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146

Cunningham Memoirs.

But we have not yet attempted to explain this descent of the Sylvian
fissure as growth advances. In the first stages of the covering-in of the
Sylvian fossa, the temporal operculum is more energetic in its growth than
the parietofrontal operculum which grows down to meet it. Before long,
however, the parieto-frontal operculum takes the more prominent share in
the enclosing of the insula, and there cannot be a doubt but that it is
this excess of growth-energy, carried on through infancy and early
childhood, which leads to the depression of the Sylvian fissure. But
the growth-energy of the fronto-parietal and the temporal opercula does
not appear to be of equal intensity along their whole length. Thus, in the
adult it is usual to observe in the anterior part of the fissure a certain
amount of overlapping of the inferior frontal convolution by the lower
bounding lip of the Sylvian fissure, whilst further back the reverse
condition, as a rule, holds good. Here, in the region of the central convolutions
of the outer surface of the hemisphere, the fronto-parietal operculum
commonly overlaps, to a slight extent, the temporal bank of the
fissure. Eberstaller* calls attention to this arrangement.

Professor D. N. Serno£f,t °f Moscow, has recently published a Paper in
which he describes a very ingenious instrument which he has constructed
for the purpose of mapping out the fissures and convolutions of the
cerebrum on the surface of the head. This is illustrated by two charts
which show, as we have stated above, that there is not only a considerable
variation in the position of the squamous suture, but also in the level of the
Sylvian fissure in different heads.

XV. Topography of the Sylvian Fissure in the Apes.—Fere, in an
exhaustive Paper,$ supplies us with a large amount of information upon
the position of the various cerebral fissures in the apes, with reference to the
cranial sutures. Unfortunately, the method which he adopted was not

* Das Stirnhirn, p. 11.

f " Encephalometer : an apparatus which determines the precise position of the several parts
of the human brain in the living body." Preliminary Report. Moscow, March, 1889.

% " Contribution a 1'etude de la topographie Cranio-cerebrale chez quelques singes," Journ.
Anat. et Pbys., vol. xvni.