172 THE GROWTH OF THE BRAIN. 



and maturity the increase in size of the brain is due 

 in part to the enlargement of the cell elements, but in 

 greater measure to the formation of the medullary 

 substance accompanying this enlargement. The loss of 

 weight in extreme age probably depends on destructive 

 processes affecting the medullary substance, accompanied 

 by a general decrease in the proportion of water. 



2. Why do tall persons have heavier brains ? Individuals 

 who are above the average height are, as a rule, also above 

 the average in weight, and it would help greatly in this 

 inquiry if the histological basis for differences in weight 

 and stature were known whether, namely, they were for 

 the most part due to an increase in the number or the 

 size of the cells. The cranial cavity and spinal canal 

 would tend to be enlarged in either case, and the size 

 of these cavities is one condition affecting the size of the 

 enclosed nerve structures. Also it seems probable that 

 the variations in number or size would affect both the 

 supporting and the nerve tissues alike, and hence that 

 the larger central system and the large cavities enclosing 

 them are the result of similar variations in both namely, 

 an increase in the size of the structural elements. 



3. What significance is to be attached to the fact that 

 the brain-weight is different in different races ? When 

 individuals belonging to different races are thus com- 

 pared, it appears probable that differences in the number 

 of the cell elements may be of importance, but there is 

 no direct evidence on this point, and since in different 

 races variations in brain-weight follow in general varia- 

 tions in the body-weight, it is necessary therefore to 

 attribute the differences to the same conditions which 

 are important in the case of the persons of different 

 stature, remembering, on the one hand, that racial dif- 

 ferences are more marked among adults than among 

 infants ; and, on the other, that variations in the absolute 



