PERCENTAGE OF WATER 135 



weight, the rate at which the percentage of water diminishes 

 will become slower and slower. At first glance it may be difficult 

 to harmonize these data on the absolute weight of water with the 

 rapid fall in the percentage of water as it appears in charts 1 and 

 2 based on body weight and on age. If, however, the precocious 

 growth of the brain and spinal cord is recalled, a reference to 

 chart 4 in which the body weight and the ages are given below 

 the brain weights, will serve to make the matter clear. 



Weight of water in the spinal cord. The foregoing relations as 

 described for the brain hold true for the spinal cord of both sexes 

 as well, with the difference that in the cord the percentage of 

 water in the total increment from the first to the final entry is less 

 than in the brain, being 68.3 per cent. The percentage of water 

 in the increment during the first 15 days of life is on the average 

 70.4 and after that 67.9. The record in the case of the cord 

 therefore is more nearly represented by a single straight line than 

 in the case of the brain, but like conclusions can be drawn from 

 the study of the data on the spinal cord as here presented. 



Explanation of the change. It still remains to attempt an 

 explanation of the course followed by the percentage of water 

 through the life cycle, and ako to explain why even at birth the 

 brain has more water than the cord, as well as why it shows a 

 smaller fall in this percentage during the life cycle. 



In the interests of such a general explanation, let us consider 

 first the condition of the brain and the cord at birth. 



In the albino rat at birth, both brain and spinal cord are un- 

 medullated, and both are very watery. Both are composed of 

 gray matter in the strict sense, and growing axones, also gray in 

 color. The nerve elements are enmeshed in supporting tissues 

 and vessels. 



In the brain the probability is that the supporting tissues, as 

 well as the vessels, form a slightly smaller fraction of the total 

 mass than in the cord. Cell division in the brain is continued 

 longer after birth than in the cord, while medullation in the brain 

 begins later than in the cord, and is less rapid. During subsequent 

 growth, medullation is most actively carried on from the age of 

 about twenty to one hundred days. 



Between birth and maturity the proportional increase in the 



