METABOLISM, NUTRITION AND DIETETICS 467 



rapidly ; and that only a relatively small and fairly constant amount 

 of food- or circulating-proteid is required to supply the waste of the 

 organ-proteid. It is assumed that the greater part of the former, 

 without being incorporated with the protoplasm, is nevertheless acted 

 upon, rendered unstable, shaken to pieces, as it were, by the whirl 

 of life in the organized framework, the interstices of which it fills. 



(b) That we have no right to draw a distinction between the 

 consumption of organ- and circulating-proteid ; that the whole of the 

 latter ultimately rises to the height of organ-proteid, and passes on 

 to the downward stage of metabolism only through the topmost step 

 of organization. An increase in the supply of nitrogenous material 

 in the blood must, on this view, be accompanied with an increased 

 tendency to the break-up, the dissociation, as Pfliiger puts it, of the 

 living substance. The actual organised elements, however, the 

 existing cells, are not supposed to be destroyed; the building 

 remains, for although stones are constantly crumbling in its walls, 

 others are being constantly built in. 



(c) That the tissue elements themselves are short-lived ; that the 

 old cells disappear bodily and are replaced by new cells ; and that 

 the whole of the proteids of the food take part in this process of 

 total ruin and reconstruction. 



Histological evidence is on the whole strongly against (c). 

 Although the cells of certain glands, such as the mammary, 

 sebaceous, and perhaps the mucous glands, are known to break 

 down bodily as an incident of functional activity, in most organs 

 there is no proof of the production of new cells on the immense 

 scale which this theory would require. There is but little evidence 

 which would enable us to decide with confidence between (a) and 

 (), although the observation of Munk, that a dog fed with proteids 

 and carbo-hydrates after a thirty days' fast used up less proteid than 

 the minimum in starvation, certainly suggests that, under those 

 conditions at least, the proteids of the food were all built up into 

 the protoplasm of the tissues. 



A second law of nitrogenous metabolism is that within 

 normal limits it is nearly independent of muscular work, that 

 is to say, the quantity of nitrogen excreted by a man on a 

 given diet is practically the same whether he rests or works. 

 Before this was known it was maintained by Liebig that 

 proteids alone could supply the energy of muscular contrac- 

 tion that, in fact, proteids were solely used up in the 

 nutrition and functional activity of the nitrogenous tissues, 

 while the non-proteid food yielded heat by its oxidation. 

 As exact experiments multiplied, it was found that muscular 

 work, the production of which is the function of by far the 

 greatest mass of proteid-containing tissue, had little or no 

 effect upon the excretion of urea in the urine. More than 



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