganglion cell layer, where they branch about the cell bodies (Fig. 454). The fiber layer contains also neuraxons which have come out from the brain to terminate in free branches among the cells of the inner nuclear layer.



In addition to the nervous elements, the retina contains blood vessels and a supporting framework of neuroglia cells. The largest vessels are toward the fiber layer (Fig. 453), in thich they travel to and from the central vessels in the papilla. The neuroglia framework consists chiefly of radial (or Miiller's) fibers, which are elongated cells extending from the internal to the external limiting membrane. Beyond this membrane they send short processes between the rods and cones, forming "fiber baskets" (Fig. 455). The radial fibers are not isolated cells but are parts of a general syncytium, being connected by a network of processes which penetrate all the layers of the retina (Fig. 454). The external limiting membrane, through which the rods and cones pass, is formed by the coa

Fiber basket.

Nucleated part of the fiber.

__. Basal pyramid.

- Precipitate.

FIG. 455. GOLGI PREPARATION OF RADIAL FIBERS IN A THICK SECTION OF THE HUMAN RETINA. The fine processes of the fibers in the outer nuclear layer appear as a compact mass. X 360.

lescence of these processes, and the internal limiting membrane is made up of the closely adjacent basal expansions of the radial fibers. The nuclei of the fibers are found in the inner nuclear layer. In addition to the radial fibers there are neuroglia cells with horizontal or tangential branches (Fig. 454, "oo"). As in the central nervous system, some of the stellate groups of fibers do not contain nuclei.

Two modifications of the retina require special description, namely, the fovea centralis, which is the region of most acute vision, and the pars ciliaris, which is the rudimentary peripheral portion.

Macula lutea and fovea centralis. When vision is centered upon a particular object, the eyes are so directed that the image of the object falls upon the macula lutea, or yellow spot of the retina, within which there is a depression, the fovea centralis. The macula sends straight slender



EYE 449

fibers to the papilla of the optic nerve, which is close by on its median side; other coarser optic fibers diverge as they pass the macula, forming an ellipse around it. The retinal layers of the macula are arranged as shown in Fig. 456. At its border the number of rod cells diminishes, and within the macula they are entirely absent. The nuclei of the numerous cone cells, which are here somewhat smaller than elsewhere, form an inner nuclear layer of twice the usual thickness. The basal portions of the cone cells make a broad Henle's fiber layer, and slope away from the fovea. The bipolar cells of the ganglion retinae are so numerous that their nuclei may form nine rows. The ganglion cells of the optic nerve are also abundant. All of these strata become thin toward the fovea, the deepest part of which contains scarcely more than the cone cells. In some individuals the slope of the sides of the fovea is less steep than in the figure; its depth is variable. The macula and fovea are saturated with a yellow pigment soluble in alcohol.

Pars ciliaris retina. The optic nerve fibers and their ganglion cells disappear before reaching the ora serrata. The cone cells extend further toward the ora than the rods, but the last of them appear to lack outer segments. By the thinning of the reticular layer, the nuclear layers become confluent (Fig. 457). Near the ora serrata large clear spaces normally occur in the outer nuclear layer, and they may extend into the deeper layers (Fig. 457). The radial sustentacular cells form a simple columnar epithelium as the other layers disappear, and they constitute the visual layer of the pars ciliaris. The pigmented epithelium is apparently unmodified as it extends from the optic to the ciliary portion. Along the inner surface of the ciliary part of the retina, the cells of the visual layer produce closely packed horizontal fibers, which form a refractive hyaline membrane.

Zonula ciliaris. Some of the fine homogeneous fibers arising from the pars ciliaris immediately in front of the ora serrata enter the vitreous body, but a much larger number pass between the ciliary processes to the lens. They are attached to the borders of its capsule, overlapping slightly its anterior and posterior surfaces. Thus they form the zonula ciliaris (suspensory ligament) which holds the lens in place (Fig. 452). The zonula is not a continuous layer, nor does it consist of two laminae, one to the anterior and the other to the posterior surface of the lens, with a space between them. It consists rather of numerous bundles, between which and the vitreous body, and among the bundles themselves, there are zonular spaces (canals of Petit) which communicate with the posterior chamber.


In its intraorbital portion the optic nerve is surrounded by prolongations of the meninges. On the outside is the dural sheath, consisting of 29



_ ^- "Vacuole.


w ^-,V *:*>'

-Radial fibers of Muller.




dense connective tissue with many elastic fibers. The outer connective tissue bundles tend to be longitudinal and the inner circular. Internally the outer sheath is connected with the arachnoid layer by a few dense strands of tissue, and the arachnoid joins the pial sheath by many branched trabeculae. The pia surrounds the entire nerve and sends anastomosing septa among the bundles of nerve fibers. The latter are slender and medullated, but without a neurolemma; they are supported by longrayed neuroglia cells, which are found between the individual fibers, but are most numerous at the periphery of the bundles and around the entire nerve. Thus the optic nerve differs from the peripheral nerves, and resembles a cerebral commissure.

At the posterior surface of the eye-ball (or bulbus oculi), the dura blends with the sclera. Continuous with both is the dense elastic lamina cribrosa which is perforated by the optic nerve fibers. The chorioid and the pia are also in relation with this lamina (Fig. 458). As the optic

Central artery. Fibers of the lamina cribrosa. | Central vein.

Hyaloid membrane t loosened.

Dural sheath

FIG. 458. LONGITUDINAL SECTION OF THB OPTIC ENTRANCE OF A HUMAN EYE. X 15. Above the lamina cribrosa is seen the narrowing of the optic nerve, due to its loss of myelin. The central artery and vein have been for the most part cut longitudinally, but above at several points transversely.

nerve penetrates the lamina, its fibers lose their myelin and radiate into the nerve fiber layer of the retina. The central artery and vein of the retina enter the optic nerve in its distal half, and appear at the fundus of the eye in the center of the optic papilla. Their branches spread in the inner layers of the retina, which are covered by the membrana limitans interna (Fig. 453).


The lens is a biconvex structure having an anterior and a posterior pole, and a vertical equatorial plane. It is enclosed in a thick transparent



elastic capsule, 6.5-25 /* thick in front and 2-7 n thick behind, which is apparently derived from the lens itself. Within the capsule the anterior surface of the lens is formed by the lens epithelium, a single layer of cells 2.5/1 thick at the pole, but becoming taller toward the equator. There they are continuous with the elongated lens fibers of the posterior layer, which collectively are called the substantia lentis.

Originally the fibers multiply throughout the lens, but in later stages the formation of new fibers, as indicated by the presence of mitotic figures, is limited to the region of transition between the lens epithelium and the mass of lens fibers (Figs. 451, E, and 460). When first formed the fibers are short, but they increase in length and become six-sided prisms, somewhat enlarged at one or both ends. The first fibers extend from one surface of the lens to the other. Later these become buried in by the new



A, Isolated lens fibers; three with smooth and one with dentate borders. X 240. B, Human lens fibers cut transversely; c, section through club-shaped ends. X 560.


C, Tangential section. D. Meridional section across the equator of the lens; i, capsule; 2, epithelium; 3, lens fibers. X 240.

fibers formed at the periphery, and thus they constitute the nucelus of the lens. This is a dense mass of somewhat shrunken fibers, which have lost their nuclei and have acquired wavy or notched margins (Fig. 459). The outer fibers of the cortical substance are softer. They have smooth borders, and nuclei which are chiefly in the equatorial plane. Their protoplasm is transformed into a clear fluid substance, said to be chiefly a globulin. The fibers are united to one another by a small amount of cement substance, which is more abundant at the poles, at each of which it forms a "lens star," usually with nine rays.

When the fibers formed at the periphery of the original nucleus elongate so as to cover it in, they do not extend from one pole to the other. Those that reach the anterior pole fall short of the posterior pole, terminating along a horizontal suture of cement substance; and conversely those that reach the posterior pole terminate anteriorly along a linear vertical suture. As the lens becomes larger, the linear sutures at either pole are replaced by tri-radiate or Y-shaped stars, one of which is inverted.

EYE 453

Lens fibers starting near the center of one star end near the tips of the rays of the other, and vice versa. When the stars become nine-rayed the arrangement of the fibers is very intricate. Without crossing one another, and without any of them being long enough to pass from pole to pole, they cover the lens with even layers. The development of the stars is described by Rabl (Ueber den Bau und Entwicklung der Linse, Leipzig, 1900). As a result of its structure the lens may be separated into concentric lamellae, but Rabl considers that the meridional segments, or "radial lamellae," of which the lens contains about two thousand, are its essential subdivisions.


