204 Cc. W. M. POYNTER AND J. J. KEEGAN 
and a posterior bifurcation can be recognized in 63 per cent of 
the cases. In several hemispheres a superficial appearance of 
bifurcation is a superior extension of the pars anterior of the 
sulcus to form the superior limb, while the pars posterior forms 
the inferior limb of the supposed bifurcation. Another source 
of error is the misinterpretation of the polar element of the 
sulcus occipitalis superior for the posterior bifurcation when 
there is a shallow communication between the two; such a con- 
dition is seen in figure 6. 
It is noticeable that in all the hemispheres in which there is 
present a typical sulcus occipitalis lateralis, the sulcus inter- 
striatus mesialis is very short. From a study of this series we 
are forced to conclude with Smith that “‘it is quite clear that the 
retrocalearine sulcus of one brain is not necessarily strictly 
homologous with the retrocalcarine sulcus of another brain.” 
The sulcus collateralis is not a single sulcus but represents 
several elements which united or separate form a fissural complex. 
The only constant portion is the central element, and it is the 
’ only part which belongs strictly to the collateral sulcus. It is 
the deepest part of the complex and possesses a prominently 
operculated lateral lip. That this lip is of late development is 
shown by an examination of the fetal brain, in which it is absent, 
it is also absent from both hemispheres in the poorly developed 
female brain; there is a striking resemblance of the sulci col- 
laterales of these two brains and very little resemblance to the 
forms present in the remainder of the series. In each the cen- 
tral element of the sulcus is shallow, non-operculated and termi- 
‘nates in an anterior arcuate bifurcation which is situated several 
millimeters posterior to a boldly curved fissura rhinalis. 
The typical form of the sulcus as described by Retzius may be 
seen in figure 4; this condition is present in 40 per cent of the 
hemispheres. While the remaining hemispheres present a num- 
ber of variations, they permit in general of the*following inter- 
pretation. As has been said, the central element is constant. 
The anterior element in the great majority of cases represents 
a part of the fissura rhinalis, but it is incorrect to interpret it 
as entirely homologous to a portion of the fissura rhinalis. In 
