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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



encephalon appears to be an important factor in the cause of death ; hence the 

 larger brain-weight found at autopsies during these years. While, in general, 

 the individual may be supposed to follow in the development of his encephalon 

 the course here indicated by the curve, this premaximal increase must be ex- 

 cepted for the reasons given. 



It appears probable, from various lines of research, 1 that individuals differ 

 widely in the length of time during which the brain enlarges, and also in the 

 time at which the atrophic changes due to old age become evident. The curve 

 for the brain-weight of eminent men also points in this direction. In this 

 latter group the atrophy of old age does not become evident until the sixtieth 

 year. To explain this, it must be remembered, as has been previously stated, 

 that the records of the weight of the brain, such as those here quoted from 

 Boyd, Bischoff, and Vierordt, are all based on hospital autopsies made in 

 densely-settled communities, and that the social status of the individuals there 

 examined was that of a class least vigorous and least favorably situated. It is 

 not surprising, therefore, that when compared with the group of eminent men, 

 both vigorous and, as a rule, more favorably situated, not only should the 

 average weight be greater in this group, but, what is more important, growth 

 should continue for a longer time and the period of senile atrophy be deferred. 



If, as appears probable, these differences depend on the favorable or unfa- 

 vorable conditions existing during growth, then it will be evident that the 

 average man is possessed of a nervous system which probably grows for a 

 longer time and resists decay to a later age than the figures of Bischoff or 

 of Boyd would suggest. 



"Weight of Brain at Birth. The older records gave the male child the 

 heavier brain at birth, while the newer records, like those of Vierordt and 

 others, give the reverse. Be this as it may, the weight at birth is seen to be 

 nearly alike in the two sexes, and the difference in weight becomes distinct and 

 increases during the period of most active growth up to maturity, from which 

 time to the end of life this difference between the sexes remains nearly constant. 



The proportional weights of the different parts according to the method of 

 subdivision practised by Boyd are here shown. The figures indicate the per- 

 centage values of the parts of the encephalon : 



Weight of the Encephalon and its Parts at Different Ages (Boyd}. 



MALES. 



Gallon : Hereditary Genius, 1 884 ; Venn : Nature, 1890. 