The corpus vitreum consists of the fluid vitreous humor, together with looser or denser strands of fibrous stroma which stretch across it in all directions. Although it is difficult to recognize any definite arrangement in the stroma, certain pathological cases suggest that it is distributed like the septa of an orange. The cells of the vitreous body are round forms, probably leucocytes, and stellate or spindle-shaped connective tissue cells, sometimes degenerating and vacuolated, which invaded the vitreous body with the blood vessels. The latter have atrophied and been resorbed, except for occasional shreds and filaments. Such opacities, which occur normally, are observed when looking at a bright light, and are frequently troublesome to those beginning to use the microscope; because of their erratic motion they are known to physiologists as muscce volitantes. In old age, in eyes otherwise normal, crystals may form in the vitreous humor and float about, " falling like a shower to the bottom of the eye when the eye is held still." Surrounding the vitreous body there is a very resistant thick fibrous layer, which is continuous anteriorly with the hyaloid membrane of the ciliary part of the retina.


Chorioid. Between the sclera and the chorioid there is a loose tissue containing many elastic fibers and branched pigment cells, together with flat non-pigmented cells. In separating the sclera from the chorioid, this layer is divided into the lamina fusca of the sclera and the lamina suprachorioidea. Internal to the latter is the lamina vasculosa, which forms the greater part of the chorioid. It contains many large blood vessels imbedded in a loose elastic connective tissue, some of its cells being branched and pigmented; others without pigment are flat and arranged in layers surrounding the vessels. A thin inner layer of blood vessels, the lamina choriocapillaris, consists of a very close network of wide capillaries. The choriocapillaris is separated from the pigmented epithelium of the retina by a structureless elastic lamella which may be 2 n thick. This lamina basalis shows the imprint of the polygonal retinal cells on its inner surface, and is associated with fine elastic networks toward the choriocapillaris.



Between the vascular lamina and the choriocapillaris, there is a boundary layer consisting of a fine elastic network, generally without pigment. Here in ruminants and horses there are many wavy bundles of connective tissue, which give to the eyes of those animals a metallic luster. Such a layer is known as the tapetum fibrosum. The similarly iridescent tapetum cellulosum of the carnivora is formed of several layers of flat cells which contain numerous fine crystals.

The ciliary body encircles the eye as a muscular band, attached to the inner surface of which there are from 70 to 80 meridional folds, the ciliary processes (Fig. 452). The equator of the eye is vertical, like that of the lens, and the meridians are antero-posterior. The processes begin low at the ora serrata and rise gradually to a height of i mm., terminating abruptly near the border of the lens. Each process consists of fibrillar connective tissue containing numerous elastic fibers and blood vessels, and is bounded toward the pars ciliaris retinae by a continuation of the

Cross and longitudinal sections of bundles of scleral fibers.

Lamina supra\ chorioidea.

Lamina vasculosa.

Boundary zone. Choriocapillaris. Basal membrane. Pigment layer of the retina.



g, Large vessels; p, pigment cells; c, cross section of capillaries.

lamina basalis, which is thrown into intersecting folds. The ciliary processes, which are compressible, may serve to prevent the increase of intraocular pressure during the contraction of the ciliary muscle; and the fluid within the eye is derived from the vessels which they contain. The ciliary muscle is a band of smooth muscle fibers about 3 mm. broad and 0.8 mm. thick anteriorly; it arises beneath the sinus venosus of the sclera and tapers toward the ora serrata (Fig. 425). It consists of three sets of fibers, the meridional, radial, and circular. The meridional fibers (Fig. 452, p. 443) are next to the sclera, grouped in numerous bundles with elastic tissue intermingled. They extend to the smooth part of the chorioid, and constitute the tensor chorioidea. The radial fibers are directed to



ward the center of the eye-ball. They form a middle layer of curving fibers which blend with the meridional fibers externally. The circular fibers, which vary in number in different individuals, form that part of the ciliary muscle which is nearest to the equator of the lens. The contraction of these muscles affects the shape of the lens, which is attached to the adjacent tissue by the zonula.

The iris consists of its stroma anteriorly, and the pars iridica retina posteriorly, and is covered by the mesenchymal epithelium of the chambers of the eye. The anterior epithelium is a simple layer of flat polygonal cells (sometimes called "endothelium"). It rests upon a loose network of stellate cells, in part pigmented, resembling the reticulum of a lymph gland. This is followed by the loose connective tissue of the stroma, likewise containing networks of stellate cells, which in blue eyes are not pigmented. The very few elastic fibers are limited to the posterior layers, where they are radially arranged in rela

Mesenchymal epithelium.

Loose connective tissue.


FIG. 462. A, FROM A TEASED PREPARATION OF A HUMAN CHORIOID. X 240. p, Pigment cells; e, elastic fibers; k, nucleus of a flat non-pigmented cell; the cell body is invisible.

B, PORTION OK A HUMAN CHORIOCAPILLARIS AND THB ADHERENT LAMINA BASALIS. X 240. c, Wide capillaries, some of which contain (b) blood-corpuscles; e, lamina basalis, showing a fine lattice work."


Vascular layer.

Spindle cell layer.



of the entire width of the iris is shown.

g, Blood vessel, with thick connective tissue sheath; m, sphincter pupilla muscle cut transversely;

pupillary border of the iris.

tion to the pupil. The stroma contains numerous radial blood vessels with thick connective tissue coats, but (in man) without musculature or elastic fibers. In the vascular layer, toward the pupillary border of the iris, there is a band of circular smooth muscle fibers, i mm. deep; this is the sphincter pupilla. It is invested with many


prolongations of the stromatic network, the polygonal meshes of which are radially elongated. The dilatator pupilla is a peculiar membrane of smooth muscle fibers on the posterior surface of the vascular layer, stretching from the connective tissue between the muscle bundles of the sphincter, to that between the ciliary muscles. Its fibers consist of an anterior contractile portion, and a posterior nucleated and pigmented portion. The anterior parts form a continuous layer, readily seen in radial sections as "Henle's spindle cell layer," which is a clear non-nucleated stripe, 2-5 p wide (Fig. 463). The nucleated portions of the fibers appear to blend with the pigmented retinal layer of the iris, from which they are derived. These muscles are therefore ectodermal.

The two layers of the optic cup are intimately blended in the thin stratum which forms the posterior layer of the iris. Except in albinos, this pars iridica retinae is deeply pigmented. Posteriorly it is covered by a continuation of the hyaline membrane of the pars ciliaris.


The sclera, toward the chorioid, is bounded by the pigmented lamina fusca. This is a loose tissue containing branched pigment cells and flattened connective tissue cells. Except for this boundary layer, the sclera consists of densely interwoven bundles of connective tissue, chiefly meridional and longitudinal. Elastic fibers accompany the bundles, and are especially abundant at the insertions of the ocular muscles. The flat irregular cells of the connective tissue are surrounded by tissue spaces as in the cornea, and anteriorly the cornea and sclera are continuous with one another. The transition, however, is quite abrupt and the boundary is oblique, so that the rim of the cornea is bevelled at the expense of its anterior surface.

The cornea (Fig. 464) consists of an outer epithelium, external basal membrane, substantia propria, internal basal membrane, and mesenchymal epithelium bounding the anterior chamber. The corneal epithelium, about 0.03 mm. thick, is stratified and consists of a basal layer of clearly outlined columnar cells followed by three or four rows of cuboidal cells and several layers of flattened superficial cells. The outer cells retain their nuclei. Peripherally the epithelium is continuous with that of the conjunctiva bulbi. The anterior basal membrane (Bowman's) is an almost homogeneous layer, sometimes as much as o.oi mm. thick. Superficially it connects with the epithelial cells by spines and ridges. Beneath, it blends with the substantia propria, of which it is a modification. Since it is not formed of elastic substance the name "anterior elastic membrane" is not justified.

The substantia propria consists of fine straight fibrils of connective



tissue, bound together in bundles of almost uniform thickness by an interfibrillar substance, perhaps fluid; these bundles are joined to one another by an interfascicular cement, so that they form a succession of

Epithelium. Anterior basal membrane.

Substantia propria.

Posterior basal membrane.

Mesenchymal epithelium.


Corneal canaliculus. Corneal space. Corneal cells.


superposed flat lamellae, parallel with the corneal surface. Oblique bundles, the so-called arcuate fibers, are found especially in the anterior layers, where they pass from one lamella to that next above or below.


Numerous tense elastic fibers are found especially in the deeper layers, where they form a fine network over the posterior elastic membrane.

Within the cement substance, there is a system of branched canaliculi, dilated in places to form oval spaces. The latter are between the lamellae, but the canaliculi extend also among the constituent fiber-bundles. Within the spaces, there are flat stellate anastomosing cells or "corneal corpuscles," the branches of which extend into the canals and tend to unite with those of neighboring cells, at right angles (Fig. 466). The cells and their processes are more or less surrounded by serous fluid. Leucocytes enter the canals, and are normally found in the cornea; if the cornea is inflamed they become abundant. Blood vessels and lymphatic vessels are absent.

