ENZYMES AND THEIR WORK 



149 



Stomach after the change from an alkaHne surrounding in the 

 mouth to an acid surrounding in the stomach. Enzymes seem to 

 be able to work indefinitely, provided the surroundings are favora- 

 ble. A small amount of digestive enzyme, if it had long enough 

 to work, could therefore digest a large amount of food. 



Salivary Glands. — We are all familiar with the substance 

 called sali'va which acts as a lubricant in the mouth. Saliva is 

 manufactured in the cells of three pairs of glands which empty 

 into the mouth, and which are called, according to their position, 

 the parot'id (beside the ear), the submaxillary (under the jawbone), 

 and the sublin'gual (under the tongue). 



Digestion of Starch. — If we collect some saliva in a test tube, 

 add to it a little starch paste, place the tube containing the mixture 



Vomer 



Eusf-achian tube 

 ■ Esophagus 



Oloffis 

 Tongue 



Mouth of a frog and mouth of a man. Notice the points of likeness 

 and the points of difference. 



for twenty minutes in tepid water, and then test with Fehling's 

 solution, we find grape sugar present. Careful tests of the starch 

 paste and of the saliva, made separately, show no grape sugar in 

 either. 



If another test is made for grape sugar, in a test tube containing 

 starch paste and saliva as above, and also a few drops of a weak acid, 

 it will be found that no starch has been changed to grape sugar. 

 You will remember that starch in the growing corn grain was 

 changed to grape sugar by an enzyme called diastase. In saliva a 



H. NEW CIV. BIOL. — 11 



