INTESTINE. 185 



E. INTESTINE. 



In the intestine we can distinguish the same number of 

 coats as in the stomach (Fig. 140), namely, mucosa, submu- 

 cosa, muscularis, and serosa. 



The surface of the mucosa is nowhere in the intestine 

 smooth. It possesses two kinds of inequalities, whose function is 

 to increase the area of the surface. There are ring-like folds of 

 the whole mucous membrane, the so-called valvulce conniventes 

 (plicae conniventes Kerkringii), which are developed especially 

 in the upper part of the intestine. Besides these there are the 

 villi, which are folds simply of the epithelial layer and the 

 tunica propria, the muscularis mucosa continuing in a straight 

 line below them. These are found only in the small intestine 

 (Plate XVII.). They reach a height of 0.2-1 mm., and vary 

 considerably in form according to the region of the small intes- 

 tine in which they occur. In the duodenum they are leaf-like ; 

 in the jejunum and ileum cylindrical and somewhat thickened 

 at the end. They lie more closely together in the duodenum 

 than elsewhere. 



We find also, in the intestine, cavities in the form of simple 

 tubular glands (Lieberkuhrts glands) which enter the depths 

 of the tunica propria at the bases of the villi. These are longer 

 in the large, than in the small, intestine. 



The mucosa of the whole intestine consists of a single layer 



o / 



of epithelium, a tunica propria, and a muscularis mucosse. The 

 cells of the epithelial layer (Figs. 131 and 143) are cylindrical 

 with a finely granular protoplasm, often containing many kinds 

 of granular inclusions. The nucleus of each cell is oval and 

 lies usually in the lower half. The sides of the cell show no 

 definite cell membrane, while at its free surface it shows a char- 

 acteristic finely striated border. These are known as border 

 cells. The opposite end of the cell often runs to a point, and 

 is separated from the underlying tissues by a thin homogeneous 

 basal membrane. 



The epithelium of the glands is not essentially different 

 from that of the villi. The cells are somewhat lower and the 

 striated border is not so well marked. Among these epithelial 



