Book - Stoehr's Histology 1-3

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Lewis FT. Stoehr's Histology. (1906) P. Blakiston's Son & Co., Philadelphia.

Stoehr's Histology 1906: 1 Microscopic Anatomy | 1-1 Cytology | 1-2 General Histology | 1-3 Special Histology | 2 Preparation of Specimens | Figures | Histology | Embryology History
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Contents

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 ftrT 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 tnyelocytes (m arrow cells). jj [n ordinary sections the marrow appears as a compact tissue of small cells 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. 174). The reticular .framework of the marrow consists of flattened cells generally seen cut across ; their nuclei then appear slender and elongated. The abundant mesh work of fibrils associated with these cells is not apparent in ordinary sections.^n the meshes are found giant cells; premyelocytes; myelocytes which are neutrophilic^ basophilic or eosinophilic; erythrocytes; lymphocytes; and mature corpuscles both red and whft^^ The giant cells of the marrow have a single polymorphous nucleus. They have been named therefore * megakaryocytes,' in distinction from the multinucleate osteoclasts or * polykarjocytes.' 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.



Fig. 174. Human Bone Marrow. Eosinophilir myelocyte ; e-b., erylhroblasl ; e-C, cr>throc>lc ; f. c, pari of the protoplasmic rim of a fat cell ; g. c., giant cell ; mv., ricutrophylic myelocyte; n-b., normoblast ; pm., premyelocyte ; r., reticulai tissue cell.

It has been said that the latteris divisible into two concentric zones, which differ from the protoplasm within the nuclear sphere. In ordinary preparations these details are not evident. A large number of centrosome granules (over one hundred) has 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 orig;in is unknown, but is said to be from the leucocyte series of cells. CCfe^ important function^f producing blood plates has but recently been estabUshed (see page 150). J

Premyelojjjg^are cells with large round vesicular nuclei containing one or two coarse chromatin masses, and surrounded hv |^p-<iir pi-ft^r^pljiywj free from specific granule s. It is possible that these cells are parents of myelocytes.

Myelocytes are cells larger than polymorphonuclear leucocytes, having roun^orcrescentic nuclei and protoplasm containing a var)dng 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 inter\'ene 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 rhast 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. 171, p. 147.

^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.

L- , ^ 17np^^7Yt^T iF^ 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 o f ^eat


interest . It has been thought that the endothelium blends with the retic ulum so that no sharp .distinction can be made between the two. It seems more probable that the endothelium is merely more permeable thjm usuaL ^ a frtftr fifparntinn rf its.cclk; The same problem is presented by th6J blood vessels and reticular tissue of the l}Tn£h^landsand spleen^ I

(T ?he f unctio ns of t he marrow are the production and dissolution of the / bone, me storing of fat, the formation of granular leucocytes (neutrophiles, eosinophiles, and mast cells), of red corpuscles, and to a less extent ofi lymphocytes; to these some would add the destruction of red corpuscles a$ indicated by ingested fragments and intercellular granules.


(^Such marrow as has just been described is called red marrow. It ocaJrsin the bones of embryos and persists in the flat bones of the adult, — those of the skull, the bodies of the vertebrae, the ribs and sternum, the epiphyses, and the heads of the humerus and femur. The shafts of the long bones contain yellow marrow which resembles ordinary fat tissue.} B^tjyeen the fat cells an occasional plasma cell or myelocyte may ocair. /Yellow marrow is formed from red by the 4evelopment of true fat cells aHd not by fatty degeneration of myelocytes. ; In disease it may resume its blood forming function and become red. In starvation it becomes mucoid like other fat tissue.


Lymph nodules and Lymph glands

Lymph glands, haemolymph glands, and the spleen have a similar origin m the embryo. Xhex.are at first small dense areas of mesenchyma developing near blood and lyn^)hatic vessels. The blood vessels extend into these areas producing a notch on one side of the mass, known as the hilus. Here in the adult the arteries enter and veins leave. After the invasion of the blood vessels the dense tissue is tranformed into reticular tissue containing lymphocytes. The lymphocytes occur especially in that part which surrounds the arteries. The veins tend to be at the periphery of the compact lymphoid tissue surrounding the arteries and to be associated with a portion of the reticulum which is comparatively free from lymphocytes. Lymphatic vessels spread over the surface and into the substance of the lymph glands, but^they are absent from haemolymph g lands an d from the spleen.

Lymph glands (also called lymph nodes) in early stages of development are shown in Fig^iys, the left half of which represents a younger stage than the right. ^ The left portion shows a mass of reticular tissue and lymphocytes penetrated by an artery and a vein which join through cagUlaries.} It is surrounded by a network" of lympHatuTTessels some of which are afferent and others, toward the hilus, are efferent. Such structures occurring in the adult are called solitary nodules [follicles]. (They are abundant in the walls of the intestine and respiratory tubes., > Each is an area of l)rmphocyte production characterized by crowded nuclei which stain deeply with haematoxylin. Under low magnification the nodule appears as a mass of dark granules (Fig. 244, p. 216) in the center of which a lighter area is sometimes seen, the germinalive center. Here the cells are larger, resembling the large mononuclear leucocytes of the blood, and are frequently found in mitosis. They are thought to give rise to lymphocytes. The reticular tissue, which is concealed by the cells


in its meshes, forms a coarser net in the geraiinative centers than in the peripheral part of the nodule. Blood vessels within the nodule are inconspicuous and the surroundmg lymphatic vessels are sometimes absent.


Fig. 175.


Fig. 176. Diagrams Representing Four Stages in the Development of Lymph Glands.

^Certain of the solitary nodules are merely transient local accumulations of lymphocytes which are diffusely distributed in the layer of reticular tissue found beneath the intestinal epithelium.


In the small intestine and in the vermiform process, lymphatic nodules occur side by side, so as to form macroscQpic areas visible on the inner surface of the intestine. They are broadly elliptical, and usually from I to 5 cms. long though occasionally much longer. From two to forty or more nodules may enter into the formation of one of these a^greg^e nodules [Peyer^ s patc hes] and they may remain distinct though adjacent, as IiiFig7"24i, p. 213, or they may be confluent. In the latter case they may be recognized by their germinative centers. Their structure is that of the sohtary nodules.

The lymph glands are round or bean shaped structures, varying in f length from a few milUmeters to a few centimeters, They occur along the • courses of the l)miphatic vesseis,^as is shown in textbooks of anatomy. In producing a lymph gland, as" seen on the right of Fig. 175, a connective tissue capsule forms around the lymphoid tissue, into which it later sends trabeculae and plate-like prolongations. These may unite with similar trabeculae from the region of the hilus, as on the right of Fig. 176, thus making colunms of connective tissue extending from one side of the gland to the other. (Such a complete trabecular system is found only in the larger lymph glands. Y The capsule consists of connective tissue with elastic elements whicff increase with age. It contains also scattered smooth muscle fibers; the trabeculae are of similar structured

(Beneath the capsule and surrounding the trabeculae, there^s areticular meshwork comparatively free from lymphocytes. This is called the lymph sinusal It is in free conmiunication with the afferent and efferent lymphatic vessels, and is also continuous with the reticulum of the dense lymphoid tissue. -Its embryological relation to the lymphatic vessels has not been satisfactorify determined. Some consider that it is a network of ^ndothehal tubes closely investing slender strands of reticular tissue; others believe that the endotheUal tubes are penetrated by the reticular tissue; / and still others that the endothelium blends inseparably with the reticulum, j into which the lymphatic vessels therefore open freely. <^It seems justifiable to maintain that endotheliuni and reticular tissueare distinct, though in close relatTon^ (All of the functions and appearances of the sinus can be explained if the endothelial lymphatic vessels are regarded as freely permeable in the gland, by separation of their cells from one another.' ) Fig. 178 shows the trabeculae highly magnified; between them and tKe Sense lymphdd tissue are the lymph sinuses.

( Several organs can be divided into an outer and an inner portion, called cortex (meaning bark) and medulla (pith) respectively. The lymph gland is one of these.^' Its cortical part, shown in Figs. 176 and 179, consists of large lymphoid masses resembhng nodules and containing germinative centers. These are sometimes called secondary nodules. The medullary portion includes cord-like prolongations of the nodules, called medullary cords. The secondary nodules often are incompletely separated from one another and the cords join to form a network. Both the nodules and the cords are enveloped by the lymph sinuses, and the trabeculae if present are in the midst of the sinuses (Figs. 177 and 178). The nodules and cords are both composed of lymphocytes in a close-meshed reticular tissue^

^The blood vessels, of the lymph gland in part enter from various points in the capsule and r un in the trabeculae^ but the chief vessels enter at the hilus. jThe artery divides into several branches which remain in


Fig. 177. Fi<;. 17S.

From Vhrtical Skctions throigh the Mkollla of a Lymph Gland of an Ox. Fig. 177. X 50, shows the medullary cords and trabeculae cut letiKthwise in its upper part, and cut across in its lower part. Both the cords and the trabeculae form continuous networks. Fig. 178, x 240. shows the fine reticular tissue of the lymph sinus, containing a few leucocytes.

the trabeculae for only a sTiort distance, and then cross the l)miph sinuses to the medullary cords.. ' They extend through the axes of the cords into the nodules, giving oflF small branches which form capillary networks and unite in veins found at the periphery of the nodules and cords. The veins soon cross thj^sinuses and enter the trabeculae in which they travel toward the hihis) VA_central artery, surrounded by lymphoid tissue together with peripheral veins, is found not only in lymph glands but also in the spleen. The l)miphatic vessels penetrate the capsule at several points and become involved in the lymph sinuses. Through these, partly in endothelial tubes, and partly in tissue spaces, the lymph flows toward the hilus which it leaves in the eflferent vessels, fewer in number than the afferent. Lymphocytes are added to the lymph as it passes across the gland.

Nerves to the lymph glands are not abundant. They consist of medullated and non-meduUated 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.



Fig. 179. LoNGiTiDi.NAL Shction uf a HiMA.N Cervical Lymph Gland, X 12.

Tlie functi on of t he lymph glands is not only to produce lymphocytes which enter the lymphatic vessels, but also to ** filter the lymph." If cerI! tain poisonous substances, inert particles, or bacteria are brought to the i gland in the lymph, they may be removed by phagocytic endotheUal or ! reticular cells. The gland at the same time may become enlarged by con' egstion, and by multiplication of its cells.


Haemolymfh glands

Haemolymph glands resemble lymph glands in form and also in size, ranging from that of a "pinhead to an almond." They occur especially in the retro-peritoneal tissue near the origin of the superior mesenteric and renal arteries, but also in the thorax and neck. ^They are darker than lymph glands, and on section yield blood in place of lymph. No lymphatic vessels are associated with typical haemolymph glands, 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 haemol)mph gland is therefore a blood filter. Many blood corpuscles fragment and are removed from the circulation by phagocytic cells which in consequence become pigmented. Eosinophilic cells which have been found in haemolymph glands have been explained as due to the ingestion of haemoglobin products. Haemol3m[iph glands have as a second function that of producing lymphocytes which may enter the blood vessels.

After accidents accompanied by extravasations of blood, the lymph sinuses of lymph glands may be filled with red corpuscles conveyed to them by afferent lymphatics. Such glands should not be confounded with haemolymph glands which have no l)rmphatic vessels. It has been said, however, that intermediate forms between the two sorts of glands occur, meaning that some normal lymphatic glands contain blood in their sinuses derived from their own blood vessels. C The embryology of haemolymph glands is unknown but it is not supposed that they are lymph glands which in the course of development have lost their lymphatic vessels. They are regarded rather as structures which are distinct from the outset, and which are closely related to the spleen. )

Spleen

The spleen, being five or six inches long and four inches wide, is much the largest organ of the l)inph gland series. It is the first of them to develop, appearing in rabbits of 14 days (10 mm.) as a condensation of the mesench)rma in the dorsal mesentery of the stomach. At this stage the only lymphatic vessels in the embryo are those near the jugular vein. Lymph glands are not indicated until six days l^ter. The blood vessels enter the spleen at its hilus and branch freely. \ It is unknown whether or not the artery ever connects with the vein. • Surrounding the arterial branches there is a zone of lymphoid tissue which is so highly developed in reptilian spleens that they resemble closely 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 spindleshaped or spherical masses called scenic nodules [Malpighian corpuscles], f r^ ipn arterial branch passes through each nodule. (Thus, as compared with ^-^ (IheJiaemolymph gland, the spleen is deficient in lymphoid tissue (Fig. i^?^ s jThe bulk of the spleen is composed of splen ic pulp , which corresponds J [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.


Fig. 180. Diagram or a Hakmoi.ymph Gland, A, and of a Part ok the Splki:n, B. The arteries are shown as slender lines (art.) and the veins as heavy on^ (v.) ; c, capsule ; b. 8., blood sinus, corresponding with the splenic pulp, p.; 8. n., secondary nodule; 8p. n., splenic nodule; tr., trabecula.


As shown in the diagram. Fig. i8i, 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 trabeculae 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 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 whiclTthey anastomose before passing on to the pulp. When the main stems are about 15 /^ 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 walk of these branches possess ellipsoid thickenings due to a longitudinal arrangement of closely applied reticular fibers. These * sheathed arteries' are 6-8 fi in diameter, and have been supposed to regulate the amount of blood which enters the distal portion of the artery. Some authorities state that this distal part connects with the terminal veins, meeting them at an acute angle. According to others such connections 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 celfe.


Fig. 181. Diagram of thk Blood Vessels of the Human Spleen. At X is shown the direct connection of terminal arteries with terminal veins (the existence of such a connection has been questioned). At XX and XXX are the free endings of the terminal veins in the pulp and near the nodules respectively,




Fig 182. Cross Skction (A) and Surfack View ^B) ok Terminal Veins from the Human Spleen. Rod shaped endothelial cells, with projecting?: nuclei, n; r., encircling reticular tissue; I., leucocytes passing between the endothelial cells. (After Weidenreich.)


The terminal veins begin as dilated structures (sometimes unfortunately called * splenic sinuses,' or * ampullae,' the latter term being applied also to the terminal arteries). Their endotheliial 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 as shown in Fig. 182. Around them are encircling reticular tissue fibers, and a continuous basement membrane-like structure has been described stretching across the intervals between the endothelial cells. The existence of such a membrane has recently been denied. A peculiar feature of the endothelial cells is their projection into the lumen of the vessel, their nuclei being at the summits of these elevations as shown in Fig. 182. 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.


Fig. 1S3. Part of a Section of the Spleen from an Adllt Man. x 15.


The splenic pulp consists of a reticular tissue framework such as has been described on p. 39. It supports the terminal arteries and the terminal and pulp veins, and in its meshes are the white and red corpuscles passing between them.

The pulp appears as a diffuse mass of cells infiltrate d 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. 183) . The nodules are not sharply separated from the pulp, so that lymphocytes are abundant in their civinity. These lymphocytes enter the terminal veins and thus are removed from the spleen. (^Jhe splenic vein the proportion of lymphocytes to red corpuscles is said to be seventy times as great as in the splenic arteryT) One for every four red corpuscles has been reported by two investigators, but later estimates are lower, ( ^t seems eviAm\ that lY^P^ft^Yf? production is an jm portant function of the spleen Another is the filtration of the blood pass in g[^ ^hp p iilp^ Baemolymph gland s granular debris is fou nd, and there are j^hagggj^i^ , pi gm ented, and ysinophilic cell s. (The phagocytes are cells with large roun^nuclei and considerable protopFa^m^ They vary in size, but the small forms are most numerous; these are called splenk cells. Some are described as multinucleate. Erythroblasts are not foundTn the normal adult human spleen; in certain blood diseases, however, they occur in it 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. The formation of granular leucoc)rtes, wh ich h as been asserted, presumably does not occur.

{^The splenic nodu les are quite like the secondary nodules of l)maph glan3s!) 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 germinative centers are sometimes distinct. The nodules have been regarded as varying in shape from time to time, being but transient accumulations of lym-j phocyte§.

(CThe capsule of the spleen is divided into two layers. The outer is the tumca serosa and the inner, the tunicajUb^ginea, \ The serosa consists of the peritoneal mesotheUum which covers the spleen except around its hilus, and of the underlying connective tissue. The albuginea is a dense layer of connective tissue, containing elastic networks and smooth muscle fibers, gimjlar tissue i?; found in the trab eculae . The iliuscle elements are less numerous in the human spleen than in those of many animals. ^y contraction they force bloodirpm the pulp ^nd cause the circul ation-la follow morf^ i'iyto'tq f |i?»nnpJs . ^>^\v i3en t hey are paralyz eajne^i ^

fiHec^mEfnEBmood corpuscles. ^

The nerves ot the ' spleen, from the right vagus and the coeUac sympathetic plexus, are meduUated and non-medullated fibers, chiefly the latter. They form plexuses around the blood vessels (Fig. 184) 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 eflFect due to areas bounded more or less definitely by trabeculae. Such areas, about i mm. in diameter, have been described by


    • •. Surface blackened by

precipitate of silver.


Fig. ^84.— Golgi Preparation of the Spleen of a Mouse. X 85,

The boundary between the spleen pulp and the lymphoid tissue is indicated by a dotted line.

The nerves are chiefly in the wall of an artery.


Professor 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. There are veins in the trabeculae, often three, at the periphery. 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

  • i — « about the lobule). Apparently, therefore, the lobules shown in the diagram, Fig. 181, except along its lower border, represent groups or pairs of Mall's

lobules. Professor 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 pharyngeal pocket of entoderm has been described and illustrated (Fig. 20). This 'pharynx' of the young embryo is to produce the fore part of the intestinal tract including the pharynx, oesophagus, and stomach of the adult. Its anterior extremity encounters the ectoderm at the bottom of a depression. The ectoderm and the entoderm there fuse to make the oral plate (Fig. 185), which becomes thin, ruptures, and disappears. Just anterior to the plate, in the median line, thfc^ctpderm sends, a j^landlike projection toward the brain. It branches and becomes detached from the oral ectoderm, - lying in the sella turcica of the adult. It is knovni as the anterior lobe of the hypophysis, and it will be described with the brain, from which the posterior lobe develops, ihe ectoderm in p^^ ^s^-diagram showing front of the oral plate forms also the epithehum ^c^^^'il'iZVr^o^^, oitht lips and of the peripheral part of the .mouth J^^alian E^?iRYo!" ^ ^ including the enamel organs, as has already been st" rij*/ iibcs^o?fhlhv^^^^^^ described. No line of separation between the fui^/'ih"phbyrl^f o"T. ectoderm and entoderm can be found in the adult. dâ„¢nfwh?chproducestheii^

and teeth of the lower and

The entoderm of the mouth and pharynx the upper jaw respectively constitutes the epithelium lining a broad cavity flattened dorso-ventrally. It produces a succession of paired lateral outpocketings which meet corresponding ectodermal depressions. Ectoderm and entoderm fuse where these meet, making plates similar to the oral plate, and in fishes these rupture to produce the branchial clefts (gill clefts). Their arrangement in a young dog-fish is shown in Fig. 186. The mouth, m, leads into a cavity, the pharynx, which opens freely on the outer surface of the fish through five gill clefts, g.c. It also opens to the surface through the spiracle, sp, a structure similar to the gill clefts but anterior to them and ha\ang a more dorsal aperture. Gill clefts and spiracle occur on both sides of the fish. In mammalian embryos epithelium may become that of the tonsil. The upper portion of the depression made by the second pouch probably becomes the pharyngeal recess [fossa of Rosenmiiller]. The third pouch, near where it meets the ectoderm, sends a tubular diverticulum (/A) down the neck behind the thyreoid gl^d; it continues into the thorax, lying ventral to the arch of the aorta (Fig. 189). The diverticulum loses its lumen and becomes detached from the pharynx; it forms the thymus. (^Besides this elongated structure, the third pouch produces a rounded clumpl)f cells which becomes separated from the upper or anterior end of the thymus.^ This nodulus thymicus has been said to produce the glomus caroiicim; but the latter is now generally regarded as a vascular mesenchymal structure. The nodulus th)rmicus has also been said to form a small body attached to the posterior surface of the thyreoid gland in the adult, and called the parathyreoid jland. The origin of the parathyreoid glands, of which there may be four in man, two on either side, is still uncertain; and the fate of the nodulus thymicus is obscure. The fourth pharyngeal pouch (4) soon becomes Y-shaped by union with the postbranchial body {p-h.). The latter ^)id^, t"of 29'mm. human embryo; p.. parathyre 15 an mdependent outgrowth of the pharynx, ans- °^**K^ndiderivedfrom ing near the fourth pouch, and considered either a SvTd^roS^ihc^i^h Luch rudimentary fifth pouch, or a structure not related tili^r^fd ! to the pouches. It elongates and fuses with the thy- JJi^crulJ* ' VAftT ver*reoid gland, from the tissue of which it is scarcely to be distinguished. Embryologists differ as to whether it forms any of the adult thyreoid gland. The fourth pouch itself produces a nodule of tissue which has been said to form the anterior pair of parathyreoid glands, but its fate is still uncertain.

Since the derivatives of the first pouch are to be described with the ear it remains to consider the palatine tonsils, as related with the second pouch; the thymus, as derived from the third; the thyreoid, from the floor of the mouth and from the postbranchial bodies; and the parathyreoid glands from the third and fourth pouches

Palatine tonsils

The palatine tonsils are two rounded masses of lymphoid tissue, one on either side of the throat, between the arches of the palate. They are covered by the mucous membrane or tunica mucosa, which throughout the digestive tract consists of several layers. The entodermal epithelium rests on a connective or reticular tissue layer, the tunica propria. A structure less basement membrane beneath the epithelium is called the membrana propria. The epithelium, membrana propria, and tunica propria together form the mucous membrane. Beneath it, and sometimes not clearly separable from the tunica propria, is the submucous layer, or tela submucosa. It is a vascular connective tissue by which the mucous membrane is attached to underlying muscles or bones. All of 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. 190). The stratified epithelium lines from ten to twenty almost macroscopic depressions called tonsillar pits or fossulae fcrypts). These are irregularly cylindrical and sometimes branched. Many lymphocytes penetrate between the epithelial cells and escape from the free surface into the saliva, to become * salivary corpuscles.' In places the tonsillar epithelium is so full of lymphocytes as to appear disintegrated. In the reticular 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 forms the bulk of the tonsil.


Fig. 190. Vertical Skction of a Himan Palatine Tonsil. a. Stratified epithelium; b, basement membrane: c, tunica propria; d, trabeculae; e, diffuse lymphoid tissue ; f, nodules ; h, capsule ; I, raucous glands ; k, striated muscle ; I, blood vessel ; q, pits. (From Radasch.)


The submucous layer forms a capsule for the organ, into which it sends trabecular prolongations. It contains many blood and lymphatic vessels, together with the secreting portions of mucous glands, and the branches of the glossopharyngeal nerve and of the spheno-palatine ganglion which supply the tonsil. Some of the small glands 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 are to be described presently. Beyond the submucosa is striated muscle, belonging to the arches of the palate and to the superior constrictor of the pharynx.


Except that the palatine tonsils lie in depressions which correspond in position with the second phaiyngeal pouches, they afford no evidence of their branchial relations. Only their epithelium is entodermal. The lymphoid tissue is mesenchymal. In these respects the palatine tonsils resemble the median lingual tonsil which forms the posterior part of the tongiie (see page 184) and the more diffuse median pharyn geal lonsil on the dorsal wall of the nasopharynx between the openings of the auditory tubes. Irregular enlargements of the latter may obstruct the inner nasal openings, producing the adenoids of clinicians ( the a djectiye adenoid being synpnymous with lymphoid}. The pits of the pharyngeal tonsil are smaller than those of the palatine.

Thymus

The thymus arises from the two tubular prolongations of the third pharyngeal pouches which meet in the median line as shown in Fig. 189, and become bound together by their connective tissue coverings lumen is lost, and the cells proliferate. They form a broad, flat, bilobed mass with a tapering prolongation up eitlier side of the neck.) The bulk of the organ isjn 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. It increases in size and .weight for some years after birth, probably until puberty, and then slowly atrophias. At i,^ years it is said to weigh from 40-50 grams. It is considered an active organ even to the fo rtieth year, losing its functions with beginning old age (50^ years). Then it becomes fibrous and fatty. The importance of the thymus has apparently been underestimated.

The thymus is subdivided by connective tissue layers \r)Xo lobes from 4 to IT mmi in dinmrtrrj and these are similarly subdivided into lobules of about one cubic millimeter eacli> On either side all the lobules are attached to a cord of medullary substance, 1-3 mm. in diameter, as may be seen if the gland is pulled apart. The medjillary uthsfnttre extends from the cord into the lobules (Figs. 191 and 192) where it is partially surrounded by a denser cortical substance.


Fig. 191. From a Cross Section of the Thymus of a Child, 1 Year and 9 Months Old. X 21.


In places the medulla is in contact with interlobular connective tissue the cortex and medulla are not sharply separated from one another.

The cells of the thymus have been variously interpreted according to a recent investigation (by Dr. E. T. Bell) the thymus is at first a compact mass of entodermal cells. By vacuolizaticm 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 cms. and is well advanced at 4.5 cms." It has already been stated that lymphocytes are first recognizable in the blood and in the lymph glands of pigs of 8 cms. ^The possible first appearance of lymphocytes in the thymus and their origin from entoderm are of great intere^^

Tfi at the thymus ceils are thymocytes however its Stohr who regards the cortex as composed of round entodermal cells deceptively similar to lym phocytes, and as forming a degeneration zone of

thymus tissue. \0i 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. 193^?^ Still another view is that the cortex consists of reticular tissue of mesenchymal derivation, containing l)nnphocytes arising like those in lymph glands. The original entodermal pouch is thought to become surrounded by dense mesenchyma and to form but an insignificant part of the medulla. The nature of the thymus then must still be considered obscure.



Fig. 192. Part of a Si-ction of the Thymls from a 5 Months' Human Fetus, x 50.




Fig. 193. Part of a Skction of the Thymus of a Child at Birth. X 50.


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 therefore is considered a blood forming organ^ (In ordinary sections it resembles a lymph gland, from which it may be distinguished by the presence of thymic cpfpy^scles [HassalPs corpuscles] in its medulla. These corpuscles jire found exclusively in the medulla of the thymus. They are rounded bodies, at first few in number and small (12-20 fi in diameter), but they increase rapidly in size (to a diameter of 180 ft) and new ones are constantly forming. At birth they are numerous, varying in size as shown in Fig. 193. To produce them , the nucleus and protoplasm of a reticular tissue cell (entodermal) are said to enlarge. The nucleus loses its staining capacity by changes in its chromatin, and 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. The nucleus becomes obliterated neighboring cells are concentrically compressed by the enlargement of this structureT^nSTBy^ hyaline 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 mucus 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 as degenerative Qduct§.Qf the entodermal epithelium; 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).



Fig. 194.Thymic Corpuscles, in Sfxtion, from a Man 23 Years Old. X 360.


The arteries of the thjinus enter it along the medullary strand and extend between the cortex and medulla, sending branches into both but chiefly into the cortex. The cortical braxiches empty into veins between the lobules; the others into those within the medulla. There are many interlobular lymphatic vessels beginning close to the surface of the gland, 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 on the vessels; a very few have free endings in the medulla.

Thyreoid gland

The thyreoid gland is a median, entodermal down growth from the tongue; its Hyreoglossal duct becomes obliterated, leaving the foramen caecum to mark its former outlet. The down growth is joined by cells, from the jostbranchial bodies, which fuse with it. This entire structure comes to he 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 fyram' idol lobe extends from the isthmus or adjacent part of the lateral lobe toward the tongue (Fig. 189). Irregular detached portions of the gland such as occur especially along the course of the thyreoglossal duct, are calle d accessory thyreoi dglands.

(The prohferating mass of entodermal cells forms at firs t 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 ^

colunmar, cuboidal, or flat. Flat cells are said to occur especially in old age, low columnar or cuboidal cells being usually founds (The mature \ thyreoid gland consists, therefore, of rounded, closed spaces, or [olHcles, \ bounded by a. jsimple entodermal epithelium (Fig. 195). The foUicles j ' vary greatly in diameter. Generally they are rounded, but sometimes they are elongated, and occasionally they branch or conmiunicate with one another,;^ Among them are cords or clumps of cells which have not acquired a lumenj) 1

(^Within the foUicles, and forming the most conspicuous feature of the ; thyreoiH gland in ordinary sections, is a htaUne material which stains deeply with _eosine and is named 'coUoid.'^^ Its chemical nature is undetermined. The hyaUne 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 th)n-eoid gland it usually does not fill the follicle but has contracted, producing a spiny border. Granules, vacuoles, detached cells, leucocytes, and crystalloid bodies may be found in it. I tjs^a product of the epithelial ^c ejls, in the protoplasm of which similar material has been detected. It has been said that it is transferred to the blood and lymphatic vessels.

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 th)n-eoid cells,


Fig. 195. Section of a Lobule of the Thyreoid Gland from an Adult Man. X aao.

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. 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 folhcles, which contains very many blood and lymphatic vessels. Denser connective tissue forms a capsule and lobular partitions. The nerves from the cervical sympathetic ganglia form perivascular plexuses, and pass to the follicles, between the epitheUal cells.


A few have been found to end


Parathyreoid glands

It is generally stated that there are four parath)nreoid glands in man the anterior or upper pair being derived from the fourth entodermal pouches, and the posterior or lowejr ^infrom the nodulus thymicm of the third (Fig. 189)? ) Although they have been repeatedly investigated, their origin is not yet established. In the adult they are round or oval bodies, said to measure from 3 to 13 nmi., 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. It is not known that two pairs always occur. The parath)n-eoid glands may be lacking on one side, where in other cases as many as four have been recorded. (^Both pairs possess a similar structure unlike that of ' either the thyreoid gland or the thymus, but resembling the corresponding epUhelial bodies of the lower vertebrates. > (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. 196), 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. I^ittle is known of the lymphatics or nerves.


Fig. 196. Section of a Human Parathyreoid Gland. (Huber.)



Glomus caroticum

The glomus caroticum (carotid gland) has already been described as 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 cells said to be chromaffine and prone to disintegrate (Fig. 197). Many nerve fibers, medullated and non-medullated, enter the glomus and a few multipolar ganglion cells are associated with them. In its arrangement of cells and blood vessels it resembles a parathyreoid gland, and also the glomus coccygeum which is far removed from entodermal structures. Since the nature of the glomus caroticum is undetermined, the three views regarding it may be mentioned. First, it has been consfdered derived from the nodulus thymicus, which is now said to form a parathyreoid gland. Recently it has been found that the 'carotid gland' of Echidna comes from the second pharyngeal pouch, and the non-en todermal origin of the human glomus is not beyond question. Second, it has been considered gangliom'c or paraganglionic in nature, so that it is classed with nervous structures. Third, it is considered essentially a vascular formation, containing strands of modified mesenchymal cells.


Fig. 197. Section of a Part of the Glomus Caroticum of Man. (After Schaper.) b.v., Blood vessels; e.v., efferent vein; tr., trabecula; c.t., connective tissue septum.


Development and Structure of the Tongue

The tongue consists of two parts, an anterior and a posterior, which differ in origin and adult structure. Separating the branchial clefts from one another there arc columns of tissue known as branchial arches. They come together in the median ventral line to form the floor of the mouth as shown in Fig. 198. 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 (tuberailum' 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^). The second and third arches unite toward the median ventral line and there produce the posterior part of the tongue (t'). 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 tfiird arch separated from the posterior part of the tongue by a curved groove


Fig. 198. Floor of the Pharynx of a 10 mm. Human Embryo. lY, Bronchial arches; t', anterior part of the tongue; t*, second arch, joinine the posterior part of the tongue toward the median line. The thyreoid gland is dotted. The epiglottis extends over the 4th arch. (From McMurrich, after His.)

.

In the adult, Fig. 199, the dorsum of the anterior part of the tongue is covered with papillae. These are chiefly the slender filiform papillae and conical papillae, but knobhke forms, the fungiform papillae, are scattered among them over the entire surface. Near the junction of the anterior and posterior parts of the tongue there is a V shaped row of larger' papillae, generally 6 to 12 in number, called vallate papillae. Their name refers to the deep narrow depression which encircles them. Behind the apex of the V, which is directe(|^oward the throat, is the foramen caecum(On either side of the tongue, as indicated in the figure, there are from 3 to 8 parallel vertical folds J2-5 mm. long) occurring close together; these are the foliate papillae. (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 contains the lingual tonsil, and has a nodular surface covered with soft epithelium. Laterally there are fold-like elevations called lenticular papillae.


FiG. 199. The Upper Surface of the Adult Tongue.

C, Conical papillae; ep., epiglottis ; f., foliate papillae; f. c* foramen caecum; n., position of the filiform and fungiform papillae; I., lenticular papillae; I. t., lingual tonsil; p. t, palatine tonsil; v.. vallate papillae.


The tongue is composed of a mucous membrane (tunica mucosa) and a submucous layer, together with the underlying striated muscle which forms the bulk of the organ. Its anterior portion may be described first.

The mucous membrane is characterized by the various papillae. The filiform papillae (Figs. 200 and 201) are cylindrical or conical elevations of the tunica propria, each with from 5 to 20 secondary papillae at its upper end. They consist of vascular fibrillar connective tissue with numerous elastic fibers and are covered by a thick stratified epithelium. The outer epithelial cells are flattend squamified, - that is they have undergone a horny hyaline degeneration, - and several slender columns of such cells may extend beyond the secondary papillae. The filiform papillae are from 0.7 to 3.0 mm. tall. Fungiform papillae.. (Fig. 201) are rounded^^vaiioiis with a somewhat constricted base. The entire outer surface of their connective tissue core is beset with secondary papillae. They contain but little elastic tissue; the epithelium is not as thick as in the filiform papillae, and its ou ter cells are not comified . In life, fungiform papillae are red since their epithelium transmits the color of the blood beneath. Their height varies from 0.5 to 1.5 mm. 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. 202). 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-hke bodies such as are generally known as ' epithelial pearls.' There are also branched serous glands which grow down from the epithelium, having ducts which open into the deep grooves (Fig, 202). The foliate papillae are parallel folds of mucous membrane, in the epithelium of which there are mdjxy ^gghuds. These structures, which occur also in the lateral walls of the vallate papillae, require a detailed description.



Fig. 200. From a Longitudinal Skction of the Dorsum of a Human Tongue, x 12.


Taste buds are round or oval groups of elongated epithelial cells which extend from the inner to the outer epithelial surface; in contact with them the nerves of taste terminate. ;; Their position in the epithelium is shown in Figs. 202 and 203. In the fetus of from 5 to 7 months they are more numerous than in the adult, occurring in many filiform papillae and in all the fungiform, vallate and foliate forms, together with both surfaces of the epiglottis. They are destroyed with an infiltration of leucocytes, excep Cthos e on the lateral walls of the vallate and foliate papillae, small numbers of those on the anterior and lateral fungiform papillae, and those on the laryngeal surface of the epiglottis. In such places they are found in the


Fig. 201. From a Longitudinal Section of the Htman Tonguf. X 25. X, Epithelium showing postmortem disintegration.



Each bud consists of two sorts of elongated epithelial cells, among which lymphocytes are frequently seen. Most of the cells are supporting cells. These may be uniform in diameter or tapering toward the ends.


Fig, 202. Vertical Section of a Human Vallate Papilla. X 25.


They are sometimes forked or branched below and at the free surface they may end in a short conical process. The peripheral halves of the cells in a taste bud converge somewhat like the segments of a melon, so that their ends are brough^together in a small area. ( This area is at the bottom of^ little pore or short canal found among the outermost flat cells of the epithelium. Sometimes it is bounded ty the supporting cells. The taste pore opens freely to the surface, but in oblique sections it may appear bridged as in Fig. 203. Besides the supporting cells which are found at the periphery of the bud and which terminate around or beneath the pore, there are more slender forms in the interior of the bud, which reach the pore. There are also a few flat ones confined to thelower half of the bud. The taste c ells are slender structures, being thickened to accommodate the . narrow nucleus. The nucleus is usually in the middle or lower part of the cell. Toward the taste pore these cells generally taper, and they end in a stiflF refractive process which is a cuticular formation. These processes extend into the deeper part of the pore but do not reach its outlet. The taste cells may have a triangular base, or end bluntly. Their protoplasm is darker than that of the supporting cells.


Fig. 203. From a Verticai, Section of a Human Foliate Papilla. X 330.


Fig. 204. From a Vertical Section of the Foliate Papilla of a Rabbit. X 220.


The nerves to the buds are branches of the glossopharyngeus, assodated with microscopic sympathetic ganglia. These nerves, both medullated and nomn^^iUated, make a thick plexus in the submucous connective tissue. '{The terminal branches probably end in part in bulbous corpuscles, but most of them, as nonmeduUated fibers, enter the epithelium. Some are found between the taste buds, extending to the outer epithelial cells generally without branching (Fig. 204). Others enter the buds, where they divide into coarse varicose branches which reach almost to the taste pore. They end freely, without uniting with the cells or anastomosing with one another^ The terminal branches are chieflyJnjgl ation with the t^e celk ; to a less extent they are said to I ramify about certain of the supporting cells* The taste cells are believed ' to transmit to the nerves the stimuli received at the taste poreT)

The tunica propria of the mucous membrane, a loose connective tissue layer containing fat, is not sharply separated from the denser stibmucosa.

At the tip, or apejc linguae j and over the dorsum, tljeLSiabmucosa is particularly firm and thick, forming the fascia lingtiae. (Three sorts of glands branch in the submucosa and may extend into the superficial part of the muscle layer. These are the serous glands found near the vallate and folliate 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 Qftierwr Ungual glands, from half an inch lo an inrfi long, each of which, empties by five of six ducts on the under surface of the apex.j The appearance of these types of glands will be described in a following section.


Fig. 305. From a Section of the Likri^ai, Tohsij. Of an ADULt MaJ*. X Mv cunlalnmg leucocytes ^bkb hiivc rnfiltmtefl its epkhelitim on the left side; thai on the tl^ht b

atmpst IriL&et.


Blood vessels are numerous in the submucos a and form extensive capillary networks in the tunica propria of both the larger and the secondary papillae* Small lymphatic vessels also form a netw^ork in the tunica propria and this is continuous with a coarser net in the submucosa. The nerves (sensory) are the terminations of the lingual branches of the mandibular nen*e antcriorlyj and of the lingual branches of the glossophaiyngeu» posteriorly. They contain nene cells which are grouped in small Ijanglia, notably beneath the vallate papillae. The glossopharyngeal end in the taste buds have been described. The others terminate in bulb ous corpuscle s or in free endings beneath or within the epitheUum.

The muscular layer consists of interwoven bunclles of striated fibers which are inserted into the submucosa or into the intermuscular connective tissue. §Gmfi^l_ these striated fibers^re bra nched . The musculature of the tongue is partly divided" into right and left halves by a dense median connective -tissue partition, the septum linguae. It begins low on the

Emigrating leucocytes. Fragments of epithelium.


Fig. 206. From a Thin Section of a Li.ngial Tonsil of a Man. x 420. On the left the epithelium is free from leucocytes, on the right many leucocytes are watidering through.

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 (genioglossuSf hyoglossus, and verticalis linguae muscles), partly longitudinal (styloglossus, chondroglossus, superior and %n\mor longitudinalis linguae muscles) and partly transverse (the transversus linguae muscle). The glosso palatine 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 of these are sensory nerves to the mucosa but many are the lingual branches of the hypoglossal nerve which supply all the tongue muscles except the inferior longitudinal; that one is supplied by fibers from the chorda tympani. Sensory spindles have been found in the lingual muscles.

