The Jejunum and Пleum form by far the longest part of the small intestine, and are not separated from each other by any sharp line of demarcation-the upper twofifths being called jejunum, on account of its being usually empty after death, the lower three-fifths being termed ileum, from its convoluted arrangement. They occupy the umbilical, hypogastric, right and left iliac regions of the abdomen, in which they are arranged in a series of coils or convolutions; one or two coils of the ileum sometimes lie in the cavity of the pelvis, between the bladder and rectum. The coils are attached to the posterior wall of the abdomen, along a line from the body of the first lumbar vertebra to the right sacro-iliac joint, by the fold of peritoneum called the mesentery. Owing to the extent of the mesentery, the coils of the jejunum and ileum can be freely moved about in the abdominal cavity, so that they are apt to be displaced from their natural position, and, when a rupture occurs, to become the most usual contents of the hernial вас. The lower end of the ileum passes into the right iliac fossa, where it becomes continuous with the large intestine, at the junction of the cœcum and ascending colon. Though the line of demarcation between jejunum and ileum is an arbitrary one, yet the upper end of the jejunum may be distinguished from the lower end of the ileum by being wider, and having a thicker mucous membrane, in which the folds called valvulæ conniventes are larger and more

numerous.

Structure of the Small Intestine.-The wall of the small intestine consists in the greater part of its extent of four coats, named, from without inwards, serous, muscular, submucous, and mucous coats.

The serous or external coat, derived from the peritoneum, forms a complete investment for the jejunum and ileum, and is continuous with the mesentery along a line of attachment, named the mesenteric border of the intestine; but the serous covering of the duodenum is incomplete.

The muscular coat consists of non-striped fibres arranged in two layers from without inwards. The outer layer consists of longitudinal fasciculi, which form a thin layer parallel to the long axis of the intestine. The inner layer consists of circular fasciculi arranged around the gut transverse to its long axis; this layer is thicker, stronger, and more highly coloured than the longitudinal layer. By the contraction of the muscular coat, the peristaltic or vermicular movement is produced, which propels the ingested materials along the intestine.

The submucous coat lies immediately subjacent to the circular layer of the muscular coat, and consists of areolar connective tissue; in it the blood-vessels ramify before they pass into the mucous membrane.

The mucous or internal coat is a soft, velvety-looking membrane, which lines the wall of the small intestine, and possesses a complex appearance and structure. The inner surface is not smooth, but is thrown into strongly-marked, transverse folds, the valvula conniventes, which are not obliterated during distension of the gut. They are very numerous in the duodenum and jejunum, but then decrease in size and numbers, until at the lower end of the ileum they have disappeared. Each valvula consists of a fold of the mucous membrane with its submucous coat. Owing to

The projecting bodies are the intestinal Villi, which jut out into the lumen of the intestine from the free surface of the mucous membrane, not only of the valvulæ, but of the intermediate surface. They are delicate, minute processes, varying in length from a fourth to half a line, and in number amount to several millions.

They are best examined when the mucous surface is placed in water or spirit, when they may be seen with the naked eye, or, still better, with a pocket lens; when the chyle-vessels or blood-vessels are injected, they become rrected, and stand out more prominently from the surface. They vary in form, being filiform, or cylindrical, or conical, or club-shaped, or leaf-shaped. They are more numerous in the duodenum and jejunum than in the ileum, and to their presence is due the velvety appearance of the mucous surface. They are not found elsewhere than in the small intestine.

As they are the parts of the mucous membrane directly concerned in the absorption of the chyle, their structure is interesting and important. Each villus is invested by a cap of epithelium continuous with the general epithelial covering of the mucous membrane. The epithelium consists of a single layer of columnar cells, compactly arranged side by side. Scattered amidst the columnar cells are cells which possess the form of microscopic goblets, and are named goblet cells. The free end of each goblet cell appears to have an open mouth on the surface of the villus, through which a mucus-like substance exudes. Various opinions have been expressed as to the nature of these goblet cells. Some regard them as special structures engaged either in the absorption of chyle, or the secretion of mucus; others look upon them as merely modifications of the columnar epithelium; whilst others again consider them to be post-mortem productions, due to the swelling out of the columnar epithelium by the imbibition of fluid. There can be no doubt, however, that they are not specially concerned in the absorption of chyle, as cells of the same character are found in the respiratory mucous membrane, and on other surfaces, where the absorption of chyle does not take place.

