NUTEITIVE DECOMPOSITION. 91 



bustible matters for the oxygen to act on, by means of which 

 they are transformed into carbonic acid gas and water, and in 

 that form eliminated from the body : from the same source 

 are derived the materials of growth and of secretion. 



But whether the carbonated and hydrogenated matters 

 thus consumed come from the aliment directly or from the 

 tissues themselves, one thing is obvious, that the loss must 

 be supplied from without, under the form of aliments. 



The alimentary matters, which contain only carbon, hy- 

 drogen, and oxygen in their ultimate composition, such as 

 fecuia or sugar, may be transformed into carbonic acid and 

 water, leaving no residue ; but the vital combustion of azotized 

 matters gives rise to other products ; and these compounds 

 by losing carbon become richer in azote, and constitute 

 peculiar organic principles, such as urea and uric acid.* 



173. Chemistry seems to prove that it is vegetables 

 which fabricate the combustible matters destined to be con- 

 sumed in animal bodies, plants alone having the power thus 

 to fix carbon under the forms of organic compounds.f 



The carbonic acid gas and water escape by respiration ; the 

 more solid products, as the urea, by the urine. In the adult 

 animal it would thus seem that there may be found nearly 

 the whole of the elements introduced into the system by the 

 food or by respiratory absorption, in the products of the 

 respiration and urinary secretion, the alvine dejections being 

 composed almost wholly of the indigestible residue of the 

 food, mingled with various secretions. 



Before the growth is completed, all the alimentary matter 

 is not burnt or consumed in this way ; a part is found in the 

 organism when the carbonaceous matters taken in exceed the 

 power of the oxygen to consume, the result being the deposi- 

 tion of fat, which may afterwards be consumed according to 

 the exigencies of the animal. J 



* When the tartrate, malate, or citrate of potass has been absorbed or 

 injected into the veins, and, so absorbed, carbonate of potass is found in the 

 urine, thus proving the combustion of the vegetable acid entering into their 

 composition. 



f According to the recent experiments of Dumas, Boussingault, and 

 Payen. 



J The fat is not deposited indifferently in every part of the body. It is 

 composed of two substances, oleine and stearine, the proportions of which 

 vary in the fat of different animals. It is abundant in animals which hyber- 

 nate at the commencement of winter, but disappears towards the close of 

 that season. The fat is supposed to be useful as a cushion for certain organs, 

 as a reservoir to meet the consuming powers of the oxygen taken into the 

 body, and as a preservative of the heat of the body. 



