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70

Cunningham Memoirs.

the end of the fifth month. It is not so constant as the external perpendicular
fissure of Bischoff, and in many cases it fails completely.

Although evanescent in the brain of man, there is some reason to
believe that it has a permanent representative in the brain of the ape. On
the outer surface of the occipital lobe of most apes a deep fissure runs
horizontally forwards from the occipital pole, and comes to an end a short
distance behind the free anterior lip of the occipital operculum. This fissure
is placed on the outer face of the hemisphere in an exactly corresponding
position to the calcarine fissure on the mesial face, but it is shorter than
the calcarine fissure; and its anterior end, in most of the numerous
specimens I have examined, just falls short of the posterior horn of the
lateral ventricle. In two species, however, both of which belong to the
genus Cercopithecus, the fissure in question reached as far forward as the
posterior part of the ventricular cavity, and formed a slight bulging on
the outer ventricular wall opposite to the calcar avis on the inner
wall (PI. i., fig. 20). The second occipital sulcus in the human brain
apparently corresponds with this fissure in the cerebrum of the ape—at
least it corresponds to it in position, but not exactly in its history.

Eberstaller * gives this fissure in the adult brain a position of high
importance. He terms it the " sulcus' occipitalis lateralis," and considers
that it should be held as forming, on the outer surface of the hemisphere,
the inferior boundary of the occipital lobe.

Whilst in the description of these two transitory fissures on the outer
aspect of the occipital lobe of the foetal brain I have inclined to the view
that they are to be regarded as distinct fissural integers, I am not blinded to
the fact that it is very possible that we may really be dealing with one and
the same fissure, which alters slightly in position according to variations
in the growth-energy of this part of the cerebrum. It is certainly significant
that, amongst the very numerous specimens which I possess, I have
never seen the two fissures present on the same brain except in one instance,
and as this was a specimen in which both fissures were almost completely
obliterated, I do not consider that it offers us absolute proof on the

* Zur Oberflachen-Anatomie der Grosshirnhemispharen. (Vorlaufige Mittheilung.)
No. H. ; Der Hinterhanptlappen. Wiener Medizinische Blatter, 1. Mai, 1884, No. 18.