336 FOOD AND DIGESTION 



The quantity secreted in twenty-four hours varies greatly, but is at least 

 i liter. 



Function of Saliva. The purposes served by saliva are mechanical 

 and chemical. 



Mechanical. (i) It keeps the mouth in a due condition of moisture, 

 facilitating the movements of the tongue in speaking and in the mastication 

 of food. (2) It serves also in dissolving sapid substances, and renders them 

 capable of exciting the nerves of taste. (3) But the principal mechanical 

 purpose of the saliva is that, by mixing with the food during mastication, it 



FIG. 252. Showing the variation of the rate of secretion of saliva, second line from 

 the top, and variation of blood pressure, top line. At a, an injection of o . 2 mgr. pilocar- 

 pine. At b, 50 c.c. oxygenated blood was injected into the jugular vein. (Jonescu.) 



makes a soft pulpy mass such as may be easily swallowed. To this purpose 

 the saliva is adapted both by quantity and quality. For, speaking generally, 

 the quantity secreted during feeding is in direct proportion to the dryness 

 and hardness of the food. 



Chemical. The chemical action which the saliva exerts upon the food in 

 the mouth is to convert the starchy materials which it contains into soluble 

 starch and then into sugar. This power the saliva owes to the enzyme 

 ptyalin. Certain investigators have of late asserted that saliva contains 

 another enzyme, known as maltase, which has the power of splitting the di- 

 saccharides into monosaccharides, or maltose into dextrose. The action of 

 this ferment is certainly very limited. The conversion of the starch under 

 the influence of the ferment into sugar takes place in several stages, and in 

 order to understand it a knowledge of the structure and composition of 

 starch granules is necessary. A starch granule consists of two parts: an en- 



