METABOLIC ACTIVITY OF NERVOUS SYSTEM 363 



For amino-acid determination, I have followed the procedure 

 of Bock ('17) using also Van Slyke's micro-apparatus. For 

 ammonia nitrogen, the solution was evaporated to nearly 10 cc. 

 in a large test tube (25 x 200 mm.) and after cooling was nearly 

 neutralized with 0.1 N NaOH. To this was added anhydrous 

 sodium carbonate to saturation and also 1 cc. of kerosene to 

 prevent foaming, and the solution was then aerated for thirty 

 minutes. For the urea determination the solution was also 

 evaporated to 10 cc. and cooled. The solution was then made 

 slightly alkaline by means of 0.1 n NaOH and then 1 cc. of 15 

 per cent arlco-urease together with 1 cc. kerosene were added. 

 The urease was allowed to act for 25 minutes at about 23°C. 

 At the end of 25 minutes anhydrous sodium carbonate to satura- 

 tion was added and the solution aerated for 30 minutes. For 

 determination of the non-protein nitrogen I have used a modi- 

 fication by Bock and Benedict ('15) of the micro-method of 

 Folin and Farmer ('12). In all cases the nitrogen was estimated 

 by means of the DuBoscq colorimeter. 



CONTENT OF NON-PROTEIN NITROGEN IN THE ENTIRE BRAIN OF 

 THE ALBINO RAT AT DIFFERENT AGES 



The rats at thirty-five days of age and older were fed with 

 regular laboratory diet twenty-four hours before they were 

 examined. We, however, kept plenty of dog biscuit in the cage 

 and therefore the rats had always enougli to eat. The rats 

 younger than thirty-five days were kept with the mother and 

 were either exclusively nourished with the milk or partially. 

 The dog biscuit was also kept in the cage. It seems therefore 

 that so far as feeding was concerned they were all in the same 

 nutritional state. Altogether 142 rats of different ages were used. 



From table 1 we see that the percentage of total nitrogen in 

 solids (column (g) ) diminishes steadily with advancing age. This 

 is to be expected since the brain during the first 350 days of life 

 rapidly acquires the so-called myelin substance, which is poor 

 in nitrogen. Therefore this diminution of the total nitrogen 

 indirectly indicates the degree of myeUnation. The non-protein 

 nitrogen content (column (i) ) in relation to the total solids also 



