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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 247

Between the first frontal furrow and the mesial margin of the hemisphere
there are usually present a series of small irregular depressions or
sulci which follow each other in linear order, and partially divide the first
frontal convolution into two parts. In some cases these are observed to
run into each other, and form a continuous furrow which effects a very
complete subdivision of the superior frontal convolution. These sulci have
not received the attention they deserve, as there is reason to believe their
degree of development gives a clue to the determination of high and low
types of cerebrum.

The different elements of the system of furrows on the outer surface of
the frontal lobe are not all equally important, and the assigning to each its
proper morphological value is no easy matter. The development of the
frontal sulci is subject to very considerable variation, and it is only when
we associate the evidence it affords with that obtained by the study of the
frontal sulci in the different groups of apes that we are able to arrive at
satisfactory conclusions. The sulcus prsecentralis inferior and the sulcus
frontalis secundus are the furrows which must be regarded as holding first
rank; the sulcus prsecentralis superior and the sulcus frontalis primus
possess an inferior status ; next in point of morphological importance
comes the sulcus frontalis medius; and lowest in the scale are the irregular
sulci which impress the first frontal convolution.

II. Sulcus Prsecentralis Inferior (fig. 53, p. c. i.).—This sulcus presents
a vertical, and a more or less horizontal portion, and these are, as a rule,
in direct and uninterrupted continuity with each other.

The vertical limb is placed in front of the lower part of the ascending
frontal convolution, and rarely extends higher than one-third of the distance
between the Sylvian fissure and the mesial border of the hemisphere. In
almost every case it is curved, so that it is convex to the front and concave
towards the back, and its general direction is somewhat variable. Sometimes
it runs parallel to the lower part of the fissure of Rolando ; in other
cases it inclines slightly forwards as it ascends.

In many cases the vertical limb of the inferior prsecentral sulcus is
shortened by a deficiency in its lower part, but it never, of itself, elongates