Freiburger historische Bestände - digital

298

Cunningham Memoirs.

stage in the growth of the human cerebrum the insula is still open in
front, the frontal and orbital opercula are only partially formed and
the resemblance between the anterior limiting sulcus of the insula and
the fronto-orbital sulcus of the anthropoid apes is very evident. The
similarity is rendered all the more striking in those cases where the
frontal operculum in the human foetal brain fails to appear and the
development is therefore tending towards the formation of a single
anterior limb of the Sylvian fissure (fig. 71, p. 297).

Figure 1, Plate iv., and figure 33, Plate I., may also be studied from
this point of view and compared with figures 9 and 11, Plate iv., which
represent the right cerebral hemispheres of a chimpanzee and an orang.

The late appearance of the frontal and orbital opercula in the human
foetal brain and the occasional complete suppression of the former are
significant facts in connexion with the total absence of both of these
opercula in the anthropoid cerebrum.

In the Gibbon brain Kohlbriigge, in the Memoir already referred to,
has made a most important observation which bears directly on this
question. He states that in three examples of Hylobates syndactylies, as well
as in the right hemisphere of a H. leuciscus the so-called anterior limb of
the Sylvian fissure (i. e. the furrow formed by the anterior free margin
of the fronto-parietal operculum) joined the sulcus fronto-orbitalis. In
these cases therefore the exposed part of the insula was completely cut
off from the surrounding surface of the frontal lobe.

But there are other facts which can be brought forward in support of
the contention that the portion of cerebral cortex which lies between the
anterior free margin of the fronto-parietal operculum of the anthropoid
brain and the sulcus fronto-orbitalis is to be regarded as belonging to the
insula. We have previously seen (Chapter II.) that the submerged portion
of the island of Reil in the chimpanzee and the orang presents very little
trace of an anterior boundary, and that it ascends gradually as we trace it
forwards along an inclined plane until it finally reaches the free surface of
the frontal lobe (fig. 11, PI. iv.). We have further noted that if the anteroposterior
length of the submerged part of the insula in the anthropoid brain
be compared with the length of the human insula it is observed to be rela-