64 THE CARBON OF THE FOOD 



69 per cent, of carbon. Consequently, if a horse 

 secretes 37 Ibs. of bile, this quantity will contain only 

 40 ounces of carbon. But the horse expires daily 

 nearly twice as much in the form of carbonic acid. A 

 precisely similar proportion holds good in man. 



Along with the matter destined for the formation or 

 reproduction of organs, the circulation conveys oxygen 

 to all parts of the body. Now, into whatever combina- 

 tion the oxygen may enter in the blood, it must be held 

 as certain, that such of the constituents of blood as are 

 employed for reproduction, are not materially altered by 

 it. In muscular fibre we find fibrine, with all the prop- 

 erties it had in venous blood ; the albumen in the blood 

 does not combine with oxygen. The oxygen may pos- 

 sibly serve to convert into the gaseous state some un- 

 known constituent of the blood ; but those well-known 

 constituents which are employed in reproduction, cannot 

 be destined to support the respiratory process ; none of 

 their properties can justify such an opinion. 



Without attempting in this place to exhaust the whole 

 question of the share taken by the bile in the vital op- 

 erations, it follows, as has been observed, from the 

 simple comparison of those parts of the food of the 

 carnivora which are capable of assimilation, with the 

 ultimate products into which it is converted, that all the 

 carbon of the food, except that portion which is found 

 in the urine, is given out as carbonic acid. 



But this carbon was ultimately derived from the sub- 

 stance of the metamorphosed tissues ; and this being 

 admitted, the question of the necessity of substances 



