Freiburger historische Bestände - digital

78

Cunningham Memoirs.

I possess (representing as they do every stage in the development of the
region) have enabled me to clear up several points which have hitherto
been somewhat obscure, and upon which also there has been some variance
of opinion.

In the previous Chapter I have expressed the view that .the Sylvian
fissure cannot be regarded as forming one of the " Totalfalten," or complete
fissures of His. The growth-projection into the cavity of the
hemisphere which forms the corpus striatum and corresponds with the
central lobe or insula on the surface is not produced by a folding in of the
mantle-wall, but by an elevation in the floor of the prosencephalon. The
surface area corresponding to this internal projection does not keep pace
with the mantle, as the latter grows around it, and in consequence the
Sylvian depression becomes evident.

As Mihalkovics has pointed out, it is towards the end of the second
month of development that the first sign of the Sylvian depression may be
detected. At the same time, it is right to state that its appearance is
frequently delayed beyond this stage, and I have observed hemispheres
well on in the third month with hardly a trace of it. We have previously
noted that the Sylvian region at this period is not unfrequently the seat of
a temporary fissure. As the fossa assumes shape, this fissure (when
present) retreats before it, and seems to exercise little or no effect upon
its development. In no instance have I ever observed a temporary fissure
encroaching upon the Sylvian fossa when the latter has once become
definitely mapped out, although one or more may approach so closely as to
cut into its boundary wall (PI. n., figs. 1 and 2).

At first the Sylvian depression exhibits a nearly circular outline, and
lies on the outer face of the cerebrum immediately above the hilum of the
bean-shaped hemisphere. The surrounding mantle is raised very slightly
beyond the level of the surface of the fossa, so that the outline is by no
means distinctly mapped out. As the foetus enters upon the third month
the depression usually becomes more apparent. The surrounding boundary
of mantle-wall becomes higher and more evident, the area enlarges, and
towards the end of this month it shows a tendency to elongate in the
vertical direction, and at the same time take a backward inclination. In