ENCEPHALON OF FISHES. 



277 



(if r,T[-li, upper eiirfacc. xxui. 



in Fishes, is called the ' epencephalon,' fig. 179,/, C, fig. 178, g, c ; 

 it is relatively larger, occupies a greater j^roportion of the cranium, 

 and is more complex and diversified in this than in any of the 

 higher classes of Vertebrata. 



The next succeeding primary division of the brain is called the 

 ' mesenceplialon,' figs. 180 & 18J, b, e,f: it is usually the largest 

 division in Osseous Fishes, and con- 

 sists of two upper spheroidal bodies, 

 called 'optic lobes," figs. 176, 177, 

 180, /; (in most of the figures, o), 

 of two lower subspherical bodies. 



called ' hypoaria,'^ figs. 178, 185, n, 

 fig. 181, e, with intervening con- 

 necting walls enclosing a cavity, 

 called the ' third ventricle,' wliich 

 is prolonged downward into the pedicle of the 'hypophysis,' or 

 pituitary gland, fig. 185, p, and u})ward into that of the ' cona- 

 rium' or pineal gland, fig. 175, zo. The prepyramidal columns 

 are continued forwards, along the floor of the fourth ventricle, 

 where they are covered by a thin layer of medullary fibres, to the 

 hypoaria and prosencephalon : some fibres Idending with the wall 

 of the third ventricle and the base of the optic lobes. The 

 transverse 'ansulate' commissure,' which unites or crosses the pre- 

 pyramids Ijcfiire they penetrate the hypoaria, may be regarded as 

 the most anterior of the arciform fila- 

 ments, wliich feebly represent the 

 pons Var(jlii in Fislies. The restiform 

 columns are exjiended chiefly in form- 

 ing the walls of the third ventricle and 

 the Ijase and exterior walls of the optic 

 lobes, a small part only being con- 

 tinued forwards to the cerebrum in most Osseous Fishes. The 

 anterior cerebellar crura are chiefly lost in the inner walls or 

 seiitum of the optic lobes. , 



These lobes are commonly of a sulispherical figure, and larger 

 than the cerebral lobes, as in figs. 177, 180, h, 171, 184, o; they are 

 often larger than the cerelDcUum, ib. ib. ; l:)ut of nearly equal size 

 with the cerebellum in the Eel, fig. 176; they are smaller than 

 the cerebral lobes, but larger than the cerebellum, in the Polypterus 

 and Lepidosiren, fig. 186, O; they are smaller than either the 

 cerebrum or cerebellum in the Amblijopsis spelmus, fig. 175, o, in 



181 



fir- 



I rif Percli, lindor surface. 



' Lobes crcux,' Ciivier. 



' Lobes inferieuis,' Ib. 



LVii. pi. ir., fig. 7, /. 



