Il8 LIFE AND DEATH. 



place, it has determined the composition of the 

 constituent materials of the organism ; then analyzing 

 qualitatively and quantitatively all that penetrates into 

 that organism' in a given time — that is to say, all the 

 alimentary or respiratory ingesta, and all that issues 

 from the organism, i.e., all the excreta, all the egesta, 

 — it has drawn up mitritive balance sheets, corre- 

 sponding to the various conditions of life, whether 

 naturally or artificially created. And thus we can 

 determine the alimentary regimes which give too 

 much, and which give too little, and which finally 

 restore equilibrium. 



We do not propose to give a detailed account of 

 this scientific movement. This may be done in mono- 

 graphs. All we wish to indicate here is the most 

 general result of these laborious researches — that is 

 to say, the laws and the doctrines which are derived 

 from them, and the theories to which they have given 

 birth. It is by this alone that they are brought into 

 relation with general science, and may therefore 

 interest the reader. The facts of detail are never 

 lacking to the historian ; it is more profitable to show 

 the movement of ideas. The theories of alimentation 

 bring into conflict very different conceptions of 

 the vital functional activity. And here we find a 

 confused medley of opinions on which it is not without 

 interest to endeavour to throw some light. 



§ I. Food, a Source of Energy and Matter. 



Definitions of Food. — Before the introduction into 

 physiology of the notion of energy, no one had 

 succeeded in giving an exact idea and a precise 

 definition of food and alimentation. Every physio- 



