WEIGHT OF THE BRAIN. 



177 



hitherto inconclusive, for although there have been observed several notable instances 

 in which superiority of intellect has been found to be accompanied by increased size 

 or complexity of the cerebral surface, in many other cases no such relation has been 

 noticed. 



Comparative measurement of the extent of surface of the cerelral convolutions. 



It will be seen from the above that the total surface, exposed and sunken, is 

 about 200,000 square mm.,, and that there is about twice as much sunken as 

 exposed surface. With this estimate the determinations made by subsequent 

 observers mostly agree. Baillarger, who dissected off and unfolded the cortex, and 

 then measured its whole extent, obtained only an area of 170,000 sq. mm. as 

 the mean of the brains thus treated. Paulier, by a modification of the method of 

 Wagner, obtained a result similar to that of Baillarger for the whole surface, and, 

 .further, found the sunken surface to measure but little more than the exposed 

 surface, and the extent of surface to bear no relation to the weight of the brain. 

 Calori measured 41 brains (Italians), and obtained the following average results (in sq. 

 mm.) for the total surface : Male, brachycephalic, 243,773. Male, dolichocephalic, 

 230,212. Female, brachycephalic, 211,701. Female, dolichocephalic, 198,210. 



Danilewsky attempted to determine the area of the whole cortex by a comparison 

 of the weight of the brain, its specific gravity (1038), the specific gravity of the 

 grey matter (1033), and white matter (1041), and the average thickness of the grey 

 matter, which he estimated at 2 - 5 mm. He obtained in this way a result of about 

 33 per cent, as the weight of the grey cortex, giving for a brain weighing 1324 grm., 

 a total surface of 169,200 sq. mm. De Eegibus made similar calculations from 

 estimation not of the specific gravity but of the amount of water in the whole brain 

 and in its two component substances. His estimates of the total area of the cortex 

 of both hemispheres are higher than those of Danilewsky, varying from about 

 217,472 to 278,940 sq. mm. (vide Donaldson "On the Brain of Laura Bridgman," 

 in the American Journal of Psychology, vols. iii.fand iv.) 



Thickness of cortex. It is clear that a measurement of surface alone without 

 taking into account the thickness of the cortex, may be entirely misleading as to the 

 amount of grey matter in the brain. This has been recognized by various observers, 

 who have accordingly endeavoured to form an estimate both of the average thickness 

 of the cortex generally, and also its thickness in special localities. The results have 

 been tabulated by Donaldson in the paper above referred to, and from them it 

 would appear that the thickness may vary from 1'55 mm. to 3'5 mm., or even 

 somewhat more than this, the average in normal brains being 2'9 mm. If 

 a section be made across a gyrus it will be found that the cortex is thickest at the 

 summit of the gyrus and thinnest at the bottom ofi the bounding sulci, so that it is 

 necessary to take a mean between these two measurements in order to arrive at the 



