104 FISHES. 



The optic nerves (second ixdr) vary in size, their strength 

 corresponding to the size of the eye ; they take their origin 

 from the loM o'ptici, the development of which again is pro- 

 portionate to that of the nerves. The mutual relation of the 

 two nerves immediately after their origin is very character- 

 istic of the sub-classes of fishes. In the Cyclostomes they have 

 no further connection with each other, each going to the eye 

 of its own side.^ In the Telcostci they simply cross each other 

 (decussate), so that the one starting from the right half of the 

 brain goes to the left eye and vice versa. Finally, in PalocicJi- 

 tJiyes the two nerves are fused together, immediately after 

 their origin, into a chiasma. The nerve is cylindrical for 

 some portion of its course, but in most fishes gradually changes 

 this form into that of a plaited band, which is capable of 

 separation and expansion. It enters the bulbus generally 

 behind and above its axis. The foramen through which it 

 leaves the skull of Teleostei is generally in a membranous 

 portion of its anterior wall, or, where ossification has taken 

 place, in the orbito-sphenoid. 



B, Nerves proper taking their origin from the train 

 (Figs. 41-45). 



The Nervus oculorum motorius (third pair) takes its origin 

 from the Pedunculus cerebri, close behind the lobi inferiores ; 

 it escapes through the orbito-sj^henoid, or the membrane 

 replacing it, and is distributed to the musculi rectus superior, 

 rectus internus, obliquus inferior, and rectus inferior. Its 

 size corresponds to the development of the muscles of the 

 eye. Consequently it is absent in the blind Amllyopsis, and 

 the Myxinoids. In Lcpidosircn the nerves supplying the 

 muscles of the eye have no independent origin, but are part 



^ XccovAmg to Langn-hans " Untersuchungen iiber Petroinyzon plaiieri" 

 (Freiburg, 1873) an optic chiasma exists in that species. 



