766 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 is acted upon by the inverting 

 enzymes (maltase) and converted into the simple sugar, dex- 

 trose. The ptyalin digestion seems, therefore, to be prepara- 

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

 to get the starch into a condition ready for nutrition. Under 

 the influence of these two enzymes, the complex starch molecule, 

 consisting of a number of C 6 Hi O 5 groups, is broken down into its 

 constituent elements or building stones, that is to say, to the 

 simple sugars of the formula C 6 Hi 2 O 6 . The simple sugars or mono- 

 saccharids (CeH^Oe), the double sugars or disaccharids (C^H^On), 

 and the dextrins, starches, and cellulose or polysaccharids, with 

 the general formula (CeHioC^n, constitute a series of increasing 

 complexity as regards the size of the molecule. By hydrolysis, 

 with enzymes or with acids, the polysaccharids are hydrated and 

 split to form the simpler members of the series, while on the other 

 hand, in the living organism the simple sugars may be synthesized 

 by combining two or more of the groups with dehydration to form 

 the higher polysaccharids, the animal and vegetable starches. 

 A question of practical importance is as to how far salivary diges- 

 tion 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 

 normally of but little value as a digestive process. Our recent 

 increase in knowledge regarding the conditions in the stomach 

 (p. 722) shows, on the contrary, that some of the food in an ordi- 

 nary meal may remain in the f undic 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 



