GROWTH OF THE CEREBRAL CORTEX 265 
of the weights of bodies, and 2) the exponent of correlation within 
the same species is for all vertebrates the 0.22 power. 
These relations were based on a series of observations, and 
this illuminating idea is now generally accepted as true. 
The brain in general consists of the white and the gray matter, 
and in higher animals the gray matter as represented by the 
cerebral cortex occupies a relatively large part of the entire 
cerebrum. This cortex is the seat of a complex series of physio- 
logical nerve centers, and the possibility that it has definite 
quantitative relations with the body as a whole is suggested by 
the following statement made by Du Bois (’13): 
If the quantity of brain does not increase proportionally to the 
volume of the body, expressed by the weight, it might be that this is 
really the case with regard to the superficial dimension, as being pro- 
portional with the receptive sensitive surfaces and with the sections 
of the muscles, thus measuring the passive and active relations of the 
animal to the outer world, for which in this way the quantity of brain 
can be a measure. 
This statement, to be sure, is applied by DuBois to the weight 
or volume of the entire brain, but if the volume of the cortex 
stands in some definite relation to the volume of the entire 
brain, then the cortical volume should be also in a definite rela- 
tion to the size or weight of the body. 
The cortica’ volume is determined by the area of surface of the 
cerebral hemisphere and the thickness of the cortex. Theformer 
factor is not easy to determine exactly, even in lissencephala, 
while in higher animals the hemispheres have many convolutions 
which increase still further the difficulty of this determination. 
In lissencephala, the surface area of the hemispheres in two 
brains, which are nearly similar in the form of cerebrum, are 
approximately comparable with squares of the corresponding 
diameters of the cerebra. 
The cortical thickness, on the other hand, is not so hard to 
determine exactly. The average thickness of the cortex in dif- 
ferent mammals is given in table 11, quoted from various sources, 
and, as seen from this table, it is not directly related to the size 
or weight of the brain, since, as Marburg’s (’12) table shows, the 
