THE ALIMENTARY CANAL 189 



The result is a fermentation and in all probability its course 

 is determined largely by enzymes which the microor- 

 ganisms develop. But here again the enzymes are not 

 themselves alive in the true sense of the word. The cells 

 of the growing culture are comparable with the cells of 

 glands, sources of enzymes which can outlast the life of 

 their producers. There is evidence that enzymes may 

 be intracellular, that they may bear a part in the chem- 

 ical changes taking place in protoplasm itself. But most 

 of the story of digestion can be told without reference to 

 this possibility. 



While the essential part of digestion is chemical there 

 are physical accompaniments that call for recognition. 

 The early observers made much of them and, indeed, 

 it could hardly be otherwise for their chemical knowledge 

 was slight. They emphasized the crushing and grinding 

 of the food by the teeth and they assumed a continuation 

 of such treatment in the stomach and beyond. The 

 mechanical factors in digestion remain interesting but 

 should be regarded as preliminary to deeper seated 

 changes. The salient fact in connection with mastica- 

 tion or any similar treatment of food is that subdivision 

 increases the surface exposed to the juices. This is 

 just as true when an oil is broken into fine drops 

 emulsified as when we have to do with solids. 



Much that has gone before can be made clearer by 

 considering what kinds of food need no digestion. , This 

 is the case with the simple sugars, the dextrose of grapes 

 and honey, the levulose in many fruits. It is also true 

 of the various mineral salts of the diet, so far as they are 

 destined to be absorbed, and of alcohol. Any other kind 

 of food can, theoretically, be predigested, but the ad- 

 vanced cleavage products in the case of proteins and 

 fats are not appetizing. 



