82 THE VERTEBRATE ANIMAL: METABOLISM 



changes take place in the mouth. However, in man, the sight, the 

 odor, or even the thought of food induces the flow into the mouth of 

 saliva, a digestive juice secreted by the sahvary glands. As the 

 food is chewed it is mixed with the saliva, which is almost neutral or 

 slightly alkaline. This results in the softening of the food, which 

 aids swallowing, and also in the initiation of the digestive changes. 

 Saliva is largely water and mucin, but contains a digestive enzyme 

 known as ptyalin, in the presence of which starchy foods are par- 

 tially digested. Starch, if it has been cooked, enters the mouth in 

 the soluble form; if not, it is changed to that form by the ptyalin. 

 This soluble starch in the presence of ptyalin reacts with water to 

 form erythrodextrine and maltose, one of the compound sugars. 

 Certain investigators believe the reactions to occur as follows: 



hot water, dilute acids 



(C6Hio05)n ;, ^ (Cl2H2oOlo)lO 



starch or any starch-splittmg enzyme goiubie starch 



ptyalin 



(Cl2H2oOio)lO+H20 >(Ci2H2oOlo)9+(Cl2H220il) 



soluble starch erythrodextrine maltose 



This reaction, which is a splitting of the starch and dextrine 

 molecules by the addition of water, continues until the action of the 

 ptyalin is halted by the acidity of the juices in the stomach. The 

 food mass, softened by the water and mucin of the saliva, mixed 

 by chewing, and with its starch components partially digested, is 

 carried down the esophagus by the muscular movements known as 

 swallowing. No digestive changes occur in the esophagus; it is 

 merely a passageway. 



The digestive juice of the stomach is known as gastric juice 

 and is thoroughly mixed with the food mass by muscular move- 

 ments of the stomach wall. Because of the contraction of the 

 pyloric sphincter, a circular band of muscle located between the 

 stomach and the small intestine, the materials are retained in the 

 stomach for a time. Gastric juice is secreted by the gastric glands 

 in the lining of the stomach, and is strongly acidic because of the 

 presence of hydrochloric acid. It contains three enzymes, pepsin, 

 rennin, and lipase. Pepsin, the action of which is dependent 

 on the presence of the hydrochloric acid, starts the digestion of 

 some of the protein foods and brings about their distintegration into 

 proteoses and peptones. Rennin, which is present chiefly in the 



