10 



THE ECONOMICS OF FEEDING HORSES 



substance of the cells of which plants and animals are 

 made up. 



The percentage composition of protein may be roughly 

 represented as — 



Thus the element nitrogen forms about 16 per cent, 

 of protein material ; and since in an animal whose weight 

 is remaining constant practically all the nitrogen is 

 excreted in the urine, it is easy to estimate the amount 

 of protein assimilated. Most of the work done in and 

 by the body is due to muscular action, and each contrac- 

 tion of a muscle means the oxidation or breakdown of 

 some of its substance, so setting free energy. Now, 

 muscle is made up of a collection of cells, each consist- 

 ing chiefly of protein. These cells are bathed by the 

 fluids from the blood containing the food materials, 

 protein, sugar, and fat, together with oxygen from the 

 air, and the cells have the vital power of attaching these 

 substances to themselves in more or less stable com- 

 bination. The object for which the muscle exists is 

 work or contraction, and this requires a supply of 

 energy in the form of food. At each contraction, then, 

 the loosely attached food substances in the muscle cells 

 are broken down by the oxygen, give up their supply of 

 energy, and are resolved into waste products, which are 

 removed from the body. Protein, sugar, and fat, can all 

 act in this way, and, in fact, the latter two serve only 

 this purpose. Protein, however, has a dual role, for with 