The sub-epithelial tissue of a villus forms its matrix or basis substance, and consists of the sub-epithelial counective tissue of the mucous membrane. When thin sections through a villus are examined, the matrix is seen to be

Fra. 4-A. transverse section through an Intestinal villas, showing its eptthelial investment and the matrix of lymphoid tissue; c, columnar epithelium; 9, goblet-shaped cell; 7, lacteal; r, r, lymphoid retiform tissue; e,, transversely divided blood-vessels. B, free ends of columnar epithelium, with mouths of four goblet-shaped cells. X 300.

composed of a delicate retiform tissue, which forms a network, in the meshes of which numbers of colourless lymphoid corpuscles are imbedded. These cells were described and figured by Goodsir, as the absorbing cells or vesicles of the villus. In the axis of the villus one, or perhaps two, minute lacteals or chyle vessels are situated, which serve as rootlets of origin of the lacteal division of the lymph vascular system. The lacteal is a capillary tube,

ileum, but diminish in size and numbers in its upper end and in the jejunum, and are absent in the duodenum.

These follicles are lymphoid organs, and are composed of lymphoid or adenoid tissue. The solitary and Peyer's glands, as is the case generally with the lymphoid organs, are more distinct and perfect in structure in infancy and childhood, than in adults or in advanced age.

The muscular layer of the mucous membrane lies next to the submucous coat, and consists of non-striped fibres which lie parallel to the surface of the membrane. It passes into the substance of the villi, and lies around the closed end of the glands of Lieberkühn.

Of the blood-vessels of the small intestine, the arteries enter the wall of the jejunum and ileum at its attached or mesenteric border, and are branches from the arcades of the superior mesenteric artery. They run in the sub-serous tissue around the wall of the intestine; then pierce the muscular coat and supply it; they then enter the submucous coat, and a form a network from which branches pass into the mucous coat. The veins accompany the arteries, and form rootlets of the superior mesenteric vein. The lymph-vessels, or lacteals, may be traced into the wall of the intestine at the mesenteric border; they form a network in the muscular coat, and then enter the submucous coat, where they are very abundant; from this submucous layer offshoots pass through the retiform tissue, which lies between the Lieberkühnian glands, into the villi. Where the solitary and Peyer's glands are situated, the lacteals, as Frey has pointed out, form a system of anastomosing vessels around the base and mesial part of each follicle. The nerves are derived from the plexuses of the sympathetic, which accompany the branches of the superior mesenteric artery. They form between the two layers of the muscular coat an important plexus, named, after its discoverer, Auerbach's plexus, in which large stellate nervecells are intermingled with nerve-fibres, and a similar nervous plexus is found in the muscular coat of the other divisions of the alimentary canal. It supplies and regulates the movements of the muscular coat.

The Large Intestine, though not nearly so long as the small intestine, is of much greater diameter. It reaches from the end of the ileum to the orifice of the anus, and is divided into the cœcum with the appendix vermiformis, the colon, and the rectum; whilst the colon is subdivided into the ascending colon, the hepatic flexure, the transverse colon, the splenic flexure, the descending colon, and the sigmoid flexure.

The Cacum, the dilated commencement of the large intestine, lies below the ileum, and occupies the right iliac fossa. It forms a large cul-de-sac, closed in below, but communicating freely above with the ascending colon. Opening on the inner and posterior wall of the cæcum is the appendix vermiformis, which is a slender hollow prolongation of the bowel, varying in length from 3 to 6 inches. It has the calibre of the stem of a common tobacco pipe, and ends in a free closed extremity, so that, like the cæcum, it is a cul-de-sac. It is not generally found in mammals, but is present in man, the orang, certain lemurs, and the marsupial wombat..

