444 H. H. DONALDSON 



It is also well known that in some mammals all the axons in 

 the central nervous system are unmyelinated at birth, while in 

 other species a greater or smaller number of them may already 

 have their sheaths. In all cases, however, an active formation 

 of myelin occurs during the period of rapid growth (Koch and 

 Koch '13). 



In a previous study on the percentage of water in the albino 

 rat I was misled by certain graphs (Donaldson '10) to the con- 

 clusion that probably both the axons and their myelin sheaths 

 changed in their water content so as to produce the well known 

 reduction of water which is observed, but further study has 

 shown that this is an incorrect view, and it is the object of this 

 paper to present the evidence of my revised conclusion. 



Since it has already been shown that the loss of water in the 

 human brain follows the same course as in the brain of the albino 

 rat — and has similar limits (Donaldson '10) — it is permissible to 

 use in the argument certain observations on the human brain. 



From the data for the human brain already in the literature 

 I have selected those published by de Regibus ('84), because 

 this author evidently examined only the outer layers of the cor- 

 tex when making his determinations for the water in the gray 

 substance and because he was able also to obtain remarkably 

 uniform results for all of his determinations. 



De Regibus tested four male brains, 25 to 76 years of age and 

 three female brains, 30 to 60 years of age. 



In Table 1 there appear also determinations of the water 

 content in the human cortex and in the fibers at birth. These 

 are based on the records of Weisbach ('68). 



This enables us to contrast in Table 1 the conditions at birth 

 with those at maturity. 



TABLE 1 



Percentage of water in the gray and white substance of the human brain at birth and 



at maturity 



CORTEX (gray) 



At birth (Weisbach) 



At matoirity (de Regibus) 



per cent 



88 



per cent 



88 

 70.4 



