HOW THE PARTS ARE SUSTAINED. 29 



But such a generality as this explains nothing. If it were 

 necessary only to introduce the elementary bodies into the 

 stomach of the animal, why make use of vegetable substances for 

 the purpose? They might be swallowed at once as they occur in 

 the soil, and animals might, to a great extent, become indepen- 

 dent of vegetable life. But the relation, in regard to chemical 

 composition, between the plant and the animal, is much closer 

 than is represented by the above general explanation, and is, in 

 reality, both beautiful and interesting. Thus 



1. The animal body contains a variable, often a large, pro- 

 portion of fat. The vegetable always contains a quantity of 

 fat, sometimes identical with the fat of the body, and always 

 closely resembling it. The fat of the plant, therefore, enters 

 ready formed, and directly contributes to the increase of the 

 fat of the body. Or if the plant do not contain a sufficient 

 amount of fat to supply the wants of the body, other substances 

 are present in it (wax, starch, sugar, &c.) which are more or 

 less readily transformed into fat in their progress through the 

 organs of digestion. Thus the animal does not deal with the 

 elements (carbon, hydrogen, and oxygen) of which fat consists, 

 but with chemical combinations already prepared by the plant, 

 and which it slightly transforms if necessary, or conveys directly 

 to the several parts of its body. 



2. So with the muscle of the animal. The gluten and other 

 protein compounds of the plant which also especially abound 

 in young plants and shoots, and in seeds are either identical 

 with the muscular fibre, or differ from it in a very small 

 degree, either in properties or in chemical composition. The 

 stomach, therefore, extracts them at once, ready formed, from 

 the vegetable food, as it did the fats, and they are sub- 

 jected, if necessary, to a slight transformation only, before the 

 vessels convey them to the parts of the body where they are 

 required. 



3. The blood, as a whole, is almost identical in composition 

 with the muscle of an animal. The same general relation, 

 therefore, exists between the protein compounds of the plant 

 and this part of the animal substance. From the food in 

 the stomach they are extracted and conveyed at once into the 



