104 THE HUMAN MECHANISM 



perhaps here that we have the strongest argument against 

 hasty eating. 



18. Mastication. Digestion in the mouth prepares for 

 digestion in the stomach, in the second place, by the com- 

 minution, or grinding down, of the food in the act of chew- 

 ing. When this is properly done the larger food masses are 

 broken up into smaller ones, so that the whole is made more 

 readily accessible to the subsequent action of digestive secre- 

 tions. The small intestine has almost no means of accom- 

 plishing this subdivision of the food ; the stomach can do it 

 for some foods easily, for others with difficulty, while against 

 others it is virtually powerless. Only in the mouth can all 

 foods be thoroughly comminuted. For this purpose it is 

 necessary to keep the teeth sound. 1 



19. Chemical action of saliva. Digestion in the mouth 

 presents a feature which is characteristic of all the digestive 

 processes ; namely, a combination of the mechanical action of 

 some form of muscular movement with the physical and 

 chemical action of some digestive juice. The muscular act 

 of chewing and the secretion of saliva, which moistens and 

 acts chemically upon the food, cooperate to reduce the food 

 to smaller particles and to change part of it into other sub- 

 stances. Neither mastication nor insalivation, acting alone, 

 would be as effective as are both when acting together. 

 We shall see the same thing more strikingly illustrated in 

 our studies of gastric and intestinal digestion. 



The chemical action of saliva is much less important than 

 that of other digestive juices, but it is typical of the charac- 

 ter of all of them, so that it is profitable to consider it at 

 some length. Upon proteins and fats saliva has no action 

 whatever, but upon starch it exerts a striking and readily 

 demonstrable influence. To demonstrate the effect in ques- 

 tion some starch paste should be prepared. This is not a 



1 The structure and care of the teeth will be described in Part II, 

 Chap. XXIII. 