The posterior part of the tongue is occupied by the lingual tonsil^ this term being a collective designation for a considerable number of rounded masses of lymphoid tissue. Each of these is from i to 4 mm. in diameter, and is situated in the tunica propria so that it causes a low, macroscopic elevation of the epithelium. In the center of the elevation there is a punctate depression, or pit, lined with stratified epithelium. Around it the lymphoid tissue; is partly separable into nodules with germinative centers (Fig. 205).^ Th e entire lymphoid structure is bounded by a sheath of connective tissue. Numerous lymphocytes enter the epithelium, and pass between its cells to the free surface where they escape into the saliva. The temporary disintegration of the epithelium, due to this cause, is shown in Fig. 206^ -In all these details the lingual tonsil is essentially like the palatine tonsils. â– 

^^ Mouth and Pharynx.

\^The lining of the mouth, like the covering of the tongue, consists of epithelium, tunica propria, and submucosa. At the lips toward the line of transition from skin to mucous membrahe> hairs disappear from the skin. The epithelium becomes abruptly thicker but more transparent as it crosses the line. Its outer cells are still comified, 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, comified cells are absent. Granules of the refractive homy substance, keratohyalin, are said to occur in the outer cells, even in the oesophagus. The outer surface of the epithelium is smooth, but its under surface is indented by many connective tissue papillae^ which are particularly long and slender in the lips (Fig. 207) and gums. Cilia occur on the epithelium in the highest part of the nasal pharynx, and in the fetus over the oral part^lso, and even in the oesophagus. They persist only in the nasal phar^x.

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


Fig. 207.— Vertical Section through the Mucous Membrane of the Lip of an Adult Man. X 30 I. Papilla; 2, excretory duct ; the lumen is cut at only one point ; 3. accessory eland ; 4, a branch of the excrctor\' duct in transverse section ; 5, gland bodies grouped into lobules oy connective tissue ; 6, a gland tubule in transverse section.

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.


Glands of the Oral Cavity

In the general account of glands (page 32) 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. 34, p. 32. T^ejoynd nucleus is generally in the basal half of the cell, not far from its center (Fig. 238). Mucous cells when fresh are much less refractive than serous cells. (^ In fixed preparations they are typically clear since the large area occupied by"mucous secretion stains




Fig. 208.— Sections of TrBri.Es, from Lingual Glands, Illustrating the Difpbrencks

BKTWKKN MlCOUS AND SeROUS GlAND CkLLS.

b, Empty mucons cells ; c» mucous cells full of secretion ; d, lumen of the tubule. X 340.

faintly. Fully elaborated mucus, however, may be colored intensely with certain aniline dyes, mucicarmine, and Delafield's haematoxylin. j In certain types of mucous cells the pale secretion area is large in alT 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. 2o8).('This diflFers from the type of mucous ceiTTound in the gastric epithelium in which the secretion area varies considerably with the elaboration and discharge of secretion (Fig. 35, p. 33).^^

Glands may consist «rtireiy 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]. They are shown in Fig. 216. The serous cells of the crescent are in connection with the lumen by means of secretory capillaries (p. 36) which branch over their surfaces, ending blindly, after passing between the mucous cells (Fig. 209). Sometimes


Fig. 200. From a Section of thk

SlBMAXILLARY GLAND OF A DoG.

X 320.


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 Whkh have been crowded from the lumen by those full of secretiorfr^ No secretory capillaries lead to such mucous crescents, which moreover are not abundant. Another sort of crescentic figure is made by the basal protoplasm in mucous cells otherwise full of secretion. 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,

^_ Serous Glands.

(I'he serous oral glands are the parotid glands and the serous glands of the tongue [v. Ebner's glands]. ; The latter are branched tubular glandt^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 ciUated. Their small tubules consist of a deUcate membrana propria or basement membrane, which surrounds the low colunmar or conical serous cells. In this simple epithelium, cell walls are lacking. With special stains and high magnification an outer dark granular zone has been distinguished from the clear basal portion of the cell which contains the nucleus. The lumen of the tubules is very narrow and receives the still narrower intercellular secretory capillaries (Fig. 210).

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; fts duct, the parotid duct [Stenson's], empties into the mouth opposite the second molar tooth of the upper jaw. The parotid gland is an organic, branched serous gland, subdivided into lobes and lobules. The qccessory parotid gland appears as a lobe separated from the others. The parotid duct is characterized by a thick membrana propria and consists of a two layered colunmar epithelium with occasional goblet cells. As the duct branches



Fig. 210. — Section of a Serous Gland FROM THE Tongue of a Mouse. X 240.

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


repeatedly, the epithelium becomes a simple columnar epithelium, after being pseudostratified, with two rows of nuclei (Fig. 27, p. 28). Possibly the epitheliiim near the outlet of the duct is also pseudostratified. The excretory portion of the duct is followed by the secretory part formed of simple columnar cells with basal striations, perhaps indicative of secretory activity. As shown in the diagram, Fig. 211, and in the sections. Figs. 212 and 213, the secretory duct becomes slender, making the intercalated ducts. They are lined by flat cells, longer than they are wide, and these form a continuous layer with the large cuboidal serous gland cells of the terminal alveoli. The gland cells when empty of secretion are small and


Fig. 211. HUMAN


End pieces.


-Diagram of the Parotid Gland.


Fig. 212.— Section of the Parotid Gland of an

Adult Man. X 252.

The very narrow lumen of the alveolo-tubular end pieces

is not shown.


darkly granular, 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?\

The alveoli of the parotid gland are somewhat elofJgated, and are branched. Between them 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 duds'. They are smaller and have less connective tissue around them than the interlobular ducts, of which however tky are the continuations. 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. Lymphatic vessels also follow the ducts and branch in the interlobular connective tissue where they terminate. Only tissue spaces have been found within the lobules. The jierve^ supply requires further investigation. (Sympathetic nerves from the plexus around the carotid artery accompany the blood vessels into the parotid, and by controlling the blood supply have an important bearing upon secretion; ^ The great auricular nerve, from the second and third cervical nerves, enters the gland, and branches of the facial nerve are involved in it, but branches from the otic ganglion are considered the essential nerves to the gland cells. In the other salivary glands which have been more thoroughly studied, nonmeduUated fibers from the sympathetic ganglia, either outside of the gland like the otic or from microscopic ganglia along its larger ducts, form plexuses beneath the basement membranes. Fibers from these plexuses penetrate the membranes, within which they form another network before terminating in contact with

the epithelial cells. Their endings may be simple or branched, and are varicose. Free sensory endings of meduUated fibers are said to occur in the epithelium of the ducts.


Fsit cell.

AlvtroluS, Intercalated ffuctj longitudinal stri:tK3h, cross accliim.



Secretory duct, lon|;;itudinal section,

cross section.


Fig. 213.— Section of the Parutid Gland from a Man of 23 Vkars. X te.


Mucous Glands.

( The pure jnucous glands of the mouth are simple branched alveolotubular glands found only 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 Uned 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. 208, I-II. The empty cells are smaller 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, submaxillar}', 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.



Fig. 214. — Diagram of thk Human Sublingi'al Gland.


Fig. 215.


-Section of the Sublingual Gland from Man of 23 Years. X i>o.


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 linguae. 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 (also called saUvary ducts). As shown in the diagram, Fig. 214, the secretory ducts are very short, and narrow intercalated ducts are absent. The tubules are surrounded by basement membranes cx)ntaiiiing stellate cells, and consist of both serous and mucous cells. The crescents are often very large and include many cells. ^ Only the serous cells are provided with the branched intercellular secretorj^capillariesTj The connective tissue between the tubules and lobules contains many leucocytes. The nerves are arranged as described for the parotid gland. The gland cells are supphed by sympathetic fibers from adjacent sublingual ganglion cells, about which fibers from the chorda tympani may arborize. The latter are said not to proceed directly to the gland cells. Sensory nerves to the ducts may come from the lingual branch of the mandibular nerve.

Besides the gland just described there are from 8 to 20 small separate




Tangential section of serous cells.


' Cross section ' with mucous / cells and (left) thick memn { brana propria.


^ Connective tissue.


Fig. 216.— Section of a Human Subungual Gland. >, 252.

alveolo-tubular glands closely joined to it, and described as part of the sublingual gland. They open by separate ducts, the ductus sublinguales minores. They all ( ?) consist almost exclusively of mucous cells.

The submaxillary glands are branched alveolar glands, in part tubuloalveolar, found within the lower border of the mandible, each being drained by a submaxillary duct [Wharton's] which opens on the sides of the frenulum linguae near its front margin. Its orifice may be Uned by stratified epithelium, but this soon gives place to the two layered form. Secretory ducts are well developed (Fig. 217) and their 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. Secretory capillaries


Wry


Excretory



Secretory duct.


Conn Intercalated tivelis^ut; ducts.



End pieces.


Fig. 217.— Diagram of the Human Fig. 218.— Section of thk Si'bmaxillary Gland from

Submaxillary Gland. a Man of 23 Years. X 80.

such as have already been described, are related only to the serous cells. Elastic tissue surrounding the alveoli has been thought to aid in expelling



Connective tissue. Lunit-n. \ ^

Crescent. Secretory duct.

Fig. 219.— Section of thk Sibmaxillary Gland ok an Adult Man. X 252.

the secretion through the ducts. It is known that the secretion is eliminated from the gland cells under high pressure, and so would not be checked by this action of the elastic membranes. The nerves are sympathetic fibers from the submaxillai^ ganglion and microscopic ganglia along the ducts. The chorda tympani does not send fibers directly to the gland cells. Sensory nerves may be derived from the branches of the mandibular nerve. 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.

The Development of the Digestive Tube.

The early development of the entoderm has been described in the section on general histogenesis (page i8). At first it forms a layer lining the blastodermic vesicle. Then by a process of folding and constriction the 'pharynx' develops from its anterior part so that the entire entoderm is shaped somewhat hke a chemist's retort. The bulbous expansion is the lining of the yolk sac. An analogous stage has been described in the chick embryo (Fig. 20), where, in place of a thin walled yolk sac^ there is a solid mass of yolk-laden entoderm. From the posterior wall of the yolk sac an entodermal outpocketing is produced, which rapidly becomes long and slender. It is called the allantois (Fig. 220, a/.). At first the allantois is directed posteriorly but soon it swings ventrally and then, as in C, it passes from the hind end of the digestive tract along the ventral body wall into the umbilical cord. The part within the cord becomes a strand of cells. Within the body, that portion of the allantois which is toward the umbilicus or navel, becomes subsequently a fibroiis remnant, the urachusy which leads from the navel to the bladder. {The bladder is the'\ dilated lower part of the allantois, and is therefore Uned with entoderm, being embfyologically a part of the digestive tube. * .

In mammalian embryos the allantois and the intestinal tract connect freely at their posterior ends, and the entodermal area common to both is called the cloaca. ^ Here the entoderm comes in contact with the ectoderm and forms the cloacal membrane^ a structure comparable with the oral membrane. After this membrane disappears there is no apparent line of separation between the ectoderm of the skin and the entoderm of the cloaca. In this region in both sexes a conical elevation, the genital papilla^ is formed, and the cloaca with its lateral walls closely approximated is found within it. Gradually the allantois becomes divided from the intestinal tract as shown in Fig. 220, B, C, and D. The mesenchymal tissue between them thus comes in contact with the ectoderm to produce the perineum which divides the cloaca into the urogenital sinus ventrally and the anus dorsally. In E, the bladder is seen to terminate in the urethra which in the male is considered to be chiefly an elongation of the ectodermal part of the urogenital sinus; only the part toward the bladder, which corresponds with the urethra in the female, is described as entodermal. As already noted there is no line of demarcation between the germ layers at this point, and a portion of the female urethra is by some considered ectodermal. The bladder is to be described with the urinary organs and the urethra with the genital organs.

Returning to the intestinal portion of the entodermal tract, it is seen that in early stages, A, the yolk sac extends from the pharynx nearly to the



Fig. 220.— Stacks in thk Dkvklopmknt of thk Digfstivk Tube. A, Rabbit of q days. B, Man

2.15 nun. (aflcr His). C, Pig, la mm. D, Man, 17.8 mm. (after Th>TiK). E, Man, about 5 months, a., Anus: al., allantois ; bl.. bladder; cae., caecum ; cl., cloaca ; du., duodenum ; I. I., large intestine; oe.,

oesophagus ; p., penis ; ne.« perineum ; ph.. pharynx ; r., rectum ; 1. 1., small intestine ; St., stomach ;

u. c. umbilical cord ; ur.. urethra ; ura., urachus ; u. 8., urogenital sinus ; v. p., venniform process ;

y. t., yolk sac ; y. St., yolk stalk.

posterior limit of the entoderm. With further growth a posterior intestine becomes formed by folding or constriction, comparable with the pharynx in front (B). The connection between the yolk sac and the intestine becomes a slender yolk stalky a part of which is shown in C and D. Later it loses its continuity and the detached yolk sac remains until birth as a small vesicle at the distal end of the umbilical cord, with which it will be described later. The yolk stalk which extends from the umbilicus to the intestine should be completely resorbed. It may persist as a fibrous cord liable to produce intestinal obstruction, or the part near the intestine may



DIGESTIVE TUBE

remain as Meeker s diveriictUum. This is a blind pouch of int usually less than four inches long but sometimes much longer, found on the small intestine some four feet from its termination.

Anterior to the yolk stalk the entodermal tube forms successively the pharynx^ oesophagus^ stomachy duodenum, and the greater part of the small ifUestine; posterior to it, the remainder of the smaU itUesUney the large intestine and the rectum. The rectum terminates at the anus which is formed as an ectodermal inpocketing closed in embryonic life by the anal membrane. Rarely this membrane or the adjacent rectum remains imperforate at birth. A transient embryonic extension of the intestine beyond the anus toward the tail is known as "post-anal intestine." It early disappears, and has not been drawn in Fig. 219. The stomach is a dilated portion of the tube at first vertically placed in the median plane ((3) but later so turned that its left side is ventral (or anterior), as in D. fflie duo- 1 denum is a subdivision of the small intestine, the remainder of which is arbitrarily divided into the jejunum (the anterior two fifths) and the ileum (the posterior three fifths). Where the ileum joins the large intestine a blind outpocketihg of the latter occurs, consisting of the caecum and its slender prolongation the vermiform process (processus vermiformis)." (At a certain stage (C) the intestines make a simple loop of which t£e large intestine forms the posterior or lower Umb. To produce the arrangement characteristic of the adult, the loop becomes twisted, as in D, so that the large intestine crosses the small intestine not far from the stomach; thus it is possible for the large intestine nearly to encircle the small intestine which becomes greatly convoluted, without, however, changing its fundamental relatio^T^ Besides the vermiform process and caecum, the large intestine includes the ascending, transverse, descending and sigmoid colon, the last terminating at an arbitrary line at the rectum. The rectum proceeds to the anus, but not straight as its name implies.

t^The entoderm fo rms only the epitheUal lining of the digestive tube and that of its assocfated glands. (Besides innumerable accessory glands these include the liver, pancreas, and the lungs.) Around the ento- ^^^ derm, the mesenchyma fo rms successively the following layers,— f !Ee tunica \ I ^v) i>ro ^ia which contains the reticular tissue and lymph nodules, and the 3) muscular is mucosae, a thin layer of muscles. The epithelium, tunica pro- 1 1 pna, and muscularis mucosae together constitute the mucou s memb ranej i j It rests on the tela submucosa, 2l vascularconnective tissue layer containing ! ' the sympathetic plexus submucosus. '.The submucous lay er is followed by 1 the tunica mu scularis . This consists 61 two or more layers of muscle fibers ' â–  between wnicn is tlie sympathetic plexus myentericus. Beyond the muscularis is the connective tissue tunica adventitia in case the intestinal tube is uncovered by peritonaeum, or the tunica serosa if the peritonaeum is present. The following account of the subdivisions of the digestive tube is essentially a description of modifications in these fundamental layers.

Oesophagus

The oesophagus is a tube about 9 inches long, the several layers of which are continuous anteriorly with those of the pharynx, and posterioriy with those of the stomach. It is lined with a stratified, many layered epithelium like that of the pharynx. The free surface which is smooth but thrown into coarse longitudinal folds, (Fig. 221) is covered with


Fig. 221. Transverse Section of the I'ppkr Third of the Htman Oesophagus. X 5 squamous cells; the basal surface is indented by papillae of the tunica propria. A muscularis mucosae, consisting of longitudinal smooth muscle fibers, arises at the level of the cricqi_d xartilage and continues into the stomach. At its anterior end it begins as scattered bundles inside the elastic layer of the pharynx, and as the muscles increase to form a distinct layer, the elastic lamina terminates. Beneath the muscularis mucosae is the submucosa, containing the bodies of the oesophageal mucous glands. They are tubulo-alveolar branched glands, with bodies about 2 mm. long, and closely resemble those of the mouth. Crescents and serous cells are absent, although empty cells may suggest the latter. Their ducts pass spirally through the muscularis mucosae and tunica propria, entering the epithelium where it projects outward between the connective tissue papillae. The ducts generally slant toward the stomach. The large ones are lined with stratified epithelium, often ciliated, and sometimes they present cystlike dilatations. The smaller ducts are of simple epithelium. Lymphocytes may be numerous along the ducts, forming solitary nodules near them in the tunica propria, and extending into the submucosa. Sometimes the glands show signs of degeneration. Their number varies greatly in different individuals. Usually they are most abundant in the upper half of the oesophagus.

A second type of oesophageal glands closely resembles the cardiac glands found in the oesophageal end of the stomach. The oesophageal cardiac glands (Fig. 222) occur in the posterior or lowest 2 to 4 mm. of the oesophagus, and also in small numbers at its anterior end between the levels of the cricoid cartilage and the fifth tracheal ring. The latter group is said to be absent in about 30% of the cases examined. The bodies of the oesophageal cardiac glands are confined to the tunica propria, and their ducts enter the epithelium at the summit of a connective tissue papilla. Their ducts have many branches, lined throughout with simple columnar epithelium, and this form of epitheUum may spread around their outlets in the lumen of the oesophagus. Because of this, when the oesophagus is opened, the anterior cardiac glands may appear macroscopically on its lateral walls as small erosions of the lining. The secreting cells of the cardiac glands contain round nuclei and granular protoplasm. Although



Fig. 222.— Longitudinal Suction throigh the Junction of THE Oesophagus and Stomach of Man. X 121. (Schd/y>r, from Bailey's Histology.)


0«., Oesophagus, its stratified epithelium, E., terminating at u ; M, stomach ; cd.dd, cardiac glands in stomach and oesophagus respectivelv ; tc, wd, dilated ducts of the cardiac glands ; S,


they are not generally considered mucous cells, it has been found that in the stomach their protoplasm responds to concentrated mucin stains, and it is quite possible that they produce a variety of mucin. Occasionally the oesophageal cardiac glands possess a few parietal cells like those found in the stomach. Cystic enlargements and dilated ducts occur, as shown in Fig. 222. No special function has been assigned to the cardiac glands.

Beneath the submucosa is the tunica muscularis^ consisting of an inner layer of circular or oblique fibers, and an outer layer of longitudinal fibers. In the anterior or upper part of the oesophagus the longitudinal fibers predominate. The muscles there are chiefly striated and are continuous with those of the pharynx. Gradually they are replaced by smooth fibers so that the striated forms are infrequent in the lower half of the oesophagus. . At its lower end the circular fiber layer is said to be three times as thick as the longitudinal. The oesophageal muscles are joined by slips from the trachea, left bronchus, aorta, and other adjacent structures.

Outside of the muscularis is the connective tissue adventitia. It contains branches of the sympathetic nerves and the oesophageal plexus of the vagus nerves. From these, the ner\'es invade the muscularis forming the ganglionated myenteric plexus between its layers, and pass on into the submucosa where they constitute a poorly developed submucous plexus. The terminal branches include free sensory endings in the stratified epitheUum, motor plates on the striated muscles and the simpler motor endings on the smooth muscle. The blood vessels form capillary networks with meshes between and parallel with the muscle fibers. They also branch irregularly in the submucosa, and form terminal loops in the papillae of the tunica propria. Lymphatic vessels are numerous.

Stomach. ' / Qlie inner surface of the stomach presents macroscopic longitudinal

folds which become coarse and prominent as the organ contracts) There are also polygonal areas from i to 4.5 mm. in extent, bounded by shallow depressions under which the gastric glands have been said to be fewer and shorter than elsewhere. The depressions are also ascribed to the contraction of the muscles in the mucous membrane. Toward the pylorus, or duodenal end of the stomach, there are small leaf-Uke elevations of the mucous membrane, called plicae villosae. They may connect with one another to form a network. The gastric mucosa is pinkish. gray since its epithelium is thin enough to transmit the color of the blood beneath; this is not true of the oesophagus, the lining of which appears white.

(Xhe epithelium of the stomach is simple and columnar, the transition from the stTatiSeT epithelium of the oesophagus being abrupt (Fig. 222).


Its cells produce mucus and may be divided into a basal protoplasmic portion containing the elongated, round, or sometimes even flattened nucleus;


Epithelium.


Tunica propria.




Parietal cells.


Chief cells.


Leucocytes,


Smooth muscle fibers.


Fig. 223.— Vertical Section of thk Mucous Mkmbrane of a Human Stomach, showing Gastric Glands (Glandulae Gastricae Propriae). X 220.

and an outer portion containing the centrosome and the secretion area. The area varies in size, sometimes being large enough to suggest goblet

Parictcl cell.



Chief cell.


Gland lumen.


Axial lumen.



Parietal cells with intracellular sc- . crelory capillar- \ ies. ^


cells. It may cause the free surface of the cell to bulge, and in preserved tissue to rupture, but this may be due to reagents. The mucus of the gastric cells responds less readily to mucin stains than that of the intestinal

goblet cells. It first appears in granular Portion of a parietal cell. form. The gastrfc epithelium lines a

great many closely adjacent gastric pits

(foveolae) into the bottom of which the

glands of the stomach empty. These

,, ^ ^ fflands are of three sorts, the gastric

hn;. 224.— Transverse Suction OP A ° °

Human Gastric Gland. X 240. ^Iflfids^ cardioc glattds, and pyloric glands.

^None of them extend through the muscularis mucosae into the submucosal The cardiac glands are limited to the oesophageal end of the stomach, occupjdng a zone from 5 to 40 mm. wide; the pyloric glands may extend from 6 to 14 cms. from its duodenal end; and the gastric glands occur throughout its body and fundus.

The gastric glands [fundus glands, peptic glands] are straight or somewhat tortuous tubular glands with narrow lumens, several of which empty into a single gastric pit (Fig. 223). The pits are sometimes considered to be the ducts of the glands. The tubules may join one another before entering a pit, so that they may be described as branched. They are somewhat narrowed toward the pits, forming the neck of the glands; their slightly expanded base is called the fundus. Each tubule consists of cells of two sorts, chiej cells^ and parietal cell^ . The chief cells in fresh tissue appear dark and filled with refractive granules; in stained specimens they are clear, cuboidal or low columnar structures enclosing round nuclei. Aft^r death the chief cells rapidly disintegrate. Their granules, which are often destroyed by reagents, are coarse toward the lumen

and fine in the basal protoplasm. In the absence of food the chief cells enlarge and the granules accumulate, but with prolonged activity the cells


Intercellular secretory capillaries.


Chief cells.



Fig. 225.— Golgi Preparation, showing thk Sf.crktory Capillaries in Gastric

Glands. > 230.

become small; granules disappear. They do not respond to mucin stains. It is supposed that the granules, called zymogen granules ^ become converted into pepsin. The chief cells form the greater part of the gastric glands,


Fig. 226. V'hktical Shctio.v of Human Pyloric Glands. > 90 parietal cells being irregularly distributed among them as in Fig. 223. The latter are fewest toward the base of the gland. Like the cells of serous crescents, they appear crowded away from the lumen with which. they are often connected only by intercellular secretory capillaries (Fig. 224). The


Mucosa.


Tunica propria.


Muscularis mucosae.


Submucosa.


Inner circular layer I of muscle. â– 



capillaries form a basket-like network within the protoplasm of the parietal cells, as may be demonstrated by the Golgi method. This produces a black precipitate wherever secretion is encountered (Tig. 225). Short intercellular secretory capillaries are found between but not inside the chief cells. In fresh preparations parietal cells are clearer than chief cells. They do not disintegrate so readily. In preserved specimens they appear as large cells with granular protoplasm which stains deeply with aniline dyes, each cell containing one or two rather large, round nuclei. After fasting, the parietal cells are smaller and their intracellular capillaries disappear. Following Epithelium. ^, ., . ^ abundant meals they

enlarge and may contain vacuoles due to the rapid formation of secretion. They are thought to produce hydrochloric acid, but this is not beyond question.

The cardiac glands (Fig. 221) arc much branched tubulo-alveolar mucous glands, often cystic, containing a few chief and parietal cells in tubules. Those furthest from the oesophagus are the least branched and resemble gastric glands. The secreting cells of the cardiac glands suggest those in the necks of the gastric glands; their mucous nature is not apparent and has been but recently determined . Although cardiac glands are developed in many animals much more extensively than in man, nothing is known of their special function.

The p yloric glands (Fig. 226) consist of very deep pits ajnd^of short ^ winding branc lie3 tu bules." Gastric glands may "Le mingled with them. The pyloric gland cells are chiefly mucous, but occasional parietal cells are found among them, and in animals there are dark thin cells apparently produced by compression. The usual t>^e is columnar with a rounded



Fig. 227.TRANSVERSE Shctio.n ok the Wall of a Human Stomach.


The tunica propria contains j^lanf^s standing so close together that its tissue is visible only at the base of the glands toward the muscularis mucosae.

nucleus near its base, and protoplasm resembling that of chief cells. In structure the pyloric glands are like the duodenal glands, but the latter extend into the submucosa.

The gastric glands are so closely packed that but little reticular and connective tissue of the tunica propria is found between them (Fig. 227). It is sufccient 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. 223). 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; they may extend through the muscularis mucosae into the submucosa. CT^^ 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^

The muscularis consists of a thick inner circular and a thin outer longitudinal layer, together with oblique fibers sometimes described as a third and innermost layer.*. -Owing to the distension and twisting in the development of the stomach the course of the fibers is disturbed, and in small sections they may appear to run in every possible direction?) The two layers are clearly marked at the M ^^s, where a great thickening of circular fibers produces the sphincter muscle. Longitudinal fibers have been said to be involved in it so that they can act as a dilator of the pylo rusj

The serosa consists of connective tissue with well developed elastic nets, and of the peritonaeal mesothelium interrupted only at the mesenteric attachments. The serosa contains the vessels and nerves which supply the stomach. The nerves are partly vagus branches (the left vagus supplies the ventral surface and the right vagus the dorsal surface owing to the rotation of the stomach during its development) and partly sympathetic nerves from the cardiac plexus. The distribution of vessels and nerves is similar to that in the intestine, which will be described in detail.

Small Intestine - Duodenum

The mucous membrane of both the small and the large intestine contains many simple tubular glands, which reach but do not penetrate the muscularis mucosae. They are called intestinal glands [crypts of Lieberkiihn]. Besides these, but in the small intestine only, there are cylindrical, club-shaped or foliate elevations of the epithelium and tunica propria,


called villi. Since the villi are from 0.2 to i.o mm. in height they may be seen macroscopically under favorable conditions. In Fig. 228, A, which represents an enlarged surface view of the hardened mucosa, the ori&ces of the intestinal glands and the projecting intestinal villi are clearly indicated. The villi of the duodenum are low (0.2-0.5 mm.) and leaf-like as seen in the reconstruction Fig. 228, B.



Fig. 228. 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.

There is no sharper line of separation between the stomach and duodenum than the sphincter muscle of the pylorus. Intestinal glands have been recorded in the stomach, and pyloric glands are said to extend into the duodenum. Moreover the leaf-like duodenal villi resemble the villous folds of the pylorus.


Fig. 229. Longitudinal Section of thk Human Duodenum. X 16.

The duodenum diflFers from the remainder of the small intestine by containing duodenal glands [glands of Brunner]. These are branched tubulo-alveolar structures which extend into the submucosa (Fig. 229). To a small extent they branch among the intestinal glands inside the muscularis mucosae, as seen in Fig. 230. Their ducts may either enter the bases


Intestinal glands

of the intestinal glands, or may pass between them to the surface. In form and position the duodenal glands suggest those of the oesophagus, but in structure they so resemble the pyloric glands as to have been considered identical with them. They produce a mucus which stains with diflBculty, and are free from goblet cells. As in the pyloric glands, occasional parietal cells have been observed, found chiefly inside of the muscularis mucosae. The dark cells due perhaps to compression, occur, and there are intercellular secretory capillaries. ^ A structureless basement membrane surrounds the tubules. 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 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.


Small Intestine— Jejunum and Ileum. As already stated, the small intestine is characterized by its glands and the viUi which impart a velvety appearance to its surface. In the jejunum the club-shaped or cylindrical villi are more slender and numerous than in the ileum; in the distal portion of the latter


Fig. 230.From a Section of a Human Duodenum. X 240.


Only the lower half of the mucosa and upper half of the submucosa are sketched. A lar;?e portion of the duodenal gland lies above the muscularis mucosae.


they are short and scattered, finally disappearing on the colic surface of the valve of the colon [ileocaecal valve]. Each villus consists of an epitheUal covering and a core of connective tissue, the tunica propria (Fig. 231). There are other and larger elevations in the lining of the small intestine, known as circular folds (plicae circular es) [valvulae conniventes]. As shown in Fig. 232, their interior is formed by the submucosa and their surface is covered by the entire mucous membrane, — villi, glands, and the muscularis mucosae. Since the tunica muscularis does not enter them they cannot be obliterated by distending the intestine. The circular folds begin in the duodenum


(Fig. 229), and beyond the duodenal papilla they are tall and close together. They are also highly developed in most of the jejunum, but distally, as in the ileum, they are lower and further apart. From the last two feet of the ileum, they may be absent. As their name implies, they generally tend to encircle the intestine. They may form short spirals, or branch and connect with one another. Some of them are so oblique as to appear cut transversely in cross sections of the intestine.


Fig. 231. Vertical Section ok the Mucous Membrane of the Jkjunum ok Adult Man. x So.

The space, a, between the tunica propria and the epithelium of the villus is perhaps the result of the shrinking 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.


There is only an arbitrary separation between the jejunum and the ileum; the latter contains fewer and shorter villi, and its circular folds are more widely separated.

The entodermal epithelium of the small intestine is of the simple columnar form and contains many goblet cells. Since that portion which covers the villi contains perhaps as many goblet cells as the part which lines the glands, it has been suggested that the latter are more properly termed pits. At the base of the glands, however, there are often some cells containing coarse granules, indicative of a special secretion. Its nature has not been determined. Such cells, known as cells of Paneth, are invariably present in the ileum, and often in the jejunum; they are not found in the glands of the duodenum, or in those of the large intestine, with the possible exception of the vermiform process. They are shown in Fig. 233.

The sides of the glands are formed of columnar cells and goblet cells, so arranged that the latter are seldom in contact with one another. It is thought that any of the cells may elaborate mucus and become goblet cells, in the manner described and figured on page ^^, Mitotic figures are often observed in the glands and seldom elsewhere. (In the stomach they occur near the necks of the glands.) From this it is inferred that the outer cells, including those of the villi, are replaced from below, and that the cells toward the fundus of the glands are renewed from above.


Fic. 232. Vertical Longitudinal Section of the Jejunum of Adult Man. x i6. 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 oetween the two. The isolated bodies lying near the villi (more numerous to the left of the plicae circulares) are partial sections of villi that were bent, therefore not cut through their entire length.


The epithelial cells of the villi are taller than in the glands, and the goblet cells are somewhat larger. The columnar cells are covered by a vertically striated top plate or cuticula, which is thinner in the outer part of the glands and is absent from their deeper parts. The striation is considcred due to protoplasmic processes lodged in pores. Terminal bars are also present. The goblet cells have a thin top plate, which in sections is

often ruptured to allow the escape of mucus. This is probably not artificial. Between the epithelial cells there are narrow spaces into which l)miphocytes often migrate (Fig. 234), and from which some of them may escape into the lumen of the intestine. The lateral walls of the epithelial cells are described as modified ectoplasm rather than true membranes. Their basal ends rest upon the tunica propria, which is a reticular tissue containing many small round cells in its meshes and supporting a central lymphatic vessel together with numerous blood capillaries (Fig. 23s). Smooth muscle fibers extend into it from the muscularis mucosae, and by contraction they shorten the villus and empty its lymphatic vessel. Eosinophilic cells, plasma cells and phagocytes may also be found in the tunica propria of the villi.

Interest in the villi centers chiefly in their relation to the absorption of nutritive material from the intestinal contents (chyme). Fat, probably in combination, is said to be received by osmosis through the cuticula. It appears in vacuoles in the outer part of the cells, as shown in osmic acid preparations, but neither within

nor in contact with the cuticula. It extends to the deeper part of the cells and is found in the intercellular spaces between the epithelial cells. It has been said that lymphocytes ingest it there and convey it to the cen


Cells of Paneth.

Fig. 233.— Three Intkstinai. Glands from Sections of THE Ilel'm, the One on the Right from a Mouse, THE Other Two from a Man. X 390.

The left j^rland was from a preparation fixed in Zenker's fluid, the other two were fixed in a potassium bichromate and formaline mixture.


Fig. 234.— From a Section of thk Small Intestine

FROM A Kitten Seven Days Old. x 250. The epithelium on the left contains many wandering

leucocytes (lymphocytes). The epithelium on the

rij^ht contains but three.


tral lymphatic, within which they break down and set free the fat, but this explanation of the transfer is not beyond question. It is well


Epithelium.


Tunica propria.


_ Portion of a capillar)' blood vessel.


Cuticula.


—^ Nucleus of a lymphocyte.


Tangential section of a goblet cell.


Mucus in a goblet cell.


Nucleus of a smooth muscle fiber. Central lymphatic vessel.

Fig. 235. Longitudinal Section through the Apex of the Villus of a Doc. X 360. The goblet cells contain less mucus as they approach the suni:nit of the villus.



known that fat enters the lymphatic vessels so that they become distended and white, their fatty contents being designated chyle. In regard to the absorption

of proteid material, the observations of Pio Mingazzini, which have been confirmed by some and denied by others, are of considerable interest. As shown in Fig. 236, he found that the basal protoplasm of the epitheUum presented an ordinary appearance before digestion (A), but that after absorption had progressed, hyaline spherules apappeared in it (B). As these became numerous they were detached from the cells, forming a reticular mass between them and the tunica propria (C).


Fig. 236. Stacks ok Intkstinal Absorption as Seen in Epithelial Cells of Villi from a Hen. (Alter Mingazzini.)

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


After the spherules had broken down and 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. Thus proteid absorptio n was ac complishe d as a se cretory process of the 1 1 qn Aelium^^ the product being eliminated from its basal portion. The ^ spherules accumulate at and near the tips of the villi in spaces which many authorities, including Professor Stohr, describe as due to the artificial retraction of the tunica propria (Fig. 231, a). The spherules have been considered a coagulum of the fluid squeezed from the reticular tissue. In part they may be the boundaries of the basal ends of epithelial cells on the distal wall of the villus. Often a delicate connective tissue artificially



Fig. 237. Diagram of a Mesentery as Seen in Cross Section of the Abdomen. (After Minot.)

a., Aorta; c. p.* cavity of the peritonaeum; Int., intestine:' met.* mesentery ; p. m. ana V. m., parietal and visceral layers of mesothelium.


Fig. 338.— Surface View of the Greater Omentum from A Rabbit. X 240. Thick and thin connective tissue bundles form meshes. The wavy striation of the bundles is obscured since the preparation is mounted in balsam. At X the epithelial cells of the opposite surface are visible.


shrinks from an epithelium, as seen in Fig. 22, p. 23. On the other hand, these considerations are familiar to those who interpret the spherules as the result of proteid absorption. It is well known that a certain amount of proteid is absorbed in the large intestine, and it has recently been found, by Dr. J. L. Bremer, that beneath its epithelium, reticular appearances simikj- to those in the small intestine occur after proteid digestion.

Crhe muscularis mu cosae of the small intestine consists of an inner circular and an outer longitudinal layer of smooth muscle. The submucosa is of loose fibrous connective tissue with few elastic fibers. The mus cularis includes an inner circular layer of smooth muscle fibers, and a much thinner outer longitudinal layer^ Between them is a narrow but important band of connective tissue, l^umerous elastic fibers are found not only on the surfaces of the muscle layers but also in their interior. Their abundance is directly proportional to the thickness of the musculature.

The serosa consists of connective tissue which is covered with mesothelium except along the line of attachment between the intestine and it^. mesentery. As shown in the diagram, Fig. 237, the mesentery is a thin layer of connective tissue bounded on either side by mesothelium, which serves to suspend the intestine from the median dorsal line of the body cavity. It is present unless adhesions occurring in the course of development have destroyed it, and in the^small intestine such adhesions involve only a part of the duodenum. \At the root 0} the mesentery (the portion attached to the trunk of the body) the mesothelium extends laterally and with the underlying connective tissue forms the parietal peritonaeum. The tunica serosa of the intestine and the lateral parts of the mesentery constitute the visceral peritonaeum (this term being applied especially to the former). The mesothelium of the entire peritonaeum consists of flat, polygonal cells shown in surface view in Fig. 238. '^he outer portions of the cells fit closely, but the deeper parts, containing the nuclei, are joined by intercellular bridges. y| Beneath the epithelium there is fibrillar connective tissue containing abundant elastic networks parallel with the surface, and having plasma cells and other free forms in its meshes., These cells are found especially along the blood vessels. The connective tissue layer is denser in the parietal than in the visceral peritonaeum. In places where the peritonaeum is freely movable there is a subserous layer of loose fatty tissue, but there is no distinct subserous layer in the intestine. The mesothelial layers on the opposite sides of the mesentery are so close together that they may both be seen in a surface preparation by changing the focus, or even simultaneously as at X in Fig. 238. The connective tissue between them is thin except where it surrounds the larger blood and l)rmphatic vessels and nerves which pass through the mesentery to and from the intestine.

Blood vessels 0} the small intestine. The arteries pass from the mesentery into the serosa in which their rnain branches tend to encircle the intestine. Smaller branches from these pass across the muscle layers to the submucosa in which they subdivide freely (Fig. 239, 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 forming 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

Fig. 239. A, 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 lympliatic vessels (atter 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. Ct intermuscular connective tissue ; I. m., longitudinal muscle ; s.» serosa, c. I.» central lymphatic ; n., nodule ; s. pl.t submucous plexus ; m. pi., myenteric plexus.

at the periphery (Fig. 239), or on one side of the villus with a vein on the other. The network of blood vessels in the villi is very abundant as shown

Fig. 240.— Vertical Section of the Mucous Membrane of the Human Jejunum. X 50. The blood vessels are injected with Berlin blue. The vein of the first villus on the left is cut transversely.

in Fig. 240. 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 muscle for the veins which penetrate it. 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 receives the blood from the intestines.

Lymphatic vessels. The intestinal lymphatics [lacteals] appear as central vessels within the villi (Fig. 239, B). Each villus usually contains one, which ends in a blind dilatation near its tip; sometimes there are two or three which form terminal loops. In some stages of digestion the distension 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. 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.


Fig. 241. 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 lo.


Lymphoid tissue. The lymphoid tissue of the intestine occurs primarily in the tunica propria, and in three forms, — diffuse lymphoid tissue, soh'tary nodules, and aggregate nodules. Solitary nodules are seen in Figs. 232 and 244. The latter shows how the nodule which arises in the propria may extend through the muscularis mucosae and spread in the submucosa, thus being as a whole, flask shaped or pyriform. A peripheral section of such a nodule may present only the part beneath the muscularis mucosae. The nodules are surrounded by small vessels, the lymphatics being drawn in Fig. 239, B. Blood vessels may make a similar net, and penetrate the outer portion of the nodule, ters are similar to those in the l)rmph glands.


