136 



PHYSIOLOGY 



cular; and in the unexcited male it is through a mechanism 

 mediated by the vagus, while in the excited rat the effect is 

 produced through splanchnic endings or fibers. 



Exercise. See Chapter 1, page 16. 



3. Nervous system, (a) Central. In a study of the metabolic 

 activity of the nervous system Hatai ('17 a) determined the 



15 



10 



MGMS. 



AGE DAYS 



50 



100 



150 



200 



250 



300 



350 



400 



Chart 15 Showing the number of milligrams of non-protein nitrogen per 

 gram of the dried brain at different ages. The chart shows also the proportional 

 value of organic extractives together with inorganic salts in the brain of the 



albino rat, as determined by W. Koch and M. L. Koch ('13). Non-protein 



nitrogen; X organic extractives and inorganic salts. 



amount of non-protein nitrogen in the brain under normal con- 

 ditions. The data appear in table 91 and chart 15. 



The number of milligrams of non-protein nitrogen per gram 

 of total solids decreases rapidly during the first 15 days, and 

 less rapidly up to 120 days, after which age it remains nearly 

 constant. This apparent decrease is largely due to the forma- 

 tion of myelin. When determined in relation to the lipoid- 

 free solids, the percentage of nitrogen is the same in the several 

 parts of the brain and in the spinal cord. 



