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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 279

are warranted in asserting that on an average the furrows in question
appear in the following order:—

1. The sulcus prsecentralis inferior.

2. The inferior frontal furrow.

3. The lower piece of the sulcus prsecentralis superior, with the basal part of the

superior frontal furrow.

4. The anterior part or parts of the superior frontal furrow, and the upper portion of

the sulcus prsecentralis superior.

5. The sulcus frontalis medius.

6. The sulcus frontalis mesialis.

X. Frontal Furrows in the Apes.—The most striking peculiarity in the
frontal lobe of the apes consists in the total absence of the frontal and
orbital opercula of the Sylvian region, and the consequent exposure on the
surface of a portion of the insula. This interpretation of the condition
present is strenuously denied by the great majority of anatomists, but a
prolonged study of the region has only served to convince me the more
fully that it is the case. But further, the most conflicting opinions are
entertained regarding the homologies of the various furrows on the outer
surface of the lobe. More particularly has the inferior frontal furrow been
a source of difficulty. The discovery by Broca of the motor centre for
speech in the basal part of the inferior frontal convolution has, undoubtedly,
had the effect of leading many observers to rashly conclude that this
convolution is either entirely absent, or only very poorly represented in
the apes. In man, however, as we shall see, the inferior frontal convolution
usually consists of an opercular and an extremely small non-
opercular part. The former, it is true, is almost entirely absent in the
apes; but the non-opercular part, which is placed above the level of the
insula, is undoubtedly present. From this it will be seen that I believe
in the truth of Gratiolet's views, in regard to the homologies of the
inferior frontal furrow in the apes, although at the present day it may
almost be said that these have been universally discarded.

In order that I may be able to develop this branch of the subject, it
will be necessary to study in detail the frontal furrows as they are
exhibited in certain of the lower apes, and from them to advance