172 PHYSIOLOGICAL CHEMISTRY. 



The amount of action in the latter case is dependent upon several 

 circumstances, such as the degree of dilution and the presence of 

 proteids or peptone (CHITTE^DEN). Of the greatest physiological 

 importance, however, is the fact that even very small quantities of 

 free acids not only a degree of acidity of about 1 p. m. HC1, 

 which often occurs in the gastric juice, but even a smaller quantity 

 of hydrochloric acid (organic acids are less active) at once pre- 

 vents the action of the ptyalin and destroys the enzyme. It is also 

 of interest that boiled starch (starch-paste) is more quickly con- 

 verted into sugar than unboiled. The time required to change 

 unboiled starch varies with the kind. 



The rapidity with which ptyalin acts increases, at least 

 tinder conditions otherwise favorable, with the amount of enzyme 

 and a temperature a little above -\- 40 C. Foreign substances such as 

 metallic salts, have a different effect. Certain salts even in small 

 quantities completely arrest the action; for example, HgCl 2 accom- 

 plishes this result by the presence of only 0.05 p. m. Other salts, 

 such as magnesium sulphate, in small quantities (0.25 p. m.) acceler- 

 ate, and in larger quantities (5 p. m.) check the action. The accu- 

 mulation of the products of the amylolytic decomposition also checks 

 the action of the saliva. 



To show the action of saliva or ptyalin on starch the three 

 ordinary tests for glucose may be used, namely, MOORE'S or 

 TROMMER'S test or the Bismuth test (see Chap. XIV). It is also 

 necessary, as a control, to first test the starch-paste and the saliva 

 for the presence of glucose. 



The quantitative composition of the mixed saliva must vary con- 

 siderably, not only because of individual differences, but also because 

 under varying conditions there may be an unequal division of the 

 secretion from the different glands. Analyses of the composition 

 of human saliva are given in the table on the opposite page. The 

 figures are parts per 1000. 



The quantity of saliva secreted during 24 hours cannot be exact- 

 ly determined, but has been calculated by BIDDER and SCHMIDT 

 to be 1500 grms. The most abundant secretion occurs during 

 meal-times. According to the calculations and determinations of 

 TUCZEK in man, 1 grm. of gland should yield 13 grms. secretion 

 in the course of one hour during mastication. These figures 