The posterior basal or elastic membrane (Descemet's membrane) is a structure clear as glass, 6 n thick. Its posterior surface is covered by a simple layer of flat polygonal cells (Fig. 464), which form a part of the lining of the anterior chamber. Toward the periphery of the cornea in adults, the posterior surface of the elastic membrane presents rounded elevations, and the posterior epithelium becomes continuous with the anterior epithelium of the iris (Fig. 452). In this "angle," the cornea receives connective tissue prolongations from the iris, which form the pectinate ligament of the iris a structure highly developed in the horse and cow, but rudimentary in man.


The central vessels of the retina supply a part of the optic nerve, and the retina; the ciliary vessels supply the rest of the eye. These two sets of vessels anastomose with one another only at the entrance of the optic nerve (Fig. 467).

The ciliary arteries include (i) the short posterior ciliary arteries; (2) the long posterior ciliary arteries; and (3) the anterior ciliary arteries. The three groups will be considered in turn.

1. After supplying the posterior half of the surface of the sclera, some twenty branches of the short posterior ciliary arteries penetrate the sclera around the optic nerve. They form the capillaries of the lamina choriocapillaris. At the entrance of the optic nerve they anastomose with branches of the central artery of the retina (c) and thus form the circulus arteriosus nervi optici. At the ora serrata they anastomose with recurrent branches of the long posterior ciliary and the anterior ciliary arteries.

2. The two long posterior ciliary arteries also penetrate the sclera near the optic nerve (Fig, 467, i). They pass, one on the nasal and the other on the temporal side of the eye, between the chorioid and sclera to the ciliary body. There each artery divides into two branches which follow the ciliary border of the iris, and connect with the corresponding branches from the artery of the opposite side, thus encircling the iris with an arterial ring. This is the circulus iridis major (Fig. 467, 2), from which



numerous branches extend to the ciliary processes (3) and to the iris (4). Near the pupillary border of the iris, the arteries form an incomplete ring, the circulus iridis minor.

3. The anterior ciliary arteries proceed from those supplying the recti muscles, penetrate the sclera near the cornea, and in part join the circulus iridis major, in part supply the ciliary muscle, and in part through recurrent branches, connect with the

Branches Branches

to the to the Sinus corneal conjunctiva venosus border, bulbi. sclerae. . V > Connection with the lamina choriocapillaris.

en" 53 } ciliaris anterior.

Venous ] episcleral

I branches of the ( anterior

Arterial J ciliary vessels.

Capillaries of the lamtna choriocapillaris.

Vena vorticoia.

"^ Venous \ episcleral branches Arterial / of the short posterior ciliary vessels.


y e " a . | ciliaris posterioris brevis.


__, Outer)

v vessels of the sheath.

Inner J Short posterior ciliary arteries.

Vena Arteria centralis retina.

FIG. 467. BLOOD VESSELS OF THE EYE. (After Leber.)

The retina, optic nerve and tunica fibrosa are stippled; the tunica vasculosa is blank. V, Connection of the anterior ciliary artery with the circulus iridis major (2).

lamina choriocapillaris. Before penetrating the sclera, the anterior ciliary arteries give off posterior branches for the anterior half of the sclera, and anterior branches for the conjunctiva bulbi and the corneal border. The cornea itself is without vessels, but at its border, between the anterior lamellae of the substantia propria, there are terminal loops.


The veins generally proceed toward the equator, uniting in four (less often in 5 or 6) vents vorticosa. These pass directly through the sclera and empty into one of the ophthalmic veins. Besides the venae vorticosae there are small veins accompanying the short posterior and the anterior ciliary arteries. The short ciliary veins receive branches from the ciliary muscle, the episcleral vessels, the conjunctiva bulbi and the periphery of the cornea. The episcleral veins also connect with the venae vorticosae. Within the sclera, near the cornea, there is a circular vein, receiving small branches from the capillaries of the ciliary muscle. This sinus venosus sclerce (canal of Schlemm) connects with the anterior ciliary veins.

Arteria centralis retina. The central artery of the retina enters the optic nerve 15-20 mm. from the eye-ball, passes to its center and proceeds to the optic papilla. There it divides into two branches directed upward and downward respectively, and these by further subdivision supply the entire pars optica retinae. Within the optic nerve the artery sends out numerous little branches which anastomose with small vessels that have entered the sheaths from the surrounding fat; and also with branches of the short posterior ciliary arteries (Fig. 467, &).

The central vein of the retina receives two main branches at the optic papilla and follows the artery along the axis of the optic nerve.


The eye contains no lymphatic vessels, but is provided with communicating tissue spaces, bounded by loose cells or mesenchymal epithelia. They include the corneal and scleral canaliculi, and the anterior and posterior chambers; the latter connect with one another through the capillary interval between the lens and iris. The posterior chamber extends into the zonular spaces; and there are irregular extensions of the anterior chamber, associated with the pectinate ligament of the iris, called spaces of the angle of the iris (spaces of Fontana). The latter are but slightly developed in man. Posteriorly the tissue spaces include the hyaloid canal of the vitreous body; the very narrow perichorioideal space between the chorioid and sclera; the subdural and arachnoid spaces of the optic sheaths named the intravaginal spaces; and finally the interfascial space (of Tenon) which surrounds most of the sclera and is prolonged as a supradural space around the optic nerve. These spaces may be filled from the arachnoid space about the brain. They contain a "filtrate from the vessels." The interfascial and perichorioideal spaces hold but little fluid; acting as bursae, they facilitate the movements of the eye.


Apart from the optic nerve, the eye is supplied by the short ciliary nerves from the ciliary ganglion, and the long ciliary nerves from the naso



ciliary branch of the ophthalmic nerve. The ciliary nerves penetrate the sclera near the optic nerve and send branches containing ganglion cells to the vessels of the chorioid. The main stems pass forward between the chorioid and sclera to the ciliary body, where they form a circular ganglionated plexus, the plexus gangliosus ciliaris. Its branches extend to the ciliary body, the iris and the cornea, and are described as follows:

The nerves of the ciliary body form a delicate network on its scleral surface; they supply its muscle fibers and those of the vessels with slender motor endings; and between the ciliary muscle bundles they have branched free endings, perhaps sensory.

The medullated nerves of the iris lose their myelin and form plexuses as they pass toward the pupillary margin. A sensory plexus is found just beneath the anterior surface, and motor fibers supply the sphincter, dilator and vascular muscles. The existence of ganglion cells in the human iris is denied.

The nerves of the cornea enter it from the plexus annularis in the sclera just outside. The annular plexus also sends fibers into the conjunctiva, where they end in networks, and in bulbous corpuscles (Fig. 154, p. 160) situated in the connective tissue close to the epithelium. Such corpuscles may be found i or 2 mm. within the corneal margin. The corneal nerves become non-medullated and form plexuses between the lamellae throughout the stroma. They extend into the epithelium and there form a very delicate plexus with free intercellular endings.

Substantia \

propria. I


n, A branching nerve penetrating the anterior basal membrane; s, subepithelial plexus beneath the cylindrical cells; a, fibers of the intraepithelial plexus ascending between the epithelial cells.


The eyelids or palpebrce (Fig. 469) are covered with thin skin provided with fine lanugo hairs; small sweat glands extend into the corium, which here contains pigmented connective tissue cells. The subcutaneous tissue is very loose, having many elastic fibers and few or no fat cells. Near the edge of the lid there are two or three rows of large hairs, the eyelashes or cilia, the oblique roots of which extend deep into the corium. Since they are shed in from 100 to 150 days they occur in various stages of development. They are provided with small sebaceous glands, and the ciliary glands (of Moll) open close beside or into their sheaths. The ciliary glands are modified sweat glands, with simpler coils, which may show successive constrictions ; " a branching of the tubules has been observed."


The central portion of the eyelids is muscular. Beneath the subcutaneous tissue there are bundles of the striated orbicularis palpebrarum extending lengthwise of the lid. A subdivision of this muscle, found behind the roots of the cilia, is called the musculus ciliaris Riolani. Posterior to the obicularis muscle are found the terminal radiations of the tendon of the levator palpebra. A part of these are lost in connective tissue; another part, associated with smooth muscle fibers, are inserted into the upper border of the tarsus and form the superior tarsal muscle. This occurs in the upper lid, but correspondingly in the lower lid the radiations from the inferior rectus muscle contain smooth muscle fibers, forming the inferior tarsal muscle.

