NON-PROTEIN NITROGEN, BRAIN OF FIGHTING RATS 71 



in other cases they assume a fighting attitude only when a shock 

 passes. 



The amount of non-protein nitrogen in the brains of these 

 fighting rats was determined and experiments were also made to 

 determine the amount of non-protein nitrogen in the brains of 

 the rats which had rested for twenty-four hours or more, follow- 

 ing a severe fight for a period of one hour. For this latter pur- 

 pose the stimulated rats were returned to their original cages 

 separately, because such excited rats continue to fight when two 

 of them are placed in the same cage. 



TECHNIQUE 



The rats were etherized and the blood removed by severing the 

 carotid artery, followed by evisceration. The brain was removed 

 quickly and the left half used for the determination of the non- 

 protein nitrogen, while the right half was taken for a water esti- 

 mation. From the dried residue the total nitrogen was deter- 

 mined by the usual Kjeldahl method. For the determination 

 of non-protein nitrogen I have employed the method adopted 

 for my former studies on the metabolic activity of the brain ('19) ; 

 that is, the brain material was finely ground with 2.5 per cent 

 aqueous solution of trichloracetic acid and then transferred to 

 an Erlenmeyer flask (50 cc.) with a small amount of distilled 

 water. The amount of trichloracetic solution taken was always 

 twenty times the weight of the sample in grams, while the 

 amount of water used was five times the brain weight similarly 

 expressed, in volume. The mixture of tissue and reagents in the 

 flask is shaken repeatedly during the first hour and then left 

 for twenty-four hours at room temperature. The clear filtrate ob- 

 tained from this extraction was analyzed by Folin and Farmer's 

 micro-method ('12) as modified by Benedict and Eock ('15). 

 In all cases the nitrogen was estimated by means of the Duboscq 

 colorimeter. The water content of the brain was determined by 

 drying the tissue at 98°C. for one week and the total nitrogen by 

 the usual Kjeldahl method. In this investigation, as in my pre- 

 vious studies, the designation on each flask was replaced by a 



