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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 167

ends of the sulcus, so that they mark it out into three equal parts. The
superior genu is usually much the weaker of the two and is directed backwards
. The upper third of the fissure inclines downwards and slightly
backwards. At the superior genu the sulcus bends suddenly in a forward
and downward direction. The inferior genu is always strongly marked
and looks forwards. Here the fissure again changes its direction and proceeds
very nearly vertically downwards.

In almost every case the educated eye is able to detect these genua.
Perhaps the most common deviation from the condition which we have
described as being typical is one in which the central piece of the fissure
of Rolando becomes considerably shortened and nearly horizontal in its
direction. The two genua are thus more closely approximated, and the
inferior may lie almost directly in front of the superior.

We have seen that when the lips of the fissure of Eolando are
drawn widely asunder, a deep annectant gyrus is sometimes seen in its
lower part. But this is not the only one which may be present. In the
neighbourhood of the superior genu there is generally a shallowing of the
fissure and a deep interlocking of its adjacent walls. Two of the inter-
digitating gyri—one projecting backwards from the anterior central convolution
and the other forwards from the posterior central convolution—are
always larger and more pronounced than the others, and in a considerable
number of cases they unite at the bottom of the sulcus in the form of a distinct
deep gyrus, which constitutes a marked interruption in its floor. All
gradations between a mere shallowing with an interlocking of the adjacent
walls of the fissure and the presence of a distinct deep annectant gyrus are
met with. This is a point of considerable morphological importance, as
we shall see later on, and it is one to which Eberstaller has specially
referred.

The following points I have specially noticed in regard to this bridging
gyrus in the human brain: (1) it is not more strongly marked in the
brain of a full-time foetus nor in the brain of a child than in the adult;
(2) it varies slightly in its position ; it may be higher or lower, but it is
usually placed in relation to the superior genu of the fissure and opposite
the first frontal gyrus; (3) the more usual condition exhibits no direct