The inner portion of the lids consists of the conjunctival epithelium and the underlying connective tissue, including the tarsus. This is a plate of dense connective tissue which gives firmness to the lid. It begins at the free edges and extends over the adjacent two-thirds of the lid, close to the conjunctiva. Imbedded in its substance in either lid, there are about 30 tarsal (or Meibomian) glands, which open along the palpebral border. Each of them cnsists of a wide excretory duct, surrounded on all sides by small acini, which empty into the duct through short stalks. In structure they resemble sebaceous glands. At the upper end of the tarsus and partly enclosed in its substance, there are branched tubular accessory lachyrmal glands. They occur chiefly in the medial (nasal) half of the lid.

The tunica propria of the palpebral conjunctiva contains plasma and lymphoid cells; the latter invade the epithelium, beneath which in some animals they form nodules. The stratified epithelium of the skin gradually changes to that of the conjunctiva, which has several basal layers of cuboidal cells and a superficial layer of short columnar cells. The latter are covered by a thin cuticula, and goblet cells are found among them. The transition from the superficial squamous cells to the columnar form may occur at the posterior edge of the lid, or quite high on the conjunctival surface. Toward the arch where the palpebral conjunctiva becomes continuous with that of the bulb, the epithelium is so folded that in sections it may seem to form glands.

The conjunctiva bulbi is similar to that of the lid. Its outer epithelial cells, however, become squamous toward the cornea and over the exposed portion of the eye, and its basal cells contain pigment. The yellow appearance of the exposed portion, often most pronounced near the medial border of the cornea, and known as pinguecula, is said not to be due to fat or to an epithelial pigment; it accompanies a thickening of the connective tissue layer. The tunica propria forms well-marked papillae near the cornea. Its lymphocytes may form nodules, as many as twenty having been found in the human conjunctiva bulbi. Occasional



mucous glands occur. (It may be noted that the entire anterior covering of the bulb of the eye is named by some the conjunctiva bulbi, which accordingly is divided into the conj. sclera and the conj. cornets.}


arsal Radiations mscle. from the tendon Orbicularis

of the levator palpebrae. palpebrarum. Skin.


Tunica propria.


tarseus ezternus.



Sweat gland.

Oblique section of a hair sheath.

Cross section of the bundles of the

orbicularis palpebrarum.



Stratum subcutaneum.

Tarsal gland

Arcus tarseus

Part of a

ciliary gland.


Posterior edge of the lid. '

Musculus ciliaris (Riolani).

FIG. 469. SAGITTAL SECTION OF THE UPPER LID OF A CHILD OF Six MONTHS. The outlet of the tarsal gland was not in the plane of section. X 15.

At the medial angle of the lids there is a thin fold of connective tissue covered with stratified epithelium; this plica semilunaris is a rudimentary third lid. The nodular elevation of tissue at the medial angle, the caruncula lacrimalis, resembles skin except that a stratum corneum is lacking;



it contains fine hairs, sebaceous and accessory lachrymal glands, and in its middle part, small sweat glands.

The blood vessels of the lids proceed from branches approaching the lateral and medial angles of the eye. They form an arch, the arcus tarseus externus, at the upper border of the tarsus, and a second arcus tarseus near the free margin of the lid (Fig. 469). They extend also into the conjunctiva bulbi, and near the margin of the cornea they pass inward to unite with the anterior ciliary vessels (Fig. 467). The lymphatic vessels form a close network beneath the palpebral conjunctiva, and a loose one in front of the tarsus. Whether the lymphatic vessels of the conjunctiva bulbi end blindly toward the cornea or connect with the canaliculi, has not been determined. The nerves form a very thick plexus in the tarsus and supply the tarsal glands. There are free endings in the conjunctival epithelium, and bulbous corpuscles in the connective tissue beneath.


The lachrymal glands are groups of compound tubular glands, and are therefore provided with several excretory ducts. These are lined with a double row of epithelial ceils, the superficial layer being columnar. The

excretory ducts pass gradually into long intercalated ducts with a low epithelium. These terminate in tubules, surrounded by a membrana propria, and containing two sorts of cells. Certain cells are tall when filled with secretion, which occupies the superficial half of the cell; when empty they are shorter. The cells of the other form are low when full of secretion, which gathers in a large


A, Gland body; a, tubule cut across; a', group of tubules cut obliquely; s, intercalated tubule; s', intercalated tubule in cross section; b, connective tissue. B, cross section of an excretory ^uct; e, two-rowed cylindrical epithelium; b, connective tissue.

round mass, leaving only a

thin basal layer of protoplasm. Intercellular secretory capillaries and secretory granules have been demonstrated. Between the gland cells and the basement membrane there are occasional flat cells, which are a continuation of the deeper layer of the epithelium of the duct. The blood vessels and nerves are similar to those of the oral glands.

At the medial angle of either eye there are two lachrymal ducts which have no connection with the lachrymal glands, but serve to convey the secretions which pass across the front of the eye to the lachrymal sacs.

EYE 465

From these sacs it passes through the naso-lachrymal ducts into the nasal cavity. The lachrymal ducts are lined with stratified squamous epithelium, resting upon a tunica propria containing an abundance of cells and elastic fibers. Externally these ducts are surrounded by striated muscle fibers, chiefly longitudinal. The lachrymal sac, which is provided with small branched tubular glands, and the naso-lachrymal duct are both lined with two-rowed columnar epithelium, surrounded by a lymphoid tunica propria. They are separated from the underlying periosteum by a thick plexus of veins.


Development and General A natomy. The ear is divided into three parts : (i) the external ear, which includes the auricles projecting from the surface of the body, and the external acoustic meatus leading from the surface to the tympanic membrane; (2) the middle ear, including the tympanic cavity or "drum" and the chain of three bones extending across it; and (3) the internal ear, which is a system of epithelial ducts and surrounding tissue spaces, imbedded in the temporal bone, and connected with terminal branches of the acoustic nerve.

On either side of the body, the internal ear first appears as a local thickening of the epidermal ectoderm near that portion of the medullary tube which later becomes the pons. The thickened areas are invaginated as shown in Fig. 471 A and B, and the pockets thus produced become separated from the epidermis in the form of auditory vesicles (otocysts). The place where they become detached from the epidermis is marked by a slight elevation on the medial surface of the vesicle, which soon elongates, producing the tubular endolymphatic duct (Fig. 471, C). The blind upper end of the duct becomes enlarged to form the endolymphatic sac, which, however, is only slightly developed in man; it appears in the models of the embryonic vesicle shown in side view in Fig. 472, A-C. ID the adult the endolymphatic duct is a very slender tube, terminating blindly (or perhaps with secondary apertures) just beneath the dura.

In two places the medial and the lateral walls of the upper half of the vesicle approach one another, and after fusing, the epithelial plates thus produced become thin and rupture, so that two semicircular ducts are formed (Fig. 472, B and C). The space encircled by each duct may be regarded as a hole through the vesicle. The two ducts are the superior and posterior semicircular ducts respectively . Th e third or lateral semicircular duct forms soon afterward. In Figs. 471, D and 472, B it is a horizontal shelf-like projection of the vesicle, the center of which is to become perforated so that its rim will become the duct. The portion of the vesicle which receives the terminal openings of the three semicircular ducts is called the 30



utriculus. Since at one of their ends the superior and posterior ducts unite in a single stalk before entering the utriculus, there are but five openings for the three ducts (Fig 472, D). Near one end of each duct there is a dilatation or ampulla, where nerves terminate.


FIG. 471. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OF THE EAR. X 9. A, 9 days, 3.8 mm.; B, 10 days, 3.4 mm.; C, I2j days, 7.5 mm.; D, 14 days, 10 mm. a., Ectoderma epithelium which forms the membranous internal ear; a. bas., basilar artery; ch. t., chorda tympanj. d. c., cochlear duct; d. e., endolymphatic duct; d. s. 1., lateral semicircular duct; d. s. s., superior semi; circular duct; ep., epidermis, fa., facial nerve; meten., metencephalon; m. t. medullary- tube; phpharynx.



INTERNAL EAR FROM HUMAN EMBRYOS. Different enlargements. (After His, Jr.) A, from an embryo of 6.9 mm.; B, 10.2 mm.; C, 13.5 mm.; and D, 22 mm. am., ampulla; c T., caecum

vestibulare of d. c., cochlear duct; d. e., endolymphatic duct; d. s. 1., d. s. p., and d. s. s. lateral,

posterior, and superior semicircular ducts; sac., sacculus; ut., utriculus.

While the formation of the semicircular ducts is occurring in the upper part of the auditory vesicle, the lower portion elongates and its end becomes coiled, eventually making two and a half revolutions. The coiled



tube is the ductus cochlearis; its distal end is the cacum cupulare, and at its proximal end is the ccecum vestibulare (Fig. 472, D, c. v.). A dilated sac formed at its proximal or upper end, opposite the caecum vestibulare, is known as the sacculus; in the adult the connection between the sacculus and ductus cochleae is relatively narrow, and is called the ductus reuniens (Fig. 481). The portion of the original vesicle between the sacculus and utriculus, from which the endolymphatic duct arises, becomes a comparatively slender tube, the ductus utriculo-saccularis (Fig. 481).

