INTRODUCTION 21 



is needed. As it would appear improbable that creatinine 

 originates in the body from the type of endogenous metabolism 

 Folin has created ; also, that, even if this were the case, it cannot 

 possibly be a measure of such metabolism, the deduction drawn 

 that, being a small and constant output, therefore only a small 

 and constant supply of protein in the diet to replace the nitrogen 

 represented by its loss is necessary must be fallacious. 



It may be concluded, therefore, that, so far as the evidence 

 goes, the output of creatinine in the urine cannot be made use 

 of as evidence to support the arguments of those who consider 

 that the protein requirements of the body would be covered by 

 the presence of a very limited amount of nitrogen in the daily 

 food. 



This finishes our summary of the recent advances that have 

 been recorded in the investigations into the intermediate stages 

 of protein metabolism. Beyond a knowledge of certain changes 

 that take place in proteins during the process of digestion, very 

 little accurate information is available. With regard to the 

 series of chemical reactions by which the products of tryptic 

 digestion are made use of by the protoplasm of the cell for its 

 growth, nutrition, and repair, we know practically nothing. 

 Still, a beginning has been made, and enough light has been shed 

 on some of the individual chemical reactions of the body to 

 enable us to look forward with hope and confidence to the 

 future ; otherwise, as Verworn puts it, the fact that wherever 

 the gross activities of the body are traced to the activity of the 

 individual cells, we always come upon an unsolved problem 

 might lead us to maintain with Bunge : "All processes in the 

 organism which may be explained mechanically are no more 

 phenomena of life than are the movements of the leaves and 

 branches of a tree that is shaken by the storm, or the move- 

 ments of the pollen that the wind wafts from the male poplar 

 to the female." 



Amongst those things that give promise of hope in the future 

 are the advances that have been made in the conception of the 

 part played by enzyme action in metabolic processes. At present 

 a great variety of chemical reactions within the body are brought 

 about in virtue of the presence of a large group of intracellular 

 enzymes. In addition to the action of the digestive enzymes, 

 which resolve protein, carbohydrates, and fats into their com- 

 ponents, many oxidizing enzymes have been found in the various 



