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170

Cunningham Memoirs.

month—there is certainly an appearance which leads me to believe that the
sulcus may have developed in the manner usually attributed to it. It is a
clean-cut straight fissure, with its extremities equally distant from the
superior border of the hemisphere and the Sylvian region; further, on
breaking the hemisphere across in the line of the fissure, the latter is seen
to present a uniform depth, and to be at no point interrupted by an
elevation of the bottom.

There is some variability in the time at which the fissure of Rolando
makes its appearance. The more usual time is the last week or ten days
of the fifth month, but it is not uncommon to meet with hemispheres well
on in the sixth month of development with no sign of the fissure.

As a general rule, the fissure of Rolando is developed in two separate
and distinct portions (PI. i. figs. 24 and 31; PI. n. figs. 12,14,17,18,19,20, and
21; r1. and r2.). The lower portion appears in the form of a shallow oblique
groove which represents the lower two-thirds of the fully-formed sulcus.
It always makes its appearance before the upper portion (PI. n., figs. 10 and
11, etc., r1.). Its lower end is placed close to the coronal suture—perhaps,
indeed, it may lie immediately subjacent to the suture—while the upper
end lies further back, and reaches a point midway between the upper
margin of the hemisphere and the Sylvian fossa. The upper portion of the
fissure makes its appearance in the form of a deep pit or depression between
the upper end of the lower portion and the margin of the hemisphere. An
eminence separates the two portions of the fissure from each other. Soon,
however, a faint furrow runs over the summit of this elevated intervening
piece of the cortex, and the two primitive portions of the sulcus are partially
united to each other (PI. n., figs. 17, 19, and 20). As development
goes on the more complete does the union become, and the more fully is the
intervening eminence borne down into the bottom of the fissure. As a
rule the confluence takes place rapidly, but in many cases the process
appears to be retarded. Amongst my specimens I have several hemispheres
in the early part of the seventh month, which still show a complete
severance of the two constituent elements of the furrow.

But the portion of cerebral cortex which intervenes between the two
parts of the fissure is never entirely obliterated. It disappears from the