104 FISHES. 



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

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

 from the Idbi optici, 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 Teleostei 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 Palceich- 

 thyes 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 

 behiad and above its axis. The fora*men through which it 

 leaves the skull of Teleostei is generally ia a membranous 

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

 place, ia the orbito-sphenoid. 



B. Nerves proper talcing their origin from the drain 

 (Figs. 41-45). 



The Nervus oculorum motorius (third pair) takes its origin 

 from the Pedunculus cerebri, close behind the lobiinferiores; 

 it escapes through the orbito-sphenoid, or the membrane 

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

 rectus internus, obUquus inferior, and rectus inferior. Its 

 size corresponds to the development of the muscles of the 

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

 the Myxinoids. In Zepidosiren the nerves supplying the 

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



^ According to Langerlmns " Untersuchungen iiber Petromyzon planeri" 

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



