GROWTH OF THE CEREBRAL CORTEX Tt 
As already shown (Sugita, 717, 717 a), the longitudinal diameter 
of the sagittal section (from the frontal pole to the occipital 
pole), that is L. F, as well as the cortical thickness, are both 
steadily increasing as the brain weight increases. The thickness 
of the cortex is one component of its area in the section, the other 
being obtained by dividing the area by the thickness, and the 
length thus found is correlated with the longitudinal diameter 
of the section (L.F') as defined above. The increase of the 
cortical area will therefore depend on the increase in cortical 
thickness and the increase in the longitudinal diameter of the 
section (Z. F). Table 4 shows these relations. Column B gives 
the average brain weight by groups, column C the average 
corrected area of the cortex (taken from table 1), column D 
the cortical thiekness (7’,) and column F the diameter L. F, 
all in the fresh condition of the brain and the last two quoted from 
the data already published (Sugita, ’17,’17 a). In column E is 
given the ratio C/D or the computed length of the long side, 
when the cortical area is reduced to a rectangle with the short 
side equal to the cortical thickness. If these computed lengths are 
compared with the actual longitudinal diameters of the cerebrum 
(L. F), given in column F, it is of interest to note that, in brains 
_ weighing more than 0.5 gram, the ratios, given in column G 
as I/F, are quite similar, ranging between 1.16 and 1.25 (average 
1.22).!. In the newborn or before birth (Group I), the ratio is 
somewhat higher. So, if necessary, the cortical area in the 
sagittal sections may be obtained by the following formula 
ele SG Alo, (L. F and T,, in millimeters) 
As the, sagittal cortical thickness in brains weighing more than 
1.17 grams increases only slowly, the cortical area in the sagittal 
section in brains older than twenty days is approximately 
proportional to the longitudinal diameter of the cerebrum (L. F). 
E. The area of the cortexin the frontal section 
Reviewing table 2 and chart 1 (graph f), we see that the 
cortical area in the frontal section increases in the same manner 
1 In making comparisons with the Norway rat in part II of this paper, the 
average ratio given by Groups XIII to XX will be that used. This average 
ns) dale 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 29, NO. 2 
