120 LIFE AND DEATH. 



vention of the notion of energy enables us more 

 completely to understand the true nature of food. 

 We must, in fact, have recourse to the energetic con- 

 ception if we desire to take into account all that the 

 organism requires from food. It not only requires 

 matter, but also, and most important of all, energy. 



Investigators so far concentrated their thoughts ex- 

 clusively on the necessity of a supply of matter that 

 is to say, they only looked upon one side of the 

 problem. The living body presents, at each of its 

 points, an uninterrupted series of disintegrations and 

 reconstitutions, the materials being supplied from 

 without by alimentation, and rejected by excretion. 

 Cuvier gave to this unceasing circulation of ambient 

 matter throughout the vital world the name of vital 

 vortex, and he rightly saw in it the characteristic of 

 nutrition, and the distinctive feature of life. 



This idea of the cycle of matter has been com- 

 pleted in our own time by that of the cycle of 

 energy. All the phenomena of the universe, and 

 therefore those of life, are conceived of as energetic 

 transformations. We now look at them in their relation- 

 ship instead of considering them individually as of old. 

 Each has an antecedent and a consequent unity with 

 which it is connected in magnitude by the law of 

 equivalents taught us by contemporary physics. And 

 thus we may conceive of their succession as the 

 cycle of a kind of indestructible agent, which changes 

 only apparently, or assumes another form as it passes 

 from one to the other, but its magnitude remains 

 unaltered. This is energy. Thus, in the living being 

 there is not only a circulation of matter, but also a 

 circulation of energy. 



The most general result of research in physiological 



