Fig. 47.—Bird-brain. A, side view; B, top view.
Fig. 48.—Mammal-brain. A, top view; B, side view.
Fig. 49.—Man’s brain. A, side view; B, top view.
3. Vertebrate Brain.—Another excellent example is the structure of the vertebrate brain. The brain of an average fish is represented in [Fig. 45]. It consists of four or five swellings, or ganglia, strung along, one beyond another. Commencing behind, these are, first, the medulla, m; then the cerebellum, cb; then the optic lobes, ol; then the cerebrum and thalamus combined, cr; and last, the olfactive lobes, of. Of these, it will be observed, the optic lobe is the largest in the brain of the fish ([Fig. 45]). In the brain of the reptile ([Fig. 46]) we have the same serial arrangement, of the same parts, only that the cerebrum has now become the dominant part instead of the optic lobes. In the average bird ([Fig. 47]) the cerebrum has grown so large that it extends backward, and partly covers the optic lobes. In the lower mammals (marsupials), the brain is much the same in this respect, as in birds—i. e., the cerebrum only partly covers the optic lobes, so that, looked at from above, the whole series of ganglia are still visible. But in the average mammal ([Fig. 48]) the cerebrum is so enlarged that it covers entirely the optic lobes and encroaches on the cerebellum behind and the olfactive lobes in front. In some monkeys, indeed, the cerebellum is nearly or even quite covered. Finally, in man ([Fig. 49]), the cerebrum has grown so enormously that it covers every other part and completely conceals them from view when the brain is looked at from above. In front it not only covers but has grown far beyond the olfactive lobes; behind it extends beyond and overhangs the cerebellum; on the sides it overhangs and covers all. Looked at from above, nothing is seen but this great ganglion. The ideal section ([Fig. 50]) represents all these stages diagrammatically in one figure. After what has been said, the figure will be readily understood.
Fig. 50.—Ideal section showing all the above stages.