300 



ANATOMY OF VERTEBEATES. 



Hake some fibres of the optic nerve, ib. 2, are derived from both 

 the hypoaria, ib. n and fig. 199, d, and from the wall of the third 

 ventricle. The relation of the hypoaria to the nerves of sight is 

 illustrated in the fishes with unsymmetrical heads and eyes, e. g. 

 Pleuronectida ; in fig. 198, the optic lobe, e, and hypoarion, g, 

 giving origin to the larger optic nerve, c, are larger than the optic 

 lobe,/, and hypoarion, g, giving origin to the smaller optic nerve, d. 

 The nerves cross one another without interchange of fibres ; some- 

 times the right nerve in its passage to the left eye passes under, 

 fig. 199, 6, a, fig. 201, sometimes over, figs. 185, 198, the left nerve:' 

 rarely does one nerve perforate the other, as, e. g. in the Herring. 

 The nerves are flattened where they decussate. In most Osseous 

 Fishes the structure of the optic nerve is peculiar ; it consists of 

 a folded plate of membrane and neurine, fig. 200, a, which usually 

 prevails throughout the length of the nerve, from its cerebral 



attachment to the eyeball : 



^^^ in some instances the inner 



— surface of the optic lobe is 



also folded : and, in all, the 



plaits may be observed to 



be faintly continued upon 



the retina, which is formed 



by the unfolding of the 



nerve. The optic nerve escapes, in Osseous Fishes, either through 



the anterior fibrous wall of the cranium beneath the orbito- 



sphenoid, or through a notch or a foramen 



in that bone. In the PleuronectidcE one optic 



nerve is usually shorter, as well as smaller, 



than the other, fig. 198. In the Eel the 



nerves form, after decussation, a very acute 



angle in the axis of the body, fig. 176, a: in the 

 Lump-fish they form an obtuse open angle. 



Since there are no muscles of the eyeball 

 in the Lancelet, the Myxinoids, the Am- 

 blyopsis, and the Lepidosiren, there are no 

 motory nerves of the orbit. In the Lamprey 

 a small third nerve and a fourth nerve, wliioli 

 arc closely connected where they quit the 

 cranium, again separate, the one to supply 

 the rectus superior and rectus intcrnus, the 

 other the obliquus superior; the filaments supplying the other 



' The writer has seen both varieties in difl'crciit inOividuals of Gadm morrlwa. 



Brain of a Hake (ilcrluccim) with tlic base upward, cuii. 



200 



Plaited optic nerve of a Mullet. 

 a, o]>tio nerve deprived uf it3 

 elieath, exhibiting tlie pliiited 



■disposition; b, scler.iiir cuat 

 of the eye tlirougli wliicli the 

 nerve is paSBing ; r, retina, 

 In ■\vliit'li tlie nerve termi- 

 nates, ecu. 



