1 84 



Physiology. 



while the folds in the lining of the stomach are temporary, 

 these are permanent. They increase the surface of the 

 lining and retard the passage of the food material, and so 

 aid the process of digestion and of absorption. 



Villuses. To increase, still further, the surface for 

 absorption, the mucous membrane of the small intestine is 



thickly covered with 

 little cylindrical pro- 

 jections, like the 

 "pile" on velvet. 

 Each of these pro- 

 jections is a villus. 



Villuses 



Intestinal Glands 



Structure of a Vil- 



Fig. 78. Mucous Membrane of Small Intestine, showing l ug A 



Intestinal Glands and Villuses. 



IS 



about a thirtieth of 

 an inch long. It is made up of four parts : (i) on the out- 

 side a layer of soft, moist, thin-walled cells; (2) plain 

 muscle fibers, running lengthwise; (3) a network of blood 



Lacteal with Valves 



Capillaries Muscles 



Fig. 79. Four Parts of a Villus. 



Epithelium 



capillaries; and (4), near the center, lymph capillaries, 

 called lacteals. Fig. 79 shows these four parts separate, 

 while Fig. 80 shows them combined, as they are in the 

 complete villus. 



