THE DIGESTIVE SYSTEM 



233 



surface. In ihe ileum, Ihe crescentic folds disappear, and villi become 

 smaller and more scattered. 



The four layers of tissue characteristic of the alimentary canal are 

 present in the small intestine. Throughout the entire length of the 

 intestine are numerous tubular mucus-secreting glands, perpendicular to 

 the surface of the intestine, the intestinal glands or crypts of Lieberkiihn. 

 Goblet-shaped cells distended with mucus are abundant in the walls of 

 these glands. The secretions of these glands are said to stimulate peristal- 

 sis of the intestine as well as lubricate its surface. (Figs. 218-219) 



r -MUCOUS EPITHELIUM 



"CRYPT 



-LYMPHOID NODULE 

 -_ MUSCULAR! S MUCOSAE 



*^r^4^<*;^t*:?:^s^-SUBMUC0SA 



■CIRCULAR MUSCLE 



i- LONGITUDINAL MUSCLE 



"^ "" "^ — SEROSA 

 Fig. 219. — A longitudinal section of the human colon. Xi5- 



Each villus is covered with a mucus epithelium containing numerous 

 goblet-cells, and each villus has a core of connective tissue filled with 

 capillaries and lymph vessels. A single lymphatic or lacteal occupies the 

 center of each villus, and a network of capillaries lies just below the base- 

 ment membrane of the mucous epithelium. Each villus is therefore a 

 mechanism admirably adapted for absorbing the digested food which 

 bathes it. Besides the peristaltic waves which pass along the intestine 

 squeezing the food backwards towards the large intestine, divisive or 

 churning movements are also carried on, bringing the digested food into 

 contact with the villi. 



