PERCENTAGE OF WATER 137 



comes— is so adjusted as to yield after the period of more rapid 

 growth of the brain, the rather simple relations of a nearly con- 

 stant weight of water for the same increment of total weight. 



In this connection the analysis of the brain and cord should how- 

 ever be carried one step further. Both are composed of gray 

 matter (substantia grisea) and axones, plus the supporting 

 elements, the axones being more or less medullated according to 

 locality and age. In the case of the rat, it has not been possible 

 to study the changes in the percentage of water in the gray matter 

 alone. We know however from a number of studies on man — on 

 the cortical gray and the gray of the corpus striatum — that 

 the change in the percentage of water in the substantia grisea 

 with age, is much less — less than one-half — that in the axones 

 (white matter). This has a bearing on the percentage of water 

 in the brain as contrasted with the cord, because the brain has 

 relatively less axone substance in it. Moreover the maturing of 

 this substance is slower in the brain than in the cord. It is worthy 

 of note as bearing on this last point that according to Watson 

 ('03, p. 91 and 105) medullated fibers in the spinal cord of the 

 rat are first found on the second day after birth, while in the 

 cerebrum, they are not found until the eleventh day. At that 

 age — eleven days — the percentage of water in the brain has 

 fallen to that of the cord at the second day, and it thus appears 

 that the medullation of axones begins in both divisions of the 

 central nervous system when these have acquired the same per- 

 centage of water. 



This suggestion, that the onset of medullation is closely related 

 to the percentage of water in the axones, fits with the common 

 observation that the fibers first medullated in any locality become 

 the largest (because they have the longest time to grow aftei 

 reaching the condition in which they can become medullated) 

 and that in any nerve containing medullated and non-medullated 

 fibers, it is the smaller (or younger) fibers which lack the sheath 

 (Roughton '06). Also, as the portion of the axone nearest the 

 cell body is the older, and hence would have the lower percentage 

 of water, this should be the portion first medullated; a conclusion 

 which fits with the observations. 



It is hardly necessary to remark that these last two facts when 



