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.