NERVOUS SYSTEJI OF FISHES. 195 



muco-ganglionic corpuscles.* In tlie Sturgeon the snout and its 

 tentacula are supplied by branches of the infra-orbital, not from the 

 supra-orbital division of the fifth: the opercular or facial branch sup- 

 plies, in addition to the gill-cover, the integuments and lips of the 

 protractile mouth, and the pseudo-branchia : it communicates with 

 the glosso-pharjngeal. 



In the Lancelet the fifth nerve {Jig. 46. oli) distributes many fila- 

 ments to the expanded sensitive integument whicli represents the 

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

 the oral tentacula. 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 as it were, i. e. 

 by fewer primary branches : it also sends filaments to the rectus ex- 

 ternus and r. inferior of the eyeball : the nerves to the muscular parts 

 of the jaws and tongue arise in the Lamprey distinct from the fifth, 

 and their trunk may be regarded as a facial nerve ; one of the fila- 

 ments of this joins a branch of the vagus to form a short 'nervus 

 lateralis.' 



Thus in reference to the motor filaments of the trigeminus 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 motory filaments of the rectus externus, 

 forming the sixth nerve of higher Fishes and Vertebrata, retain an 

 associated origin with tlie trigeminal. The opercular or facial di- 

 vision of the fifth forms the hindmost portion of its apparent origin 

 in the Perch "j", it supplies the mandibular, opercular, and branchios- 

 tegal muscles ; and sends off the branch to form, with a branch of the 

 vagus, the dorsal division of the ' nervus lateralis.' In the elongated 

 'medulla oblongata' of the Sander {Lucioperca) the facial nerve has 

 a distinct origin between the trigeminal and acoustic. 



The acoustic nerve rises so close to the fifth, in the Skate, as to 

 appear to be a primary branch of that great nerve ; its distribution 

 on the labyrinth is beautifully shown by Mr. Swan in iA\. pi. x. fig. 2. 

 It communicates on tlie great otolithic sac with a motor branch i'rom 

 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. Mr. Swan calls this branch 



* Lxxvi. f XXIII. torn. i. p. 325. pi. vi. fig. v. fi. 



o 2 



