466 A MANUAL OF PHYSIOLOGY 



gelatin, at least for a few days (Munk) ; so that gelatin economizes 

 proteid in a much greater degree than fat and carbo-hydrates do. 



The Laws of Nitrogenous Metabolism. Within the limits of 

 nitrogenous equilibrium, which is the normal state of the 

 healthy adult, the body lives up to its income of nitrogen ; 

 it lays by nothing for the future. In the actual pinch of 

 starvation the organism becomes suddenly economical. 

 When a plentiful supply of proteid is presented to the 

 starving tissues, they pass at once from extreme frugality to 

 luxury ; some flesh may be put on for a short time, some 

 nitrogen may be stored up; but the tissues soon pitch 

 their wants to the new scale of supply, and spend their 

 proteid income as freely as they receive it. This is the first 

 great law of nitrogenous metabolism, and we may formu- 

 late it thus : Consumption of proteid is largely determined by 

 supply (p. 529). 



Various hypotheses have been offered to explain this remarkable 

 fact. It has been suggested that a large proportion of a heavy 

 proteid meal may be broken up into leucin and tyrosin in the 

 alimentary canal, and may pass by this short-cut to the stage of urea 

 without ever joining the proteid of the blood, much less that of the 

 organs. This would be a form of true luxus-consumption^ of really, 

 and not apparently, wasteful expenditure. The surplus proteids 

 would be shunted out of the main metabolic current at its very 

 source ; and it is conceivable that in this short-cut from proteid to 

 urea we have a kind of physiological safety-valve to protect the 

 tissues from the burden of an excessive metabolism. But it is 

 doubtful whether such a process occurs to any great extent in normal 

 digestion. If it does occur, it may bear a different interpretation, 

 and in any case it probably plays only a subordinate part, and cannot 

 of itself explain all the facts of nitrogenous equilibrium. 



Then, again, it has been said that the luxus-consumption takes the 

 form of oxidation of the surplus proteids in the blood and lymph. 

 Here the shunting would take place farther down the stream, but 

 still high enough up to shield the tissue elements from excessive 

 metabolic work. This theory of luxus-consumption breaks down, 

 however, under the accumulating evidence that the oxidative changes 

 go on chiefly in the living cells and not in the extra-cellular fluids. 



We seem driven to locate the metabolism of actually absorbed 

 proteids, as well as of other food substances, within the cells of the 

 body ; and there are three chief views as to the manner of this 

 metabolism : 



(a) That the actual protoplasmic substance, the living framework 

 of the cell, is comparatively stable ; that it does not break down 



