138 TlMEHRI. 
individual, of longer duration and more complete and 
searching in their character, clearly established that, 
though some increase of egested nitrogen did occur during- 
the period of work, it was due to increased nitrogenous food, 
the small excess of egested over ingested nitrogen which 
was noticed simply representing the additional wear of 
the tissues in converting the chemical energy of the food 
into potential energy. 
There is thus an exact analogy between the operation 
of the animal system and that of a machine. In the one 
the fuel supplies the source of power, in the other the 
food. In the working of a machine a certain amount of 
wear goes on, necessitating renewal and repair. With 
the animal system a certain amount of tissue-waste re- 
quires renovation, this waste being dependent on the 
amount of work done in converting the force capability 
of the food into the manifestation of muscular power. 
In preparing, then, a dietary adapted for work, the 
following points have to be attended to : — 
lstly. That there should exist in the food sufficient force-value to 
yield the amount of work to be undertaken. 
2ndly. That sufficient nitrogenous food should be included, so as to 
supply material not only for the renewal of the normal tissue waste, but 
also for the compensation of the increased tissue wear and tear. 
Hrdly. That the food chosen should be of such a nature that, while 
possessing as much force-value as possible, it should be easily digested 
and assimilated, and thus diminish the tax upon the system. 
As agents for supplying muscular power the non-ni- 
trogenous foods are especially adapted. The ultimate 
oxidation products of all these are carbonic-acid gas and 
water, substances easily eliminated from the system ; 
and as complete oxidation occurs with these bodies un- 
der physiological conditions, their full force value is 
obtained. In addition they possess, as a rule, the ad- 
