THE CHIMPANZEES AND ORANGS. 79 
the optic platform (0), beyond which the presphenoidal part of the floor (9) describes 
a slight convexity upwards before it sinks into the deep and narrow rhinencephalic 
fossa (rh). 
The lower boundary dividing the anterior from the middle lobes of the cerebrum, 
formed by the orbitosphenoids or ‘lesser alz,’ which in Man, after overarching the 
‘foramen lacerum anterius,’ is continued outwards as a ridge upon the inner surface of 
the frontal, has no existence in the Gorilla, and the fossa for the ‘ natiform protube- 
rances’ (n, Pl. XXVIII.) of the cerebral hemispheres is much less deep; so that the 
division of the prosencephalic compartment into the fossa for the anterior, and that for 
the middle, lobes of the cerebrum is so feebly indicated in the Gorilla, that we may 
safely aver it would not have been recognizable without the indications afforded by the 
better-developed boundaries in the Human subject. By the light of the same com- 
parison we are able to recognize, reciprocally, through the better-defined rhinence- 
phalic compartment in the Gorilla and Orang, the true nature of that almost effaced 
primary natural division of the cranial cavity in our own species. 
In the Gorilla the boundary of the epencephalic chamber is more complete and defi- 
nite than in Man, the tentorial ridge being continued from the petrosal (16, Pl. XXVIII.) 
outwards and backwards to the superoccipital (2). The epencephalic compartment of 
the cranium, which, in Man, is bounded behind by the impression of the lateral sinus in 
the higher races,—which impression is feebly, if at all, marked in Papuan skulls,—is 
relatively larger and especially deeper in all the races than in the Gorilla; and this 
greater depth, with the more central position of the foramen magnum at the base of the 
skull, is associated with a production of the walls of the epencephalic compartment 
downwards in an infundibular form to the foramen magnum, and with a remarkable 
difference in the aspect of the plane of the basioccipital and basisphenoid in the Human 
cranium. 
The posterior boundary of the epencephalic compartment is only half an inch above 
the foramen magnum in the Gorilla, whilst in the Papuan it is one inch two thirds, and 
in the European nearly two inches; and this difference depends not only on the greater 
vertical extent of the cerebellum, but on the above-mentioned characteristic position 
of the foramen magnum which relates to the erect posture and gait of the Human 
species. 
By no method, indeed, is the strongly marked distinction afforded by the foramen 
magnum between Man and the highest Quadrumana so clearly demonstrated as by the 
vertical bisection of the skull here described. In the first place, as to the size of the 
foramen, it is, relatively to the capacity of the cranium, much larger in the Gorilla than 
in Man. Taking the antero-posterior diameter crossed by the section, we find this to be 
precisely the same in the skulls of the Gorilla and the Papuan compared. ‘The differ- 
ence in the aspect of the plane of the foramen (D -. - . - D) is, perhaps, best shown by 
comparing such plane with that of the bony palate (B - -- - B). 
