182 



MOUTH DIGESTION. 



ciurn phosphate and carbonate, magnesium phosphate, and potas- 

 sium chlorid. Although it is commonly stated that it contains 

 also sodium carbonate, yet, as we have seen, this is denied by 

 Chittenden and Richards, whose investigations are the most recent 

 on this subject. Calcium carbonate and phosphate occasionally 

 form salivary calculi in the glands or their ducts, and may require 

 removal by the surgeon. These salts also contribute to the forma- 

 tion of the tartar on the teeth. 



The following table gives four analyses of human mixed saliva, 

 by as many chemists. It should be remembered, however, that 

 even in the same individual the composition of this secretion 

 varies during the day, and that, too, independently of the taking 

 of food. Observation has shown that between 7 and 11 A.M., 

 provided no food is taken, its composition is very constant. 



Mixed Human Saliva. 



Office of Saliva. This is twofold : (1) chemical and (2) mechan- 

 ical. 



The chemical action of saliva is due to the enzyme ptyalin, 

 which is undoubtedly the most important constituent of the 

 mixed saliva. This ingredient exists in the human parotid gland 

 at birth, but does not appear in the submaxillary gland until the 

 age of two months. It is an amylolytic or diastatic enzyme /. e. 9 

 one having the property of changing starch into sugar. Although 

 it resembles diastase, which is the enzyme obtained from malt, in 

 its power to convert starch, still the enzymes are not the same. 

 Thus, the optimum temperature for ptyalin is 46 C., and its power 

 is destroyed between 65 C. and 70 C. ; while the optimum 

 temperature for diastase is between 50 C. and 56 C., and it is 

 destroyed at 80 C. 



Although the optimum temperature of ptyalin is at or near 

 46 C., still it acts vigorously at from 30 C. to 40 C. The con- 

 version of starch into maltose takes place as follows : The starch 

 grains being acted upon by hot water, take it up, and soluble 

 starch or amylodextrin is produced. The action of ptyalin upon 

 this is to convert it into erythrodextrin and maltose ; the ptyalin, 

 continuing its hydrolytic action, changes the erythrodextrin into 