The germinative cen

Fig. 242. A, Surface view of the plexus myentericus of an infant. X 50. g. Groups of nerve cells ; f, laver of circular muscle fibers recognized by their rod-shaped nuclei. B, Surface view of the plexus subraucosus of the same infant. X 50- 9* Groups of nerve cells ; b, blood vessel visible through the overlying tissue.

Aggregate^ nodules [Pe^er^s^ pat ches] are oval macroscopic areas, usually from i to 4 cms. long but occasionally much larger, composed of from 10 to 60 nodules placed side by side (Fig. 241). The nodules may be distinct or blended by intervening lymphoid tissue. 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. There are from 15 to 30 of them in the human intestine (rarely as many as 50 or 60) and they occur chiefly in the lower part of 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 branches of the jjujgjjjgf mesenteriir ; p^^^^^ pf the sympathetic system. This plexus is ventral to tJxe aorta, and sends branches through the mesentery into the serosa. (The manner in which they penetrate the other layers, forming the myenfeng^^feflgi ^Auerbaf^^'^s pky^isj i n the intermuscular connective tissue, and the submucous jkeocus. p^eiss ner^s plexus] in the submucosa is shown in Fig. 239, C. In surface view, obtained by stripping the layers apart,, these plexuses are seen in Fig. 242. 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. 239, C); it has been suspected, however, that some of these 'nerve cells' are portions of the reticular tissue. Their terminations require further investigation. Most of the intestinal nerves are nonmeduUated but they include a few large meduUated fibers said to have free endings in the epithelium.



Fig. 243.~Transvhrse Section of the Human Vermiform Process. X 20. (Sobotta.)

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

Only a part of the circular layer of the musculans has been drawn.

Large Intestine - Vermiform Process

The entire large intestine is characterized by the presence of intestinal glands associated with the absence of villi. ^ In human embryos of from


4 to 6 months there are villi in the large intestine, but they disappear before birth, by becoming flattened out. The vermiform process is distinguished from the colon by its small diameter and by the abundance of lymph nodules in its tunica propria. They are often confluent (Fig. 243). In old age the lumen of the vermiform process is frequently obliterated; this has been recorded in 50% of persons over 60 years old, and appears to be a normal retrogression. The epithelium with its glands, and the nodules disappear and are replaced by an axial mass of fibrous tissue. This is surrounded by the unaltered submucosa, muscularis, and serosa.

Large Intestine - Caecum and Colon

The intestinal glands of the caecum and colon are longer than those in the small intestine, — sometimes twice as long (0.4-0.6 mm.). They contain more goblgLcells, but cells of Paneth are absent. Striated cutic


Fig. 244. Vertical Shction of the Mucous Membrane of the Descending Colon of an Aduit Man. X 80.

Compare the length of the glands with those of the small intestine (Fig. 230), from the same individual

and drawn under the same magnification.

ular 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. Apart from the muscularis the remaining layers resemble those of the small intestine. The outer longitudinal layer of the muscularis is thin except where its fibers are gathered into three longitudinal bands or taen iae, nearly equidistant from one another, which terminate in the corresponding layer of the vermiform process. The latter is uniform and not separated into taeniae. V^^Siiice the longitudinal bands are shorter than the iimer layers of the colon, internal transverse folds are produced, called plicae 5gwi/unarg5?; (flnasmuch as the cirailar muscle layer is included in them, they differ from the plicae circulares o f the small intestinep (They occur at considerable intervals and between two successive plicae the wall of the colon exhibits a saccular dilatation or haustrum. The valve of the colon (valvula coli) is a pair of folds or labia J which are similar in structure to the semilunar folds; that is, they include fibers of the circular muscle layer, but the shorter layer of longitudinal fibers passes directly from the ileum to the colon without entering the valves. Figures of the bands, folds, pouches, and valves of the colon may be found in the text-books of anatomy, and to these the student should refer. The serosa of the colon contains lobules of fat which form pendulous projections known as appendices epiploicae.

Rectum and Anus

The rectum agrees in its general structure with the colon, and has even longer glands (0.7 mm.). Its lining presents transverse folds (plicae transversales) and in the anal region there are several longitudinal folds, the rectal columns. In this region the musculature is highly developed. The muscularis mucosae becomes thicker and enters the columns. The circular layer of the muscularis terminates as a special accumulation of fibers, the internal sphincter of the anus. Just beyond it is the external sphincter, a striated muscle of the perineal group. The three taeniae of the colon unite so as to form two in the rectum, a ventral [anterior] and a dorsal [posterior], but by the development of fibers between them the longitudinal layer becomes essentially complete and uniform. It terminates by joining the internal sphincter and neighboring muscles, and by ending in the subepithelial tissue.

A short distance within the internal sphincter the simple columnar epithelium abruptly becomes a thick stratified layer with flat outer cells. Its base rests upon vascular papillae. The rectal glands extend for a short distance into the region of stratified epitheUum. The circumanal glands which appear as modified sweat glands occur beyond the anus, in the skin.

The vessels and ner\'es of the large intestine are distributed essentially as in the small intestine, except for the absence of vilU. The great abundance of veins in the submucosa of the anal part of the rectum should be noted because of its clinical importance.

Liver

The liver is one of the three organic glands which develop from the digestive tube, the others being the pancreas and the lungs.

Deuelopment of the liver. . The liver arises as a clump of rounded masses of entodermal cells which proliferate from the ventral surface of the "pharynx^just anterior to the yolk sac. It is shown in the diagram Fig. 245, A. The liver at this stage lies between the vitelline veins, in the connective tissue which extends from the mesothelium of the pericardial cavity to the entodermal layer of the yolk sac. Since this connective tissue forms


Fig. 345. Diagrams of thk Development of the Liver.

A, The condition in a 4.0 mm. human embryo. B. A 12 mm. pig. C, The arrang^ement of ducts in the human a<luU. c. d.. Cystic duct ; c. p., cavity of the peritonaeum ; d.* duodenum : d.C.« ductus choledochus ; dlt., diaphragm : dlv., diverticulum ; f. I., falciform li^ment ; g. b., gall bladder ; g. 0., greater omentum; h.d.. n^P^tic duct; ht., heart ; Int, intestine; II., liver; I. 0., lesser omentum ; m., mediastinum; oe.. oesophagus ; p. c. pericardial cavity; p. d., pancreatic duct; ph., pharynx: p. v.* portal vrin ; St., stuniacn ; tr., trabecula ; v. C. I., vena cava inferior ; v. v., vitelline vein ; y. s., yolk sac.


a septum across the body, separating the cavity of the yolk sac from that of the pericardium, it is called the sej>tum transversumT^ Witii further growth the liver becomes divisible into two parts; first, a more or less cylindrical diverticulum of the intestine (Fig. 245, B, div.); and second, a mass of branched columns of entodermal cells, the hepatic trabeculae, which grow out from the diverticulum and form the essential part of the liver (Fig. 245, B, tr,). The trabeculae are not irregular detached islands as seen in single sections, but through anastomosis with one another they form a single complex network of solid cellular cordsi At first they are connected with the diverticulum by several strands of cells, as in B, but later all of these atrophy and disappear except one, which forms a permanent communication between the trabeculae and the diverticulum. After acquiring


The septum is bounded


a lumen it is known as the hepatic duct, C, h,d. The diverticulum becomes enlarged at its distal end to form the gall bladder, g.b. This has a tapering neck leading to the cystic duct, c.d. After receiving the hepatic duct, the diverticulum forms the common bile duct (ductus choledochus) which enters the duodenum. (Just before the entrance it is joined by the pancreatic duct, p. d.fj

From its development the liver is seen to be an entodermal organic gland with branched and anastomosing terminal pieces. Cll.^^^^^^P^ ^° the septum in close relation with the vitelline veins^ Before describing the structure of the adult liver the transformations of the septum and of the veins should be considered.

Transformation 0} the septum transversum. anteriorly, that is, toward the head, by the mesothelium of the pericardium and of the pleurae. Beneath the mesothelium, striated muscle spreads out in the septum transversum, producing the diaphragm. Ventrally the septum extends from the liver to the subcutaneous tissue of the abdominal wall; dorsally it passes from the liver to the lesser curvature of the stomach and the first part of the duodenum. Posteriorly, as shown in Fig. 245, B, a rupture occurs through it so that the cavity of the peritonaeum extends from side to side betwegp the diverticulum and the small intestine. (The gall bladder is thus provided with a seroriscpat, similar to that of the intestine, and it extends over the sides of the liver. It forms the lateral walls of the connective tissue layers passing from_^ the liver to the stomach, the diaphragm and ventral body wall. T These connective tissue layers with their mesothelial covering are mesenteric structures known as the ligaments of the liver. J The primary ones which represent the original septum transversum are the falciform ligament between the liver and ventral abdominal wall and diaphragm, in the median plane; the lesser omentum between the liver and the duodenum and lesser curvature of the stomach, also in the median plane (the blood vessels to the liver and the common bile duct are within the lesser omentum) ; and finally, the right and left triangular ligaments between the liver and the diaphragm. The triangular ligaments are compressed dorso-ventrally, so that their line of attachment to the liver is across the body from right to left. The relation of the right triangular ligament to the coronary ligament will be described with the



Fig. 246. The Lhft Side of an Adult Liver. (Compare wilh Fig. 244, B.)

d. C, Ductus choledochus; 0. b.,

f[all bladder; f. I., falciform ij^ment ; I. o., lesser omentum ; 1. t. i., left triangular ligament ; p. v.. portal vein ; r. I., round ligament ; v. C. I.* vena cava inferior.


blood vessekC^ Surrounding the ennre liver close to the hepatic trabeculae, the septum transversum produces the dense fibrous capsule [capsule of Glisson]. The fibrous capsule is covered by the tunica serosa everywhere except at the mesenteric or ligamentous attachments. Thus the transverse septum produces the diaphragm, the falciform and triangular ligaments, the lesser omentum, the fibrous capsule of the liver and the connective tissue portion of the serosa. It also gives rise to the connective tissue found within the liver. ,

Development of the veins of the liver. As seen in Fig. 245, A, the liver at once comes into close relation with the vitelline veins. The latter branch about the ramifications of the hepatic trabeculae producing sinusoids (described on page 125). At first there are two vitelline veins, a right and a left, one on either side of the intestinal tract. They anastomose ijv-ith one another dorsal to the duodenum as shown in Fig. 247, A. Ventral to a

more distal portion of the duodenum they fuse and thus proceed to the yolk sac. By the obliteration of the portions of these veins indicated in Fig. 247, B, the portal vein is ioTm^, and its adult relations to the duodenum are explained. It receives the blood from the intestines, stomach, spleen and pancreas, through branches which develop later, and conveys it to Fig. 247. the liver. It follows the hepatic duct and its

The formation of the portal vein *

p. v.. from the right and left branches into the liver, where it is resolved into

vitelline veins, r. v. and I. v. ; '

^; Ju"^rio?mk!^\ii^rvkin': sinusoids. Thcse unite anteriorly to form that part of the vena cava inferior which passes from the liver to the right atrium of the heart. As may be seen in Fig. 245, this part of the vena cava is essentially a persistent portion of the \dtelline veins. Three other veins connect with the vitelline sinusoids in the liver, namely the right subcardinal vein which forms a large part of the vena cava inferior, and the right and left umbilical veins.

The distal portion of the vena cava inferior is derived from the right vein of a pair which pass up the back near the aorta; their position may be understood since their anterior parts persist as the azygos and hemiazygos veins of the adult. In the embr\'0 these veins are prolonged posteriorly (in part) as the right and left subcardinal veins, shown in the cross section, Fig. 248. This figure indicates also that the liver fuses with the dorsal wall of the abdominal cavity on the right side of the body (at x). No corresponding adhesion occurs on the left. After the fusion has taken place, the right subcardinal vein anastomoses with the hepatic sinusoids, and all of the blood from the limbs which it formerly took to the heart by way of the azj'gos vein, now passes to the vitelline portion of the inferior vena cava. The original connectimi between the right subcardinal vein and the azygos vein is destroyed, (in^short the vena cava inferior represents a combination of different veinsj) The part from the hepatic sinusoids to the heart is derived from the original vitelline veins as shown in Fig. 245^ B. The distal part includes another vein secondarily joined to the former through the adhesion of the right lobe of the liver to the dorsal abdominal wall . This adhesion is of fundamental importance. It appears on the dorsal surface of the adult liver (Fig. 249, c, /.) as a somewhat triangular area, uncovered by peritonaeum, known as the coronary ligament. This ligament is in reality an extension downward of the right triangular ligament. It is usually described, however, as interposed between the right and left tri


Fig. 24S. Cross Section of a Mammalian Embryo, to show the Adhesion, x, between THE Right Lobe of the Liver and THE Dorsal Abdomi.nal Wall.

ao.. Aorta : f. C. fibrous capsule and serosa : f. I., falciform lif^inent ; g. o., greater omentum : 1. 0.« lesser omentum ; I. s-C. v.. left subcardinal vein ; o. b., omental bursa : r. i-C. v., ris^ht subcardinal vein ; St., stomach ;. v. um., left umbilical vein.


Fig. 249.— Dorsal Si'Rface of the Adult

Liver.

. I., Coronary li{2:ament: f.l., falciform ligament;

f|. b., Rail bladder ; I. o., lesser omentnm ; 1. 1. 1., eft triangular ligament ; o. b., caudate lobe bounding the omental bursa venlrally ; p. v., portal vein ; r. I., round ligament ; r. t. I., right triangular ligament ; v. c. I., vena cava inferior.


angular ligaments. The coronary adhesion leads to the formation of the foramen epiploicum [of Winslow] and bounds the superior recess of the omental bursa (Figs. 248 and 249, o,b.). The foramen and bursa are further described in text books of anatomy. The development of the vena cava is figured with the veins of the WolflSan body, on page 246.

The umbilical veins, one on either side, pass from the umbilical cord through the lateral abdominal walls to the liver, which they enter through the septum transversum. They connect with the sinusoids. The right umbilical vein subsequently becomes small and loses its connection with the liver (Fig. 248). The left umbihcal vein is gradually shifted to the median ventral line and passes from the umbiUcus to the liver along the free margin of the falciform ligament. It maintains a distinct channel across the liver, apparent on the lower hepatic surface. From the ventral margin to the lesser omentum, the umbilical vein is on the left of the gall bladder from which it is separated by the quadrate lobe. After the umbilical cord is severed the vein becomes a fibrous strand, known as the round ligament of the liver, Fig. 249, r. /. It extends from the umbiUcus along the free margin of the falciform ligament, and under the Uver to the lesser omentum. From this point to the vena cava the umbilical vein is called the ductus venosus. It follows the hepatic attachment of the lesser omentum, and there it forms after birth, the ligament 0} the ductus venosus.

Development 0} the connective tissue and hepatic artery. The history of the liver has been described to that point where it consists of a great network of ; entodermal trabeculae connected with the intestine by a single


Fig. 250, From a Tangential Section of the Human Liver. X 4oThe 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.


duct. Its trabeculae are separated by a very small amount of connective tissue from the endothelium of the sinusoids. The latter are essentially subdivisions of the portal vein which reunite in the vena cava inferior. Later in development the connective tissue around the principal branches of the portal vein increases so as to be conspicuous; to a less extent that which surrounds the main hepatic branches of the vena cava is also increased, (^nce the portal branches are associated with the bile ducts they may be cfistinguished from the caval branchep Moreover the hepatic artery which develops rather late, grows into the connective tissue along the bile ducts. It suppUes the fibrous capsule and the connective tissue layers with capillaries, which empty into the adjacent sinusoids and into the portal capillaries limited to the connective tissue. Thus there is j a

capillary circulation in the liver, in addition to the sinusoidal, but the former is essentially confined to the connective tissue.

^^Microscopic appearance, 0} the adult liver. In Sections of the adult human liver there will be seen clumps of connective tissue which contain branches of the portal vein, hepatic artery, and bile ducts, the last being easily distinguished by their columnar or cuboidal epithelium (Fig. 250). Lymphatic vessels and nerves (non-medullated fibers but no nerve cells) may also be found in this connective tissue. There is a tendency for the connective tissue areas to anastomose with one another. Pathologically


Fig. 251. Liver ok 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, trabeculae; f, central vein.

in man, but normally in certain animals, as in the pig, this anastomosis is complete and polygonal areas of hepatic trabeculae are thus made prominent (Fig. 25]^(These are the lobules of the liver, and the connective tissue around them is the interlobular connective tissue, containing interlobular veins (the branches of the portal). In the center of each lobule is a large sinusoid, the central vein (sometimes there are two). Toward it the sinusoids converge from the interlobular veins on all sides (Fig. 252), and from it the hepatic trabeculae radiaje." 'The central veins open, usually at right angles, into the larger sublobular veins (Fig. 253) The latter, being derived from sinusoids, have notably little connective tissue in their walls. The sublobular veins unite to form the hepatic tributaries

of the vena cava inferior, ^he path of the blood through the liver is then briefly as follows: portal vein, interlobular veins, sinusoids, central veins, sublobular veins, hepatic veins, vena cavainferifl^ The hepatic artery tlrrough capillaries connects with the interlobular veins and with the sinusoids at the periphery of the lobules. Certain pathological conditions suggest that the cells near the center of

the lobules are not^s w^ell nourished as those at the periphery. He^qtw_C€Us, (^The hepatic cells are arranged in anastomosing tra


Fig. 252. From a Section of the Human Adult Liver Injected through the Portal Vein.


Hepatic lobules.



Fig. 253. From a Vertical Section of a Cat's Liver, Injected through THE Vena Cava Imkrior.

The central veins and the sublobular vein Into which they empty are cut longitudinally. X 15.


beculae as shown in Fig. 254. Near the central veins they form terminal loops. The cells composing the trabeculae are polygonal or cuboidal with an exoplasmic layer which sometimes resembles a cell membrane. The cells contain round nuclei which are variable in their staining capacity;


Fig. 254. From a Cross Sbction of a Human Hepatic Lobule, x 300.

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

precipitates of the silver.


they may be dark or paleT^ Often a cell contains two nuclei, and rarely large cells with several nuclei produced by amitosis, have been found. The protoplasm is granular. It 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 well nourished individuals. Pathologically the vacuoles may be large and have a wider distribution. Glycogen (page 51) 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 clear central part and dark periphery (Fig. 255). In man both conditions may be found in one liver. The bile, secreted by the hepatic cells, frequently contains drops of fat and pigment granules such as occur within the cells. It is eliminated through the bile capillaries. *

Bile capillaries. ^The hepatic trabecule, as they proliferate from the diverticulum, form a network of solid cords. Within the cords a slender lumen develops later, apparently beginning at the periphery of the lobule and extending toward the center. It causes such cords as consist of only two rows of cells to resemble the tubules of other glands, as shown in the diagram Fig. 256. In uninjected sections the lumen, if cut across, appears as a minute circle midway along the line of contact between two hepatic



Fig. 255. Livkr Cells of Man.

A, Isolated liver cells containing smaller and laiRcr fat drops, f.; b., imprint from contact with a blood vessel. B, From a section ; 1, empty cells ; 2, cells filled with secretion.



^ Gland lumen (bile capillary).


Blood vessels.


Fig. 256. Diagram of a Tubule of THE Liver (Disregarding the Anastomoses with Adjoining Tubules).


Fig. 257. — Section of the Liver of a Rabbit with the Bilk Capillaries Injected. X 560. (AW a Diagram.) Two of the cells are each in contact with four sinusoids

(1, 2, 3, 4) and four bile capillaries. X, a bile capillar}*

where three cells meet.


cells. Its sharp contour is ascribed to a cuticular formation belonging to the cells which bound it. In longitudinal view it appears as a dark intercellular line suggesting a cell wall. Both views are shown in the injected specimen Fig. 257, where, however, those seen longitudinally seem to disregard cell boundaries; this is because they lie in intercellular spaces turned toward the observer, the cells beneath presenting an uncut surface. Sometimes (as at x) a lumen occurs at the angle where three hepatic cells meet, but usually sinusoids are found at the comers of the cells and as seen in the figure, a lumen tends to be placed as far from the blood vessels as possible.


Fig. 258.— From a Rabbit's Livkr. X 570.

The cells 1, 2, a and b, are cut in halves ; their four bile capillaries (includine I and II) may be intercellular branches of trabecular bile capillaries shown in the group 3, 4, C and d. The latter are necn in surface view, the plane of section there beine between the cells. Tne actual arrangement can be determined only by reconstruction.



^lCTnle capillary , as the lumen

is called, follows the trabeculae, branching and anastomosing as they do (Fig. 254). Between the hepatic cells, the bile capillaries send off branches at right angles. These inlercelltdar capillaries are similar in diameter and structure to the trabecular capillaries. They are unbranched and end blindly before reaching the vascular surface of the cells (Fig. 254). In cases of pathological obstruction

of the bile ducts, however, the intercellular capillaries are said to be prolonged to that surface and to rupture, so that bile enters the tissue spaces and the vessels, producing jaundice.^Nj

Intracellular bile capillaries also occur; several have been found to extend from the trabecular capillary into a single hepatic cell. As seen in

Golgi specimens they may terminate in knobs interpreted as vacuoles of secretion (Fig. 259). Since neighboring capillaries are free from these branches, the intracellular capillaries are regarded as phases of functional activity, accompanying the discharge of secretion. They have been reported as forming baskets similar to the secretory capillaries of parietal cells. In studying intracellular capillaries, care must be taken to exclude surface views of intercellular forms.

Sinusoids and perivascular tissue. The endothelium of the smusoids is separated from the hepatic cells by a thin layer of reticular tissue. With special methods this tissue is seen to consist of innumerable fine fibers free


Fig. 259. From a Golgi Preparation, of the Liver of a Dog. X 490.


from elastic elements. The meshes of the reticular tissue are drained by the lymphatic vessels of the capsule and interlobular tissue; the reticular tissue itself contains no vessels. Unlike other reticular tissue, that of the lobules is free from cells in its meshes. In the embryo, however, it contains large numbers of er>'throblasts and is for a time an important source of blood corpuscles. A few ner\'e fibers which terminate upon the hepatic cells, have been found in it.

The endothelium of the sinusoids is easily penetrated by injections, which spread in the reticular tissue, and even enter the hepatic cells. The


Fig. 260. Golg! Preparation of the Liver of a Doc. X 240.


blood vessels are not supposed to extend into the cells; the injection mass probably invades the trophospongium or other intracellular canals. In chloride of gold preparations the endothelial cells of the sinusoids appear stellate and have been mistaken for connective (retiailar) tissue cells. They are phagocytic. Often they are called the stellate cells [of Kupflfer]. The ducts of the liver. The ducts in an island of interlobular connective tissue drain the bile capillaries from all the surrounding lobules. If lines are drawn connecting the central veins with one another they will bound areas (structural units) comparable with the lobules of other organs:

in their centers the ducts are found. The actual connection between the trabeculae and bile ducts is very difficult to observe in ordinary sections, although it is easily seen after the ducts have been injected, or in Golgi preparations (Fig. 260). The transition from hepatic cells to the low cuboidal cells of the small ducts occurs abruptly at the borders of the lobule. The cuticula of the bile capillaries is continuous ^^'ith that of the ducts. The larger interlobular ducts have a simple columnar epithelium. They are said to anastomose with one another.

The cystic , hepatic , and common bile d ucts all have a simple columnar epithelium, containing occasional goblet cells. It rests on an elastic tunica propria, surrounded in turn by a submucosa. In the cystic duct the mucosa is thrown into coarse transverse folds, containing muscle fibers, known as the spiral valve. In the hepatic and common bile ducts especially, branched mucous glands extend into the connective tissue layer (glandulae mucosae biliosc^). Outside of them is a tunica muscularis consisting chiefly of circular fibers. These form a sphincter around the bile duct at the duodenal papilla (and there are similar sphincters around the outlets of the pancreatic ducts). The parts of the ducts exposed on the under surface of the liver are covered bv a serosa.


In the gallbladder iht mucosa forms a network of folds. The columnar epithelial cells are twice the height of those in the common bile duct. Goblet cells are absent and glands are infrequent. Solitary nodules may be found in the mucosa. The muscular layer is of obliquely circular fibers in a plexiform layer. Among them are groups of sympathetic nerve cells, which innervate the muscle. There are also meduUated ner\'e fibers in the gall bladder which terminate in its epithelium.; The subserous portion of the serosa is highly developed and contains largclymphatic vessels. \.

The vasa aberrantia of the liver are bUnd ducts which extend beyond the territory of the trabeculae. They are found about the left lobe, and especially around the vena cava, the porta hepatis and the left triangular ligament, and represent portions of the liver from which the hepatic cells have degenerated and disappeared.

The porta hepatis, meaning *gate of the liver' is the place where the vessels enter and the ducts leave, thus corresponding with the hilus of other organs. There the lymphatic vessels and the nerves are very numerous. The latter, chiefly non-meduUated, form networks around the vessels and ducts. They extend into the capsule and interlobular tissue, chiefly supplying the blood vessels. Some, however, continue into the lobules to the hepatic cells. The Ijonphatic vessels anastomose freely in the capsule and in the interlobular tissue, these sets connecting with one another. They do not enter the lobules.


Pancreas

The pancreas is a large entodermal gland consisting of lobes and lobules and resembb'ng in its general structure^the parotid gland. It arises

as two distinct outgrowths of the digestive tract, as seen in Fig. 261, A. The smaller of these, called the ventral pancreas , develops from the duct u s choledochus near its intestinal orifice. Its duct, called the pancreatic duct [of Wirsung], opens beside the common bile The papilla is a hollow eleva


FiG. 261. A, Diagram of tub Pancrkas from a 15 mm. Human Embryo. B, Dissection of thk Diodenlm and Pancrkas of an Adult. (After Schiomer.)

a. p. d.. Accessory pancreatic duct ; c. d., cystk duct ; d., duodc-num ; d. C, ductus choledochus; d. p., dorsal pancreas; h. d., hepatic duct; p., duodenal papilla ; p. d., pancreatic duct ; tt., stoniach ; v. p., ventral pancreas.


Slood ve««e]^


duct at the base of the duodenal papilla. tion of the mucosa, which has been spread open in Fig. 261, B. The larger part of the pancreas grows out separately, from the dorsal wall of the duodenum between the papilla and the stomach. The duct of this dorsal pancreas is the accessory pancreatic duct [of Santorini]. The dorsal pancreas fuses ^ith the ventral so as to make a single gland of uniform structure, the former producing its body and tail, and the latter contributing to the head. The two ducts anastomose as shown in Fig. 261, B, and the outlet of the ventral duct becomes predominant.





Fig. 262. An Island of thk Pancreas with the Slrroi'Nuing Alvkoli, from an .Adult. X 4«>


The intestinal end of the accessory duct is sometimes obliterated, but it may remain pervious and be of clinical importance in case of obstruction of the main duct. It opens about an inch above the papilla. (In the pig, which is often studied embryologically, the dorsal pancreas enters the duodenum distal to the papilla; its duct persists whereas that of the ventral pancreas is obliterated.)

As is true of most glands, the developing tubules of the pancreas are at first solid, but in the pancreas alone certain portions of the proliferating tubules become detached from the rest, forming islands of_ solid cords of cells. These islands [of Langerhans] were not found in a human embryo of 28 mm. (S3 days) but have been recorded at 54 mm. (73 days). Tjjey are then roimd or oval masses of cells rich in finely granular eosinophilic protoplasm, which are still connected with the developing alveoli. ) Later they become detach^, and by the invasion of capillaries of large diam. eter they are irregularly subdivided into cords as seen in Fig. 262. The islands are said to appear first in the tail and body of the pancreas, and later in the head where they are always relatively fewer. In an early stage they are

at the peripher}' of the lobules which are bounded by abimdant connective tissue, but subsequently they are surroimded by the proliferating alveoli which reduce the connective tissue to interlobular septa (Fig. 263). Cl^ is not now supposed that the islands arise from connective tissue, or that they are produced in adult life by the degeneration of alveoli. The islands have neither ducts nor lumen. Their secretion, which is internal, is received by the blood vessels. It is essential for the metabolism of sugar as shown by experiment. After removal of the pancreas, sugar appears in the urine; on the other hand if the pancreatic ducts are tied the alveoli degenerate but the islands remain intact, and sugar does not appear in the urine. Thus the islands constitute an organ within but functionally independent of the pancreasTN



Fig. 263.— Section of Human Pancreas, showing Several Islands, f.

a, Interlobular connective tissue containing an interlobular duct, c; b, capillary; d, interlobular duct; e* alveoli. (Radasch.)


In sections of the adult pancreas the islands are areas from .07 to 0.3 mm. in diameter, occupied by cords or groups of polygonal cells, the boundaries of which are often indistinct. The nuclei, round or oval, contain chromatin in many small granules, together with a few larger ones. The protoplasm is finely granular and in certain of the cells only, it is said to stain intensely with saffranin. Sometimes the protoplasm appears reticular. The islands may be separated from the alveoli by a considerable layer of connective tissue in which the elastic elements are infrequent, or by a thin basement membrane. Sometimes even the latter is absent. The endothelium of the capillaries is surrounded by a very small amount of reticular tissue. The pancreatic and accessory pancreatic ducts are lined with simple columnar epithelium which is surrounded by an inner dense, and an outer loose layer' of connective tissue. The latter contains some smooth muscle fibers which are gathered into sphincters at the outlets of the ducts. Occasional goblet cells, and small glands resembling mucous glands,



Tubule. Fig. 264.— Diagram of the Pancreas.



Zymogen granules.


A B

Fig. 265. — From Sections of a Hlman Pancreas. X 50a

In section A the granules are wanting, the elements of the intercalated duct are flat and dark ; in section B the granules are distinct, the cells of the intercalated duct are cubical and clear.


have been found in the mucosa. ( The epithelial cells become lower! in the smaller ducts, and are cuboidal or flattened parallel with the^ long axis in the intercalated ducfer^ There are no secretory ducts in the pancreas. The long intercalated oucts terminate in the alveoli (or acini) in a peculiar manner. /^As seen in Fig. 265, the cells of the duct seem prolonged into the center~of the alveolus, where they are known sscentroacinal cells^ In development the duct is not invaginated into the alveolus, but the latter develops so as to consist of two layers, only the outer of which produces the pancreatic secretion. Sometimes the inner cells are lacking. The lumen of the intercalated ducts and alveoli is very small and in many parts of a section it cannot be seen. Intercellular secretory capillaries extend from it between the centro-acinal cells to the secreting cells, as seen in Fig. 266. They may be prolonged between the latter, but they do not reach the basement membrane.


Intercellular secretor>' , • capillary-.



Inter- Centroacinal cells,

calated

duct. \ Cells of the

\ 1 alveolus.

\



Fig. 266.— a. From a Section op the Pancreas of Adult Man X 320; B, An Interpretation OF the Right Lower Portion of A.

The secreting or pancreatic cells are mostly arranged in alveoli but in part they form tubules. Toward the lumen their protoplasm contains a zone of coarse granules of zymogen, which accumulate while the cell is inactive and are eliminated during secretion. Apparently they are transformed into fluid as they 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 reticular. 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 with 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 blood and lymphatic vessels and the nerves 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 nonmedullated sympathetic fibers from the coeliac plexus, associated with scattered nerve cells within the pancreas. Lamellar corpuscles may be found in the connective tissue.



Development of the Respiratory Tract. The respiratory system, consisting of the larynx, trachea, bronchi, and lungs, arises as a gland-like subdivision of the entodermal tract. Beginning opposite the third or fourth branchial arch, two longitudinal

grooves develop, one on either side of the embryonic * pharynx.' They deepen posteriorly and unite, thus separating the ventral trachea from the dorsal oesophagus. The trachea and oesophagus open anteriorly into the pharynx of the adult, ^he anterior end of the trachea, with the epiglottis, thyreoid, cricoid and other cartilages which develop in the connective tissue around it, constitutes the larynx.) Posteriorly the trachea bifurcates, as seen in the front view of the embryo, Fig. 267, A, and these primary subdivisions or bronchif further subdivide as shown in B. In side view the right lung of an older embryo is shown in Fig. 268; the left lung has been cut away. (The entodermal outpocketings are seen to lie in abimdant connective tissueWhjch is invaded by blood vessels from tlmee sources, — the pulmonary arches, the left atrium and the thoracic aorta^ Some branches which grow from the azygos veins are ^ot shown. .

The pulmonary arches are two arteries, one on either side, extending from the ventral to the dorsal aorta. Approximately midway in its course each sends a branch to the lung of the corresponding side. The part of the arch between this branch and the dorsal aorta is early obliterated on the right side, but on the left it persists until birth'as the ductus arteriosus (Fig. 268, d.o,). After birth it is reduced to a fibrous cord which sometimes retains a minute lumen. The spiral division of the ventral aorta into the proximal parts of the permanent aorta and pulmonary artery, has been referred to


Fig. 267.— Reconstructions of the Lungs of Young Embryos, seen FROM THE Ventral Sirfack.


A, A younger staee than B bronchus; 1,11 (His.)


ep, apical primary bronchi.

in connection with the heart. The pulmonary artery of the adult leaves the heart as a subdivision of the ventral aorta; it divides into right and left rami, apparently simple vessels, but in reality each of them consists of the proximal part of a pulmonary arch together with a branch of that arch. In Fig. 268, there is no indication that the left ramus, /.r. includes a part of the left pulmonary arch.

The pulmonary veins grow out from the left atrium as a single vein with four main branches. By expansion of the atrium the proximal part of the vein is incorporated in its wall and the four branches, two from each lung, then open separately. The capillary subdivisions of the veins anastomose with those of the pulmonary artery to form the principal blood supply of the lungs.

The small bronchial arteries which supply the connective tissue of the lungs are branches of the thoracic aorta, one or two on each side. Their capillaries join those of the bronchial veins derived from the azygos veins.- In part they connect with the pulmonary veins. Since the bronchial arteries convey 'arterial blood' whereas the pulmonary arteries contain Venous blood,' the former may be compared physiologically with the hepatic artery in the liver.

The connective tissue in which the entodermal part of the lungs ramifies, occurs as a pair of lateral swellings of the mediastinum. The mediastinum is the connective tissue surrounding the oesophagus and extending between the heart and the dorsal aorta. It is bounded on either side by the mesothelium of the body cavity, and so has the structure_of_ji broad m esentery of th e hea rt The pair of mediastinal swellings or 'pulmonary wings'

project into that portion of the coelom which connects the median pericardial cavity, on either side of the mediastinum, with the peritonaeal cavity. These portions of the coelom become cut off, first from the pericardium and later from the peritonaeum, thus producing two closed sacs, the pleural cavities. Each of these is lined with a continuous layer of mesothelium, which, with the underlying connective tissue, con


Fig. 268.— Reconstruction of a Part of a Human Embryo of 13.8 MM. (Dr. F. W. Thyng.)

ao.. Aorta ; d. a., ductus arteriosus; l.,entocIermal part of the lung; I. at., left atrium; I. br., left bronchus; I, r., left ramus of pulmonary artery, p. a.: r. r.. Its right ramus; oe.» oesophagus; p. c, pericardial cavity; p. v., pulmonary vein: a. t.» septum transversum ; ih. ao., thoracic aorta ; tr., trachea.


stituteS the pleura. The parietal pleura is the part attached to the body wall; the pulmonary pleura covers the lungs; other subdivisions 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 ner\'es extend. This is the root of the lung.

Development of the alveoli. Fig. 269, A, from an embryo of four months, shows a portion of the lung adjacent to the pleura. The terminal subdivisions of the bronchi are lodged in an abundant, vascular connective tissue. They are lined with a simple cuboidal epithelium and are glandlike in form. This appearance is retained until birth when they become distended with air. Then their cuboidal cells are flattened, and many of them are transformed into thin non-nucleated plates (Fig. 269, B). The


Fig. 269.— Sections of the Visckral Pleura, pi., and Adjacent Alveoli, tl., from the Lung of A Four Months Embryo, A, and from an Adult, B.

tr.. Artery ; b. v., blood vessel ; cap., capillary ; |y., lymphatic vessel ; •., surface view of alveolar wall ; v., vein.

connective tissue between the alveoli is compressed into strands scarcely wider than the diameter of a capillary. In fact the capillaries which they contain are in contact with the respiratory epithelium of both of the adjacent alveoli. A section of the adult lung is essentially a network of these slender partitions, scattered among which are islands of connective tissue containing the bronchi and vessels. There are also connective tissue septa, dividing the lung into lobules.

^Summary. The lungs develop as a branched entodermal gland with the trachea and bronchi as its ducts. The terminal alveoli become greatly distended and their cells form flat plates adapted for respiration but not for secretion. The lungs have two sets of blood vessels, both capillary in t\pe, — the pulmonary and the bronchial vessels. The connective tissue forms a peripheral layer which is part of the pleura, and a large mass at the root of the lung. Within the lung it forms interlobular septa, and the thin interalveolar layers, but it is most conspicuous around the bronchi. In the following sections the structure of the respiratory tract will be considered beginning with the larynx, and proceeding posteriorly^^

Larynx

The mucous membrane of the larynx is a continuation of that of the pharynx, and likewise consists of an epithelium and tunica propria. A submucosa connects it with the underlying parts. In most places the epithelium appears to be stratified and colunmar, but it is said to be pseudostratified, with nuclei at several levels. It is difficult to determine whether or not all of 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. (j\ stratified epitheUum with squamous, non-ciliated outer cells is found on the vocal folds [true vocal cords], the anterior surface of the arytaenoid cartilages and the laryngeal surface of the epiglottisT^The distribution of the two sorts of epithelium anterior to the vocal foBsis subject to individual variation. The squamous epitheUum often occurs in islands. The tunica propria consists of numerous elastic fibers and fibrillar connective tissue, which in the lower animals forms a dense membrana propria under the epithelium. It also includes reticular tissue containing a variable number of leucocytes; solitary nodules may be found in the ventricle of the larynx [sinus of Morgagni]. PapiUae in the tunica propria are chiefly in the region of the squamous epithelium. At the free border and on the under surface of the vocal folds, 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 nmi; they are abundant in the ventricle but are absent from the middle part of the free border 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 cartilagejsjound in the entire epiglottis, the cuneiform and comiculate 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 in 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 encircle individual taste cells. The nerves and vessels of the larynx are numerous, except in the dense elastic tissue of the vocal folds. The ventricular folds [false vocal cords] consist of loose fatty, glandular tissue rich in vessels.

Trachea

The trachea consists of a mucosa, submucosa, and a fibrous outer layer containing the tracheal cartilages. The outer layer is continuous with the tissue of the mediastinum. It forms the perichondrium surrounding the succession of hyaline C-shaped cartilages, the free ends of which are toward the oesophagus. In the interval between these ends, there is a layer of transverse smooth muscle fibers, usually accompanied by bundles of outer longitudinal fibers. As in the intestine, elastic fibers are abundant among the muscle cells. The tracheal cartilages may become partly calcified in old age.

The submucosa is a layer of loose fatty connective tissue, continuous on its outer side with the perichondrium. It contains the bodies of the branched, mixed tracheal glands. On the dorsal or oesophageal wall of the trachea, these glands are larger than elsewhere and extend into or through the muscle layers.

