(2) Buccal (bucc.VII): lies close to maxillary division of V. and passes to the sensory canals of the lower side of the snout.
(3) External mandibular (md.ex.): lies in close association with the mandibular division of V., supplies the sensory canals of the lower jaw and hyoid region.
Lateralis vagi (l.n.X.) becomes closely associated with the vagus. It supplies the lateral line organs of the trunk.
In the lamprey and in Dipnoans the lateralis vagi loses its superficial position in the adult and comes into close relation with the notochord.
In Actinopterygians and at least some Selachians a lateralis set of fibres is associated with IX., and in the former fishes a conspicuous trunk of lateralis fibres passes to some or all (Gadus) of the fins. This has been called the lateralis accessorius and is apparently connected with V., VII., IX., X. and certain spinal nerves.[53]
Vagus Group (IX., X., XI.).—The glossopharyngeus (IX.) forks over the first branchial cleft (pretrematic and post-trematic branches) and also gives off a palatine branch (pn.IX.). In some cases (various Selachians, Ganoids and Teleosts) it would seem that IX. includes a few fibres of the lateralis group.
Vagus (X.) is shown by its multiple roots arising from the medulla and also by the character of its peripheral distribution to be a compound structure formed by the fusion of a number of originally distinct nerves. It consists of (1) a number of branchial branches (X.¹ X.² &c.), one of which forks over each gill cleft behind the hyobranchial and which may (Selachians) arise by separate roots from the medulla; (2) an intestinal branch (v.n.X.) arising behind the last branchial and innervating the wall of the oesophagus and stomach and it may be even the intestine throughout the greater part of its length (Myxine).
The accessorius (XI.) is not in fishes separated as a distinct nerve from the vagus.
With increased development of the brain its hinder portion, giving rise to the vagus system, has apparently come to encroach on the anterior portion of the spinal cord, with the result that a number of spinal nerves have become reduced to a less or more vestigial condition. The dorsal roots of these nerves disappear entirely in the adult, but the ventral roots persist and are to be seen arising ventrally to the vagus roots. They supply certain muscles of the pectoral fins and of the visceral arches and are known as spino-occipital nerves.[54]
These nerves are divisible into an anterior more ancient set—the occipital nerves—and a posterior set of more recent origin—(occipito-spinal nerves). In Selachians 1-5 pairs of occipital nerves alone are recognizable: in Dipnoans 2-3 pairs of occipital and 2-3 pairs of occipito-spinal: in Ganoids 1-2 pairs occipital and 1-5 pairs occipito-spinal; in Teleosts finally the occipital nerves have entirely disappeared while there are 2 pairs of occipito-spinal. In Cyclostomes no special spino-occipital nerves have been described.