402 PHYSIOLOGICAL CHEMISTRY. 



lent of the accomplished work. A part of the increased heat 

 liberation is called for by increased internal work also. It 

 follows, therefore, that the working animal is warmer than 

 the passive animal, but the increase is not proportional to the 

 food consumed or oxygen absorbed. 



External Work Equivalent. Although the animal is able 

 to convert but a limited portion of the potential energy of the 

 food into external work, as a machine it is still much more 

 perfect than the steam engine. This is especially true of man. 

 In the best steam engines not more than about 12 per cent of 

 the potential energy of the fuel can be recovered in the form 

 of work. In animals, through a short period, the transforma- 

 tion may amount to as much as 35 per cent of the net available 

 potential energy. In making such comparisons, however, it 

 must be remembered that the animal can work but a limited 

 time. In the rest periods of the animal the loss of heat, with- 

 out any corresponding mechanical gain, goes on. 



The law defining the maximum conversion of heat into work through 

 the steam engine is well known. The extent of the limitations in the 

 animal are not known. In the steam engine the limitation depends on 

 the relation of the highest heat of the steam to the temperature of the 

 condenser. If T is the absolute temperature of the live steam and t the 

 temperature of the condenser the maximum transformation of heat into 

 work cannot be greater than 



T t 

 T 



THE INTERNAL WORK OF THE ANIMAL. 



Even when the animal appears passive a great deal of work 

 is going on which may be described as internal work. The 

 nature and extent of some of this is known with a fair degree 

 of accuracy, while for the extent of the metabolism correspond- 

 ing to other kinds of work we have not much beyond conjec- 

 ture. It is possible to calculate approximately the work done 

 in maintaining the circulation of the blood, and in respiration, 

 and some attempts have been made to estimate the work of the 

 other muscles at rest; but to approximate the work done in 

 masticating, digesting, transporting and transforming the food 



