140 MANUAL OF PHYSIOLOGY. 



Our food is generally composed of materials some of which 

 are soluble in water, and some are not. 



While in the mouth the saliva dissolves a great quantity of the 

 more readily soluble materials, such as sugar and salt, which may 

 be either mingled with the insoluble substances, and swallowed 

 together with the bolus, or swallowed separately in a fluid form. 

 Solution, then, is an important item in mouth-digestion. 



In many carnivorous animals the use of the mouth-fluid is 

 chiefly mechanical, dissolving some insignificant part of the food, 

 and aiding mastication and deglutition. In man, however, and 

 other animals that make use of much vegetable food, it has a 

 chemical function, and acts on the insoluble starch, converting it 

 into soluble sugar. 



The active principle which brings about this change is Ptyalin. 

 This is one of a series of ferments which exist in the body, and to 

 which most of the chemical changes in digestion are due. Although 

 each possesses certain peculiarities, yet as a group they may be 

 described as effecting by their presence various alterations in the 

 substances on which they act, while they themselves do not undergo 

 any perceptible change, and a small quantity will do as much 

 work as a larger quantity if allowed a proportionately longer time. 



Ptyalin acts exclusively on starch, and hence is spoken of as an 

 amylolytic ferment, its action consisting in causing the starch to 

 unite chemically with one molecule of water thus: 



Starch. Grape-sugar. 



During this process, which takes at the least a few minutes to 

 complete, various stages can be detected : first, two substances 

 are formed which together are commonly spoken of as dextrin ; 

 one, erythro-dextrin, which gives a red color with iodine, and 

 easily passes into soluble sugar ; and the other, achroo-dextrin, 

 gives no color with iodine, and is with difficulty converted into 

 sugar. As it gives no color with the ordinary test, its presence is 

 often overlooked. 



The sugar thus formed has been called Ptyalose, which, how- 

 ever, can be converted into ordinary grape-sugar (glucose) by the 

 action of sulphuric acid. Some say the product is all maltose. 



