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the villi are absent and a lateral displacement of the glands of Lieberkühn is observed. The lymph-nodule is usually pyriform in shape. The thinner portion protrudes somewhat in the direction of the lumen of the intestine, while the thicker portion extends outward to the muscularis mucosa, the latter being frequently indented or even perforated if the lymph-nodules be markedly developed. Their structure is similar to that of the lymph-follicles (see under these), and consists of reticular adenoid tissue, supporting lymph-cells. It should be remembered that every nodule may possess a germ center. Peyer's patches are collections of these lymph-follicles. The surface of the nodule presenting toward the lumen of the intestine is covered with a continuous layer of intestinal epithelium. In man the summit of that portion of the

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Fig. 209.-Section of solitary lymph-nodule from vermiform appendix of guineapig, showing crypt; about 400 (Flemming's fluid).

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nodule projecting into the lumen of the intestine presents but a slight depression of the intestinal epithelium, while in some animals (guinea pigs), and especially in the nodules composing Peyer's patches, there is a deeper depression, even leading to the formation of a so-called crypt" or "lacuna" (vid. Fig. 209). At the summit, the intestinal epithelium where it comes in contact with the lymph-nodule, is peculiarly altered. In most cases there is an absence of a basement membrane, the epithelium resting directly upon the lymphoid tissue. No clearly defined boundary between the two is distinguishable (intermediate zone of v. Davidoff); they are therefore in the closest relationship to each other. The basal surfaces of the epithelial cells are fibrillar, the fibrils seeming to penetrate into the adenoid reticulum of the follicles.

4. THE LARGE INTESTINE, RECTUM, AND ANUS.

The small intestine ends at the ileocecal valve. At some distance from the margin of the valve the villi of the ileum become broad and low. In the immediate vicinity of the valve their basilar portions become confluent, forming a honeycomb structure which supports a few villi. At the base of the honeycomb open the glands of Lieberkühn. On the cecal side of the valve the villi become fewer in number and finally disappear, while the folds which give the honeycomb appearance persist for a considerable distance.

Intestinal epithelium.

In

[graphic]

Lumen of gland.

Goblet cell.

Muscularis mucosa.

Fig. 210. From colon of man, showing glands of Lieberkühn; X200.

the adult cecum the villi are absent. The mucosa and glands present a structure similar to that of the remainder of the large intestine. In the mucosa of the vermiform appendix is found a relatively large number of solitary lymph-follicles, occasionally forming a continuous layer. The marked development of the lymph-follicles encroaches upon the glands of Lieberkühn, so that many are obliterated; they are penetrated by the adenoid tissue, the epithelial cells of the glands mingling with the lymph-cells. What finally

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becomes of the secretory cells has not been definitely ascertained (Rüdinger, 91).

In the colon the villi are wanting, while the glands of the mucosa are densely placed and distributed with regularity.

The glands of Lieberkühn in the colon are somewhat longer, and as a rule contain many more goblet cells than those in the small intestine. Only the neck and fundus of the glands show cells devoid of mucus. Transitional stages between the latter and the goblet cells have been observed in man (Schaffer, 91). Solitary lymph-follicles are found throughout the colon. They are situated in the mucosa, only the larger ones extending into the submucosa. The glands of Lieberkühn are displaced in the regions of the lymphfollicles.

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Gland.

Epithelium of intestine.

Germ center.

Submucosa.

Fig. 211.-A solitary lymph-follicle from the human colon: At a is seen a pronounced concentric arrangement of the lymph-cells.

The tania and plica semilunares cease at the sigmoid flexure, and are replaced in the rectum by the plica transversales recti. Permanent longitudinal folds, the so-called columnæ rectales Morgagni, are present only in the lower portion of the rectum. Here the intestinal glands are longest but disappear simultaneously with the rectal columns. At the anus the mucous membrane of the rectum forms a narrow ring devoid of glands, covered by stratified pavement epithelium, and terminating in the skin in an irregular line. The transition from the mucous membrane to the skin is gradual, yet reminding one of the appearance presented at the junction of the esophagus with the cardiac end of the stomach.