The ectodermal vesicle thus produces a complex system of connected epithelial ducts, namely the superior, posterior, and lateral semicircular

Semicircular duct.

^ f^^-^^^^W~ * Epithelium of the duct.

.' {'^ : .Ivr;^^;':'^ym

Blood vessel. k t3

Wall of the semi- r^g

circular duct.

/ "" Ligament of the duct.

. Bone of the semicircuS3f / lar canal,

me '


Perilymph spaces ..

Blood vessel.


(B6hm and von Davidoff.)

ducts; the utriculus, and utriculo-saccular duct with the endolymphatic duct connected with it; the sacculus, ductus reuniens and ductus cochleae. They all contain a fluid called endolymph. The acoustic nerve sends branches between the epithelial cells in certain parts of the ducts. Round areas of neuro-epithelium, in which the nerves terminate, are called macula acustica; there is one in the sacculus and another in the utriculus. Elongated areas are crisia, and there is one in each of the three ampullae. The axis, or modiolus, about which the cochlear duct is wound, contains the nerves which send terminal fibers to the spiral organ of the adjoining epithelium. In this they form a line of terminations along the medial wall of the cochlear duct, following its windings from base to cupola.



The mesenchyma immediately surrounding the entire system of ducts becomes mucoid in appearance, and cavities lined with mesenchymal epithelium are formed within it. They contain a tissue fluid called perilymph. Around the semicircular ducts the perilymph spaces are so large that the tissue between them is reduced to strands as shown in Fig. 473 ; these are sometimes called ligaments. The perilymph spaces around the semicircular ducts are irregularly arranged and communicate with one another at various points; they connect also with the perilymph cavities of the vestibule, which is the central part of the internal ear, from which the semicircular, cocblear and endolymphatic ducts proceed outward. All of these structures are surrounded by spaces, connecting with those of the vestibule which enclose the sacculus and utriculus. At the distal



Scala vestibuli.

of the nervus


Meatus acusticus interims.


The winding ductus cochlearis, x, crossed the plane of section five times. Above it in every case is the scala vestibuli, and below it is the scata tymponi.

end of the endolymphatic duct, the spaces communicate with those of the cerebral arachnoid, and the perilymph mingles with cerebro-spinal fluid.

Around the cochlear duct the perilymph spaces form a single tube. Starting from the vestibule, it ascends to the cupola, following the windings of the cochlear duct, to which it is closely applied. It is known as the scala vestibuli (i.e., "staircase of the vestibule," from which it passes out) At the apex of the cochlea it turns and becomes the descending scala tympani, which ends blindly at the base of the cochlea, close against the wall of the tympanum. The two scalae bear a constant relation to the coils of the cochlear duct. If the cochlea is so placed that its apex is upward, the scala vestibuli is always found on the upper side of the duct, and the scala tympani on the lower side, as shown in Fig. 474. In the body, the apex of the cochlea is directed forward and outward.

The temporal bone develops from the mesenchyma surrounding the

EAR 469

ducts and their perilymph spaces, so that when the membranous labyrinth which they form, is removed by maceration, the bone still contains a corresponding arrangement of cavities and canals. These constitute the bony labyrinth. Casts of it, made in soft metal, may be seen in all anatomical museums. Instead of subdivisions to correspond with the utriculus, sacculus, and utriculo-saccular duct, the bony labyrinth has a single space, already referred to as the vestibule. Into it the semicircular and cochlear canals open, together with the aquaductus vestibuli which contains the endolymphatic duct.

The middle ear and external ear arise in connection with the first or spiracular gill cleft. In common with the other clefts, this includes an entodermal pharyngeal outpocketing (Fig. 206, p. 217) and an ectodermal depression (Fig. 205, sp.}. At an early stage these meet one another and fuse, but later, the primary epithelial connection breaks down, and mesenchyma intervenes. In the adult, however, the two parts are still close together, being separated by only the drum membrane, which is covered on one side with ectoderm and on the other with entoderm.

The ectodermal groove becomes surrounded by several nodular elevations of skin, which coalesce in a definite manner to make the projecting auricle (pinna). Its depression deepens, becoming the external acoustic meatus, which extends inward to the tympanic membrane. The entodermal portion of the spiracular cleft becomes in the adult an elongated outpocketing of the pharynx, known as the auditory tube (Eustachian tube). As seen in the section Fig. 475, the tube is separated from the bottom of the meatus by a very thin layer of mesenchyma, which is later included in the drum membrane.

In the mesenchyma behind the spiracular cleft, a chain of three small bones (the malleus, incus, and stapes} develops; it extends from the meatus to the vestibule. The bony wall of the vestibule is deficient at the small oval area where the stapes reaches it, so that the chain of bones comes directly in contact with the fibrous covering of the perilymph space. This area of contact is thejenestra veslibuli (i.e., window of the vestibule). When the chain of bones vibrates back and forth, the motion of the stapes is transmitted through the fenestra vestibuli to the perilymph, and waves may pass up the scala vestibuli and down the scala tympani, stimulating the nerves of hearing in the cochlear duct. The blind termination of the scala tympani rests against the lateral wall of the vestibule, where also the bone fails to develop; the round fenestra cochlea is thus produced. Its fibrous membrane may yield somewhat to the perilymph waves, thus relieving tension in the cochlea.

In Fig. 475 the fragments of the chain of bones together with neighboring nerves are imbedded in a mass of mesenchyma. In a later stage the outer end of the auditory tube expands, filling all the space between



the vestibule and the bottom of the meatus. Thus it forms the tympanic cavity. It encounters the chain of bones and the chorda tympani, and wraps itself around them so that they lie in its folds or plica. Thus all structures which extend into the tympanic cavity, or appear to cross it, are covered with a layer of entodermal epithelium derived from the auditory tube. The original contact between the ectoderm and entoderm of the spiracular cleft forms only an insignificant part of the tympanic membrane. The latter becomes greatly enlarged, extending somewhat along the upper surface of the ectodermal auditory meatus. The portion of the malleus lying near it becomes imbedded in its mesenchymal layer,


d.c.' /:;.;:.;.

FIG. 475. HORIZONTAL SECTION THROUGH THE EAR OF A HUMAN EMBRYO OF ABOUT 5 CMS. an., Auricle; au.t., auditory tube; ch.t., chorda tympani; d.c., cochlear duct; d.s.l., and d.s.p., lateral and posterior semicircular ducts; e.a.m., external acoustic meatus; fa., facial nerve; f.c., fenestra cochlea; p.s., perilymphatic space; St., stapes;, transverse sinus; t.b., temporal bone.

and its inner entodermal layer is made by the expansion of the tympanic cavity. The enlargement of the tympanic cavity continues after birth, when it invades the spaces formed within the mastoid part of the temporal bone.

In spite of these modifications the course of the spiracular cleft is retained in the adult. The ectodermal depression and its surrounding elevations constitute the external ear; the pharyngeal outpocketing persists as the auditory tube and the tympanic cavity of the middle ear. It opens freely into the pharynx and contains air.


The walls of all these structures consist of three layers. On the outside there is connective tissue with many elastic fibers and occasional pig

EAR 471

ment cells. This is followed by a narrow basement membrane said to form small nodular elevations toward the third and innermost layer, the simple flat epithelium. Near the maculae and cristae the connective tissue and the basement membrane become thicker, and the epithelial cells are columnar with a cuticular border. In the neuro-epithelium of these areas there are two sorts of cells, sustentacular and hair cells. The sustentacular or fiber cells extend clear across the epithelium and are somewhat expanded at both ends; they contain oval nuclei. Hair cells, which receive the stimuli, are columnar cells limited to the superficial half of the epithelium; they have large spherical nuclei near their rounded basal ends, and a clump of fine agglutinated filaments projecting from their free surface. The nerves lose their myelin as they enter the epithelium and ascend to the bases of the hair cells. There **

they bend laterally, forming a dense network which ^

appears as a granular layer in ordinary preparations; ^ ^

the granules are optical sections and varicosities. ^y*

The horizontal fibers terminate like their occasional F IG . 47 6. OTOCONIA branches, by ascending between the hair cells, on the LUSO/AN INFANT" sides of which they form pointed free endings. They do not reach the free surface of the epithelium. This surface is covered by a continuation of the cuticula, a "membrana limitans," which is perforated by the hairs. Over the two maculae there is a soft substance containing very many crystals of calcium carbonate, 1-15 ju long, which are named otoconia. (Large "ear stones" of fishes are called otoliths.) Over the cristae of the semicircular ducts there is a gelatinous substance, transparent in fresh preparations, but coagulated and rendered visible by reagents.

