NERVES OF FISHES. 



301 



muscles of the eyeball cannot be separated from the fifth pair. In 

 all other fishes the sixth or abducent nerve, fig. 185, 6, has its proper 

 origin, as well as the fourth and third. The third, or oculomo- 

 torius, ib. 3, rises from the base of the mesencephalon, behind the 

 hypoaria, ib. n, or from the commissura ansulata ; it escapes through 

 the orbito-sphenoid (Carp), or the unossified membrane beneath it 

 (Cod, fig. 196, 3), and is distributed constantly to the recti superior, 

 inferior, and internus, and to the obliquus inferior ; it also sends 

 filaments into the eyeball : the ciliary stem, or a branch of it, 

 usually unites with a branch of the fifth nerve, and sometimes, 

 as in the Mackerel, Gar-pike, and Lump-fish, developes a small 

 ciliary ganglion at the point of communication. 



The fourth nerve, or trochlearis, fig. 196, 4, rises from the back 

 of the base of the optic lobes, between these and the cerebellum ; 



201 



Brain and origins of the fifth nerves of the Cod. covin. 



it escapes either through the orbito-sphenoid (Carp), or the con- 

 tiguous membrane (Cod), and is constantly and exclusively dis- 

 tributed to the superior oblique eye-muscle, ib. g. 



The sixth, or abducent, nerve, figs. 185, 196, 6, rises from the 

 prepyramidal tracts of the medulla oblongata, fig. 185, a, beneath 

 the fifth, and, in most Osseous Fishes, by two roots, as in the Cod, 

 ib. 6. It usually closely adheres to the ganglionic origin of the 

 fifth. In the Carp and Lump-fish it receives a filament from the 

 sympathetic, before its final distribution to the rectus externus, 



