28 COMPOSITION OF THE ANIMAL BODV. 



1. All parts of an animal, the solid and fluid parts equally, 

 like those of a plant, consist of an organic or combustible, and 

 an inorganic or mineral portion. This organic part is com- 

 posed, as in the plant, of the four elementary bodies carbon, 

 hydrogen, nitrogen, and oxygen ; but, with the exception of the 

 fat, the body of the animal, as a whole, is much richer in nitro- 

 gen than the substance of the plant. It is chiefly to this 

 abundance of nitrogen that animal substances owe the peculiar 

 phenomena that attend upon their decomposition or decay- 

 such as the production of ammonia, the evolution of disagreeable 

 odours, &c. 



2. The mineral matter of the animal body consists of the 

 same substances as are found in the plant ; and, as in the plant, 

 they usually form only a small percentage of the whole weight of 

 the several parts of the body. The bones form the only excep- 

 tion. In them, when dry, the mineral matter forms about two- 

 thirds of the whole weight ; while in other parts of the body it 

 does not amount to more than from three to five per cent. 



Phosphoric acid and lime are the main ingredients found in 

 the mineral matter of the bones ; and the importance of the bone 

 to the animal may be regarded as a measure of the importance 

 of these two substances to the animal economy. 



The potash and soda are for the most part diffused through 

 the fluids of the body. The silica, which appears so necessary 

 to many plants, holds only a subordinate place in the animal. 

 In the hair and nails it is met with in minute quantity, and in 

 the feathers of birds. It is probably a necessary of life to most 

 animals, though present in them only in small proportion, and 

 for purposes as yet by no means well understood. The same 

 may be said also of fluorine, about one per cent of which is 

 found in the bones and in the teeth. 



Consisting of the same elementary bodies as the plant, though 

 in somewhat different proportions, it is not difficult to understand 

 generally how the parts of animals are built up and sustained. 

 The vegetable food conveyed into the stomach contains all the 

 elementary substances of which the body is composed ; and out 

 of these the absorbing vessels select, we may say, what is wanted, 

 and convey it to the part of the body where it is required. 



