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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 41

the posterior horn of the lateral ventricle in the adult brain. The parietooccipital
fissure above the " stem " has no relation whatever to the ventricular
cavity, because above the level of its junction with the " stem " the
wall of the hemisphere has become solid through the increase of the white
substance ; nor has the posterior calcarine fissure {%. e. the calcarine fissure
behind the anterior cuneo-lingual gyrus) anything to do in the production
of this intra-ventricular bulging. The calcar avis is formed entirely by
the back part of the " stem," or, as we have termed it, the anterior
calcarine fissure.

VII. Parieto-occipital and Calcarine Fissures in the Ape.—In the ape

the stability and great depth of the calcarine fissure shows that it is of
vastly greater morphological importance than the parieto-occipital fissure,
which is extremely variable, and often insignificant. In the chimpanzee,
the gyrus cuneus is on the surface (fig. 22, p. 43); in this manner the two
fissures are completely separated from each other. This condition may also
occasionally be seen in the human brain (3"9 per cent.).* In the orang and
gibbon the gyrus cunei may either be on the surface as in the chimpanzee,
or it may be concealed at the bottom of the fissure. In an orang's brain
in my possession, the right hemisphere shows a superficial communication
between the parieto-occipital and calcarine fissures (fig. 21, p. 43), whilst
on the opposite side the two fissures are completely separated by the rise
of the gyrus cuneus to the surface. Under no circumstances whatever do
we ever find the gyrus cunei completely absent in the anthropoid brain,
and thus there never occurs a perfectly free communication between
the calcarine and parieto-occipital fissures. The question now comes to
be : Is there any evidence which would seem to indicate that the calcarine
fissure in the ape is composed of two fundamentally distinct parts—a
" stem," or anterior calcarine fissure, and a posterior calcarine fissure ?
Throughout its whole length the calcarine fissure in both the chimpanzee
and the orang presents very nearly the same depth. Its walls are smooth,
and there is not a vestige of a deep annectant gyrus to be seen crossing its

* Parker lias also observed the gyrus cunei on the surface in one negro brain. Proc. Acad.
Nat. Sci. Pbila., 1878.

ROYAL IRISH ACADEMY. — CUNNINGHAM M15MOIBS, NO. VII. [6]