THE DIGESTION OF FOOD 97 



molar may also occasion an animal much discomfort and 

 cause imperfect mastication. 



133. The saliva. — ^During mastication there is poured 

 into the mouth a liquid called the saliva, which has two 

 important functions: (1) It moistens the food, and (2) 

 with several species of animals it causes a chemical 

 change in certain of the constituents of the food. 



134. Origin of saliva. — The saliva has its origin in 

 several secretory-glands that are adjacent to the mouth 

 cavity, and from these this liquid is poured into the 

 mouth through ducts that open in the cheek and under 

 the tongue (Fig. 1). The chief of these glands are located in 

 the side of the face, below and somewhat back of the jaws 

 and beneath the tongue, and are called respectively the 

 parotid, the submaxillary, and the sublingual. Other 

 glands of this character are scattered in the cheeks and 

 at the base of the tongue. The proportions of these glands 

 in the several species of farm animals are as follows: 



135. Properties and oflSce of saliva. — ^The saliva is a 

 transparent and somewhat slimy liquid, and contains 

 generally not less than ninety-nine parts in one himdred 

 of water, and one part or less of solid matter. It is alka- 

 hne in reaction, because of the presence of compounds 

 of the alkalies. One important organic compound present 

 is mucin. The specific chemical effect exerted by this 

 liquid on the food constituents may be illustrated by sub- 

 jecting starch to its action. When this is done, the starch 

 gradually disappears as such and is replaced by dextrin 



