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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 253

exhibited in Eberstaller's schematic representation of the outer surface of
the hemisphere. It is also seen in figure 54, p. 250, in Plate i., figure 33,
and in Plate iv., figure 1. The condition of the sulcus, which I consider
typical, although it is not the one which is most frequently met with,
is seen in figure 53, page 248, and also, to a less extent, in figure 55,
page 251.

Fifty adult hemispheres were examined with the view of determining
the frequency of this ramus horizontalis of the inferior prsecentral sulcus
and its relations to the vertical stem of the fissure. The following are the
results obtained:—

1. Present in complete and uninterrupted union with the vertical stem,
72*3 per cent.

2. Present in incomplete union with the vertical stem—a deep annectant
gyrus marking the point of junction—8*5 per cent.

3. Present, but completely separated from the vertical stem by a superficial
gyrus, 2*1 per cent.

4. Present, but presenting a deep annectant gyrus at the base of its
anterior branch, 4"3 per cent.

5. Present, but presenting a deep annectant gyrus at the base of its
posterior branch, 4*3 per cent.

6. Completely absent, 8*5 per cent.

The fact that it may be completely or partially cut off from the vertical
limb, and also the occasional partial separation of its anterior or posterior
branches, may be explained, as we shall afterwards see by a study of the
development of the furrow.

I am inclined to believe that in certain cases where the ramus horizontalis
has been observed altogether separate from the vertical stem, it has
been described as the sulcus prsecentralis medius. Such appears to me to
be the case in the " second type " of prsecentral sulcus described by Sernoff
—the figure of which I am able to introduce through the courtesy of the
author (fig, 56, p.r. m., p. 254). Again, in two other figures, which he gives
in page 16 of his work on the " Individual types of the convolutions of the