The Colon extends from the cæcum to the rectum, and forms the longest part of the large intestine. The transverse part of the colon lies immediately below the great curvature of the stomach, but owing to the length of the transverse meso-colon, which forms its peritoneal attachment, it not unfrequently undergoes some change in its position, and may hang downwards towards the pelvis, or be elevated in front of the stomach, or thrown to the right or left side.

The sigmoid flexure of the colon is situated in the left iliac fossa, but as the sigmoid meso-colon, which forms its

peritoneal attachment, is of some length, it is freely movable, and not unfrequently hangs into the pelvis, or even extends across into the right iliac fossa.

The Rectum is the terminal segment of the large intestine, and extends from the sigmoid flexure to the orifice of the anus. It lies in the cavity of the pelvis. It commences opposite the left sacro-iliac joint, and passes at first obliquely downwards and to the right until it reaches the middle line of the sacrum; secondly, it closely follows the curvature of the sacrum and coccyx, lying in relation to their anterior surface; thirdly, when it reaches the tip of the coccyx its terminal or third part inclines downwards and backwards for about 1 inch to the anal orifice. The anus opens on the surface of the middle line of the perineum, midway between the two ischial tuberosities, and the skin surrounding the orifice is thin, and wrinkled when the opening is closed. Immediately beneath the skin is the sphincter ani externus muscle, which forms a thin layer of fasciculi, arranged in a series of ellipses around the orifice. The sphincter in its normal condition of contraction simply closes the opening, but, under the influence of the will, a more powerful contraction can be induced, so as to resist the entrance of foreign bodies into the rectum.

The large intestine is arranged in the abdominal cavity in the form of an arch, the summit of which is the transverse colon, whilst the cæcum and rectum are the right and left piers. Within the concavity of this arch the coils of the jejunum and ileum are situated. The large intestine is not, except in the rectum, a cylindriform tube, but is dilated into three parallel and longitudinal rows of sacculi, which rows are divided from each other by longitudinal muscular bands, whilst the sacculi in each row are separated externally by intermediate constrictions. In the rectum the sacculi have disappeared, and the intestine assumes a cylindrical form, but at its lower end it dilates into a reservoir, in which the fæces accumulate prior to being excreted.

At the junction of the large with the small intestine a valvular arrangement, termed the ileo-cacal or ileo-colic valve, is found. This valve is due to the peculiar manner in which the ileum opens into the large intestine.

The opening is bounded by two semi-lunar folds, which project into the large bowel. These folds are the two segments of the valve; one situated above the opening is the ileo-colic segment, the other, below the opening, the ileocæcal. The two segments become continuous with each other at the ends of the elongated opening, and are prolonged for some distance around the inner wall of the large intestine as two prominent ridges, named the frana of the valve. The use of the ileo-cæcal valve is to impede or prevent the reflux of the contents of the large into the small intestine. When the cæcum and colon are distended the fræna of the valve are put on the stretch, and the two segments are approximated, so that the opening is reduced to a mere slit, or even closed, if there is great distension of the bowel.

Structure of the Large Intestine.-The wall of the large intestine consists in the greater part of its extent of four coats, named, from without inwards, serous, muscular, sub mucous, and mucous coats.

The serous or external coat, derived from the peritoneum, forms a complete investment for the flexures of the colon, the transverse colon, and the first part of the rectum, but not for the cæcum, or the ascending and descending colon. The second part of the rectum has only a partial serous investment, and the third part has no serous coat. Numerous pedunculated processes invested by the serous membrane, and containing lobules of fat, named appendices epiploica, are attached to the large intestine.

