Freiburger historische Bestände - digital

80

Cunningham Memoirs.

fossa is fairly established. It rarely occurs earlier, and frequently it is
later in assuming this outline (fig. 24, i. to vn. ; PL n., figs. 1 to 9).

From this stage on two different lines of development may be followed,
and these lead to different results in the formation of the Sylvian fissure.
In certain cases the anterior angle of the fossa becomes acute and sharp
(fig. 24, vm. and ix.; see also PI. n., fig. 11), and the further history of such a
condition of the bounding wall of the fossa is that, in all probability as the
opercula are formed, a single anterior limb of the Sylvian fissure will appear.
In the majority of cases, however, a different course is pursued. The anterior
rounded angle becomes depressed and flattened, so that the superior more
or less horizontal bounding wall is united to the anterior vertical boundary
by a short intervening straight portion of varying length, which joins each
of the other two portions at an angle (fig. 24, x.; see also PI. i., fig. 24 ; and
PI. ii., figs. 17, 20, 21, 24, &c). The Sylvian fossa in such a case is
therefore bounded below and behind by an oblique temporal wall—the
original posterior wall of the fossa bent backwards and elongated. Above,
it is limited by the posterior more or less horizontal portion of the original
anterior boundary. This is formed by both the parietal and frontal lobes.
In front we recognise two short portions, an oblique part on the outer face of
the frontal lobe, and a vertical part on the orbital face of the frontal lobe. At
this stage it must be remembered that the orbital surface of the frontal lobe
is not so sharply marked off from the outer surface of the hemisphere as
in the adult. These divisions of the bounding wall of the Sylvian fossa
may be distinguished as the temporal, parieto-frontal, the frontal, and the
orbital, and the further development of such a Sylvian region leads to the
formation of two distinct anterior limbs of the Sylvian fissure, or perhaps
to the Y condition of these limbs.

The change in the direction of the originally vertical Sylvian fossa is,
no doubt, largely due to the backward growth of the entire hemisphere,
and is associated with the appearance and mapping out of an occipital lobe.
At the same time it must be remembered that the Sylvian area itself
extends more rapidly than would be the case were its growth ordered by,
and proportionate to, the general growth of the hemisphere surface. This
is rendered very evident by measurement at different periods of its