METABOLISM 183 



of the chief classes of organic nutrients. For the sake of sim- 

 plicity some of the intermediate steps mentioned on previous 

 pages have been omitted. The central portion of the diagram 

 includes the feed substances taken up into the blood. At the 

 extreme left are shown the main groups of tissue ingredients, 

 and at the extreme right the excretory products. 



It cannot be too strongly emphasized that any such diagram 

 as the foregoing is of necessity in the highest degree schematic. 



For one thing, neither the enzymatic nature nor the revers- 

 ibility of the changes indicated in the diagram has been estab- 

 lished except in a few cases. As already pointed out (212), 

 this conception of the nature of metabolism is still to a large 

 extent hypothetical, although the hypothesis harmonizes well 

 with the present state of our knowledge. 



Moreover, aside from the mere omission from the diagram of 

 certain recognized products of the intermediary metabolism, 

 the chemical processes in the body are doubtless infinitely more 

 complex than can be indicated in any such way. A vast num- 

 ber of different substances have been identified in the animal 

 body, many of which are known to have important functions 

 in keeping the organism in running order but which are not 

 even hinted at in this scheme. 



In brief, the scheme is concerned with the results of the 

 metabolic processes so far as they are related to nutrition 

 rather than with the mechanism by which these results are 

 brought about. It seeks to show in outline how the principal 

 groups of nutrients are related, on the one hand, to the building 

 up of body tissues, and, on the other hand, to the formation of 

 excretory products, and to indicate the mutual relations of the 

 several groups. For this purpose it may perhaps serve a use- 

 ful end as an aid to memory, provided its limitations are clearly 

 understood. 



263. Dual function of feed. As pointed out in i of this 

 chapter (207), the animal body may be regarded in the light of 

 a transformer of energy. By the agency of the protoplasm of 

 its cells, in ways largely hidden from us, it converts the chemical 

 energy supplied in its feed into the various forms characteristic 

 of living matter. From this point of view the feed has a two- 

 fold function. 



First, the feed ingredients are carriers of energy. The higher 



