METABOLISM IN VERTEBRATES 



thought of food induces the flow of saliva, a digestive juice secreted by the 

 saHvary glands; the average daily output is about 1500 ml. As the food is 

 chewed, it is mixed with saliva which softens and lubricates the food. This 

 initiates the digestive changes and aids in swallowing. Although saliva is 

 largely water and mucin, it contains a digestive enzyme, a salivary amylase 

 known as ptyalin. This enzyme is most active in a neutral medium and is 

 responsible for the partial digestion of glycogen and starch. Starch, if it has 

 been cooked, enters the mouth in soluble form; if not, it is made soluble by the 

 ptyalin. Soluble starch in the presence of ptyalin reacts with water to form 

 dextrins and maltose, one of the compound sugars. The food mass, with its 

 complex carbohydrates 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. 



Once the food mass is in the stomach, salivary digestion may continue for 

 15 to 20 minutes until the acid of the stomach penetrates the mass. Digestion 

 of carbohydrate is resumed in the small intestine. 



Food is retained in the cavity of the stomach because of the contraction of 

 the pyloric sphincter. Muscular movements of the stomach wall thoroughly 

 mix the mass with the digestive juice of the stomach which is known as gastric 

 juice. This juice, secreted by the gastric glands of the stomach lining, is 

 strongly acidic because of the presence of hydrochloric acid. It contains 

 pepsin, an enzyme responsible for most of the digestive activity in the stomach. 

 Pepsin is secreted by the gland cells in an inactive form known as pepsinogen. 

 In the presence of hydrochloric acid, pepsinogen is converted to pepsin. For 

 its effective action this enzyme requires a strongly acid medium which is 

 normally present in the stomach. Pepsin breaks certain of the peptide 

 linkages of proteins, producing fragme"nts of considerable size called pro- 

 teoses and peptones. 



In the stomachs of young milk-feeding animals, including the human infant, 

 another enzyme, called rennin, is found in the gastric juice. The action of 

 this enzyme is to clot casein, the protein of milk, forming paracasein. Para- 

 casein combines with calcium to form an insoluble compound which remains 

 in the stomach for some time, permitting more prolonged digestion by pepsin. 

 Rennin does not occur in the gastric juice of adults. 



Degradation of proteins does not go to completion in the stomach. Once 

 the food is liquefied it is pushed through the pyloric sphincter, without regard 

 for the degree of protein digestion, into the small intestine. 



The muscular activities of the small intestine are of two types, known as 

 peristalsis and segmentation. In peristalsis a muscular contraction begins at 

 the upper end of the intestine and passes with wave-like effect toward the 

 lower end. This motion has a tendency to bring about the movement of the 

 food mass toward the large intestine. Segmentation, on the other hand, 

 consists of a series of contractions occurring close together and simultaneously 

 at different levels of the intestine. This results in a pinching of the food 

 mass into segments; and, since these segmentation contractions disappear and 



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