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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 265

the orbital border. This furrow shares the peculiarity of a hinder transverse
branch, with the upper and lower frontal furrows only; in the case
of the latter the transverse branches receive the special names of the upper
and lower prsecentral furrows."

In my endeavours to ascertain the characters presented by the sulcus
frontalis medius, I have followed the same plan which I have adopted in
the case of the other furrows I have studied. I have examined sixty-
nine hemispheres, and in each case I have sketched the fissure, and taken
care to note its component parts, and the connexions it established with
neighbouring sulci. The drawings thus obtained I have then classified
into groups according to the correspondence in general features presented
by the sulcus in different hemispheres. The degree of variability of this
sulcus, as established by this method, is very bewildering. In the sixty-
nine hemispheres examined, I have met with no less than twenty-seven
different varieties of the middle frontal sulcus. It would serve little
purpose to give these in detail. I shall therefore confine myself to the
conditions which produce this variability—remarking, however, in passing,
that a sulcus which assumes so many different forms is not likely to be
one of such leading and conspicuous morphological value as Eberstaller and
Herve- would have us believe.

A very essential point to note in regard to the sulcus is the fact that it
lies in front of the horizontal portion of the inferior prsecentral furrow,
although, owing to the manner in which the anterior part of the latter is
tilted upwards in the human brain, it can hardly be said to show in the
adult an exactly similar direction (fig. 53, p. 248). Both, however, tend to
cut the middle frontal convolution into an upper and lower portion—the
latter in its posterior part, and the former in its fore part. Eberstaller has
referred to a small transverse branch with which its hinder end is in continuity
. When this is the case (fig. 53,/ m., p. 248), it will be noticed that a
deep annectant gyrus always intervenes, and prevents a free union between
the two; indeed, in the majority of cases the annectant gyrus is on the
surface, and the transverse furrow is thus completely cut off from the
sagittal portion of the middle frontal sulcus. A second small transverse
fur row, behind that already mentioned, is also almost invariably present,

KOYAL IRISH ACADEMY. — CUNNINGHAM MEMOIRS, NO. VII. [34]