METABOLIC ACTIVITY OF NERVOUS SYSTEM 367 



When the data given by W. Koch and M. L. Koch were com- 

 puted as indicated in table 2, and the results plotted (chart 1) 

 we at once find that the organic extractives as a whole (including 

 the inorganic salts) diminish with advancing age, and further- 

 more all the observations, except one case, fall very close to the 

 curve described by the non-protein nitrogen. We infer from this 

 result that the non-nitrogenous fraction as well as salts show, 

 during the growth of the brain, changes similar to the nitrogenous 

 fraction. 



THE NITROGEN VALUES GIVEN BY NON-PROTEINS, THE AMINO 

 ACIDS, AND THE UREA AND AMMONIA 



So far I have dealt wdth the changes in the relative amount of 

 the total nitrogen, as well as of the non-protein nitrogen during 

 the growth of the brain. I now wish to consider the extent of the 

 normal variation of various extractive bodies found in the adult 

 brain. The data showing the nitrogen content of these four 

 components are given in table 3. 



The data given in table 3 were obtained during the course of 

 investigations connected \\ith other problems. The rats used 

 for these investigations had been without food wdth the exception 

 of water, for forty-eight hours before killing in the hope of put- 

 ting them in a uniform nutritional state. It w^as found that a 

 period of forty-eight hours was about long enough to remove the 

 contents from the entire digestive tract. The rats at the end of 

 the forty-eight hours w-ere as \dgorous and active as the rats fed 

 in the ordinary way. In this case I have given the nitrogen in 

 milligrams as found in 100 grams of fresh tissue. This manner 

 of presentation has some advantage when the water content of 

 tissue is nearly constant, and in fact this procedure has been 

 widely adopted by most bio-chemists. 



NON-PROTEIN NITROGEN 



The number of milligrams of non-protein nitrogen per 100 

 grams of moist brain is exceedingly uniform. The average 

 value (157 mg.) is slightly low^er than that given by some other 

 organs of the same animal (table 4) as well as those given by the 



