UKINK 



previously. This form of metabolism is called endogenous or tissue 

 metabolism, and the final product is not urea to any great extent, but 

 the waste nitrogen finds its way out of the body in other substances, 

 of which creatinine appears to be the most important. This form of 

 metabolism sets a limit to the lowest level of nitrogenous requirement 

 attainable ; the protein sufficient to maintain it is indispensable- 

 Whether the amount of protein which is exogenously metabolised 

 can be entirely dispensed with is at present questionable, and those 

 who seek to replace it entirely by non-nitrogenous food are living 

 dangerously near the margin. A point of considerable importance in 

 this connection is, that the nitrogen of the protein is split off from it 

 by hydrolysis without oxidation. There is thus very little loss of 

 potential energy, the energy of the products being nearly equal to 

 that of the original protein ; it is, however, the non-nitrogenous 

 residue which is mainly available for oxidation, and thus for calorific 

 processes. The fact that muscular work does not normally increase 

 nitrogenous metabolism becomes intelligible in the light of the con- 

 sideration that protein kataboliam, in so far as its nitrogen is concerned, 

 is independent of the oxidations which give rise to heat, or to the 

 energy which is converted into work. Those who in the past have 

 endeavoured to study the relation of muscular work to nitrogen 

 excretion have usually estimated the urea. Now that we know urea 

 is the chief end product of exogenous and not of tissue katabolism, 

 we see that estimations of urea can give us but little real information 

 on this point. The substance which ought to be estimated is 

 creatinine, and it has been found recently that even this is not notably 

 increased in the urine, provided the muscles receive their normal 

 supply of fat and carbohydrate. The body is very economical in so 

 far as protein is concerned, and tissue or endogenous katabolism is 

 kept at a low level. 



What is the proportion between exogenous and endogenous 

 nitrogen katabolism ? It is very difficult to give any exact estimate. 

 We do know that in ordinary diets, the former is far in excess, and 

 probably in a man excreting 16 grammes of nitrogen daily (that is, 

 the amount corresponding to an intake of 100 grammes of protein), 

 only a quarter of this or even less represents tissue breakdown. 



The view we have advanced concerning urea formation, then, is, 

 that it is mainly the result of the conversion, by the liver, of amino- 

 acids absorbed from the intestine into that substance. This view 

 receives confirmation from experiments in which certain amino-acids, 

 such as glycine, leucine, and arginine, have been injected direct into 

 the blood-stream. The result is an increased formation of urea. In 



