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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 17

ficial furrow, but a deep cleft, which separated the upper and front part of
the cerebrum from the lower and back portion almost as effectually as
the clefts of the lung separate its various lobes. Altogether this is a
most striking infolding. It is seen in great perfection in one of the specimens
displayed in the museum of University College, London (PI. i.,
figs. 6, 7, and 8). All the brains in which it occurred I reckoned to be about
the same period of development, viz. from the thirteenth to the fourteenth
week. In brains younger than this (PI. i., fig. 1), or older (PI. l, figs. 4 and
11), there is not a trace of it. At the same time, I am not prepared to say
that this remarkable fissure is always present. It is clear, however, that
there is a marked tendency towards the formation of such a fissure under
certain conditions of growth-restraint, and that these conditions appear
to be frequently present at the period indicated. In those brains in
which this infolding is present there seems to be some delay in the formation
of the Sylvian fossa. As the fossa assumes shape the fissure retreats
before it, although in two cases I have seen it cutting through the upper
boundary of the fossa, and encroaching to a very slight degree within its
limits.

In one hemisphere a second deep cleft was present, below and parallel
to that just described (PI. i., fig. 8). It therefore occupied the ground
of the future parallel sulcus of the fully developed brain. It has nothing
whatever to do with the development of the permanent sulcus.

Again, it is by no means unusual to observe short isolated fissures
removed a short distance from the upper free border of the hemisphere.
In many cases, these are doubtless fissures in process of obliteration.
They are chiefly noticed in specimens between the third and fourth month.

In certain hemispheres the fissure on the mesial face, which we have
named, provisionally, the precursor of the calcarine fissure, is carried
horizontally round the occipital pole, cutting it deeply, and appearing on
the outer surface in the form of a fissure, which we may term the external
calcarine (PI. i., fig. 10, e. c.). This fissure has a most interesting history,
which I purpose tracing at a later part of this chapter. In the meantime,
I may merely call attention to the fact that this external calcarine fissure
was present in the great majority of the hemispheres examined, although

ROYAL LRISH ACADEMY.-CUNNINGHAM MEMOIRS, NO. VII. [3]