NERVES OF FISHES. 305 



the head, and forms the sides of the wide oval opening ; it also 

 supplies the oral tentacnla. In the Myxinoids the same nerve 

 supplies both the muscles and the integuments of the head, the 

 tentacula, the nasal tube, the mucous membrane of the mouth 

 and tongue, the hyoid and palatal teeth, and the pharynx. The 

 trigeminus supplies the same parts in the Lamprey, but in a more 

 compact manner, i. e. by fewer primary branches : that which 

 sends filaments to the rectus externus and rectus inferior of the 

 eyeball is continued forward beneath the skin and resolves itself 

 into a rich plexus, which supplies the thick cirrate border of the 

 suctorial lip : the nerves to the muscular parts of the jaws and 

 tongue arise distinct from the fifth, and their trunk may be 

 regarded as a ' facial ' nerve ; one of the filaments of this joins a 

 branch of the vagus to form a short ' nervus lateralis.' 



Thus in reference to the motor filaments of the trio-eminus or 

 great spinal nerve of the head, those that form the portio dura or 

 facial nerve in higher Vertebrata are not distinct from the rest of 

 the trigeminus at its apparent origin, except in the Lamprey ; in 

 which, on the other hand, the rnotory filaments of the rectus 

 externus, forming the sixth nerve of higher Fishes and Vertebrates, 

 retain an associated origin with the trigeminal. The ' facial ' part 

 of the operculo-lateral division of the fifth, in the Perch, 1 is that 

 which supplies the mandibular, opercular, and branchiostegal 

 muscles. In the extended medulla oblongata of the Sander 

 (Lucioperca) the facial nerve has a distinct origin between the 

 trigeminal and acoustic. 



The acoustic nerve appears to be a primary branch of the fifth, 

 in the Skate, fig. 201, 7: its distribution on the labyrinth is beau- 

 tifully shown by Swan in Liv. pi. x. fig. 2. It communicates on 

 the great otolithic sac with a motor branch from the vagus, which, 

 after giving filaments to the posterior semicircular canal, passes 

 out to supply the first and the adjacent surface of the second gill, 

 and the faucial membrane. Swan calls this branch the ' s;losso- 

 pharyngeal,' and says, ' this nerve, on being touched near its 

 origin in a recently-dead animal, immediately produces a contrac- 

 tion of the muscular appendages of the gills ' (ib. p. 41). In the 

 Cod the acoustic nerve, fig. 185, 7, which here as in all fishes 

 above the Dermopteri is of large size, rises close behind, but 

 distinct from, the fifth pair, ib. 5, between it and the vagus, ib. 8 : 

 the acoustic nerve receives a filament from the vagus, extends in 

 a crescentic form, fig. 196, s, upon the labyrinth, expands upon 



1 xxm. torn. i. p. 325, pi. vi. fig. v. fx. 

 VOL. I. X 



