SALIVARY GLANDS 77 



(6) From a chemical standpoint, the function of the saliva 

 is to convert starch into sugar. It does this through the agency 

 of its enzyme, ptyalin, which conforms to the characteristics 

 of enzymes already noted. Maltose (C 12 H 22 O 11 +H 2 O) is the 

 form of sugar produced, but there are several intermediate sub- 

 stances formed before maltose finally results. The starch mole- 

 cule (C 6 H 10 O 5 ) was formerly supposed to simply appropriate a 

 molecule of water to form dextrose (grape sugar, glucose, 

 C 6 H 12 O 6 ), but it is now thought that there is a succession of 

 hydrolytic changes with the production of dextrin and maltose. 

 That is, the starch molecule appropriates a molecule of water; 

 this new molecule splits into a certain kind of dextrin and mal- 

 tose; the dextrin left itself appropriates water and splits up into 

 another kind of dextrin and maltose; this last dextrin goes 

 through a similar process with a like result, until finally only 

 maltose is produced. Some dextrose may be produced. It will 

 be seen under gastric digestion that mineral acidity will also 

 convert starch into sugar, but in this case the form of sugar is 

 dextrose. 



The effect of temperature on the action of enzymes has been 

 noticed. The optimum for ptyalin is 100 Fahrenheit. The 

 reaction of saliva is alkaline and its effect on starch is stopped 

 by an acid medium, since the enzyme is thereby destroyed. 

 However, ptyalin has been shown to act even a little better in 

 perfectly neutral than in alkaline solutions (Chittenden). The 

 action of this substance on starch is very much facilitated if 

 the starch be cooked; in fact, its action on uncooked starch is 

 so slow that probably it is inconsequential in digestion. Cooked 

 starch becomes hydrated, and furthermore has its cellulose 

 capsule removed from the granulose, both of which circumstances 

 make it much more susceptible to salivary influences. 



However, it must be admitted that the practical effect of pty- 

 alin in digestion is not very considerable in the mouth mainly 

 because the food is not kept in the mouth long enough. How- 



