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112

Cunningham Memoirs.

resemble the corresponding convolutions in man during their early stages
of development. In the chimpanzee the sulcus, which is generally supposed
to be the anterior limiting furrow of the insula is often very feebly
marked (PI. iv., fig. 9), and the surface of the island of Peil passes almost
continuously over the deep wall of the so-called anterior limb of the Sylvian
fissure on to the free surface of the mantle. When a well-marked anterior
limiting sulcus does exist for the submerged portion of the insula, it lies
in the same line as the prsecentral sulcus of the mantle.

As a rule, there are two oblique furrows on the surface of the insula.
One occupies a plane, and presents a direction more or less similar to that
of the fissure of Kolando, and therefore may be regarded as the fissura
centralis; the other, more faintly marked, lies in the line of the postcentral
portion of the intraparietal sulcus, and, therefore, is the representative
of the postcentral sulcus of the human insula. There are,
therefore, only three convolutions, as a rule, on the surface of the
submerged portion of the insula of the chimpanzee, and these are homologous
with the gyrus centralis anterior (gyrus brevis tertius of Eberstaller),
gyrus centralis posterior (gyrus longus of Eberstaller and Giacomini), and
a narrow strip, the gyrus posterior secundus (PI. iv., fig. 9).

In the orang, the island of Reil is somewhat more extended, and a
nearer approach to man is attained. The anterior limiting sulcus of the
submerged portion lies slightly in front of the line of the prsecentral sulcus
on the mantle, and there are three radiating furrows which represent the
three radial " Primarfurchen " of the mantle, and which exhibit a close
similarity to the condition observed in the early human foetal brain. The
convolutions are from before backwards : (1) the gyrus brevis primus ;
(2) the gyrus centralis anterior ; (3) the gyrus centralis posterior ; (4) the
gyrus posterior secundus (PI. iv., fig. 11).

The insula in the orang, therefore, in so far as its convolutions are
concerned differs from the insula in man in the absence of the gyrus
accessorius, and the gyrus brevis secundus; whilst the chimpanzee differs
from the orang in the absence of the gyrus brevis primus. Further, in
the anthropoid brain the insula is not divided so perfectly by the central
sulcus into an anterior frontal and a posterior parieto-limbic part.