134 



HENRY H. DONALDSON 



approximately constant for the remainder of the life cycle, hav- 

 ing an average value of 71.8 per cent. This value forms a limit 

 towards which the percentage of water in the entire brain slowly 

 falls. 



WEIGHT OF WATER 

 GMS. 



WEIGHT OF WATER 



SPINAL CORD 



ALBINO RAT 



ACCORDING TO 



CORD WEIGHT 



CONSTANT PERCENTAGE 

 SPINAL CORD 



To show the absolute increase in the weight of water corresponding to the 

 increase in the weight of the spinal cord. The data for the two sexes are plotted 

 separately. Below are given the body weights for the corresponding spinal cord 

 weights. The dotted line indicates the amount of water which would be required 

 to maintain the percentage at the initial value. 



Fourth. During the period of most active medullation, i. e., 

 from 20-100 days, the percentage of water in the increments of 

 brain weight does not indicate that the medullary sheaths which 

 are being rapidly formed, possess a percentage of water less than 

 that of the axones on which they appear. 



It follows from the foregoing that as the amount of water in the 

 brain at any time after birth is the sum of the amount present at 

 birth (a constant) plus the successive increments, the percentage 

 of water will diminish most rapidly at first. As the brain becomes 

 heavier, and the increments form a greater proportion of the total 



