{381}The first six cranial nerves resemble those of the higher Craniates in their mode of origin from the brain, in the physiological nature of their component fibres, and in their peripheral distribution, and therefore they need not be specially referred to here. The principal branches of the fifth or _trigeminal_ nerve are shown in Fig. 218. Comparing this nerve with a typical branchial nerve it would seem that the _profundus_ and _superficialis ophthalmic nerves_ are dorsal nerves; the _maxillaris_ and _mandibularis_, pre- and post-branchial branches, respectively, in relation with the modified gill-cleft which forms the mouth, while the branch to the oral surface represents a _palatine_ nerve. The most important of the distinctive features in the cranial nerves of Fishes are to be found in the relations of nerves vii., ix., and x. to branchial clefts, and in the lateralis system of nerve components and its association with the lateral line sensory organs. The seventh or _facial_ nerve is an exceptionally interesting nerve. Besides the usual components of a typical branchial nerve certain of its so-called branches are wholly or largely derived from the lateralis system. For this reason the nerve may be said to consist of two portions, the _facial proper_, or those fibres which constitute the facial nerve in air-breathing Craniates, and the _lateralis branches_ which solely innervate lateral line sense-organs, and are therefore peculiar to aquatic forms. The facial proper has a ganglion (_the facial or geniculate ganglion_) on its root, and on entering the orbit after traversing the cranial wall it gives off a _palatine_ nerve. Just over the spiracle a _pre-branchial_ nerve, the representative of the _chorda tympani_ of Mammals, leaves the main trunk, and passes ventrally in relation with the anterior wall of the spiracle to its ultimate distribution in the walls of the mouth cavity. The main trunk, now called the _ramus hyomandibularis_, then pursues a ventral course behind the spiracle as a post-branchial nerve, and certain of its mainly motor branches which pass downwards in connexion with the hyoid arch supply the muscles of that arch, and, if an operculum is present, the opercular muscles as well. The lateralis portion of the facial includes the following principal branches, {382}each of which may have a ganglion on its root: (1) an _ophthalmicus superficialis_; (2) a _buccalis nerve_ with its _ramus oticus_; and (3) _external mandibular_ nerves which course in the ramus hyomandibularis. The addition of the great _lateralis_ nerve, which is usually described as the lateral branch of the tenth nerve, and of the eighth or _auditory_ nerve which supplies the auditory organ, completes the enumeration of the main factors of the lateralis system. The ninth or _glosso-pharyngeal_ nerve, perhaps the most typical of all the branchial nerves, has pre- and post-branchial branches which enclose the hyo-branchial cleft.