2. In the anthropoid apes the cerebrum is smaller, relatively to the cerebellum, than in man.
3. In the anthropoid apes the sulci and gyri are generally less complex, and those of the two cerebral hemispheres are more symmetrical, than in man.
4. The hemispheres are more rounded and deeper in man than in the anthropoid apes, and the proportions of the lobes to one another are different. Furthermore, certain minor gyri and fissures, present in the one, are absent or rudimentary in the other.
The evidence of the first of these differences has, I believe, been universally admitted since the time of Sœmmering. The second and fourth clearly result from the observations of Schroeder van der Kolk and Vrolik, and those of Gratiolet (Mem. sur les plis cerebraux des Primatès, 1854), as will appear from the following extracts. The first citation is taken from the work of the first-named authors, which seems to be so little known in this country, that I make no apology for length of the extract:—
"According to very precise investigations which the first named of us has carried out with reference to this point, the difference between the brains of the higher apes and that of man is to be sought, not only in the smaller size of the hemispheres, but also in a totally different relation of the lobes. Relatively, the under surface of the first lobe of the cerebrum, in the chimpanzee, is much larger than in man; while, on the other hand, the distance from the most anterior point of the middle lobe to the hindermost point of the posterior lobe is much smaller. In our chimpanzee the distance from the root of the olfactory nerve to the anterior margin of the brain is about 44 millimetres, from the point of the middle lobe to the extreme end of the posterior lobe, 69 mm. In the adult man, according to measurements which the first of us has instituted, and which wholly agree with those of the ninth plate of Foville, the first named measurement is 57 mm., the second, 145 mm. In the brain of a new-born child, examined by us, the first dimension amounted to 33 mm., the second to 70 mm. The length of the base of the anterior lobe was thus to the distance from the point of the middle lobe to the end of the posterior lobe, in the chimpanzee, as 1 : 1⋅52; in the adult man as 1 : 2⋅54; in the child, as 1 : 2. Hence it appears that the relative proportions of the lobes of the child's brain hold just the mean between the chimpanzee and the adult man; and that in the course of the growth of the child to manhood, the posterior and middle lobes increase more in length than the base of the anterior lobe. In the orang, the same proportion obtains as in the new-born child or 1 : 2, a result which is certainly remarkable, and proves that, in this respect, the brain of the orang stands higher than that of the chimpanzee. The second point to which we would direct attention is, that in comparing the brain of man with that of animals, and especially in determining in what manner the cerebellum becomes covered we too exclusively attend to the posterior elongation of the cerebral hemispheres, while the varying size of the cerebellum itself ought to be taken into account. On comparing the perpendicular section of the brain of the new-born child (pl. ii., fig. 3.) with fig. 1, the brain of the three-year-old chimpanzee, and with fig. 2, that of the orang of a like age, it is at once apparent that the cerebellum of the orang, and especially of the chimpanzee, is much larger than that of the child; so that, supposing one could place the cerebellum of the chimpanzee behind the medulla oblongata of the child, it would be even less covered.
In fact, the distance from the anterior edge of the most anterior part of the cerebellum, close to the corpora quadrigemina, to its posterior margin, measures, in the chimpanzee, 38 mm.; in the orang, 35 mm.; in the child, 22 mm. If we compare the measurements with the whole distance from the anterior to the posterior lobe of the cerebrum, we obtain, according to measurements taken by the first named of us,—
