440 COMPARATIVE PHYSIOLOGY. 



A large number of feeding experiments have been made by 

 different investigators, chiefly, though not exclusively, on the 

 lower animals. Some such method as the following has usu- 

 ally been pursued : 1. The food used is carefully weighed and a 

 sample of it analyzed, so that more exact data may be obtained. 

 2. The amount of oxygen used and carbonic anhydride exhaled, 

 as well as the amount of water given off in any form is esti- 

 mated. 3. The amount of the nitrogenous excreta is calculated, 

 chiefly from an analysis of the urine, though any loss by hair, 

 etc., is also to be taken into account. 



It has been generally assumed that the nitrogen of the ex- 

 creta represents practically the whole of that element entering , 

 the body. This has been denied by some investigators. 



The respiratory products have been estimated in various 

 ways. One consists in measuring the quantity of oxygen sup- 

 plied to the chamber in which the animal under observation is 

 inclosed, and analyzing from time to time samples of the air as 

 it is drawn through the chamber ; and on these restdts the total 

 estimates are based. 



It will appear that even errors in calculating the composi- 

 tion of the food — and this is very variable in different sam.ples, 

 e. g., of flesh; or any errors in the analysis of the urine, or in 

 the more difficult task of estimating the respiratory products, 

 may, when multiplying to get the totals, amount to serious de- 

 partures from accuracy in the end ; so that all conclusions in 

 such a complicated case must be drawn with the greatest cau 

 tion. But it can not be doubted that such investigations have 

 proved of much practical and some scientific value. The labor 

 they entail is enormous. 



Nitrogenous Ec[uilibriiim. — It is possible to so feed an ani- 

 mal, say a dog, that the total nitrogen of the ingesta and egesta 

 shall be equal; and this may be accomplished without the ani- 

 mal losing or gaining weight appreciably or again while he is 

 gaining. If there be a gain, it can usually be traced to the 

 formation of fat, so that the proteid, we may suppose, has been 

 split up into a part that is constructed into fat and a part which 

 is represented by the urea, the fat being either used up or stored 

 in the body. Moreover, an analysis of a pig that had been fed 

 on a fixed diet, and a comparison made with one of the same 

 litter killed at the commencement of the experiment, showed 

 that of the dry nitrogenous food only about seven per cent in 

 this animal, and four per cent in the sheep had been laid away 



