300 GENERAL ZOOLOGY 



The hands are probably the best organs for seizing and 

 holding that any animal possesses. They are equally 

 effective for strangling a rabbit, holding a spear, or grasp- 

 ing a hoe, and the flat nails make it possible to pick up 

 minute objects. The instinct for survival is strong in 

 man and impels him to keep his body in training so that 

 he may not fail in the chase. His superior mentality gives 

 him a great advantage over other animals in .securing food 

 and he has been able to invent many means to insure a 

 continuous supply. 



Food, once in the hand, is placed in the mouth and goes 

 the usual course being chewed, swallowed, mixed, digested 

 and absorbed. In the mouth food is mixed with saliva 

 which contains the ferment ptyalin, capable of changing 

 starch to sugar. As soon as the chewed food enters the 

 stomach, gastric juice begins to pour out, but since the 

 food remains in the upper part of the stomach for a time, 

 ptyalin continues to act for about half an hour. Later it 

 is neutralized by the free hydrochloric acid in the stomach. 

 In addition to its digestive functions the acid acts as an 

 antiseptic, killing bacteria and rendering certain other 

 injurious substances innocuous. Gastric juice contains a 

 protein-digesting ferment (pepsin) which liquefies the food 

 by dissolving the nitrogenous substances. Most of the 

 mixing and churning is done near the muscular outlet of 

 the stomach and the liquefied food is gradually permitted 

 to pass the sphincter* muscle which guards this opening. 

 In the intestine muscular mixing movements continue and 

 secretions from two great digestive glands (liver, pancreas) 

 are mixed with the liquefied food. Pancreatic juice con- 

 tains three powerful digestive ferments: (1) trypsin, for 

 digesting proteins; (2) amylopsin, converting starches to 

 sugars; and (3) steapsin, which breaks up and emulsifies 

 fats. In the mouth the saliva has a slightly alkaline reac- 

 tion. In the stomach the food becomes acid; is slightly 

 alkaline or neutral after entering the intestine, but later 



* A sphincter is a ring-like mass of muscle which surrounds an opening. 



