INTRODUCTION. 



THE processes in the animal organism which are concerned with the 

 destiny of the protein, whether of the food or of the tissues, are 

 collectively known as protein metabolism. The metabolic changes are 

 again arbitrarily divided into two phases, the anabolic and the cata- 

 bolic, although at present there is but little evidence to show when 

 one stage ceases and the other begins, or what products are the result 

 of the action of the one and what the result of the other. 



The problem of the metabolism of protein is one of the most 

 complex and obscure in physiology, because the causes which bring 

 about these changes are practically unknown. Carl Voit, even in 

 1902, after forty years of strenuous work in this field, could say no 

 more than that " the unknown causes of metabolism are found in the 

 cells of the organism. The mass of these cells and their power to 

 decompose materials determine the metabolism." 



We have at present a more or less detailed knowledge of the 

 preliminary processes, which occur in the preparation of the food 

 protein as a suitable pabulum for the tissues, during gastro-intestinal 

 digestion. We know that the ^protein is practically completely dis- 

 integrated into very simple compounds before absorption and utiliza- 

 tion take place. We have also a fairly complete knowledge of the 

 products excreted in the urine the waste products of protein meta- 

 bolism and we have been able to associate many of the variations in 

 the metabolism, or, at least, alterations in the conditions affecting the 

 organism, with variations in the output of these waste products. We 

 know, for instance, that an animal fed on a food poor in protein 

 excretes very much less nitrogen than one fed on a nitrogen-rich diet, 

 and that there is a great diminution in the proportion of the nitrogen 

 excreted as urea. We further know that creatine is never present, or 

 only in minute amount, in normal urine, when the food taken contains 

 no creatine, yet after a comparatively short period of fasting creatine is 

 always to be found, and again we know that a high output of ammonia 

 is closely associated with the production of acids in the tissues. Of 



