NERVES OF FISHES. 



299 



Lancelet the little ciliated olfactory sac is brought into close 

 contact with the rhinencephalic extremity of the neural axis. 

 When the olfactory lobe or ganglion, in other Fishes, is near 

 the organ of smell, as in the Cod, fig. 196, o, it sends off the 

 nerves by numerous very short fasciculi. This multiplicity of 

 virtual origins of the proper nerve is less conspicuous where 

 the rhinencephalon is near the rest of the brain ; but a careful 

 analysis of the long olfactory nerve in the Eel, fig. 176, will 

 show that it is a fasciculus of filaments distinct from their 



origin. 



The optic nerves, like the eyes, are of large relative size in most 

 fishes ; but where the organs of sight are small, the nerves are 

 slender, as in the Silurus : they are still more slender in the 

 Myxinoids, and they are scarcely discernible filaments in the 



197 



193 



a 



Brain of Skate (Eaia}, base 

 view. ecu. 



Brain of a Halibut (Hippoglossus), A upper, B under 

 view. ecu. 



Amblyopsis, fig. 175, 2. In the Plagiostomes, fig. 197, Holoce- 

 phali, Ganoidei, and Protopteri, the optic nerves, ib. a, a, arise in 

 part from the optic lobes, ib. d, in part from the hypoaria, ib. c, c, 

 closely adhering to the fore part of the base of the mesencephalon, 

 and are there connected together by a transverse commissure, ib. b, 

 or close interblending of substance : they do not freely cross each 

 other. In ordinary Osseous Fishes, figs. 181, 185, the exterior white 

 fibres of the optic lobes converge to their under and anterior part, 

 to form the chief part of the origin of the optic nerves ; but a portion 

 of the origin may be traced through the septum opticum to the 

 cerebellum ; and in the Eel, the Garpike, and the Lump-fish, a 

 portion may be traced to the hypoaria : in the Cod, fig. 185, and 



