NERVOUS SYSTEM OF FISHES. 1P3 



the Eel the nerves form, after decussation, a veiy acute angle in the 

 axis of the body : in the Lump -fish they form an obtuse open angle. 



Since there are no muscles of the eyeball in the Lancelet, the 

 Myxinoids, the Amblyopsis, 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 sujjply the rectus superior and rectus 

 internus, the other the obliquus superior ; the filaments supplying 

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

 In all other fishes the sixth or abducent nerve has its proper origin, 

 as well as the fourth and third. The third, or oculo-motorius, {fig. 53. 

 55. 3.) rises from the base of the mesencephalon, behind the hypoaria, or 

 from the commissura ansulata ; it escapes through the orbito-sphenoid 

 (Carp), or the unossified membrane beneath it (Cod), and is distri- 

 buted 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 uniting with a branch of the fifth 

 nerve, and sometimes, as in the Mackerel, Gar-pike, and Lump-fish, 

 developing a small ciliary ganglion at the point of communication. 



The fourth nerve, or trochlearis, rises from the back of the base 

 of the optic lobes, between these and the cerebellum : it escapes 

 either through the orbito-sphenoid (Carp), or the contiguous mem- 

 brane (Cod), and is constantly and exclusively distributed to the 

 superior oblique eye-muscle. 



The sixth, or abducent, nerve (fig. 53. 6) rises from the prepyra- 

 midal tracts of the medulla oblongata, beneath the fifth, and, in most 

 Osseous Fishes, by two roots, as figured by Mr. Swan (liv. pi. viii. 

 fig. 2), in the Cod. 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 : 

 it escapes by the foramen or anterior notch of the alisphenoid, in 

 advance of the fifth nerve. 



This nerve, the trigeminal (fig. 53. 55. s), enormous in all Fishes, 

 from the Lancelet to the Lepidosiren, rises, often by two or more 

 roots, from the restiform, or from the anterior angle between the 

 olivary and restiform tracts ; in some fishes ( Clupeidce, 52, i. Cyprinidte) 

 from a special ganglion or enlargement of that part of the medulla 

 oblongata : in a few (Conger, Lump-fish) by a smaller origin resolved 

 into several roots. The trigeminus shows well its spinal (myelonal) 

 character in Fishes, but its double root is more deeply buried in the 

 medulla oblongata. In Coitus, Blennius, Cobitis, and Leuciscus, the 



VOL. II. o 



