The Auditory Organs.—In its more typical condition each auditory organ consists of a membranous sac or vestibule, partially constricted into an upper portion or utriculus and a lower or sacculus (Fig. 221, A). Three semicircular canals are connected with the utriculus, of which two are vertical and at right angles to one another, and the third is horizontal. One end of each canal is dilated into an ampulla. A slender tube, the ductus endolymphaticus, leaves the sacculus, and ends in a sac-like swelling, the sinus endolymphaticus, which apparently represents a portion of the embryonic epidermic involution from which the auditory organ is formed. A smaller sac-like outgrowth from the sacculus, the lagena, corresponds to the cochlea of the higher Vertebrates. The epidermic lining of this system of cavities is differentiated into patches or ridges of sense-cells (maculae or cristae), separated by supporting cells and innervated by the terminal branches of the auditory nerve. There is a crista acustica in each ampulla; and maculae acusticae are present in the utriculus, sacculus, and lagena. A fluid, the endolymph, fills all the cavities, and a similar fluid or perilymph occupies the spaces in the periotic capsule in which the various chambers are lodged. Among the more notable deviations from this type of auditory organ the Cyclostome Myxine, apparently, has but a single semicircular canal with an ampulla at each end, and the vestibule is a simple sac (Fig. 221, B). Petromyzon has two canals, but lacks the horizontal canal. In Elasmobranchs, including Chimaera (C), the ductus endolymphaticus retains its primitive connexion with the exterior by means of a pore on the dorsal surface of the head. In the Dipnoi (e.g. Protopterus) the paired endolymphatic sinuses divide into a number of caecal branches containing otoliths, which meet and interlace over the fourth ventricle (Fig. 217).[[456]] Otoliths, either in the form of fine, mucus-connected, calcareous particles, as in Elasmobranchs, or as massive solid concretions in Teleosts, are present in relation with the sensory areas of the utriculus, sacculus, and lagena.
Fig. 221.—Auditory organs of Fishes. A, of a typical Fish; B, of Myxine; C, of Chimaera; and D, of Perca. a.c, Anterior canal; am', am", am'", ampullae; am.n, nerves to ampullae; c, semicircular canal (in Myxine); d.e, ductus endo-lymphaticus; h.c. horizontal canal; l, lagena; mc, macula acustica; m.s, macula acustica of the sacculus; n, nerves to ampullae; o, external aperture of the ductus endo-lymphaticus; p.c, posterior canal; s, sacculus; s.e, sinus endo-lymphaticus; sk, superficial skin; s.s, sinus superior; u, utriculus; viii, auditory nerve. (From Wiedersheim, after Retzius.)
In a few marine and in a large number of freshwater Teleosts the auditory organ enters into a more or less intimate connexion with the air-bladder by one of three different methods.
Fig. 222.—Cavity of the air-bladder of a Siluroid (Macrones nemurus) exposed by the removal of its ventral wall. a.c, Anterior chamber; b.o, basioccipital; b.w, body wall, here reduced to the external skin; cl, clavicle; l.c, lateral chamber; l.s, longitudinal septum; pt, post-temporal; tr.a, anterior portion of the tripus; tr.c, crescentic portion of the tripus; t.s, transverse septum; t.s', shorter transverse septum. (From Bridge and Haddon.)
The first and simplest is by the apposition of the extremities of a pair of caecal tubular prolongations from the air-bladder to the outer surfaces of the fibrous membranes which close a pair of vacuities in the outer bony walls of the periotic capsules, the inner surfaces being bathed by the perilymph surrounding the auditory organs. This method is characteristic of certain Serranidae, Berycidae, Sparidae, Gadidae, and Notopteridae,[[457]] and probably in the Hyodontidae. In the second method, of which several Clupeidae (e.g. Herring, Pilchard, etc.) furnish examples, the periotic vacuities are open instead of closed, and the sac-like ends of the tubular extensions from the air-bladder are in actual contact with protruding outgrowths from the utriculus.[[458]] The third method, by far the most elaborate, is by the intervention of a series of movably connected "Weberian" ossicles, of which the most posterior on each side (the tripus) is inserted into the dorsal wall of the air-bladder (Fig. 223), while the anterior one (scaphium) forms the outer wall of a median backward prolongation (sinus impar) of the perilymph-containing spaces surrounding the two auditory organs. This in turn encloses a similar median prolongation (sinus endolymphaticus) from the two sub-cerebrally united endolymphatic ducts (Fig. 223).[[459]] This complex mechanism is present in the Cyprinidae, Siluridae, Characinidae, and Gymnotidae; and hence the term "Ostariophysi"[[460]] as a collective name for these families.[[461]] The physiological raison d'être of the connexion between the air-bladder and the auditory organ cannot yet be regarded as satisfactorily determined. It is possible, as Weber thought, that it may be an auxiliary to the function of hearing by transmitting to the ear sound-waves impinging on the surface of the body and affecting the gases in the air-bladder.[[462]] On the other hand, it may be urged with perhaps greater probability that the connexion exists for the purpose of conveying to the ear stimuli due to the varying degrees of distension of the air-bladder, such as, it may be presumed, are naturally brought about by the variations of hydrostatic pressure which a Fish encounters in the course of its ascent or descent in the water.[[463]] Whether regarded as an accessory to hearing, or as a means of regulating the distension of the air-bladder, the physiological value of the connexion must be considerable, and on this point it is at least significant that the Weberian mechanism is characteristic of the dominant families of freshwater Teleosts at the present day.[[464]]
Fig. 223.—Diagram to show the Weberian ossicles and their relations to the ear and the air-bladder. at, Atrium, an extension of the sinus impar; a.v.c, anterior vertical canal; b.w, bony wall of the periotic capsule; d.e, the medianly-united endolymphatic ducts; h.c, horizontal canal; in, intercalarium, a third ossicle imbedded in the ligament (i.lg) connecting the scaphium with the tripus; n, bony nodules on the sides of the complex vertebral centrum; p.v.c, posterior vertical canal; s, sacculus; sc, scaphium; s.e, sinus endolymphaticus; s.i, sinus impar; tr.a, tr.c, the anterior and crescentic parts of the tripus; ut, utriculus. The radial lines represent the fibres of the dorsal wall of the air-bladder. (From Bridge and Haddon.)
The Olfactory Organs.—These organs are essentially a pair of pit-like inpushings of the skin of the ventral side of the head in front of the mouth, with their lining epidermis differentiated into sensory cells separated by supporting cells, and connected with the olfactory lobes of the brain by olfactory nerves.