The nerves of the Fish.—The nervous system in the fish, as in the higher vertebrates, consists of brain and spinal cord with sensory, or afferent, and motor, or efferent, nerves. As in other vertebrates, the nerve substance is divided into gray matter and white matter, or nerve-cells and nerve-fibres. In the fish, however, the whole nervous system is relatively small, and the gray matter less developed than in the higher forms. According to Günther the brain in the pike (Esox) forms but 1/1305 part of the weight of the body; in the burbot (Lota) about 1/720 part.
The cranium in fishes is relatively small, but the brain does not nearly fill its cavity, the space between the dura mater, which lines the skull-cavity, and the arachnoid membrane, which envelops the brain, being filled with a soft fluid containing a quantity of fat.
The Brain of the Fish.—It is most convenient to examine the fish-brain, first in its higher stages of development, as seen in the sunfish, striped bass, or perch. As seen from above the brain of a typical fish seems to consist of five lobes, four of them in pairs, the fifth posterior to these and placed on the median line. The posterior lobe is the cerebellum, or metencephalon, and it rests on the medulla oblongata, the posterior portion of the brain, which is directly continuous with the spinal cord.
In front of the cerebellum lies the largest pair of lobes, each of them hollow, the optic nerves being attached to the lower surface. These are known as the optic lobes, or mesencephalon. In front of these lie the two lobes of the cerebrum, also called the hemispheres, or prosencephalon. These lobes are usually smaller than the optic lobes and solid. In some fishes they are crossed by a furrow, but are never corrugated as in the brain of the higher animals. In front of the cerebrum lie the two small olfactory lobes, which receive the large olfactory nerve from the nostrils. From its lower surface is suspended the hypophysis or pituitary gland.
Fig. 78.—Brain of a Shark (Squatina squatina L.). (After Dean.)
- I. First cranial nerve (olfactory).
- P. Prosencephalon (cerebrum).
- E. Epiphysis.
- T. Thalamencephalon.
- II. Second cranial nerve.
- IV. Fourth cranial nerve.
- V. Fifth cranial nerve.
- VII. Seventh cranial nerve.
- V4. Fourth ventricle.
- M. Mesencephalon (optic lobes).
- MT. Metencephalon (medulla).
- EP. Epencephalon (cerebellum).
Fig. 79.—Brain of Chimæra monstrosa. (After Wilder per Dean.)