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352

Cunningham Memoirs.

end of the parallel sulcus is, as just stated, not readily determinable, inasmuch
as the sulcus is not always continuous, but may be interrupted, either
by bifurcating, or by the appearance at greater or lesser depth of bridging
gyri.

If it bifurcate, the anterior of the two branches turns upwards and
forwards to run at right angles, or even less, to the middle line. In any
case, it invariably lies behind the centre of the parietal eminence.

Inferiorly the posterior limb of the angular gyrus runs downwards
parallel to the lambdoid sutures, and practically midway between the latter
and the central point of the parietal eminence, until crossing beneath the
squamo-parietal suture, it appears beneath the centre of the squamous bone,
and forms the second and broadest gyrus of the temporal region. The
degree to which it occupies the surface of the lobe varies with the development
of the superior temporal gyrus. As already stated, this proportion
usually amounts to the breadth of the second gyrus, being equal to the
combined breadths of the superior gyrus and the upper half of the third,
as exposed on the outer surface of the lobe above the lower margin of the
hemisphere. Therefore, in the adult, the temporal portion of the superior
temporal sulcus lies approximately opposite the junction of the upper and
middle thirds of the squamous bone.

In the child, up to seven years, owing of course to the relation of the
squamous suture to the hemisphere, the highest point of the suture, at first
opposite the centre of the broad middle temporal gyrus, gradually comes to
coincide with the superior temporal sulcus. The hinder part of the sulcus
maintains, however, at all ages, the direction and course described above
in relation to the lambdoid suture and parietal eminence respectively.

The Middle Temporal Sulcus.—The anterior two-thirds of this sulcus
are readily fixed topographically, namely, as running parallel to the
inferior border of the hemisphere, and distant from it one-fourth of the
breadth of the temporal lobe. Posteriorly the sulcus courses under the
squamous suture, and most frequently terminates just above the asterion,
i.e. at or in front of the lambdoid suture. So often is this sulcus divided
into three portions, an anterior, middle, and posterior, each roughly corresponding
to thirds of its length, that the asterial portion may appear as a