Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/671

653 ORGANS OF SENSE.] ICHTHYOLOGY 653 and three semicircular canals, the vestibule dilating into one or more sacs, which contain the otoliths. A tympanum, tympanic cavity, and external parts are entirely absent in the class of fishes. In the Chondropterygians and Dipnoi, the labyrinth is enclosed in the cartilaginous substance of the skull. In the former the excavation in the car tilage is larger than the membranous labyrinth, but nearly corresponds to it in form ; the part which receives the mem branous vestibulum is called vestibulum cartilagineum, from which a canal issues and penetrates to the surface of the FI skull, where it is closed by the skin in sharks, but opens by a minute foramen Iimer aspect. in rays. The otolithic contents are soft and chalklike. In the Teleosteans the sac which contains the otoliths lies on each side of the / //? base of the cranial cavity , and is often divided by a septum into two compart ments of un equal size, each containing a firm and solid otoiith (fig. 44) ; these bodies possess indente.d mar gins, frequent ly other im pressions and grooves, in which nerves from the N. acousticus are lodged; they vary much in size and form, but in both respects show a remarkableconstancyin the same kind of fishes. Out wards the vestibule is in contact with the osseous side wall of the skull, in wards with the metence- phalon and medulla ob- longata; it contains another firm concretion, and opens by five foramina into the three semicircular canals. The terminations of the acoustic nerve are distri buted over the vestibular concretion and the ampulli- form ends (fig. 45) of the semicircular canals, without being continued into the latter, which are filled with fluid. The semicircular canals (fig. 45) are some times lodged in the cranial bones, sometimes partly free in the cranial cavity. Many Teleostei have fontanelles m the roof of the skull, closed only by skin or very thin bone at the place where the auditory organ approaches the surface, by which means Fio. 45. Communication between auditory organ and air-bladder in the Carp. (After E. H.Weber.) a, bnsisphenoid; b, occi pital ; c, supraocdpital ; d, exoccipital ; e, paroccipital ; /, alisphenoid ; g, neural arch of first vertebra ; h, i, *, second,third, and fourth vertebra? ; h, i , parapophysea of second and third vertebras ; i&quot;, process of the third vertebra for the attachment of the air-bladder ; *, /, m, chain of ossicles ; n. air-bladder ; o, vestibulum ; p, p, am pulla!; g, q, caaales semicirculares ; r, siuus impur. sonorous undulations must be conducted with greater ease to the ear. In many Teleostei a most remarkable relation obtains between the organ of hearing and the air-bladder. In the most simple form this connexion is established in Percoids and the allied families, in which the two anterior horns of the air-bladder are attached to fon tanelles of the occipital region of the skull, the vestibulurn occupy ing the opposite side of the membrane by which the fontauelle is closed. The condition is similar, but more complicated, in many Clupeoids. The anterior narrow end of the air-bladder is produced into a canal at the base of the skull, and divided into two very narrow branches, which again bifurcate and terminate in a globular swelling. An appendage of the vestibulum meets the anterior of these swellings, and comes into close contact with it. In addition, the two vestibules communicate with each other by a transverse canal, crossing the cranial cavity below the brain. The connexion is effected by means of a chain of ossicles in Siluridce, Characinidce, Cyprinidce, and Gymnotidce. A canal issues from the communication between the vestibule and its sac, and meeting that from the other side forms with it a common sinus impar (fig. 45, r), lodged in the substance of the basioccipital ; this communicates on each side by a small orifice with two sub- spherical atria, on the body of the atlas, close to the foramen mag num. Each atrium is supported externally by a small bone (m) ; a a third and larger bone (&) completes the communication with the anterior part of the air-bladder. From the sinus impar a bifid canal penetrates into the alisphenoids, in which it terminates. In Cobitis and several loach-like Siluroids the small air-bladder consists of two globular portions placed side by side, and wholly included within two bullai formed by the modified parapophysea of the second and third vertebrae. The three ossicles on each side are present, but concealed by the fore part of the osseous bulla. Organ of Taste. Some fishes, especially vegetable feeders, or those provided with broad molar-like teeth, masticate their food ; and it may be observed in carps and other Cypriuoid fish that this process of mastication frequently takes some time. But the majority of fishes swallow their food rapidly, and without mastication, and therefore we may conclude that the sense of taste cannot be keen. The tongue is often entirely absent, and, even when it exists in its most distinct state, it consists merely of ligamentous or cellular substance, and is never furnished with muscles capable of producing the movements of exten sion or retraction as in most of the higher vertebrates. A peculiar organ on the roof of the palate of Cyprinoids is per haps adapted for the perception of this sense; in these fishes the palate between and below the upper pharyngeal bones is cushioned with a thick, soft, contractile substance, richly supplied with nerves from the nervus vagus and nervus glosso-pharyngeus. Organs of Touch. The faculty of touch is more de veloped than that of taste, and there are numerous fishes which possess special organs of touch. Most fishes are very sensitive to external touch, although their body may be protected by hard horny scales. They perceive impres sions even on those parts which are covered by osseous scutes, in the same manner as a tortoise perceives the slightest touch of its carapace. The seat of the greatest sensitiveness however, appears to be the snout and the labial folds surrounding the mouth. Many species possess soft and delicate appendages, called barbels, which are almost constantly in action, and clearly used as organs of touch. Among the Triglidce and allied families, there are many species which have one or more rays of the pectoral fin detached from the membrane, and supplied with strong nerves. Such detached rays (also found in the Polynemidas and in Bathypterois) are used partly for locomotion, partly for exploring the ground over which the fish moves. Some fishes appear to be much less sensitive than others, or at least lose their sensitiveness under peculiar circum stances. It is well known that a pike whose mouth has been lacerated and torn by the hook continues to yield to the temptation of a bait immediately afterwards. The Greenland shark, when feeding on the carcase of a whale, allows itself to be repeatedly stabbed in the head without abandoning its prey. A pair of congers are so