METABOLIC ACTIVITY OF NERVOUS SYSTEM 403 



the conclusion that the tissues rapidly absorb amino acids from 

 the blood when their concentration in the fluid is increased. 



The products thus absorbed in excess from the blood are 

 probably utilized in part for rebuilding broken-down tissue, 

 while at the same time the surplus which is not utilized, as well as 

 the products formed by the catabolism of the brain tissue, are 

 carried away by the circulation. This double process goes on in 

 the brain until all the wear and tear is restored. The second 

 period is represented by a marked reduction in the amount of 

 non-protein nitrogen during the three to eight or nine hours 

 after feeding. At the same time the amount of material to be 

 absorbed from' the intestine diminishes. 



Here we encounter a difficulty arising from the fact that the 

 amount of non-protein nitrogen in the test brains falls below that 

 in the brain of the controls. It seems, however, probable that 

 during the process of resynthetization of the polypeptides, some 

 of the missing amino acids might be supplied by the amino acids 

 that appear in the catabolic products constantly present in the 

 brain tissue. The withdrawal of these amino acids from the 

 normal content of metabolites in the brain tissue for utilization 

 might be responsible in part at least for this smaller amount of 

 non-protein nitrogen in the test brains at this period of active 

 reconstruction. 



This is, however, a pure hypothesis and therefore must await 

 experimental examination. 



When the minimum has been reached, and when no fresh 

 supply of non-protein bodies is coming into the brain, catabolism 

 becomes evident, and as a consequence, the amount of the metab- 

 olites again shows an increase. The slow rise from eight or 

 nine hours up to twenty-four hours after feeding may represent 

 this last period. 



From the observations of Van Slyke and Meyer ('13-' 14), 

 that fasting increases the content of amino acids in the tissues 

 and organs owing probably to autolysis, we may expect that the 

 rise in the content of non-protein nitrogen after it has reached a 

 minimum was also due to autolysis or to the phenomena of fasting. 



Taking all these facts together, we conclude that after feed- 



