186 HUMAN PHYSIOLOGY 



real cause of the arrest in such cases is the gathering 

 of the products of the change in the field of operations. 

 If the products can be removed the reaction will be 

 resumed. We must remember that the conditions in a 

 glass vessel must always be much less favorable to the 

 action of enzymes than those prevailing in the ali- 

 mentary canal. Nothing can escape from the flask or 

 the test-tube while there is the possibility of withdrawing 

 the digestive products from the canal and enabling the 

 change to proceed. 



There is a certain temptation to speak of enzymes as 

 though they were living. This is to be guarded against ; 

 they are secreted by living cells but all that they do can 

 be explained without assuming that they have life. 

 Yeasts, moulds, and bacteria which are simple living 

 things may work upon solutions in which they are grow- 

 ing very much as enzymes in suitable variety would. 

 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- 



