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272

Cunningham Memoirs.

not lose sight of the possibility that this early furrow may be a transitory
precursor of the prsecentral sulcus which becomes obliterated before the
true furrow makes its appearance. There is one fact, however, which
militates against this view, and it is this: it is not a complete fissure, and
it would be quite exceptional to find a furrow which was not due to an
infolding of the cerebral wall transitory in character.

As a general rule the sulcus prsecentralis inferior is seen in hemispheres
which have reached a stage of development corresponding to the
early part of the sixth month. Figures 12 to 15 (PI. n.) represent specimens
which were obtained from female twins about this period of development.
In all of these the inferior prsecentral furrow (p.c. and p.c.i.) is well marked,
but whereas in two (figs. 13 and 14) the vertical stem is alone formed,
the others show both the vertical and the horizontal portions of the furrow.
Indeed, it will be seen that in figure 12 the typical T-shaped form of
sulcus has been already attained. A very remarkable point in connexion
with this early inferior prsecentral sulcus is the high position which it holds
on the side of the cerebrum. A subsequent lowering of the level which is
occupied by the furrow at this period must take place, and an interesting
field for speculation is opened up in the question as to how this is brought
about. Possibly, the growth of the fronto-parietal operculum produces a
general downward locomotion of the whole cerebral surface above the
insula. Such a growth-change would also affect, but to a less extent, the
lower piece of the fissure of Rolando, and also, very slightly, the sulcus
postcentralis inferior should it happen to be in place (fig. 13, p1.).

It is a remarkable circumstance that in those cases where the frontal
furrows appear at an early date, they present the straight linear form so
characteristic of the fissures of lower apes. It is impossible, in these cases,
to analyze the different steps of the process of formation. When they
come into existence, however, at a later stage they assume, in the first
instance, the form of shallow depressions and furrows which ultimately
run into each other. In this manner we have an opportunity afforded of
determining the mode of origin of the furrows involved. A study, therefore
, of hemispheres somewhat advanced in development, but in which
the sulcus prsecentralis inferior is not fully formed (PI. n., figs. 17 to