Freiburger historische Bestände - digital

84

Cunningham Memoirs.

sixth month the first transverse convolution of Heschl appears on the deep
surface of the back part of the temporal operculum. (PI. n.3 figs. 20, 21,
and 24:h.)

The four opercula of the Sylvian region are in their fully developed
condition separated from each other by the three limbs of the Sylvian fissure
which appear on the outer face of the hemisphere. In other words, these
limbs of the fissure are formed by the meeting of the margins of the
different opercula. The posterior horizontal limb intervenes between the
temporal and fronto-parietal opercula; the anterior ascending limb separates
the contiguous margins of the fronto-parietal and the frontal opercula;
whilst the anterior horizontal limb intervenes between the frontal (pars triangularis
) and the orbital opercula.

As is well known the complete enclosure of the insula is not affected
until the end of the first month of infantile life, although I have met with
cases in which it has been delayed until the third month of infancy.

Of all the four opercula the frontal, or pars triangularis, is the one which
is subject to the greatest variations in growth energy. Many instances
occur in which its apex fails to meet the temporal operculum, and in these
cases the margins of the orbital and fronto-parietal sections meet below it,
and form a common stem, from the summit of which the shortened anterior
ascending and anterior horizontal limbs of the Sylvian fissure proceed.
This is the well-known Y condition of the anterior limbs of the fissure
(fig. 25, in.).

Should the fronto-parietal and the orbital opercula fail to meet below
the stunted pars triangularis an open condition of the Sylvian fossa results.
A well-marked example of this is seen in PI. viii., fig. 16, in the brain of
an old man aged 70. The space which should be occupied by the frontal
operculum, or pars triangularis, is vacant, and the corresponding portion of
the insula is exposed.

There appears to be a wide-spread impression amongst anatomists (in
America, especially), that the exposure of a portion of the island of Reil in
the adult brain is a mark of inferiority. Dr. Charles K. Mills* quotes a

* Presidential address delivered at the Meeting of the American Neurological Association, ir-
June, 1886.—Journal of Mental and Nervous Diseases, New York, September and October, 1886,
vol. xxxiii.