Lateral Line Organs.[52]—Epidermal sense buds are scattered about in the ectoderm of fishes. A special arrangement of these in lines along the sides of the body and on the head region form the highly characteristic sense organs of the lateral line system. In Lepidosiren these organs retain their superficial position; in other fishes they become sunk beneath the surface into a groove, which may remain open (some Selachians), but as a rule becomes closed into a tubular channel with openings at intervals. It has been suggested that the function of this system of sense organs is connected with the perception of vibratory disturbances of comparatively large wave length in the surrounding medium.
Peripheral Nerves.—In the Cyclostomes the dorsal afferent and ventral efferent nerves are still, as in Amphioxus, independent, but in the gnathostomatous fishes they are, as in the higher vertebrates, combined together into typical spinal nerves.
As regards the cranial nerves the chief peculiarities of fishes relate to (1) the persistence of the branchial clefts and (2) the presence of an elaborate system of cutaneous sense organs supplied by a group of nerves (lateralis) connected with a centre in the brain which develops in continuity with that which receives the auditory nerve. These points may be exemplified by the arrangements in Selachians (see fig. 31). I., II., III., IV. and VI. call for no special remark.
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| From Bridge, Cambridge Natural History, vol. vii. “Fishes” (by permission ofMacmillan & Co., Ltd.). After Wiedersheim, Grundriss der vergleichenden Anatomie(by permission of Gustav Fischer). | |
| Fig. 31.—Diagram of Cranial nerves of a Fish. Cranial nerves andbranchial clefts are numbered with Roman figures. Trigeminusblack; Facialis dotted; Lateralis oblique shading; Glossopharyngealcross-hatched; Vagus white. | |
bucc, Buccal. c, Commissure between pre- and postauditory parts of lateralis system. d.r, Dorsal roots of spinal nerves. g.g, Gasserian ganglion. gn.g, (Geniculate) ganglion of VII. hy, Hyomandibular. l.n.X, Lateralis vagi. m, Motor branches of hy. md, Mandibular. md.ex, External mandibular. mk.c, Meckel’s cartilage. | mx, Maxillary. oc, Occipitospinal. ol.o, Olfactory organ. op.p, Ophthalmicus profundus. op.s, Ophthalmicus superficialis. pn, Palatine. pq., Palatopterygo-quadrate cartilage. s, Spiracle. st, Supra-temporal branch of lateralis system. t.a, Lateralis centre in brain. v.n, Visceral nerve. v.r, Ventral roots. |
Trigeminus (V.).—The ophthalmicus profundus branch (op.p.)—which probably is morphologically a distinct cranial nerve—passes forwards along the roof of the orbit to the skin of the snout. As it passes through the orbit it gives off the long ciliary nerves to the eyeball, and is connected with the small ciliary ganglion (also connected with III.) which in turn gives off the short ciliary nerves to the eyeball. The ophthalmicus superficialis (cut short in the figure) branch passes from the root ganglion of V. (Gasserian ganglion), and passes also over the orbit to the skin of the snout. It lies close to, or completely fused with, the corresponding branch of the lateralis system.
The main trunk of V. branches over the edge of the mouth into the maxillary (mx.) and mandibular (md.) divisions, the former, like the two branches already mentioned, purely sensory, the latter mixed—supplying the muscles of mastication as well as the teeth of the lower jaw and the lining of the buccal floor.
The main trunk of the Facialis (VII.) bifurcates over the spiracle into a pre-spiracular portion—the main portion of which passes to the mucous membrane of the palate as the palatine (pnVII.)—and a postspiracular portion, the hyomandibular (hy.) trunk which supplies the muscles of the hyoid arch and also sends a few sensory fibres to the lining of the spiracle, the floor of mouth and pharynx and the skin of the lower jaw. Combined with the main trunk of the facial are branches belonging to the lateralis system.
Lateralis Group of Nerves.—The lateralis group of nerves are charged with the innervation of the system of cutaneous sense organs and are all connected with the same central region in the medulla. A special sensory area of the ectoderm becomes involuted below the surface to form the otocyst, and the nerve fibres belonging to this form the auditory nerve (VIII.). Other portions of the lateralis group become mixed up with various other cranial nerves as follows:
(a) Facial portion.
(1) Ophthalmicus superficialis (op.s.VII.): passes to lining of nose or to the lateral line organs of the dorsal part of snout.