CHAPTER XI 

 ABSORPTION, STORAGE, AND ASSIMILATION 



THE dissolved nutrients, to reach the cells, must be trans- 

 ferred from the alimentary canal to the blood stream. 

 This process is known as absorption. In general, absorption 

 means the penetration of a liquid into the pores of a solid, 

 and takes place according to the simple laws of molecular 

 movements. The absorption of food is, however, not a 

 simple process, and the passage takes place through an 

 active (living) membrane. Another difference is that cer- 

 tain foods undergo chemical change while being absorbed. 



Small Intestine as an Organ of Absorption. While ab- 

 sorption may occur to a greater or less extent along the 

 entire length of the alimentary canal, most of it takes 

 place at the small intestine. Its "great length, its small 

 diameter, and its numerous blood vessels all adapt the 

 small intestine to the work of absorption. The transverse 

 folds in the mucous membrane, by retarding the food in 

 its passage and by increasing the absorbing surface, also 

 aid in the process. But of greatest importance are the 

 minute elevations that cover the surface of the mucous 

 membrane, known as 



The Villi. Each single elevation, or.villus, has a length 

 of about one fiftieth of an inch and a diameter about half 

 as great (A, Fig. 76), and contains the following essential 

 parts : 



i. An outer layer of epithelial cells, resting upon a con- 

 nective tissue support. 



