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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 101

corresponding part of the inferior frontal convolution. A disturbance in
the condition of the one is generally accompanied by a disturbance in the
condition of the other.

V. Development of the sulci and gyri on the surface of the insula.—

A study of the manner in which the convolutions and sulci of the insula are
developed reveals many points of high importance and interest. Up to the
middle of the fifth month the surface of the insula remains perfectly
smooth, but long before this there is a marked indication of its division
into a frontal and a parieto-limbic portion. From its very earliest appearance
the fossa Sylvii is encroached upon below by the vallecula Sylvii.
When the hemisphere is viewed in profile the latter region appears as a
semilunar depression which indents the lower part of the Sylvian fossa and
occupies a position between the extremity of the temporal lobe and the
under aspect of the frontal lobe. This notch or depression is surrounded
above by a rim which represents the external root of the olfactory lobe,
and it divides the lower part of the Sylvian area into two nearly equal
portions, viz. an anterior portion which joins the under surface of the
frontal lobe and a posterior part which runs into the extremity of the
temporal lobe (PI. n., figs. 2, 3, 4). As the temporal operculum begins
to take shape and the extremity of the temporal lobe grows forwards
so as to overshadow the Sylvian vallecula, the hinder part of the lower
portion of the insula becomes hidden from view, whilst the fore-part
developes into the pole of the insula. At the same time the wide semilunar
notch in the lower part of the insula, or, in other words, the outer
extremity of the vallecula Sylvii becomes gradually reduced in width
and converted into a narrow cleft or incision (PI. i., figs. 21, 22, and 23).
Further, with the growth of the temporal operculum the lower limiting
furrow (lower part of the sulcus circularis Reillii of Schwalbe) becomes
apparent.

When the front extremity of the temporal lobe of a foetal brain (say at
the fifth month) is examined, twTo very distinct gyri, which traverse it from
above downwards immediately external to the uncus, are evident. The
outer of these is the extremity of the temporal operculum (PI. i.,