RELATIONS TO NUTRITION. 473 



subject, we must not fall into the error of assimilating the 

 respiratory phenomena too closely to those with which we are 

 acquainted as they occur in inorganic bodies. It must be re- 

 membered that in the organism we are dealing with principles 

 which have the remarkable property of self-regeneration ; 

 and which, as a simple condition of vital existence, consume 

 oxygen, when it is presented to them, and exhale carbonic 

 acid. Without a proper supply of oxygen, the tissues die, 

 lose these peculiar properties, and finally disappear by putre- 

 factive decomposition. This consumption of oxygen cannot 

 be regarded in any other light than as the appropriation by 

 a living part, of an element necessary to supply waste ; in 

 the same way as those materials which are ordinarily called 

 nutritive are appropriated. That waste is continually going 

 on there can be no doubt ; and as the production of urea, 

 creatine, creatinine, cholesterine, etc., is to a certain extent 

 independent of the absorption of food, so the production of 

 carbonic acid is to a certain extent independent of the ab- 

 sorption of oxygen. This has been fully demonstrated by 

 the experiments of Spallanzani, Edwards, Geo. Liebig, and 

 others, who have noted the exhalation of carbonic acid in at- 

 mospheres which contained no oxygen. How different are 

 these phenomena from those which attend combinations and 

 decompositions of inorganic matters ! As an example, ]et 

 oxygen be brought in contact, under proper conditions, with 

 iron. Under these circumstances, a union of iron and oxy- 

 gen takes place, and a new substance, oxide of iron, is formed, 

 which has peculiar and distinct properties. In the same 

 way, carbonic acid may be disengaged from its combinations 

 by the action of a stronger acid, which unites with the base 

 and forms a new substance, in no way resembling the origi- 

 nal salt. To make the contrast still more striking, let a hy 

 dro-carbon, like fat, be heated in oxygen or the air, until it 

 undergoes combustion ; it is then changed into carbonic acid 

 and water, by a definite chemical reaction, and is utterly de- 

 stroyed as fat. 



