126 DIGESTION 



less. Hence, if substances are insoluble, they are devoid of 

 taste. 



20. Finally, the saliva has the property of acting chemically 

 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 contact 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. (Read Notes 2 and 3.) 



2. The First Step of Digestion. " The digestive process begins in 

 the mouth ; among civilized people it begins in the plate, or even before. 

 Undoubtedly mastication is the natural method of mincing meat, and not 

 the least of its value lies in the faet that it takes time. A man who is 

 eating a tough, and therefore not very digestible chop, will be slow in 

 eating, if he is careful to masticate it well. There will be a long interval 

 between each mouthful, and the stomach will run no risk of being hastily 

 loaded. 



" Now, a hastily-loaded stomach is as bad almost as, or, rather, is the 

 same thing as, an overloaded stomach ; and there can be no doubt that 

 artificial mastication becomes a snare when it leads any one to introduce 

 a large quantity of finely-minced meat suddenly and rapidly into an 

 unprepared stomach, especially into the feeble stomach of an invalid, 

 under the idea that, because the meat is so nicely minced, and so very 

 tender, it can be no possible burden to that sorely-tried organ. Natural 

 mastication has, besides, another advantage over the artificial process, 

 which is perhaps not always recognized. Whenever food enters the 

 mouth, it gives rise to what is called a flow of saliva. This saliva is 

 secreted by certain glands, which pour into the mouth the fluid they 

 strain off from the blood, and which are excited or stimulated to action 

 by the presence of food in the mouth, as well as by other causes. Saliva 

 rapidly changes starch into sugar, and sugar is pre-eminently a soluble 

 body, passing with the greatest ease from the alimentary canal into the 

 blood." People's Magazine (London). 



3. Experiment. Saliva. The chemical action of saliva upon boiled 

 starch may be demonstrated by the following simple experiments : 



1. Prepare a thin solution of boiled starch ; three parts of starch to 

 one hundred parts of water ; boil for a few minutes. 



W. Its final importance ? Starch? How effected? Ptyaliu? 



