CHAPTEE VI. 



THE FERMENTS. 



IN the foregoing chapters we have considered in a general way 

 the more important characteristics of the three great classes of 

 food-stuffs, and have studied in some detail also the various decom- 

 position-products to which they give rise in their passage through 

 the animal body. We have also pointed out that with few excep- 

 tions the food-stuffs, which the animal derives either directly or 

 indirectly from the plant, cannot be utilized by the animal directly, 

 but that they must previously undergo certain changes, which vary 

 with the character of the individual substances. The native albu- 

 mins must first be transformed into albumoses and simpler products; 

 the disaccharides and polysaccharides must be inverted to mono- 

 saccharides, and the fats must first be decomposed into glycerin 

 and fatty acids. We have seen also that in the chemical laboratory 

 these changes can, for the most part, be brought about through the 

 action of superheated steam, by boiling with acids and alkalies, etc. 

 that is, through agencies which manifestly are not at work in the 

 living world. The question therefore suggests itself: What are the 

 means at the disposal of living bodies to bring about these changes? 

 This question has, in a measure, been answered in the introductory 

 remarks, where it was pointed out that the animal is capable of 

 bringing about a large number of analytical changes by means of 

 certain ferments, or enzymes, which are furnished by the animal 

 cells themselves. At the present time there is a tendency indeed 

 to assume that most of the vital phenomena are referable to the 

 action of ferments, and we know as a matter of fact that ferment 

 action is not necessarily only destructive, but may also be construc- 

 tive. The discovery of oxidation ferments has thrown further light 

 on many processes which were formerly referred to " vital " forces 

 inherent in the living protoplasm. The existence of such " vital " 

 forces has become the more doubtful the more closely vital phenomena 

 are studied. Only a few years ago the ferments furnished by the 

 digestive glands were the only known ferments in the animal body, 

 and our knowledge of the mechanism of the various metabolic 

 processes was practically nil. To-day we know that ferments are 

 present in probably every cell and are intimately concerned in all 

 its manifestations of life. In the liver-cell, for example, not less 

 than a dozen different ferments have been demonstrated. Still we 

 are only on the threshold of our knowledge of intracellular chemi- 

 cal processes, and there are still many obscure problems which 

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