140 HENRY H. DONALDSON 



in this transformation, we find a larger proportion of white 

 substance than at the beginning. The white substance having a 

 lower percentage of water than the gray, tends of course to bring 

 down the general average. We know from previous studies that 

 in the albino rat the weight (and length) of the spinal cord in- 

 creases so long as the animal grows (Donaldson '09). 



It is therefore the relative increase in the white substance due 

 to this continuous passive lengthening — which is so marked in the 

 cord — that can justifiably be held responsible for the more rapid 

 decrease in the percentage of water in the cord after maturity. 



In brief then, the more rapid diminution in the percentage of 

 water in the cord up to maturity, and the greater rate of diminu- 

 tion after maturity, are due, aside from the excess of supporting 

 tissues and vessels, to the greater amount of axone substance in 

 the cord and the peculiar form of growth designated as passive 

 lengthening. 



General significance of this change. If we are correct in conclud- 

 ing that in the percentage of water we have a character corre- 

 lated very closely with the age of the animal, and but slightly 

 influenced by the conditions which modify general growth, it 

 follows that this change must depend on processes intimately asso- 

 ciated with the span of life or longevity of the animal concerned. 



Broadly speaking, the changes in the percentage of water indi- 

 cate progressive chemical modifications which take place in those 

 constitutents of the cell that are most stable. 



A comparison of the albino rat with man in respect to the per- 

 centage of water in the brain. In connection with such a compari- 

 son, I have examined the entire literature on the percentage of 

 water in the nervous system. This literature needs to" be sum- 

 marized, but for such a summary, this is not the occasion. Out 

 of the data available, I have selected however the findings of 

 Weisbach ('68) and of Koch ('09) to be used in the present 

 instance, as from these we get the best series of determinations 

 of the water in the human brain at different ages. The data from 

 Weisbach are as follows: 



He determined the percentage of water in each case for six 

 different localities in the encephalon: (1) white substance (cal- 

 losum); (2) gray substance (corpus striatum); (3) gyrus (white 



