476 APPLIED BIOLOGY 



saliva, (2) effect of acid, (3) effect of low temperature? Boiling 

 and low temperature have the same effect on all the secretions of the 

 stomach, intestine, and pancreas. 



(D) In order to show that saliva changes starch into sugar capable 

 of being absorbed (i.e., osmose through a membrane), take some 

 starch paste which has been acted upon (digested) by saliva for two 

 or three hours, place in a membrane bag or osmose-apparatus, allow 

 half-hour for osmose, then test water in tumbler for sugar. 



The above experiments simply prove that saliva digests 

 starch to a sugar; and that while starch does not osmose, 

 the sugar formed from starch does. Saliva, then, prepares 

 starch for absorption into the blood which flows in blood- 

 capillaries beneath the lining membrane (epithelium) of the 

 alimentary canal, especially abundant in the stomach and 

 intestines. The sugar derived from starch osmoses through 

 the epithelium into the blood, just as it is demonstrated by 

 the above experiments that it will osmose through the dead 

 membrane of the osmose-apparatus. 



It is evident that the above experiments do not prove 

 anything regarding the action of saliva on other nutrients. 

 If time permitted, we might try similar experiments with 

 proteins and fats, using these nutrients in place of starch, 

 and, of course, applying the appropriate tests ( 377). Such 

 experiments have been performed many times by physi- 

 ologists, and their conclusion is that saliva has no digestive 

 power for foods other than starch. 



No simple experiment will show that there is in saliva a 

 substance (ptyalin) which causes the digestion of starch, but 

 chemists have proved it to be present, and the cause of the 

 action of saliva. Moreover, it has been shown to be an 

 enzyme, because it acts by its presence, without entering 

 into the sugar formed, and a very small amount of it can 

 digest a large quantity of starch. 



The amount of starch food which will be digested in the 

 mouth-cavity depends upon the length of time it is held 

 there. Hence, prolonged mastication is advisable. It has 



