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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 67

there cannot be a doubt (PI. i., figs. 17 and 31 ; also PI. n., figs. 8, 9, 12,
17, &c., e. p.). Further it is a complete fissure, and is produced by a
distinct infolding of the hemisphere wall. A corresponding elevation on
the ventricular aspect of the mantle-wall is consequently present during the
time that this fissure exists. It differs from the other complete fissures,
which we have examined, in its late appearance. It never assumes form
until all the other transitory fissures have disappeared. Ecker* has very
rightly pointed out, however, that it is not between the seventh and
eighth months of foetal life that it is to be seen, as Bischoff has said,
but much earlier. According to Ecker it appears in the fifth month.
This authority further adds:—"That this furrow disappears at a later
period is quite certain; for in the sixth and seventh month there is not
a trace of a fissure in this position visible." Whilst I agree with
Ecker as to the time at which the external perpendicular fissure appears, I
consider that in most cases it has a longer duration than he is inclined to
give it. I find it as a rule throughout the whole of the sixth month of
development, although towards the end of this month it is clearly
showing signs of commencing obliteration. It is rarely to be seen
beyond the first week of the seventh month. Whilst this is the case, it
must be admitted that the fissure in question is subject to a good deal of
variation. Sometimes it appears to be entirely absent, whilst at other
times its duration is cut short, and it has disappeared before the beginning
of the sixth month.

There are many of the foetal brains, which are figured in PL n., which
exhibit the fissura perpendicularis externa in various phases of its growth
[e.p.]. It will be noted that it is situated on the outer surface of the
occipital lobe, and presents a direction similar to that of the parietooccipital
fissure on the medial face of the hemisphere. It is important to
note, however, that it is placed in a plane which is distinctly behind that
of the parieto-occipital fissure. It therefore occupies a position which
corresponds exactly with that of the " Affenspalte " in the ape's brain, and
I have little doubt that Bischoff is right in supposing it to be the

* " Zur Entwicklungsgeschichte der Furchen und Windungen der Grosshirnheinispharen ira
Fcehis des Menschen," Archiv f. Anthrop., Dritter Band, 3. und 4. Heft, 1869.

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