ALIMENTATION AND DIGESTION 107 



of the salivary juice is typical of the action of other of the 

 digestive juices and of enzyme action in general. All the 

 other juices of the alimentary canal, with the single excep- 

 tion of the bile, contain enzymes, and it will greatly help 

 our understanding of the digestive action of these enzymes 

 if that of the salivary enzyme be first mastered. 



Digestion in the mouth, then, consists first, of a mechanical 

 process of chewing, by which food is crushed or comminuted; 

 second, of a physical process of moistening, by which dry 

 foods are prepared for the act of swallowing; and third, of 

 a chemical process, the chief part of which is the conversion 

 of starch into sugar by enzyme action. In addition to this 

 the stimulation of the sense of taste reflexly starts the 

 secretion of the gastric juice, which now becomes the main 

 chemical agent in carrying on the work of digestion. To 

 the consideration of the digestive processes in the stomach 

 we may now devote our attention. 



C. DIGESTION IN THE STOMACH 



According to popular ideas the stomach is the chief organ 

 of digestion; in fact, however, it is an organ in which the 

 food which has been swallowed is temporarily stored while 

 undergoing a preliminary preparation for the more impor- 

 tant changes which are to take place in the intestine. In this 

 preparatory process, to be sure, some of the food is inciden- 

 tally changed into those forms in which it passes into the 

 blood, but this action is incidental and subordinate to the 

 main function. 



20. Form and structure of the stomach. The stomach is a 

 large pouch into which open two tubes the oesophagus 

 (gullet) toward the left side and the intestine on the right 

 (see Fig. 54). The two regions into which these tubes open 

 are different in structure and are known as the cardiac (left) 

 and pyloric (right) portions of the stomach; the cardiac 



