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Mr. Victor Horsley—Cranio-Cerebral Topography.

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constancy in the anthropoid and man. Prof. Cunningham has accounted
for the cases of its apparent absence by following Eberstaller in believing
that the fissure of Rolando in such cases opens into the fissure of Sylvius,
more or less completely, by joining with the sulcus in question. This condition
, however, does not always prevail; and I have seen absence of the
sulcus transversus and termination of the fissure of Rolando in the usual
position in the orang.

On account of its functional importance in the apes I made special
observation of its occurrence in man. Of the sixty-three hemispheres
before mentioned it was present in 48, i.e. 76 per cent, of the cases
examined. Of 37 hemispheres in which the variations of the furrow
were drawn, the sulcus in two cases was double, i. e. in addition to
the customary notch proceeding from the Sylvian fissure (for the
development of the sulcus, see Prof. Cunningham's statements, p. 173),
the condition invariably seen in the Macacque was coexistent ; in six
other cases the latter alone was present. The remainder, the large
majority, opened more or less deeply into the fissure of Sylvius.

It is difficult, in the absence of exact knowledge of the differentiation
of the facial region of the human cortex, to appreciate at its proper value
this subordinate sulcus, but as the Macacque monkey's brain has proved
such a trustworthy guide before, it will be well now to remark upon the
topography of this apparently insignificant furrow. Lying as it does,
subdividing the foot of the ascending frontal gyrus, its topographical
(frontal lobe) projection demands only the ascertaining (according to
description already given) of the posterior limb of the fissure of Sylvius,
the lower end of the fissure of Rolando, and inferior prsecentral sulcus
respectively. Midway between the two latter is the position of the sulcus
transversus inferior.

Sulcus Orbitalis.—The differentiation of the orbital sulci with the
exception of the spinal fissure, is as yet not of known functional
importance. The orbital sulcus has therefore been considered under
the heading of the orbital surface of the hemisphere.

(B) Parietal Lobe.—The topography of the parietal lobe at the present
time leaves a great deal to be desired. In fact the differentiation of the

ROYAL IRISH ACADEMY.-CUNNINGHAM MEMOIRS, NO. VII. [44]