DIGESTION AND ABSORPTION 



361 



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. 



Problem XLVII. A study of where and how digested foods 

 pass into the blood. {Laboratory Manual, Prob. XL VII.} 



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, ac- 

 tually 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 intes- 

 tine in man is a slender tube 

 nearly twenty feet in length 

 and about one inch in di- 

 ameter. Its walls contain 

 muscles which, by a series of 

 slow waves of contraction, 

 force the fluid food gradually 

 toward the posterior end of 

 the tube. The movements of 

 the muscles of the coat are 

 of very great importance in 

 the process of absorption, and 

 these movements are caused 

 to a great extent (as is the 

 secretion of the various glands 

 of the food tube) by the me- 

 chanical stimulus of tfre food 

 within the food tube. If the 

 chief function of the small 

 intestine is that of absorp- 

 tion, we must look for adap- 

 tations which increase the 

 absorbing surface of the tube. 

 This is gained in part by the 



Diagram of a bit of the wall of the small intestine, 

 greatly magnified, a, mouths of intestinal 

 glands; b, villus cut lengthwise to show blood 

 vessels and lacteal (in center) ; e, lacteal sending 

 branches to other villi; i, intestinal glands; m, 

 artery ; , vein; I, t, muscular coats of intestine 

 wall. 



