doubt that this ferment is elaborated by these cells. The process consists either in a direct change of the cellular protoplasm into the ferment, or in a preliminary stage before its final transformation into the finished ferment. It is assumed that the parietal cells secrete the acid of the gastric juice, although, in spite of all efforts, it has not yet been definitely proved that these cells possess an acid reaction.

The vascular and lymph-vessels of the stomach, and also its nerve supply, will be considered in a general discussion of these structures pertaining to the entire intestinal canal.

3. THE SMALL INTESTINE.

The mucous membrane of the small intestine is characterized by the presence of villi. These are more or less elongated elevations of the mucous membrane projecting into the lumen of the intestine. They greatly increase the surface of this portion of the intestine and are actively concerned in the absorption of its contents. The mucous membrane also forms permanent folds in both the duodenum and the remainder of the small intestine, the valvulæ conniventes (Kerkring). Upon these the villi rest, and, indeed, it is probable that the very existence of the plicæ is due to the blending of the basilar ends of the villi. The latter are leaf-shaped in the duodenum, columnar in the jejunum, and club-shaped in the ileum.

The epithelium of the intestinal mucous membrane covers the villi in a continuous layer, and penetrates into the mucosa to form the glands. Its structure is essentially the same in all regions of the small intestine, the cells being of the high columnar variety with free surfaces covered by wide, striated cuticular borders. The basilar portions of these cuticular borders are nearly always homogeneous, and upon vertical section give the appearance of a fine line. The cuticular borders of adjacent cells blend with each other, forming a continuous membrane, large areas of which may be detached from the villi (cuticula). The body of the cell consists of a granular, reticular, or striated protoplasm, containing, especially at the beginning of mucous secretion, clear vacuoles of different sizesmucus. If the cuticular margin be intact, a confluence of the vacuoles may form a large drop of mucus. The nuclei lie usually in the basilar third of the cells, and only where they show mitoses, as for instance in the tubular intestinal glands, do they pass to the free ends of the cells. The basal ends of the epithelial cells in the small intestine are also seen to be pointed, and the probability is that they rest upon the basement membrane. The question has, however, not been fully settled.

The epithelial cells undergo a special metamorphosis, after which, by an increased production of mucus, they change into goblet cells. From recent investigations it would seem that any epithelial cell, whether it be situated upon the upper surface of a

villus or deep down in one of the tubules of the intestinal glands, is capable of transformation into a goblet cell. The number of goblet cells is subject to great variation; they are found singly in small numbers, or are very numerous, according to the stage of digestion and quantity of food in the intestine. The manner in which an ordinary epithelial cell changes into a goblet cell is very easily explained if the mechanical action on the cell caused by an accumulation of secretion be taken into consideration. As the secretion increases in quantity the upper portion of the cell becomes distended,

and the remains of the protoplasm, together with the nucleus, are pushed toward the narrow base of the cell; the cuticular zone is stretched, bulges into the lumen of the intestine, and is finally perforated, and perhaps even thrown off. In this way the cell loses its mucous secretion, collapses, and then appears as a thin, almost rod - like structure, with a long nucleus. It is the generally accepted theory that such an empty goblet cell may again assume the shape of an ordinary epithelial cell and repeat the process just described.

Leucocytes are sometimes found within the epithelial cells, but more usually between them, and according to Stöhr (84, 89, 94), when seen in these positions, are in the act of migrating

into the lumen of the intestine. That some of these cells actually pass into the lumen is probably true; but as yet no leucocytes have ever been observed in the cuticula itself, and neither is the number of cells found in the lumen of the intestine proportionate to the leucocytes present in the epithelium. Since many are seen in the epithelium undergoing karyokinetic division, it is more probable that a part of them actually wander into the epithelium for the purpose of

division (chemotaxis ?), only to return to the mucosa after the completion of the process (compare p. 54).

