266 NAOKI SUGITA 
cortical thickness in several primates ranges within rather nar- 
row limits (2.3 mm. to 2.8 mm.), while the brain weight shows a 
distinetly wider range (82 grams to 400 grams) (table 11). In 
some cases indeed the smaller brain has a thicker cortex, even in 
the same family (e.g., the smaller hapale has a thicker cortex than 
the larger lemur). But in general we may conclude with Brod- 
mann (’09) that, within one and the same order or family of 
mammals, the large brain tends to have a larger average value 
for the cortical thickness. 
The relative cortical volume has been formerly computed by 
me, employing the formula especially devised for this purpose, 
in the albino and the Norway rat brains, so that the two forms 
may be compared directly (Sugita, ’18b). The ratio of the 
cortical volumes in the adult Albino (brain weight, 2.0 grams) 
and the Norway (brain weight, 2.3 grams) is 1.31, as the rela- 
tive cortical volumes are, respectively, 393 and 517 (Sugita, 
18 b, table 15), and the ratio of the body surfaces in the two 
animals amounts also to 1.30, when the body weights of the 
adult albino and the Norway rats are taken as 300 grams and 
450 grams, respectively. Moreover, the ratio of cortical volumes 
in the two forms at any given age will prove to be almost equal 
to the ratio of body surfaces of the two at the same age.* 
As above tested, the body weight and the cortical volume of 
the animals in the same family stand in a definite relation, at 
least in this instance. But, as we cannot compute the volume 
of the cortex in other mammals from the data given in table 11, 
the relation can not be tested further. 
4 For example, according to my former presentation (Sugita, “18 b), the com- 
puted cortical volume in the Albino Group XV (brain weight, 1.54 grams) is 
about 346 and that in the Norway Group N XVIII (brain weight, 1.83 grams) 
is about 423, and according to another determination (Sugita, 718 a) these two 
groups may be regarded nearly equal in age, as the Albino brain weight would 
be about 18 per cent less than the Norway brain weight of the like age. The 
ratio in cortical volume of the above two is 1.22. The body weight correspond- 
ing to the brain weight of 1.54 grams in the albino rat is 64 grams and that cor- 
responding to the brain weight of 1.83 grams in the Norway rat is 90 grams 
(‘The Rat,’ Donaldson, °15). The ratio of the body surface in the above two, 
therefore, is about 1.25, quite near to the ratio in cortical volume. 