The muscular coat consists of non-striped fibres arranged

in two layers from without inwards. The outer layer consists of longitudinal fasciculi, which are not as a rule distributed uniformly in the wall, but in the cæcum and colon are collected into three longitudinal bands, which start from the cæcum, where it is joined by the appendix vermiformis, and extend along the colon to the rectum. As these bands are not so long as the colon itself, they occasion the puckerings which separate the sacculi, so that when the bands are cut through the sacculi disappear. The colon then becomes more elongated and cylindriform.

In the appendix vermiformis the longitudinal layer is not collected into bands, but arranged uniformly along the wall. In the rectum, also, the longitudinal layer is spread uniformly along the wall, and forms a well-defined red-coloured layer.

The inner layer of the muscular coat consists of circular fasciculi distributed around the wall of the large intestine. In the rectum this layer increases in thickness, and in proximity to the anus forms a circular muscle, the sphincter ani internus, which is a strong band, about half an inch broad, around the lower end of the rectum. In the large, as in the small intestine, the muscular coat occasions the peristaltic movements, and its increased thickness in the rectum is for the purpose of expelling the fæces.

The submucous coat has similar relations and structure to the corresponding coat in the small intestine.

The mucous or internal coat is not thrown into valvula conniventes, but presents a series of well-marked permanent ridges, lying transversely or somewhat obliquely to the long axis of the gut, and corresponding internally to the constrictions, which, on the outer surface of the colon, separate the sacculi from each other. The mucous membrane of the large intestine is covered by a layer of columnar epithelium. It is devoid of villi, and consists of a glandular and a muscular layer. The secreting glands of the glandular layer have the form and structure of the Lieberkühnian glands of the small intestine (fig. 7); they open on the free surface of the mucous coat, and, owing to the absence of villi, their mouths are more closely set together than is the case with the corresponding glands in the small intestine; the tubular glands are separated by a retiform tissue with lymphoid corpuscles. Solitary glands, similar to those in the small intestine, are also present, but no Feyer's patches. The muscularis mucosa resembles generally that of the small intestine.

Of the blood-vessels of the large intestine, the arteries are principally derived from branches of the superior and inferior mesenteric arteries, but the lower end of the rectum receives the hæmorrhoidal branches of the internal iliac and the pudic. The veins which correspond to these arteries for the most part join the superior and inferior mesenteric veins, and are consequently rootlets of the portal. But the veins which belong to the middle and inferior hæmorrhoidal arteries form a plexus about the anal orifice, which partly joins the superior hæmorrhoidal vein, and through it the portal vein, and is partly connected through the middle and inferior hæmorrhoidal veins with the internal iliac vein, and through it with the inferior vena cava. The veins about the anus are very apt to become varicose, and to form the excrescences termed hæmorrhoids or piles. The lymph vessels are arranged as in the small intestine, except that they are not prolonged into villi. Nervous plexuses with ganglion cells are found in both the muscular and submucous coats. They proceed from the superior and inferior mesenteric plexuses, but the rectum receives branches from the hypogastric plexus, and from the third and fourth sacral spinal nerves.

The LIVER is the biggest of the abdominal viscera, and the largest gland in the body. It is the organ in which the secretion of bile takes place, and is the chief seat in the

body of the formation of glycogen, a substance like dextrin, which readily undergoes conversion into sugar. It lies in the costal zone of the abdomen, fills up the greater part of the right hypochondrium, and extends, through the epigastrium, into the left hypochondrium. In its long or transverse diameter it averages about 12 inches, in its antero-posterior diameter about 6 inches, in the vertical diameter of its thickest part about 3 inches. Relatively to the size of the body the liver is bigger and heavier in the foetus than in the adult; soon after birth the relative weight declines, and that of the left lobe diminishes much more rapidly than the right lobe. Frerichs states that the relative weight of the healthy liver fluctuates in adults betweenth and th of that of the body, and the absolute weight varies from 18 to 4.6 pounds avoird. During the digestion of the food the liver increases both in size and weight, partly from the greater quantity of blood flowing through it, and partly from the new material in the secreting cells; whilst after a long fast it becomes smaller and lighter.

