NERVOUS SYSTEM OP FISHES. 



179 



or Plagiostome fislies. External to these tubercles the floor of the 

 ventricle usually rises into a curved eminence with its convexity 

 outwards; this is the 'torus semicircularis' of Haller* (^^. 52. ?t'.) 

 In the Carp, where the great physiologist first described and named 

 them, they are large, and much curved : in general the ' tori' describe 

 only a small portion of a circle ; and in some bony fish, as the Gar- 

 pike, Loach, and Lump-fish, they are scarcely raised above the 

 level of the floor of the ventricle. They are not developed in the Po- 

 plyterus, the Lepidosiren or the higher Plagiostomes ; and both tori 

 and globuli are peculiar ichthyic developments in the ventricles of the 

 optic lobes. The bottom of the optic ventricle anterior and external 

 to the tori, is grey, and usually prominent i^fig. 52. v), with, white 

 fibres radiating through it to rise and expand upon the walls of the 

 lobes. The optic lobes have almost coalesced in the Polypterus, Lepi- 

 dosiren, Amblyopsis, Eel, and Loach (^Cobitis). Where they appear 

 distinct externally, as in most osseous fishes, they are brought into 

 mutual communication by one or two commissures, besides the so- 

 called ' corpus callosum ; ' the anterior ' commissura transversa ' is 

 shown in the Herring {fig. 52. s) ; it crosses in front of the entry to 

 the third ventricle, f 



In the Myxine and Lepidosiren 

 the prepyramidal fibres curve sud- 

 denly forwards and upwards be- 

 fore expanding into the floor and 

 sides of the third ventricle, and 

 they thus form a small protube- 

 rance beneath the basis of the 

 optic lobes {fig. 54. n). In the 

 Shark the same columns swell 

 out laterally and form two small 

 protuberances {fig. 55. n), separated below by the vascular (hypo- 

 physial) floor of the third ventricle. In most osseous fishes the 

 corresponding fibres of the prepyramidal tracts swell out suddenly, 

 beneath the optic lobes, into two protuberant well-defined oval 



Optic ventricles ; 

 Herring. 



Base of brain ; Cod. 



* Lix. t. iii. p. 201. It is analogous to, but not, as Gottsche supposes, liomolo- 

 gous with, the ' thalamus opticus ' of the INIainmalian brain. It is neither analogous 

 to nor homologous with the 'corpus striatum.' 



f Cuvier affirms (xxiii. t- i. p. ,'516.) that this "necessarily answers to the an- 

 terior commissure of the cerebrum ;" but it has only a remote analogy with it, in 

 so far as the mech;mism of the whole mesencephalon of Osseous Fishes resembles 

 that of the cerebrum in IVIammals, wliilst the true homology of the mesencephalon 

 does not extend beyond the parts immediately surrounding the third ventricle and 

 the 'iter' to the fourth in the IMammaliau cerebrum. 



