AMYLOLYTIC ACTION OF SALIVA. 413 



investigators has emphasized the important influence which 

 the presence of proteids may exercise a fact apparently over- 

 looked by Kubel. Proteid matter tends to prevent the de- 

 structive or inhibitory influence of acids by combining with 

 them ; in comparing the rate of digestive action under different 

 conditions, it is obviously necessary to take into consideration 

 the amount of proteid present. Thus the dilution of the saliva, 

 or the differences in the concentration of the secretion col- 

 lected at different times or from different individuals may 

 bring about digestive variations when all the other condi- 

 tions are constant, owing to the varying quantities of proteid 

 thus introduced. Further progress was made when it was 

 demonstrated by Chittenden and Smith that free acid, i. e., 

 acid which reacts with tropseolin 00, readily destroys the 

 enzyme of the saliva. The difference between free and com- 

 bined acid in their relative destructive or inhibitory power 

 appears to have been overlooked by Kubel. It will be shown 

 in the experiments to follow that digestion never proceeds at 

 all in the presence of more than the merest traces of free hy- 

 drochloric acid ; and it would seem probable from our repeti- 

 tion of Kiibel's experiments that in those instances in which 

 he observed complete inhibition of digestive action, free acid 

 was usually present. The conception of free acid as an in- 

 hibitory agent towards ptyalin carries with it a definite idea ; 

 it is independent of any consideration of the accompanying 

 conditions, such as amount of saliva used, concentration of 

 saliva or strength of starch paste. Whenever the digestive 

 mixture gives a reaction for free acid, no amylolysis is to be 

 expected. On the other hand, to speak of a definite total 

 acidity of the fluid as influencing amylolysis in a definite 

 way is scarcely permissible without defining carefully the 

 conditions of the reaction, in particular the amount of ,'proteid 

 present. Even the different proteids vary widely in their 

 combining power with acids. 