For descriptive purposes the liver may be regarded as having two surfaces, two borders, and two extremities.

The superior or diaphragmatic surface is smooth and convex, and attached to the diaphragm by the falciform ligament.

The posterior or vertebral border is comparatively thick, and attached by the coronary ligament to the diaphragm. The anterior border of the liver is unattached, thin, and attenuated, and is marked by a deep notch, opposite the anterior edge of the falciform ligament, which lodges the round ligament of the liver.

Of the two extremities of the liver the right is thick and massive, and lies deep in the right hypochondrium, in contact with the diaphragm; the left is thin and attenuated, and overlaps the oesophageal opening and fundus of the stomach.

The inferior or visceral surface of the liver is much more complex in form than the upper. The longitudinal or umbilical fissure, continuous with the notch in the anterior border of the liver, and much nearer to the left than the right extremity of the gland, divides it into a large right

Fig. 8.-Under surface of the liver. E, right labe; L, left lobe; Q, lobus quadratus; S, lobus Spigelli; C, lobus caudatus; p, pons hepatis; V, longitudinal fissure;, transverse fissure; cf. caudate fissure; ef, fossa for vena cava; kf, fossa for right kidney; G, gall bladder in its fossa; u, obliterated umbilical vein; v, obliterated ductus venosus; IV, inferior vena cava; h, h, hepatic veins; P, portal vein; A, hepatic artery; D, bile duct; c, coronary ligament; and ri, left and right lateral ligaments; s, suspensory ligament; r, round ligament.

and a small left lobe. In the anterior part of the fissure the round ligament, formed by the obliteration of the umbilical vein of the foetus, is lodged; whilst the posterior part contains a slender fibrous cord formed by the obliteration of a vein of the foetus, named ductus venosus. The longitudinal fissure is often bridged across by a band of

and bile-ducts, with perhaps nerves and lymphatics. The blood-vessels will first be considered.

liver substance called pons hepatis. The under surface of the left lobe is smooth, and overlaps the anterior surface of the stomach. The under surface of the right lobe is divided into smaller lobes by fissures and fossæ. Starting from about the middle of the longitudinal fissure is the portal or transverse fissure, which extends for from 3 to 4 inches across the under surface of the right lobe. It is the gate (porta) of the liver, the hilus or fissure of entrance into the organ of the portal vein, hepatic artery, hepatic duct, and hepatic nerves and lymphatics. A short distance to the right of that part of the longitudinal fissure in which the round ligament lies, is the fossa for the gall bladder, which is a depression on the under surface of the right lobe extending from the anterior border to the transverse fissure: in it the gall bladder lies. Extending somewhat obliquely from the posterior border of the liver, towards the transverse fissure, is a deep fossa for the inferior vena cava. Opening into the vena cava as it lies in this fossa are the trunks of the large hepatic veins from the substance of the liver. A portion of liver substance, which is bounded by the gall bladder, the longitudinal fissure, the transverse fissure, and the anterior border, forms a four-sided lobe called lobus quadratus. Another portion, bounded by the transverse fissure, the posterior border, the vena cava, and the longitudinal fissure, is the lobus Spigelii. A thin prolongation of liver substance continuous with the lobus Spigelii, and running obliquely between the fossa for the inferior cava and the transverse fissure, is the lobus caudatus.

Structure of the Liver.-The liver is a solid organ, of a brownish-red colour. It is composed of the ramifications of the portal vein, of the portal capillaries, the hepatic vein, the hepatic artery, the hepatic duct, of secreting cells, nerves, and lymphatics. These several structures are bound together by connective tissue, and the organ is invested by the peritoneum. The liver possesses two coats, a serous and a fibrous.

The serous or external coat is a part of the peritoneal membrane, and forms an almost complete investment for the liver. It is reflected from the transverse fissure as the gastro-hepatic omentum, and from the upper surface and the posterior border as the falciform, coronary, and right and left lateral ligaments of the liver.

