1 NAOKI SUGITA 
the corresponding Albino brain. The ratio tends to increase as 
the brain weight increases, showing roughly the relative growth 
in the Norway cortex. 
Since, as has been shown (Sugita, 718 a), the cortex in the 
mature Norway is about 8 per cent thicker (average of the sagittal 
and frontal sections) than in the Albino, and since this value 
enters as 7’ into the formula under discussion, this would tend 
to give a greater volume of the cortex in the Norway than in the 
Albino. The mean value found for the ratio of the cortical 
volume—1.07—is about that to be expected, in view of the 
relatively smaller value of L. F in the Norway. 
S. Computed number of nerve cells in the entire cortex. Norway 
rat compared with the Albino 
As described in part I, the computed number of nerve cells 
in the entire cerebral cortex may be obtained by the following 
formula :° 
NCEE XW. DX XC GE. FW. D and 71m millimeters) 
where L. F X W. DX T XC is the computed volume of the 
Norway cortex made comparable directly with the correspond- 
ing volume for the Albino, as explained in the foregoing chapter, 
and N is the cell density, represented by the sum of the num- 
bers of cells in a unit volume in the lamina pyramidalis and in a 
unit volume in the lamina ganglionaris (two unit volumes alto- 
gether), given separately in table 10 and combined in table 16. 
Table 16 gives the computed value of the cell number in the 
entire cerebral cortex for each brain weight group of the Norway 
rats (column FE), calculated by the use of the above formula, 
and also in the corresponding case of the Albino (column G). 
On examining table 16, column H, we find the computed num- 
ber of nerve cells in the cortex to be nearly completed in a brain 
weighing 1.37 grams (Group N XIII), while in the Albino this 
condition was reached in a brain weighing 1.17 grams (Group 
XI). The value of the completed cell number is indicated in 
6 The formula for the total number of nerve cells in the Norway cortex is 
like that for the Albino cortex with the addition of the factor C (footnote 4). 