The mucosa is separated from the submucosa by a distinct dense layer of elastic fibers, chiefly longitudinal. This layer has been compared with the muscularis mucosae of the intestine. Between it and the epithelium there is a thin layer of tissue, containing elastic fibers and having leucoc)rtes in its meshes. A basement membrane is found beneath the epithelium. As in the larynx the epithelium is pseudo-stratified and columnar, with cilia proceeding from distinct basal bodies. It contains goblet cells. On the oesophageal surface there have been found areas of non-ciliated, stratified epithelium, _with connective tissue papillae beneath, and squamous cells on its surface^


Bronchi and Bronchioles

The primary bronchi have the same structure as the trachea. In their subdivisions changes occur, the C shaped cartilages being replaced by irregular plates found on all sides of the tube (Fig. 270). These diminish in size and thickness as the branches of the bronchi become smaller, and disappear in those about i mm. in diameter. Branched tubulo-alveolar glands occur as far as the cartilages extend. They are situated in a loose connective tissue layer containing many nerves, blood and lymphatic vessels, together with small lymph glands. The bodies of the bronchial glands lie outside of a rather loose smooth muscle layer with fibers chiefly circular. The mucosa is thrown into longitudinal folds. It consists of a pseudo-stratified ciliated epithelium in the larger bronchi, changing gradually to a simple epitheUum in the small ones. The stroke of the cilia, as in the trachea, is toward the pharynx. The epithelium contains goblet cells, and rests on a tunica propria which has many elastic fibers and lymphocytes. The latter may accumulate in nodules.




Fig. 270.--Cross-section of a Bronchus 2 mm. in Diameter, from a Child.


Bronchioles are the small subdivisions of the bronchi, measuring from 0.5 to i.o mm. in diameter. They are free from cartilage and glands but have a columnar ciliated epithelium throughout. Obviously the distinction between the smaller bronchi and the bronchioles is arbitrary. The terminal branches of the latter are called respiratory bronchioles.

Respiratory Bronchioles, Alveolar Ducts, Alveolar Sacs,

Alveoli.

An arrangement of the ultimate branches of a bronchiole is shown in the diagram, Fig. 271. The respiratory bronchioles^ 0.5 mm. or less in diameter, at their beginning contain a simple columnar ciliated epithelium.


Fig. 271. Diagram of a Lobule of thr Ling, showing the Blood Vkssels and the Terminal Branches of a Bronchiole.


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 together with the isolated cuboidal cells remaining among them 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 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. 272, A). The respiratory epitheUum 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 ducis^ from I to 2 nmi. long, differ from the respiratory bronchioles in that they contain only the respiratory epithelium and are thickly beset with alveoli.


A Border of an alveolus. B Fundus of an alveolus.

Fig. 272. From Sections of the Human Lung. X 240. A, Mixed epithelium of a respiratory bronchiole ; B, an alveolus sketched with chanjfe of focus ; the border of the alveolus is shadwl ; 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.)

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.


Fig. 273. — Camhra Licida Drawing from a Skction of a Calf's Lung. (Miller.)

The stippling indicates smooth muscle and cuboidal epithelium; the lines, respiratory epithelium. B. R., Respirator>' bronchiole ; D. A., alveolar duct ; A., atrium ; A. 8., alveolar sac.


The alveolar ducts branch to produce alveolar sacs [infundibula] which are cavities in the center of clusters of alveoli. The sacs resemble the ducts as shown in Fig. 271. According to Professor Miller, who has made reconstructions of these structures in the human lung, an at rium or round cavity should be recognized between the alveolar duct and the alveolar sacs. The alveolar duct opens sometimes into five atria from each of which several alveolar sacs proceed (Fig. 273). If the student in examining this figure questions why the atria are not alveolar ducts, and the alveolar ducts are not respiratory bronchioles, it may be said that these terms are variously employed by different histologists, and that atria are not recognized by German writers. It seems questionable that the final ramifications of the lung are so definitely arranged as to justify the cumbersome nomenclature in current use. ,f^. 273 shows, however, exactly what may be expected in any section of the lung, namely (i) alveoli; (2) spaces bounded by alveoli (alveolar sacs, atria, alveolar ducts, the last being supposed to have muscle fibers associated with them) ; (3) small bronchioles with alveoli along their walls, therefore consisting of a mixed epithelium (respiratory bronchioles); and (4) bronchioles with no respiratory epithelium^ The alveolar walls have been described as consisting of respiratory epithelium (Fig. 272, B). The non-nucleated plates are presumably derived from the flattened nucleated cells scattered among them, and large plates arise from the fusion of small ones. In amphibia, nuclei in small amounts of protoplasm are found attached to the edges of the plates, and projecting into the connective tissue between the capillaries.

The abundant capillary network of the alveolar walls is shown in Fig. 274; lymphatic vessels are absent. Elastic tissue is highly IBflSIRSSBSiiSBHi^^ — Artery, developed around the alveoli and forms rings encircling their outlets. In inspiration an alveolus may expand to three times the diameter to w^hich it returns during expiration (o.i to 0.3 nma.).

Pores have been described, leading from one alveolus to another (Fig. 272, B).

The pleura is essentially similar to the peritonaeum, consisting of a connective tissue layer covered with a flat epithelium (mesothelium). Per


Fig. 274. From a Section of the Lung of a Child, Injected through the Pulmonary Artery, x So. Of the five alveoli drawn the three upper ones arc fully injected.


manent apertures (stomata) in the epithelium probably do not exisy The connective tissue of the pulmonary pleura contains many elastic fibers; these are less abundant in the parietal pleura. Fat is found, sometimes forming folds (plicae adiposae) and the vascular elevations suggestixe of synovial vilU are called pleural villi. These may be sought toward the median wall, beneath the lung. The nerv^es of the pleura, derived from the phrenic, sympathetic and vagus are said to possess small ganglia. In the parietal pleura typical lamellar corpuscles and some of their varieties (Golgi-Mazzoni corpuscles) have been found. The blood vessels of the pleura are said to include branches both of the pulmonary and the bronchial vessels. Lymphatic vessels are numerous and small lymph glands occur.

Septa extend from the pleura into the lung thus dividing its superficial portion into lobules from i to 3 cms. in diameter. They are visible on the surface as polygonal areas bounded by pigmented lines. Since these lobules consist of smaller subdivisions also called lobules, the former are designated as secondary and the latter as primary lobules (slructural units).

In the connective tissue between the secondary or larger lobules, lymphatic vessels make their way to the pleura and thence over the surface of the lung to its root. These lymphatic vessels constitute the superficial system. The deep lymphatic vessels begin along the small bronchioles and the adjoining vessels, and they accompany the arteries, veins, and bronchi to the root of the lung. To some extent the superficial and deep systems communicate. No lymphatic vessels are found beyond the alveolar ducts, within the lobules. Along the larger bronchi and toward the root of the lung lymph glands are numerous.

Black pigment is generally abundant along the course of the Ijmaphatic vessels. It is not melanin but soot, which is absent from the lungs at birth but accumulates with age, especially in certain environments. It penetrates the pulmonary epithelium chiefly in the smallest bronchioles, apparently passing between the cells. Some of it is taken up by phagocytes. Having entered the lymphatic vessels it becomes distributed along their courses.


The blood vessels accompany the bronchi. In the primary or ultimate lobules the arteries are central, producing a terminal branch for each atrium or alveolar sac (Fi^. 271). The veins arising from the alveolar capillaries pass over the peripheral surface of the structural units as shown in the figure. The distribution of the bronchial vessels has already been noted.


The nervei of the lung include a pulmonary plexus from the sympathetic system, which, entering at the root, accompanies the bronchi and vessels; to them it is chiefly distributed. Small ganglia are found within it. The vagus also sends important branches to the lung, which mingle with the perivascular and peribronchial nerves. They contain both medullated and non-meduUated fibers.

Urinary Organs

Wolffian Body

The Wolffian body or mesonephros is the "kidney" of adult amphibia and of certain fishes. It is one of the largest organs found in the human embryo of the second month, but subsequently its renal fimctions are performed by another structure of later development, — the kidney {metanephros). As the Wolffian body degenerates it becomes transformed in the male into the ductus deferens and the epididymis^ essential portions of the genital tract. Some vestigial remnants may produce pathological growths. In the female the entire organ is vestigial, with pathological possibilities. During its development and regression the Wolffian body is a controlling factor in the arrangement of the large veins of the abdomen.


In an embryo of 35 days (Fig. 275) the Wolffian bodies are seen as a pair of long, rounded elevations, one on either side of the root of the mesentery. They extend the length of the abdominal cavity and each empties through its Wolffian duct into the allantois (described on p. 193). The excretion of the Wolffian bodies accumulates in the allantois, which in man is a slender but very long tube. In the pig at a certain stage, it is an elongated, thin- walled sac many times the size of the entire embryo; the large amount of fluid which it contains is due to an unusual development of the Wolffian bodies. After the urogenital sinus opens to the exterior, the contents of the allantois may mingle with the amniotic fluid in which the embryo is immersed.


Development of the Wolffian body. In a previous section (p. 22) the development of the mesoderm has been described to that stage when it presents a series of segments (protovertebrae), connected by stalks (nephrotomes) with the layers which line the body cavity. From several of the anterior nephrotomes there arise rounded elevations which grow posteriorly and unite with one another to form a longitudinal cord of cells on either side of the body. This later becomes hollow and is known as the Wolffian duct. In a rabbit embryo it is shown in Fig. 276, A. As the Wolfl5an duct extends posteriorly it lies so close to the ectoderm that the latter has been said to participate in its formation. Finally it reaches and fuses with the entodermal allantois. The posterior nephrotomes are not thought to contribute to the formation of the duct. As seen in Fig. 276, B, they become separated both from the segments {my) and the coelomic epithelium. The nephrotomes form vesicles (WA.) which become tubular and coiled; each acquires connection with the Wolffian duct (Fig. 276, C). By branching or fission the tubules become more numerous than the corresponding segments.


Fig. 275. Dissection of a Human Embryo of 35 Days. (After Costc.) aI., Allantois ; I. , lung; ft., stomach ; s.tr., septum transversum ; u. C. umbilical cord ; W. by. Wolffian body ; W. d.. Wolffian duct.




Fig. 276. A, Transverse Section of a Rabbit Embryo of Nine Days; B, Human Embryo, 4 mm.; C, Human Embryo, 10 mm. ao. Aorta; c, posterior cardinal vein; coe., coelom ; ol., glomerulus ; 0. r., genital ridj^e ; Int., intestine; mes., mesentery; met. teg., mesodermic segment ; my., myotome; nch., "olochord; neph.. nephrotome; t-C. v., subcardinal vein ; si., sinusoid ; sy., sympathetic nerves ; u. v., umbilical vein ; W. d„ Wolffian duct ; W. t., Wolffian tubule.


The aorta sends a succession of branches to the ventro-median border of the Wolffian body. There they terminate in round knots of capillaries known as glomeruli (Fig. 276, C). A glomerulus is at first lodged in a cup shaped depression on one side of a Wolffian tubule, at its bhnd end. The tubule then grows around the glomerulus so that the latter appears invaginated into its globular distal extremity (Fig. 277). The tubule is said to form the capsule of the glomerulus, consisting of an outer and an inner layer between which is an extension of the lumen of the tubule. The layers are continuous with one another at the stalk of the glomerulus. There the efferent vessel may be found near the afferent artery as in the figure, or, as has been described in the pig, several radiating eflferent vessels may leave the capsule at different points. Whether these all emerge through one crescentic aperture in the capsule, or whether, by coalescence of its edges between the vessels, they leave through separate openings, has not been determined. The stalk and its tubule may both be on one side of the capsule, and not at its opposite poles as in the figure. From the blood circulating through the glomerulus, fluid "filters" into the tubule, forming the greater part of the urine.

The tubules, starting from the ventro-medial glomeruli, follow a convoluted course to the Wolffian duct. In the pig two tubules have been foimd to unite before entering the duct, and near the glomeruli they may fork so as to connect with two capsules. A blind diverticulum is shown in Fig. 277. The tubules are lined thoughout with simple epithelium. It is flat in the capsule where, in the pig, it is said to be thinner in the outer layer; the reverse condition has been figured for the human embryo. The remainder of the tubule may be divided into conducting and secretory portions. The latter, found in the middle part of the tubule, has low columnar epithelium with dark basal protoplasm and a clear vacuolated appearance toward the lumen. These cells are supposed to excrete a portion of the urine. The conducting tubules have a cuboidal epithelium without indications of glandular activity. The secreting and conducting portions of the kidney tubules have been more thoroughly studied than those of the Wolffian tubules.

Veins of the Wolffian body. Early in embryonic life two vessels arise from the vitelline veins close to their entrance into the atrium and grow forward into the head, one on either side. These are the anterior cardinal veins, and from each of them a posterior cardinal vein grow^ along the aorta toward and into the tail. (Veins and arteries in its path contribute to its formation.) Duct 0} Cuvier is the name of the single vessel on each side which conveys the blood from the cardinal veins to the right atrium; the left duct of Cuvier crosses the dorsal surface of the heart in the atrioventricular groove. The early arrangement of the cardinal veins is shown in Fig. 278, A. A Wolffian body has developed in the path of each posterior cardinal vein, and has been a factor in causing the vein to form^the elongated loop shown in the figure. The dorso-lateral limb of the loop is the main stem of the posterior cardinal vein; it receives the intersegmental veins (lumbar and intercostal). The ventro-medial limb of the loop is the subcardinal vein found near the root of the mesentery, as seen in the cross section, Fig. 276, C. Sinusoids extending among the Wolfiian tubules connect the cardinal and subcardinal limbs with one another. (They are shown only on the right of Fig. 278, A.) The sinusoids are less numerous in mammals than in selachians and reptiles.



Fig. 277. Reconstruction of a Wolffian Tubule from a Human Embryo of 10.2 mm. (Except the glomerulus, after Kollman.) C., Inner layer, and c. a., outer layer of the capsule of the glomerulus: div., diverticulum; gl., glomerulus ; W. d., Wolffian duct.


Fig. 278. The Transformation of the Posterior Cardinal Veins of Man, C Rkfreshnting the Adult. The Wolffian Body is Dotted.

a. C, anterior cardinal ; at. I., ascending lumbar ; azMazygos; c, caudal; c. C, cistema chyli ; c.h., com mon hepatic; c. il., common iliac; c. S., coronary sinus ; d. C., duct of Cuvier; g., spermatic or ovarian ; h., hepatic ; n-az., hemiazygos : h-az.ac, accessory hemazygos; I. j., internal jugular; I.C.I. left common iliac; I. in., left innomnale; m. S., median sacral; p. c, posterior cardinal; r., renal; r. a., renal anastomosis; r. C. I., right common iliac; r. In., right innominate; s., suprarenal ; t-C, subcardinal ; 8-cl., subclavian ; tl., sinusoids ; v. C. I., vena cava inferior ; v. c. S., vena cava superior.


The hepatic veins (Fig. 278, A) are ventral to the subcardinals, which are at the root of the mesentery. When, however, the right lobe of the liver fuses with the dorsal body wall making the coronarj- ligament, the right subcardinal connects with the hepatic system, as shown in Fig. 278, B, thus making the inferior vena cava. The vena cava consists of the right subcardinal vein from the liver to an anastomosis between the two subcardinals, known as the renal anastomosis; beyond this point it is continued through WolflSan sinusoids into a portion of the posterior cardinal. The part of the subcardinak distal to the anastomosis is apparently the source of the cistema chyli, and the associated lymphatic vessels (Fig. 161, p. 138).


With the formation of the vena cava and the regression of the WolflSan body, the network of WolflSan sinusoids becomes separated from the veins which entered it posteriorly, and from those which drained it anteriorly. From the network one large vein is differentiated (derived in part from the posterior cardinal) called the spermatic or ovarian vein according to sex; the remnants of the sinusoids are tributaries of this vein. The kidneys come to lie opposite the renal anastomosis, from which the renal veins grow out to enter them. The reduction of the posterior cardinal veins to form the azygos system of the adult, and the formation of the superior vena cava from the anterior cardinals are shown in Fig. 278.


The arteries of the Wolffian body are a series of branches of the aorta, each of which supphes one or more glomeruli. They pass between the posterior cardinal and the subcardinal veins as seen in Fig. 276, C. The vessels formed by the imion of the capillaries of a glomerulus empty into the WolflSan sinusoids. With the regression of the mesonephros one of these arteries, - the future spermatic or ovarian - sends branches into the neighboring genital gland (Fig. 276, C, g. r.). There it unites with veins which grow in from the WolflSan sinusoids to make a capillary circulation.

-==Pronephros==-

Anterior to the WolflSan body there occurs, in the lower vertebrates especially, another renal organ known as the pronephros. Its development precedes that of the WolflSan body. The pronephric tubules are segmental structures derived from the nephrotomes and characterized by retaining their connection with the coelom and by having their glomerulus (glomus) on the side of the tubule instead of at the end. Since the WolflSan duct is considered to be primarily the duct of the pronephros it is often called the pronephric duct; the WolflSan tubules become connected with it secondarily.


In mammals the pronephros is scarcely distinguishable. Its tubules are said to begin with the 4th or 5th segment and to extend to the 9th in sheep or the nth in rabbits. They are transient structures imperfectly formed. In human embryos of 3 to 5 mm. one or two rudimentary pronephric tubules have been described. In one case a detached portion of the Wolfl&an duct opposite the 6th, 7th and 8th segments has been thought to be associated with the pronephros.

Kidney

The kidney develops after the Wolfl&an body has been formed. It arises in two parts, one an outgrowth of the Wolfl&an duct; and the other, ajmass of dense mesenchyma which is said to be derived from the posterior nephrotomes. In this mesenchyma tubules are formed, which have at one end glomeruU similar to those of the Wolfl&an body, but smaller. The tubules follow a contorted course and acquire their openings into the outgrowth of the WolflSan duct. The kidney is a more complex organ than the Wolfl&an body, yet it is constructed on a similar plan.



Fig. 279. The Development of the Renal Pelvis and Ureter. (After Kcibel.) A, Human embryo of 11.5 mm. (4)^ weeks); B, 25 mm. (8J^-9 weeks), a., Anus; al. d.^ allantoic duct; bl., bladder; cl., <^loaca; M. d., Mullerian duct ; r., rectum ; ur., ureter; u. S., urogenital sinus ;W.d.. Wolffian duct

Development. An outpocketing of each Wolffian duct near its entrance into the allantois becomes elongated and dilated at its distal end (Fig. 279, A). The tubular part becomes the ureter and the lobed terminal expansion is the renal pelvis. As the allantois expands to become the bladdery a portion of the Wolfl&an duct is taken up into its wall so that the ureters acquire orifices independent of the Wolfl&an ducts; the latter are carried toward the median Une and the outlet of the bladder, as shown in Fig. 279, B. The figure shows their permanent relation to the ureters.


In later stages the lobes of the renal pelvis become deeper and form 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. 280) 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. From 2 to 9 primary pyramids are said to fuse to form a macroscopic pyramid of the adult kidney (Fig. 281). The nipple-like apex of the pyramid projects into the renal calyxforming a renal papilla. Each papilla is covered by the pelvic epithelium, which is continuous with that which lines the collecting tubules. The trunks of these tubules near the papilla are called papillary ducts and their outlets are named foramina. Each papilla has from 15 to 20 foramina. Sometimes two papillae project into one calyx.


Fig. 281. Cross Section ok an Adult Kidney. (With modifications, after Brodel.)


Fig. 280. Reconstruction of the ureter, Renal Pelvis, and its Branches in a 20 mm. Human Embryo. (Huber. Amer. Journal of Anat., Suppl. to vol. iv.)


The renal pyramids constitute the medulla of the kidney. Except toward their apices they are surrounded by cortical substance. The cortex forms the peripheral part of the kidney, and it also dips down between the pyramids almost to the pelvis. In this way the cortex forms the renal columns [of Bertini], one of which is shown in Fig. 281. The outgrowing collecting tubules derived from the pelvis do not stop at the base of the pyramid but continue in tapering cones through the cortex almost to its surface. They constitute an essential part of its radiate portion (pars radiata) [medullary rays, pyramids of Ferrein].



Fic. 2S2. From a Skction of a Kidney of an i8 MM. IIiMAN Embryo. X 234. (Huber, Amer. Journal of Anat., Suppl. to vol. iv.)

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



Fig. 283.— a Series of Models showing Successive Stages in the Development of a Urinif EROL's Tubule, Including the Associated Portion of the Collecting Tubule. From a human embryo of ihc seventh month. X 160. (Huber, Am. Jour, of Anat., Suppl. to vol. iv.)


Thus far the development of the outgrowth of the Wolffian duct has been considered. The dense mesenchyma which surrounds the pelvis has the following history. It becomes subdivided into masses enveloping the enlarged tips of the branching collecting tubules. Some of its cells become arranged so as to form vesicles as shown in the section Fig. 282, and in the reconstruction Fig. 283, A. In these the vesicle is independent of the collecting tubule. In B and C it has become elongated making an

S-shaped tubule, and has united with the collecting tubule. A glomerulus develops in the lower curve of the S and, as shown in the figures, it gradually becomes enclosed in its capsule — the terminal part of the tubule. The glomeruli begin to form near the surface of the kidney and become buried in the advancing cortex; the oldest_glomeruli are nearest the medulla.

(^Between the capsule and the collecting tubule, the tubule of mesenchymal origin becomes contorted or convoluted. One of the loops in the midst of the coil elongates downward toward the medulla, l)dng close beside and parallel with the collecting tubules. This Henle^s loop (shown only in J of Fig. 283) is lodged in the radiate part of the cortex, and extends into the medulla.

Three tubules of the adult, with capsules situated in the outer, middle, and inner part of the cortex respectively, are shown in the diagram Fig. 284. ^ach capsule connects with a proximal convoluted lubule which is continuous with the descending limb of Henle's loop, after having extended toward the surface of the kidney in the convolute part of the cortex^VThe descending limb is essentially a straight tube of small diameter, owing to the.flMnes&-Qlits cells and not_t^ anarrowing of the.l ugieQr>(1rhe portion of the proximal convoluted tubule which descends in a straight course^Jo join the descending hmb is called the *end segment' or 'spi ral tubul e. y The descending hmb generally becomes of large diameter before it Turns to become the ascending limb of Henle*s loop. Qlhis returns to the immediate neighborhood of its capsule, where it forms the distal convoluted tubule [intercalated tubule]. By means of the * junctional' or 'arched collecting tubule' the distal convoluted joins the straight collecting tubule. The uriniferous tubule has no branches between the capsule and the collecting tubule, but ^gre are many branches connected withjihejatter, as shown in the figure. The "rounded "tuning fork crotches have become angular. (The straight tubules, including Henle's loops and the collecting tubules, consHfute the njedulla and radiate part of the cortex. The remainder of the cortex {pars convoluta) [labyrinth] contains the capsules together with proximal and distal convoluted tubules and arched collecting tubules



Fig. 2S4. Diagram of Th«ke URIMFEROUS Tl'BUl.ES AND THEIR Relation to a Collecting Tubule. (Huber.) a. I., Ascending limb of Henle's loop ; c., capsule ; c. t., collecting tubule: d. Cm distal convoluted tubule; d. I., descending limb; p. c., proximal convoluted tubule; p. d., papillary duct.




Fig. 285. Part of a Radial Section of a Human Kidney. X 25. At X a renal corpuscle has dropped out.

Since a radial section of the kidney shows both the cortex and the medulla, it is the kind made for pathological examination. Under low magnification such a section is shown in Fig. 285. The renal corpuscles [Malpighian corpuscles] are the glomeruU together with their capsules. With higher magnification the various tubules of the radiate portion may be identified (Fig. 286) ; they may be studied to better advantage, however, in tan gentia l scc t^j,Qn& of the kidney, one through the cortex and one through the medulla. In these the tubules appear in cross section. The radiate parts of the cortex are seen as islands of circular sections surroimded by the irregular convoluted tubules and renal corpuscles. The greater part of such an island is shown in Fig. 287. ^

Finer structure of the renal tubules, (The renal tubules are lined throughout with simple epithelium. In the inner layer of the capsule of the glomerulus, itjsa flat syncydaHayeLblending with the small.aaiQunt of perivascular connective tissue beneath. Theo^ter layer of the capsule is also flat and is composed of polygonal celjsj) ( Term inal bars which occur in all other divisions of the renal tubules have not been demonstrated in the capsSe) The flat epithelium of the outer layer of the capsule changes at the 'neck' of the capsule to^e low columnar epithelium of the pjToximal convolute d tub ule. Here cell boundaries are indistinct. The nuclei are towar d _the base of t he ce ll s which re st on a struc tureless basement membrane continuous with that of the capsule. The protoplasm contains granules arrange in vertical rows which toward the base of the cell appear as rods (Fig. 289). In certain animals plaitings in the cell wall have been found to cause a rodded appearance in these cells. Toward the irregular lumen there is a brush border (Fig. 289) suggestive of short non-motile cilia. It is uncertain whether this is normal or due to 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; reverse conditions occur when the urine is scanty. It is in the two convoluted portions of the tubules that urea and pigments are believed to be excreted; the fluid part of the urine comes chiefly from the glomeruli. )


Fig. - Surface view of an ascending limb.




The descending limb both in the radiate cortex and in the medulla (Figs. 287 and 288) is a thin walled conducting tube from 9 to 16 /i in diameter. (The proximal convoluted tubule measures from 40 to 60 /i). Cell boundaries are absent. Often in sections the flat nucleus causes


Fig. 286.— Tubulks of the Pars Radiata. From a radial section of a human kidney. X 240.


a local thickening of the cell, but this is perhaps a post mortem appearance. The descending limbs may suggest capillaries, as seen in the figures.

Cjhe ascendi ng limb s, 23-28 jjl in diameter, resemble the distal convoluted tubules said to measure from 39 to 44 //. (The cells in the distal convoluted tubule are taller than in Henle's loop and they may have basal


Fig. 287. From a Section through the Cortex of a Human Kidney (Parallel with the Surface). The pars radiata is seen in the lower left corner. X 200. (Schaper.)

striations. Thus they are much like the proximal convoluted tubules except that their markings are less distinct and their size is smalie^."^

(The collecting tub ules are a distinct type, having a round, well defined l lumen and distinct cell walls. The round nuclei are arranged with striking \ r^ularitj^ The cells are columnar in the papillary ducts which may ' be 0.3 mm. in diameter. Although some cells of the collecting tubules appear darker than others, they are thought to form only conducting tubes.


' . From the preceding account it is evident that some parts of the urinary tubules are easily recognizable and that others are not. The capsules, descending Umbs and the collecting tubules have distinctive characters.


Fic. 288. From a Transverse Section through the Medulla of a Human Kidney. X 320. (Schaper.)

In the medulla, since convoluted tubules are absent, the ascending limbs (including the part of the descending limb which is of large diameter) are likewise easily identified. In the cortex the proximal and distal convoluted tubules wind about one another and cannot be absolutely distinguished except by reconstructionsT) In Fig. ^^ . 287, the tubules labelled ascending limb (?),

found in the radiate part of the cortex, have also been labelled distal convoluted and end segment of the proximal convoluted; they cannot be distinguished from these in a single section, but their position in the radiate portion is in favor of regarding them as ascending limbs.

The connective tissue about the kidney

FROM A foHtts 2l fatty capsule, capsida adipos a, which

Rabbit. (Szymonowicz.)

surrounds the renal pelvis, and its calyces except where they receive the papillae. A dense fibrous capsule, tunica Abrosa , is closely appUed to the outside of the kidney, from which it may be stripped ofiF. It contains elastic fibers which increase in abundance with age, and also smooth muscle fibers. Within the kidney each tubule is surrounded by a small amount of connective tissue, in part reticular. It is more abundant around the vessels, in the papillae, and about the renal corpuscles than elsewhere. The normal amount should be carefully studied since an increase in this ** interstitial tissue" is indicative of disease.

Lohes and l obule s. In embryon ic Ijfe the kidney is divided into lobes, bounded by the renal columns and indicated by grooves upon the outer surface (Fig. 290). The grooves become obliterated during the first year. (In the ox similar grooves are permanent; in most mammals they never exist, as 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. They are not bounded by connective tissue septa.

Blood vessels. The kidney has a capillary circulation. The renal artery^ from the aorta, passes to the hilus or notch on the medial border of the kidney. It divides into several branches most of which pass over the ventral surface of the pelvis into the fat around the calyces (Fig. 281). >^As interlobar arteries they pass to the boundaiyjayer between_ thecortex and medulla where thej" are designated ajrijornijjieries (Fig. 291). ; These send interlobular arteries through the convolute part of the cortex and their terminal branches enter the fibrous capsule. It will be noted that the kidney is exceptional in having ^^^ ^ — kidnky its arteries at the periphery of its lobules. From the tAher'^Honwig^!^

interlobular arteries small stems pass to the glomeruli, each of which receives a single twig (Fig. 292). This is resolved into a knot of capillary loops, the endothelium of which seems to blend with the surrounding syncytium and possibly 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 divides into small branches on leaving the capsule. These spread among the convoluted and straight tubules of the cortex and some continue into the medulla . The latter is supplied by other straight branches {arteriolae rectae) from the interlobular, efferent and arciform arteries as shown in Fig. 291. The veins of the medulla begin around the papillae and as venulae rectae empty into the arciform veins. The cortical veins are the interlobular vessels which are beside the corresponding arteries. They arise from converging veins in the renal capsule which on surface view form a stellate figure {venae stellatae). The interlobular veins drain the capillaries of the cortex, but have no direct relation with the glomeruli. IfUerlobar veins follow the arteries, passing out from the hilus of the. kidney over the ventral surface of the renal pelvis.

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.


Fig 291. Diagram of thr Course of the Rknai. Blood Vessels.


The nerves a re medullated and non-medullated. There is a sympathetic plexus at the hilus associated with small ganglia, and from it


Fig. 292. From a Section of thk Injecthd Cortex of an Adult Human Kidnev. X 30 interlacing nerves extend into the kidney around the vessels (Fig. 293). Fine branches supply the epithelial cells, pgpeciallYjJKi^^/^f thp <jyQYQhit£»H


Fig. 393. From the Kidney of a Mouse. Golgi Pkrparation. X 180.


Fig. 294. Transverse Section of the Lower Half of A Human Ureter. X 15. ., Epithelium : t., tunica propria ; I, inner longitudinal muscle bundles r, circular layer of muscle bundles; 1|, outer lonj^itudinal muscle bundles.

tubules . Thfiy-iorm plexuses beneath and above the basement membrane j nd hdve free inte rcellular endings.


Renal Pelvis and Ureter

The renal pelvis and ureter both consist of a mucosa (and submucosa), muscularis and adventitia (Fig. 294). 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. 295), and its cells when found detached in urine are not distinguishable from bladder cells. Thejepithelium 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 underljdng cells, as is particularly the case in the contracted bladder (Fig. 296). Two nuclei are often found in a superficial cell and 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 are true glands. Capillary blood vessels, which arc abundant in the mucosa, are found directly beneath the epithelium and present the deceptive appearance of becoming intraepithelial. The tunica propria consists of fine connective or reticular tissue with few elastic fibers. It contains many cellular elements and some leucocytes and passes without a definite boundary into the loose connective tissue of the submucosa.


Fig. 295. Vertical Section of thk Mucols Membrane of a Human Bladder. X 560.




Fio. 290. A Superficial Epithelial Cell and Two club-shaped Cells from A Contracted Bladder. (Koelliker.)


The tunica muscularis is not compact since there is 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. Aiymnd the papillae of the kidney the circular fibers form a ** sphincter. "CThe 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 .J) 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.


Fig. 297. Section through the Fundus of the Urinary Bladder of an Adult Man. X 48.

Bladder

The development of the bladder from the proximal end of theallantois has been described on page 193. Since the allantois is a part of the entodermal tract, the epithelium of the bladder is entodermal whereas that of the ureter is mesodermal. There is however no demarcation between the layers in the adult, since both produce the same sort of " transitional e^khelium.!' ~

The bladder consists of a mucosa, submucosa, muscularis and serosa

The epitfielium of^the mucosa i s two-layered in the distended bla dder, the outer cells having terminal bars; in the contracted condition it becomes several-layered and the bars form a net extending into the epithelium. Some of 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. 297). Some have no lumen or are detached from the epithelium, but others are pits containing a colloid substance. The pits are the first stages of gland formation. In the adult, branched tubules lined with cylindrical epithelium may sprout from the bottom of the pits, thus forming true glands. Their occiurence is limited to the fundus (the dorsally bulging lower part of the bladder) and to the neighborhood of the urethral outlet. In the latter position they present transitions to well developed prostatic glands.

The tunica propria sometimes contains s olitary nod ules. It blends with the submucosa, as in the ureter, and contains l)rmphatic and blood vessels the latter extending very close to the epithelium.

mi^grnlayiR consists of smooth musclc fibers arranged in three interwoven layer s, which are seldom separable in sections. They are an jnp^r Inngrjtu^jn^l^ i middle circular and outer longitudinal layer. The 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 aT connective tissue layer covered with mesotEelium. In the non-peritonaeal part of the bladder it is replaced by an adventitia or fibrous layer.

Non-medullated nerves, with scattered g roups of g anglion cell s, are found outside of and among the muscles. Medullated fibtrs terminate around the ganglion cells; others pass through the ganglia to intra-epithelial sensory endings. ^

Urethra (in the Female)

The male urethra will be described with the genital organs; only its upper portion is homologous with the urethra of the female which 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 form in different individuals. The lumen is inregularly crescentic with longitudinal folds, as seen in Fig. 298. ) Branched tubular urethral glands are found only in small numbers excej)t near the outlet. Their secretion is mucoid, but is not typical mucus. (In the submucosa there are many thin walled veins constituting the corpus sfongiosumi jli is comparable with the upper part of the more highly developed corpus cavemosum urethrae of the male. (Compare with Fig. 322, p. 283), The muscularis consists of imier longitudinal and outer circular smooth muscle fibers, among which the veins extend. Connective tissue with many elastic fibers is abundant in the mucularis. The striated constrictor urethrae is outside of the smooth muscle layer, as showirnTthe'figure.



Fig. 298. Cross Section of the Female Urethra. (Koelliker.) d.t Gland-like diverticulum; e., epithelium ; L., lumen of the urethra; m., striated muscle; g., corpus spongiosum, containing venous spaces, v., and smooth muscle.

Male Genital Organs

Development

The Wolffian body becomes an important part of the male genital organs and its duct serves to transmit the products of the testis to the urogenital sinus. Another duct, parallel with the Wolffian and close beside it, develops later, and is called the Mullerian diict. It arises as an inpocketing of the coelomic epithehum near the anterior end of the Wolffian body. The orifice into the peritonaeal cavity becomes surrounded by irregular folds known as fimbriae. 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. 299, but the Wolffian duct is said not to contribute toward its formation. The two MuUerian ducts reach the bladder side by side and acquire openings into it, between those of the Wolffian ducts. Near the bladder the two Miillerian 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. 279. B, page 249). In the female the united portion becomes the vagina and uterus, and the separate parts are the uterine [Fallopian] tubes. In the male the united portion becomes a small blind pocket, the prostatic utricle, opening into the prostatic urethra. Each fimbriated extremity persists in 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. 301).


FiG. 290. From a Reconstri ction of a 13.6 mm. Human Embryo. (F. VV. Thyng.) bl., Bladder; f., fimbriae; g. g., genital gland ; g. p., genital papilla; M. d.. Miillerian duct; p., renal pelvis; r., rertuin ; u.r., ureter; u. 8.« urogenital sinus; W. d., Wolffian duct.



Fig. 300. Diagram of the Development of the Testis, based iron Figures by MacCalli'm and B. M. Allen.

C., glomerular capsule; I. c., inner or sex cords; M. d., Mullerian duct ; 0. C outer or rete cords; W. d., W. t., Wolffian duct and tubule.


The genital glands in either sex begin as a thickening on the ventromedial border of each Wolffian body (Fig. 299). A section of this genital ridge is shown in Fig. 276, C, page 245. The ridge is a dense mass of mesoderm covered by the peritonaeal mesothelium which here consists of columnar cells. In forming the testis, cords of cells which later become tubules, appear in the dense mesenchyma (Fig. 300). These are considered to be invaginations of the peritonaeal layer rather than segregations of mesenchyma. The cords near the surface of the genital ridge become the convoluted tubules of the testis (tubuli contorti) and their continuations into the substance of the organ are the straight tubules (tubuli recti). Both the convoluted and straight tubules (Fig. 301) arise from the cords of cells in the outer part of the genital ridge. The cords in the interior of the ridge are similar and have recently been described as the posterior extensions of the rudimentary peripheral cords formed in the anterior end of the genital ridge. These inner cords produce a net of anastomosing tubes, the reie testis, into which the straight tubules empty. The tubes of the rete acquire openings into the glomerular capsules of


Fig. 301. Diagram of the Male Sexi'Al Organs. (Modified from Eberth, after Waldeyer.) (The course of the Mullerian duct is indicated by dashes.)

the';Wolffian body (Fig. 300). The glomeruli atrophy and disappear. The products of the convoluted tubules thus pass in turn through the straight tubules and rete testis into the Wolfl5an tubules.

0/ the Wolffian tubules about fifteen persist as the ducttUi efferentes. Each of these is a greatly convoluted tube which if straightened measures 8 inches (20 cms.). When coiled it forms a conical mass or lobule 0} the epididymis, with its apex toward the rete, and its base toward the Wolffian duct which it enters (Fig. 301). 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. Along the testis the Wolfl&an 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, Uke 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; an out-pocketing forms the seminal vesicle.

It has been noted that only about fifteen of the Wolffian tubules persist as efferent ducts. Some of the others become detached, producing the paradidymis; and some which are partly detached remain as blind tubes extending from the rete or ductus epididymidis, — they are called ductus aberrantes. The one of these labelled in Fig. 301 is quite constant and may be from 5 to 30 cms. in length. The appendix epididymidis in the figure contains a tube connected with the Wolffian duct. The nature of this appendix is obscure; it has been thought a derivative of the Miillerian duct.

The urethra. At an early stage (Fig. 299) the aUantois is arbitrarily divisible into a * temporary bladder' which extends to the genital ducts, and a urogenital sinus which receives both urinary and sexual outlets and extends to the surface of the body. A portion of the urogenital sinus is ectodermal having formed from a depression in the outer surface; its inner part is entodermal and the boundary between these portions is no longer apparent. At a later stage the 'temporary bladder' forms the permanent bladder together with a limited portion of the urethra. In the female it forms the entire urethra, but in the male only that portion of the prostatic urethra which extends to the genital ducts. The remainder of the male urethra is urogenital sinus. By the anatomists the male urethra is divided into the prostatic^ membranous and cavernous [penile] portions.

The penis and scrotum. In Fig. 299 the outer portion of the urogenital sinus is seen to be a cleft-hke space in an elevation known as the genital papilla (or tubercle). In Fig. 302, A, the papilla has lengthened to form the penis; its enlarged distal end is the glans. On the lower surface of the penis the urogenital sinus has an elongated opening. Apart from the condition of arrested development called hypospadias, the opening is bridged over, except at its distal end; thus it forms the cavernous part of the urethra. The embryonic penis is covered with a layer of skin described as forming two lateral folds, the lesser genital jolds. They meet beneath the penis as the urogenital sinus becomes closed, and a raphe (seam) remains to indicate their place of fusion. A reduplication of the lesser folds over the glans forms the prepuce. Outside of these folds there are two larger elevations of skin, one on either side of the root of the penis. They extend toward the anus, between which and the penis they fuse in the median line forming a continuation of the raphe already mentioned. These larger genital folds thus produce the scrotum.

Descent of the testes. The peritonaeal cavity sends a prolongation, the processus vaginalis, over the pubic bone into each half of the scrotum. The testis and epidid)rmis at this stage lie behind the peritonaeum of the abdominal cavity (Fig. 302, B). A large retroperitonaeal colunm of connective tissue, the gubemaculum 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. 302, C). The Wolffian duct is bent over the ureter as shown in Fig. 301. Except on its dorsal border the testis is closely invested by the peritonaeum 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. 302, D. The descent of the testes is completed shortly before birth and the obliteration of the stalk of the processus follows.



