CHAPTER VI. 



ALIMENTATION. 



49. IT lias been already mentioned that the tissues of the 

 body are constantly parting with particles which enter into 

 their composition, and are receiving new materials to replace 

 Avhat is lost. We have seen that increased muscular exertion 

 is accompanied with increased loss of substance; and the 

 same is true of increased mental exertion, and of increased 

 evolution of animal heat, as in exposure to extreme cold. 

 Muscular and mental effort, and the maintenance of the 

 temperature of the body, as well as regeneration of the 

 tissues, all involve losses of substance which require to be 

 made good; and thus, as has been said at the outset, the body 

 may be regarded as a vortex whose component particles are 

 ever changing while its form remains. 



The processes by which materials are altered and thrown 

 off are all processes of oxidation, by which complex chemical 

 combinations are reduced to others of simpler descriptions ; 

 and this is sometimes expressed, though not very accurately, 

 by using the word combustion in speaking of such changes. 

 The ultimate products of combustion of organic matter are, 

 as will be recollected, carbonic acid, water, and ammonia; 

 that is to say, that, keeping sulphur, phosphorus, and 

 mineral matters out of account, and confining our attention 

 to the carbon, hydrogen, oxygen, and nitrogen of organic 

 matters, the most complete oxidation of suck substances 

 which can be effected is into carbonic acid, water, and 

 ammonia. The oxidation within the body falls short of this; 

 by far the greater part of the matters which escape from 

 it, after having circulated in the system, being eliminated 

 as carbonic acid, water, and urea. No doubt, as has already 

 been pointed out, there is a continual casting off of horny 



