PART VII 



METABOLISM 



CHAPTER LX 



METABOLISM 



Introductory. The object of digestion, as we have seen, is to render 

 the food capable of absorption into the circulatory fluids the blood and 

 lymph. The absorbed food products are then transported to the various 

 organs and tissues of the body, where they may be either used at once 

 or stored away against future requirements. . After being used, certain 

 substances are produced from the foods as waste products, and these pass 

 back into the blood to be carried to the organs of excretion, by which they 

 are expelled from the body. By comparison of the amount of these ex- 

 cretory products with that of the constituents of food, we can tell how 

 much of the latter has been retained in the body, or lost from it. This 

 constitutes the subject of general metabolism. On the other hand, we may 

 direct our attention, not to the balance between intake and output, but to 

 the chemical changes through which each of the foodstuffs, must pass be- 

 tween absorption and excretion. This is the subject of special metabolism. 

 In the one case we content ourselves with a comparison of the raw ma- 

 terial acquired and the finished product produced by the animal factory; 

 in the other we seek to learn something of the particular changes to which 

 each crude product is subjected before it can be used for the purpose of 

 driving the machinery of life or of repairing the worn-out parts of the 

 body. 



In drawing up a balance sheet of general metabolism, we must select 

 for comparison substances that are common to both intake and output. In 

 general the intake comprises, besides oxygen, the proteins, fats and car- 

 bohydrates; and the output, carbon dioxide, water and the various nitrog- 

 enous constituents of urine. This dissimilarity in chemical structure be- 

 tween the substances ingested and those excreted limits us, in balancing the 

 one against the other, to a comparison of the smallest fragments into which 

 each can be broken by chemical agencies. These are the elements, and of 

 them carbon and nitrogen are the only ones which it is possible to measure 



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