100 



HUMAN BIOLOGY 



This movement, while favorable to digestion, diminishes absorp- 

 tion, because th6 liquefied food does not remain long enough in one 

 place to be absorbed by the blood. 



140. Absorption in the small intestine. — We, therefore, 

 find that most of our food passes through the pylorus before 

 it is absorbed. In the structure of the small intestine, how- 

 ever, we seem to find every possible provision for gathering 

 up the nutrients. In the first place, the lining of this tube at 

 intervals is elevated to form ridges that run two thirds "of 

 the way around the interior wall, and some of them project 

 about a third of an inch into the cavity of the intestines 

 (Fig. 2). Like little dams, they delay the onward flow of 

 the food, and they also increase considerably the large surface 

 for absorption. 



The absorbing surface is multiplied still further by the 

 viUi. If one were to examirie with a hand lens the mucous 

 lining of the small intestine, one would 

 see that the ridges and the depressions 

 are covered with tiny, hairlike pro- 

 cesses that give a velvety appearance 

 to the surface. Each of these minute 

 elevations is called a villus (Latin, villics 

 = a tuft of hair). The villi are ex- 

 ceedingly numerous in the small intes- 

 tine of man, the total number being 

 estimated at four millions. The ab- 

 sorbent action of the villi may be com- 

 pared with the absorption that takes place through the 

 walls of the root hairs of plants. In structure, however, 

 a villus is much more complicated than is a root hair. 



Each villus (Fig. 32) when highly magnified, is found to 

 contain a network of minute blood vessels, and since they are 

 covered only by a thin layer of cells on the outside of the 



Fig. 32.— Two villi, highly 

 magnified. 



