142 



HENRY H. DONALDSON 



value for the percentage of water in the encephalon at two year? 

 81.1 per cent. Thus we are able to obtain approximate values 

 for the percentage of water in the human encephalon at birth, 

 two years, nine and one-half years and at maturity, twenty-five 

 years. 



TABLE 6. 



Comparison of the percentage of water in the encephalon of man and the albino rat at 



corresponding ages 



In order to compare these determinations, it is necessary to 

 recall that the span of life in man is about thirty times as long as 

 that in the rat, and if this relation holds throughout the life 

 cycle, it follows that each determination for man is to be compared 

 with that for the rat having one thirtieth the human age taken. 



The data for the rat are based on the entries in table 2 giving 

 the percentage of water according to age. 



This table shows that when we compare brains of correspond- 

 ing ages, the diminution in the percentage of water in the two 

 forms has similar limits, and would be expressed by a curve of 

 like form in both instances. 



When we examine the records for other mammals, we find almost 

 no determinations for the water in the encephalon at birth, but 

 we do find determinations for this character at maturity, and the 

 values are very similar to those for man and the rat. Remember- 

 ing that the relative amount of white matter in the encephalon 

 varies somewhat in different species, and must therefore modify 

 this result, we reach the interesting conclusion that probably 

 in all mammals we shall find approximately the same range in the 

 percentage of water between birth and maturity, and that the 

 loss of water in them occurs in the same manner but that the 



