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258

Cunningham Memoirs.

pieces may be developed in relation to the upper and lower extremities
of the sulcus prsecentralis superior. Of these, the latter is named the sulcus
prsecentralis medius, and will require separate notice. The other is a small
furrow which is very frequently observed between the upper end of the
superior prsecentral sulcus and the mesial margin of the hemisphere (fig. 53,
p. 248 ; fig. 54, p. 250 ; and fig. 59, p. 262, a.). It may either stand
separate, or it may join the upper extremity of the superior prsecentral
sulcus. A deep annectant gyrus always marks the point of union. We
may distinguish this little furrow by the name of sulcus prsecentralis
marginalis. It must not be confounded with the sulcus prsecentralis
medialis, or the anterior limiting furrow of the paracentral lobule which
cuts the mesial border of the hemisphere in front of the upper extremity
of the superior prsecentral furrow (fig. 53, p. 248, s. m.).

IV. Sulcus Prsecentralis Medius.—This is not a constant sulcus, and
the term is not always applied to the same fissural element. As we have
noted, the separation of the horizontal limb of the inferior prsecentral
sulcus, by the appearance on the surface of the deep annectant gyrus,
which occasionally intervenes between it and the vertical limb, produces a
furrow which holds an intermediate position, and to which the term sulcus
prsecentralis medius has been applied. An increase in the obliquity of the
furrow which brings it more into a line with the vertical portions of the
fissural system is very common, and tends to obscure its real nature. As a
rule its lower end lies behind the upper extremity of the vertical limb of
the inferior prsecentral sulcus, whilst its upper end is placed in front of the
inferior extremity of the sulcus prsecentralis superior (fig. 56, p. r. m.,
p. 254). The sulcus prsecentralis medius, described by Eberstaller, is of this
nature.*

The second form of sulcus prsecentralis medius is of a totally different
character. It, apparently, belongs more to the upper than to the lower
main portion of the prsecentral sulcus. It is well seen in figure 57. It is
usually placed altogether behind the upper part of the sulcus prsecentralis

* Das Stirnhirn, p. 61. Leipzig, 1890.