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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum, 19

Schmidt has stated that, at the places where the bending-in of the cerebral
wall takes place, the wall of the hemisphere is thinner than elsewhere.
This is well seen in fig. 2, although on the one side of the fissure it is
better marked than on the other. At the same time it must be noted
that this condition is not universally present. In the shallow parts of a
fissure, or where a fissure is shallow throughout, it is the bottom of the
fold that becomes thinned (figs. 3, and 4). When a continuous series of
sections is made through the entire length of one of the transitory fissures
the central part is seen to be very deep (fig. 2), while at its two extremities
it is shallow, and the bottom gradually rises to the surface (fig. 3).
The folds, therefore, as seen from the ventricular aspect, present a semilunar
form (PI. i., fig. 13,/.).

A study of the various specimens would seem to indicate that the
process of obliteration consists in a shortening of the fissure from both
extremities towards the centre. More and more of the bottom of the
furrow rises to the surface, until at last the deep central part alone remains
in the form of a round knob-like elevation on the ventricular aspect of the
wall (PI. i., fig. 18, e. <?.). Ultimately this disappears also. During this
process the fissures retreat from the hemisphere margin, and in several of
the figures which are given in Plates i. and n., one or more of these may
be observed. But it is necessary to mention here what we shall have to
insist upon more fully hereafter, viz. that this obliteration does not seem
to depend solely upon a process of unfolding, but that a certain amount of
absorption of the fold appears also to take place.

On the outer surface, just as on the mesial surface of the hemisphere,
the number of transitory fissures present varies very considerably. Fifteen
hemispheres ranging from the tenth week or so up to the end of the fourth
month gave the following results :—

In 1 there were 2 fissures.

l ^

i i it u ii

» * ii 6

1 7

ii x ii 1 ii

[3*]

In none was the surface smooth.
,, 4 there were 8 fissures.
».« ,, 3 ,,

n 2 ii 10 ,,

» 2 » 4 „