JO THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



as peptones for example, have an inhibitory action upon the 

 enzyme. If these substances be removed as they are formed, 

 the action of the enzyme continues. (4) The particular re- 

 sult is independent of the amount of the enzymes (unless 

 it be very small) no matter how large a quantity of the sub- 

 stance to be acted upon is present. 



Manner of Action. These enzymes are supposed to bring 

 about their respective changes through hydrolysis that is, 

 by causing water to be taken up by the molecules of the af- 

 fected substance and by the subsequent splitting of the 

 newly formed molecule into two or more simpler ones. How 

 they cause this appropriation of water is as yet undeter- 

 mined. It was formerly supposed to be brought about by 

 contact merely, and the enzymes were called catalytics; but 

 this term offers no explanation of the real change which oc- 

 curs. 



Digestive Processes. The digestive processes may be con- 

 sidered under the heads of (i) prehension, (2) masti- 

 cation, (3) salivary digestion, (4) deglutition, (5) gastric 

 digestion, and (6) intestinal digestion. Prehension, mastica- 

 tion and deglutition cannot properly be looked upon as di- 

 gestive processes, inasmuch as they involve no chemical 

 change. They are, however, necessary occurrences, and can- 

 not be disregarded. Of course, absorption and "internal di- 

 gestion" follow gastro-intestinal or "external digestion," and 

 assimilation or cell appropriation follows absorption. 



Prehension. 



Prehension is simply the taking of food into the mouth. 

 Its mechanism in the human adult is so familiar that it needs 

 no description. In the sucking child it is more complex. 

 The buccal cavity is closed posteriorly by the application of 

 the velum palati to the base of the tongue. The tip of the 

 tongue is applied to the hard palate, and successive portions 

 of it (going backward) being applied in the same way leave 



