.100 



ANATOMY OF VERTEBRATE?. 



Hake some fibres of the optic nerve, ib. 2, arc derived from botli 

 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. 

 Pleuronectidce ; in fig. 198, the optic lobe, e, and hypoarion, //, 

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

 lobe,/, and hypoarion,^, 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, Z>,«,fig. 201, sometimes over, figs. 185, 198, the left nerve: 1 

 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 Arn- 

 blyopsis, and the Lepidosiren, there are no 

 motory nerves of the orbit. In the Lamprey 

 a small third nerve and a fourth nerve, which 

 are closely connected where they quit the 

 cranium, again separate, the one to supply 

 the rectus superior and rectus internus, the 

 other the obliquus superior; the filaments supplying the other 



Brain of a Hake {ilerluccius) with the base upward, ecu. 



200 



Plaited optic nerve of a Mullet. 

 a, optic nerve deprived of its 

 sheath, exhibiting the plaited 

 disposition; b, sclerotic coat 

 of the eye through which the 

 nerve is passing ; c, retina, 

 in which the nerve termi- 

 nates, ecu. 



1 The writer has seen both varieties in different individuals of Gadus morrhua. 



