Fig. 21.—Brain of Marsupial (Opossum). A, side view; B, dorsal view; of, olfactory lobes; cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla. Thalami concealed from view by the backwardly extended cerebrum; also the optic lobes are partially covered by the cerebrum.
Fig. 22.—Brain of Lemur (Lemur nigrifrons). A, dorsal view; B, side view; cr, cerebrum; cb, cerebellum; m, medulla.
In the marsupial ([Fig. 21]), a more intelligent animal still, the cerebrum, cr, has grown so large that it extends backwards and partially covers the optic lobes. It is to be observed that in the marsupial the cerebellum, cb (like the cerebrum, cr), has evolved to a higher phase. It consists of a median lobe, cb, which is larger than the median cerebellum of the lower creatures mentioned, and of two lateral lobes, one on either side, which have been acquired in the course of evolution. The median lobe, the homologue of the single, median cerebellum of lower animals, is larger than the lateral ones. The cerebellum of the marsupial has its surface increased by fissures, while that of the fish and reptile is smooth. The fissured cerebellum is a higher evolution than the smooth ones. In the groups of animals referred to so far the cerebrum is smooth and the olfactory lobes are still in front, though much encroached upon in the marsupial by the enlarging cerebrum. In those animals still higher in the scale of life, such as the prosimiæ (Lemurs), the cerebrum has reached yet larger proportions and complexity, and has grown still farther backwards towards the medulla, so that it hides from view a considerable portion of the cerebellum ([Fig. 22]); it has also grown forward, thus concealing largely the olfactory lobes. The cerebrum is no longer smooth, but has a number of simple fissures and convolutions (the higher animals have numerous complex fissures and convolutions). The lateral lobes of the cerebellum have increased relatively more than the central lobe, and the whole organ has advanced in complexity of fissures. In the higher simiæ (monkeys and apes) the cerebrum has grown so far backwards as to almost completely cover the cerebellum and medulla, and its convolutions have become much more numerous and complex. The cerebellum has also grown greatly, and its lateral lobes are now larger and more complex than the central lobe.
Plate XII.—Brain of man: dorsal and side views. The cerebrum has grown so far backwards and forwards as completely to hide the other segments of the brain when looked at from the dorsal surface. From Carus’s “The Soul of Man.” By courtesy of The Open Court Publishing Company.
Finally, in man ([Plate XII]), the whole brain has grown so enormously that it is three times larger than the brain of the highest simian creature. The cerebrum, especially, has increased enormously in size. It has grown not only backwards (overlapping cerebellum), upwards, and downwards on the sides, it has grown so far forwards as not only to cover the olfactory lobes, but also to project far beyond them. The cerebellum has also increased in size and complexity, especially the lateral lobes. The ideal vertical section ([Fig. 23]) shows diagrammatically in one figure all these stages in the evolution of the human brain through the geologic ages.
Fig. 23.—Ideal, vertical and sagital section, representing the ontogeny and phylogeny of the human brain. of, olfactory lobe; crf, cerebrum of fish; ol, optic lobes of fish; cbf, cerebellum of fish; m, medulla of fish; cbr, cerebellum of reptile; cbo, cerebellum of opossum; cbl, cerebellum of lemur; cbm, cerebellum of man; cr, cerebrum. Cerebrum convoluted in lemur; much more convoluted in man. Cerebellum convoluted from opossum upwards; mm, medulla of man.