2 PHYSIOLOGICAL CHEMISTRY. 



the changes, the chemical expression of which appears often 

 so extremely simple. It may be the part of wisdom to admit 

 at once that this question is one beyond our power to answer. 

 Then, again, we find ourselves attracted by questions of form, 

 function and general conditions of the existence of organisms 

 upon the earth in addition to those of composition and mode 

 of formation. In touching these we enter upon the field of 

 General Biology and soon recognize that in this vast and inde- 

 pendent science there is much contained which has no possible 

 bearing on our problems of chemistry. But some knowledge 

 of biological science is certainly essential to a proper under- 

 standing of the chemistry of living beings. 



As proper subjects of inquiry in Physiological Chemistry 

 we recognize mainly the following: (a) The nutrition of 

 plants and animals and the composition and properties of 

 the nutrient substances, (b) The changes which the nutrients 

 undergo before and during the processes of assimilation, (c) 

 The agents of preparation for assimilation and the general 

 conditions of their activity, (d) The fate of the assimilated 

 nutrients and the nature of the products of degradation, (e) 

 The absorption or liberation of energy. 



In the broader sense the discussion is extended to the con- 

 ditions appearing in the life history of plants and the lower 

 animals as well as of man, but ordinarily the narrower field 

 of the life of man and the higher animals alone is considered, 

 and in the present work the latter limits will be observed. But 

 a brief discussion of the general relations of plants to animals 

 will not be out of place. 



At one time it was very generally held that the cell activ- 

 ities in the plant are essentially different from those in the 

 animal, the work in the one case being looked upon as wholly 

 synthetic, while in the other it was assumed to be disinte- 

 gration or analysis. But this is not quite correct. The reac- 

 tions in the two classes of structures are qualitatively much 

 the same, although the quantitative differences are so great 

 that we almost lose sight of qualitative similarities. The 



