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Show 1899.] DR. A. KEITH ON THE CHIMPANZEES. 311 measures on an average 25 mm. high and 25 mm. broad; in the female Gorilla 29 m m . high and 31 m m . broad; 34 m m . high and 38 m m . broad in the adult male Gorilla. The orbits vary according to age, sex, and the individual, but they measure, almost without exception, most in the trans\rerse diameter in the Chimpanzee, in the vertical in the Gorilla, a feature dependent on teeth development. Certain features in the foramina for the exit of nerves in the facial part of the skull separate the Gorilla aud Chimpanzee. The infra-orbital foramen in the Gorilla is divided into tAvo or more compartments by a vertical bar, that in the Chimpanzee by a horizontal bar. This difference depends on the fact that the infraorbital nerve in the Gorilla sinks clown within the maxilla from the margin of the orbit; iu the Chimpanzee it passes horizontally inwards from the maxillary-malar suture. The foramina for the nasal nerves are always to be seen in the nasal bones of the Gorilla, never in those of the Chimpanzee ; in the Chimpanzee these foramina occur in, or at the side of, the upper part of the premaxilla. Malar foramina occur only occasionally in the Gorilla ; they are always present in the Chimpanzee. The inferior palatine foramen of the Chimpanzee is divided by a process of bone-a division not seen in the Gorilla. The supra-orbital ridges in Johanna project from the frontal bone to a depth of 22 mm., and are separated by a glabellar notch. This notch is very seldom seen in the female and never in the male Gorilla. It rarely occurs in the male Chimpanzee and is variable in the female, but does not appear to be con fined to any particular race. The supra-orbital ridges keep on growing through life : in 5 young adult Chimpanzees their average depth was 14 mm., in 4 old adults 18 mm.; at corresponding periods of the same sex of the Gorilla they measure 20 and 25 m m . In this feature Johanna resembles the Gorilla. These ridges begin to form before the milk-dentition is completed, and the part they play in the animal economy is to strengthen the facial portion of the skull to give a firm dental support. The skull of the Chimpanzee is the more brachycephalic. The average length of 10 skulls of Gorillas, excluding from the measurements the prominence due to the frontal air-sinuses and the external occipital protuberance, Avas 118 mm., the corresponding measurement in 10 skulls of Chimpanzees 103*6 ; the breadth of skull, from one parietal eminence to another, Avas in the first 94 mm., in the second'89 m m . The breadth of the skull in Gorillas is 80 per cent, of the length, in Chimpanzees 86 per cent. But the measurements overlap, and many of the measurements of the female Gorilla correspond to those of the female Chimpanzee. The temporal ridges in Johanna are about 25 m m . apart on the crown of the head, a Chimpanzee character. In only 1 out of 5 adult female Gorillas had these ridges not fused into a median crest, and in that particular case the cranial capacity was uncommonly great. In it the temporal ridges were 20 m m . apart. 21* |