The brain of fishes is relatively small; in the Burbot (Lota) it has been estimated to be 1/720th part of the weight of the entire fish, in the Pike the 1/1305th part, and in the large Sharks it is relatively still smaller. It never fills the entire cavity of the cranium ; between the dura mater which adheres to the inner surface of the cranial cavity, and the arachnoidea which envelops the brain, a more or less considerable space remains, which is filled with a soft gelatinous mass generally containing a large quantity of fat. It has been observed that this space is much less in young specimens than in adult, which proves that the brain of fishes does not grow in the same proportion as the rest of the body ; and, indeed, its size is nearly the same in individuals of which one is double the bulk of the other.
The brain of Osseous fishes (Fig. 41) viewed from above shows three protuberances, respectively termed prosencephalon, mesencephalon, and metencephalon, the two anterior of which are paired, the hindmost being single. The foremost pair are the hemispheres, which are solid in their interior, and provided with two swellings in front, the olfactory lobes. The second pair are the optic lobes, which generally are larger than the hemispheres, and succeeded by the third single portion, the
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Fig. 41. — Brain of Perch.
a, cerebellum ; b, optic lobes ; c, hemispheres ; e, lobi inferiores ; f, hypophysis ; g, lobi posteriores ; i, Olfactory lobes ; n, N. opticus ; o, N. olfactorius ; p, N. oculo-motorius ; q, N. trochlearis ; r, N. trigeminus ; s, N. acusticus ; t, N vagus ; u, N. abducens ; v, Fourth ventricle.
cerebellum. In the fresh state the hemispheres are of a grayish
colour, and often show some shallow depressions on theirII