Fig. 302. a, Diagram of the Embryonic External Genital Organs in the Male; B, C, D, Diagrams op the Descent of the Testis. (After Eberth.)

a., Anus ; tp., epididymis ; g., glans penis ; g. f., lesser genital folds ; g. g. f., greater genital folds ; p. c.» peritonaeal cavity ; p. v.. processus vaginalis; r., raphe ; t., testis ; t. v., tunica vaginalis (p. I., parietal ; V. I., visceral layer) ; ii. 8.* urogenital sinus.

Testis

Sustentacular and sexual cells. Among the cells of the cords which develop in the genital ridges there are some which are larger than the rest, and are further characterized by abundant clear protoplasm and large round nuclei. Two of these sexual cells are shown in Fig. 303, from a testis at birth. At this stage the lumen of the convoluted tubules is imperfectly developed or absent. The sexual cells multiply slowly by ordinary mitosis, until puberty when their increase in number becomes rapid. In a somewhat smaller form with round nuclei containing abimdant chromatin, in granules or encrusted at the nuclear membrane, they are called spermatogonia. From them the mature sexual cells are derived. The cells which in Fig. 303 constitute the larger part of the tubule are called sustentacular cells (SertoU's cells, vegetative, or follicular cells). They form a syncytium and with the increase in the number of spermatogonia their protoplasm is resolved into a network of strands. Their nuclei are radially compressed into ovoid shapes and lie in colunms of protoplasm extending from the periphery of the tubule toward its lumen and moulded by the surrounding cells. 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 colunms, the polygonal bases of which are in contact with one another beneath the spermatogonia. Within the protoplasm fat droplets occur, together with brown granules; cr}^stalloid bodies in pairs may also be found. The appearance of the sustentacular cells in ordinary sections is shown in Figs. 305 and 309, in which it is evident that they may be recognized by their characteristic nuclei.



Fig. 303. Cross Section of a Convoluted Tubule of thk Testis at Birth. (Eberth.)


Fig. 304. Sustentacular Cells. Isolated (Koelliker) ; b., Golgi preparations. (Bohm and von Davidoff.)



There are two views as to the origin of the sexual cells. According to the first they arise from the mesoderm quite like the cells of other organs; the second regards them as a race of undifferentiated celb set apart from the outset of development. In the worm, Ascaris, it has been obser\'ed that the fertilized ovum divides into two cells, one of which produces only somatic cells (those of the various tissues) and the other divides into a somatic and a sexual cell. In the mitoses which follow, the sexual cell at first continues to give rise to a somatic and a sexual cell, but later its products are wholly sexual. In certain fishes large cells situated in the entoderm and mesoderm before the genital glands have formed, are regarded as sexual cells (germ cells). Later they are scattered about in the mesothelium of the abdominal cavity and finally they migrate into the genital ridges to become spermatogonia in the male, or the corresponding oogonia in the female. Similar cells have been found in reptiles and in the older mammalian embryos. The early segregation of the sex celb has been cited in favor of the opinion that acquired characters cannot be transmitted; the cells have been considered as quite independent of the body in which they are lodged, which serves as their " trustee." It has not been established, however, that these cells in mammals are earlier or more completely separated from the rest of the body than are those of other organs. Although in consideration of the variety of cells to which it may give rise the fertilized ovum is classed as the least differentiated of cells, yet the sexual cells which unite to produce it are highly differentiated both in form and function.


Fig. 305. Cross Sections of Seminiferous (Convoluted) Tubules of a Mouse. X 360.


Neither of them normally has the power for further mitosis, yet when combined they produce a cell in which this capacity is unsurpassed. The rate of cell division falls as the embryo grows, and is restored only in the sexual cells differentiated for this purpose.

The development of the mature sexual cells in the male, the spermalozoa, occurs in the convoluted tubules of the testis, beginning at puberty and continuing throughout life. 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 process of their formation from spermatogonia is known as spermatogenesis. In place of the name spermatozoon which was applied to seminal filaments when they were considered parasitic organisms, the term spermium has been proposed.

Spermatogenesis

The spermatogonia which are found at the periphery of the convoluted tubules, divide by ordinary mitosis for a variable number of times. Some of the resulting cells move toward the lumen and increase considerably in size. The chromatin in the nucleus of each forms a thread which is resolved into one half the usual number of chromosomes. Before this takes place the chromatin may be gathered at one side of the nucleus, a condition named synapsis; ordinarily it appears as a convoluted thread or spireme. The reduced number of chromosomes in man is said to be twelve, in favor of which the drawing copied as Fig. 306 has recently been published. The chromosomes are seldom so arranged as to allow a conclusive count. Instead of being of the usual elongated form they are block-like bodies which in certain animals are each clearly groups of four granules or subdivisions. They become ring shaped before dividing in halves (the ring-shaped arrangement characterizing the heterotypic form of mitosis) and each half contains two of the four granules. The cells produced by this division are somewhat smaller and pass toward the lumen . Fig. 306.-PRIMARY spkrm-

Within their nuclei the chromatin returns to the ?ng^2[?^*chromo«>1>!Jes" spireme form, and possibly to a network. The ues rg. chromatic thread again is resolved into twelve chro mosomes, each in some animals consisting of two granules. In the mitosis which follows, these divide into single granules and each of the cells produced receives twelve. They then form a network in a small nucleus, the entire cells being reduced in size. These cells border upon the lumen. The generations of cells which have been described are named as follows. Those which proceed from spermatogonia, and which first present the reduced number of chromosomes are called primary spermatocytes. They are large cells in the outer part of the tubule, sometimes with vacuolated protoplasm containing rows of granules. Each of them divides into two secondary spermatocytes (praespermatids) which are similar cells, though smaller and nearer the lumen. They also have the reduced number of chromosomes. Every secondary, spermatocyte divides into two decidedly smaller sperm^ids^ giving them the reduced number of chromosomes. The spermatids without further division are transformed into spermatozoa. Thus each primary spermatocyte produces spermatozoa.


Stages in the transformalion oj a spermatid into a spermatozoon are shown in the diagram Fig. 307. The twelve (?) chromosomes of the spermatid disappear in a dense chromatic network which becomes an apparently homogeneous mass. This deeply staining nucleus passes to one end of the protoplasm of the spermatid and becomes the essential part of the head of the spermatozoon. In man it is a flattened structure, oval on surface view, and pyriform with its apex forward, when seen on edge (Fig. 308). 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 proloplasm bet\%^eii the two parts of the posterior centmsome is reduced to a thin layer in which a spiral filament develops, winding about the axial filament* The axial filament, which consists of fine fibrils in some forms at least, distal to the centrosome ring is sur* rounded by a thin membrane which terminates or becomes very thin near the extremity of the filament. This membrane whicli in salamandei^ forms a conspicuous tmdulatbg frill, is thought to be a product of the filament and not an extension of the protoplasm* In man it is inconj_spicuaus. In fact most of the detail which is seen in ordinan^ sections containing ^ermatoaoa is shown in Fig* 308,


Fig. 307. Diagrams of the Development of Spermatozoa.' (After M eves.) a. C, anterior centrosome; a. f., axial filament; C. p., connecting piece ; ch. p., chief piece ; g.C, galea capitis; n., nucleus; nk.,neck; p., protoplasm ; p. c, posterior centrosome.



Fig. 308. Spermatozoa: 1, 2, 3, Human; 4, from A Bull. a. Head ; b, connecting piece, and Cf chief piece of the tail. 1, 3, and 4, Surface views; 2, side view. X 360.


Mature spermatowa are divided into three parts, the head neck and tail. The head (3-5 ft long and 2-3 ,ei 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 po&terior centrosome. The neck in man is not constricted as in some forms, yet it is a place where the head may become detached. The tall includes three parts, the c&nnecting f^iect^ chief pkce and md piece. The connecting piece (6 u long and scarcely 1 /^ wide) consists of protoplasm, axial and spiral filaments and the two parts of the poslerior ceotrosome. The chief piece (40-^ « long) is axial filament with a surrounding membrane, and the end piece (10 /i) is a prolongation of the filamentIn the convoluted !ubules the heads of the spermatozoa are attached to, or buried in the protoplasm of the sustentacular cells which are supposed nourish them. Their tails project into the lume^. Later they become [rtached and float in the albuminous fluid secreted in small quantity by br tubutes. They pass through the straight tubules and rete to the epididymis, in which they accumulate and where they first become motile. Their molility is greater, however, in the seminal fiuid which is a mixture of the products of the epididymis, vesicles, prostate and bulbo-urethral glands. Then by an undulating movement of the tail the head is propelled against such a current as is made by a ciUa, at a rate of J of an inch in a minute. Water inhibits the motion, which is favored by alkaline fluids; it occurs also in those faintly add. Spermatozoa may retain their activity three days after death and 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 of unknown significance.


Fig. 309.



Fig. 310. Section of the Human Rbtb Testis. X 96. (KSUiker.) A, Artery ; C, rete tubules ; L, lymphatic vessels ; 8, connective tissue partly surrounded by rete tubules ; 8k, part of a convoluted tubule, to the left of which are sections, probably of straight tubules ; V, vein. (From Bailey's *' Histology.")


The convoluted tubules of the testis consist therefore of a complex stratified ciliated epithelium, the basal cells being spermatogonia and the superficial cells spermatozoa The columnar sustentacular cells are scattered through this epithelium. Spermatogenesis occurs in "waves" along these tubules as is seen when they are cut lengthwise (Fig. 309).


The superficial cells show alternating areas of mature and immature spermatozoa. In cross sections all the superficial cells may be of one stage, which differs from that of the adjoining tubule (Fig. 305). Toward the periphery of the testis the convoluted tubules (140 // in diameter) present many loops and they may anastomose forming a network. Blind endings are also observed, and investigators disagree as to the nature of the usual termination. As they pass toward the epidid)nnis 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.


Fig. 311. Cross Section of the Tkstis ok a Child at Birth. X lo.


A distinction between the reie and straight tubules seems superfluous histologically since both 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. 310. They contain spermatozoa and immature sexual cells together with pigment granules and broken down cells.

Connective tissue of the testis. The rete possesses no basement membranes, such as surround the convoluted tubules, but is imbedded in a mass of connective tissue known as the mediastinum testis (Fig. 311). From the mediastinum, layers of tissue, the septula testis, extend radially toward the periphery of the testis, dividing the convoluted tubules into pyramidal lobules with apices toward the rete. The periphery of the testis is covered with a dense connective tissue layer, the tunica albuginea. It contains numerous elastic fibers which increase with age. The visceral layer of the tunica vaginalis rests upon its outer siurface. The inner portion of the albuginea is very vascular, forming a distinct layer at birth (the timica vasculosa). 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, shown in Figs. 312 and 309. They are said to 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. (Rod and spindle shaped crystalloids are also found in the spermatogonia at all ages, and after puberty octahedral forms occur. Rod shaped forms in the sustentacular cells have already been mentioned. The composition and significance of all these are imknown, but they are not considered post mortem formations.)


Fig. 312. From a Cross Section of the Testis ok a Man Twenty-two Years Old. X 50.


The interstitial cells, although not intimately related with the vessels, are thought to produce an internal secretion, and there is evidence that upon it the sexual instinct depends. 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, leaving the sustentacular cells.


The vessels and nerves of the testis enter the mediastinum and tunica albuginea, having followed the ductus deferens in the spermatic cord. The convoluted tubules are surrounded by capillary networks derived from branches of an artery to a Wolffian glomerulus, and are drained by capillary branches of the Wolffian sinusoids. The main stems of these vessels are called internal spermatic. Lymphatic vessels are numerous in the tunica albuginea and they extend among the tubules. Nerves from the spermatic plexus accompany the blood vessels; the presence of intraepithelial endings has not been estabUshed with certainty.

Epididymi s. \ £h^ efferent ducts which pass fro m the rete to the duct of the epididymis are hned with an epithelium in which groups of colunmar cells alternate with those which are cuboidal (Figs. 313 and 314). Thus the inner surface of the epithehum has depressions suggesting glands, but the basal surface is free from outpocketings. The epithehum is generally simple, although in the tall parts it may appear 2 or 3 layered. The cells contain fatty, pigment, and other granules, and ^ oduc e a secretion which 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 muscles, several


Fig. 313. From a Section of the Hkad of a Human Epididymis, showing Sections of the Ductus Epididymidis in the Center and of Ductuli Efferentes on the Sides, x 50.


Fig. 314. Transverse Section of a Ductulus Efferens Testis of an Adult Man. The rieht-hand end of the illustration is schematic. No cilia could be seen, although those of the epithelium of the epididymis were well preserved. X 360.


Before puberty and in old age



cells thick. The muscle columnar cells.

layer is thickest toward the ductus epididymis. Among the muscle cells there are elastic fibers which, like those of the ductus epididymidis and 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. these irregularities are slight. The ductus epididymis two-rowed epitheliuni with rounded basal >x:£lls_,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. 33, b, p. 31). The epithelium may contain round cavities opening into the lumen or forming closed cysts. The delicate membrana propria and a 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 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 tunica propria to bulge toward the epithelium. The nerves, besides perivascular nets, form a thick plexus myospermaiicus provided with sympathetic ganglia. It is found in the muscular layer and occurs more highly developed in the ductus deferens and seminal vesicles. Its fibers supply chiefly the smooth muscles, and to a less extent, the mucosa.



Fig. 315. Transverse Section of a Human Ductus Epididymidis. X 80.


Ductus Deferens

The ductus deferens begins as a convoluted tube continuous with the ductus epididymidis; it becomes straight and passes to its termination in the ductus ejaculatorius. Shortly before reaching the prostate it exhibits a spindle shaped enlargement, about ij in. long and | in. wide, known as the ampulla (Fig. 317). The ductus deferens consists of a

furiica mucosa, muscu fv laris and adventitia.

â– 'â– ^^'^*t^i^c^^^'^ij^ The epitHellum 'Is gen .;,^ - :. V^-->^.,i Kpilhelium. gj.^yy ^ ^^^ ^^^^ ^^

^^i ' tall inner cells producing

tA^' .â– 'r:>\^ Tunica propria. ,

';; yHi-
f roimd masses of secre

B ^-{ Inner longiiu- ^ion, Toward the epi ( V : ';//^ : v/*^^ 'â– ^" '" '*'"^' '""" ^^^y^s it may also have

ir, U'J-^' -^ ci . Vt:'V Circular non - motile ciha. To ^^tL^m- ■ muscles. , ,, „ .. ""

^IP ward the ampulla it may .

•^ be several rowed, resem c - Outer longitu- bUng the epithelium of

  • ^ ^ dinal muscles. o , .>

I the blacTder. It res^g ^ti ,

^^ ^i^"^ '^^Tsu'i!^ a conne ctive tissue jAUil^

^O^ '•_- ^-n- propria which is surrounded by the three

Fic. 316. CROSS SBCTioN OF THE HuMAK DUCTUS 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 of the spermatic cord which contains numerous arteries, veins, lymphatics, nerves, striated muscle fibers of the cremaster muscle, and the rudiment of the processus vaginalis.

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

Seminal Vesicles and Ejaculatory Ducts

The seminal vesicles grow out from the ductus deferentes at the prostatic ends of their ampullae. Each consists of a number of saccular expansions arranged along the main outgrowth which is irregidarly coiled. The lining of the sacs is honeycombed with fold s as s hown, jfi Figs. 317 and 31b. 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 dining sexual excitement they are absent from the vesicles of rodents; this and other facts indicate that the function of the organ is primarily glandular. Pigment granules in varying quantity occur in the epithelial cells and in the underlying connective tissue. They may impart a brownish color to the secretion.



Fig. 317. Seminal Vesicle and Ductus Deferens. (This is natural size.) (After Eberth.) ad., Adventitia; am., ampulla; d., diverticulum; d. d., ductus deferens; d. e., ductus ejaculatorius; m., muscularis; s. v., seminal vesicle; t. p., tunica propria.


Fig. 318. Vertical Section of the Wall of A Seminal Vesicle. (After Kolliker.) ep.. Simple epithelium; g., ^land-like depression; in., muscularis ; t. p., tunica propria.


The ductus ejaculatorii on their dorso-median side 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_^imicous ^membrane of the ductus ejaculatorii is like that of the se minal vesicles, except that its folds are not so compGcate cT" 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).

Appendices and Pakadidymis

The appendix testis [hydatid of Morgagni, sessile hydatid] is a small vascular nodule of connective tissue covered with peritonaeum of the tunica vaginalis, except at its stalk of attachment. It contains one or more fragments of a small canal, closed at both ends, occasionally having blind outpocketings. The canals are lined with simple colimmar epithelium sometimes ciliated. The peritonaeal cells over a portion of its surface are columnar and have been interpreted as the evaginated end of the Miillerian duct.


The appendix epididymidis (stalked hydatid) is not always present. Among 105 cases examined by Toldt it was found 29 times. It consists of loose vascular j[_t.v: connective tissue covered by the vaginalis, and contains a dilated canal lined with columnar epithelium sometimes ciliated. The canal has no connection with the tubules of the epididymis. Its embryonic history is obscure. ^'A^'?MTi"s?M^nirurai ^y^^ ^ound in the vicinity of the epididymis may

sire. (After lEbcrth.) arise from pockets of the tunica vaginalis. '• midfs'!'l?t? aSi^ndS The paradidymis is found "frequently but not

testis; c.'e.V caput epi- always in older embryos and children, as an elongated, tunura vaginal whitish Structure on ihe ventral side of the spermatic cord. It is sometimes just above the head of the epididymis, sometimes higher, but always in front of the venous plexus. A second, lower part of the paradidymis is found in late childhood, but not as a rule in the adult. It is a macroscopic coiled canal with outpocketings, found behind the head of the epididymis and in front of the pampiniform plexus." (Eberth.) The upper portion represents the anterior part of die Wolffian body, which is not involved in the formation of the testis. It contains pigment derived from the degenerated Wolffian glomeruli. Cilia, which occur at birth, later disappear. The lower section may connect with the tubules of epididymis and contain spermatozoa, or it may be completely detached. Its tubules are made of columnar epithelium, simple or stratb5ed, sometimes ciliated, and they show elevations suggesting those of the efferent ducts. Often they become cystic.

Prostate

The prostate consists of from 30 to 50 branched alveolo-tubular serous glands, which grow out from the prostatic urethra, and surround it together with the ejaculatory ducts and the prostatic utricle. The prostatic urethra is embryologically the neck of the bladder, and as the glands grow out they become surrounded by the smooth muscle fibers of the bladder or urethra. 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 glandular epithelium is simple and either cuboidal or colunmar. It may appear stratified as it passes over the folds in theValls of the tubules. Near the outlet of the larger ducts the epithelium is li ke that of th e bladder and prostatic urethra. In the prostatic alveoli, of older persons especially, roimd or oval colloid masses from 0.3 to i.o mm. in diameter occur; as seen in sections (Fig. 321) 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 which is absent from the other constituents of the seminal fluid.


The smooth muscle fibers are found ever3rwhere 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 lymph vessels. The numerous nerves form gangUonated 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 parts of the gland in dogs and cats, end in cyUndrical lamellar corpuscles.



Fig. 320. From a Section of the Prostate OF A Man Twenty-three years old. X 50.

Urethra and Penis

The form of epithelium found in the bladder extends through the prostatic to the membranous part of the urethra. Its outer cells gradually 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 navicularisy 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 Littr^]. Often these pockets are on the sides of epithelial pits so that the glands are branched. Non-glandular pits also occur, known as urethral lacunae, and the "paraurethral ducts near the external orifice are large lacunae of various sorts.


Fig. 321. From a Shction of thk Prostate of a Man Twenty-threb years old. X 360. The epithelium is cut obliquely at X, and has artificially separated from the connective tissue at XX.



The two principal glands empty by irregularly dilated ducts, an inch and a half long, into the beginning of the cavernous urethra. The bodies of these bulbourethral 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 musctdaris of the prostatic part of the urethra consists 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 obUque and longitudinal bimdles in its distal part.


Mucous membrane of the urethra. Epithelium. Tunica propria. Urethral glands. Submucosa.


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

Fig. 322. Transverse Section of the Pars Cavernosa Urethrae of Man. X28.


Corpus cavernosum urethrae. In the submucosa of the cavernous urethra there are many veins (Fig. 322) which become larger and more numerous in and beyond the muscularis. This vascular tissue which surrounds the urethra is hmited by a dense elastic connective tissue layer, the tunica cUbuginea, and the structure which is thus bounded is the corpus cavernosum urethrae. Toward the perineum it ends in a roimd enlargement, the bulbus urethrae^ 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 arteriea bulbi and the urethral arteries, penetrate the albuginea, and the former pass the length of the cavernous body and end in the glans. These arteries have particulariy thick walls of circular muscle and in cross sections the intima may be seen to form coarse rounded projections into the lumen. These contain longitudinal muscles and circular subdivisions of the inner elastic membrane (Fig. 323). The arteries in the corpus cavemosum produce capillaries found chiefly toward the albuginea. (Jhe capillaries empty into thin walled v enous spaces which appear a^ endpthelium-Uned clefts, in a connective tissue containing many smooth muscle fibers . The cavernous body is permeated with these veins which, at times of sexual excitement, become distended with blood, reducing the tissue between them to thin trabeculae. Some arteries connect directly with the venous spaces, and such as appear coiled or C shaped in a collapsed condition are called arteriae helicinae. The venae cavemosae 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.


Fig. 323. Cross Section OF AN Artery of thk BULBUS U R F T H R A E, SHOWING Thickenings OF THE iNTIMA AT X. Elastic tissue stain. (After Eberth.)



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. 324). 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 union of these tunicae. The cavernous or erectile tissue of which the corpora are composed, is essentially like that around the urethra.

All three cavernous bodies are surroimded by subcutaneous tissue and fascia, containing blood vessels, lymphatics and nerves, especially along the upper surface of the penis. The lymphatic vessels form a superficial and a deep set, the latter receiving branches from the urethra. The



Fig. 324.— Cross Section of a Penis.

Skin; b., subcutaneous tissue; c.» subfascial tissue; d., dorsal vein; e.* corpora cavernosa penis ; f., urethra; g., corpus cavemosum urethrae. (Bailey.)


principal sensory nerves are the meduUated dorsal nerves of the penis. They terminate in many tactile corpuscles in papillae of the skin, in bulbous and genital corpuscles in the deeper connective tissue, and 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

During early embryonic development sex is indistinguishable and perhaps imdetermined. Since it is well known that the sex of ma ture insects may^ be_largely controlled by the amount of nutriment whiclulhe larva receives^ it has been thought that the sex of mammals may become established in the course of their embryonic development. All attempts to find the controlling factors have failed and it is possible that the sex is determined when the egg becomes fertilized!)

In both the male and female there are similar primitive sexual cells, genital ridges, WolflSan and MuUerian ducts, elongated urogenital sinuses and prominent genital papillae. The structures shown in Fig. 299, page 264, may belong with either sex. The two Mullerian ducts reach the allantois side by side, between the Wolffian ducts. They fuse with one another, beginning at a short distance from their outlets and extending toward the allantois (Fig. 325, a detail from Fig. 279, B, page 249). The fused portion becomes divisible into the vagina below and the uierus above; a thick layer of smooth muscle in the mesenchyma surroimding the Mullerian ducts characterizes the uterus (Fig. 326). A fold of membrane, the hymeny which is found in the adult at the orifice of the vagina, may mark the termination of the Mullerian ducts. Some authorities, however, consider that more or less of the vagina is an outpocketing of the urogenital sinus and that the hymen has nothing to do with the Mullerian ducts. This opinion seems to rest on the inconclusive evidence that' persistent Woffian ducts in the adult may open into the vagina at some distance above the hymen.


Fig. 325. Reconstruction showing the fusion of THE MOllhrian Ducts. (After Keibel.) bl., Bladder; M.d., Mullerian duct; u., ureter; ur., urethra; U.S.. urogenital sinus; W.d.. Wolffian duct.


The portions of the Miillerian ducts which do not fuse remain as the uterine tubes (Fallopian tubes). Each opens freely through its fimbriated extremity info The abdominal cayity. Cystic appendages of the fimbriae have been described, and rarely there are accessory openings into the peritonaeal cavity. The uterine tubes, instead of being vertical as in the embryo, tend to become horizontal. The change in position is associated with the partial descent of the ovaries. The ovarian Ugament and the roimd ligament of the uterus represent the lower portion of the genital ridge and the gubemaculiun testis of the male. The round ligament is a cord of connective tissue, containing smooth and striated muscle fibers.


The Wolffian tubules in the female remain as from 8 to 20 transverse ducts, corresponding with the ductuli ef ferentes. They follow a tortuous course from the longitudinal duct (a part of the Wolffian) to I the ovary, near which they terminate, sometimes in small cystic enlargements. The longitudinal duct, which corresponds with the ductus epididymidis, ends blindly at both ends. In from 20 to 60 % of cases it terminates distally in a Uttle cyst, the appendix vesiculosa, which is lodged ^ a nodule of tissue attached to the broad ligament by a slender pedicle. Sometimes there are two or three such appendices. The structure consisting of the transverse and longitudinal ducts, which corresponds with the epididymis, is called the epoophoron [no longer parovarium or organ of Rosenmliller]. It is a functionless remnant of the Wolffian body lodged in the mesentery of the uterine tube, where it may easily be found. The paroophoron, a small vestige of Wolffian tubules, occurs nearer the uterus than the epoophoron; it has been found in various mammals and detected in the human adult. Except for the longitudinal duct, the Wolfl&an duct is ordinarily obliterated in the female. Fragments may persist in the musculature of the uterus and these "canals of Gaertner" sometimes open into the vagina.



Fig. 327. Part of the Ovary at Birth. (After Waldeyer.) a. Epithelium ; b, epithelial cord ; c. sexual cell ; d, detached cord ; e, jH'oup of follicles ; f, a single primarv follicle ; g, olood vessel. ( From McMurricVi.)



The ovary, like the testis, develops from the middle part of the genital ridge. The upper end of the ridge is said to be reduced to the band of tissue (fimbria ovarica) connecting the ovary with the uterine tube (Fig. 326); except for its ovarian attachment this fimbria resembles the others. The ovary is covered by a layer of columnar peritonaeal cells containing scattered large sexual cells. From this layer, cords including cells of both sorts, extend into the deeper tissue of the genital ridge (Fig. 327); toward the epoophoron their arrangement has been found to suggest a rete. Instead of forming tubules which empty into the WolflSan body as in the male, the sexual cords of the female produce detached islands of cells. The islands become subdivided into groups usually containing a single sexual cell, and known as primary follicles. Their later history will be considered with the adult ovary. The rete cords become vestigial or disappear.

The urogenital sinus which receives the urethra and vagina becomes a shallow space called the vestibule (Fig. 326). The genital papilla, tipped by its glans, 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 minora. (Compare

Fig. 328 with Fig. 302, page 267.) The labia form a prepuce for the clitoris but do not imite beneath it making 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 roimd Ugaments of^the uterus which correspond, with the gubernacula testis, and sometimes the peritonaeal cavity is prolonged into them forming a processus vaginalis. In late stages of development they become large enough to conceal the clitoris and labia minora which previously projected between them.


FiG. 328. Diagram of the External Genital Organs of a Female Embryo.

a., Anus; g., glans clitoridis ; g. f., lesser genital folds (labia minora); g.g. ff., greater genital tolds (labia majora); u. s.« uro-genital sinus (vestibule).



Ovary

The ovary is an oval body about an inch and a.half long, covered by a modified portion of the peritonaeum. 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 mesentery is continuous with that of the ovary, and the mesenteric connective tissue joins the mass which forms the central part of the ovary. This tissue, rich in elastic fibers and tortuous blood vessels accompanied by strands of smooth muscle fibers, is sometimes called the medulla of the ovary but may perhaps be better named the med iastinum. The peripheral part, except at the hilus, consists of the connective tissue stroma ovarii together with the primary and large vesicular follicles which it surroimds. Just beneath the ovarial epithelium it forms a dense layer consisting of two or more strata, the tunica albuginea.


Fig. 329. Cross Section of the Ovary of a Child Eight Years Old. X 10. I, Germinal epithelium; a, tunica albuginea; 3. peripheral rone with primary follicles; 4, vesicular follicle; 5, stroma ovarii ; 6, mediastinum ; 7. 8, peripheral sections of vesicular follicles ; 9, hilus, containing large veins.


The formation of follicles. The germinal or peritonaeal epithelium of the ovary consists of a single layer of small cells which may become low colunmar or flat. Even after birth sexual or "egg cells" may be found in it (Fig. 330). The egg cells divide by ordinary mitosis in the epithelium and in the detached islands of peritonaeal cells in the stroma. At sexual maturity nearly all of these islands have been separated into primary follicles each being a singje egg cell surrounded by a simple layer of fat cells derived from the peritonaeum. Sometimes a follicle contains two or more egg cells, all but one of which may atrophy; or the egg cell may have two nuclei the significance of which is obscure. The number of follicles in an ovary has been estimated to be from 8,000 to 16,000. Some consider that no new ones are formed after birth, but others believe that they may be produced in the adult. At all events only about 200 of them become mature; the others degenerate at various stages of development.


With further growth the follicular cells become columnar and then stratified (Fig. 331); the egg cells enlarge as their protoplasm becomes charged with nutritive material (yolk granules or deutoplasm). The connective tissue around the folli cle is compressed to form a distinct layer, the theca joUictdu Later the theca is divisible into a dense fibrous tunica externa, and a vascular tunica interna containing many cells with abun

Follicular cells

Fig. 330. From a Section of the Ovary of a Child Four Weeks Old. X 240.


Fig. 331. From a Section of a Rabbit's Ovary. X 240.



dant protoplasm (Fig. 332). A delicate membrana propria is found between it and the follicular cells. ( After the follicles have attained a certain size a crescentic cleft appears among their stratified cells.^ By distention of the cleft and enlargement of the follicle the condition shown in Fig. 332 is produced. These vesicular jollicles [Graafian follicles] vary in diameter from 0.5 to 12.0 mm. Besides the theca, the follicle includes a stratum granulosum or peripheral layer of follicular cells, and the cumulus oophorus or heap of such cells containing the immature ovum. The cumulus is connected with one side of the follicle although in certain sections (such as a horizontal section near the top of the cumulus in Fig. 332) it would appear completely detached. The columnar cells of the cumulus adjacent to the ovum are radially arranged, forming the corona radiata. The cavity of the follicle, at first crescentic, becomes so distended with fluid as to be nearly spherical. The fluid, or li^uorJgUiC' ulj^ is an aqueous transudate from the blood vessels. Certain appearances (Call - Exner bodies) in the stratum granulosum have been ascribed to cells undergoing liquefaction, and also to spaces containing a dense liquor. The structure of the egg cell within the cumulus will be considered under oogenesis.




FiG. 332. Section of a Large Vesicular Follicle of a Child Eight Years Old. x 90. The clear space within the follicle contains the liquor foUiculi.


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 12 mm. in diameter, and one surface of it is so close to the ovarial epithelium as to cause it to bulge macroscopically and then to rupture. Through the opening thus made the liquor folliculi and the egg cell, surrounded by more or less of its corona, are expelled into the peritonaeal cavity. ^Qiejiischarge of^the ovum from the follicle is known as ovular tion^ Blood escapes from the tunica interna and Torms a clot within the ffiipty folhcle (Fig. 333). On_all sides the clot is surrounded by proliferating cells which contain a yellow fatty pigment; thus they form a corpus luteum. The lutein cells increase in size and number and the clot"which may'show haematoidin crystals, is gradually absorbed. Between the lutein cells there are strands of vascular connective tissue as shown in Fig. 334. If pregnancy does not occur the corpus luteum attains its maximum development in 12 days and degenerates within a few weeks.



Fig. 333. Ovary, Cut Across. Slightly Rkduckd.

a.. Aperture through which the ovum escaped; c. a., (-^^rpus albicans ; cl., bl(X)d clot in a corpus luteum of ovulation ; th.. thcca folliculi ; v. f., vesicular follicle. (Alter RiefTel.)




Fig. 334. a. Portion of a Corpus Luteum of a Rabbit. B, Portion of a Corpus Luteum of a Cat. X 260. In B the lutein cells have become fatty and contain large and small vacuoles.

Connective tissue increases and the lutein cells disintegrate; the newly formed vessels are obUterated and the mass becomes a nodule of dense "scar tissue," the corpus albicans. If however, ovulation is followed by pregnancy the corpus luteum enlarges even to a diameter of from 1.5 to 3 cms., reaching the height of its development in five or six months. It persists until the end of pregnancy. Thus the corpus luteum of pregnancy must. he distinguished from the corpus luteum of ovulation.


As to the origin of the granular, vacuolated lutein cells there is a difference of opinion. Some consider that they arise from the stratum granulosum, and others from the tunica interna. They have been compared with the interstitial cells of the testis, and there is_experimental evidence that they produce an internal secretion without which an embryo cannot deyebp '^thin the uterus.


Many follicles degenerate without discharging their egg cells. Cells from the stratum granulosum and leucocytes are said to invade them and after absorbing the egg protoplasm they disintegrate. The zona pellucida, a clear layer aroimd the egg cell, becomes conspicuously folded and persists for some time (Fig. 331). The basement membrane of the stratum granulosum has been said also to thicken and become convoluted. These degenerating or atretic follicles are finally reduced to inconspicuous scars or they disappear. After the menopause the degeneration of the egg cells becomes general.


Fig. 335. The Ovum as Discharged from a Vesicular Follicle of an Excised Ovary of A Woman Thirty Years of Age. Examined fresh in liquor folHcuH. (Nagcl.) C. r., Corona radiata ; n., nucleus ; p., granular protoplasm ; p. s., perivitelline space ; y., yolk ; z. p., zona pellucida. (From McMurrich's *' Embryology.")

Oogenesis

The maturation of the ovum is comparable with that of the spermatozoon. Just as an indefinite number of generations of spermatogonia produced by ordinary mitosis, terminates in primary spermatocytes, so the oogonia terminate in primary oocytes. Both the primary spermatocyte and oocyte give rise by two reduction divisions, in 'which one half the somatic number of chromosomes is involved, to four mature sexual cells. In case of the ovum, however, only one of the four is capable of fertilization.

The sexual cells in the germinal epithelium and in the islands of the ovary are chiefly oogonia. The vesicular follicles contain ooc)rtes which may be recognized by their great size (about 200 /^ in diameter). As seen in Fig. 335, the nucleus is large and vesicular [and is often called the germinative vesicle]. It contains a nucleolus [germinative spot] which in fresh liquor folliculi exhibits amoeboid movements. The nucleus has a distinct membrane; usually it is near the center of the cell, but it may migrate to the periphery. The central part of the protoplasm contains coarse granules of yolk derived from the follicular cells; it is surroimded by a finely granular zone, and this is followed by a very narrow layer free from granules. The protoplasm of oocytes may contain a "yolk nucleus," a structure formed by the centrosome and archoplasm or idiozome. Yolk nuclei are not found in mature ova. The oocytes probably possess no distinct cell wall. They are surrounded by a broad, clear, radially striated band, the zona pdlucida. The striations are said to be canals containing processes of the follicular cells. It is still doubtful whether the zona is a product of the oocytes or of the foUicle. The egg cell may become separated from it by a narrow perivUelline space as shown in Fig. 335.


When the primary oocyte divides into the secondary oocytes the nuclear material is equally distributed between them. One of them, however, receives nearly all the protoplasm; consequently the other is a small cell and is known as the first polar globule. In becoming a mature ovum the secondary oocyte divides for the second and last time, thus giving rise to the ovum and second polar globule. The first polar globule may divide in two. Thus the primary ooc)rte produces a mature ovum, and three polar globules which from their lack of protoplasm are generally f unctionless. As they occur in the mouse they are shown beneath the zona pellucida in Fig. 336. It is imknown when the polar globules are formed in man, whether in the ovary before ovulation, or later. In the mouse one forms in the ovary and the other in the uterine tube.



Fig. 336. Ovum of White Mouse, Surrounded by zone pellucida

Above the ovum are two polar globules; within It are two nuclei, one belonging to the ovum, the otner beine derived from the heaa of the spermal07oon. X 500. (After Sobotta, from Minot's "Embryology.")


Fertilization

The ovum passes from the peritonaeal cavity into the fimbriated end of the uterine tube, and in the upper part of the tube it may be fertilized. The process in man is unknown, but from observations in other animals it is probable that several spermatozoa enter the zona pellucida, and that only one passes into the protoplasm of the ovum. It loses its tail piece as it enters. The head is resolved into twelve (?) chromosomes which become arranged beside the tw^elve (?) in the nucleus of the ovum. The centrosomes of the fertilized ovum may be derived from that of the spermatozoon, or from that of the ovum, or arise anew; the evidence is conjflicting. Each of the two cells into which the fertilized ovum divides, receives one half of each of the twenty-four chromosomes, tr^v^elve from either parent, and in all subsequent mitoses 24 (?) chromosomes appear. This remarkable distribution of chromatin has caused it to be considered the bearer of hereditary qualities. The spermatozoon, however, contributes protoplasm to the fertilized ovum and possibly the centrosome also.


Vessels and nerves. Branches of the ovarian and uterine arteries follow a tortuous course from the hilus to the capillary networks of the timica interna. They branch freely in the stroma. The veins form a dense plexus at the hilus. Thin walled lymphatic vessels arise in the tunica externa of the corpora lutea and larger follicles, and become more numerous toward the hilus. Their course is independent of the blood vessels, periVascular lymphatics being absent. There are no lymphatics in the albuginea. Medullated and non-medullated nerves supply chiefly the vessels, but they form terminal nets in the thecae. It is uncertain whether any extend among the foUicular cells. Ganglion cells have been recorded near the hilus, but in man the existence of an ovarian ganglion is denied. The principal nerve supply is the plexus 0} the ovarian artery.

Epoophoron

The tubules of the epoophoron presumably vary in structure. They have been described as cords of cells and as tubules lined with simple cuboidal or columnar epithelium, sometimes ciliated. A layer of circular muscles may surround them and internal longitudinal fibers have been found. The epoophoron is of interest as a source of cysts of the broad ligament. Peritonaeal cysts may also occur.

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 injundibulum] the ampulla or distensible outer two thirds, 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 tube includes a tunica mucosa, (submucosa) , muscularis, and serosa. The mucous membrane is thrown into folds which are low in the isthmus but are tall and branch in the ampulla, the lumen of which they seem to fill (Fig. 337). The branches may anastomose; glan ds arc absent. The ampuUa^has been compared_ with a seminal vesicle; in it the ovum is probably fertilized normally and the development of large embryos within it is not a rare occurrence. The epithelium is chiefly simple columnar and ciliated the stroke of the cilia being toward the uterus. Small areas of flat non-ciliated cells may occur near the infundibulum and non-ciliated cells have been found in the isthmus. The tunica propria is a vascular tissue often containing lymphocytes. It extends into the folds. In some places the presence of strands of longitudinal smooth muscles (a muscularis mucosae) separates the mucosa from a submucosa. The muscularis consists of a thick inner layer of circular fibers and a thin outer longitudinal layer. The layers are thin toward the infundibulum where the longitudinal fibers may be absent. The loose inner tissue of the serosa is sometimes called the adventitia. Abundant elastic fibers occur in it, and except in childhood and old age they are numerous in the muscularis also. Blood vessels are highly developed between the muscle layers and in the mucosa. The lymphatics form large ves,Tube , Fundus sels in the mesentery of the tube. Nerves supply the muscles and after branching freely in the mucosa ascend to the epithelium.



Fig. 337. The Mucosa of the Uterine Tube A, Near its Fimbriated End; B, Near the Uterus. (After Orthmann.)


Fig. 538. Cross Section, near the Ampulla, of a Uterine Tube from an Adult Woman.


Fig. 330. The Dorsal Half of A Virgin Uterus natural size. (After Rieflel.)

Uterus

The uterus is a muscular, pyriform 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 triangular cavity of the corpus and fundus opens into the canal 0} the cervix through the internal orifice; the canal commimicates 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 its ventral surface; these are the plicae palmatae. The walls of the uterus consist of a tunica mucosa, muscularis and serosa.


