THE SALIVARY GLANDS. 701 



reaction with iodin; later, instead of a blue, a red reaction is obtained 

 with iodin, and this has been attributed to a special form of dextrin, 

 erythrodextrin, so named on account of its red reaction. Still later 

 this reaction fails and chemical examination shows the presence of 

 maltose and a form of dextrin which gives no color reaction with 

 iodin and is therefore named achroodextrin. While the number 

 of intermediate products may be large, the main result of the action 

 of the ptyalin is expressed by the following simple schema : 



The products formed in this reaction are probably not absorbed as 

 such. The absorption takes place mainly no doubt after the food 

 reaches the small intestine, and we have evidence, as will be stated, 

 that before absorption the maltose and the dextrin are acted upon by 

 the inverting enzymes (maltase) and converted into the simple 

 sugar, dextrose. The ptyalin digestion seems therefore to be pre- 

 paratory, and the combined action of ptyalin and maltase is necessary 

 to get the starch into a condition ready for nutrition. By way of 

 comparison it is interesting to remember that when starch is boiled 

 with dilute acids it is hydrolyzed at once to dextrose. ' A question of 

 practical importance is as to how far salivary digestion affects the 

 starchy foods under usual circumstances. The chewing process in 

 the mouth thoroughly mixes the food and saliva, or should do so, 

 but the bolus is swallowed much too quickly to enable the enzyme to- 

 complete its action. In the stomach the gastric juice is sufficiently 

 acid to destroy the ptyalin, and it was therefore supposed formerly 

 that salivary digestion is promptly arrested on the entrance of the 

 food into the stomach, and is therefore normally of but little value 

 as a digestive process. Our recent increase in knowledge regarding 

 the conditions in the stomach (p. 663) shows, on the contrary, that 

 some of the food in an ordinary meal may remain in the fundie 

 end of the stomach for an hour or more untouched by the acid 

 secretion. There is every reason to believe, therefore, that salivary 

 digestion may be carried on in the stomach to an important extent. 

 Conditions Influencing the Action of Ptyalin. Temperature. 

 As in the case of the other enzymes, ptyalin is very susceptible to- 

 changes of temperature. At C. its activity is said to be suspended 

 entirely. The intensity of its action increases with increase of 

 temperature from this point, and reaches its maximum at about 

 40 C. If the temperature is raised much beyond this point, the 

 action decreases, and at from 65 to 70 C. the enzyme is destroyed. 

 In these latter points ptyalin differs from diastase, the enzyme of 

 malt. Diastase shows a maximum action at 50 C. and is destroyed 

 at 80 C. 



