256 ELEMENTS OF GENERAL SCIENCE 



produced by special organs, known as salivary glands. There 

 are three groups of these. The largest are the parotid glands, 

 situated one below each ear, each having a duct through 

 which saliva enters the mouth just opposite the second pair of 

 the upper molar teeth. The other glands (submaxillary and 

 sublingual) are the smallest. They lie in the floor of the 

 mouth and discharge saliva through various openings into 

 the lower part of the mouth. It is evident that saliva may 

 be introduced into the mouth at several points, thus moisten- 

 ing the food from different sources. 



280. Mastication : use of the saliva. When food is intro- 

 duced into the mouth the amount of this secretion is increased, 

 and while chewing proceeds, the saliva becomes mixed with 

 the food. Very thorough mixing renders the food almost or 

 quite liquid before it is swallowed. Hasty chewing makes 

 swallowing difficult, and sometimes choking results from at- 

 tempting to swallow before the food is thoroughly mixed with 

 saliva. Such hasty swallowing often results in passing the 

 food into other parts of the digestive tract without enough 

 liquid in it. The sali va is equally important because of another 

 effect which it produces. A chemical known as ptyalin, an 

 enzyme which composes part of the saliva, affects some of the 

 carbohydrates, as starch and some of the sugars, in such a 

 way as to change them into grape sugar. Starch is not easily 

 absorbed, but the grape sugar made from the starch is readily 

 taken up by the walls of the digestive system. There is an- 

 other starch-digesting secretion that enters the food after it 

 leaves the stomach. The advantages of thorough chewing and 

 mixing the saliva with the food are apparent. An experiment 

 in eating carbohydrate food for a given length of time, on one 

 day by a very careful chewing and on another by swallowing 

 as soon as possible, will produce some first-hand data upon 

 the advantage of thoroughness in this matter. The food of 

 herbivorous animals, being coarser than that of carnivorous 

 animals, needs to be chewed longer. 