The "ligaments" of the ducts, the thin periosteum of the bony semicircular canals, and the perilymph spaces lined with mesenchymal epithelium are seen in Fig. 473.


The relation between the ductus cochleae and the scalae tympani and vestibuli is shown in Fig. 474. The ductus is triangular in cross section, being bounded on its peripheral surface by the thick periosteum of the bony wall of the cochlea; on its apical surface (toward the cupola) by the membrana vestibularis (Reissner's membrane) ; and on its basal or medial surface by the lamina spiralis. These three walls may be described in turn.

The peripheral wall of the cochlear duct is formed by the dense fibrous periosteum attached to the bone, together with a large mass of looser tissue crescentic in cross section, the ligamentum spirale (Fig. 477). The spiral ligament is covered by a layer of cuboidal epithelial cells belonging



to the cochlear duct. Close beneath the epithelium there are blood vessels which are said to give rise to the endolymph. The thick plexus which they form is described as a band, the stria vascularis, which terminates more or less distinctly with the vas prominens. The latter occupies a low elevation of tissue which has its maximum development in the basal coil of the cochlea (Fig. 477).

The apical wall, or membrana vestibularis, consists of a thin layer of connective tissue bounded on one side by the mesenchymal epithelium of the scala vestibuli, and on the other by the simple flattened ectodermal epithelium of the cochlear duct.

Blood vessels.



Ganglion spirale.

Scala vestibuli.

Ductus cochlearis.

Vas prominens

Ligamentum spirale.

Scala tympani

Lamina spiralis ossea

Lamina spiralis membranacea.


The basal wall or lamina spiralis extends outward from the modiolus to the bony wall of the cochlea. Near the modiolus it lies between the two scalae, but peripherally it is between the cochlear duct and the scala tympani. Toward the modiolus it contains a plate of bone perforated for the passage of vessels and nerves; this part is the lamina spiralis ossea. The peripheral portion is the lamina spiralis membranacea. Both parts are covered below by the mesenchymal epithelium of the scala tympani, and above by the epithelium of the cochlear duct, including its complex neuro-epithelium known as the spiral organ (of Corti).

Where the membrana vestibularis meets the osseous spiral lamina, there is an elevation of tough connective tissue called the limbus spiralis (Fig. 477). It consists of abundant spindle-shaped cells, and blends below with the periosteum of the spiral lamina. Superficially it produces irregularly



hemispherical papillae covered with simple flat epithelium, found within the cochlear duct near the vestibular membrane. Further within the cochlear duct the papillae give place to a single row of flat ridges or plates, directed peripherally. These are "Huschke's auditory teeth" (Fig. 480). Beneath them the limbus terminates abruptly in an overhanging labium vestibulare, which projects over an excavation the sulcus spiralis (Fig. 478). The basal wall of the sulcus is the labium tympanicum, found at the peripheral edge of the osseous spiral lamina. As the epithelium of the limbus passes over the labium vestibulare into the sulcus, it becomes cuboidal. A remarkable non-nucleated structure projects from the labium

Membrana tectoria.

Capillaries of the

Labium vestibulare.

Nerve bundle.


Deiter's Membrana Connective

- cells. basilaris. tissue.

Pillar cells. X 240. x, Intercellular "tunnel" traversed by nerve fibers.

vestibulare over the neuro-epithelium of the membranous spiral lamina. It is called the membrana tectoria and is considered to be a cuticular formation of the labial cells to which it is attached. Hardesty describes it as composed of "multitudes of delicate fibers of unequal length, embedded in a transparent matrix of a soft, collagenous semi-solid character, with marked adhesiveness" (Amer. Journ. Anat., 1908, vol. 8, pp. 109-179). The lamina spiralis membranacea, or lamina basilaris, consists of four layers. The mesenchymal epithelium of the scala tympani is followed by a layer of delicate connective tissue, prolonged from the periosteum of the scala. Its spindle cells are at right angles with the fibers of the overlying membrana basilaris. This membrane, which is beneath the epithelium of the cochlear duct, consists of coarse straight fibers extending from the labium tympanicum to the ligamentum spirale. They cause it to appear finely striated (Fig. 479). Peripherally (beyond the bases of the outer pillar cells) the fibers are thicker, and are called "auditory strings"; they are


shortest in the basal part of the cochlea and longest toward the apex, corresponding in length with the basal layer of the cochlear duct. These fibers have been thought to vibrate and assist in conveying sound waves to the nerves, but theories which assume that the basilar membrane is a "vibrating mechanism" are considered untenable by Hardesty; he finds it more probable that the membrana tectoria vibrates and transmits stimuli to the neuro-epithelium.

The epithelial cells covering the basilar layer occur in rows of highly modified forms, which extend up and down the cochlear. duct, constituting the spiral organ (organ of Corti). Next to the cuboidal epithelium of the sulcus spiralis there is a single row of inner hair cells (Fig. 480). These are short columnar cells which do not reach the bottom of the epithelium; each has about forty long stiff hairs on its free surface. The inner hair cells are followed peripherally by two rows of pillar cells, the inner and outer, which extend the whole length of the cochlear duct. As seen in cross section F IG . 479. SURFACE they are in contact above, but are separated below by

VIEW OF THE . . , ., , . - .

LAMINA SPIRALIS a triangular intercellular space or tunnel, which is


A CAT. x 240. filled with soft intercellular substance. Thus they rest

Drawn with

change of focus. upon the basilar membrane in A -form. Each pillar

e, Epithelium ( cells of r ,,..,,. 11 , j ,

Claudius) of the cell may be subdivided into a head, a slender body,

ductus cochleans J

o" thTmembrana an ^ an ex P an< ied triangular bas'e. The greater portion

bf s nuc?ei n of oc the ^ eac ^ ce ^ nas been transformed into a resistant band,

nec d t!ve y t^ue con " at the base of which, within the tunnel, there is a mass

of protoplasm containing the nucleus. A protoplasmic

sheath extends up from the base around the body of the cell. Dark round

structures which may be found in the heads of the pillars, and at the

foot of the outer ones, are not nuclei, but are "probably of horny nature."

The heads of the pillars interlock. Both pillars produce "head-plates"

directed outward, and so arranged that the plate from the inner pillar

overlies that from the outer pillar (Fig. 480). Moreover, the round head

of the outer pillar is fitted into a concavity in the head of the inner pillar,

as shown in the figure.

On the peripheral side of the outer pillars there are several rows (usually four) of outer hair cells separated from one another by sustenlacular cells (Deiter's cells). The outer hair cells have shorter hairs than the inner ones, and are characterized by the presence of "Hensen's spiral bodies," one of which occurs in the outer half of each cell. These bodies, shown as dark spots in Fig. 480, probably represent a trophospongium. The centrosomes of the hair cells are always in their upper ends. Like the inner hair cells, the outer ones do not extend to the basilar membrane, thus leaving unoccupied the communicating intercellular spaces between



the deeper portions of the sustentacular cells. These Nuel's spaces connect with the tunnel.

Deiter's sustentacular cells are slender bodies, each containing a stiff filament, and having at its free end a cuticular formation referred to as a "phalanx." The phalanges come between the outer hair cells, separating them from one another (Fig. 480), and the inner hair cells are similarly separated by short processes the inner phalanges, derived from the inner pillars. (The inner phalanges are not shown in the figure.) The phalanges of Deiter's cells connect with one another, forming a trim reticular membrane. As a whole Deiter's cells resemble the pillar cells, but their transformation into stiff fibers has not proceeded so far; the cuticular border is comparable with the head plate.


Tun- | \IT Membrana Tympanal

nel. Nuel's Deiter's basilaris. lamella.

Vas spirale space. cells. Inner Outer

Pillar cells.

FIG. 480. DIAGRAM OF THE STRUCTURE OF THE BASAL WALL OF THE DUCT OF THE COCHLEA. A, View from the side. B, View from the surface. In the latter the free surface is in focus. It is evident that the epithelium of the sulcus spiralis. lying in another plane, as well as the cells of Claudius, can be seen distinctly only by lowering the tube. The membrana tectoria is not drawn. The spiral nerves are indicated by dots.

The most peripheral of the sustentacular or Deiter's cells are followed by elongated columnar cells (cells of Hensen), which gradually shorten, and are succeeded by the low "cells of Claudius" which extend to. the limit of the membrana basilaris. In both the columnar and the low forms there are single stiff filaments which are less developed than in the sustentacular cells. The centrosomes of all these cells lie near their free surfaces. Beyond the basilar membrane the epithelium is continued over the ligamentum spirale as a layer of cells with branching basal processes extending deep into the underlying tissue.



