278 



ANATOMY OF VERTEBRATES. 



the Chimasra, fig. 179, o, and in the Sharks, fig. 187, O. In 

 the latter they bear the same proportion to the optic nerves 

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

 on the advanced developement of both cerebellura and cere- 

 brum : in the blind Amblyojjs of the svibterraneous 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 jjerceptible as ideas by the animal. 



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

 exterior surface shows Ijlended 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 medvdlary 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 exposetl : 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 dift'erent 

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

 optici,' figs. 182, 183, c, 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. 18-1, t, two: in the Carp they are oblong, 

 juxtaposed, and were called 'tuberculura cordiforme' by Haller;' 

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

 Plao'iostome fishes. 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,^ fig. 18-4, u\ 

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

 ' corpus striatum ' of the mammars brain. In the Carp, where 

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

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



' In Salmo Umhla Ilallcr calls tlicm 'coipora ciuadrigeniiua,' as ilocs Ciivior, in 

 Perca flmiatiUs : they arc analogous in foim to the pai'ts so named, in JUamnials; 

 but are not homologous therewith. 



- nx. t. iii. p. 201. - 



Brain of Terrli, with tlic optic 

 open, and tlie certbcilum turned to the 

 right side, xxiii. 



