138 HENRY H. DONALDSON 



interpreted in this way, constitute indirect evidence for the view 

 that the axone is an outgrowth from the cell body. 



The medullation process as such does not reduce the 'percentage 

 of water. This statement, already made in the " fourth" conclu- 

 sion on p. 19 is here repeated because there is a more or less 

 widely diffused opinion that the medullary sheath is a structure 

 containing less water than the axones, and that it is the addition 

 of the myeline, as it appears in the medullary sheaths, which 

 largely serves to reduce the percentage of water in the white sub- 

 stance, and thus in the entire mass of the central system. For 

 this there is no evidence. Charts 4 and 5, exhibiting the increase 

 in the weight of water with increasing brain weight and cord 

 weight, show no changes in the increment of water which would 

 warrant such an explanation. It appears most probable therefore 

 that the medullary sheaths when first formed have approximately 

 the percentage of water characteristic for the axones at the time 

 of their myeli nation, and after that, in company with the axones, 

 they undergo a slow but steady diminution in water content. 



A few separate determinations of the percentage of water have 

 been made by various investigators on the white and gray sub- 

 stances of man and other larger mammals. These show without 

 exception that between birth and maturity there is a greater loss 

 of water in the white substance than in the gray. In these cases 

 of course the white substance at maturity is always medullated, 

 and thus the results do not answer the question whether the forma- 

 tion of the medullary sheaths has contributed to the diminution 

 in the percentage of water. That the axones previous to 

 medullation do show a reduction in the percentage of water with 

 advancing age, is indirectly indicated by my own observations 

 in the following way: 



At birth (i. e., 5 grams body weight) the average percentage of 

 water in the rats' brains of both sexes is 87.8 per cent (table 1). 

 At eleven days of age, as shown in chart 2, it is about 84.8 per 

 cent, a loss of three units, yet it is not until after the eleventh day 

 that medullation in the brain begins. The percentage of water 

 in the brain therefore falls off before medullation begins, and the 

 nerve substance, cell bodies and axones, are the portions in which 

 this diminution chiefly occurs. 



