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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 221

regarded as an independent fissure or as a dependency of the ramus
occipitalis of the intraparietal sulcus. • Upon phylogenetic as well as upon
ontogenetic grounds, I have been led to believe that the furrow in question
cannot be regarded as being in man the homologue of the " Affenspalte "
in the apes ; nor can it be looked upon as constituting an entirely independent
fissural element. It belongs in part, if not in whole, to the intraparietal
fissure-system, and is simply the hinder bifurcation of the ramus
occipitalis. In support of this contention we shall, in the first instance,
bring forward some facts in connexion with the condition present in the
apes, and then study its mode of development in the foetal human brain.

Let us begin with the simple arrangement which is present in the
brain of Cebus capucinus. The intraparietal sulcus ascends obliquely
in an upward and backward direction, and approaches the upper margin
of the hemisphere where the parieto-occipital fissure makes its appearance
on the outer surface. Here it bends upon itself and turns sharply
downwards for a short distance behind the summit of the angular gyrus,
and under cover of the occipital operculum. The parieto-occipital fissure,
as it cuts through the upper hemisphere border, opens freely into the
angle of bending. This is seen in figure 45, p. 222. In the left hemisphere
of the cerebrum represented, the operculum has been removed, so that the
entire fissural arrangement is exposed; in the right hemisphere, where the
operculum is in place, the bottom of the " Affenspalte,1' as well as the short
descending part of the intraparietal sulcus are represented by dotted lines.
Three parts, then, are here recognisable, viz. a long ascending limb, a short
descending limb, and the parieto-occipital fissure, as it appears on the outer
surface, and all the three are directly and uninterruptedly continuous.
The descending limb represents a part, but not the whole, of the ramus
occipitalis of the intraparietal system of sulci in man, and it should be
noticed that in Cebus capucinus it does not reach the bottom of the
"Affenspalte," although it usually lies under cover of the operculum.

In the five cerebral hemispheres of this species of new-world apes which
I have in my possession, I notice that there is a considerable difference in
different individuals in the degree of development or forward projection
of the operculum. In one specimen, indeed, it is so short that the entire