420 MANUAL OF PHYSIOLOGY. 



nitrogenous substance eliminated (urea) is greatly in excess of the 

 low standard found during complete abstinence from food. From 

 this it would appear that even when supplied with an amount of 

 nitrogenous material equal to that drawn from the tissues during 

 starvation, an animal still takes a further supply from its own 

 textures. Thus the body subsists on the scanty allowance of nu- 

 triment it borrows from the tissues, during starvation, only so 

 long as there is absolutely no food income, and the moment any 

 food is supplied an increased expenditure is set up, the income is 

 exceeded, and a deficit occurs, which is best seen in the nitrogen 

 balance. 



It follows, then, that feeding an animal on an amount of food 

 stuffs exactly corresponding to the quantity of nutriment ab- 

 stracted from its own textures during total abstinence is only a 

 slower form of starvation. 



With regard to nitrogenous substances, it has been proved that 

 nearly three times as much as the amount eliminated during 

 starvation is required to establish an equilibrium between the 

 income and expenditure of those special substances, and that 

 any less than this leads to a distinct nitrogenous deficit. 



The third case mentioned in a previous paragraph (viz., in 

 which the nutritive equilibrium is exactly maintained, so that 

 the body weight remains unaltered, the gain and loss being equal) 

 is the most important one for us to determine, since its final set- 

 tlement would enable us to fix the most beneficial standard of 

 diet. Unfortunately, this case is also the most difficult upon 

 which to come to a satisfactory conclusion, for the following 

 reasons : 



1. The elaborate nature of the conditions imposed during the 

 experiment makes it difficult to carry on the investigation with 

 scientific accuracy. 



2. Even when the amounts of gain and loss exactly correspond 

 we cannot say that we have the best dietary ; because some of the 

 income may be quite useless, and pass through the economy with- 

 out having any function to perform in it, and yet appear in the 

 output so as to give an accurate balance. 



