2/4 THE DIGESTIVE ORGANS. 



while the nuclei return to the center of the cells. Since chemic 

 examination has shown that the amount of pepsin found in the gas- 

 tric mucous membrane increases with the enlargement of the chief 

 cells of the body of the fundus glands, and decreases with their 

 diminution in size, there can be hardly any doubt that this ferment 

 is elaborated by these cells. 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. The villi vary in size and shape in the 

 different mammals. In man, in the upper portion of the small in- 

 testine, they are distinctly leaf-shaped, being three to four times as 

 broad in one direction as they are in the other, often showing a 

 narrowing at their bases. This has been shown by reconstruction 

 of the muc'osa and a number of villi from the duodenal region of 

 a well-preserved human intestine. The villi are of columnar shape 

 in the jejunum, and club-shaped in the ileum. The muc.ous mem- 

 brane also forms permanent folds in both the duodenum and the 

 remainder of the small intestine, the valvulae conniventes (Kerk- 

 ring). Upon these the villi rest, and, indeed, it is probable that the 

 very existence of the plicae is due to the blending of the basilar 

 ends of the villi. 



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 homo- 

 geneous, and upon vertical section give the appearance of a fine line. 

 The cuticular borders of adjacent cells blend with each other, form- 

 ing a continuous membrane, large areas of which may be detached 

 from the villi (cuticula). The body of the cell consists of a fine 

 fibrillar' structure (spongioplasm) with the main threads parallel to 

 long axis of the cell. This is more distinct in the free portions of 

 the cell. In the interfibrillar substance are found fine granules. 

 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. 



