300 



ANATOMY OF VERTEBRATES. 



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, y, 

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

 lobe,y, 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, &,#, fig. 201, sometimes over, figs. 185, 19 8, 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 Pleuronectida 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 



Plaited optic nerve of a Mullet. m/vfniMr TIPWPCJ nf flip nvVn't Tn flip 

 a, optic nerve deprived of its HlOtOry HeiVCS Lt. 



a 



Brain of a Hake (Merluccius) with the base upward, ecu. 



200 



a sma11 ^^ lierve anc l a fourth nerve, which 



of the eye through winch the are closely connected where they quit the 



nerve is passing ; c, retina, > > 



in which the nerve termi- cranium, again separate, the one to supply 



nates. ecu. _ , _ . _ 



the rectus superior and rectus internus, the 

 other the obliquus superior ; the filaments supplying the other 



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



