88 DIGESTION. 



in digegtion. In the first place, it facilitates the motions 

 of mastication, by moistening the food and lubricating the 

 various organs of the mouth. Secondly, it prepares the 

 way for other digestive acts: by the action of the teeth, 

 the saliva is forced into the solid food, softens the harder 

 substances, and assists in converting the whole morsel into 

 a semi-solid, pulpy mass, that can be easily swallowed, and 

 readily permeated by other digestive fluids. The saliva 

 also, by dissolving certain substances, as sugar and salt, 

 develops the peculiar taste of each ; whereas, if the tongue 

 be dry and coated, they are tasteless. Hence, if substances 

 are insoluble, they are devoid of taste. 



17. Finally, the saliva has the property of acting chemi- 

 cally upon the food. As we have before stated (Chap. IV.), 

 starch, as starch, cannot enter the tissues of the body ; but, 

 in order to become nutriment, must first be changed to 

 grape sugar. This change is, in part, effected by the saliva, 

 and takes place almost instantly, whenever it comes in con- 

 tact with cooked starch. This important function is due 

 to an organic ingredient of the saliva called ptyalin. 

 This substance has been extracted from the saliva by 

 the chemist, and has been found, by experiment, to 

 convert into sugar two thousand times its own weight of 

 starch. 



18. Importance of Mastication and Insaliva- 

 tion. Each of these processes complements the other, 

 and makes the entire work available; for, by their joint 

 action, they prepare the food in the best possible manner 

 for further digestive changes. The knowledge of these 

 preliminary functions will appear the more important, 

 when we reflect that they are the only ones which we can 

 regulate by the will. For, as soon as the act of swallowing 

 begins, the food not only passes out of sight, but beyond 



IT. Its final importance ? Starch ? How effected ? Ptyalin ? 

 18. Each of the processes? Why is a knowledge of the digestive function 

 important ? How shown ? 



