CHAPTER IX. 
THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 
TuRroucGHout the preceding chapter, particularly in considering 
the indirect methods of animal calorimetry, it has been assumed 
that the law of the conservation of energy applies to the animal 
body. This is the fundamental postulate upon which all study of 
nutrition from the standpoint of energy is based, and it is of prime 
importance, therefore, to examine into the experimental evidence 
upon which it is based. 
The processes of metabolism are essentially chemical processes, 
and, like other chemical reactions, are accompanied by thermal 
changes, resulting as a whole in a liberation of kinetic energy. 
From this point of view, then, the subject may be regarded as a 
branch of thermo-chemistry. 
The applicability of the law of the conservation of energy, and 
in particular of the law of initial and final states, to the most diverse 
chemical reactions has been amply demonstrated by the investiga- 
tions of Hess, Berthelot, Thomsen, and others. It might seem, then. 
in view of the chemical nature of metabolism, that we were justified 
in assuming the same law to apply also to the reactions taking place 
in the body, especially since investigations in other fields of science 
have led us to regard it as one of the fundamental laws of the uni- 
verse. On the other hand, however, the reactions occurring in the 
body are vast in number, are of the most varied character—oxida- 
tions, reductions, syntheses, cleavages, hydrations, etc.—are infi- 
nitely more complex than those which the chemist can produce in his 
laboratory, and finally, our knowledge of them is as yet but very 
partial and fragmentary. Moreover, the matter composing the 
body is living matter, and whatever view we may take as to the 
nature of life the properties of living matter differ from those of 
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