78 SPECIAL SENSES. 



mechanism described in amphibious reptiles. In osseous 

 fishes, the membranous labyrinth lies for the most part full 

 within the cranial cavity, and adjacent to the brain ; or it 

 is only imperfectly and partially enclosed in bones, as the 

 skin and muscles are continued over the skull. The sonorous 

 vibrations propagated by the water are communicated through 

 the walls of the cranium, as no openings exist for the special 

 reception of waves of sound. The labyrinth consists 1st, of 

 a simple vestibule, or transparent sac, which receives the am- 

 pullse of the arched canals, and is provided with nervous 

 expansions : 2nd, the auditory sac is separated from the 

 vestibule by a partition, and divided into two chambers, which, 

 with the vestibule, contain the ossicles and calcareous parts, 

 surrounded by the fluid of the labyrinth: 3rd, the semicircular 

 canals, which are more or less developed in different genera, 

 and open by ampullae into the vestibule. In the rays and 

 sharks, the labyrinth is separated from the cranial cavity, and 

 imbedded in a mass of cartilage, which is more solidified around 

 the membranous labyrinth. We find two openings, closed by 

 membranes, on each side of the skull, which communicate with 

 the internal ear, and represent the round and oval foramina of 

 the labyrinth. Between each of these openings and the integu- 

 ment a membranous sac is placed, which is filled with a calca- 

 reous mass, and extends into the membranous vestibule. A pair 

 of otoliths, composed of the carbonate of lime, are appended to 

 the walls of the sacs. Osseous fishes are furnished with three 

 of these concretions, almost as hard as porcelain : one is lodged 

 in the vestibule, the others occupy the chambers of the sac. 

 In the cyclostome fishes, as the petromyzon, the ear is simple, 

 consisting of a cartilaginous part, and a pair of hard yellow 

 oval capsules, connected with the skull, and enclosing, like a 

 bony labyrinth, a membrane lining the same, and having in- 

 terposed between them a fibro-membranous layer. The mem- 

 brane of the labyrinth consists of a small sac, divided into two 

 cells, two wide depressed semicircular canals, which enter the 

 vestibule by one common ampulla, a rudimentary auditory sac, 

 which appears as an appendage to the vestibule. The auditory 

 nerve sends two branches to supply the labyrinth. In the 

 myxine the ear is still more simple : the auditory capsule is filled 

 with a membranous labyrinth, within which a single arched 

 canal is blended with the vestibule. Otoliths and calcareous 



