70 THE PHYSIOLOGY OF DIGESTION AND ABSORPTION 



the amount of the enzyme (unless it be very small) no matter 

 how large a quantity of the substance 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 affected 

 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 undetermined. 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 occurs. 



Digestive Processes. The digestive processes may be con- 

 sidered under the heads of (i) prehension, (2) mastication, (3) 

 salivary digestion, (4) deglutition, (5) gastric digestion and (6) 

 intestinal digestion. Prehension, mastication and deglutition 

 cannot properly be looked upon as digestive processes, inasmuch 

 as they involve no chemical change. They are, however, neces- 

 sary occurrences, and cannot be disregarded. Of course, ab- 

 sorption and " internal digestion" are supposed to follow gastro- 

 intestinal or " external digestion," and assimilation or cell 

 appropriation to follow 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 a partial vacuum in front, 

 and liquids are drawn into the mouth. The mechanism of 

 drinking is the same. 



