680 PHYSIOLOGY OF DIGESTION AND SECRETION. 



of the enzyme the starch molecules take up water and undergo 

 cleavage into simpler molecules. The steps in the process and the 

 final products have been investigated by a very large number of 

 workers, but much yet remains in doubt. The following points 

 seem to be determined: The end-result of the reaction is the 

 formation of maltose, a disaccharid, having the general formula 

 C 12 H 22 O n , and some form of dextrin, a non-crystallizable poly- 

 saccharid. When the digestion is effected in a vessel some dextrose 

 (C 6 H 12 6 ) may be found among the products, but this is explained on 

 the assumption that there is present in the saliva some maltase, an 

 enzyme capable of splitting maltose into dextrose. So far as the 

 ptyalin itself is concerned, its specific action is to convert starch to 

 maltose and dextrin. It seems very certain, however, that a number 

 of intermediate products are formed consisting of a variety of dex- 

 trins, so that the hydrolysis probably takes place in successive 

 stages. There is little agreement as to the exact nature of the in- 

 termediate dextrins. The following facts, however, may be easily 

 demonstrated in a salivary digestion carried on in a vessel and ex- 

 amined from time to time. The starch at first gives its deep-blue 

 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 



