44 SCIENTIFIC FEEDING OF ANIMALS 



(2) Methods of investigation. 



To ascertain whether any given susbtance is a 

 nutrient in the above sense a careful examination 

 of the quantity and nature of the food and faeces 

 has to be made. The same method is employed for 

 finding out what quantity of the nutrient material 

 is necessary for an animal under certain condi- 

 tions, and, further, under what circumstances a 

 gain or loss of body tissue takes place. 



i. The increase of body tissue, or flesh, can be 

 reckoned from the amount of nitrogen which is 

 retained in the animal body when a known ration 

 is fed. It is true that the growth of hair, horns, 

 hoofs, etc., also claims a little nitrogen, but this 

 amount is so small as to be negligible, except in the 

 case of sheep. The computation of the quantity 

 of nitrogen which an animal retains in its body is 

 relatively simple, owing, in the first instance, to 

 animals not having the power to make use of the 

 nitrogen of the atmosphere. The air coming from 

 the lungs contains the same amount of nitrogen 

 that it did when taken in. Secondly, the nitro- 

 genous components of the food do not split off free 

 nitrogen, for all the nitrogen which is not stored 

 in the body passes into the urine or faeces in 

 the form of organic compounds. The amount 

 of nitrogenous matter which leaves the body 

 through the skin in the form of perspiration is 



