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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 285

4. The sulcus fronto-orbitalis is the homologue of the sulcus frontalis
secundus in the human brain. It follows from this that, as the sulcus
fronto-orbitalis is only developed in a few of the lower apes and in the
anthropoids, the sulcus frontalis inferior, according to Eberstaller, is absent
in the majority of the lower apes.

I fully agree with Eberstaller in the views which he has expressed
regarding the sulcus arcuatus, the sulcus frontalis primus, and the sulcus
prsecentralis superior, but I cannot accept the theory he has advanced upon
the homologies of the sulcus rectus, the sulcus frontalis medius, and the
sulcus fronto-orbitalis, although I recognize and appreciate the plausibility
and force of the various arguments which he has adduced in favour of his
contention.

The constancy of the sulcus arcuatus in the lower apes, and the early
appearance in the human foetus of a furrow similar in form and similar in
position, afford us both phylogenetic and ontogenetic evidence in favour
of the view that it is the sulcus prsecentralis inferior. In the human cerebrum
the sagittal limb of this furrow, as we have noted, may undergo
considerable modification. In many cases, however, it retains its early,
primitive, and typical form (fig. 53, p. 248).

The first frontal furrow is one of late phylogenetic origin. In several
of the lower apes it is totally absent, and even in its highest state of
development it is only represented by two or three oblique or transverse
shallow furrows which are quite distinct from each other (see the sketch of
the baboon's cerebrum, fig. 66). This permanent condition in the lower
apes is fully in accord with the transitory conditions exhibited in the
development of the furrow in the human cerebrum. In man it is relatively
late in making its appearance on the surface of the frontal lobe,
and it always comes into existence in the first instance in the form of a
series of isolated furrows, which, as a rule, run together, although it is
not uncommon, especially in the negro, to find them remaining separate.

The close association which exists between the basal part of the first
frontal furrow and the superior prsecentral sulcus renders it extremely
difficult to say at what point in the ape-series the latter sulcus makes its
appearance. It is unmistakably present in the chimpanzee and in the