External to the anus, and at a distance of about one centimeter from it, are numerous highly developed sweat-glands, the circumanal glands, which are almost as large as the axillary glands.

5. BLOOD, LYMPH, AND NERVE SUPPLY OF THE INTESTINE.

In general, the following holds true with regard to the bloodvessels of the intestinal tract (further details will be discussed in dealing with the vessels of the various regions of the intestine): The arteries enter along the line of the mesenteric attachment and penetrate the longitudinal muscular layer. Between the two muscular layers branches are given off which form an intermuscular plexus, from which, in turn, smaller branches pass out to supply the muscles themselves. The arterial trunks penetrate the circular muscular layer and form an extensive network of larger vessels in the deeper layer of the submucosa. This is known as Heller's plexus (F. P. Mall). From this, radiating branches are

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Fig. 212.-Section through fundus of cat's stomach. The blood-vessels are

injected; X 60.

given off which supply the muscularis mucosa, forming under the latter a close network of finer vessels. This plexus, together with that of Heller, gives rise to vessels which penetrate the muscularis mucosa and break up into capillaries in the mucous membrane. The veins of the mucous membrane form beneath the muscularis mucosa a plexus with small meshes, giving off many radiating branches; these in turn unite to form an extensive network of coarser vessels. Veins extend from the latter and unite to form larger trunks, which then lie side by side with the arteries. According to F. P. Mall, delicate retia mirabilia occur here and there in the venous network in the submucosa of the intestine of the dog.

In the esophagus the arteries end in a capillary network situated

in the mucosa and extending into the connective-tissue papillæ of the mucosa.

The vessels of the stomach are arranged in plexuses in the muscular coat, submucosa, and beneath the muscularis mucosæ, as previously described. From the plexus beneath the muscularis mucosa, small branches are given off which pass through this layer and in the mucosa form a capillary network, consisting of relatively small capillaries, which surround the gastric glands, this plexus being particularly well developed in the region around the body and neck of the glands, where the parietal cells are most numerous. The capillaries of this network are continuous with capillaries of a much larger size, forming a network surrounding the gastric crypts and situated immediately under the epithelium lining the mucosa of the stomach. The blood is collected from this capillary plexus by small veins which pass nearly perpendicularly through the mucosa, forming a plexus above the muscularis mucosa, from which small veins pass through the muscularis mucose to the venous plexus in the sub

mucosa.

The blood-vessels of the mucosa of the small intestine may be divided into (1) the arteries of the villi and (2) the arteries of the intestinal glands. The former arise principally from the deep arterial network in the submucosa, then penetrate the muscularis mucosæ and give off branches at acute angles which continue without further branching into the summits of the villi. Within the villi themselves the arteries lie in the axes. The broader villi may contain two arteries. The circular muscle-fibers of the arteries gradually disappear inside of the villi (dog), and at the summit of the latter the vessels break up into a large number of capillaries. These form a dense network extending beneath the basement membrane and into its marginal layer. These networks give rise to venous capillaries which unite to form small vessels and finally end in two or more larger veins inside of the villi. These latter are connected with the venous network in the mucosa.

The glandular arteries, derived principally from the superficial network of the submucosa, also pass through the muscularis mucosa and break up internally into capillary nets which encircle the intestinal glands; these give rise to small veins which empty into the venous plexus of the mucosa. The veins of the plexus in the mucosa unite to form larger branches, which connect with the plexus in the submucosa (compare Fig. 213). In the dog these trunks inside of the muscularis mucosæ are encircled by bundles of muscle-fibers (sphincters, F. P. Mall). The capillaries of the solitary lymph-nodules do not always penetrate into the centers of the latter, but often leave a central nonvascular area.

The blood-vessels of the mucosa of the large intestine are, in their distribution, similar to the glandular vessels of the small intestine and stomach.

The lymph-vessels begin in the mucosa near the epithelium, pass

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