INTRODUCTION 15 



may therefore be looked on as the result of antagonistic reactions, 

 the balancing of which results in nitrogenous equilibrium. 



The only other point it is necessary to refer to in this summary 

 of the intermediate products of nitrogenous metabolism is the 

 importance that has been attached to the constancy in the 

 amount of certain of the waste products that are eliminated in 

 the urine. Folin has shown, and his results have been confirmed 

 by many observers, that the output for the same individual of 

 creatinine and neutral sulphur in the urine is constant, that this 

 output does not alter with the change from a nitrogen-rich to 

 a nitrogen-poor diet, and that the output seems only to vary 

 with the mass of the nitrogenous tissues of the body. (Cathcart, 

 experimenting on himself, found a distinct difference in the 

 output of creatinine whilst fasting and on an ordinary mixed 

 diet.) Folin holds that these substances, creatinine and neutral 

 sulphur, are characteristic of endogenous or tissue metabolism, 

 whilst urea and other nitrogenous constituents, the output of 

 which varies with changes in the composition of the diet, are 

 indicative of an exogenous or intermediate metabolism. From 

 the results obtained for the output of creatinine in the urine 

 which seem to vary for the same individual under different 

 dietaries from 0-353 to 0-534 gramme, expressed as nitrogen a 

 whole theory of protein metabolism has been built up, and great 

 importance has been attached to it as affording strong evidence 

 that only very small quantities of nitrogen are necessary to 

 maintain the organism in full bodily vigour just that amount, 

 in fact, that is required to make good the ravages of endogenous 

 metabolism, of which the constant output of creatinine and 

 neutral sulphur in the urine is in some way a measure. The 

 underlying suggestion is, although Folin himself does not state 

 so, that these two forms of metabolism are absolutely distinct 

 from one another, and that urea is almost, if not entirely, an 

 excretory product of exogenous metabolism. 



According to the older views of Liebig and Pfliiger, the products 

 of digestion circulate in the blood-stream, or are built up into the 

 bioplasm, the proteins of the blood plasma and lymph forming 

 the source from which the cells derive their supply. All kata- 

 bolism of protein material must, therefore, be preceded by 

 anabolism. The discovery that a large proportion of the 

 nitrogen of a protein meal appeared in a few hours as urea entailed 

 the belief that the protein absorbed from the food has been very 



