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, 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. Its palatine nerve usually extends forwards and anastomoses with the corresponding branch of the seventh, thus forming a connexion (Jacobson's anastomosis) between the two cranial nerves. In some Elasmobranchs and Teleosts fibres derived from the dorsal branch of the ninth nerve innervate a few sense-organs of the lateral sensory canal of the head, and hence that nerve sometimes contains lateralis fibres. The tenth or vagus is a compound nerve. Besides the great lateralis nerve generally associated with it, the vagus includes as many typical branchial nerves as there are branchial clefts behind the hyo-branchial cleft, and in Elasmobranchs and in Chimaera these nerves have independent origins from the medulla oblongata. Each nerve has the typical structure, a ganglionated trunk which forks over a gill-cleft into the usual pre- and post-branchial branches, and palatine branches to the pharyngeal walls. In the Dipnoi the lateralis nerve is connected with the superficial ophthalmic branch of the seventh nerve by a commisural nerve which curves across the outer face of the auditory capsule. A somewhat similar anastomosis is also present in Petromyzon. The vagus also includes a large ramus intestinalis, which in Elasmobranchs, at all events, has a distinct ganglionated root. The nerve forms characteristic plexuses on the oesophagus and stomach, and in Cyclostomes its branches may extend nearly the whole length of the intestine. In Ganoids and Teleosts there is an interesting nerve known as the "lateralis accessorius." It is a compound nerve, and owes its formation to the union of somatic sensory fibres derived in succession from dorsal branches of the v., vii., ix., and x. nerves, and also from the corresponding branches of a variable number of spinal nerves. The finer branches of the nerve are distributed to the skin of one or more of the fins, or even, as in Gadus, to all the fins, especially to the numerous "end-buds" which are present on those organs. In many Fishes a variable number of the anterior spinal nerves (spino-occipital) perforate the occipital region of the skull. They probably represent the ventral roots only of the ordinary spinal nerves of this region.
Sense-Organs
The Cutaneous Sense-Organs.—These organs, the most remarkable and certainly the most characteristic of the sense-organs of Cyclostomes and Fishes, are bud-like groups of epidermic cells in relation with the ends of sensory nerve fibres. Each consists of a central core of sensory cells, provided with terminal cuticular sensory hairs, and surrounded by a zone of supporting and mucus-secreting cells which leave the hairs exposed at the apex of the bud. Two kinds of these organs can be distinguished, which differ in their innervation and in their position in the skin. Of the two, the so-called end-buds are the more primitive. They occupy a superficial position in the epidermis, and their sense-cells are as long as the supporting cells. They are present in Cyclostomes and Elasmobranchs, and especially in Teleosts, where they are irregularly distributed over the surface of the body, on the fins, lips, and barbels, and also in the epithelium of the mouth and pharynx. In the Dipnoi they are limited to the oral cavity, and in the higher Craniates they become taste-buds.[[450]] Their somatic sensory nerves[[451]] are derived from the vii., ix., and x. cranial nerves, and the lateralis accessorius. In the second type, usually called "nerve-eminences," the sensory cells are shorter than the supporting cells, and they are always innervated by the lateralis system. When first developed in the embryo they are quite superficial, like end-buds, but later the epidermis in which they lie sinks inwards so as to line a series of pits, closed sacs, tubes, open grooves, or closed canals. Pit-organs, so abundant on the head and trunk of Teleosts (Fig. 220), are simple epidermic pits with insunken nerve-eminences, disposed in groups or in lines (accessory lateral lines) or irregularly distributed. The "Spalt-papillen" of Elasmobranchs are pit-organs in which the orifice of the pit is reduced to a slit. The more deeply-seated Savi's vesicles on the ventral surface of the Torpedo, and the nerve-sacs of Ganoids, are similar organs converted into closed sacs and pinched off from the rest of the epidermis. Lorenzini's ampullae or mucus canals, which are found in definitely located groups on the lateral and upper surfaces of the head in Elasmobranchs, may perhaps be compared to pit-organs prolonged inwards to form subcutaneous tubes, each of which terminates in a radially-septate, chambered dilatation or ampulla, containing groups of sensory cells.
