OCR Text |
Show 1880.] MR. W. A. FORBES ON THE UAKARI MONKEYS. 641 (scissure occipitale externe) first appears externally. It is at slightly concave forwards, then convex. At about two thirds of its course it is joined by the well-marked antero-temporcd sulcus (a.t.) (scissure parallele, Gratiolet; premier sillon temporal, Broca), which commences near the lower margin of the temporal lobe. This is a condition not found in Ateles, Lagothrix, Mycetes, or Pithecia monachus ; it is represented by Gratiolet as existing in Cebus capu-cinus (Mem. Plis Ccr. Atlas, pl. x. figs. 7 & 8), and likewise occurs in Cynocephalus, Macacus, and other Old-world genera. Embracing the upper extremity of the Sylvian fissure is a somewhat Y-shaped sulcus, the "stem*' of the Y being short and springing from the median line, whilst the two arms are much longer and run downwards and outwards, and in the case of the anterior one forwards as well, onto the external surface of the hemispheres, appearing there one on each side of the upper part of the Sylvian fissure (fig. 9). The posterior of these is, no doubt, the aforesaid temporo-occipital sulcus (t. o.) (scissure perpendiculaire externe of Gratiolet, scissure occipitale externe of Broca). The anterior limb no doubt corresponds to the anterior part of the "sulcus (4) bounding the upper border of the angular gyrus, having the form of a broad pointed arch," described by Prof. Flower in Pithecia monachus (I. c. p. 330). A comparatively slight modification of the condition of these parts in that species as represented in his fig. 1 would bring about that which obtains in Brachyurus, which also is represented in Gratiolet's figures of Cebus capucinus and C. apella (I.e. pl. x. figs. 7 & 11). This "supraangular" (s.a.) sulcus defines anteriorly the well-developed angular gyrus. The union of the temporo-occipital and occipitoparietal (the two parts of the scissure occipitale of Broca) sulci divides off perfectly the occipital and parietal lobes ; so that there is here no such superficial "pli de passage" between these two lobes as exists in Cebus capucinus and Pithecia satanas. In P. monachus there is, in addition, a second, more superior passage-fold between these two lobes (Flower, I. c.)1. In Cebus apella (according to Gratiolet's figure, I.e. pl. 10. figs. 12, 12 bis), as in Brachyurus, the " pli de passage " is concealed, so that superficially the two lobes appear perfectly distinct. Anterior to the supraangular sulcus, the fissure of Rolando (p.p.) (postero-parietal, Huxley) is seen as a well-developed sulcus forming a sigmoid curve extending very nearly to the middle line. Anterior to this is the superofrontal sulcus (s.fl), also curved, though roughly parallel with the external border of the hemispheres. The orbital surface of the hemispheres is also marked by a somewhat H-shaped complex sulcus (incisure en-H). The occipital lobe is almost entirely smooth ; below a sulcus is observable, curving upwards, and ending in a bifurcate manner in front of the lower termination of the occipitotemporal sulcus. Posterior to this is a very 1 As regards this and sundry other differences between Pithecia monachus and P. satanas, it must be remembered that the two are not perhaps strictly congeneric, P. satanas having been separated, with P. chiropotes, as a genus Chiropotes. |