The thick muscularis consists chiefly of interwoven circular and oblique fibers. A thinner outer longitudinal layer continuous with that of the tube, is more or less separated from the circular layer by connective tissue containing many large blood vessels. The outer layer borders upon the serosa and is sometimes considered a s belonging with the subserous tissue. Inside of the circular layer an inner longitudinal layer is described by Professor Stohr, and the three layers are said to be quite distinct in the cervix. More generally only two layers are recognized, an inner oblique and circular, and an outer longitu dinal. The uterine muscles are smooth sometimes branched. During pregnancy they increase in number and in length to three or four times their ordinary dimensions. Except in the peripheral part of its lower half the uterus contains little elastic tissue. There the elastic elements are at riglit angles with the course of the muscle fibers. They increase during the first half of pregnancy and decrease in the latter half (except in the outer connective tissue).


Fig. 340. From a Transversk Skction ok the Middle of the uterus of a girl fifteen years old. X lO. a. Epithelium ; b. tunica propria ; c, glands ; i, inner muscular layer; 2. middle muscular layer; 3, outer muscular layer.


There is no submucosa; epithelia l pits or uterine gland s extend to the muscle layer and occasionally enterTt They are vertical tubes, sometimes branched, which have a tortuous course in their deeper part. Often two or three unite so as to have a common outlet. Their distance from one another, the extent of their flexures and their relation to themuscularis are features subject to pathological changes. Cystic dilatations are common especially in older persons. The glands produce no specific secretion. They are lined with simple columnar epithehum sometimes ciliated, in all respects like that of the uterine cavity.


Often cilia are absent from the uterine epithelial cells, which is said not to be due to faulty preservation but to the fact that the ciliated cells occur singly or in groups. According to a recent estimate only -^^ or -2V oi the cells are ciliated; and from observations on certain animals it is suggested that cilia are present only in certain functional conditions, at other times being absent.

In the cervix, mucus-producing cells occur, especially in the outpocketings of epithelial pits, thus forming the branched cervical glands. They discharge a secretion which occludes the canal of the cervix during pregnancy. Often they produce macroscopic retention cysts, due to the accumulation of secretion [ovules of Naboth, named for the Leipzig anatomist who mistook their nature in 1707]. WTien empty of secretion the cervical glands are said to resemble the uterine glands. Toward the external orifice of the uterus the epithelium becomes stratified, resting on papillae and having its outer cells squamous. Such epithelium is found in the vagina, and after the first child-birth it may extend into the lower half of the cervix.



Fig. 341. Mucous Membrane of the Resting Uterus of a Young Woman. X 35- (After Bohin and von Davidoff.)



The tunica propria of the uterus is a very vascular reticular tissue with abundant nuclei. It contains many free lymphocytes and its lymphatic vessels form a wide meshed network with blind extensions. They empty into a network of larger vessels in the subserous tissue. Medullated nerves are said to extend to the epithelium and many nonmedullated fibers supply the muscularis. Ganglion cells detected within the uterus by the Golgi method are believed to be not more ganglionic than those of the intestinal villi found by the same method. In the utero- vaginal plexus which is the source of the sympathetic nerves of the uterus, ganglion cells have been found in the vicinity of the cervix.

Menstruation

Menstruation is the periodic degeneration and removal of the superficial part of the mucosa of the uterus, accompanied by haemorrhage from the vessels of the tunica propria. For four or five days before the discharge occurs, the thickness of the mucosa increases 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 vessels and form subepithelial masses. This stage of tumefaction is followed by one of haemorrhage and desquamation lasting about four days. 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 which requires about seven days, the epithelium spreads from the glands over the exposed reticular tissue, the congestion diminishes, and the mucosa returns to its resting condition. In about twelve days the cycle begins anew. The cervix takes no part in menstruation except that the secretion of its glands may increase during the stage of congestion.


Beginning at puberty (12-15 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 also by those which cause the delivery of the child. Thus tiie duration of pregnancy is described as ten menstrual cycles. The significance of menstruation is still obscure. In mammals generally, a period of congestion accompanied by uterine changes which are sometimes closely comparable with those of menstruation, precedes sexual intercourse and ovulation. Ovulation ordinarily occurs at that time, independently of coitus. (In the rabbit and ferret, also in pigeons, ovulation may fail to occur in the absence of the male.) 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 ovulation is independent of coitus and to some extent of menstruation. The spermatozoa of rabbits retain their activity and are capable of fertilizing the ovum for about ten days, and it is perhaps true that if human ovulation takes place within some such period after coitus, fertilization may occur. The ovum is said to take four days in the rabbit and eight or ten in the bitch to pass through the tube to the uterus. The condition of the mucosa of the human uterus when the fertilized ovum enters it is unknown. The stage of development of many young human embryos suggests that their growth began nearer the time of the first menstruation which lapsed than the last which occurred. This may be due to the frequency of human menstruation, which may still be preparatory to coitus as in other mammals.


Fig. 342. Mucous Membrane of a Virgin Uterus During the First Day of Menstruation. X 30. (Schaper.)

The Development of the Decidual Membranes

Before describing the mucosa of the uterus during pregnancy, it is necessary to consider the membranes of the embryo which are in contact with it. Fig. 343, A, represents a blastodermic vesicle in which the three germ layers are present. (The formation of such a vesicle by the segmentation of the ovum has been figured on page 19.) In a thickened portion of the outer layer of the vesicle a cleft occurs, which in B has widened and become the amniotic cavity. It is bounded below by the ectoderm which covers the body of the embryo, and above by a layer which is soon divided into two parts by an extension of the body cavity. This has occurred in C. The inner layer or amnion consists of ectoderm toward the embryo and mesoderm away from it. It is a membrane continuous with the skin of the embryo. The outer layer or chorion surrounds the entire vesicle and is characterized by shaggy villi. It consists of ectoderm [trophoblast] on its peripheral surface and mesoderm within. A stalk of mesenchymal tissue surrounding the allantois extends from the embryo to the chorion. It lodges the umbilical (allantoic) vessels through which the blood of the embryo passes to the chorionic villi and back to the embryo. These villi enter into close relation with uterine mucosa, being bathed in maternal blood, and the embryo receives such nutriment as is absorbed through their walls. Human embryos of the stage C are well known, but the youngest which have been obtained are more advanced than B; therefore the stages A and B are hypothetical.



Fig. 343 Three D!agr,\ms of thk Hypothetical Development of the Human Decidual Membranes. (After Minot.) al., Allantois ; am., amnion ; am. c* amniotic cavity; cho.. chorion ; coe., coclom ; y. 8., yolk sac. The mesoderm is stippled, the ectoderm is shaded with lines and the entoderm with dots.


In further development the amniotic cavity enlarges so that the amnion is in contact with the inner surface of the chorion (Fig. 344). The yolk sac and its attenuate stalk are brought close to the allantois. The mesenchymal tissue surrounding the yolk stalk and allantois and covered by a layer of ectoderm, forms the umbilical cord. At first it contains an extension of the body cavity around the yolk stalk but later this is obliterated by adhesions. The ectoderm of the cord is continuous distally with that of the amnion and proximally with the epidermis of the embryo. There is an abrupt transition from the skin with its capillaries to the nonvascular covering of the cord, which at birth is about 8 mm. from the abdominal wall.


Only one side of the chorionic vesicle becomes implanted upon the uterine mucosa. The villi on that side of the vesicle proliferate and constitute the chorion frondosum. Elsewhere the villi become scattered and low, finally disappearing; the resulting smooth part of the chorion is called the chorion laeve.


The appearance of a human embryo at a stage intermediate between the last two diagrams considered, is shown in Fig. 345. The greater part of the villous chorion has been cut away together with half of the thin smooth anmion, thus exposing the embryo with its umbilical cord and yolk sac.


Relation between the membranes and the uterus. That portion of the uterine mucosa against which the chorionic vesicle rests and into which its villi proliferate, is called the decidua basalis [serotina]. A portion which grows over the vesicle completely enclosing it is the decidua capsularis [reflexa]. The remainder of the mucosa is the decidua vera (Fig. 346). As the embryo increases in size so as to fill and distend 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 upon the decidua vera (Fig. 346, B). The chorion frondosum together with the inseparable part of the decidua basalis into which its vilU have grown, form the placenta. Thus the placenta consists of a uterine and a fetal portion. It is a discoid mass of vascular tissue which at birth is about 7 inches in diameter, i inch thick, and weighs a pound. The distal end of the cord is usually but not always inserted near its center. From the end of the cord the anmion spreads over the placenta and is lightly adherent to it; the free surface of the amnion is smooth and glistening. The chorion laeve, beginning at the placental margin, continues clear around the cavity of the uterus and, as before mentioned, the amnion adheres to it. The amniotic cavity is filled with fluid in which the embryo is immersed. Shortly before birth the cervix dilates and the membranes thus exposed, rupture. The amniotic fluid escapes and the child follows, its umbilical cord extending through the vagina to the placenta. In the course of half an hour the placenta and membranes are expelled, the sac which they form being inverted in the process. Thus the smooth or amniotic surface of the placenta is exposed. The very thin membranes attached to its margin consist of amnion, chorion and fragments of decidua vera. The denuded uterine mucosa is gradually restored to its normal condition, as after menstruation. Epithelium spreads over its surface from the bases of the glands. In the following account the histology of the decidua vera and adjacent membranes will be considered first, then the placenta and finally the cord.



Fig. 344. Diagram of the Formation of the Umbilical Cord, Lettered as in

Fig. 343


Fig. 345. A Normal Human Embryo of 10.0 mm., Removed Surgicaliy with the Uterus. Six Weeks after the Last Menstruation. The embryo has been exposed by cutlins: away most of the chorion, cho., and part of the amnion, am.; U.C., umbilical cord ; v., chorionic villi ; y.t., yolk sac.



Fig. 346, The Uterus and Decidual Mhmbranbs 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 cora ; y. 8., yolk sac.

Decidua Vera, Amnion, and Chorion Laeve

On the upper surface of the section Fig. 347, is seen the amnion, having its simple cuboidal or flat epithelium toward the embryo, and its mesodermic connective 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 sUght irregularities but is without villi. The superficial uterine epitheUum 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 epithelium of the glands in the compact layer degenerated and was resorbed, the connective tissue came together obUterating the gland cavities. The compact layer is therefore

Placenta

The chorionic villi, the interlacing branches of which form the fetal portion of the placenta, are shaped as shown in Fig. 349. 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 containing dark roimd or flattened nuclei. Since cell boundaries are lacking, this is called the syncytial layer. Mitoses are seldom seen in it. Generally its nuclei are in a single layer but they



Fig. 349.— Isolatkd Terminal Branches of Chorionic Villi; that on the Left is from an Embryo of Twelve Weeks; on the Right at Full Term. (Minot.)


may accumulate in "knots" or "proliferation islands," especially in late stages. 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 yoimg 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 decidual basalis degenerates and disappears, and the distal parts of



Syncytium.


Cuboidal cells of the basal layer.


Connective tissue.


Blood vessel containing nucleated red corpuscles.


Oblique section of the epithelium. Fig. 350.— Cross Section of a Human Chorionic Villus of the Fourth Week of Prkgnancv.

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


Fig. 351.— Diagram of the Human Placenta at the Close of Pregnancy. (Scha|>cr).

intervillous spaces as shown in the diagram Fig. 351, 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 otiierwise of maternal origin, but this view is not accepted.)

The placenta at birth, being an inch thick, presents in cross section a vast number of the branches of villi cut in various planes. In the villi of Fig. 352 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. Within the villus are the blood vessels of the embryo; their blood never mixes with the maternal



Fig. 353.— Cross-section through a Smaller (A) and a Larger (B) Chorionic Villus of a Human Placenta at the End of Pregnancy, x 250. (Schapcr.)

blood which surrounds the villi, as is easily seen in the early stages when the fetal blood contains nucleated red corpuscles.

The embryonic surface of the placenta is shown in Fig. 353. Along the chorionic epitheUum there are generally areas of hyaline material which stain deeply with eosin and have the appearance of fibrin. In the outer part of the placenta also, the villi may seem to terminate in hyaline masses attributable to the degeneration of the inner epithelium. The hyaline masses [canalized fibrin] are a conspicuous feature of the mature placenta.

The decidua basahs consists of compact and cavernous layers, thinner but similar to those of the vera (Fig. 354). It sends septa into the fetal part of the placenta dividing it into coarse lobes or "cotyledons."


The maternal arteries are in the septa but the veins are in the spaces between them.



^*>G. 353.— From a Cross-section of a Ma^ l re Human pLACiiNTA. X 260.


Umbilical Cord

The umbilical cord is a translucent glistening white or pearly rope of tissue about two feet in length, extending from the umbilicus to the placenta. It consists of mucous tissue (p. 37) covered with epithelium and containing at birth three large blood vessels, two arteries and a vein (Fig. 355, 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


354.— From a Cross Section of a Mature Human Placenta. X 260.

in the cord and the vessels do not anastomose. The arteries contain many muscle fibers but very little elastic tissue and they are usually found



Fic. 355.— Cross Sections op Umbilical Cords. A, X 20, from an embryo of 2 mos. ; B, X 3, at birth. al., Allantois ; art., artery ; coe., coelom ; v., vein ; y. 8., yolk stalk.

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 within the embryo as the principal terminal branches of the aorta; parts of them in the adult are called the common iliac and hypogastric [internal iliac] arteries. They end in the capillaries of the chorionic villi. The single umbilical vein is due to a fusion of two; within the body only the left remains, passing from the umbilicus along the under surface of the liver (as the ductus venosus) to the vena cava inferior.

The allarUois which the umbilical vessels accompany, extends the entire length of the cord as a slender tube or strand of cells. At birth it is rudimentary but may be found usually between and equidistant from the arteries. It is more conspicuous when Mallory's stain is used. Within



Fig. 356.— Yolk Sac and Persistent VITELLINE Vessels, Exposed by Reflecting the Amnion at the Distal End of the Cord. (Lonnbcrg.)




Fig. 357.— Part of a Human Amniotic Villus. X 330. Ep., Epitrichium ; 8. C, stratum corneum ; S. g., stratum ^anulosum ; S. G., stratum germinativum ; M. B., homogeneous layer; F. T., fibrous tissue ; A. T., areolar tissue.


the body the allantois dilates to make the bladder, and if its prolongation into the cord remains tubular, urine may escape at the umbilicus (through a "urinary fistula").

The yolk stalky surrounded by an extension of the body cavity, is found in young umbilical cords (Fig. 355, A). 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 yolk stalk remains pervious the intestinal contents may escape at the umbilicus (fecal fistula). Ordinarily the stalk and its vitelline vessels, together with the coelom of the cord, have been obliterated before birth and no trace of them remains in sections of the cord.


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. 356), 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.

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. 357 they are areas of imperfectly developed skin; since epithelial elevations occur abundantly over the cords of certain mammals, these structures of unknown significance are probably normal.

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 rugae. Glands are absent. The tunica propria is a delicate connective tissue with few elastic fibers, containing a variable number of leucocytes. Occasionally there are solitary nodules, above which numerous leucocytes wander into the epithelium. The submucosa consists of strong elastic and looser white 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 l)nnphatic vessels are found in the connective tissue layers, and wide veins form a close network between the muscle bundles. There is a gangUonated 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 ^^' ^^ 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 bulbourethral glands in the male and are of similar structure. The h)nnen consists of fine fibered, vascular connective tissue covered with mucous membrane. The clitoris is a somewhat erectile body, resembling the penis. It includes two smaU corpora cavernosa. The glans cUtoridis contains a thick net of veins. It is not, as in the male, at the tip of a corpus cavemosum 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.


Skin

The skin (cutis) consists of an ectodermal epithelium, the epidermis and a mesodermal connective tissue they orium (Fig. 358).: The ectoderm is at first a single layer but soon it becomes double, the outer cells

staining more deeply, and


1 ectoderm ffidenms



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 imderlying epithelium do not penetrate it, but cause it to be cast ofiF. The epitrichium has been found on the umbilical cord and in places on the amnion. It may possibly be related with the chorionic syncytium. The deeper layer of ectoderm becomes stratified, and it 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 through the skin; 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


Fig. 358. Skin from the Occiput of an Embryo OF 2j^ Months. (After Bowen.)

The outer layer of dark cells is the fpitrichinm.


Fig. 359. Vertical Section from the Sole of the Foot of an Adult. X 25.


many little furrows 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 along the summits of which the sweat glands open. The ridges are most highly developed over the pads of tissue at the finger tips and in the interdigital spaces at their bases. Here the tactile function is most perfect. The pads. are very prominent in the embryo and correspond with the "walking pads" of camivora. Similar structures occur on the soles.

Corium

The corium is a layer of densely interwoven bundles of connective tissue extending from the epidermis to the fatty, areolar sub Depressions which were occupied by cutaneous tissue (Fig. 359). Its epidermal surface exhibits papillae which are tallest and most numerous on the palms and soles. Their height may be 0.2 mm. In the skin of the face they are poorly developed and in old age they tend to disappear entirely. As seen in Fig. 360 they may be definitely arranged beneath the ridges of the finger tips, fonning a double row under each; the grooves between the ridges correspond with epithelial depressions between the papillae. In Fig. 361, which represents the under surface of the epidermis, the relation of the papillae to the rectangular markings may be seen. The papillae are formed of tunica propria, a cellular connective tissue; and each papilla contains terminal capillary loops or a tactile corpuscle (Fig. 126, p. 105). The corpuscles are most numerous in the sensitive finger where they may occupy one papilla in every four.



Fig. 360. Vertical Section from the Sole of the Foot of an Adult, showing Four Ridges (A-D) WITH A Pair of Papillae beneath Each. Between the papillae of D is the duct of a sweat gland. X 25.



Fig. 361. Epidermis from the Skin of the Dorsum of the Human Foot, seen FROM the Lower Surface. X 120.



Beneath the papillae the connective tissue bundles are closely interwoven but toward the subcutaneous tissue they form a coarse network [hence the coriiun is sometimes divided into a stratum papillare and a deeper stratum reticulare]. The subcutaneous tissue is areolar, with large areas of fat cells; where the fat forms a continuous layer it is called the panniculus adiposus. Columns of areolar tissue which extend to the hair follicles and glands of the skin, may become paths for infection from the surface to the subcutaneous tissue. The elastic fibers of the skin are said to form a subepithelial net, a thick plexus of fine fibers beneath the papillae, and layers of coarse fibers along the vessels in the deeper part of the corium and in the fascia. The subcutaneous tissue contains relatively little elastic tissue. In the skin of the face and joints, elastic elements are most abundant; in old age, throughout the skin, they decrease notably. Smooth muscle fibers constitute the arrector muscles of the hairs; as a membranous layer they occur only in the tunica dartos of the scrotum, and in the nipple. Striated muscle fibers in the skin of the face represent the insertions of the muscles of expression. The vessels and nerves of the corium are described on page 327.


Fig. 362. From a Skction through thk Skin op the Sole OF thk Foot of an Adult Man. x 360.



Epidermis

The epidermis is stratified epithelium, the many layers of which are divisible into a stratum ^ermin fLtiimpi and £i stratum c orneum . The former includes a basal row of columnar cells without membranes, which rest on the papillae of the corium. Although mitoses are seldom seen, these cells multiply and produce the several layers of polygonal cells which overlie them. The latter are connected by numerous slender intercellular bridges, as seen in Fig. 31, p. 30. Because of this striking feature the stratum germinativum was formerly called the stratum spinosum [and rete Malpighii]. The transition to the stratum corneum or outer layer of homy flat cells is quite abrupt, except in the thick skin of the palms and soles. An incomplete layer of coarsely granular cells may intervene. In the corneum the cells acquire a homy exoplasmic membrane; the bridges become short stiflF spines; the protoplasm and nucleus are dried and shrunken and in the outermost cells the nucleus may wholly disappear. The cells become flatter toward the surface, from which they are constantly being desquamated.

The process of comification presents a more elaborate picture in sections of the palms and soles. Passing 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 haematoxyline and eosine 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. They are followed by a thick stratum comeum. «  (^Except in the palms and soles the granulosum is thin and the lucidum is ' absentX Chemic a^ the coarse granules of the stratum granulosum resemble ' 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 comeum 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 surfaced

The color of the skin is due to fine pigment granules in and between the lowest layers of epidermal cells; a few smaller granules occur in the corium. Pigmented connective tissue cells are found near the anus, but they are generally infrequent and are absent from the palms and soles. The possibility of the mesenchymal origin of epithelial pigment


is stated on page 46. It is probable that the epidermal pigment arises in the cells In which it occurs. The origin of the granules found between the epithelial cells is obscure*

Nails

The nails are areas of modified skin consisting of corium and epithdium. The corium consists of fibrous and elastic tissue, the bundles of which in part extend vertically from the periosteum of the phalanx to 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 skinAt the proximal end or root of the nail the corium has tall papillae.


Fig. 5&3. Dorsal Half of a Cross Sectiok of the Thtrd Phai.amjs or A Cwii,D. K ts. The ridgts of the nail btd in cms* secUou Appear like papillae.


The epithelium consists of a stratum gtrminathmm and a stratum carmunty but the latter corresponds with a thick stratum lucidum. In the embryo the homy 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, 363), The cponychium is the stratum corneum of the adjoining skin* Although the nail cells are formed by the entire underlying stratum germinativum, as is shown by the increasing thickness of the nail toward its distal edge, yet the principal production is at its proximal root beneath the crescentlc white area, the lunula. The opacity of the nail at the lunula has been attributed to keratohyalin; an imperfect stratum granulosum occurs there. The pink color of the outer portion is due to blood beneath, which is seen through the transparent straItim luddum- The cells of the nail may be separated by heating to boiling a fragment placed in a strong solution of caustic potash. The cells retain their nuclei as is seen in Fig. 364. The forward movement of the nail is due to the production of new cells from behind.


Fjg. .

Hair

The hairs arise as local thickenings of the epidermis. They soon become roimd colunms of ectodermal cells extending downward into the corium (Fig. 365). As the columns elongate the terminal portion becomes enlarged, forming the bulb of the hair, and a mesodermic papilla occupies the center of the bulb. On that side of the epitheUal colunm which from its obliquity may be called the lower surface, there are found two swellings (Fig. 366 and 368). The outer is to become a sebaceous gland discharging its secretion into the epithelial colunm; the inner or deeper swelling is called the hair matrix and its cells, which increase by mitosiscontribute 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 bulbus 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 oi the shaft become cornified just above the bulbus, 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 imme, diately 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. 369). The shaft enters the canal, breaks up the overlying epitrichium, and projects from the surface of the body (Fig, 370). That portion of the hair which remains beneath the epidermis is called its root. In addition to the epithelial sheaths, the root of all larger hairs possesses a connective tissue sheath derived from the corium. This serves for the insertion of a bundle of smooth muscle fibers which arise in connection with the elastic elements of 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


Fig. 365. Vertical Section of the Skin of the Back of a Human Fetus of Five Months. X 230.





Fig. 366 Vertical section of the skin of right Glutaeai region of a fetus of about.


Fig. 367. Vertical section of the skinTlflG BACI£ nr A HUMAM Fetus op Fjvb awp a Haui'


Fig. 368. Vertical section of the skin QF t MB Folef^tiKAtl OK A HUMAN

Fh it's OF FivH Months. X 330. Difieretitialion of thesheath&ol the hair.

Fig. 369. Vertical Section of the Skin of the Back of a Human Fetus of Five and a Half Months. X lao. The staining with iron haeniatoxylin has made the horny parts so black their details are invisible.


as shown in Fig. 370. The hairs which cover the body of the embryo and which to a variable extent persist after birth, are soft and downy; they are known as lanugo, Arrector muscles are absent from the lanugo of the nose, cheeks and Ups, and also from the eyelashes (cilia) and nas^Jiairs (vibrissae). In describing the development of hairs it has been stated that a hair consists of a papilla, bulb. and shaft and that the part of the shaft beneath the epidermis is covered with a connective tissue sheath, an outer epithelial sheath , and below the sebaceous gland, with an inner epithelial sheath The finer structure of the shaft and its sheaths is shown in the cross section. Fig. 371, and the longitudinal s e c tion. Fig. 372; it is described in the following p a r a graphs.

The connective tissue sheath is derived from the corium. It is found about the larger hairs where it may be divisible into three layers. The outer layer is a loose connective tissue with longitudinal bundles, containing elastic fibers and numerous vessels and nerves. The middle layer, which is thicker, consists of circular bundles of connective tissue without elastic fibers. The inner layer together with the basement membrane of the outer epithelial sheath may form a single, transparent hyaline membrane. The connective tissue portion of the membrane is sometimes longitudinally fibrous; the epitheUal part is homogeneous and provided with small pores.


Fig. 370. From a Thick Section of the Human Scalp. X 20.



The outer epUhdial sheath is an inpocketing of the epidermis. The stratum comeum extends to the sebaceous gland; the stratum granulosum continues somewhat deeper, but only a thinned stratum germinativum can be followed to the bulb.


Fig. 371. From a Horizontal Section of the Human Scalp. X 240. Cross section of a hair and its sheaths in the lower half of the root.


The inner epithelial sheath extends from the sebaceous gland to the bulb. It begins as a layer of comified cells below the termination of the stratum granulosum; it is, however, not a continuation of that layer. Toward the bulb the inner sheath is divisible into three 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 middle or Huxley's layer is a row of nucleated cells, and the inner layer or cuticula of the sheath is formed of non-nucleated comified scales. Toward the bulb both the cuticule and Henle's layer are nucleated and the three layers become indistinguishable as seen in Fig. 372. Kerato-hyalin granules which occur in Huxley's and Henle's layers extend nearer the papilla in the latter.


Hair cuticle. Cortical substance,


Fig. 372. Longitudinal Section op the Lowest Division of the Root of a Hair; the keratohyaline granules are colored red. From a vertical section of the human scalp. X 200.


The shaft of the hair is entirely epithelial. Its surface is covered by a thin cuticula which is formed of transparent scales directed from the center of the shaft outward and upward, and overlapping like shingles. These are non-nucleated comified cells. The greater portion of the shaft is included in the cortex. Toward the bulb the cortex consists of soft cells, but distally they become comified, elongated and compact; their nuclei are then linear. Except in white hairs pigment occurs both between and in these cells. Very small intercellular air spaces are found in the cortex of fully developed hairs. The medulla when present, occupies the center


Fig. 373 Elements of a Human Hair and its Sheath. X 240. 1, White hair ; 2, scales of the cuticle ; 3, cells of the cortical substance of the shaft ; 4, cells of Huxley's layer; 6, cells of Henle's layer, having the appearance of a fenestrated membrane; 6, cells of the cortical substance of the root.



of the shaft. It is generally a double row of cells containing keratohyalin granules and degenerate nuclei. A medulla is found only in large hairs and it terminates before reaching their tips.

The shedding of hairs. Shortly before and after birth there is a general shedding of hair. In the adult the loss and renewal of hairs is not periodic but constant. The Ufe of a hair in the scalp may last 1600 days. The process of removal begins with a thickening of the hyaUne membrane and circular fiber sheath. The matrix ceases to produce the inner sheath and consequently the cuticula and hair. The bulbus becomes comified, forming a solid frayed end of the shaft as seen in Figs. 375 and 376. The


increase of undififerentiated cells in the outer sheath and matrix forces

the degenerating hair with its inner sheath outward (Fig. 376). The

comified bulb remains near the

sebaceous gland at the outer

Umit of the matrix; after a

variable time the hair falls

out. The deep portion of the i„„er sheath, outer sheath, emptied of its hair, collapses and shortens, drawing the atrophic papilla upward. The matrix cells proliferate causing the epitheUal cord to return to its former depth and a new hair develops in the old sheath. This hair in growing toward the surface its predecessor.


Fig. 374. Four Stages in the Shrdding of a Hair, FROM A Section of the Nasal Skin of yH Months Embryo. X 50.

X, beginning of the new hair.


may complete the expulsion of


Remains of inner sh^th.


Fig. 375. Lower Part of Fig, 374, A. X 230.


Fig. 376. Lower Part of Fig. 374, B. X 230.

Sebaceous Glands

The sebaceous glands are simple, branched or imbranched alveolar structures situated in the superficial layer of the corium and usually appended to the sheath of a hair (Fig. 370)/ In connection with the lanugo, a large gland may be associated with a ver\" small hair (Fig. 377), and in exceptional case s as at the margin of the lip or on the labjajii nora, th^ occur independ ent^^ of hairs. They %'ary in size from 0.2 to a.2 mm.* the largest being found in the skin of the nose where the ducts are macroscopifi^ None are fouind in the palms or soles where hairs also are absent. (The short duct is a prolongation of the outer sheath of the hair and is formed of stratified epithelium, the number of layers of which decreases toward the alveoH, The alveoli consist of small cuboidal basal celis, and of large rounded inner cells in aU stages of fatty metamorphosis. .\s the cell becomes full of vacuoles the nucleus degenerates, and the cell is cast off with its contained secretion. '^Ms in life is a semi-fluid material composed of laTand broken down cells.


Ftg. 377. A* Fhom a VitdTiCAi, Section thboucm the Ala Nasi of a Cmii-1>, /. a^- C, Btraium toftieiim ; U. stratum geruiiiiativuni ; t, scbafeows gland contininK of four sacks, â– .duel ol ibe same; w» latmj^ hmr, about to be shed* 1i, sbeath of |he same, »t I be base of which a new hatr» %^\s, lonnmK.

Ftg. 378. Ffias* A Vkktical ?*iiCTtow OF THK Skik op Titif Ala Nasi oi* ak Isj'ant, X M*^- Sack of m. sebaceous Kland conlainifig inland cella In various stages of sccttftion.


Glandidae prarpuiiaies 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 J to i cm. long which regularly occur near the frenulum praeputii. Praeputial glands and cry|)ts 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 orpts are absent from the praeputium and glans clitoridis.

Sweat Glands

The glanduiae sudoriparae are long unbranched tubes terminating in a simple coil (described by Oliver Wendell Holmes as resembling a fairy's intestine, Fig. 378). Thej;;oil ia foundili.the defJipart.pf .Uije^gprijaiji, or in the subcutaneous ^tissue (Fig. 359). Clhe duct pursues a straight or somewhat tortuous course to the epidermis which it enters between the connective tissue papilU^^I^ Within the epidermis its spiral windings are pronoimced; 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 covered by longitudinal connective tissue fibers. Within the epidermis its walls are made of ceUs 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.



Fig. 378. Model OF the Coiled Part op a Sweat Gland FROM THF. Sole of the Foot. (After Hubcr.)


Membra na propria


Cuticula.


Muscle fibers.


A. Duct in cross section.

D. Low epithelium from a coiled tubule.

Membrana propria.

Muscle fibers.

Muscle nucleus. Cuticula.

Membrana propria. Muscle fiber.

B. Columnar epithelium from the coiled tubule.

C. Surface view of the coiled tubule.

Cross section of coiled tubule.


Fig. 379. A-D, from a Section of the Skin of the Axilla; E, from the Finger Tip of a Man OF 23 Years. X 230. E is not a tme cross section.


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 large forms with 30 mm. of coiled tube. They acquire their large, size at puberty and have been considered as sexual "odoriferous gland In the vicinity of the anus there are branched sweat glands, and large unbranched "circumanal glands" together with other modified forms.



Fig. 380. Part of a Vertical Section of the Injected Skin of thk Sole of the Foot. X 20. The veins are not completely filled by the injection.

Vessels and Nerves of the Skin

The arteries proceed from a network above the fascia and branch as they ascend toward the surface of the skin. Their branches anastomose, forming a horizontal 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 before sending terminal arteries into the papillae. The superficial plexus is called subpapillary, and from it the branches to the sebaceous glands and hair sheaths are derived. The papilla of a hair receives an independent branch. 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 lymphatic vessels form a fine meshed plexus of narrow vessels beneath the subpapillary network of blood vessels. It empties into a wide meshed subcutaneous plexus. There are lymphatic vessels around the hair sheaths and both sorts of glands.


The nerves form a wide meshed plexus in the deep subcutaneous tissue, and secondary plexuses as they ascend through the skin. The sjrmpathetic, non-meduUated 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. MeduUated sensory nerves end in the various corpuscles already described (page 105), and in free terminations, some being intraepithelial. MeduUated 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 sheath.) There are no nerves in the hair papilla. The corium beneath the nails is rich in meduUated nerves, the non-meduUated endings of which enter the Golgi-Mazzoni type of lamellar corpuscle (having a large core and few lamellae), or they form knots which are without capsules. Elsewhere the skin contains tactile corpuscles in its papillae and lamellar corpuscles in the subcutaneous tissue, together with free endings in the corium and epidermis (as far out as the stratum granulosum).

Mammary Glands

In young mammalian embryos generally, the mammary glands are first indicated by a thickened line of ectoderm extending from the axilla to the groin. Later much of the line disappears, leaving a succession of nodular thickenings corresponding with the nipples. In some mammals 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. In an embryo of 25 cms. (Fig. 381) several soUd cords have grown out from the ectodermal proliferation. There are ultimately from 15 to 20 of these in 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 alveoU 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 duct-like tubes.



Fig. 381. Section Through the Mammary Gland of an Embryo of 25 cms. 1, Connective tissue of the gland. (After Basch, from McMurrich.)


Toward the end of pregnancy each of the 15 or 20 branched glands forms a mammary lobe and its alveolo-tubular end pieces are grouped in lobules . The secretory epithehum is a simple cuboida^ or flattened layer in which fat accumulates at the seventh or eighth month. It first appears as granules at the basal end of the cell, where it is received in combination from the siurounding tissue. This fat is not produced bv the gland celL The lumen of the alveoli contains leucocytes which have passed between the epitheUal cells, from the connective tissue. Some of them degenerate; others receive fat from the gland cells, either in combination, or in drops which are devoured by phagocytic action. The fatty leucocytes grow to considerable size and are called colostrum corfniscles. ^[Seneath \ ^^*^^**^?L, the alveolar epithelium there are basal or basket cells which have been compared with the piuscle fibers of sweat glands basement membrane separates them from the connective tissue which contains many mononuclear leucocytes 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 (F ig. 383). After two days of lactation some of tlie giand ceus are nat and empty of secretion. Others are tall columnar, with a rounded border toward the lumen; often they contain two nuclei. ^The fat within them is not a degeneration as in sebaceous glands , nor a secretion produced by the nucle us: it is a product of protoplasmic acti xto, and may fill the cell several times before it perish Transitions between the low empty cells and the colunmar forms occur, but mitoses are absent from the lactating gland. Mitotic divisions are numerous during pregnancy.



Fig. 382. Section of a Human Mammary Gland at the Period of Lactation. X 50.


Milk consists of fat droplets , 2-5 fJL in diameter, floating in a clear fluid containing nuclein derived from degenerating nuclei, and occasionally a leucocyte or colostrum corpuscle?) The interstitial connective tissue, greatly reduced by the enlarged glands, also contains very few leucocytes and eosinophilic cells.


At the end of lactation the connective tissue increases and the leucocytes reappear; as during pregnancy, they form colostrum corpuscles. The lobules become smaller and the alveoli begin to disappear. In old persons all the end pieces and lobules have gone and only the ducts remain.


The diicts are Uned 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 lactiferous. The epitheUum of the outer part of the ducts is stratified and squamous.


The skin of the nipple and of the areola at its base contains pigment in the deepest layers of its epidermis. The corium forms tall papillae and contains smooth muscle fibers, some of which extend vertically S'^^'n%^ through the nipple and others are circularly arranged around the ducts. There are tactile corpuscles in the nipple, which becomes rapidly elevated upon irritation, due to muscular rather than to vascular action. There are many sweat and sebaceous glands in the areola and occasional rudimentary hairs. The areolar frlands [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.


Fig. 383. From a Section of the Mammary Gland of a Nursing Woman. X 250.


Fig. 384. a.. Milk Globules from Human Milk. X 560. B., Elements OF THE Colostrum of a Pregnant Woman. X 560.

I, Cell containing uncolored fat globules ; 2, cell containing minute colored fat globules; 3, leucocyte; 4, milk globules.


Fig, 385. From a Thick Section op the Mammary Gland of a Woman Last Pregnant Two Years Before. X 50. I, Large excretory duct; 2. small excretory duct; 5, gland lobules, separated from one another by connective tissue.


Blood vessels enter the breast from several sources and form capillaries around the alveoli. Lymphati c y^qspIs are found in the areola, around the sinuses, and in the interlobular tissue. The collecting vessels pass chiefly toward the axilla ; a few penetrate the intercostal spaces toward the sternum. The nerves are like those of sweat glands.

Suprarenal Glands

The suprarenal glands are two flattened masses of strands of cells, without lumen or ducts, situated in the retroperitoneal tissue above the kidneys. The right is generally described as triangular and the left as crescentic. They are between one and two inches long, not quite so wide, and about a quarter of an inch thick. On section they present macroscopically a yellowish cortical substance which becomes dark brown toward the center of the gland, (jn the thicker portions there is a vascu lar medullary s ubst ance also dark colored, related to the cortex as seen in Fig. 3^6?^ In many lobes the medulla is lacking so that the deep portions of the cortex of the two sides are in contact. The suprarenal glands produce a secretion received by the blood (some have said by the lymphatic vessels also). (^^Death follows the removal of the glands, and their pathological conditions may be fatal. Intravenous injection of suprarenal extract causes a great rise in blood pressure.


The development of the suprarenal gland indicates a radical difference between the cortex and medulla. In the sharks these components form separate organs. The interrenal gland which corresponds with the cortex, consists of cords of mesodermal cells and has apparently a sinusoidal circulation. The medulla is represented by a pecuUar development of the sympathetic gangUa. (^In mammals the medulla likewise arises by the development of chrom^fjine cells in relation with the sympathetic nerves. The position of the involved nerves, between the aorta and the WolflSan body, is shown in Fig. 276, C, page 245. The sympathetic portion of the gland becomes surrounded by dense mesenchyma in which the cords of the cortex are differentiated^ ; Opinions are divided as to whether this mesenchyma is derived from the Wolffi^ body or from the coelomic epitheUum. As the kidneys attain their permanent position the suprarenal glands are foimd above them; they are structurally as independent of the kidneys as are the Uver and spleen.


Fig. 386. Section of the Suprarenal Body op A Child, x 15.


The cortical substance consists of cuboidal cells which in the outermost zone are arranged in rounded masses; in the middle zone they form cylindrical colimi|is; and in the deepest layer the cords unite in an irregular network, (xhejcortex is therefore divided into a zona glomertUosa, zona fasciculata and zona reticularis (Fig. 387)^ The cells of the cortex are about 15 /i in diameter and contain fatjroplets causing the macroscopic yellow appearance. (The drops are especially large in the zona fasciculata (Fig. 388), and are small or even absent in the zona reticularis. The dark brown col or of the latter is due to pigment which becomes conspicuous only in the adult. Besides vacuoles the protoplasm of the outer cells contains granules; the nuclei of the glomerular zone may be denser than those of the fascicular layer. The cell columns are in close relation \^thj^e endothelium of the blood vessels. They have no^ basement membrane, and are separated from the vessels by a very slight amoimt of reticular tissue.



Fig. 387. Section of a Human Suprarenal Gland. X so.


The medullary substance consists of chromaffine cells arranged in elongated strands which unite and form a network. The cells are very delicate and easily become stellate by shrinkage even in well fixed preparations. They have round nuclei and granular protoplasm but their specific


Fig. 388. From a Section of the Suprarenal Gland of an Adllt. y 360.


qharacteristicjs^^ by which

they axe colored brown, (^milax

cells occur in some s)nnpathetic

ganglia and in the glomus carot icum^

The capstde of the suprarenal glands is connective tissue, said to contain smooth muscle fibers, blood and lymphatic vessels, nerves and small ganglia. It sends prolongations into the interior. Elastic fibers are found in the medulla but they are very few or absent in the cortex.


