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Dr. Cunningham—Surface Anatomy of the Primate Cerebrum. 219

narrower sense. The sagittal furrow-segment pursues a backward course
and describes an arch which is convex towards the mantle-border. The
first half of this arch ascends slightly, the second descends more or less
sharply. At the highest point of the arch the furrow sends a constant branch
towards the mantle-border which bounds that convolution-loop which limits
externally the parieto-occipital fissure. Immediately in front of this branch
the sulcus is interrupted in typical cases, and here begins the descending
portion of the furrow which bounds the arcus parieto-occipitalis laterally.
This descending part . . . proceeds behind the parieto-occipital fissure, and
opens without any intervening bridge into the hinder transverse segment.
This hinder transverse segment (the sulcus transversus of Ecker) is the analogue
of the 1 Affenspalte'; it is the fissura perpendicularis externa of the primate
brain, and thus the boundary which limits the occipital lobe in front."

The descending portion of the sagittal segment of the furrow to which
Eberstaller refers is the ramus occipitalis, and in many cases it is completely
cut off from the ramus liorizontalis (Eberstaller's ascending portion) by a
superficial bridging gyrus. Indeed Wilder would seem desirous of placing
it upon an independent footing altogether. He applies to it the special name
of paroccipital fissure.* The grounds upon which he bases this view are
briefly : (1) the separate development of this element; and (2) that " in
the adult these two fissures (ramus occipitalis and ramus liorizontalis)
remain independent in about half of the cases, more often on the right
side."

The separate development, however, of this element cannot be urged as
a reason why we should place it outside the intraparietal system of furrows,
because we might apply the same argument, with very nearly equal force,
to all the other segments of this furrow-system. Further, although in its
ontogeny it may appear as a distinct element, and even remain in this
condition throughout life, its phylogeny shows in the clearest manner that
it is a part of the intraparietal sulcus. The continuous intraparietal
sulcus of the apes, presenting in itself all the elements which tend to break
asunder from each other in man, affords us sufficient proof of this.

* "The paroccipital, a newly-recognized fissural integer."—Journal of Nervous and Mental
Disease, vol. xiii., No. 6, 1886.

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