METABOLISM, NUTRITION AND DIETETICS 



535 



in which the nitrogen was supplied in the form of plasmon, a com- 

 mercial preparation of casein. 



When, instead of protein, the cleavage products of pancreatic 

 digestion are given to an animal, nitrogen 

 equilibrium is also maintained. This 

 has been shown for casein. But if the 

 casein has been hydrolysed by acid, 

 the products will- not preserve nitrogen 

 equilibrium, perhaps because the acid 

 has broken up all the polypeptides (p. 2), 

 which the cells may need as the starting- 

 point of protein synthesis. 



The Laws of Nitrogenous Metabo- 

 lism. 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 protein is pre- 

 sented to the starving body, it seems 

 to 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 consumption is soon 

 adjusted to the new scale of supply, 

 and the protein income spent as freely 

 as it is received. This is the first 

 great law of nitrogenous metabolism, 

 and we may formulate it thus : Con- 

 sumption of protein is largely determined 

 by supply (Practical Exercises, p. 612). 



The explanation of this remarkable 

 fact, or at least an essential part of the 

 explanation, has already been given in de- 

 nning the differences between exogenous 

 and endogenous protein katabolism 

 (p. 497). It is to the exogenous process, 

 and practically to that alone, that the law 

 applies. We have seen that a large pro- 

 portion of the nitrogen of the food-pro- 

 teins split off by hydrolysis in the lumen 

 of the alimentary canal, the intestinal 

 wall, and probably also in the liver, 

 passes by a short-cut to the stage of urea 

 without ever joining the protein of the 

 blood, much less that of the organs, 

 true ' luxus- consumption.' The expenditure is apparently but not 



FIG. 186. CURVES CON- 

 STRUCTED TO ILLUSTRATE 

 NITROGENOUS EQUILIB- 

 RIUM (FROM AN EXPERI- 

 MENT OF VOIT'S). 



The loss of flesh in grammes 

 is laid off along the horizontal 

 axis. The income and ex- 

 penditure corresponding to a 

 given loss are laid off (in 

 grammes of 'flesh') along 

 the vertical axis. The con- 

 tinuous curve is the curve of 

 income ; the dotted curve, of 

 expenditure. With no in- 

 come at all the expenditure is 

 190 grammes ; with an in- 

 come of 480 grammes the 

 expenditure is 492 and the 

 loss 12 grammes. Nitro- 

 genous equilibrium is repre- 

 sented as being reached with 

 an income of about 530 

 grammes ; here the two curves 

 cut one another. 



This is not a form of 



