i 4 4 THE CHEMISTRY OF CATTLE FEEDING 



required for maintenance. Two questions arise in this 

 connection, viz. (i) the kind, and (2) the quantity of the food 

 that must be added to the maintenance ration when an animal 

 does work. 



As regards the former, Liebig argued that since animal 

 work is done by the contraction and relaxation of the muscles, 

 it must involve the deterioration of the muscular (nitrogenous) 

 tissues of the body. These tissues can only be restored from 

 the nitrogenous matter of the food, and he concluded, there- 

 fore, that the additions to the maintenance rations should 

 consist mainly, if not wholly, of nitrogenous matter. It is 

 true that animal work is done by contraction and relaxation 

 of the muscles ; but it does not follow that the energy which 

 causes them to contract and relax is of necessity derived 

 from the oxidation of their substance. The work of an engine 

 is done by the thrust of the piston ; but the energy which causes 

 the thrust is derived from the combustion of fuel, not from the 

 oxidation of the piston itself or of the cylinder in which 

 it acts. 



At any rate, Liebig's theory is untenable. The experi- 

 ments of Voit and Pettenkofer l and they have been confirmed 

 by many others clearly show that the amount of nitrogenous 

 matter is practically the same when the animal does work as 

 when it is resting. On the other hand, larger quantities of 



1 The following results were obtained in experiments on a man in the 

 respiration chamber. In each case the figures refer to periods of twenty- 

 four hours. 



