DIGESTION IN THE MOUTH 129 



function by reason of the enzyme ptyalin which it contains. 

 Ptyalin is an enzyme which acts on starches. Little is known of 

 its chemical nature, but evidence seems to indicate that it is 

 made up of a substance resembling a protein combined with a 

 carbohydrate gum. Perhaps ptyalin is not a single enzyme, but 

 a mixture of two or more enzymes, each of which is responsible 

 for one of the steps in the digestion of starch. Ptyalin is 

 secreted by the parotid glands. It acts best at a temperature of 

 40-45 C. and is destroyed by heating rapidly to 75 C. 

 Ptyalin acts best in a very faintly acid solution, the optimum 

 acidity being 10- 6>7 X normal. If the acidity is sufficient to turn 

 congo red to violet (N X 10 -4 ) the action of ptyalin is inhibited. 

 It also will act in a weakly alkaline solution such as the saliva. 

 The presence of some salt favors its action. 



Starch is attacked by ptyalin and broken down into simpler 

 substances. The physical condition of the starch is important, 

 for raw starch is digested only with difficulty, whereas cooked 

 starch is digested very rapidly and completely under favorable 

 conditions of temperature, acidity, etc. Various products are 

 produced in the breaking up of starch, first the dextrins, of 

 which there undoubtedly are several, and finally maltose and 

 isomaltose. The progress of the decomposition may be followed 

 by testing small portions of the liquid with iodine. The blue 

 color characteristic of starch gives place to a purplish, then a 

 red color, and finally no color whatever is produced. The 

 products corresponding to these stages are as follows : 



Color with Iodine 



Starch > Amylodextrin > Erythrodextrin > Achroo-dextriii 

 blue blue red (no color) 



-^ Maltose 

 (no color) 



Maltose undoubtedly is split off even in the first stages of the 

 process. 



The saliva also contains small amounts of an enzyme maltase, 

 which breaks up some of the maltose into glucose. Perhaps, as 



