98 FISHES. 



surface ; a narrow commissure of white colour connects them 

 with each other. The optic lobes possess a cavity {ventriculus ' 

 lobioptici),a,ttliehottomoiwhieh 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 lohi 

 inferiores, which slightly diverge in front for the passage of 

 the infundihulum, from which a generally large hypophysis or 

 pituitary gland is suspended. The relative size of the cere- 

 bellum 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 contaias the fourth ventricle, which 

 forms the contiauation of the central canal of the spiaal 

 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 {Idbi postcriores), 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 hemi- 

 spheres 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 {fisswra cerebri magna), may be 

 seen. A vascular membranous sac, containing lymphatic fiuid 

 {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, mark- 



