A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

In none of the fishes is the epiphysis more than a nervous enlargement, and neither in fishes nor in amphibia is there the slightest suggestion of its connection with vision. It seems probable, as suggested by Hertwig and maintained by Dean that the original function of the pineal body was a nervous one and that its connection with or development into a median eye in lizards was a modification of a secondary character. On consideration of the evidence, Dr. Dean concludes that "the pineal structures of the true fishes do not tend to confirm the theory that the epiphysis of the ancestral vertebrates was connected with a median unpaired eye. It would appear, on the other hand, that both in their recent and fossil forms the epiphysis was connected in its median opening with the innervation of the sensory canals of the head. This view seems essentially confirmed by ontogeny. The fact that three successive pairs of epiphyseal outgrowths have been noted in the roof of the thalamencephalon[6] appears distinctly adverse to the theory of a median eye."[7]

The Brain of Primitive Fishes.—The brain of the hagfish differs widely from that of the higher fishes, and the homologies of the different parts are still uncertain. The different ganglia are all solid and are placed in pairs. It is thought that the cerebellum is wanting in these fishes, or represented by a narrow commissure (corpus restiforme) across the front of the medulla. In the lamprey the brain is more like that of the ordinary fish.

In the lancelet there is no trace of brain, the band-like spinal cord tapering toward either end.

The Spinal Cord.—The spinal cord extends from the brain to the tail, passing through the neural arches of the different vertebræ when these are developed. In the higher fishes it is cylindrical and inelastic. In a few fishes (headfish, trunkfish) in which the posterior part of the body is shortened or degenerate, the spinal cord is much shortened, and replaced behind by a structure called cauda equina. In the headfish it has shrunk into "a short and conical appendage to the brain." In the Cyclostomes and chimæra the spinal cord is elastic and more or less flattened or band-like, at least posteriorly.

The Nerves.—The nerves of the fish correspond in general in place and function with those of the higher animals. They are, however, fewer in number, both large nerve-trunks and smaller nerves being less developed than in higher forms.

The olfactory nerves, or first pair, extend through the ethmoid bone to the nasal cavity, which is typically a blind sac with two roundish openings, but is subject to many variations. The optic nerves, or second pair, extend from the eye to the base of the optic lobes. In Cyclostomes these nerves run from each eye to the lobe of its own side. In the bony fishes, or Teleostei, each runs from the eye to the lobe of the opposite side. In the sharks, rays, chimæras, and Ganoids the two optic nerves are joined in a chiasma as in the higher vertebrates.

Other nerves arising in the brain are the third pair, or nervus oculorum motorius, and the fourth pair, nervus trochlearis, both of which supply the muscles of the eye. The fifth pair, nervus trigeminus, and the seventh pair, nervus facialis, arise from the medulla oblongata and are very close together. Their various branches, sensory and motor, ramify among the muscles and sensory areas of the head. The sixth pair, nervus abducens, passes also to muscles of the eye, and in sharks to the nictitating membrane or third eyelid.

The eighth pair, nervus acousticus, leads to the ear. The ninth pair, glosso-pharyngeal, passes to the tongue and pharynx, and forms a ganglion connected with the sympathetic system. The tenth pair, nervus vagus, or pneumogastric nerve, arises from strong roots in the corpus restiforme and the lower part of the medulla oblongata. Its nerves, motor and sensory, reach the muscles of the gill-cavity, heart, stomach, and air-bladder, as well as the muscular system and the skin. In fishes covered with bony plates the skin may be nearly or quite without sensory nerves. The eleventh pair, nervus accessorius, and twelfth pair, nervus hypoglossus, are wanting in fishes.

The spinal nerves are subject to some special modifications, but in the main correspond to similar structures in higher vertebrates. The anterior root of each nerve is without ganglionic enlargement and contains only motor elements. The posterior or dorsal root is sensory only and widens into a ganglionic swelling near the base.

A sympathetic system corresponding to that in the higher vertebrates is found in all the Teleostei, or bony fishes, and in the body of sharks and rays in which it is not extended to the head.