196 Cc. W. M. POYNTER AND J. J. KEEGAN 
has been noted in the description of the sulcus frontalis inferior. 
It is found in every hemisphere but one, which disagrees with 
Cunningham’s statement that it occurs only in exceptional cases. 
In a few hemispheres it cuts across the supraciliary border to 
the orbital surface. In three cases it communicates with an 
internal element and so forms a superficially continuous sulcus 
extending from the frontal pole to the pars orbitalis of the gyrus 
frontalis inferior. 
The internal element of the sulcus fronto-marginalis has been 
a topic for much discussion. The contention has been upon 
the identification of this sulcus with the sulcus rectus of lower 
apes, and from the evidence at hand the homology seems to be 
established. The explanation of the displacement of the sulcus 
rectus to a polar position occupied by the sulcus fronto-margin- 
alis in the human brain involves a recognition of a growth 
process in the frontal lobe or frontal association area, which 
has resulted in a marked displacement of cortex and sulci towards 
the mesial border of the hemisphere. In this series the internal 
~ element of the sulcus fronto-marginalis can be divided in almost 
every instance into three parts, a posterior stem, an internal 
longer limb and an external shorter limb. Duckworth recog- 
nized this form in 71.4 per cent of Australian brains, however 
he found the external limb the longer in the majority of cases; 
he says (’07): ‘‘The fact that of these two terminal branches 
the outer is the longer more frequently than is the inner in 
_ Australian hemispheres, may be held to support Cunningham’s 
view that the outer or lateral is developmentally the older, and 
the real continuation of the original sulcus.”” 
This series of Negro hemispheres with the longer internal Hen 
and its fuller and more direct continuity with the stem upholds 
the claim of the greater morphological importance of this part 
of the sulcus. 
The sulcus diagonalis merits no special attention in this series; 
it is undoubtedly present in 50 per cent of hemispheres. In 
the remaining hemispheres it is represented in one of three ways, 
by an unusual inferior extension of the ramus verticalis of the 
sulcus praecentralis inferior, by a long inferior posterior limb 
