136 HENRY H. DONALDSON 



weight of the brain is only about two fifths that of the spinal 

 cord (Donaldson '08, p. 355 and p. 358) and at maturity the rela- 

 tive amount of white matter in the brain is much less than in the 

 spinal cord (Donaldson and Davis '03; Watson '03). Such are 

 the characteristics of these two portions of the central nervous 

 system which are of interest to us in connection with the per- 

 centage of water. 



Explanation of the greater percentage of water in the brain at 

 birth. The greater percentage of water in the brain at birth may 

 be connected with some of the facts just enumerated, namely, the 

 lesser maturity of the brain, as indicated by the longer continuance 

 of cell division, by the later onset of medullation, and by the lesser 

 proportion of supporting tissues and other non-nervous constit- 

 uents. All of these conditions would tend to give the brain a 

 higher percentage of water. 



During the subsequent growth, the slower diminution of the 

 percentage of water in the brain is due to the fact that the relative 

 increment of water is greater than in the case of the cord (see 

 charts 4 and 5). This however is again no explanation and 

 leaves the conditions which control the increment of water in 

 each division of the system still to be described. 



As can be seen from inspecting chart 4 it is possible to express 

 the events taking place by assuming that the initial weight of 

 material in the brain maintains its initial percentage of water 

 and that each of the subsequent increments in weight from just 

 after birth to old age has a relatively low and slightly diminish- 

 ing percentage of water. Such a statement however is purely 

 formal. 



What probably takes place is this: Starting with a given per- 

 centage of water in the brain or cord, this percentage continually 

 diminishes as the formed material becomes older— at the moment 

 of formation, however, the young material subsequently added 

 most probably has a relatively high percentage of water, and the 

 percentage we obtain at any given age is therefore the mean 

 of these several values. The rate at which the percentage is 

 falling off at any moment, together with the general slowing of 

 the growth process — requiring a longer and longer time to add the 

 same increment of weight to the brain the older the brain be- 