Into the spaces between the villi open numerous tubular glands. These are seldom branched, and are known as Lieberkühn's glands, or crypts. Their length varies from 320 μ to 450 μ. They are regularly arranged in a continuous row, and often have an ampullalike widening of their lumina extending almost to the muscularis mucosa, but never quite reaching it. They are uniformly distributed not only throughout the small intestine, but also throughout the large intestine and rectum. Their cells are somewhat lower than those of the villi, and pos

sess a very narrow cuticular zone. The cells are, however, conical,-a condition probably due to the curvature of the glandular wall, -the base of each cone lying toward the basement. membrane, the apex toward the lumen of the gland-a condition opposite to that found in the villi. Numerous goblet cells are also present. They vary only slightly in shape during mucous secretion, and never, as in the villi, assume the form of goblets with distinct pedicles. Mitoses are always seen in the intestinal glands, especially in cells which do not contain mucin. They are readily distinguished, since the nuclei in process of division, as we have seen, lie outside of the row formed by the remaining nuclei. The plane of division in these cells lies horizontal to the long axis of the gland, so that an increase in the number of cells results in an increase in the area of the glandular Mitoses are very rarely observed in the epithelium covering the villi. If, therefore, any cells be destroyed on the surface of the villi, it must be assumed that the loss is replaced by new elements pushed up from the glands below (Bizzozero, 89, 92, I).

b

с

Fig. 207.-Longitudinal section through summit of villus from human small intestine; X900 (Flemming's solution): At a is the tissue of the villus axis; b, epithelial cells; c, goblet cell; d, cuticular zone.

The entire duodenum, as well as that part of the pylorus in the immediate vicinity of the pyloric valve, is characterized by the presence of glands of a second type. In the duodenum these are

seen intermingled with the glands of Lieberkühn, and in the pylorus with the pyloric glands. These glands, Brunner's glands, have a diameter of from 0.5 to 1 mm., and are compound, branched tubular glands, with tubules provided with alveoli, especially along their lower portions. The bodies of the glands are situated principally in the submucosa, although a part may be in the mucosa. In the stomach they open into the gastric crypts, in the intestine either independently between the villi, or into the glands of Lieberkühn. Here the glandular cells are in general similar to those of the pyloric glands, although, as a rule, somewhat smaller than the latter. Just as the duodenal glands extend into the stomach, so also the pyloric glands of the latter are found in the upper portion of the duodenum. Besides short villi, there are also present in the duodenum depressions of the mucous membrane analogous to the gastric crypts. The glands of Lieberkühn begin at a certain distance from the pylorus ; at first they are short, and do not attain their customary length until a point is reached at which the gastric glands extending into the duodenum finally disappear (vid. Fig. 208). It is therefore obvious that a transition zone exists between pylorus and duodenum, and that a distinct boundary line can not be drawn between the two, at least so far as the mucous membrane is concerned. The duodenal glands, as their name would indicate, are present only in the duodenum. Between the jejunum and ileum there is no distinct boundary, not even when microscopically examined. The differences are mostly of a quantitative nature; in the jejunum the valvulæ conniventes are more numerous than in the ileum, and the villi more slender and closer together. The glands of Lieberkühn also appear to be more numerous in the jejunum.

The mucosa of the small intestine consists of reticular adenoid tissue containing lymph-cells. It supports the glands and extends into the villi whose axes it forms. The mucosa is separated from the glands, from the epithelium of the villi, as well as from that of the remaining surface of the intestine by a peculiar basement membrane. The latter somewhat complicates a proper histologic analysis, and as a consequence opinions regarding its structure and significance vary considerably. By some it has been described as a homogeneous, hyaline, and exceedingly fine membrane containing nuclei, by others as a lamella consisting entirely of endothelial cells. At all events, there are certainly nuclei in the basement membrane. Beneath the basement membrane is a marginal layer of a more fibrillar character. This is closely associated with the mucosa, and may be regarded as a differentiation of the latter. Toward the muscularis mucosæ the mucosa is terminated by a reticulated elastic membrane (F. P. Mall, in the dog), containing openings for the entrance of vessels, nerves, and muscle-fibers.

The muscularis mucosa consists of two layers of unstriped muscular fibers arranged in a manner similar to that in the external muscular tunic-i. e., having an inner circular and an outer longi

tudinal layer. The fibers are frequently gathered into bundles, which appear to be separated from each other by connective tissue. From both layers, but more especially from the inner, muscle-fibers are given off at right angles, which enter the tunica propria and pass between the glands of Lieberkühn, and even into the villi. In the latter these muscle-fibers are arranged in bundles, and lie

Fig. 208.-Section through the junction of the human pylorus and duodenum; X about 15: At a the pyloric glands extend into the duodenum.

near their axes around the lacteal vessels.

The contraction of these

fibers causes a contraction of the entire villus.

Lymph-nodules are distributed throughout the mucosa of the small intestine, occurring either singly, as solitary follicles, or aggregated, as Peyer's patches. At the points where they occur,