PERCENTAGE OF WATER 139 



As there is every reason to think from what we know concern- 

 ing the relatively small reduction in the gray substance that the 

 percentage of water in the cell bodies in this case is not progress- 

 ing more rapidly than it does in the entire brain, it follows that 

 in the remaining nerve structure — the axones — the percentage 

 of water is reduced at least an equal amount. Since however 

 the diminution in the percentage of water is found to be much 

 greater in the mature white substance than in the mature gray, 

 it seems probable that the axones are subject to a more extensive 

 reduction in the percentage of water than are the cell bodies. 3 



From this it follows that the greater proportion of gray sub- 

 stance in the brain would tend to maintain in that organ a higher 

 percentage of water at maturity, and the lesser proportion of 

 gray in the cord, a lower percentage (see the measurements on the 

 areas of the gray and white matter in the spinal cord as given by 

 Watson, '03, p. 101). 



But there is still one more peculiarity of the spinal cord which 

 is important in this connection. This I have called (Donaldson 

 '09, pp. 166-167) passive lengthening. The segments of the cord, 

 especially in the thoracic region, undergo during growth a length- 

 ening which is largely passive, and which does not imply any 

 marked increase in the structural complexity of the cord, but 

 serves mainly to keep the spinal nerves nearly opposite to their 

 intervertebral foramina. 



In the course of this lengthening, we have evidence that the 

 volume of the gray substance is but slightly increased, while the 

 proportions of the gray column are much modified in the sense 

 that the diameter alters but slightly (it may even diminish) while 

 the length is correspondingly increased (see the measurements on 

 the areas of the gray and white matter in the spinal cord of the 

 rat, Watson '03, p. 101, and Donaldson and Davis '03). 



At the same time that this is occurring in the gray columns, 

 the white tracts not ODly lengthen (passively) but also increase 

 in the area of their cross sections, and thus at the end of any step 



3 The question whether a growing fiber at any age of the animal becomes medul- 

 lated as soon as its percentage of water falls below the value at which medullation 

 first begins after birth, cannot at the moment be answered. It is conceivable 

 however that with advancing age this critical point for medullation is lowered. 



