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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 245

in the course of development they usually become united. The two portions
of the vertical part of the prsecentral furrow are termed the sulcus
prsecentralis inferior and the sulcus prsecentralis superior. These differ
from the corresponding parts of the intraparietal sulcus, in so far, that in
their further development they, as a rule, remain distinct.

But, in the intraparietal sulcus we also recognize a sagittal part, which
is developed in two portions, viz. a ramus horizontalis and a ramus occipitalis
. These stand in more or less direct connexion with each other, and
also with the sulcus postcentrals inferior. In the frontal lobe similar sulci,
which divide its outer surface into an upper and a lower field, may be
recognized. These are—(1) a short horizontal or oblique furrow in direct
connexion with the upper end of the sulcus prsecentralis inferior, which
extends forwards into the middle frontal convolution, and which may
therefore be compared with the ramus horizontalis of the parietal lobe; and
(2) in front of this, and usually in the same line with it, the sulcus frontalis
medius of Eberstaller. A still further feature of similarity between these
systems of sulci may be noted in the fact, that the sagittal portion of each
furrow, as it approaches the extremity of the cerebrum, ends in a terminal
bifurcation, developed in a precisely similar manner in each instance. In
the case of the intraparietal sulcus this forms the sulcus occipitalis transversa
of Ecker; in the case of the sulcus frontalis medius it forms
the sulcus fronto-marginalis of Wernicke (fig. 52, p. 245).

There is one point, however, in which this comparison between the
intraparietal and prsecentral systems of sulci fails. In the parietal lobe the
ramus horizontalis and the ramus occipitalis are developed at the same time,
and in the lower primates they are directly continuous with each other.
They must, therefore, be considered as possessing an equal morphological
value. In the frontal lobe, however, the sulcus frontalis medius is on a
different morphological footing from the ramus horizontalis of the inferior
proecentral sulcus. In the first place it is, as a general rule, later in making
its appearance, and in the second place, although it may be recognized as
a distinct element in certain of the anthropoid apes, it is totally absent in
the lower apes. From this statement it will be seen that the conclusions at
which I have arrived regarding the ontogeny and phylogeny of the sulcus