2 INTRODUCTION TO STUDY OF METABOLISM 



ing change are unfolded splendid open stretches in full bright 

 light, and dark cloaks of vapor with only here and there a 

 ray breaking through ; thick, still banks of clouds, and rolling 

 veils of mist through which the picture, before clear and 

 transparent, becomes in the next moment dark and uncertain. 



We view with wonder and admiration the vast amount 

 of labor which has been devoted to the study of metabolism 

 since the days when Lavoisier first recognized the vital 

 combustion processes and since, long after, Eobert Meyer 

 and Herman v. Helmholtz inflamed with their genius a torch 

 destined to cast its beams of light into the darkness of all 

 fact and error in the domain of vital phenomena. Under 

 the dominance of the law of conservation of energy the newer 

 physiology of nutrition, founded by Liebig, Pettenkof er, Voit 

 and Pfliiger, is seen to arise; modern chemistry at the task 

 of uncovering the secrets of the intermediate metabolism; 

 and pathologists in earnest endeavor to open to physiology 

 the experiments on man which nature provides. But, on 

 the other hand, how slow the steps of progress to one who 

 never turns to observe the long path already traversed but 

 keeps ever before him the goal constantly receding into the 

 distance before his advance. 



In undertaking to present a clear outline of the problems 

 which mainly occupy the attention of metabolic physiologists 

 to-day it seems best to the author, in conformity with the plan 

 followed in the preceding volume upon the Chemistry of the 

 Tissues, which started with a discussion of the protein prob- 

 lem, to begin this study of metabolism also with the proteins. 

 The proteins and their catabolic products will be first con- 

 sidered, tracing them from their ingestion into the stomach 

 as food, in their course through the body until their deriva- 

 tives disappear in the unknown processes of intermediate 

 metabolism. In turn the carbohydrates and fats will be 

 taken up in the same way ; in preparation for the considera- 

 tion, finally, of the vital combustion processes, the most im- 

 portant phases of metabolic physiology. It is no short and 



