506 
DR. J. BARNARD DAYIS ON THE WEIGHT OF THE BRAIN 
capacities and deducing from those capacities the average volume of the brain, affords, 
in some respects, more available data for determining this relative volume for any par- 
ticular race than the weighing of the brain itself. It might be less easy in this way to 
fix the exact weight of any individual encephalon, which might be much changed by 
some lingering and wasting disease (a large portion of Dr. Peacock’s cases died of 
Phthisis) ; but, practically, this method is more sure to yield an accurate average size of 
the organ, because we have it in our power to use an unchangeable substance with 
which to gauge the capacity of the skull. And we thus arrive at conclusions the same 
in result as if we had the brain in all skulls at a uniform density, which, in fact, is the 
true basis of comparison. 
The method followed in the present researches has been to fill each skull, as uniformly 
as possible, with clean and dry Calais sand of a definite specific weight, 1425 *. Then 
to pour out the sand, and weigh it carefully. The great difficulty which has always 
stood at the threshold of ascertaining the volume of the brain from the mere internal 
capacity of a skull, has arisen from the necessity for, and the variable mode of, making 
allowance for the other contents of the cranium besides the encephalon, and the uncer- 
tainty of the proper amount of such allowance in each special case. This difficulty being 
once overcome and a uniform rate of allowance being fixed upon a reliable basis, it will 
be almost as satisfactory in all investigations of this kind to obtain the weight of the 
brain by gauging the capacity of the skull as by weighing the encephalon itself. 
In order to get the proper allowance, or tare , to be deducted from the weight of sand 
which filled the cranium, as a compensation for the weight of the dura mater, the fluids 
of the membranes and ventricles, and also of the blood contained in the large vessels 
(after much inquiry and deliberation and consulting the best information to be got 
upon the absolute weight of the membranes and fluids of the human brain), it has been 
decided to allow a proportionate or percentage deduction as a tare upon the capacity of 
the skull and its contents ; and this deduction has been fixed at 15 per centum. By this 
means the allowance will vary in exact proportion to the size of the brain itself, increasing 
as this increases. And when we come to compare the brain-weights acquired by dif- 
ferent observers from actual metrological experiments with the results obtained by 
such a uniform deduction of 15 per cent, they will be found to agree upon the whole 
accurately *f\ 
The weight of the sand, after this deduction of 15 per cent., has then to be converted 
into its equivalent of cerebral matter of the specific gravity of 1040, which is the nearest 
average datum carefully determined by different competent observers J. 
* The weight of a given bulk of dry Calais sand, moderately shaken down, is, to distilled water, as 1425 to 
1000. 
t In the ‘ Crania Britannica,’ p. 224, Note *, it was proposed to allow 5 ounces av. in the skulls of men, 
and 4-5 oz. in those of women. This was the result of some observations made by Dr. Thurnau:. 
j In a subsequent part of this memoir a reference will be made to actual metrological experiments on indi- 
vidual brains, where the cranial capacity has likewise been determined, when it will he seen that the results 
