212 PISCES. 



mesocerebrum that is open superiorly into the lesser optic thai ami 

 and quadrigeminal bodies. The open cavity corresponds to the 

 third ventricle. A pineal gland is not always distinct, but probably 

 occurs at least in the Rays. The middle cerebral ganglia or lobes 

 (lobi optici and corpora quadrigemina) are largest in the foetal state 

 in proportion to the hemispheres, but are always much smaller than 

 in the Osseous Fishes ; they are hollow in their interior, but simpler 

 in structure, and covered in great part by the cerebellum. A pair of 

 large inferior ganglia are siuated in front of (not, as in the Osseous 

 Fishes, behind) the very large pituitary body (that is provided with 

 membranous appendages, and supplied by vessels of some size), 

 and partly beneath the narrow part of the brain enclosing the third 

 ventricle. The cerebellum is very large, of varied form, and for the 

 most part crucially divided by a longitudinal and transverse groove ; 

 more rarely, as in some of the Sharks and Rays, by several trans- 

 verse grooves, as in the higher animals. The medulla oblongata 

 forms a considerable ganglionic enlargement, and the corpora resti- 

 formia are usually much corrugated, and provided with ganglionic 

 dilatations, which may be viewed as posterior lobes or ganglia, such 

 as were described in the Osseous Fishes, and which correspond in 

 position to the clava of the human brain, as also to the funiculi 

 cuneati. The fourth ventricle is prolonged far backward into the com- 

 pletely open rhomboidal sinus. The roof of the fourth ventricle often 

 sends off several divisions into the cerebellum, e. g. in the large trans- 

 versely-furrowed braia of many Rays. 



The cerebellum not uitfrequently exhibits an asymmetrical posi- 

 tion, as in many Osseous FisLes, e. g. Scyllium catulus and canicula, 

 and in other Rays and Sharks. The anterior or posterior lobe is in- 

 determinately placed in one instance more to the right, in another to 

 the left side. 



In rare cases the posterior lobes are developed to such a degree as 

 completely to cover the fourth ventricle, as is the case in the Electric 

 Rays (Torpedo). The lobes of the fourth ventricle are here connect- 

 ed with the electric organ, and may be called electric lobes ; they are 

 characterized by their yellow color, and present under the microscope 

 a peculiar structure, consisting of numerous ganglionic corpuscules 

 surrounded by dense sheaths. 



The structure of the brain in Chimara (at least C. monstrosa) is 

 very different from that of the Sturgeon, and approaches in a stri- 

 king manner to that of the Sharks ; only the hemispheres are far 

 removed from the mesocerebrum by the very long and narrow divi- 



