AMERICAN NEGRO BRAIN 189 
and this leads in the first instance to a greater prominence of the gyrus 
and ultimately to a partial overlapping of the frontal ascending con- 
volution by the ascending parietal convolution. It is more obvious ~ 
in the lower two-thirds of the fissure. It is owing to this that the 
adult fissure cuts into the cerebral surface in an oblique direction from 
before backwards. 
From the study of this series of hemispheres the greater growth 
process appears to be in the gyrus centralis anterior and this 
gyrus is uniformly more prominent than the gyrus centralis 
posterior. There is a superficial appearance of an operculation 
by the gyrus centralis posterior, but this appearance is produced, 
not by overgrowth of the gyrus centralis posterior, but by the 
posterior extension of the gyrus centralis anterior in the deeper 
two-thirds of the fissure. This growth has displaced the floor 
of the sulcus centralis and so undermined the posterior gyrus. 
This quite clearly indicates that an apparent operculum does 
not always represent an excessive growth of the region. Further 
evidence of the greater growth energy of the anterior gyrus is 
seen in the prominence of the superior genu and the weakness 
of the inferior genu as shown in figure 3. This is in accord with 
Symington’s (’13) observation ‘‘that the sinuous course of the 
fissure is secondary to growth in the anterior wall.”’ 
The data in regard to the superior and inferior termination of 
the sulcus centralis may be compared with those furnished by 
Cunningham and Retzius, although the value of such data is 
very doubtful. In 70 per cent of the hemispheres the sulcus 
cuts across the border and terminates on the mesial surface; 
Cunningham 60 per cent, Retzius 64 per cent. In 24 per cent 
the sulcus just reaches the border; Cunningham 21 per cent, 
Retzius 16 per cent. The fetal hemispheres possess a sulcus 
centralis which extends slightly beyond the border but at right 
angles to it, which suggests that the growth process which 
produces the typical diagonal arcuate sulcus centralis has not 
yet been completed. 
In only one hemisphere is there a very noticeable tendency 
toward the establishment of communications between the sul- 
cus centralis and adjoining sulci. In this hemisphere there are 
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