362 



DIGESTION AND ABSORPTION 



inner surface of the tube being thrown into transverse folds which not 

 only retard the rapidity with which food passes down the intestine, but 

 also give more absorbing surface. But far more important for absorp- 

 tion are millions of little projections which cover the inner surface of 

 the small intestine. 



The Villi. So numerous are these projections that the whole 

 surface presents a velvety appearance. Collectively, these struc- 

 tures are called the villi (singular villus). They form the chief 

 organs of absorption in the intestine, several thousand being 

 distributed over every square inch of surface. By means of the 

 folds and villi the small intestine is estimated to have an absorb- 

 ing surface equal to twice that of the surface of the body. Between 

 the villi are found the openings of many small tubelike glands, 

 the intestinal glands. These glands manufacture a digestive fluid, 

 strongly alkaline, which aids in diges ing fats, and acts some- 

 what like the pancreatic fluid. 



Internal Structure of a Villus. The internal structure of a villus 

 is best seen in a longitudinal section. We find the outer wall made 

 up of a thin layer of cells, the epithelial layer. It is the duty of these 

 juguiar^vein ce \\ s ^ o absorb the semifluid food from within the 

 intestine. Underneath these cells lies a network 

 of very tiny blood vessels, while inside of these, 

 occupying the core of the villus, are found spaces 

 which, because of their white appearance after 

 absorption of fats, have been called lacteals. 



Absorption of Foods. Let us now attempt 

 to find out exactly how foods are passed from 

 the intestines into the blood. Food substances 

 in solution may be soaked up as a sponge would 

 take up water, or they may pass by osmosis into 

 the cells lining the villus. These cells are 

 alive, and therefore have the power of select- 

 ing certain substances and rejecting others. 

 Once within the villus, the sugars and digested 

 proteids pass through tiny blood vessels into 

 the larger vessels comprising the portal circu- 

 cuiatipn. lation. These pass through the liver, where, 



