124 LIFE AND DEATH. 



Chemical energy is the only energy, according to the 

 second law of energetics, which may be transformed 

 into vital energy. It is true at any rate for animals ; 

 for in plants it is otherwise. There the vital cycle has 

 neither the same point of departure nor the same final 

 position. The circulation of energy does not take 

 place in the same manner. 



On the other hand, and this we are taught by the 

 third law, energy brought into play in vital pheno- 

 mena is finally liberated and restored to the physical 

 world in the form of heat. We have just said that 

 this release of heat is employed in raising the tem- 

 perature of the living being. It is animal heat. 



Thus there are two forms of energy supplied by 

 food, chemical and thermal. 



It must be added that these are not the only forms, 

 but the principal, and by far the most important It is 

 not absolutely true that heat is the only outcome of the 

 vital cycle. It is only so in the subject in repose, con- 

 tented to live idly without doing external mechanical 

 work, without lifting a tool or a weight, even that of its 

 own body. And again, speaking in this way, we neglect 

 all the movements and all the mechanical work which 

 is done without exercise of the volition, by the beating 

 of the heart and of the arteries, the movements of 

 respiration, and the contractions of the digestive tube. 



Mechanical work is, in fact, another possible termi- 

 nation of the cycle of energy. But there is no 

 longer anything necessary or inevitable in this, since 

 motion and the use of force are in a certain measure 

 subordinated to the capricious volition of the animal. 1 



1 There is another reason why the role of mechanical energy, 

 compared with that of thermal energy, is reduced, in the partition 

 of afferent, alimentary energy at least, in animals which have not 



