Freiburger historische Bestände - digital

6

Cunningham Memoirs.

Fig. 1.

Section through the forepart
of the fissura arcuata.
The hemisphere from which
this specimen was obtained
is figured in PI. I., fig. 13.

hippocampal fissure (h) and two shorter anterior portions which are placed
upon the fore-part of the mesial surface of the hemisphere. Of the latter
the one is a straight fissure which takes an oblique
course downwards and forwards toward the lower
and fore-part of the frontal lobe (e); the other lies
immediately behind this, and on a higher level (/ a.).
The same condition was present in both hemispheres.
Not only then has the front part of the fissura arcuata
liberated itself from the hinder hippocampal part, but
it has broken up into two parts, a sure sign that it is
in course of obliteration. On removing the roof of
the left cerebral vesicle the infolding of the hemisphere
wall, corresponding to the posterior of the two
anterior portions, is seen to project horizontally outward
like a shelf across the greater part of the width
of the cerebral vesicle (PI. i., fig. 13, f. a.). In
this situation it lies over the anterior part of the corpus striatum. The
depth of this fold may be appreciated by examining fig. 1, which
represents a transverse section through the fold as seen under a low
power.

The detachment of the anterior part of the fissura arcuata in this specimen
and the similarity which it presents to temporary infoldings in the
course of their obliteration is highly suggestive, and favours the view that
the posterior hippocampal part of the arcuate fissure alone is retained; or
in other words, that the front part is wiped out, and that the callosal sulcus
in the fully formed brain is a new fissure which is called into existence at
the time when the callosal fibres cross from one hemisphere to the other.
In this opinion I am confirmed by the examination of brains somewhat more
advanced, and in which the corpus callosum is seen in different stages of
formation. It would appear that in the process of disappearance the
arcuate fissure does not necessarily break up into separate pieces as exhibited
in fig. 12, PI. i. It may become obliterated by a gradual shallowing
which travels forward from the hippocampal part. The last part to
disappear in such a case is the foremost portion on the mesial face of the