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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 93

Some very interesting points are indicated in this Table. The most
common condition is clearly that in which the two limbs are distinct and
separate. Further, it is important to note that this condition is more
frequently present in the male, and on the left side, than it is in the female,
and on the right side. Whether this has any significance in connexion
with the development of the motor centre of speech, which is placed in the
pars basilaris, or that portion of the lower frontal convolution which is
situated between the prsecentral sulcus and the anterior ascending limb of
the Sylvian fissure, it is impossible to determine at present. Next in point
of frequency comes the Y condition of the anterior limbs, and this is peculiar
in being more common on the left side, and in the female. Lastly,
the remarkable point about the single anterior limb is its marked preference
for the right side. It, apparently, occurs with nearly equal frequency in
the two sexes.

III. The Orbital Limbs of the Sylvian Fissure.—In certain cases the
orbital operculum of the island of Reil will be found to be cut into by one
or two fissures which cut through its entire thickness, and extend straight
forwards for a short distance on the orbital face of the frontal lobe. These
are distinguished from the true anterior horizontal limb in so far that they
both lie to the inner side of the posterior end of the external orbital
sulcus. They may be termed the external and internal orbital limbs of
the Sylvian fissure. The external is much more frequently present, and is
longer than the internal. It cuts the orbital operculum immediately to the
inner side of the posterior extremity of the external orbital sulcus, and its
length varies from a few millimetres up to 1 cm. The internal orbital
limb is much shorter as a rule, and is often little more than a notch. When
present it may be looked upon as the anterior branch of a terminal bifurcation
of the furrow, which bounds the island of Reil anteriorly, and it
therefore gives to the orbital operculum a short free internal border which
it does not possess when this little fissure is absent.

These little incisions into the orbital operculum were first observed and
described by Benedikt.* He says : " Als Typus der ausseren Orbitalfurche
* Anatoniische Studien an Yerbrecber-Gehirnen. Wien, 1879, Epilogomena X., p. 107.