Besides the more diffusely scattered sense-organs there are others which become disposed in definite lines along the sides of the body and on the head, and, enclosed in grooves or closed canals, constitute the highly characteristic lateral line system of Cyclostomes and Fishes.[[452]] The auditory organ must also be included as a specialised portion of this system. Both organs are innervated by the lateralis system, and both arise from a common rudiment in the embryonic epidermis in the position of the future auditory organ. This rudiment grows backwards along the side of the body in the form of a cord of cells differentiated from the epidermis, and also forwards, where it soon divides into the rudiments of future supra-orbital and infra-orbital canals. Sense-organs are differentiated at intervals along the line of the cord; and in the body, but not on the head, they frequently exhibit a segmental disposition. Each sensory organ then sinks down into a short epidermic groove, which by the subsequent meeting of its lips becomes a canal detached from the epidermis. The short canals then become continuous, leaving, however, an externally opening primary pore between every two consecutive canals, and the result is a continuous canal having sense-organs imbedded in its epidermic lining and connected with the exterior by pores at intervals (Fig. 219).[[453]] The enclosure of the canals in the scales of the lateral line of the trunk or in special drain-pipe ossicles on the head, and the dichotomous subdivision of the primary pores into groups of surface-pores, complete the evolution of the system in its more advanced condition.
Fig. 219.—Vertical longitudinal section through the lateral canal of Amia calva. l.n, Lateralis nerve with its branches, n, n, to the sensory organs, s.o, s.o; p, p, p, external pores; s.c, sensory canal; s, s, scales of the lateral line. (From Wiedersheim, after Allis.)
Typically, the lateral line system consists of certain canals or grooves, usually but not invariably continuous, and defined by their innervation, (i.) a lateral canal extending along the side of the body and the hinder part of the head, and having its sensory organs supplied by the great lateralis nerve (Fig. 220); (ii.) a supra-orbital canal passing forwards over the eye and innervated by the superficial ophthalmic branch of the facial nerve; (iii.) an infra-orbital canal supplied by the buccalis and otic branches of the same nerve; and (iv.) a hyo-mandibular or operculo-mandibular canal, situated on the outer side of the hyoid region, and thence prolonged downward and forward in relation with the lower jaw, and innervated by the external mandibular branches of the facial nerve. The hyo-mandibular canal is sometimes distinct from the other canals, as in Elasmobranchs and some Teleosts (Fig. 220); and in certain North American Siluroids the same may be said of the supra-orbital. But, as a rule, the infra-orbital is continuous behind both with the lateral and the supra-orbital canals, while the hyo-mandibular canal joins the infra-orbital, or, exceptionally, the supra-orbital canal. Transverse commissural canals often connect the lateral and supra-orbital canals of opposite sides across the dorsal surface of the head, and the corresponding infra-orbital and hyo-mandibular canals may also be continuous at the extremity of the snout or at the mandibular symphysis.
Fig. 220.—Sensory canals of the left side of the head of Gadus virens. e, Eye; i.o, infra-orbital canal (dotted); l.c, lateral canal (oblique shading); n, nasal apertures; op, operculum; op.m, operculo-mandibular canal (longitudinal shading); p.o, pit-organs; s.o, supra-orbital canal (cross-hatched); s.o.c, supra-orbital commissure; s.t, supra-temporal branch; t.t, tubuli by which the canals communicate with the exterior. (From Cole.)
Throughout their extent the canals communicate with the exterior by pores, or short canals terminating in pores, or by branched canals ending in groups of pores. In Cyclostomes[[454]] the lateral line system is represented by pit-organs disposed as in Fishes, and innervated by a true lateralis nerve. Some Elasmobranchs have the lateral canal of the trunk represented by an open groove protected by marginal denticles. Chimaera is more primitive still in this respect, for on the head as well as on the body the sensory organs are in open grooves. Amongst Fishes these organs are most primitive in the Dipnoi, where they retain their superficial position in the epidermis. In Teleostomes the lateral canals perforate the scales of the lateral line, and at intervals they open externally by simple or multiple pores which perforate the scales. On the head they are more or less completely enclosed in special ossicles which either remain distinct or fuse with certain of the adjacent dermal or cartilage bones of the skull. The use of the lateral line organs is not certainly known. They occur only in Fishes and Amphibia, and as blind Fishes are able to avoid obstacles with the greatest ease when swimming, it is possible that these organs enable their possessors to appreciate undulatory movements in water in the shape of reflex waves from contiguous surfaces or objects.[[455]] Their great antiquity is shown by their existence in most of the Heterostraci, and in the Antiarchi and Arthrodira, although they have not yet been discovered in the Osteostraci.