The acoustic nerve is a purely sensory nerve passing between the pons and internal ear through a bony canal, the internal acoustic meatus. It is divided into vestibular and cochlear portions (Fig. 474). The vestibular nerve proceeds from the vestibular ganglion and has four branches the utricular nerve and the superior, lateral, and posterior ampullary nerves; according to Streeter (Amer. Journ. Anat., 1906, vol. 6, pp. 139-165) it produces also the branch to the saccuhis, usually regarded as derived from the cochlear nerve. If this is true, the cochlear nerve supplies only the spiral organ of Corti. The ganglion of the cochlear nerve is lodged within the modiolus at the root of the lamina spiralis, and is known as the spiral ganglion (Figs. 474 and 477). The ganglion cells remain bipolar, like those of embryonic spinal ganglia. They are surrounded by connective tissue capsules; and their neuraxons and single peripheral dendrites receive myelin sheaths not far from the cell bodies.

The peripheral fibers extend through the lamina spiralis ossea, within which they form a wide-meshed plexus, and after losing their myelin they emerge from its outer border in the labium tympanicum through the foramina nervosa. In continuing to the spiral organ they curve in the direction of the cochlear windings, thus producing spiral strands. Those nearest the modiolus are on the axial side of the pillar cells; the middle ones are between the pillars, in the tunnel; and the outer ones are beyond the pillar cells. From these bundles, delicate fibers pass to the hair cells, on the sides of which they terminate.


The internal auditory artery is a branch of the basilar artery. It arises in connection with branches which are distributed to the under side of the cerebellum and the neighboring cerebral nerves, and passes through the internal acoustic meatus to the ear. It divides into vestibular and cochlear branches (Fig. 481). The vestibular artery supplies the vestibular nerve and the upper lateral portion of the sacculus, utriculus and semicircular ducts. The cochlear artery sends a vestibulo-cochlear branch to the lower and medial portion of the sacculus, utriculus, and ducts. This branch also supplies the first third of the first turn of the cochlea. The capillaries formed by the vestibular branches are generally wide meshed, but near the maculae and cristae the meshes are narrower. The terminal portion of the cochlear artery enters the modiolus and forms three or four spirally ascending branches. .These give rise to about thirty radial branches distributed to three sets of capillaries (Fig. 482); i, those to the spiral ganglion; 2, those to the lamina spiralis; and 3, those to the outer walls of the scalae and the stria vascularis of the cochlear duct.



The veins of the labyrinth form three groups (Fig. 481). i. The vena aqu&ductus vestibuli receives blood from the semicircular ducts and a part of the utriculus. It passes toward the brain in a bony canal along with the ductus endolymphaticus, and empties into the superior petrosal sinus. 2. The vena aquaductus cochlea receives blood from parts of the utriculus, sacculus and cochlea; it passes through a bony canal to the internal jugular vein. Within the cochlea it arises, as shown in Fig. 482, from

Ductus semicircularis superior.

Ampulla lateralis.

f Arteria vestibulans. [ Arteria cochlearis.

/ I

Superior Inferior


\ Posterior

Vena spinalis.

Vena vestibularis.

Ampulla posterior.

Ductus semicircularis posterior.

Arteria cochlearis.


branch of the arteria cochlearis.

Vena aquseductus cochlea.

PIG. 481. DIAGRAM OF THE BLOOD VESSELS OF THE RIGHT HUMAN LABYRINTH. MEDIAL AND POSTERIOR ASPECT. D, c., Ductus cochlearis; S., sacculus; U., utriculus; i, ductus reuniens; 2, ductus utriculo-saccularis. The saccus

endolymphaticus is cut off.

small vessels including the vas prominens (a) and the vas spirale (6). Branches derived from these veins pass toward the modiolus. (There are no vessels in the vestibular membrane of the adult, and the vessels in the wall of the scala tympani are so arranged that only veins occur in the part toward the membranous spiral lamina; thus the latter is not affected by arterial pulsation.) Within the modiolus the veins unite in an inferior spiral vein, which receives blood from the basal and a part of the second turns of the cochlea, and a superior spiral vein which proceeds from the



apical portion. These two spiral veins unite with vestibular branches to form the vena aquaeductus cochleae (Fig. 481). 3. The internal auditory vein arises within the modiolus from the veins of the spiral lamina; these anastomose with the spiral veins (Fig. 482). It receives branches also from the acoustic nerve and from the bones, and empties "in all probability, into the vena spinalis anterior."

Lymphatic spaces within the internal ear are represented by the perilymph spaces, which communicate through the aquaeductus cochleae with

Scala tympani. Scala vestibuli.

Stria vasculars.


Cross section of a spiral artery of the modiolus.

--Vena laminae spiralis.

Ganglion spirale.

Vena spiralis superior.

Cross section;of a spiral artery of the modiolus.

Vena laminae spiralis. Anastomosis.

Vena spiralis inferior.


the arachnoid space; the connecting structure, or "ductus perilymphaticus," is described as a lymphatic vessel. The saccus endolymphaticus, which is the dilated distal end of the endolymphatic duct, is in contact with the dura, and there are said to be openings between it and the subdural space. In the internal ear perivascular and perineural spaces are found, and they probably connect with the arachnoid spaces.


The tympanic cavity, which contains air, is lined with a mucous membrane closely connected with the surrounding periosteum. It consists of



a thin layer of connective tissue, covered generally with simple cuboidal epithelium. In places the epithelial cells may be flat, or tall with nuclei in two rows. Cilia are sometimes widely distributed and are usually to be found on the floor of the cavity. In its anterior part, small alveolar mucous glands occur very sparingly. Capillaries form wide-meshed networks in the connective tissue, and lymphatic vessels are found in the periosteum.

The auditory tube includes an osseous part toward the tympanum, and a cartilaginous part toward the pharynx. Its mucosa consists of fibrillar connective tissue, together with a ciliated columnar epithelium which



M ucosa of the pharynx.


Glands. ---ZIZ~


X 12.

becomes stratified as it approaches the pharynx. The stroke of the cilia is toward the pharyngeal orifice. In the osseous portion, the mucosa is without glands and very thin; it adheres closely to the surrounding bone. Along its floor there are pockets containing air, the cellules pneumatics. In the cartilaginous part the mucosa is thicker; near the pharynx it contains many mucous glands (Fig. 483). Lymphocytes are abundant in the surrounding connective tissue, forming nodules near the end of the tube, which blend with the pharyngeal tonsil. The cartilage, which only partly surrounds the auditory tube, is hyaline near its junction with the bone of the osseous portion; it may contain here and there coarse fibers which are not elastic. Toward the pharynx the matrix contains thick nets of elastic tissue, and the cartilage is consequently elastic.




Between the middle ear and the external ear is the tympanic membrane, which consists, from without inward, of the following strata: the cutaneum, radiatum, circular e and mucosum (Fig. 484). The stratum cutaneum is a thin skin without papillae in its corium, except along the handle or manubrium of the malleus. There it is a thicker layer, containing the vessels and nerves which descend along the manubrium and spread from it radially. In addition to the venous plexus which accompanies the artery in that situation, there is a plexus of veins at the periphery of the membrane, receiving tributaries from both the stratum cutaneum and the less vascular stratum mucosum. The radiate and circular strata consist of compact bundles of fibrous and elastic tissue, arranged so as to suggest tendon. The fibers of the radial layer blend with the perichondrium of the hyaline cartilage covering the manubrium. Peripherally the fiber layers form a fibro-cartilaginous ring which connects with the surrounding bone. The stratum mucosum is a thin layer of connective tissue covered with a simple non-ciliated flat epithelium



a, Stratum cutaneum (showing _the corneum and germinativum) ; b, stratum radiatum, its fibers cut across; c, stratum circulare; d, stratum mucosum.

Hair sheath. Corium

Excretory duct

Young hair

Coil of ceruminous gland .


Membrana propria.

Nuclei of smooth muscle fibers.


Gland cells.


, Cuticular border. . Gland cells. Nuclei of smooth muscle

fibers. ' Membrana propria.


A, Cross section, from an infant; B, longitudinal section, from a boy 1 2 years old.

continuous with the lining of the tympanic cavity. Peripherally, in children, its cells may be taller and ciliated. As a whole the tympanic membrane is divided into tense and flaccid portions. The latter is a relatively small upper part in which the fibrous layers are deficient.

EAR 481

The external acoustic meatus is lined with skin continuous with the cutaneous layer of the tympanic membrane. In the deep or osseous portion the skin is very thin, without hair or glands except along its upper wall. There and in the outer or cartilaginous part, ceruminous glands are abundant. "They are branched tubulo-alveolar glands" (Huber) which in many respects resemble large sweat glands. Their ducts are lined with stratified epithelium. The coils consist of a single layer of secreting cells, general cuboidal, surrounded by smooth muscle fibers and a well-defined basement membrane. They differ from sweat glands in that their coils have a very large lumen, especially in the adult; and their gland cells, often with a distinct cuticular border, contain many pigment granules and fat droplets. Their narrow ducts in adults end on the surface of the skin close beside the hair sheaths; in children they empty into the sheaths (Fig. 485). It has not been shown that the ceruminous glands are more directly concerned in the production of cerumen than the sebaceous glands. The cerumen obviously is an oily rather than a watery secretion, and it contains fatty cells and pigment.

