470 A MANUAL OF PHYSIOLOGY 



quantity in the tissues or the food, they constitute the main 

 source of the energy of muscular contraction (p. 584). 



Experience has shown that the minimum quantity of 

 nitrogen required in the food of a man whose daily work 

 involves hard physical toil is higher than the minimum 

 required by a person leading an easy sedentary life. This is 

 evidently in accordance with the view that proteid is actually 

 used up in muscular contraction ; but it is not inconsistent 

 with the opposite view. For the body of a man fit for con- 

 tinuous hard labour has a greater mass of muscle to feed 

 than the body of a man who is only fit to handle a com- 

 posing-stick, or drive a quill, or ply a needle ; and the 

 greater the muscular mass, the greater the muscular waste. 

 But if an animal just in nitrogenous equilibrium on a diet 

 of lean meat when doing no work, is made to labour day 

 after day, it will lose flesh unless the diet be increased. 

 This must mean that some of the proteid is being diverted 

 to muscular work, and that the balance is not sufficient to 

 keep up the original mass of ' flesh ' (see p. 479). 



(2) Income and Expenditure of Carbon. This division of the 

 subject has been necessarily referred to in treating of the 

 nitrogen balance-sheet, and may now be formally completed. 



Carbon Equilibrium. A body in nitrogenous equilibrium 

 may or may not be in carbon equilibrium. It has been 

 repeatedly pointed out that the continued loss or gain of 

 carbon by an organism in nitrogenous equilibrium means 

 the loss or gain of fat ; and, since the quantity of fat in 

 the body may vary within wide limits without harm, carbon 

 equilibrium is less important than nitrogen equilibrium. It 

 is also less easily attained when the carbon of the food is 

 increased, for, although the consumption of fat is to a certain 

 extent increased with the supply of fat or fat-producing food, 

 there is by no means the same prompt adjustment of ex- 

 penditure to income in the case of carbon as in the case of 

 nitrogen. 



Carbon equilibrium can be obtained in a flesh-eating 

 animal, like a dog, with an exclusively proteid diet ; but 

 a far higher minimum is required than for nitrogenous 

 equilibrium alone. Voit's dog required at least 1,500 



