136 



BIOLOGY AND HUMAN LIFE 



115. Absorption. Very small outgrowths project into the 

 cavity of the small intestine, so that the surface exposed to con- 

 tact with the food mixture is increased several hundred times. 

 Each of these tiny projections, called a villus (plural, villi) 

 (see Fig, 77), acts as a special absorbing and transforming or- 

 gan. The mixture in the intestine now consists of (i) many 



Esophagus 



Prove 11 

 ulti's 



Gizzard 



■ v. - ^ I Gnll 

 jA^- WA\ / bladder 



Liver 



Large Intestine 

 and Rectum 



Fig. 78. Digestive system in fish and in bird 



The main features of the digestive system are alike in all backboned animals. In the 

 birds the gullet has a curious pouch, the crop, in which food may be retained indefi- 

 nitely and later either swallowed to the stomach or regurgitated through the mouth. 

 The glandular portion of the stomach, or proventriculus, is distinct from the grinding 



part, or gizzard 



crystalloids in solution, (2) many colloids in the process of being 

 converted into crystalloids, and (3) solid substances that are not 

 capable of changing under conditions that exist in the gut. 



When the dinner that you have eaten reaches the end of the 

 small intestine, most of its carbohydrates, proteins, and fats 

 have been absorbed by the villi and passed into the blood and 



