276 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The method of weighing the brain used by Dr. Boyd 1 was as follows : 

 The skull-cap being removed and the pia intact, the hemispheres were sliced 

 away by horizontal sections as far down as the tentorium. The parts of the 

 hemispheres still remaining were then removed by a section passing in front 

 of the quadrigemina. The cerebellum was next separated from the stem, this 

 latter being represented by the quadrigemina, the pons, and the bulb. Each 

 hemisphere, the cerebellum, and the stem was then weighed separately. 



If groups of similar ages and corresponding statures, as entered in the 

 Table, are compared according to sex, it is at once seen that the male pos- 

 sesses the heavier encephalon, and that all the subdivisions of it are likewise 

 heavier. 



When individuals of the same sex and falling within the same age-limits 

 are compared according to stature, those having the greater stature are found 

 to have the greater brain-weight, though in the case of the subdivisions of the 

 encephalon, and especially among the females, there are some irregularities, 

 but these would probably disappear could the number of observations be 

 increased. Finally, within the groups of those having the same stature, but 

 different ages, the weight decreases with advancing age. The middle group, 

 forty-one to seventy years of age, is in one way unfortunate, because, while 

 the brain is probably still growing (see curve of growth. Fig. 1 18) during the 

 first third of that period, and is nearly stationary (males especially) during 

 the second, it begins to diminish so rapidly during the last third that the 

 average weight is lower for the cases between sixty-one and seventy years 

 than for the twenty years between forty-one and sixty years. Between 

 seventy-one and ninety years the involutional 1 }' changes in the central system 

 arc most marked, and the decrease in weight during this period is clearly 

 indicated. 



Before suggesting an explanation of these variations according to age, sex, 

 and stature, it is to be noted that they occur in other mammals as well as in 

 man. As regards the difference in the weight of the encephalon due to sex, 

 it has been shown to obtain among the apes, 2 the male having the heavier 

 brain ; and from the genera] relation of size according to sex among the mam- 

 malia, where the male as a rule has the greater body-weight and iargerskull, 

 it is to be anticipated that a similar difference in the weight of the brain will 

 be shown in other genera. 



A i none; individuals of the same species, but of differenl races or of different 

 statures and weights, the law holds good that the larger races have the heavier 

 brains, as do the larger and heavier individuals. 3 Here, as in the case of 

 man, it is always assumed that the differences in body-weights are mainly 

 correlated with the active ti.-sues like muscle, and not with fat. As to the 



1 Philosophical 'I'm tismi inns <>/ th< ll>\jnl S„rirhj, London, 1860; see also Marshall : Journal 

 of Anatomy and I'tnjxioloipj, ls'.t'i. 



2 Keith: Journal of Anatomy and Physiology, 1895. 



:; Imi Bois, in the Arch, fur Anthropol., Bd. xxw. maintains that among forms which may be 

 fairly compared, the formula E = S ' 56 , will give the weight of the encephalon, — E being the 

 encephalic weight and S the body-weight. 



