854 THE SIZE AND WEIGHT 



hours before putting the organ into the scales. By this 

 latter process its total weight may in some cases be 

 diminished by from 1 to 2 oz.* 



These being almost the only possible sources of varia- 

 tion, where ordinary care is exercised in the process of 

 weighing, the Brain-weights of different observers are 

 more strictly comparable with one another than are the 

 estimations of * cranial capacity ' by different observers, 

 using, as they mostly have done, very different methods, 

 whose relative indices of variation have not yet been 

 determined. 



Of course most of the causes which affect the cranial 

 capacity of individuals would also affect their Brain- 

 weights, and vice versa. But, except in regard to the 

 comparison of ancient with modern races, these con- 

 ditions have been much more fully worked out in terms 

 of Brain-weight than in terms of cranial capacity. 



Some of the principal modifying conditions will now be 

 briefly referred to. 



Age. — It was believed by the earlier anatomists, and 

 even by Tiedemann and Sir William Hamilton, that the 

 human brain attained its greatest development at about 

 the seventh year. We now know this to be incorrect ; yet 

 from the extensive researches of Dr. Boyd as tabulated 

 by Thurnam (loc. cit.. Tab. ix.), it would appear that it 

 does in the male actually reach about -l^ths of its ultimate 

 weight by the end of the seventh year, and in the female 

 about yjths of its ultimate weight by the same period. 

 According to this table, moreover, the maximum weight 

 of Brain, for both sexes, was met with in individuals not 

 exceeding their twentieth year. 



♦ See an excellent paper by Dr. Thurnam." On the Weight of the 

 Human Brain and on the Circumstances affecting it," Journ. of 

 Ment. Science, 1806. 



