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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 261

is placed so as to block the free communication between the sagittal portion
of the furrow and its transverse terminal part.

According to the presence or absence, and also the degree of development
of these annectant gyri, we may meet with several varieties of the
inferior frontal sulcus. The examination of fifty-four hemispheres afforded
the following results :—

1. In rather more than half of the hemispheres examined (50*8 per
cent.), the furrow was equally deep throughout, and not interrupted by
annectant gyri.

2. The anterior deep annectant gyrus was present in 30 per cent., and
was so placed that it partially cut off the transverse terminal part from
the sagittal stem. In only one hemisphere did this gyrus rise to the
surface and produce a complete divorce of the terminal transverse portion
from the main part of the sulcus.

3. In eleven cases (or 19*2 per cent.) the middle annectant gyrus was
present. In five of these (8*7 per cent.) it remained below the surface,
and only partially interrupted the sulcus ; in six (10-5 per cent.) it reached
the surface and completely broke up the sagittal stem into an anterior
and a posterior portion.

But there are two additional furrows which must be associated with
the inferior frontal sulcus, seeing that they undoubtedly belong to the
same system, and not unfrequently are directly connected with it. I refer
to two short transverse sulci, which are frequently placed in front of the
inferior frontal sulcus, in series with and parallel to its transverse terminal
part. Eberstaller has recognized the presence of these furrows, but he
gives them separate names and an independent position. The first he
terms the sulcus radiatus (die radiare Stirnfurche). By its lower end it
cuts, as a rule, into the fore-part of the pars triangularis; by its upper end
it frequently joins superficially the sulcus frontalis medius. I have found
it present in twenty-nine out of fifty-seven hemispheres examined. Of
these it was completely separate from the inferior frontal sulcus in twenty-
two cases, whilst in seven it was joined to its anterior extremity—a deep
annectant gyrus marking the place of union.

The second and more anterior of the two transverse furrows which lie