536 A MANUAL OF PHYSIOLOGY- 



really wasteful, since the greater part of the energy of the protein 



molecule is still obtained by the oxidation of the carbon-rich 



residue.- The surplus nitrogen is shunted out of the main metabolic 



current at its very source. Some writers conceive that in such 



a short-cut from protein to urea we have a kind of physiological 



safety-valve to protect the tissues from the burden of an excessive 



metabolism. And if by this is meant that it is advantageous to 



the tissues that a special mechanism should exist to eliminate a 



surplus of nitrogen which they do not require, and which they cannot 



store, and to present them with a residue which they can utilize, 



the conception is certainly correct. But there is no good evidence 



that in the presence of an over-abundant supply of protein the 



endogenous protein metabolism would be essentially modified. 



And another interpretation of the preliminary splitting off and 



elimination of nitrogen, which must under ordinary conditions of 



diet account for a considerable formation of urea, has been alluded 



to in discussing the influence of the food on the serum-proteins 



(p. 498). The surplus of those amino-acids which the food-proteins 



contain in greater quantity than the body-proteins cannot be 



utilized for building protein in the tissues. Also at any moment 



;the magnitude of this non-utilizable surplus will depend upon the 



; quantity of that one of the indispensable amino-acids which is 



: present in the smallest amount. For the proper proportion must 



'be preserved between the different ' stones ' out of which the mole- 



, cule is built. When a single amino-acid is introduced into the body, 



;it is at once changed into urea and excreted, since it cannot be 



utilized by itself for building up protein. 



' } The relatively small and constant amount of the endogenous 

 metabolism indicates that the actual protoplasmic substance, 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. 



Other explanations of the relation between consumption and 

 supply of protein have been put forward. Although some of these 

 may now possess merely a historical interest, they must be mentioned, 

 all the more as it has not been shown that the production of urea 

 in the liver from decomposition products of proteins brought to it 

 in the portal blood is the only form of exogenous protein katabolism. 

 The famous theory of Voit assumes 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 in the organized framework, the interstices of which 

 it fills. 



Pfliiger, on the other hand, has 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-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 dissociation, as Pfluger puts it, of the living substance. 

 The actual organized elements, however, the existing cells, are not 



