THE CHEMISTRY OF DIGESTION 993 



takes place. The velocity of the reaction is then proportional to the 

 square of the amount of the substance. 



The Function of Saliva. — Saliva possesses a two-fold action, namely, 

 a physical one and a chemical one. It moistens the mucous surfaces 

 of the mouth as well as the food, thereby facilitating its mastication and 

 deglutition. In addition, it acts as a solvent allowing sapid substances 

 to excite sensations of taste, and as a cleaning agent of the oral cavity. 

 The former is of special importance, because it serves to evoke those 

 stimuli which give rise to the psychic secretion of gastric juice and a 

 certain satisfaction in eating. These are its only functions in those 

 animals, such as the horse, sheep, ox and dog, in which a true digestive 

 ferment is not present.^ The substance more particularly concerned 

 in this purely mechanical process is mucin . In man and some of the 

 herbivora, however, it also possesses a moderate chemical action by 

 virtue of its enzyme ptyalin. This ferment acts exclusively upon 

 starch, converting it into maltose through several intermediary stages, 

 such as soluble starch or erythro-dextrin which gives a red color with 

 iodin, and achroo-dextrin which gives no color with iodin. But since 

 ptyalin does not attack cellulose, it is imperative that the starch be 

 well cooked beforehand so as to destroy its capsular investments. 

 Moreover, while the warmth of the mouth causes the starch to be re- 

 duced very rapidly, a considerable portion of even the boiled starch 

 invariably escapes salivary digestion, because mastication is usually 

 practised in a hasty and careless manner. The solid starch which is 

 ingested in farinaceous foods, bread, and biscuits, is only slightly 

 affected by the saliva and- practically no hydrolysis is instituted by 

 this secretion. 



Ptyalin is most effective at 37°C. and in a neutral or weak acid 

 medium, but a slightly alkaline medium is not unfavorable to its 

 activity. Inasmuch as its action is destroyed by such small amounts 

 of acid as 0.003 per cent. HCl, it might be supposed that it must lose 

 its effectiveness as soon as it enters the stomach. This is not the case, 

 because the freshly swallowed food forms a coherent mass which is not 

 easily penetrated by the gastric juice, and besides, some time must 

 elapse before a sufficient quantity of the latter has been secreted to fill 

 the relatively inactive cardiac end of the stomach. During the interim, 

 the ptyalin continues its reductions and it is safe to say that from 30 to 

 40 minutes must elapse before its action is stopped completely. Mean- 

 while, the largest part of the available starch has been hydrolyzed and, 

 while a certain proportion of unreduced starch may escape salivary 

 digestion, it is later on subjected to the action of the amylopsin of the 

 pancreatic juice. Unboiled starch, on the other hand, escapes even 

 this powerful diastatic ferment and enters the feces unutilized. 



The Function of Gastric Juice. — The action of the gastric juice 

 is due partly to its acid and partly to the combined action of its acid 

 and ferments. It may be said that : 



iKuss, Ref. Maly, 1898, Zebrowski, Pfliiger's Archiv, ex, 1905, 105, and 

 Palmer, Am. Jour, of Physiol., xli, 1916, 483. 

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