GROWTH OF THE CEREBRAL CORTEX 83 
thicknesses (7', and T',,). For the present purpose, the mean 
cortical thickness (7') may be substituted for both the foregoing 
values of the cortical thickness, when the brain weight is the 
same, because 7’ falls at the mean of the 7, and T7',, so that 
the gain in 7’, would be compensated by the loss in 7’, (fig. 3). 
As a consequence the volume of the cortex may be represented 
by an index value, the formula for which follows.® 
ale, WED Xe SE (all in millimeters) 
Fig. 3. The solid lines show the simplified geometrical form used to indicate 
the volume of the entire cortex, which is assumed to be proportional to the rect- 
angular form designated by dotted lines in the figure. The volume of the rect- 
angular figure, which was obtained by the value: L. F X W. V X T, has been 
tabulated in column F, table 6, and plotted as graph LWT in chart 2. 
The values thus obtained—which mean the actual volume of 
the rectangle denoted by dotted lines in figure 3—stand in a 
fixed relation to the true cortical volume which is denoted by 
solid lines in the same figure, as far as the latter retains a similar 
form during growth. 
’In this formula, the coefficients 1.22 and 0.91, which were empirically de- 
termined, were eliminated, because these coefficients are fixed throughout all 
the groups to be compared. For Groups XIII to XX, the coefficients are re- 
spectively 1.21 and 0.93, and these will be taken into consideration when com- 
parison is made between the Albino and the Norway rats. 
