The Digestion of Food 103 



property of being able to change starch into a form of 

 cane sugar known as maltose. 



Thus, while the food is being chewed, another process 

 is going on by which starch is changed into sugar. The 

 saliva moistens the food, thus fitting it for swallowing, and 

 it also aids in speech by keeping the mouth moist. 



The activity of the salivary glands is largely regulated 

 by their abundant supply of nerves. Thus, the saliva may 

 flow into the mouth, even at the sight, smell, or thought 

 of food. This is popularly known as " making the mouth 

 water." The flow of saliva may be checked by nervous 

 influences, as sudden terror and undue anxiety. 



Experiment 41. To show the action of saliva on starch. Saliva 

 for experiment may be obtained by chewing a piece of India rubber 

 and collecting the saliva in a test tube. Observe that it is colorless 

 and either transparent or translucent, and when poured from one 

 vessel to another is glairy and more or less adhesive. Its reaction is 

 usually alkaline to litmus paper. 



Experiment 42. Make a thin paste from pure starch or arrowroot 

 and boil it for a minute. Dilute a little of the saliva with five volumes 

 of water. Label three test tubes A, B, and C. In A, place starch 

 paste ; in B, saliva ; and in C, one volume of saliva and three volumes 

 of starch paste. Place them for ten minutes in a water bath at about 

 104 Fahr., or in any place where the temperature is about that of 

 the body. 



Test portions of all three for sugar, by means of Fehling's solution 

 or tablets. 1 A and B give no evidence of sugar. C gives a yellow or 

 red deposit of cuprous oxide, thereby proving that starch is thus con- 

 verted into sugar by the saliva. This is done by the ferment ptyalin 

 contained in saliva. 



1 Tablets and other material for Fehling's test and additional tests for 

 sugar can be purchased or ordered at any drug store. 



The practical details of these, and other tests which assume some knowl- 

 edge of chemistry, may be taught orally by the teacher or learned from 

 some manual on the subject. 



