94 STUDIES IN PHYSIOLOGY 



slowly forced on toward the large intestine. This succession 

 of contractions and relaxations is called per-i-stal 'sis. 



Adaptations for Absorption. In the small intestine the 

 mucous membrane is developed to an extraordinary degree. 

 In the first place this lining is elevated into 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 



FIG. 34. Cross Section of Intestine of a Mouse. (Ridges are not present.) 



Magnified fifteen times. Photographed through the microscope. A large 

 number of villi project toward the interior of the intestines. The dark 

 lines show the blood vessels filled with a colored mixture. 



intestines. These crescent-shaped ridges are most numer- 

 ous near the stomach and gradually disappear in the region 

 near the large intestine (Fig. 30). Like little dams, they 

 delay the onward flow of the food, and they also increase 

 considerably the surface for absorption. The absorbing 

 surface is multiplied still further by the vil'li. 



The Villi. If one puts into water a piece of the small 

 intestine of a sheep, and examines with a hand lens the 

 mucous lining, one sees that the ridges and depressions are 



