278 



ANATOMY OF VERTEBRATES. 



a. 



the Chimaera, fig. 179, o, and in the Sharks, fig. 187, o. In 

 tlie latter they bear the same proportion to the optic nerves 

 and eyes as in other fishes, their small relative size depending 

 on the advanced developemcnt of both cerebellum and cere- 

 brum : in the blind Amblyops of the subterraneous waters, 

 the diminution of the optic lobes relates to the almost total 

 abrogation of the visual organ ; but since both in the Amblyops 

 and the equally blind Myxine these lobes are present, they 

 cannot be exclusively the central ganglion of the optic nerve, nor 

 their sole function that of receiving the impressions of the sense 

 of sight, and making them perceptible as ideas by the animal. 



The optic lobes are hollow in most Fishes, fig. 182, b. The 



exterior surface shows blended grey and white matter, the white 



182 fibres usually converging to the optic 



nerves ; some of the fibres unite with 

 the anterior crura of the cerebellum 

 to form the septum of the optic lobes, 

 fig. 184, r, which consists of two or 

 four medullary fasciculi, decreasing in 

 the Tench, increasing in the Cod, as 

 they pass forward. On divaricating 

 the optic lobes from above, as in fig. 

 182, or by a horizontal section, as in fig. 183, their cavity, d, or 

 ventricle, is exposed : it communicates with the expanded myelen- 

 cephalous canal, called ( third ' and ( fourth ' ventricles, as shown 

 by the bristle, q. Its floor is variously configurated in different 

 fishes. There are one or two small white tubercles, 'tuberculi 

 optici,' figs. 182, 183, e, on each side of the back part of the septum; 

 the Cod, Salmon, Pike, and Perch, show four of these bodies; the 

 Carp and Herring, fig. 184, t, two: in the Carp they are oblong, 

 juxtaposed, and were called 'tuberculum cordiforme' by Haller; 1 

 they are not present in the Polypterus, Lepidosiren, Sturgeon, or 

 Plaodostome fishes. External to these tubercles the floor of the 



o 



ventricle usually rises into a curved eminence, with its convexity 

 outwards; this is the ( torus semicircularis ' of Haller, 2 fig. 184, w. 

 It is not homologous with either the ' thalamus opticus ' or the 

 ' corpus striatum ' of the mammal's brain. In the Carp, where 

 the great physiologist first described and named them, the f tori ' 

 are large, and much curved; in general they describe only a 



1 In Salmo Umbla Haller calls them ' corpora quadrigemina,' as does Cuvier, in 

 Perca fluviatilis : they are analogous in form to the parts so named, in Mammals; 

 but are not homologous therewith. 



2 LIX. t. iii. p. 201. 



Brain of Perch, with the optic lobes laid 

 open, and the cerebellum turned to the 

 right side. xxm. 



