CENTRAL NERVOUS SYSTEM. -J 7, 



loss of the brain in weight after maturity, observations on animals are scanty, 

 but point to decrease in weight toward the natural close of life. 



Interpretations of Weight. — Assuming as the simplest case that the 

 number of the nerve-elements composing a given portion of the central system 

 is constant within the limits of the same species, then differences in the weight 

 of these portions in different individuals imply variations in the size of the 

 component cells. The significance of variations in the size of the nerve-ele- 

 ments must be, primarily, that the larger the cells, and especially the larger 

 the cell-bodies, the greater the 'mass of cell-substance ready at any moment to 

 undergo chemical change leading to the release of energy, and the more nu- 

 merous the probable connections. On the other hand, if the number of ele- 

 ments is variable, an increase in the number must, in view of the law of 

 isolated conduction, also provide a larger number of conducting pathways. 

 Whether this increase in the number of pathways shall further add to the 

 complication of the system depends on the localities at which it occurs. 



In the absence of fuller data, the explanation of the series of differences 

 shown in Boyd's table is in a very high degree tentative. The loss of weight 

 in advanced years appears to be due to a general atrophy of the nerve-ele- 

 ments. The greater brain-weight associated with greater stature appears to 

 depend on the variations in the size of the elements rather than in their num- 

 ber, and, so far as can be seen, the distinction according to sex is also sus- 

 ceptible of the latter explanation. 



Weights of Different Portions. — A study of the proportional weights of 

 the several subdivisions of the encephalon according to the sex, stature, and 

 age, shows that there is very little difference caused by variations in these 

 conditions. This, too, so far as it goes, suggests that the absolute weight is 

 dependent rather on variations in the size than in the number of the elements, 

 since an harmonious variation in number would be less probable than an har- 

 monious variation in size. 



Social Environment. — It is not to be expected that the weight of the 

 brain among the least-favored classes in any community will be the same as 

 that of those who, during the years of growth, are under favorable conditions. 

 All extensive series of observations which we possess relate to the leasts- 

 favored social classes, and hence it is not improbable that the figures id the 

 foregoing tables, which are based on data obtained mainly at the Marylebone 

 Workhouse in London, are decidedly below those which would be obtained 

 from the more fortunate classes in the same community. We have a li>t "t 

 brain-weights which contains the records for a number of men of acknowl- 

 edged eminence, and also for others who attained recognition as able persons 

 without being exceptionally remarkable. This list shows the persons thus 

 selected to have brains on the average heavier than the usual hospital sub- 

 ject. 1 



Brain-weight of Criminals. — The observations of Manouvrier have shown 

 that among French murderers the brain-weight is similar to that of the indi- 



1 Donaldson: The Orowth of the Brain, 1895. 



