OCR Text |
Show 2 0 ON THE BRAINS OF MONKEYS. [Jan. 20, The parallel sulcus extends considerably beyond the Sylvian fissure, and does not- as is so commonly the case with the Macaques- join that fissure dorsally. An interesting fact about this fissure is that it bends forward at its dorsal extremity, instead of being continued on in a straight line. In this bending I see a point of likeness to the Semnopitheci (including Nasalis). No absolute distinction between Cynopithecus and Macacus can be, however, drawn on account of this furrow, since Kiikenthal and Ziehen figure a brain of Macacus inuus in which there is this same bending forwards, and, moreover, a bifurcation of the furrow superiorly, such as I note in my example of Cynopithecus niger. The fissure of Rolando presents no noteworthy characters ; it does not nearly cut the inter-cerebral groove. The postcentralis is better developed on the right side of the brain than on the left, and is transverse in both cases. The precentralis superior is, on the other hand, better developed upon the left side than upon the right. It is transverse in position. Among Monkeys this fissure is more commonly parallel with the long axis of the brain. The median parieto-occipital sidcus, visible when the brain is bisected longitudinally, presents what I regard as rather an interesting and suggestive character. This fissure, in the Macaques, &c., has a distinctly forward inclination, often making an angle of quite 45° with the vertical. On the other hand, in the Semnopitheci this furrow is nearly vertical (a brain of Nasalis) or with a distinctly backward inclination. In Cynopithecus niger this same furrow is nearly vertical, but with a slightly backward inclination, thus resembling the Semnopithecidae more than the Cercopithecidse. The calcarine sulcus of Cynopithecus is not at all like that furrow in the brains of Cynocephalus, Macacus, Cercopithecus, and Cercocebus. In the four last-named genera it is a T-shaped sulcus, the cross of the T appearing almost, sometimes in fact quite, upon the upper surface of the occipital lobe. This furrow is very characteristic of those genera. In Cynopithecus the furrow is simple and oblique in direction, as it is, as a general rule, among the Semnopithecidae. It may be convenient to tabulate the likenesses shown in the brain of Cynopithecus to that of Semnopithecus. The brain resembles that of the Semnopitheci in :- (1) The form of the intraparietal fissure. (2) The backward direction of the internal parieto-occipital. (3) The absence of any junction between the Sylvian and parallel fissures. (4) The simple form of the calcarine fissure. (5) The shortness of the inferior occipital fissure. (6) The presence of a well-marked superior portion, and the absence of an inferior portion of the inferior temporal fissure. The first two characters are absolutely distinctive of Cyno- |