DIGESTION AND ABSORPTION 307 



of lime salts that collects on the teeth especially near the 

 gums. 



The teeth should be brushed with water after every meal 

 to remove every particle of food from the mouth. Once 

 or twice a week a good tooth powder like precipitate of 

 chalk should be used. This is not hard enough to injure 

 the enamel but removes the tarter. A dentist should examine 

 the teeth about once a year. 



Salivary glands. — The sahvary glands open into the 

 mouth. There are three pairs. These are named from 

 their location: parotid, lying in front of and below the ears; 

 submaxillary, lying beneath the lower jaw, and sublingual, 

 lying beneath the mucous membrane in the floor of the 

 mouth. The position and openings of these glands are 

 shown in fig. 156. 



Chemical action of saliva. — ^The salivary glands secrete 

 a digestive juice called saliva. This is composed mainly 

 of water and a certain enzyme called ptyalin. 



An enzyme is an organic substance which acts chemically 

 upon another substance so as to change its nature without 

 itself becoming changed. 



Ptyalin changes starch, an insoluble food, to sugar, a 

 soluble food, that is, it digests starch. 



The chewing process in the mouth thoroughly mixes the 

 saliva with the food so that the enzyme can reach the starch. 

 This is the first act of digestion. The mechanical action 

 of moistening the food and thus preparing it for swallowing 

 is quite as important as its chemical action. Food thus 

 moistened stimulates the sensation of taste. 



Experiment to Show Digestion of Starch. — Chew a piece of par- 

 affin; this will start the flow of saliva in the mouth. Collect the saliva 

 in a test tube. Test its chemical reaction with litmus paper. If 

 it changes blue litmus to red it is alkaline. If it changes red litmus 

 to blue it is acid. Add a little vinegar to the saliva and test 

 again. Account for the different result. Partly fill a test tube 

 with saliva, another with water, a third with saliva to which vinegar 