The arteries divide in the capsule into many small branches which penetrate the cortex and there form a long-meshedjcapillary network; toward and within the medulla the meshes become round. Some arteries pass directly from the capsule to the medulla, without branching in the cortex. The larger of the numerous veins which arise in the medulla are accompanied by longitudinal bundles of smooth muscle fibers. Before leaving at the hilus they unite to form the suprarenal vein, (^Xymphatic vessels have been recorded in the capsule and medulE)

The numerous mostly non - medullated nerve by of which a human suprarenal gland receives about thirty small bundles, proceed chiefly from the coeUac plexus and pass with the arteries from the capsule into the medulla. Branches from the plexus in the capsule descend between the cell groups of the cortex and terminate on the surface of the cells in the two outer zones; they do not enter between the separate cells. The plexus in the zona reticularis is more abundant, but here also onl y groups of cells are

the medulla the nerves are extraordinarily abundant and 

each cell is surrounded by fibers) Groups of sympathetic ganglion cells may be found, but these rarely occur in the cortex. A part of the nerves terminate in the walls of the vessels.



Fic. 389. From an Injected Section of the Suprarenal Gland of a Child, x 50.


In the vicinity of the ductus deferens and in the broad ligament of the uterus, suprarenal bodies may occur, consisting only of cortical substance. Groups of chromaffine cells have been found in relation with the paroophoron and paradidymis.

BRAIN AND SENSE ORGANS

Brain.

DEVEL0PB4ENT AND GENERAL AnATOMY.

In a previous section the formation of the medullary tube from the primitive ectoderm has been described, and it has been stated that the posterior portion of the tube becomes the spinal cord and that the anterior portion forms the brain. In a human embryo of 4.0 mm., the tube still opens freely through a large anterior neuropore, the extent of its connection with the epidermal ectoderm being indicated in Fig. 390, A. The tube has become bent in two places; the posterior or neck bend is near the junction of the cord and brain, the Une of separation between which must be arbitrarily drawn both in the embryo and in the adult; the anterior or head bend occurs in a part of the tube called the mid-brain (mesencephalon). In front of the mid-brain is the fore-brain (prosencephalon) and behind it is the hind-brain (rhombencephalon). The entire brain is therefore divided into fore-brain, mid-brain, a iid^hincj-Hra^'ji. In an early stage the forebrain produces two lateral outpocketings, one on either side, called the oj^tic 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 inner wall of the brain which marks the position of the stalk is called the optic recess.

The hind-brain soon becomes rhomboid or kite-shaped as seen from its dorsal surface. This is due to a widening of the cavity of the medullary tube; its lateral walls spread apart and the roof plate becomes thin and transparent. The dilated cavity of the hind-brain is called the fourth ventricle; the cavity of the mid-brain in the adult is a slender passage called the aqueduct [of Sylvius]; it becomes vertically expanded in the fore-brain to form the third ventricle. These two ventricles and the aqueduct are continuous with the central canal of the spinal cord and represent the original cavity of the medullary tube.

In an embryo of lo mm. (Fig. 390, B) the hind-brain may be subdivided into the mydencephalon posteriorly and the metencephalon anteriorly. The constriction between the hind-brain and mid-brain is called the isthmus. The mesencephalon remains undivided; the fore-brain is represented by the diencephalon posteriorly and the telencephalon anteriorly. Thus there are six fundamental subdivisions of the brain. Their further development is illustrated in the median sagittal sections of the brain, Figs. 392 and 393, and may be briefly described as follows.

The myelencephalon becomes the medulla oblongata. It transmits the fibers passing between the cord and the brain; it receives the sensory roots of the vagus and glossopharyngeal nerves and contains the groups of cell bodies from which their lateral roots arise [the lateral root of the vagus being called the accessory nerve]. It also contains the cell bodies from which arise the ventral roots which make the

hypoglossal nerve. (These nerves are shown in Fig. 113, p. 96, and in Fig. 391, B.) The medulla also includes groups of cell bodies, the processes of which do not leave the central nervous system. Such groups are called nuclei; the gray substance in most of the ventral portion of the brain is in the form of separate nuclei and not in continuous columns as in the cord.

The metencephalon produces the pons ventrally and the cerebellum dorsally. The pons receives the sensory roots of the trigeminal, intermediate and acoustic nerves; it gives rise to the lateral roots of the tri


ple. 390.— A, The Brain op a 4.0 mm. Human Embryo (after Bremer); B, thb Brain op a 10.2 mm. Embryo (after His).

Except the isthmus, is., the principal subdivisions of the brain are indicated by prefixes of the term encephalon\ sp.C, spinal cord ; h.. hemisphere ; 0. v., optic vesicle ; r., rhinencephalon ; v., roof of the fourth ventricle.


geminus and intermedius (facial) and to the ventral root which makes the abducens. The pons transmits the ascending and descending fibers between the cord and the anterior portion of the brain, together with fibers to and from the medulla. Many fibers of the pons pass through the lateral wall of the brain-tube into the cerebellum, forming a large bundle on each side, called the brachium poniis (Fig. 391). The cerebellum also receives on each side a bundle from the anterior part of the brain, the brachium conjunctivumy and another from the medulla, the restiform body. These three bimdles not only contain fibers to the cerebellum but also those passing from it. The cerebellum (Fig. 393) is a large lobular mass


-bx:.



Fic. 391.— A, Dorsal and B, Ventral Vikw of the Posterior Part of the Adult Brain. The Cerebellum and Roof of the Fourth Ventricle has been Removed from A.

b. C, Brachium conjunctivum ; b. p., brachium pontis ; c. m., corpus mamillare ; c. p., cerebral peduncle ; C. q. a. and c. q. p., anterior and posterior corpora quadrij^emina ; inf., infundibuluni ; med., medulla ; oi., olive; p., pons; p. b., pineal body ; pyr., pyramid ; r. b., restiform body ; ven., floor of fourth ventricle. The nerves are— oc, oculomotor ; tr., trochlear ; trl., trigeminal ; abd., abducens; Int. intermedius, fa., its facial portion ; ac, acoustic ; gl., glossopharyngeal ; va., vagus, acc., its accessory portion ; hjf., hypoglossal.

of nerve tissue, consisting of an arborizing medulla of white substance, and a cortex composed of special forms of nerve cells.

The isthmus presents on its dorso-lateral surfaces the brachia conjunctiva. Beneath the floor of the central cavity or aqueduct it contains the motor cells from which the fibers of the trochlear nerve arise. After crossing to the opposite side above the aqueduct, these fibers emerge from the dorsal surface of the isthmus. Ventrally the tracts of fibers extending between the hind -brain and the fore-brain form projecting elevations which diverge as they pass forward; the elevations are called the peduncles of the cerebrum.

The mesencephalon forms dorsally four rounded elevations, the corpora quadrigemina. The superior or anterior pair receives fibers fomr


Fig. 39a.— Sagittal Section of the Brain of a Three Months Embryo. (After His.)


cbl.. Cerebellum; h. , . ..,..., ,. ,. - .

uUa oblongata ; met., mesencephalon: ol. D., olfactory bulb; 0. r., optic recess; pineal body ; p. s., pars subthalamica ; th., thalamus.


hemisphere; hy., hypophysis (posterior lobe); Isth., isthmus; • t., olfa<


med.: med pons; p. b



Fig. 393.— Median Sagittal Section of an Adult Brain. ebl.. Cerebellum ; c. C, corpus callosum ; c. q., corpora quadrigemina ; hy., posterior lobe of the hypophysis ; m^d., medulla obloneata ; 0. b., olfactory bulb ; 0. r., optic recess ; p., pons ; p. b., pineal boay ; p. s., pars subthalamica ; th., thalamus.

the optic tract, and gives rise to some which connect with the motor cells of the nerves to the eye muscles; others pass down the spinal cord close beside the median ventral fissure. Thus the anterior corpora are centers of optic reflexes. The posterior or inferior corpora, which are smaller, are in relation through an intervening group of neurones, with the acoustic nerves; thus they are centers of auditory reflexes. The mesencephalon gives rise to the ventral root which forms the oculomotor nerve. The cerebral peduncles which begin in the isthmus extend imder the mesencephalon.

The diencephalon has on its median dorsal surface the pineal body [epiphysis]. This is a small nodular structure which is thought to represent a rudimentary median eye, such as is more clearly indicated in reptiles. The upper part of the lateral walls of the diencephalon are each thickened by a mass of nerve tissue called the thalamus. The thalami of the two sides bulge inward so that their most prominent parts adhere across the third ventricle. Fibers from the retina connect with nerve cells in the thalamus, the latter sending their processes to the hemispheres; thus the thalami have an important relation with the optic tracts. The walls of the diencephalon below the thalamus form the pars mamiUaris hypothalami. This part of the hypothalamus includes the two mamillary bodies found side by side on the ventral wall of the diencephalon (Fig. 391, B).

Telencephalon. The fibers from the posterior part of the brain pass outside of the thalami to terminate in the dorso-lateral walls of the telencephalon. As seen in Fig. 390, B, this part of the fore-brain forms a hemispherical outpocketing on either side, into each of which a prolongation of the third ventricle extends; the extensions are called lateral ventricles (and are counted as the first two). The hemispheres enlarge, growing back so as to cover the posterior portion of the brain. Their waUs, which externally are subdivided by grooves into convolutions, constitute the pallium of the hemispheres. The olfactory bulb is the expanded termination of the part of the hemispheres which receives the olfactory nerves. The e ntire olfactory tract is called the rhinence^halon. The coipusjtrialum is a deep portion of the hemisphere found outside of the thalamus; anteriorly it forms the outer wall of the beginning of the lateral ventricle. The hemispheres are connected with one another by a great transverse commissure, the corpus callosum, through which fibers pass from one to the other. The principal subdivisions of the hemisphere are therefore the palUum, rhinencephalon, corpus striatum and corpus callosum.

Besides the hemispheres, the telencephalon forms! the pars optica hypothalami. This includes the optic recess in front on either side, and the injundibtUum in the mid- ventral line. QThe infundibulum terminates in an expansion which is the posterior lobe of t hp. fiypnp^ysis This body, together with the anterior lobe derived from the oral ectocTerm but later severed fronTll) is lodged in the sella turcica of the sphenoid bone. The development of the brain is summarized in the following table (after His).

Myelencephalon Medulla oblongata.

""^-^'^•" ] Metencephalon { S^Uum.

, Isthmus Isthmus.

Mid-brain Mesencephalon { Cerebral peduncles.

^ I Corpora quadrigemma.

Mamillary part of the hypothalamus. Thalamus. Pineal body. Fore-brain i * f Optic part of the h3rpothalamus.

H)rpophysis (posterior lobe). Hemisphere:

Palliiun.

Rhinencephalon.

Corpus striatum.

Corpus callosum.


f Diencephalon.


Telencephalon.


Medulla Oblongata.

Before considering the medulla the student should review the arrangement of fiber tracts in the spinal cord (Fig. 147, p. 121). The cerebrospinal fasciculi, both ventral and lateral, consist of the fibers which descend from the hemispheres. These four fasciculi of the cord arise from two in the medulla, which there produce a pair of ventral sweUings {pyramids) shown in Fig. 391, B. In the section. Fig. 395, it is seen that the p)rramids are in the position of the ventral cerebro-spinal tracts of the cord. In the lower or posterior part of the medulla the greater number of fibers in each pyramid crosses through the ventral conunissure to the opposite side; thence they proceed across the gray substance to the lateral cerebro-spinal fasciculus, which they form (Fig. 394). The crossing is called the decussation 0} the pyramids, or, since these fibers terminate about motor cells, it is called the motor decussation. The relatively small number of pyramidal fibers which do not decussate in the medulla, form the ventral cerebro-spinal fasciculi of the spinal cord.

The fibers from the spinal gangUa ascend to the medulla in the cuneate and gracile fasciculi. Within the medulla their fibers terminate, but their course toward the hemispheres is prolonged by a second group or " relay" of nerve cells, the bodies of which form four nuclei. These nuclei appear as additional columns or horns on the dorsal part of the gray H (Fig. 395); the inner pair are the nuclei of the gracile fasciculus, and the outer ones are nuclei of the cunecUe fasciculus. In them the fibers from the cord terminate and others arise which cross beneath the central canal to the opposite side of the medulla (Fig. 395). Then they pass forward in right and left bundles known as lemnisci or fillets. The decussation of the lemnisci occurs higher up in the medulla (that is, more anteriorly) than that of the pyramids; and, after crossing, the fillets remain internal to the pyramids.

With the sensory and motor decussations the resemblance between



Fig. 394.— Section of the Cord at the Level of the First Cervical Nerve.

The rieht half of the section shows the effect of Weigert's stain, the myelinated portions being dark; the left half shows the gray substance stippled and the white is blank. f. C, Fasciculus cuneatus; f. c. I., fasciculus cerebro-spinalis lateralis ; 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.


Fic. 395.— Section of the Medulla. (After Dcjerine.)

d. C, Dorsal column ; d. I., decussation of the lemnisci ; f. c., fasciculus cuneatus ; n. acc.«  nucleus of the accessory portion of the vagus ; n. C, cuneate nucleus : n. g., gracile nucleus ; py., pyramid ; t. S. n. t., spinal tract of the trigeminal nerve ; v. c, ventral column.


the medulla and spinal cord is lost. The gray substance no longer forms an H, and the dorsal fiber tracts have become ventral; the central canal expands to make the fourth ventricle, as seen in Fig. 396. The lemnisci form vertical bands of white substance on either side of the median ventral raphe. The pyramids cause protrusions of the ventral surface. Dorsal to each there is a large nucleus, the olive, which also makes an external elevation (Fig. 391, B). Its gray substance forms a convoluted capsule; it receives fibers from the cord and cerebellum, and gives rise to some which cross through the median raphe and ascend to the cerebellum in the restiform body. The restiform body, which forms the dorso-lateral portion of sections of the upper part of the medulla, contains olivary fibers, thoseof the cerebello-spinal fasciculus of the cord, some from the gracile and cuneate nuclei, and some from other nuclei in relation with the sensory roots of the cranial nerves.

The cerebral nerves of the medulla (and pons also) are arranged in general as follows. The ventral roots arise from groups of cell bodies, — the niiclei of the nerves, situated beneath the floor of the ventricle near the median line. The nucleus of the hypoglossal nerve is seen in Fig. 396. The lateral roots arise from nuclei more deeply placed and further from the median line; their fibers may pass upward and inward toward the ventricle before turning downward and outward to leave the brain. The nucleus ambiguus (Fig. 396) gives /ise to the lateral roots of the vagus and glossopharyngeus. Like the motor cells of the ts nh v,

spinal cord, those of the brain are also in connection with descending fibers of the pyramidal tract. The dorsal roots on entering the brain generally divide into a short ascending branch and a longer descending one. The iractus solitar ius ( Fig. 396) contains the descending fibers of the vagus and glossopharyngeus; the large spinal tract composed of the descending fibers of the trigeminus is shown in Figs. 395 and 396. The dorsal root fibers end in nuclei corresponding with the gracile and cuneate nuclei of spinal nerves. Fibers from the internal nuclei of the cerebral sensory nerves join the lemniscus and proceed toward the hemispheres.

Pons. The ventral swelling characteristic of the pons is due to the trans\ferse fibers of the brachium pontis (Fig. 391). These cross beneath and through the pyramidal bundles. Some of them arise from numerous groups of nerve cells scattered among them, the nuclei pantisy and pass to the same or opposite side of the cerebellum; others descend from the cerebellum to the same or opposite side of the pons. The fibers of the lemniscus and



Fig. 396.— Section of the Medulla. (After Dejerine.) C. r.. Corpus restiforme ; f. c. o.t ccrebello-oltvary fibers ; lein.«  lemniscus; n. am., nucleus ambiguus; n. h., nucleus hypoglossi ; ol., olive ; py., pyramid ; t. s., tractus solitarius ; t. S. n. i, iractus spinalis nervi trigcmini ; v., fourth ventricle.


pyramidal or cerebrospinal tract traverse the pons, together with a bundle which ascends beyond the trigeminal fibers and then turns back to enter the cerebellum through the brachium conjunctivum. (This group of fibers ascending to the cerebellum is found in the superficial ventrolateral fasciculus of the cord, and is known as Gowers' bundle.) The brachium conjunctivum contains fibers from the cerebellum, which decussate in the mid-brain; some of them terminate in nuclei which send branches through

the pons and down the lateral bundles of the cord; others pass into the cord directly


C^tay fitialum. Gnnfillonic


_ i^rnniilar stratum. Cortex.


— Medulla.



Cerebellum. The medullated nerve fibers of the brachia and restiform body form an arborizing medulla which extends into the small subdivisions of the cerebellum as shown in Fig. 397. This medulla of white substance is surrounded by a cortex consisting of an inner granular stratum^ a middle ganglionic stratum (presenting in section a single row of large cell bodies), and an outer gray stratum.

The inner granular stratum, which is rust-colored, 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 neurogUa cells, two sorts of nerve cells are present, the small and large granule cells; the former (Fig. 398) are multipolar gangUon cells with short dendrites having claw- like terminations, and a slender non- medullated neuraxon which ascends perpendicularly to the outermost layer and there divides in T-form into two branches. The branches run lengthwise of the transverse folds or convolutions of the cerebellum so that they are cut across in sagittal sections (Fig. 398) ; they are parallel with the surface and have free unbranched endings (Fig. 399). The small granule cells form the bulk of the granular stratum. The less


Fig. 397.— From a Sagittal Section of the Cerebellum OF AN Adult Man. X 12.


frequent large granule cells (Fig. 398) are more than twice the size of the small ones; their branched dendrites reach even into the gray stratum, and their neuraxon, going in the opposite direction, is soon resolved into very numerous branches which penetrate the entire granular stratum.



Fig. 398.— Diagram of a Sagittal Section of the Cerkbixlum. 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.


The granular layer also contains a thick network of meduUated fibers which proceed chiefly from the white substance. A part of these fibers end in the **eosine bodies" of the granular stratum, which are heaps of


Fig. 390.— Diagram of a Skction of the Cerebellum Lengthwise of the Convolutions. Golgi's Method. (Kolliker.)

gr., Cells of the granular stratum : n., their ncuraxons in the granular layer and n'., in the gray stratum ; p., p\, Parkinje's cells. (From Bailey's '^Histology. ")


stainable particles found between the small cells (Fig. 400). Another

part of the fibers forms 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 gangUon 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. 398). Their many

branches do not extend in aU 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.

399. From the deep surface of the

cell bodies the neuraxons arise, 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 return to Purkinje's

cell bodies.

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 chiefly pass toward the surface. Their long ncuraxons, 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 inten^als produce finebranches which descend and terminate in baskets


Eosine bodies.


Nuclei of small cells of the granular stratum.



Fig. 400. — From a Thin Suction of thi«  Ckkhbkllum of an Adult. > 400.

around the cell bodies of Purkinje's cells (Fig. 398). Often the basket surrounds also the beginning of their neuraxons.

The small cortical cells, distinguished 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. Its two to five dendrites lie in the sagittal plane Uke those of Purkinje's cells; its slender neuraxon, i nmi. long or more, sometimes forms loops, and is characterized by abundant branches in its proximal part. The terminal branches are few. The second type is in general somewhat smaller; the shorter neuraxons of its cells branch in their immediate vicinity. 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 abimdant in its superficial part. The second type appears everywhere in the gray stratum.

The meduUated nerve fibers found in the gray layer are prolongations from the granular stratum. In part they proceed toward the surface where, after losing their myelin, they end in branches among the dendrites of Purkinje's cells; in part they run between the bodies of Purkinje's cells lengthwise of the convolutions.

The neurogUa of the cerebellum consists of short-rayed stellate cells found in all the layers; of long-rayed cells found in the white substance; and of peculiar cells with small bodies found at the outer boundary of the granular layer. 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 yoimg animals the partly developed gray stratum is covered by a superficial granular layer, the cells of which later become nerve and neurogha cells of the cortex.

Hemispheres. The ascending fibers of the lemniscus and the descending cerebrospinal or pyramidal tracts, continue from the medulla through the pons and peduncles of the cerebrum into the hemispheres. They enter them on each side between the thalamus and the lentiform nucleus (a subdivision of the corpus striatum) as seen in Fig. 401. (The fibers of the ascending tract ha ve received acce ssions from the cerebral nerves, the thalamus, corpora quadrigemina, and other special nuclei near which they pass!) Many of the fibers which arise in the gracile and cuneate nuclei terminate before reaching the hemispheres and their course is prolonged by a new set of nerve cells.

The central portion of the hemispheres is a mass of white substance. The peripheral zone of gray in which these fibers arise or terminate is called the ^ojtex; it is divided into four ill-defined layers , an outer molecular or neuroglia layer, a layer of smdljj^ainidal cells, a layer of large pyram idal cells, and next the white substance, a layer of polymorphous cells . From the pyramidal cells the fibers of the descending tract arise. The layers are shown in Figs. 403 and 404.

The molecular layer which in ordinary sections appears finely punctate or reticular contains, besides many neuroglia cells, a network of meduUated

fibers parallel with the surface, the tangential fibers. The Golgi method shows that these fibers are partly neuroglia and partly the dendrites of pyramidal cells. The

  • ' cells of Retzius" found in this layer have

irregular bodies, and processes some of which are parallel with the surface; their branches, together with other processes from the cell body, descend into the p)Tamidal layer (Fig. 402). These cells are probably neuroglia.

The layer of small pyramidal cells is characterized by large ganglion cells with pyramidal bodies measuring 10-12 ,u. 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 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. 402, 3). Sometimes the neuraxon turns toward the molecular layer, joining the tangential fibers; infrequently an inverted pyramidal cell is found. The neuraxons and collaterals are medullated.

The layer of large pyramidal cells contains those with bodies 20-30 ft 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.



Fig. 401. — Transvkrsk Section of THE Brain. About ^ natural size.

The eray substance is stippled; the white is blank. «. t.. Ascending tract, including the fillet; c. c, corpus callbsum; d.t.. descending tract, entering the hemisphere from the cerebral peduncle ; n. I., nucleus lentiformis; th., thalamus; v., third ventricle.


where it arborizes freely;


The layer of polymorphous cells includes oval or polygonal cells which lack a chief dendrite directed toward the surface; their slender


Cell of Retzius. Short-rayed neuroglia cell. Bloodvessel.



Long-rayed neuroglia cell.


Fig. 402.— Diagram of the Ckrebral Cortex. The cells on ihe right are drawn from Golgi preparations of an adult man. X 120. The left portion of the diagram is X 60.

neuraxons produce collaterals, and enter the white substance where they may divide into two branches in T form. Polymorphous cells with

Blood vessel.


Layer of large

pyramidal

cells.


Layer of polymorphous nerve cells.


f 1": *•'• :.â– /.♦ H' ••:•♦• .V. >.••■.!

•<• >'■ ■.'. .*.^V;.' ;■>• ..■"■ '^^.- V ,', •■ *■ ■ . ■•V.o, •■•.*. ■• •

.:TT-- '^v ■• .'v. *; • •• •• ;■


"M ^ . •<




Fig. 404.


Kigs. 403 and 404 are from vertical sections of the corio^ (central convolution) of an adult. Fig. 403 is a Wc.:. ri preparation; Fig. 404 is from a section stained mi'I haematoxyline and eosine. X 45.


I'u.. 4,3


HEMISPHERES. 349

branched neuraxons limited to the vicinity of the cell body are found in this layer and in the p)rraniidal layers also. The neuraxon may branch in the molecular layer (Fig. 402, 6).

Many meduUated fibers are found in the deeper pyramidal and pol)rmorphous layers. They are grouped in tapering radial bundles which are resolved into separate fibers toward the layer of small pyramidal cells (Fig. 403). The bundles include the descending medullated neuraxons of the pyramidal cells, and the asc ending mediillatftH fih^rsfrgn^ tfiP white substance, w hich end after branrhin g lyppgT^Hly m jKa ysnpra-raHjal and tang ential networks. The medullated collaterals of the pyramidal cells nm at right angles with the radial bundles; they form an inter-radial network, or a band of fibers which near the calcarine fissure is macroscopic. A similar supra-radial band may be detected elsewhere in thick sections.

In the gyrus hippocampi and ifs hook (uncus) the tangential fibers are so abimdant 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 peculiarities are considered in the larger special works on the nervous S)rstem.

The neuroglia of the hemispheres, like that of the cord, is at first a S)mcytium with strands extending from the ventricle to the periphery. Later, the S)mcytium is divisible into short-rayed neuroglia cells found chiefly in the gray substance, long-rayed cells foimd chiefly in the white, and ependsrmal 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 periphery of the cerebral cortex is particularly rich in neuroglia fibers.

Hypop hysis. , - * '*. ^

The development of the two lobe s of the hypophysis [pituitar y body], the anterior from the oral ectoderm and the posterior from the felencephalon,Tias' "already been described (Fig. 185, p. 165). (The smaJler posterior lobe^ which is at the tip of the infundibulum, contains fine bran ching nerve fibers which form a delicate network, together with cells closely resembling bipolar and multipolar ganglion cells, and many blood vessels. The nature of the cells is, however, uncertalnoCThe larger anterior lobe consists of loose connective tissue with many blood vessels and nerveSj and of solid branched epithelial cords varying in caliber and frequently anastomosing. Near its border toward the posterior lobe a few of the columiis are hollow, and sometimes they contain masses similar to the colloid of the thyreoid gland. This does not come from the granules which occur in varying quantity in all the epithelial cells, giving them sometimes a lighter and sometimes a darker appearance. The granules in some cells are eosinophihc; most of them are not, and a portion may be fat. Ciliated epithelial ceUs have been recorded. (The part of the anterior lobe which is near the posterior is sometimes called medullary substance"; in children it may be represented by a cleft- like cavity containing colloid) < Frqmjhe relation of the hypophysis to certain diseases^ it is quite_c ertain that it produces an important internal secretion.

Eptthelkl cord.


Porlitm r anlerior lube. \


- f^


Portion oflhe pt!»terior lob«.


EpiihcllKl rolUcle,


Blood vcsed conlainln^ blood

  • ' Colloid"

subsUitce.


Multipolar cdh


-Connective ttaaiie fibers.


S>#-^ff*'t)ti


Fiti. ios.— PORTJOlt* OF A Horizontal Sectjon of a Human HvpoI'Hysis, tbowlnif the bouiularv line between the unterfor afid the posterior lob^. Two glaiid lollicles on the left cAch cotitftln a dark epkhelUil eel L X uo.


Pineal Body.

The pineal body [epiphysis] is a median dorsal outpocketing of the dicnccphalon, which has preserved its original epithcUal character. It consists of a layer of neuroglia cells thrown into folds and is covered by a connective tissue capsule sending prolongations between the folds. In the pineal body there is found generally "brain sand," acenndus cerebri, which consists of round or mulberry-like concretions 5/^ to i mm. in diameter In specimens preserved in glycerin or balsam they show distinct concentric layers. They consist of an organic matrix containing calcium carbonate and magnesium phosphate, and are sometimes surrounded by a thick connective tissue capsule.

Not infrequentlVj especially in old age^ there are found in the brain substance round or elongated bodies distinctly stratified, which are colored violet by tincture of iodine and sulphuric add, and therefore are related to amyloid. These corpuscula amylacea are found almost always in the walls of the ventricles of the brain, and also in many other places both in the gray and white substance and in the optic nerve. They have a homogeneous capsule with occasional processes, composed of neuroglia cells transformed by amyloid infiltration.

Meninges

The meninges are connective tissue membranes investing the central nervous system. They are usually divided into three layers, the durh mater y arachnoid, and pia mater.

The dura m^er 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. The dura mater cerebralis or dura mater of the brain, includes the periosteum of the inner surface of the j^rv

cranium and consists of two lamellae. The inner is like c^yy

the dura mater of the cord but contains more elastic *^

fibers; the outer corresponds with the periosteum of |^> ., ) the vertebral canal and consists of the same elements ( {'^ ^ ^^

as the inner layer, but its fibers nm in a different direction. It contains many blood vessels, some of which ,;^^ extend into the cranial bones. The very large thin- fig. 406. - acervulus walled veins of the dura are called sinuses. The dura body oV^a woman

, !• r 1 1 1 1 Shventy Years

has many nerves, some ending freely and others supply- old. x 50.

ing the vessels.

The arachnoid of the cord and brain is but loosely connected with the dura, being generally limited externally by a mesenchymal epithelium. Between the arachnoid and the dura there is a capillary cleft containing a very small amoimt of fluid. This subdural space in the rabbit and dog is in communication with the deep cervical lymphatic vessels and glands, with the lymphatic spaces around the peripheral nerves, with the lymphatic vessels of the nasal mucosa, with the tissue spaces in the dura, and with those around the arachnoid granulatious.

The arachnoid granulations [Pacchionian bodies] are elevations or outpocketings of the arachnoid in definite places, especially along the sides of the superior sagittal sinus. Covered by a thin portion of the dura and by the endotheUum of the vessel, they project into the cavity of the sinus.

The subarachnoid space between the arachnoid and the pia mater, is traversed by strands and layers of tissue and bounded by mesenchymal epithelium. It connects with the lymph spaces of the peripheral nerves, with the lymph vessels of the nasal mucosa, and with the ventricles of the brain through apertures in the roof of the fourth ventricle. It contains an abimdant fluid called the liquor cerebrospinalis. (The direct connection of the subdural and subarachnoid spaces with both lymphatic vessels and tissue spaces, is not in accord with recent embryological studies and requires further investigation.)

The pia mater of the cord and brain is a delicate vascular connective tissue which extends into their substance along with its blood vessels. Its nerves may remain outside. Pericellular lymphatic spaces around the nerve cells, and the epicerebral space between the pia and the brain, do not communicate directly with the lymphatic vessels. The blood vessels form narrow-meshed capillaries in the gray substance and coarser ones in the white. Capillaries in the cerebral cortex empty into veins which

Blood vessels. x Epithelium.



Fig. 407.— Portion of the Plexus Chorioidbus of an Adult Man. X 80. X, Blood vessel in optical section. The large dots in the epithelium are not nuclei, but pigment and

fat granules.

arise in the white substance beneath, and from there pass through the cortex to the pia; the blood in the capillaries therefore passes through the entire cortex before emptying into the veins. The blood vessels generally have a second so-called '^adventitial sheath" consisting of a mesenchymal epithelium. Within the sheath is an ** adventitial lymph space" connecting with the subarachnoid space; outside of it is a perivascular tissue space.

Chorioid plexuses. In certain places where the wall of the medullary tube is very thin, as in the roof of the fourth ventricle, it becomes invaginated into the central cavity by the vascular pia, thus forming a chorioid plexus. The epithelial cells of the brain covering the plexus, contain pigment granules and sometimes fat droplets. The chorioid plexuses extending into the third, fourth, and both lateral ventricles, are essentially similar in structure. A part of the network of blood vessels within them is shown in Fig. 407.


EYE

Development and General Anatomy. The eyes first appear as a pair of optic vesicles j which are lateral outpocketings of the fore-brain. They are shown in the model, Fig. 390, A (p. 325) and in section in Fig. 409, A. The vesicles are connected with the brain by the optic stalks j which become relatively slender as the vesicles enlarge. The epidermal ectoderm immediately overlying the vesicles, thickens and becomes invaginated (Fig. 409, B and C). The invaginated f)ortion then becomes 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 surface is pressed in, transforming the vesicle into the optic cup. At first the cup is not complete, being deficient on its lower side (Fig. 408). The arteria centralis retinae is seen passing through the 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. ^'""sT^L'ii^oJ' a ^human Theatery is shown in section in Fig. 409, D. fAT/Koiimanir ^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 are often more or less separated. They constitute the retina, which includes a t hin outer ^gmented la yer, and a thick inn er irisual /aw comf)osed of several strata of nerve cells and fibers. 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 Ught it is stated 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 seen to be continuous with the outer surface of the skin. Since in mammals the optic vesicles bep^in to form b efore the relaj^ ted portion of the me dullary groove h as closed, they appe ar as degressions in ^ j:h^qt ^ ^^pd epidermal eclioclerfe ^

Nerve fibers grow from the mner surface of the visual layer toward the central artery and vein of the retina, around which they pass out of the optic cup (Fig. 409, 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 I figure that the optic nerve at its origin interrupts the retinal layers, pro' ducing a "blind spot." The part of the nerve which forms the blind spot, with the vessels in the center, is called the papilla oj the optic

The lens (Fig. 409, D) loses its central cavity by the elongation of the cells in its posterior layer. These become the fibers oj the lens. The anterior layer remains throughout life as a simple epithelium, called the epithelium oj the lens (Fig. 410). The lens becomes covered by an elastic capsula lentis and in embryonic life it possesses a vascular capsul e Fig. 409, 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 strpnuj. and _a humor . Extending through it, from the papilla of the optic nerve toward tnelens, is the hy^nid 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 surroimdcd 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 oj 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. 309, E, c.p).

^The retin al cup is surrounded by two lay ers ofjnesenchymalorigin. The inner (umca vasculosa corresponds with the pia mater and forms the chorioid coat of the eye; the outer tunica fibrosa corresponds with the djara and forms the sclera^ into which the muscles of the eye are inserted. QThe portion of the retinal cup which forms a curtain, circular in front view, between the anterior and posterior chambers, is called the iris^^li consists of the tunica vasculosa together with a thin pigmented prolongation of the retina over its posterior surface (Fig. 410). This pars iridica retinae 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 retinae. At the ora serrata (Fig. ; 425) an abrupt thickening of the visual layer of the retina marks the boun-/



Fig. 409. — Sections op Rabbit Embryos to show the Dfa'klopment of thh Eye. A,95^ days, 3.0 mm.; B, 10 J4 days, 5.4 mm.; C, 11 days, 5.0 mm.; 0. 14 days, 18 hours, 12.0 (?) mm.; E, 20 days, 29 ram.

a. c. r., Arteria centralis retinae ; c, cornea ; c. a., anterior chamber ; conj.. conjunctiva ; c. p., posterior chamber; c. v., corpus vilreum ; e. I., eyelid ; I. b., fore-brain ; I., lens ; I. e., lens epithelium ; I. I., lens fibers; 0. C optic cup ; 0. n., optic nerve ; 0. v., optic vesicle; r. p., pipnented layer of the rctma; r. v., visual layer of tne retina.


dary between its ciliary and optic portions. The pars optica retinae extends from the ora to the optic nerve, covered successively by the chorioid and scleraT^

^The cornea is the tissue in front of the anterior chamber, consisting of a noi^Tascular mesenchymal tissue bounded posteriorly by mesenchymal epithelium and anteriorly by the epidermal ectoderm. The cornea is extremely transparent. The epidermal ectoderm extends from the cornea over two folds which form the eyelids. They have met in Fig. 409, D, and fused temporarily. Externally the lids are covered by skin, internally by the conjunctiva palpebrarum , or conjunctiva of the lids. The latter is continuous witii tiie conjunctiva bulbi which foims the opaque, vascular


Fig. 410.— Mkridional Section of a Part op the Eye. X 15. The radial fibers of the ciliary muscle cannot be distinguished with this magnification.

"white of the eye.*' It surrounds the cornea^the epithelium of the two structures forming an uninterrupted layer. — — —

'fhe parts of the eye to be examined histologically are therefore the retina, the optic nerve, the lens, and the vitreous body, all of which are ectodermal; then the tunica vasculosa including the chorioid, ciliary body, and iris; next the tunica fibrosa, including the sclera and cornea; and finally the accessory structures, — the lids, conjunctivae and glands.

Retina

The retina extends from the papilla of the optic nerve to the pupillary border of the iris, and is divisible into three parts; the pars optica retituie 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 cUiaris and the pars iridica retinae 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. 411, the cells of which are related to one another as in the diagram, Fig. 412. The nute^ lav^r of tlip npfiV ^np formg thepigmented epithelium of the retina which consists of a simple layer of sixsided 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 /-« long) brown granules of the pigment fuscin. In albinos the pigment is lacking. ^From the inner surface of the pig mented 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 two 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. 411). The rods are elongated cylinders (60 /i long and 2 // 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 ellipsoid, 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 threadlike and terminates in a small club-shaped enlargement without processes (Fig. 412).

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.

Beneath the outer nuclear layer there is a zone of fibers called the outer reticular layer [outer molecular layer]. It contains but few nuclei. The basal fibers of the visual cells are sometimes described as forming its outer part; more specifically they are called Henle's fiber layer. The remaining portion is a dense network of the branching processes from underlying nerve cells. Occasionally a cell body is displaced outward from the deeper layer and comes within the reticular layer. One of such

    • subepithelial ganglion cells" is seen in Fig. 412, x. The nervous elements are supported by a fibrillar network derived from non-ner\'ous

ectodermal cells, corresponding with neuroglia. Some of the supporting cells found in the reticular layer are concentrically arranged (Fig. 412, 00). The inner nuclear layer, which underlies the outer reticular layer, contains the cell bodies of both nerve and sustentacular cells. The nuclei of the latter belong chiefly with radial fibers [M tiller's fibers]; these extend from the inner surface of the retina to the membrana limitans externa. which they form. Slender fibers which arise from the outer surface of this membrane and surround the bases of the rods and cones in the form of baskets, may be regarded as prolongations of the radial fibers. The inner ends of the radial fibers form pyramidal expansions which unite with one another to make a membfana limitans interna, — the innermost layer of the retina. Throughout their course the radial fibers give off lateral expansions and processes, for the support of the nervous elements; these are especially numerous in the outer nuclear layer. Their nuclei are among those of the inner nuclear layer. The nerve cells of this layer are chiefly small bipolar ganglion cells constituting the ganglion retina The dendritic process of each extends into the outer reticular layer, where by forking it breaks up into very fine fibers parallel with the surface. They form a subepithelial feltwork and have been said actually to anastomose. All the bipolar ganglion cells send their longest dendrite between the visual cells where it ends in a little thickening near the membrana limitans. The neuraxons of the bipolar cells pass into the underlying inner reticular layer and there break up in fine varicose branches.


Fig. 411. Vertical Section of a Human Retina. X 36.



Fig. 412. Diagram of Human Retina. Supporting Substance Red.



The inner nuclear layer near its outer boundary contains stellate cells, sometimes large, which send many dendrites into the subepithelial feltwork where they anastomose. Their neuraxons extend horizontally, and may pass inward to join the fibers of the optic nerve (which is denied by some) or they may terminate in horizontal branches which ascend to the bases of the visual cells (Fig. 412,+). Toward the inner surface of the inner nuclear layer there are large ganglion cells which send branched processes into the inner reticular layer. Neuraxons of these "amakrine cells" have not been found. Some fibers extending out from the brain through the optic nerve terminate in free endings within the inner nuclear layer.

The inner reticular layer consists of a very fine supporting network, lodging the processes of the bipolar and amakrine cells, together with the dendrites of large multipolar cells of the ganglion layer beneath.



Fig. 413. Golgi Preparation op 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.

The ganglion cell layer or ganglion 0} the optic nerve consists of a single layer of large multipolar cells containing Nissl bodies. Giant forms occur at quite regular intervals. "Twin cells" have been described as joined by a short bridge, only one of the pair having a neuraxon. The branched dendrites of these gangUon cells extend into the inner reticular layer; their neuraxons pass toward the papilla of the optic nerve and except for the internal limiting membrane which covers them, they form the innermost layer of the retina. Collaterals have been detected returning from this nerve fiber layer to branch about the cell bodies of the ganglion layer. The nerve fiber layer also contains the centrifugal fibers which terminate in the inner nuclear layer. The fibers are all non-medullated. "Nummary. The elaborate subdivision of the retina into eleven layers should noT'be allowed to obscure the essential features, namely, that it consists of an outer pigmented and an inner visual layer. The latter includes an outer liyer of visual cells, — rod cells and cone cells. The bipolar cells of the ganglion retinae receive dendritic fibers which have free endings between the visual cells. They give rise to branching neuraxons which communicate with the ganglion cells of the optic nerve. The neuraxons of the latter converge at the papilla of the nerve and extend to the brain. The retina also receives fibers from the brain. It contains an ectodermal supporting tissue, blood vessels in its inner layers, and nerve cells perhaps commissural, the significance of which is still obscure.

