Freiburger historische Bestände - digital

200

Cunningham Memoirs.

running in the sagittal direction, with its convexity directed outwards, and
its extremities slightly bent inwards in front and behind the upper end of
the parieto-occipital fissure.

At first the ramus horizontalis and the ramus occipitalis stand well
apart, but as they extend they gradually approach each other, and finally
establish a superficial connexion. This confluence, although the rule, fails
in many cases, so that even in the adult the two sulci may still be found
distinct and separate. According to Mihalkovics the union of these two
elements takes place at the eighth month. But this is by no means an
invariable rule, because the connexion is in many cases delayed until after
birth. Thus, in foetuses of the eighth month, I found the two sulci still
separate in 66*7 per cent, of the hemispheres examined; in full-time
foetuses in 42*2 per cent.; and in the adult in 36'3 per cent.

But the ramus horizontalis extends forwards as well as backwards, and
the result is that a union is usually effected with the upper end of the
sulcus postcentralis inferior. This coalescence when it occurs takes place
early in the eighth month, although the deep annectant gyrus which marks
the junction remains very high until the time of birth.

There are many instances, however, in which the ramus horizontalis is
developed in direct continuity with the sulcus postcentralis inferior. In
these cases the upper end of the latter arches backwards, so as to occupy
the ground usually held by the ramus horizontalis. This is a reversion to
the primitive type (PI. i., fig. 31, jk*.1 and/>.3), and is the mode of development
which Pansch considered to be the most common. The arching
fissure which results gives no indication of that tendency to the divorce of
the horizontal ramus from the postcentral sulcus which is so marked a
characteristic of the human brain.

Lastly, the upper part of the postcentral sulcus (sulcus postcentralis
superior) makes its appearance in the form of a shallow depression placed
between the mesial border of the hemisphere and the superior end of the
sulcus postcentralis inferior. It is in every respect comparable with the
upper piece of the fissure of Rolando and the sulcus prsecentralis superior
(Plate ii., figs. 17, 18, and 24, p.2). Peichert includes this part of
the intraparietal sulcus, as well as the ram as occipitalis, amongst the