120 HENRY H. DONALDSON 



directions, and to all of these gentlemen I wish here to express my 

 obligations for assistance. 



Technique. The determination of water has been made for the 

 entire encephalon severed from the cord at the level of the first 

 spinal nerve, and for the entire cord, the spinal nerves having 

 been clipped away at their origin from the cord. The rats used 

 were chloroformed, eviscerated and rapidly dissected. No spe- 

 cial device for preventing evaporation during dissection was used. 



The percentage of water applies therefore to the nerve struct- 

 ures proper, surrounded by the meninges and containing such 

 blood as usually remains after the foregoing treatment. 



The details of the technique according to which the brain and 

 spinal cord were removed have been already given (see Donald- 

 son '08, p. 346). Each brain or cord was placed in a small glass- 

 stoppered weighing bottle, and after being weighed in the fresh 

 state, was dried in a closed water bath which had a temperature 

 ranging from 85°-95° C. and then was cooled in a dessicator over 

 sulphuric acid, and reweighed. 



The brain took somewhat longer to dry as a rule than the spinal 

 cord, but usually seven days in the water bath served to bring it 

 to a constant weight. At various times objections have been 

 raised to the determination of the percentage of water by the use 

 of heat. The other method which is most approved is that of 

 drying the material at the room temperature or somewhat above, 

 in a vacuum over sulphuric acid. 

 A comparison of these two methods has been made for the brain 

 and cord of the rat, but no significant differences have thus far 

 been found. I shall, however, reserve the discussion of the data 

 on which this statement is based for another occasion. 



The percentage of water in the brains of albino rats of different 

 body weights. The number of cases is 409 males and 212 females. 

 The mean values for the percentage of water in the brain for given 

 body weights differing by 10 grams, as determined by a correla- 

 tion table, are entered in table 1. 



The examination of table 1 shows for the brain a relative loss 

 of water amounting to about ten units between birth (body 

 weight 5 grams) and the end of the series. This loss is most 