The fibrous coat, or tunica propria, is immediately subjacent to the serous coat. When carefully raised from the liver delicate processes of areolar tissue may be seen to pass from its deep surface into the substance of the organ. At the transverse fissure it is prolonged into the liver as a very distinct sheath, enveloping the portal vein, hepatic artery, hepatic duct, nerves, and lymphatics. This sheath. is named the capsule of Glisson, and is prolonged throughout the substance of the organ, along the ramifications of the portal vein and the structures that accompany it. Lobules of the Liver.-To the naked eye the substance of the liver does not present a homogeneous aspect, but is mottled, and mapped out into multitudes of small areas or lobules, the hepatic lobules or leaflets. The lobules of the liver are irregular polygons, and vary in size fromth to ths of an inch. In man and the mammalia generally the lobules are imperfectly separated from each other by the interlobular vessels and duct, and a scarcely appreciable quantity of areolar connective tissue. In the pig, camel, and polar bear, each lobule is circumscribed by a definite capsule of connective tissue.

As a lobule of the liver is a liver in miniature, and as the structure of the entire liver is the sum of the structure of its lobules, it will be necessary to examine with care the constituent parts of a lobule, and the arrangement of the vessels, duct, and nerves which pass to and from it. An hepatic lobule is composed of blood-vessels, secreting cells,

The portal vein conveys to the liver the venous blood from the stomach, spleen, pancreas, gall bladder, and small and large intestine. It ascends to the transverse fissure, and before it enters the liver divides into two branches, one for the right and one for the left lobe. In its course within the liver, the portal vein divides and subdivides after the manner of an artery. It is closely accompanied by the hepatic artery and duct, and, along with them, is invested by the fibrous sheath, called Glisson's capsule. The terminal branches of the portal vein run between the lobules, and are named, from their position, the interlobular branches. The interlobular branches lie around the circumference of a lobule, and anastomose with each other. They partly terminate directly in a capillary network situated within the lobule, and partly give off fine branches, which enter the lobule before they end in the capillary network. The intralobular capillaries form a close network, and converge from the periphery of the lobule, where they spring from the interlobular branches of the portal vein, to the centre of the lobule, where they terminate in the intralobular or central vein, one of the rootlets of the hepatic vein, In man, where the lobules are not separated from each other by a distinct capsule, the capillaries of one lobule to some extent communicate with those of adjacent lobules.

The hepatic artery closely accompanies

by

the portal vein, and FIG. 9.-Transverse section through the hepatic lobules. i, i, i, interlobular veins ending in the divides into two intralobular capillaries; c, c, central veins joined branches, for the the intralobular capillaries. At a, a the right and left lobes. It is the nutrient artery of the liver, and gives off three series of branches :-(a) vaginal branches, which are distributed to the walls of the portal vein, the hepatic duct, and to Glisson's capsule, probably also to the wall of the hepatic vein; they end in a capillary network in these structures, from which vaginal veins arise that terminate in the portal vein; (b) capsular branches, which are distributed to the fibrous coat of the liver, and end in a capillary network, from which arise capsular veins that join the portal vein; (c) interlobular branches of the hepatic artery lie along with the interlobular branches of the portal vein, and end in the capillary network within the lobules. The hepatic vein arises within the substance of the liver from the intralobular capillaries. In the centre of each lobule is the intralobular or central vein. It traverses the axis of the lobule, and leaves it to join a small vein running immediately under the bases of adjacent lobules, which, from its position, is named the sublobular vein. Adjacent sublobular veins then join together, and form larger vessels, which are the trunks of the hepatic vein, or

capillaries of one lobule communicate with those adjacent to it.

Fig. 10.-Vertical section through two hepatie lobules of a pig. c. c. central veins receiving

the intralobular capillaries; s, sublobular vein; ct, interlobular connective tissue forming the capsules of the lobules; i, i, interlobular veins.