The sense of pain is very feeble among fishes, A trout has been known to bite at its own eye, plated on a hook and similar insensi bility has been noted in the pike and other fishes. °The Greenland shark, when feeding on the carcass of a whale, allows itself to be repeatedly stabbed in the head without aban doning its prey)) The Nervous System.— The nervous sys tem 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, its structures feeble and the gray matter less de veloped than in the higher forms. According to Gunther the brain in the pike (Esox) forms but 1-1305th part of the weight of the body; in the burbot (Lola) about 1-720th 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.
It is most convenient •to examine the fish brain in its higher stases of development, as seen in the sun-fish, striped bass or perch. As seen from above, the brain of a typical fish seems to consist of five lobes or ganglia, four of them in pairs, the fifth posterior to those on the median line. The posterior lobe is the cere bellum or metencephalon and it rests on the broadened termination of the spinal cord, called the medulla oblongata.
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 mesen cephalon. In front of these lie the two lobes of the cerebrum, also called the hemispheres, the prosencephalon. These lobes are usually smaller than the optic lobes and solid. In some fishes they are covered by a fold, but are never cor rugated, as is the brain of the higher animals. In front of the cerebrum lie the two small olfactory lobes, which receive the large olfac tory nerves from the nostrils.
In the hollow of the optic lobe are small pro tuberances, supposed to represent the corpora quadrigemina of the higher vertebrates. From the lower surface of the brain is suspended the hypophysis or pituitary gland.
In most of the bony fishes the structure of the brain does not differ materially from that seen in the perch. In the sturgeon, however,
the parts are more widely separated, so that the connecting nerve-substance is more clearly seen between the several parts. In the dipnoans the cerebral hemispheres are united, while the optic lobe and cerebellum are very small. In the sharks and rays the large cerebral hemispheres are usually coalescent into one, and the olfac tory nerves dilate into large ganglia below the nostrils. The optic lobes are smaller than the hemispheres and also coalescent. The cerebel lum is very large, and the surface of the medulla oblongata is more or less modified or specialized.
Besides the structures noted in other fishes, the epiphysis or pineal organ is largely de veloped in sharks and traces of it are found in most or all of the higher vertebrates. In some of the lizards this epiphysis is largely developed, bearing at its tip a rudimentary eye. This leaves no doubt that in these forms it has an optic function. For this reason the structure, wherever found, has been regarded as a rudi mentary eye, and the "pineal eye° has been called the "unpaired median eye of chordate° animals.
It has been supposed that this eye, once pos sessed by all vertebrate forms, has been grad ually lost with the better development of the paired eyes, being best preserved in reptiles as "an outcome of the life-habit which concealed the animal in sand or mud and allowed the fore head surface alone to protrude, the medium eye thus preserving its ancestral value in enabling the animal to look directly upward and back ward? This we may doubt, as in none of the fishes is the epiphysis more than a nervous en largement and neither in fishes nor in amphibia is there the slightest suggestion of its connec tion with vision. It seems probable, as sug gested by Dr. Hertwig, that the original func tion 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.
The brain of the cyclostomes (hagfishes and lampreys) differs widely from that of the higher fishes and the homologies of the dif ferent 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 lancelet there is no trace of brain, the band-like spinal cord tapering toward either end.