Freiburger historische Bestände - digital

48

Cunningham Memoirs.

fifth, month the calcarine fissure may exist alone on one hemisphere, whilst
on the other the parieto-occipital is alone developed.

There cannot be a doubt that the precursor which most commonly
becomes obliterated is the parieto-occipital. It is very questionable, as we
shall see later, if the calcarine is ever obliterated from one end to the
other. Its anterior end is preserved, and cases where the parieto-occipital
precursor is seemingly alone retained are generally found to be in
reality a conjunction of parieto-occipital with the retained portion of the
calcarine. This is quite in keeping with the phylogenetic evidence we
have on the question. In the ape there cannot be a doubt but that the
calcarine is the more stable fissure of the two.

From what has been said the whole process of the development of
these fissures will be seen to be subject to a considerable amount of
variation. We shall, in the first instance, therefore, trace what appears
to us to be the more common course of development, and then take
note of the chief deviations from the normal process which we have
observed.

At first, the precursors of the parieto-occipital and calcarine fissures
are in direct connexion with the posterior fissura arcuata (hintere Bogenfurche
of His) or hippocampal fissure (figs. 5 and 12, PI. i.). During the course
of the fourth month the gyrus fornicatus assumes shape, and as it
gradually rises it cuts off the precursors from the hippocampal fissure.
In the fifth month the hippocampal part of the gyrus fornicatus is very
prominent, and interposes a very effectual barrier between the fissures
in question.

The precursor of the calcarine fissure, after it is cut off from the hippocampal
fissure, begins to shorten from its posterior end forwards (examine
figs. 1 to 6 in PI. in.) until there only remains a portion which represents
about the anterior third of the original infolding. The posterior two-
thirds are completely obliterated. The portion which is preserved forms
the " stem" of the adult -^-shaped fissure, and alone assumes the responsibility
of maintaining the intra-ventricular elevation which results in the
calcar avis. In a considerable number of the figures in Plate in., this
preserved portion of the calcarine fissure is to be seen. More particularly