308 DIGESTXON AND ABSORPTION 



oxidized ; then it is changed to sugar and carried off by the blood 

 to the tissue which requires it, and there used for this purpose. 

 Glycogen is also stored in the muscles, where it is oxidized to release 

 energy when the muscles are exercised. 



The Absorption of Digested Food into the Blood. — The object 

 of digestion is to change foods from an insoluble to a soluble form. 

 This has been seen in the study of the action of the various diges- 

 tive fluids in the body, each of which is seen to aid in dissolving 

 solid foods, changing them to a fluid, and, in case of the bile, 

 actually assisting them to pass through the wall of the intestine. 

 A small amount of digested food may be absorbed by the blood 

 in the blood vessels of the walls of the stomach. Most of the 

 absorption, however, takes place through the walls of the small 

 intestine. 



Structure of the Small Intestine. — The small intestine in man is a 

 slender tube nearly twenty feet in length and about one inch in diameter. 

 If the chief function of the small intestine is that of absorption, we must 

 look for adaptations which increase the absorbing surface of the tube. 

 This is gained in part by the 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 absorption are millions of Uttle 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 the intestinal glands. 



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 cells to absorb the semifluid food from within the 

 intestine. Underneath these cells lies a network of very tiny blood 



