178 LECTURE VIII. 



walls of the optic lobes, a small part only being continued forwards 

 to the cerebrum in most Osseous Fishes. The anterior cerebellar 

 crura are chiefly lost in the inner walls, septum, or longitudinal 

 commissure of the optic lobes. 



These lobes are commonly of a subspherical figure, and larger 

 than the cerebral lobes (as in figs. 47. 49. 51, 52. o) ; they are often 

 larger than the cerebellum, (^6. ^6.) ; they are of equal size with the 

 cerebellum in the Eel ; are smaller than the cerebral lobes, but 

 larger than the cerebellum, in the Polypterus and Lepidosiren (yfig. 

 54. o.) ; they are smaller than either the cerebrum or cerebellum in 

 the Amhlyopsis spelceus {fig. 50. o.), and in the Sharks {fig. 55. 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 development of both cerebellum and cerebrum : in the 

 blind Amblyopsis 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 Amblyopsis 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 im- 

 pressions of the sense of sight, giving them form, and making them 

 perceptible and tenable as ideas by the animal. 



The optic lobes are hollow in most fishes. The exterior surface 

 shows blended grey and white matter, the white fibres converging 

 to the optic nerves on the outer side of the lobes, and passing trans- 

 versely from one lobe to the other from their inner sides across the 

 ventricle. * Some of these fibres unite with the anterior crura of 

 the cerebellum to form the ' longitudinal commissure' (fig. 52. r), 

 which consists of two or four medullary fasciculi, decreasing 

 in the Tench, increasing in the Cod, as they pass forwards. | 

 On divaricating the optic lobes from above, their cavity or ventricle 

 is exposed : its floor is variously configurated in difierent fishes. 

 There are one or two small white tubercles ('tuberculi optici,'^^. 51, 

 52. t) on each side of the back part of the septum : the Pike and Perch 

 show four of these bodies, the Carp and Herring two ; in the Carp they 

 are oblong, juxtaposed, and were called 'tuberculum cordiforme' by 

 Haller|; they are not present in the Polypterus, Lepidosiren, Sturgeon, 



* These transverse fibres are analogous to the ' corpus callosum,' but not homo- 

 logous with it, as Carus (xciv. ), Cuvier (xxiu.), and Gottsche (lvii. ) supposed. 



•}• Analogous to the ' fornix,' but not homologous with it, as Gottsche contends 

 (lviii. p. 266.) 



J Lix. In the Salmo Umbla, where they are four in number, Haller called them 

 ' corpora quadrigemina : ' Cuvier, also, regards them as answering to the ' corpora 

 quadrigemina ' of Mammals (xxiii. i. p. 317.), mistaking a relation of analogy for 

 one of homology. 



