I. Upper aspect. II. Lower aspect.
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.
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 cerebellum. In the fresh state the hemispheres are of a grayish colour, and often show some shallow depressions on their surface; a narrow commissure of white colour connects them with each other. The optic lobes possess a cavity (ventriculus lobioptici), at the bottom of which some protuberances of variable development represent the corpora quadrigemina of higher animals. On the lower surface of the base of the optic lobes, behind the crura cerebri, two swellings are observed, the lobi inferiores, which slightly diverge in front for the passage of the infundibulum, from which a generally large hypophysis or pituitary gland is suspended. The relative size of the cerebellum varies greatly in the different osseous fishes: in the Tunny and Silurus it is so large as nearly to cover the optic lobes; sometimes distinct transverse grooves and a median longitudinal groove are visible. The cerebellum possesses in its interior a cavity which communicates with the anterior part of the fourth ventricle. The medulla oblongata is broader than the spinal chord, and contains the fourth ventricle, which forms the continuation of the central canal of the spinal chord. In most fishes a perfect roof is formed over the fourth ventricle by two longitudinal pads, which meet each other in the median line (lobi posteriores), and but rarely it remains open along its upper surface.
The brain of Ganoid fishes shows great similarity to that of the Teleostei; however, there is considerable diversity of the arrangement of its various portions in the different types. In the Sturgeons and Polypterus (Fig. [42]) the hemispheres are more or less remote from the mesencephalon, so that in an upper view the crura cerebri, with the intermediate entrance into the third ventricle (fissura cerebri magna), may be seen. A vascular membranous sac, containing lymphatic fluid (epiphysis), takes its origin from the third ventricle, its base being expanded over the anterior interspace of the optic lobes, and the apex being fixed to the cartilaginous roof of the cranium. This structure is not peculiar to the Ganoids, but found in various stages of development in Teleosteans, marking, when present, the boundary between prosencephalon and mesencephalon. The lobi optici are essentially as in Teleosteans. The cerebellum penetrates into the ventriculus lobi optici, and extends thence into the open sinus rhomboidalis. At its upper surface it is crossed by a commissure formed by the corpora restiformia of the medulla.
Fig. 42.—Brain of Polypterus. (After Müller.)
I., Upper; II., Lateral; III., Lower aspect.
a, Medulla; b, corpora restiformia; c, cerebellum; d, lobi optici; e, hypophysis; f, fissura cerebri magna; g, nervus opticus; g’, chiasma; h, hemispheres; i, lobus olfactorius; k, sinus rhomboidalis (fourth ventricle).
As regards external configuration, the brain of Lepidosteus and Amia approach still more the Teleosteous type. The prosencephalon, mesencephalon, and metencephalon are contiguous, and the cerebellum lacks the prominent transverse commissure at its upper surface. The sinus rhomboidalis is open.
The brain of the Dipnoi shows characters reminding us of that of the Ganoids as well as the Chondropterygians, Ceratodus agreeing with Protopterus in this respect, as in most other points of its organisation. The hemispheres form the largest part of the brain; they are coalescent, as in Sharks, but possess two lateral ventricles, the separation being externally indicated by a shallow median groove on the upper surface. The olfactory lobes take their origin from the upper anterior end of the hemispheres. Epiphysis and hypophysis well developed. The lobi optici are very small, and remote from the prosencephalon, their division into the lateral halves being indicated by a median groove only. The cerebellum is very small, overlying the front part of the sinus rhomboidalis.