608 METABOLISM. NUTRITION AND DIETETICS 



dictum, often dignified as a ' law ' of nitrogenous metabolism that : 

 Consumption of protein is largely determined by supply (Practical 

 Exercises, p. 720). 



To explain this many hypotheses were invented. The famous theory 

 of Voit assumed that the food-protein after absorption (the so-called 

 ' circulating-protein ') is carried to the tissues and taken up by the 

 cells, where the greater part of it, without being incorporated with the 

 protoplasm, is nevertheless acted upon, rendered unstable, shaken to 

 pieces, as it were, by the whirl of life (by the intracellular enzymes we 

 might now say less dramatically) in the organized framework, the 

 interstices of which it fills. 



Pniiger, on the other hand, maintained that we have no right to 

 draw a distinction between the consumption of organ- and circulating- 

 protein; that the whole of the latter ultimately rises to the height of 

 organ- or tissue -protein, 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 dis- 

 sociation, as Pfliiger put it, of the living substance. The actual organ- 

 ized elements, however, the existing cells, were not supposed to be 

 destroyed ; the building remained, for although stones were constantly 

 crumbling in its walls, others were being constantly built in. 



A much less plausible view was 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 proteins of the food take part in this 

 process of total ruin and reconstruction. Histo logical evidence, as soon 

 as the methods of examining tissues with the microscope became 

 sufficiently refined, told strongly against this idea. Although the cells 

 of certain glands, such as the mammary, perhaps the mucous glands, 

 and especially the sebaceous glands (p. 565), exhibit changes which, 

 hastily interpreted, might seem to indicate that they break down 

 bodily, as an incident of functional activity, no proof could be obtained 

 of the production of new cells on the immense scale which this theory 

 would require. The relatively small and constant amount of the 

 endogenous metabolism indicates that the actual protoplasmic sub- 

 stance, the living framework of the cell, is comparatively stable; 

 that it does not break down rapidly; and that only a small and 

 fairly constant amount of food- or circulating-protein, or of the 

 decomposition products of protein, is required to supply the waste 

 of the organ -protein. 



We have referred to these theories because there could scarcely be a 

 more instructive instance of the way in which theories become obsolete 

 with the advance of knowledge and of the way in which, with the 

 advance of knowledge, a phenomenon which appears an absolute riddle 

 to one generation may become fairly intelligible to the next, perhaps 

 childishly simple to a third. The student will not derive much benefit 

 from the perusal of this page should he fail to recognize that the 

 hypotheses of the twentieth century are mortal too, and bound for the 

 same bourne as those of the nineteenth. 



It is apparent in the first place from our study of the metabolism 

 of the proteins that the conclusion, ' consumption of protein is pro- 

 portional to supply,' cannot be drawn from the equality of nitrogen 

 intake and nitrogen output. The amino-acids derived from proteins, 

 except that relatively small fraction employed in repairing the 



