Fig. 80.—Brain of Protopterus annectens. (After Burckhardt per Dean.)
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. 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 olfactory nerves dilate into large ganglia below the nostrils. The optic lobes are smaller than the hemispheres and also coalescent. The cerebellum is very large, and the surface of the medulla oblongata is more or less modified or specialized. The brain of the shark is relatively more highly developed than that of the bony fishes, although in most other regards the latter are more distinctly specialized.
The Pineal Organ.—Besides the structures noted in other fishes the epiphysis, or pineal organ, is largely developed 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 rudimentary eye, and the "pineal eye" has been called the "unpaired median eye of chordate" animals.
Fig. 81.—Brain of a Perch, Perca flavescens. (After Dean.)
- R. Olfactory lobe.
- P. Cerebrum (prosencephalon).
- E. Epiphysis.
- M. Optic lobes (mesencephalon).
- EP. Cerebellum (epencephalon).
- ML. Medulla oblongata (metencephalon).
- I. First cranial nerve.
- II. Second cranial nerve.
- IV. Fourth cranial nerve.
- V. Fifth cranial nerve.
- VII. Seventh cranial nerve.
- VIII. Eighth cranial nerve.
- IX. Ninth cranial nerve.
- X. Tenth cranial nerve.
Fig. 82.—Petromyzon marinus unicolor (Dekay). Head of Lake Lamprey, showing pineal body. (After Gage.)
It has been supposed that this eye, once possessed by all vertebrate forms, has been gradually 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 forehead surface alone to protrude, the median eye thus preserving its ancestral value in enabling the animal to look directly upward and backward." This theory receives no support from the structures seen in the fishes.