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 receives straight slender fibers from 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 show in Fig. 414. 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 ner\^e 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.

Lewis1906 fig415.jpg

Fig. 415.

Pars ciliaris retinae. The optic nerve fibers and their ganglion cells disappear before reaching the ora serrata. The cone cells extend further 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. 415). Near the ora serrata large clear spaces normally occur in the outer nuclear layer and they may extend into the deeper layers (Fig. 415)- The radial sustentacular cells form a simple columnar epithelium as the other layers disappear, arid 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 visual layer of the ciliary retina the cells produce horizontal fibers closely packed, which form a refractive hyaline membrane.

Zonula ciliaris. Some 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 ciUaris [suspensory ligament] which holds the lens in place (Fig. 410). The zonula is not a continuous layer, nor does it consist of two laminae, one to the anterior and the other to the posterior surface of the lens with a space between them. It consists rather of numerous bundles, between which and the vitreous body, and among the bundles themselves, there are zonular spaces [canals of Petit] which communicate with the posterior chamber.

Optic Nerve

In its intraorbital portion the optic nerve is surrounded by prolongations of the meninges. On the outside is the dural sheath, consisting of thick outer longitudinal and inner circular bundles of connective tissue with many elastic fibers. Internally it is connected with the arachnoid layer by 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 layers among the bundles of nerve fibers. The latter are slender and medulla ted but without a neurolemma; they are supported by long-rayed neurolgia cells which extend between the individual fibers, but are most numerous at the periphery of the bundles and of the entire nerve. Thus the optic nerve diflfers from the peripheral nerves and resembles a cerebral commissure.


Lewis1906 fig416.jpg

Fig. 416. Longitudinal Section of the Optic Entrance of a Human Eye. X15. Above the lamina cribrosa is seen the narrowing of the optic nerve, due to its loss of myelin, artery and vein have been for the most part cut longitudinally, but above at several points transversely.


The central


At the posterior surface of the eye 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 the lamina (Fig. 416). As the optic nerve passes 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, outside of the membrana limitans interna.

Lens

The lens is a biconvex structure having an anterior and a posterior pole, and a vertical equatorial plane. It is enclosed in a thick transparent elastic capsule which is 6.5-25 /i thick in front, and 2-7 /i thick behind. Within the capsule the anterior surface of the lens is formed by the lens epithelium, a single layer of cells 2.5 /^ thick at the pole but becoming taller



Fig, 417.— Lens Fibers of a New-born Infant.

A, Isolated lens fibers, three with smooth, one with dentate borders. X 240. B, Human lens fibers cut transversely ; c, section through club shaped ends. X560.


Fig. 418.— Capsule and Epithelium of a Lens ok Adult Man.

C, Inner aspect. D, Lateral aspect, from a meridional section through the equator of the lens; i, capsule ; 2, epithelium ; 3, lens fibers. X 240.



at the equator. There they are continuous with the elongated lens fibers of the posterior layer, which collectively are called the substantia lentis. New fibers are formed by the mitosis of cells at the periphery of the posterior layer. The lens fibers are generally six-sided prisms, somewhat enlarged at one or both ends. The central fibers have lost their nuclei; their boundaries are wavy or notched. These, which were the first to form, constitute a dense mass, the nucleus oj the lens. 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 amoimt of cement substance, which is more abundant at the poles; after maceration of the lens it generally radiates from either pole, forming a stellate figure around each. These have three rays in older embryos and ordinarily nine rays in the adult. The lens fibers all run in the meridional direction from the anterior stellate rays to the posterior. The nearer the anterior pole they arise, the further from the posterior pole they terminate, and vice versa, since no fiber is long enough to extend from one pole to the other. The fibers of the cortical substance are said to form about 2000 radial lamellae comparable with the segments of an orange. Owing to the differences in consistency of fibers of various ages, concentric lamellae may be separated in hardened lenses.

Vitreous Body

The corpus 'vitreum consists of the fluid vitreous humor and loose fibrous strands of stroma. Although some recent pathological cases suggest that the latter are arranged like the septa of an orange, it has not been established that they have any definite arrangement. The cells of the vitreous body are round forms, probably leucocytes, and stellate or spindle shaped forms representing the connective tissue which invaded the vitreous • body with the blood vessels. The latter have atrophied and been resorbed. Opaque flakes which occur normally and float into the field of vision as "muscae volitantes," have been ascribed to fragments of degenerated tissue ; vacuolated degenerating cells have been observed. Crystals may form and settle in the lower part of the bulb. The vitreous body is bounded by a very resistant thick fibrous layer which does not justify the term hyaloid membrane.

Tunica Vasculosa

Chorioid. Between the sclera and the chorioid there is a loose tissue containing many elastic fibers and branched pigment cells, together with flat non-pigmented cells. In separating the sclera from the chorioid this layer is divided into the lamina fusca of the sclera and the lamina suprachorioidea. Internal to the latter is the lamina vasculosa which forms the greater part of the chorioid. It contains many large blood vessels imbedded in a loose elastic connective tissue, some of its cells being branched and pigmented; others without pigment are flat and arranged in layers surrounding the vessels. A thin inner layer of blood vessels, the lamina choriocapUlarisy 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 fi 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 of fine elastic networks 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 camivora is formed of several layers of flat cells which contain numerous fine crystals.



Fig. 419.— Vertical Sbction through a part of thb Human Sclera and the entire thickne&s

OF the Chorioid. X 100. g, Large vessels ; p, pigment cells ; c, cross sections of capillaries.


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. 410). The equator of the eye is vertical, like that of the lens, and the meridians are anteroposterior. 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 ciHaris retinae by a continuation of the lamina basalis which forms intersecting folds. The ciliary processes, which are compressible, may serve to prevent the increase of intraocular pressure during the contraction of the ciliary muscle. 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. 410). It consists of two sets of fibers, the meridional and

circular. The meridional fibers as seen in section (p. 356), form a triangular group converging toward the sinus venosus. Their numerous outermost bundles mixed with elastic tissue are applied to the scleral surface. Anteriorly the bundles become gradually shorter and more radially placed so that those in the front of the muscle are perpendicular to the sclera. The radial fibers are classed as a separate group by Professor Stohr. The circular fibers which vary in number indifferent individuals form that part of the ciliary muscle which is nearest to the equator of the lens.

The iris consists of its stroma anteriorly and the pars iridica retinae



Fig. 420.— a. From a Tkased Prkparation 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. Portion of a Human Choriocapillaris and thk Adherent Lamina Basalis. X 240. c. Wide capillaries, some of which contain (bi blood corpuscles ; e, lamina basalis, showing a fine " lattice work."


Fig. 42t— Vertical Skciion of thk Pupillary Portion of a Hu.man Iris. X 100. About onefifth of the eniire width of the iris is shown, g. Blood vessel, with thick connective tissue sheath; m, sphincter pupillae mu.scle cut transversely; p,

pupillary border of the iris.


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 [unfortunately named endothelium]. The stroma consists anteriorly of a network of stellate cells in part pigmented. It is followed by a vascular layer of fine loose connective tissue with few elastic fibers. Its stellate cells, which in blue eyes are not pigmented, form elongated polygonal meshes. The vessels are radial, and have a thick connective tissue externa but a very weak circular musculature. Among the vessels near the free border of the iris, there are smooth muscle fibers which form a band i mm. wide encircling the pupil. This is the sphincter muscle of the pupil, A few radial muscle fibers also occur among the vessels. The dilator muscle oj the pupil is behind the vascular layer. It is a continuous layer of radially arranged smooth muscle fibers, beginning near the pupil and extending to the ciliary body. The contractile portion of the spindle shaped muscle cells forms a membrane-like layer resting against the pars iridica retinae, with which the pigmented nucleated portion of the cells seems to unite. These muscle cells have been thought to arise from the outer layer of the retinal cup. Except in albinos both layers of the retina are here heavily pigmented, and apart from their embrj'ological development they would be regarded as a single layer.

Tunica Fibrosa

The sclera consists of interwoven bundles of connective tissue, chiefly meridional and longitudinal. Elastic tissue acconipanies the bundles and is 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. Next to the chorioid, the sclera forms a pigmented lamina fusca which has already been described. The sclera becomes thinner anteriorly where it is absolutely continuous with the transparent cornea. The corneal boundary is oblique, being bevelled at the expense of its anterior surface.

The cornea (Fig. 422) consists of an outer epithelium, external basal memorane, substantia propria, internal basal membrane, and mesenchymal, epithelium bounding the anterior chamber."^ The corneal epithelium, about .03 mm. thick, is stratified and consis ts of a basal lay e_r j)f clear ly^ outlined columnar celp follawed by three or four rows of cuboidal cells and several" layers of Battened 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 .01 mm. thick. Superficially it connects with the epithelial cells by bands and processes. Beneath it blends with the substantia propria, of which it is a modification. Since it is not formed of elastic substance the name ** anterior clastic membrane" is not justified.

The substantia propria c onsistsjo f deb'cate _straight connective tissue fibrils which are united in bundles of an almost uniform thickness by a



Fig. 422. Vertical Skction of a Human Cornea. X 100.


Fig. 423. Corneal Spaces and Canai.iculi (in VVhitk) fko.m a Horizontal Skction of THE Cornka of an Ox. Silver preparation. X 240.


Fig. 424. Corneal Cells from a Horizontal Section of the Cornea of a Rabbit. X 240.


(fluid?) interfibrillar cement. The bundles are .cemented together, forming superposed flat lamellae parallel with the surface. The layers are


connected by an interlamellax cement substance, and by occasional oblique fiber-bundles. The latter so-called arcuate fibers are to be found especially between the anterior layers. In the cement substance, there is a system of' branched canaliculi, dilated in places to form oval spaces. The Jatter are between lamellae but the canaliculi extend among their constituent fiberbundles. Within the spaces there are flat stellate anastomosing cells, the branches of which extend into the canals and tend to unite with those of neighboring cells at right angles. The cells and their processes are more or less surrounded by tissue fluid. Leucocytes enter the canals and are normally found in the cornea; if the cornea is inflamed they become abimdant. Blood vesse ls and^lymphatic vessels are absent.

The posterior basal membrane [Descemet's membrane] is a clear elastic lamina, 6 fi thick." Its inner'surface in the adult shows hemispherical elevations. The mesenchymal epitheli um is a si mple layer of flat polygonal gells. The iris sends connective tissue prolongations over the peripheral part of the inner corneal surface. Collectively they are called the ligamenium pectinatum of the iris. As compared with those of the ox and horse, in man they are rudimentary.


Blood Vessels

The central vessels of the retina supply a part of the optic nerve and the retina; the ciliary vessels supply the rest of the eye. These two sets of vessels anastomose with one another only at th e entrance of the optic nerve (Fig. 425).

The ciliary arteries are (i) the short posterior ciliary arteries to the chorioid; and (2) the long posterior ciliary arteries which with (3) the anterior ciliary arteries supply chiefly the ciliary body and iris. 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 ci rculus 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 (j). 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 diverging branches extending along the ciliary border of the iris. By the anastomosis of these four branches a vascular ring is formed, the circulus iridis major {2), from which numerous branches proceed to the ciliary processes (j) and to the iris (4), Near the pupillary border of the iris the arteries form an incomplete ring, the circularis m^ minor.



Lewis1906 fig425.jpg

Fig. 425. 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 ciliar> artery with the circulus iridis major.


3. The anterior ciliary arteries arise 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 lamina choriocapillaris. Before penetrating the sclera the anterior ciliary arteries give off posteriorly branches for the anterior half of the sclera, and anteriorly 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 4 (less often in 5 or 6) venae voriicosae . 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 ciliarj' muscle. This sinus venosus sclerae (canal of Schlemm) connects with the anterior ciliary veins.

After ia centralis retinae. From 15 to 20 mm. from the eye the central artery of the retina passes to the axis of the optic nerve 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. The branches are chiefly in the inner layers but may extend into th e outer reticular layer; they are absen t from the fundus o f the fovea centralis. Within the optic nerve the artery sends out numerous little branches which anastomose with small vessels which have entered the sheaths from the surrounding fat; and also with branches of the short posterior ciliary arteries (Fig. 425, b).

The central vein of the retina receives two main branches at the optic papilla and follows the artery along the axis of the optic nerve.

Chambers and Spaces of the Eye

The eye contains no lymphatic vessels but is provided with communicating tissue spaces, bounded by mesenchymal cells or epithelia. These include the canaliculi of the cornea and sclera; and the anterior chamber of the eye which through the capillary interval between the lens and iris connects with the posterior chambery and the latter is prolonged into the zonular spaces. Irregular extensions of the anterior chamber, associated with the pectinate ligament of the iris, are called spaces of the angle of the iris [spaces of Fontana]. They are but slightly developed in man. Posteriorly the tissue spaces indode tbe hymkM amai of tbe Titreaus bodf ; die veij oaiTDw pmchoriekkoi space betvreeEi the dioricsid and sdaa; the subdmal and subaraduioid spac^ of the optic sheaths, aamed die miravaginal spaces; and finaBj the imicrfasciai spa€€ [of Teooo] iriikli suoouods most of the sclera and is prolonged as a sopiaditial spmot aftMOid the optk Derre. These spaces mar be fiUed &om die swhaTadmoid of die brain- They contain a *^ filtrate from the \iesscls,*' The intez&sdal and penchonoideal spaces hold but Uttk fitiid; actii^ as buisae, diej onj fadtitate die movements of die eye.

Apart from the optic nen e, the eye is supplied by the dimi cUiary ncfves {mm the ciliary ganglic^, and the iomg cUmry ncrvts faom the iiaso^ ciltaiy branch of the ophthalmic nerve- The ciliary nerwes penetrate the

sdera near the optic nerve and send brandies contaming giy lion cdkto the ^ma^ of the

dborkid. The nerves pass forward between the dmrioid and sclera to the ciliary hoif^ where diey fonna cinrnlar gangiiocated pkxi^ the piexus gamgimsmM ciiiaris. Its tranches extend In (i) the ciliaiy body, (2) the iris and (j) the cornea.

The oerv^ of the ciliazj

body form a delkaie network qq

its scleral surface; Ihey sii|^ly its mtisck fibers and those of the vessels

with slender jnotor endings, and between the ciliary muscle bundles th^

have branched free endings, perhaps sensory.

The medullated nerves of the iris lose their myelin and form plexus^ as they pass toward the puptllaiy marj^n. A sensoiy pkztis is found just beneath the anterior surface, and motor fibers supply the sphincter, dilator and vascular mt^des. Theeristoiceof gangBoticeOsinthehitmap iris is dented.

The nerves of the cornea enter it from the plexus amnmlaris in the sdera just outside^ The annular plexus also sends fibers into the conjunctiva, where they end in networi^, and in bulbous corpuscles (Fig- i2S» p. 106) situated in the connective tissue dose to the epifhrfinm. Such corpusdes may be found i or a inm. within the corneal mazgin. The oocn^ nerves become noQ-medullated and form pleanises berween die


Fig. Fftoii ji SficrrKw op tsk Mcauuf Cmma.


lamellae throughout the stroma. They extend into the epithelium and there form a very deUcate plexus with free intercellular endings.

Eyelids

The eyelids or palpebrae (Fig, 427) are covered with thin skin provided with fine lanugo hairs; small sweat glands extend into the corium. The latter contains pigmented connective tissue cells, which are rare elsewhere in the corium. The subcutaneous tissue is very loose, having many elastic fibers and few or no fat cells. Near the edge of the Ud there are two or three rows of large hairs, the eyelashes or cilia, the roots of which extend obhquely, 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 striated bundles of the 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 orbicularis muscle are found the termmal radiations of the tendon of the levator palpebrae, A part of these are lost in connective tissue; another part associated with smooth muscle fibers, is inserted into the upper border of the tarsus and forms 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 glands [Meibomian], which consist of a wide excretory duct opening along the palpebral border and of small acini with short stalks which enter it from all sides. In structure they resemble sebaceous glands. At the upper end of the tarsus and partly enclosed in its substance, there are branched tubular accessory lachrymal glands. They occur chiefly in the medial (nasal) half of the lid.

The timica 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


Superior

tarsal Radiations

muscle. from Ihe tendon Orbicularis

Conjunctiva. of Ibe levator palpcbrae, palpebrarum.


Cross s^rlJHaii qf the bundles cif the

orhkiilHris. palpebrarum.


Cor i urn.

subcutaneum.


-*'- Sweat gland.


Obli<|ue section W it hair sheath.




oi the li




Musculus ciliaris Riolani.


Fir.. 4?7.— Sacittal Skction op thk Ippkr Lid of a Six Months Child. The outlet of the tarsal Rland was not in the plane of section. X 15.


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 hd or quite high on the conjunctival surface. Toward where the palpebral conjunctiva arches to form that of the bulb, its epithehum is so folded that in sections it may seem to form glands.

The conjunctiva bulbi is similar to that of the lid. Its outer epitheUal cells, however, become squamous toward the cornea and over the exposed portion of the eye. Its basal cells contain pigment, except in the European races. The yellow color, 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. sclerae and the conj. corneae.)

At the medial angle of the lids there is a thin fold of connective tissue covered with stratified epithehum; 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 comeum 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 tarsens externus, at the upper border of the tarsus (Fig. 427). They extend into the conjunctiva bulbi, and near the margin of the cornea they pass inward to unite with the anterior ciliary vessels (Fig. 425). 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 bhndly toward the cornea or connect with the canalicuU, has not been determined. The nerves form a very thick plexus in the tarsus and supply the tarsal glands. There are free endings in the conjunctival epithelium, and bulbous corpuscles in the connective tissue beneath.

Lachrymal Glands

The lachrymal glands are compound tubular glands with several excretory ducts. These are lined with a double row of epithelial cells, the superficial layer being columnar. The excretory ducts pass gradually into long intercalated ducts with a low epithelium. These terminate in tubules presenting two forms of cells and surrounded by a membrana propria. The cells of one form 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 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, a continuation of the deeper layer of the epithelium

of the duct. The blood vess ff _^^ sels and nerves are similar

V^* T\ to those of the oral glands.


Lewis1906 fig428.jpg

Fig, 428. From a Section of a Human Lachrymal Gland. X 420.


The two lachrymal ducts which at the medial angle of the eye connect with the nasolachrymal duel, must not be mistaken for the excretory ducts of the lachrymal glands. The former consist of stratified

A, Gland body ; .. tubutcuTacross'".', group of tubules cut epithclium with SqUamOUS

obliquely; s, intercalated tubule; 8', intercalated tubule ne^Uo. or\A an plnQtir tnnirn

in cross section ; b, connective tissue. B, cross section of ^^"^ ^^^ ^^ ClclbLlL lUIUCd

an excretory duct ; e, two-rowed cylindrical epithelium ; TM-rxTMn'o TViow or** cur

b, connective tissue. piopnd. X UCy dXC &ur rounded by striated muscle fibers, chiefly longitudinal. The lachrymal sac, which is provided with small branched tubular glands, and the nasolachrymal duct, are lined with two-rowed columnar epithelium and a lymphoid timica propria, which is separated from the underlying periosteum by a dense plexus of veins.

Ear

Development and General Anatomy

The ear is divided into three parts: the external ear, including the auricle which projects from the surface of the body, and the external auditory meatus which is the passage leading to the tjmipanic membrane or "drum"; the middle ear, including the tjmipanic cavity and the chain of three bones extending across it; and the internal ear which is a system of epitheUal ducts in connection with the terminal branches of the acoustic nervgj^found imbedded in the temporal bone.

The internal ear begins 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. 429 A and B, and the pockets thus produced become separated from the epidermis in the form of vesicles [otocystsjA From near the center of the medial surface of each, an ascending tubular outgrowth, the endolymphatic duct, arises, and its blind termination becomes enlarged to form the endolymphatic sac. The duct is seen in section in Fig. 429, C, and its upper end projects above the rest of the vesicle in Fig. 430, A. In the adult it terminates 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 they become thin and rupture so that two semicircular ducts are formed (Fig. 430, B and C). The space encircled by each duct may be regarded as a hole in the vesicle.


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Fig. 429. Sections of Rabbit Embryos to show the Development of thk Ear. x 9, 9 days, 3.8 mm.; B, 10 days, 3.4 mm.; C, laj^ days, 7.5 mm.; D. 14 days, 10 mm. a.. Ectodermal epithelium which forms the membranous internal ear ; a. bas.* basilar artery : ch. t.. chorda tympani ; d. C, cochlear duct ; d. e., endolymphatic duct : d. 8. I., lateral semicircular auct ; d. 8. 8., superior semicircular duct; ep.t epidermis ; fa., facial nerve; pharynx. metenccphalon ; m. t, medullary tube ; ph..


The two ducts in question are the superior and posterior semicircular ducts respectively. The third or lateral semicircular duct forms soon afterwards. In Figs. 429 D and 430 B it is a horizontal shelf-like projection of the vesicle, the center of which is to become perforated so that its rim forms the duct. The portion of the vesicle which receives the terminal openings of the three semicircular ducts is called the 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. 430 D). Near one end of each duct there is a dilatation or ampulla, where nerves terminate.


While the formation of the semicircular ducts is occurring in the upper part of the vesicle, the lower portion elongates and its end becomes coiled, eventually making two and a half revolutions. The coiled tube is the ductus cochleae; its distal end is the caecum cupulare and at its proximal end is the caecum vestibulare (Fig. 430 D, c. v,), A dilated sac formed at its proximal or upper end opposite the caecum vestibulare is the sacculuSf-^m the adult the connection between the sacculus and ductus cochleae is relatively narrow and is called the ductus rfunipna (Fig. 439). 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. 43"9T)


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Fig. 430, Lateral or External Surfaces of Modkls of the Mkmbrangis Portion (w ihk Left Internal Ear from Human Embryos. Different enlarifemenls. (After His, Jr.) A. from an embrx o of 6.g mm.; B. 10.2 mm.; 0,13.5 mm.; and D, 22 mm. am., ampulla ; c. v., caecum vestibulare of d. C, cochlear duct ; d. e., endolymphatic duct ; d. 8. 1., d. S. p.. and d. S. 8., lateral. posterior, and superior semicircular ducts; sac, siicculus; ut., utriculus.

The ectodermal vesicle thus produces a complex system of connected epithelial ducts, which are the superior, posterior, and lateral semicircular ducts, the utriculus, the 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 terminates in branches between the epithelial cells in certain parts of the ducts. fKound areas of ncuro-cpithelium are called maa i lae a€iisticae2M .heTe is one in the sacculus and another in the utriculus. EIongate d areas are ( fr/^/ag and there is one in each of the three ampullae The axis or modiolus, about which the cochlear duct is wound, contains the nerv^es which send terminal fibers to the spiral organ of the adjoining epithehum. In this they form a Une of terminations along the medial wall of the cochlear duct, following its windings from base to cupula.

The mcsench yma immediately surrounding the 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 showTi in Fig. 431; these are sometimes called ligaments. The perilymph spaces around the semicircular ducts are irregularly arranged and communicate with one another at various points, but those around the cochlear duct form a single tube. It arises from the other spaces at the base of the cochlea and covers the lateral or outer surface of the ductus cochleae as it ascends to the

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Fig. 431. Cross Skction of a SK.MiciRCti.AR Drcr and thk Adjackni Pkrilvmph Spacis to GKTHKK WJTH THE SEMICIRCULAR CaNAL OF BoNK IN WHICH THKV ARK LODGKD. From a human adult. X 50. (Bohm and von Davidoff.)


cupula; there it turns and descends along the medial or inner surface of the ductus cochleae, ending blindly at the base not far from its origin. The ascending perilymph space excavated in the mesenchyma around the cochlear duct is the scala vestibuli. The descending space with w^hich it connects at the cupula is the scala tympani. The arrangement of the cochlear duct and its scalae is shown in the section through the axis of the spiral.

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Fig. 432. The upper side of the figure is directed forward and outward in relation to the body.

The temporal bone develops from the mesenchyma surrounding the

of the malleus lying near it becomes imbedded in its mesenchymal layer, 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 tympaniccavity of the middle ear. It opens freely into the pharynx and contains air

Sacculus, Utriculus, and Semicircular Ducts

The walls of all these structures consist of three layers. On the outside there is connective tissue with many elastic fibers and occasional pigment 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; the epithelial cells are columnar with a cuticular border. In the neuro-epitheliu m of these areas t here are^ ^two sorts of cells, sustentacular and hair .cqIIs. The sustentacular or fiber cells extenH^ 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 epithehum; they have large spherical nuclei near their rounded basal ends, and a clump of fine agglutinated filaments projecting from their free surface. The ner\'es 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 The horizontal fibers terminate like their occasional branches, by ascending between the hair cells, on the 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 fi 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.


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Fig. 434. Otoconia from the SAtcrLi-s of an Infant. X560.


and varicosities.


The "ligaments" of the ducts, the thin periosteum of the bony semicircular canals, and the periljmiph spaces lined with mesenchymal epithelium 2^re seen in Fig. 431.

Ccchlea

The relation between the du^liI5Tnd5leae and the scalae tympani and vestibuli is shown in Fig. 435. 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 cupula) by the membrana vestibularis Rcissner's membrane and on its basal or medial surface by the Inmitm ^fyimll^ These three walls may be described in turn. The periphery 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. 435). The spiral ' ligament is covered by a layer of cuboidal epithelial cells belonging to the cochlear duct. Close beneath the epitheUum 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 protninens. The latter occupies a low elevation of tissue which has its maximum development in the basal coil of the cochlea (Fig. 435).


Lewis1906 fig435.jpg

Fig. 435. The Portion of Future 432 marked "Scala vestibuli" and " Scala tympani." X 50.


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.

The basal wall or lamina spiralis extends from the modiolus peripherally to the bony wall of the cochlea. Near the modiolus it lies between the two scalae but peripherally it is between the ductus cochleae 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-cpithelium known as the spiral organ [of Corti].

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Fig.. 436. Portion of Figure 435. X 240. x. Intercellular "tunnel " traversed by nerve fibers.


Where the membrana vestibuli meets the osseous spiral lamina there is an elevation of tmip;h r^pnertivp f jc;QnP called the limbus spiralis (Fig. 435). It consists of abundant spindle-shaped cells and blends below with the periosteum of the spiral lamina. Superficially it produces irregularly hemi

  • The familiar terra lamina spiralis membranacea employed by Professor Stohr is not

inrluded among the Nomina Anatomica. In place of it is lamina bdsilaris. Whether the Litter should be considered synonymous with the former, or should refer to the entire basal layer into a |)ortion of which a lamina spiralis ossea projects, is not apparent.


spherical papillae found within the cochlear duct near the vestibular membrane. Further within the ductus cochleae there is a row of flat elongated forms directed from the modiolus toward the periphery; these are sometimes called Huschke's auditory teeth (Fig. 438). The papillae are covered by a simple layer of flat epitheUum. As the limbus extends from the vestibular membrane toward the peripheral part of the cochlea, it terminates abruptly in an overhanging labium vestibulare beneath which is an excavation, the sulcus spiralis (Fig. 436). 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 formation, non-nucleated, soft and elastic, projects from the labium 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.

O'he lamina spiralis membranacea, or lamina basilaris (?), consists of four layers. The mesenchymal of the scala tympani is followed by a layer Tof delicate connectiv e tissue prolonged from the periosteum of the scala. Its spindle cells are at right angles with the fibers of the overlying membrana basilart s. 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. 437). Peripherally (beyond the bases of the outer pillar cells) the fibers arc thicker and are called auditory strings; they are shortest in the basal part of the cochlea an a longe st t()\yard the j4>c^> corresponding in length with the basal layer of tihc cochlear duct. These fibers have been thought to vibrate and assist in conveying sound waves to the nen'cs. j

The epithelial cells covering the basilar layer, present rows of highly modified forms extending up and dowTi or lengthwise of the cochlear duct, and constituting the spiral organ [of Corti]. Next to the cuboidal epithelium of the sulcus spiralis there is a single row of inner hair cells (Fig. 436). 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 they are in contact above, but are separated below by a triangular intercellular space or "tunnel." The space is filled with soft intercellular substance. Thus they rest upon the basilar membrane in A-form. The mner pillar cells are said to be more numerous than the outer. Both forms are stiff bands with triangular expanded bases, which are associated with nucleated masses of protoplasm within the "tunnel." The "heads" or upper ends interlock, since the inner pillars are concave to receive the convex surface of the outer pillars. From the superficial surface of both, plates extend peripherally or outward, that of the inner pillar partly covering the head-plate of the outer pillar (Fig. 438). The dark bodies in the heads of both pillars, and in the basal part of the outer ones, are not nuclei.

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Fig. 437. Surface view of the Lamina Spiralis Mkmbranacka of A Cat. X 240. Drawn with change of focus. e, Ejjithelium (cells of Claudius) of the ductus cochlearls in focus; f. fibt-rsof the membrana basilaris in focus ; b, nuclei of the underlying connective tii.sue.



Diagram of the Structure of the Basal Wall of thh Duct of the Cochlea


Fig. 438. 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 he seen distinctly only by lowering the tube. The memhrana tectoria is not drawn. The spiral nerve bundles arc indicated by dots.


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 (Deiters's cells). The outer hair cells have shorter hairs than the inner ones, which otherwise they resemble. They do not extend to the basilar membrane, thus leaving unoccupied the communicating intercellular (NuePs) spaces between the deeper portions of the sustentacular cells. NuePs spaces connect with the tunnel. The sustentacular cells are slender bodies each containing a stiff filament. They have a caplike cuticular border so that they remotely resemble the distal phalanges of the fingers. The spaces between the "phalanges" are occupied by the outer hair cells. The cuticular expansions connect with one another forming a reticular membrane, into the apertures of which the hair cell processes extend. The sustentacular 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. The most peripheral of the sustentacular cells are followed by elongated columnar cells (cells of Hensen) which gradually shorten and pass into the undifferentiated epithehum of the cochlear duct. The low cells following Hensen's cells are the cells of Claudius. They are said to have branching bases which extend deep into the underlying tissue. 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.

Nerves and Vessels of the Labyrinth

The acoustic nerve is a purely sensory nerve passing between the pons and internal ear through a bony canal, the internal auditory meatus. It is divided into vestibular and cochlear portions (Fig. 432). The vestibular nerve proceeds from the vestibular ganglion and has four branches; the utricular nerve and the superior y lateral ^ and posterior ampullary nerves. Their terminations have already been described (p. 384). The cochlear nerve, which has a saccular branch, proceeds from the spiral ganglion lodged within the modiolus at the root of the lamina spiralis (Figs. 432 and 435). The ganglion cells remain bipolar like those of embryonic spinal ganglia. The neuraxon and the single peripheral dendrite are medullated except near the cell body. The peripheral fibers extend through the lamina spiralis ossea, within which they form a wide meshed plexus, and after losing their myeUn they emerge from its free border 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 timnel; 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 int^fjt ^Lauditory artery. is a branch of Jh^ b'i Stiffs 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-auditoy meatus to the ear. It divides into vestibular and cochlear branches. The vestibular artery supplies the vestibular nerve and the upper lateral portion of the sacculus, utriculus and semicircular ducts. The cochlear artery sends a vestibulocochlear 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. 440); 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.


Diagram of the Blood Vessels of the Right Himan Labyrinth

Fig. 439. Diagram of the Blood Vessels of the Right Himan 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.


The veins of the labyrinth form three groups, (j^ The vena aquae ductus vesiibuli 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. The vena aquaeductus cochleae receives blood from parts of the utriculus, sacculus and cochlea; it passes throug h a bony canal to the internal jugular vein. Within the cochlea it arises, as shown in Fig. 440, from small vessels including the vas prominens (a) and the vas spirale (b). 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 l at ter is not affected by arterial puls ationj Within the modiolus the veins unite in an inferior spiral vein which receives blood from the basal and a part of the second turn 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. 439). 3. The internal auditory vein arises within the modiolus from the veins of the spiral lamina; these anastomose with the spiral veins (Fig. 440). It receives branches also from the acoustic nerve and from the bones, and empties *'in all probability, into the vena spinalis anterior." (The transverse and petrosal sinuses are often said to receive this vein; and the vena aquaeductus vestibuU has been described as entering the inferior instead of the superior petrosal sinus.)


Fig. 440. Diagram of a Skction of the First (Basal) and Seco.nd Turns of the Cochlea. a, Vas prominens ; b, vas spirale.



/Lymphatic spaces within the internal ear are represented by the perilymph spaces which communicate through the aquaeductus cochleae with the subarachnoid space; the connecting structure or "ductus perilymphaticus'* is described as a lymphatic vessel. The salens endolymphaiicus which is the dilated distal end of the endolymphatic duct, is in con tact with the dura, and t here ar e said to be openings between it ^ m^ the subdural space . In the internal ear perivascular and perineural spaces are found, and they probably connect with the subarachnoid sp ace.^


Fig. 441. Cross Section of the rARTii.A(;iNors Part of thk Ai'ditory Tubk. X 12. (Bohm and von DavidoflF.)

Middle Ear

The tympanic cavity which contains air, is Uned with a mucous membrane closely connected with the surrounding pe riosteum. It consists of a th in layer of connective tissue covered generally with a simp le cuboida l 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 t he , co nnective tissue, and lymphatic vessels are found in the periosteu m^ Crhe auditory tube i ncludes 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 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 aHht the mrffpji ng bone. Along its floor there are pockets containing air, the cellulae pneumaiicae. In the cartilaginous part the mucosa is thicker; near the pharynx it contains many mucous glands (Fig. 441). Lymphocytes are abundant in the surrounding connective tissue, forming nodules near the end of the tube and blending with the pharyngeal tonsil. The cartilage, which only partly surrounds the auditory tube, is hyaline near its junction with the j bone of the osseous portion; it may contain here and there coarse fibers which are not elastic. Distally the matrix contains thick nets of elastic tissue, and the cartilage is consequently elastic.

External Ear

Between the middle ear and the external ear is the tympanic membrane, which consists, from without inward, of the following strata: the cutaneum, radiatum, circulare and mucosum (Fig. 442). 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. The latter receives vessels both from 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 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.



Fig, 442. Cross Section of the membrana tympani below the manubrium. X 450. (After Kolliker.) a, Stratum cutaneum (showing the corneum and gerniinativum) ; b* stratumradiatum, its fihcrs cut across ; c, stratum circUlare ; d, stratum mucosum.


The external auditory 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" (Ruber) 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. 443). The secretion consists of pigment, fat, and fatty cells, the latter derived probably from the hair sheaths.


Fig. 443. From a Vertical Skction through the Skin of the External Auditory Meatus of an Infant. X 50. The excretory duct opens into the hair foflicle.


Fig. 444. Tubules of the Ceruminous Glands. A Cross section, from an infant ; B, longitudinal cection, from a boy 12 years old.


The cartilage of the external auditory meatus and of the auricle is elastic.


Nose

The nasal cavities are formed by the invagination of a pair of epidermal thickenings similar to those which give rise to the lens and auditory vesicle. The pockets thus produced in the embryo are called "nasal pits" (Fig. 187, n, p. 166). Their external openings remain as the nares of the adult. Temporarily, from the third to the fifth month of fetal life, they are closed by an epithelial proliferation. Each nasal pit acquires an internal opening, 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. 445). As the latter extends posteriorly it is joined by the palate processes which grow toward it from the sides of the maxillae. Thus the choanae recede toward the back of the mouth while the embr}'onic condition of cleft palate is being removed. The lateral walls (not the medial) of the nasal cavities produce three curved folds one above another; they are concave below, and in them the conchae [turbinate bones] develop. The nasal mucosa covers these and extends into the sphenoid, 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 fig 445 -the roof of the

Mouth of a Human Em olfactorj^ neurpiepithelium is derived irom .the fAfur

nasal pit.^ In man the olfaclory region is limited "■» Nans; ch. choana; ai. p., I. p., and pa. p., alveolar.

to the superior and middle concha and the part of the septum which is opposite them. This region oljactoria is covered by a yellowish-brown membrane which may be distinguished macroscopically from the reddish mucosa of the regio respiraioria. 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 Uned with a continuation 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 (vibrissae) 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. It consists of fibrillar tissue with many elastic elements especially 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. 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 Hned with the superficial epithelium. The mucosa of the several paranasal sinuses is thin ( — 0.02 mm.), with less elastic tissue and but few small glands. A pocket which extends into the lower part of the median septum and is named the vomero-nasal organ [Jacobson's organ], is in man the rudimentary remnant of an important sense organ supplied by the olfactory nerves. From the fifth month of fetal Ufe it is Uned with a tall columnar epithelium which is not olfactory.



Fig. 446. Vertical Skction through the Mlcosa of thk Infkrior Concha of Man. X 4S. On the left is a funnel-shaped depression receiving an excretory duct; nearby on the right is the section of a large vein.


Fig. 447. Isolated Cells of the Olfactory Mucosa of a Rabbit. X 560. St, Supporting cells ; g, extruded mucus resembling cilia ; r, olfactory cells, from r', the lower process has been torn of! ; if, ciliated cell; b, cells of olfactory glands.



Fig. 448. Vertical Section of the Olfactory Mucosa of a Rabbit, x 50. zo, Zone of oval ; zr, zone of round nuclei : dr, olfactory glands j a, excretory duct ; k, body ; g, fundus ; n, sections of the olfactory nerves ; v, vein ; ar, artery ; b, connective tissue.


Epithelium.


In the regio oljactoria the mucosa consists of 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. 447). 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" (Figs. 448 and 450). 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 which is gathered immediately about the nucleus,


Fig. 449. Golgi Preparation of the Olfactory Region of a young Rat.


each olfactory cell sends a slender cylindrical process towards the surface, where it terminates in small hairs. Basally the olfactory cells pass directly into the axis cylinders of the olfactory ner\'es (Fig. 449). C^hus they are ganglion cells, their basal processes being neuraxonst) Cells intermediate between the olfactory and sustentacular forms may be found. At the free surface of the olfactory epithelium there are terminal bars, and small masses of mucus sometimes suggesting cilia. The mucus is the product of the sustentacular cells. Near the tunica propria there is a network of so-called "basal cells** (Fig. 450).

The tunica propria is a network of coarse fibrous tissue and fine


Vertical. Section through the Olfactory Regions

Fig. 450. Vertical. Section through the Olfactory Regions of an Adult.T. X 400


elastic fibers associated with many connective tissue cells. In some animals ffor example, the cat) it forms a structureless membrane next to the epithelium, ^ll surrounds the numerous qljactory glands [Bow man *s glan ds]. In man these are branched cavities consisting of excretory ducts extending through the epithelium, and of gland bodies beneath. Oblique sections of the ducts have been mistaken for "olfactory buds." The glands in man appear to be serous but they sometimes contain mucus in small quantity. They are found not only in the olfactory region but also in the adjoining part of the respiratory region.


The nerves of the nasal mucosa consist of groups of non-meduUatcd olfactory fibers, which unite in larger bundles in the tunica propria and pass through the lamina cribrosa of the ethmoid to enter the olfactory bulb. They are covered by prolongations of the dura. Medullatcd branches of the trigeminal nerve occur both in the olfactory and respiratory mucosa. After losing their myelin they form terminal ramifications in the tunica propria and may ascend into the epithelium. Thus they differ from the olfactory fibers which generally do not branch.

The arteries are found in the deeper layers of the tunica propria, and they form a thick capillary network just beneath the epithelium. 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 subarachnoid space follow the perineural sheaths of the olfactory nerves into the nasal mucosa.



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