The cartilage of the external acoustic meatus and of the auricle is elastic.


The nasal cavities are formed by the invagination of a pair of epidermal thickenings similar to those which give rise to the lens and auditory vesicle. The pockets thus produced in the embryo are called "nasal pits" (Fig. 205, n, p. 216). Their external openings remain as the nares of the adult, but temporarily, from the third to the fifth month of embryonic life, they are closed by an epithelial proliferation. Each nasal pit acquires an internal opening, the choana, in the roof of the pharynx. The choanae are at first situated near the front of the mouth, separated from one another by a broad nasal septum (Fig. 487). As the latter extends posteriorly, it is joined by


the Palate processes which grow toward it from MOUTHOF A HUMAN EMBRYO

OF 8 WEEKS. X 4. (After

the sides of the maxillae. Thus the choanae re- Koiimann.)

na, Nans; ch., choana; al. p., i. p.,

Cede tOWard the back Of the mOUth While and pa. p., alveolar, intermax illary, and palate processes.

the embryonic condition of cleft palate is being

removed (Fig. 488). The lateral walls of the nasal cavities produce three curved folds one above another; they are concave below, and in them the concha (turbinate bones) develop. The nasal mucosa covers these and extends into excavations in the adjacent bones, forming the sphenoid, 31



maxillary, and frontal sinuses, and the ethmoidal cells. The boundary between the epithelium of the nasal pit and that of the pharynx early disappears, and the extent of each in the adult is uncertain. Presumably the olfactory neuro-epithelium is derived from the nasal pit. In man the olfactory region is limited to the upper third of the nasal septum and nearly the whole of the superior concha (Read). This regio olfactoria is covered by a yellowish-brown membrane, which may be distinguished macroscopically from the reddish mucosa of the regio respiratoria. The latter includes the remainder of the nose. The two regions may be considered in turn.

The vestibule, or cavity of the projecting cartilaginous portion of the nose, is a part of the respiratory region which is lined with a continuation

Cartilaginous nasal septum.

Dental ridge

of the upper iaw.

Oral epithelium.

Dental ridge

of the lower jaw.

Nasal cavity.


Oral cavity.




The palate processes have united with the nasal septum. The conch are developing along the lateral walls of the nasal cavity. In the lower part of the nasal septum the vomero-nasal organs are seen as a pair of tubes, each of which is partly surrounded by a crescentic cartilage.

of the skin. Its stratified epithelium has squamous outer cells and rests upon a tunica propria with papillae. It contains the sheaths of coarse hairs (vibrissa) together with numerous sebaceous glands. The extent of the squamous epithelium is variable; frequently it is found on the middle concha, less often on the inferior concha.

The remainder of the respiratory mucosa consists of a pseudo-stratified epithelium with several rows of nuclei. It may contain few or many goblet cells. The tunica propria is well developed, being even 4 mm. thick on the inferior concha (Fig. 489). It consists of fibrillar tissue with many elastic elements, especially abundant in its deeper layers. Beneath the'epithelium, it is thickened to form a homogeneous membrana propria,



perforated with small holes. Lymphocytes are present in variable quantity, sometimes forming solitary nodules and often entering the epithelium in great numbers. Branched alveolo-tubular mixed glands extend into the tunica propria. Their serous portions have intercellular secretory capillaries, and both mucous and serous cells contain a trophospongium. The glands often empty into funnel-shaped depressions, which are macroscopic on the inferior concha, and are lined with the superficial epithelium.



FIG. 489. VERTICAL SECTION THROUGH THE MUCOSA OF THE INFERIOR CONCHA OF MAN. X 48. On the left is a funnel-shaped depression receiving an excretory duct; near-by on the right is the section of a large vein.

The mucosa of the several paranasal sinuses is thin ( o.o2mm.), with less elastic tissue and but few small glands. A pocket which extends into the lower part of the median septum has already been described as the vomero-nasal organ (Jacobson's organ). In man it is the rudimentary remnant of an important sense organ, supplied by special branches of the olfactory nerves and by the nervus terminalis (cf. p. 141). It is lined with a tall columnar epithelium, and contains, at least in the



cat, "sensory cells apparently identical with those of the olfactory mucosa." In man sensory cells are said to be lacking in the adult and in embryos older than five months.

In the regio olfactoria the mucosa includes a tunica propria and an

olfactory epithelium. The latter consists of sustentacular cells and olfactory cells. The superficial halves of the sustentacular cells are cylindrical, and contain yellowish pigment, together with small mucoid granules often arranged in vertical rows (Fig. 490) . The more slender lower halves have dentate or notched borders, and branched basal ends which unite with those of neighboring cells, thus forming a protoplasmic network. Their nuclei, generally oval, are in one plane and in vertical sections they form a narrow "zone of oval nuclei" (Fig. 491). The olfactory cells generally

have round nuclei containing nucleoli. They occur at different levels and so form a broad "zone of round nuclei." From the protoplasm


st, Supporting cells; s, extruded mucus resembling cilia; I, olfactory cells, from r' the lower process has been torn off; f, ciliated cell; b, cells of olfactory glands.

Excretory duct

Wandering cell. Mucus.


rT.y _ Pigment


Oval nucleus of

a sustentacular


Round nucleus of an olfactory cell. Basal celL



Sections of olfactory glands.


Dilated duct. Mucus.

which is gathered immediately about the nucleus, each olfactory cell sends a slender cylindrical process toward the surface, where it ter

NOSE 485

minates in a variety of ways. It may end in a small knob-like swelling, or in a single slender spine; sometimes the terminal knob sends out a small cluster of divergent olfactory hairs or spines. Basally the olfactory cells pass directly into the axis cylinders of the olfactory nerves (Fig. 492). Thus they are ganglion cells, their basal processes being neuraxons. Cells intermediate between the olfactory and sustentacular forms may be found, and these are doubtless imperfectly developed sensory cells. At the free surface of the olfactory epithelium there are terminal bars, and small projecting strands of mucus, sometimes suggesting cilia (Fig. 490, s).

Central ependymal cells.

Fibers of the olfactory tract.

Mitral cells.


Olfactory nerves.

Olfactory fibers in

the nasal mucous


Olfactory cells.

PIG. 492. CHIEF ELEMENTS OF THE OLFACTORY BULB. (Gordinier, after Van Gehuchten.)

The mucus, which is the product of the sustentacular cells, may appear to form a continuous superficial membrane (Fig. 491). Near the tunica propria there is a network of so-called "basal cells" (Fig. 491).

The tunica propria is composed of fibrous tissue and fine elastic fibers, associated with many connective tissue cells. In some animals (for example, the cat) it forms a structureless membrane next to the epithelium. It surrounds the numerous olfactory glands (Bowman's glands). In man these consist of excretory ducts extending through the epithelium, and of branching gland bodies beneath. They have the appearance of serous glands, but sometimes contain mucus, generally in small quantities. They are found not only in the olfactory region, but also in the adjoining part of the respiratory region.


The deeper layers of the tunica propria contain the arteries of the mucous membrane, which send branches toward the epithelium, and form a thick sub-epithelial plexus of capillaries. The veins are very numerous, especially at the inner end of the inferior concha, where the tunica propria resembles cavernous tissue. Lymphatic vessels form a coarse meshed network in the deeper connective tissue. Injections of the arachnoid spaces around the olfactory bulbs follow the perineural sheaths of the olfactory nerves into the nasal mucosa, but these tissue spaces are not lymphatic vessels.

The olfactory nerves, as already stated, are formed of the basal processes of the olfactory epithelial cells, which become non-medullated nerve fibers. This is a primitive type of nervous apparatus (cf. p. 132), such as is not found elsewhere in the human body. After a tangential course beneath the epithelium, the fibers unite in bundles, and pass through the cribriform plate of the ethmoid bone to the olfactory bulb just above it, which they enter. They spread tangentially and branch, finally terminating in the glomeruli. The glomeruli are round or oval groups of arborizing fibers, in which the processes of the olfactory cells end in relation with the dendrites of the mitral cells. The latter are nerve cells with triangular bodies, which form a characteristic layer of the olfactory bulb, and send their neuraxons through the olfactory tracts to make various connections within the hemispheres.

In addition to the olfactory nerves, the nasal mucous membrane contains medullated branches of the trigeminal nerve, distributed both to the olfactory and respiratory regions.

Next : Part II. Microscopical Technique 2.1. The Preparation of Microscopical Specimens

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

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

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