220 NUTRITIVE ELEMENTS. 



the atmosphere, in the form of ammonia. On the other 

 hand, nitrogenous substances form a large proportion 

 of the constituents of the blood of animals, and appear 

 in their whole system. As there is a constant waste in 

 the animal, and a continual formation of new tissues, — 

 as the whole body is constantly renewed through the 

 agency of the blood which is converted into flesh and 

 muscle, — there must be a never-failing supply of nour- 

 ishment ; and this nourishment for the higher animals is 

 found, as already intimated, to a considerable extent, in 

 the nitrogenous elements of plants. 



For every ounce of nitrogen which the animal re- 

 quires to sustain life, and build up the muscular and 

 fleshy parts of his body, he must take into the stom- 

 ach, in the shape of food, such a quantity of vege- 

 table substances as will furnish him with an ounce of 

 nitrogen in combination with other essential elements. 

 If we suppose one kind of hay to contain one ounce 

 of nitrogen to the pound, and another to have only 

 half as much, or only an ounce in two pounds, the 

 pound which contains the ounce of nitrogen would go 

 as far to nourish the animal — other things being equal 

 — as the two pounds which contain only the same quan- 

 tity of nitrogen. The importance of woody fibre to act 

 mechanically in giving bulk to the food is not, of course, 

 to be overlooked. 



Nor is this a mere deduction of theory. The experi- 

 ment has frequently been made, and it is now fully 

 established, both by science and experience, that the 

 greater the proportion of nitrogen which any vegetable 

 contains, if it also contains other important constitu- 

 ents in proper combination, the smaller will be the quan- 

 tity of that vegetable required to nourish the animal 

 body, and the less nitrogen any vegetable contains, the 

 greater will be the quantity of it required. 



Muscle and flesh are composed of nitrogenous princi- 



