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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 303

part of the first frontal furrow. It is usually developed along with it. It
consists of two pieces—an upper and a lower—which may be partially
or completely separated from each other, as well as from the basal part
of the first frontal furrow by an annectant gyrus.

4. Two additional furrows belonging to the prsecentral system are
occasionally present, viz. the sulcus prsecentralis medius and the sulcus
prsecentralis marginalis.

5. The sulcus prsecentralis medius may arise in two different ways :
(a) it may be formed by the ramus horizontalis of the inferior prsecentral
sulcus divorced from the vertical stem and assuming a very oblique or an
almost vertical direction; (b) it may consist of a new element placed
between the superior and inferior prsecentral furrows, but showing a closer
connexion with the former.

6. The sulcus prsecentralis marginalis is a small variable furrow which
appears on the cerebral surface between the upper end of the sulcus
prsecentralis superior and the mesial border of the hemisphere.

7. The inferior frontal sulcus may be developed in three portions which
afterwards run into each other.

8. Sometimes these parts remain separate; and frequently its mode of
development is indicated in the adult brain by the presence of deep annectant
gyri.

9. The basal part of the furrow may be developed in direct continuity
with the inferior prsecentral sulcus; more frequently it is developed independently
. In the latter case it often establishes a secondary and partial
connexion with the sulcus prsecentralis inferior.

10. The sulcus radiatus and the lateral part of the sulcus fronto-
marginalis of Eberstaller should be regarded as furrows which belong to
the inferior frontal system.

11. The sulcus frontalis superior may be composed of three or four
elements which may be partially or completely separate from each other.

12. The superior frontal gyrus and the middle frontal gyrus are each
partially subdivided into two tiers or subdivisions by furrows which maybe
respectively termed the sulcus frontalis mesialis and the sulcus frontalis
medius.