CHAPTER VII. 

 SALIVA AND SALIVARY DIGESTION. 



It has already been said that the saliva contains an enzyme 

 known as ptyalin, the function of which is to begin the diges- 

 tion of starchy foods. It remains now to look into the nature 

 of this process a little more closely, and to study the conditions 

 of this kind of fermentation. The saliva as secreted by the 

 three large pairs of glands of the mouth is a thin liquid with 

 slightly alkaline reaction. Because of the constant presence 

 of mucus and epithelial cells it is never clear but presents al- 

 ways an opalescent appearance. The amount secreted daily 

 varies between i and 2 liters. 



In the older literature several complete analyses of saliva 

 are given, but less importance is now attached to these than 

 formerly, since a great degree of exactness is not possible in 

 such tests and besides the composition of the secretion cannot 

 be a constant one. In the mean the amount of water present 

 is 99.5 per cent. In the 0.5 per cent of solids about 0.2 per 

 cent consists of inorganic salts and the remainder of organic 

 substances, including the ferment. Among the salts there is 

 a minute trace of potassium thiocyanate, KSCN, which may 

 frequently be recognized by the test with ferric chloride. It 

 is not known that this substance exerts any specific function, 

 and in different individuals it is present in different amounts. 

 Some of the important properties of saliva may be illustrated 

 by simple experiments. 



Ex. After washing out the mouth thoroughly with water chew a piece 

 of rubber or other neutral insoluble substance to stimulate the flow of 

 saliva. Collect 25 to 50 cc. in a clean breaker and after diluting with an 

 equal volume of distilled water allow to stand a short time to settle. Then 

 filter through a small filter paper into a clean vessel and use the filtrate 

 for the following tests : 



To a few cc. of the clear saliva in a test-tube add several drops of a 



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