CHEMICAL COMPONENTS OF THE HUMAN BODY. 51 



CHAPTER II. 



OP THE CHEMICAL COMPONENTS OP THE HUMAN BODY, AND THE 

 CHANGES WHICH THEY UNDERGO WITHIN IT. 



THE body of Man, like that of every other Organized being, is composed of 

 elementary substances that exist abundantly in the Inorganic Universe, in the 

 midst of which it is placed ; and it is from that Universe, that all its materials 

 are derived, either directly or indirectly. The atmosphere supplies the Plant 

 with water, carbonic acid, and ammonia ; and at the expense of the oxygen, hy- 

 drogen, carbon, and nitrogen, which it appropriates from these simple binary 

 compounds, the Plant generates certain peculiar substances, of more complex 

 composition, which are applied in the first place to the extension of its own 

 structure, but which are destined afterwards to constitute the materials of Ani- 

 mal nutrition. On the other hand, the Animal, availing itself of these supplies, 

 gives back to the Inorganic world, not only by the decay of its body after death, 

 but also by the continual decomposition taking place in it during life, the ele- 

 ments which the Plant had withdrawn ; and this, for the most part, in the very 

 forms in which they were originally combined, viz. water, carbonic acid, and 

 ammonia. Most Plants also take up mineral substances of some kind from the 

 soil, which are united more or less closely with the organic compounds formed 

 by them, and which enter with these into the bodies of Animals ; to be in like 

 manner restored to the earth beneath them, by the decay of the organisms of 

 which they have formed part. As this remarkable sequence has been more 

 fully traced out elsewhere, 1 and as the Physiologist has much less to do with 

 the Ultimate components of the Animal body, than he has with the Organic 

 Compounds which are supplied to it as nutriment and of which its fabric is 

 composed, it will not be requisite to dwell any longer upon this part of the sub- 

 ject; and we may at once pass on to consider the nature and properties of these 

 compounds, and to inquire into the metamorphoses which they undergo within 

 the living system. In doing this, it would seem most appropriate to adopt a 

 classification primarily founded rather upon their physiological than upon their 

 purely chemical relations upon the position they hold in the vital economy, 

 rather than upon their ultimate composition or their resemblances to inor- 

 ganic compounds. We should thus be led to arrange them in four principal 

 groups : 



I. The histogenetic substances, which have been introduced into the body as 

 the materials of its fabric, or are generated from these compounds subsequently 

 to their introduction into it, and are on their way to become part of its organized 

 structure by progressive metamorphosis. These are either organic or inorganic 

 compounds ; and of the former, all, save fatty matters, belong to the azotized or 

 nitrogenous group. 



II. The calorific substances, which are either introduced into the body as 

 components of the food, or which are formed within it by the metamorphosis 



* See the Author's "Principles of Physiology, General and Comparative," chap, v., 

 Am. Ed. 



