FIG. 2.—Skull of Man, one-fourth natural size.
a, spinous processes of cervical vertebræ.
It is in this excessive development of the brain that the principal difference between man and the anthropoid apes must be sought. We know in fact from the researches of numerous anthropologists (see [Chapter II.]) that the average weight of a man’s brain in European races (the only races sufficiently known in this respect) is 1360 grammes, and that of a woman’s is 1211 grammes. These figures may rise to 1675 grammes in certain instances, and fall to 1025 in others.[15] Brains weighing less than 1000 grammes are generally considered as abnormal and pathological.
On the other hand, the brains of the great anthropoid apes (gorilla, chimpanzee, and orang-utan), the only ones comparable to man in regard to weight of body, have an average weight of 360 grammes. This weight may rise to 420 grammes in certain isolated cases, but never exceeds this figure. And even in these cases, with the orang-utan, for example,[16] it only represents one half per cent. of the total weight of the body, while with European man the proportion is that of at least three per cent., according to Boyd and Bischoff.[17]
The excessive development of the brain and of the brain-case which encloses it is correlative, in the case of man, with the reduction of the facial part of the skull. In this respect the difference is also appreciable between him and the animals. In order to convince ourselves of this we have only to compare the human skull with that of any ape whatever, placing both in the same horizontal plane approximately parallel to the line of vision.[18]
Viewed from above, or by the norma verticalis, as the anthropologists say, the bony structure of the human head leaves nothing of its facial part to be seen (Fig. [11]); at the very most may be observed, in certain rare instances, the lower part of the nasal bones, or the alveolar portion of the upper jaw (Fig. [10]). On the other hand, with apes, anthropoid or otherwise, almost all the facial part is visible. Examined in profile (norma lateralis), the bony structure of the heads of man and monkeys presents the same differences.
With the anthropoid apes, the facial portion forming a veritable muzzle rises, massive and bestial, in advance of the skull, while with man, very reduced in size, it is placed below the skull. The facial angle, by means of which the degree of protuberance of the muzzle may, to a certain point, be measured, exhibits notable differences when the skulls of man and animals are compared in this particular. On continuing the examination of the profiles of the bony structures of the two heads in question, we notice also the slight development of the facial part of the malar bone in man, as compared with its temporal part, and the contrary in the ape; as well as the difference in the size of the mastoid processes, very strong in man, very much diminished proportionately to the dimensions of the head in the anthropoid apes.
Seen from the front (norma facialis), the human skull presents a peculiarity which is not observed in any anthropoid skull, namely, that the top of the nasal opening is always situated higher than the lowest point of the lower edge of the orbits (Fig. [12]); while in the anthropoid apes it is always found below this point. Lastly, if the skulls in question, always placed on the horizontal plane, are compared from behind (norma occipitalis), it will be noted that on the human skull the occipital foramen is not seen at all; on the skulls of monkeys it is plainly visible, if not wholly, at least partly.[19]