CHAP. XVIII.] THE ORGAN OF HEARING IN THE ANIMAL SERIES. 65 



In fishes the organ of hearing acquires a considerable increase in the complex- 

 ity of its organization. It consists of a vestibular sac, with the accession of, in 

 general, three semicircular canals. In the myxine, however, a fish of very low or- 

 ganization, there is only one of these canals. In the lamprey there are only two. 

 The vestibular sac consists of a large sac (utriculus of JBreschet), into which the 

 semicircular canals open, and with the walls of which they are continuous, and of 

 a small offset from this larger one (the sacculus of Breschet). This apparatus is 

 composed of a thin, transparent, elastic membrane. It is filled with fluid, and 

 contains in each sac, either porcelainous bodies (otolithes), of beautiful structure 

 and great diversity, as in the osseous fishes, or masses of pulverulent deposit, like 

 powdered chalk (otokonia), as in the cartilaginous fishes. These, whether hard or 

 soft, consist of carbonate of lime, and therefore may be quickly decomposed by a 

 mineral acid. The whole of this auditory apparatus is deposited in an excavation 

 of the cranial wall, which communicates with the cavity of the cranium itself, 

 excepting in the rays and sharks, in which it is enclosed by the cranial cartilages. 

 It is suspended in fluid (part, probably, of the cerebro-spinal fluid, which con- 

 stitutes the analogue of the perilymph in the higher animals. In some fishes, 

 according to Breschet, an additional offset from the larger sac exists, to which he 

 gives the name cysticule. All these parts are analogous to the membranous laby- 

 rinth of the higher animals, there being nothing to represent the tympanum or 

 the cochlea. In many of the osseous fishes the auditory apparatus has no com- 

 munication whatever with the exterior. In rays and sharks, however, a prolonga- 

 tion of the labyrinth extends through an opening in the occipital portion of 

 the skull to the surface just beneath the skin. In many fishes, according to 

 Weber, there is an intimate connection between the auditory apparatus and 

 swimming bladder, although their cavities have no communication with each 

 other. 



In Amphibia, the auditory apparatus is closed off from the cranial cavity ? 

 and is contained in the cranial bones. It consists of a vestibule with three 

 semicircular canals. In some, there is placed external to this labyrinth a tym- 

 panic cavity, closed on the exterior by a membrane, which is intimately united 

 with, or a portion of, the integument, or a thin layer of cartilage. An osseous 

 pillar (the columella) or a chain of two or three ossicles, extends from the wall of 

 the vestibule to this tympanic membrane, analogous to the tympanic bones in 

 the human subject. In the Reptiles, there is a short canal connected with the 

 vestibule, analogous to the cochlea. The existence of this canal establishes that 

 of a second external opening belonging to the labyrinth, orfenestra cocklece, in 

 addition to the fenestra vestibuli. Some of the Reptiles, as the serpents, are de- 

 void of a distinct tympanic cavity ; but the existence of a columella beneath 

 the skin indicates a rudimentary state of it. In others, as the tortoises, croco- 

 diles, and lizards, such a cavity exists, with its usual canal of communication 

 with the fauces, the Eustachian tube, and with a columella. The fluid of the 

 labyrinth contains crystalline particles in place of otolithes. 



In Birds, the organ of hearing has the same parts as in the higher reptiles. 

 Its labyrinth has the cochlea and semicircular canals, and the two fenestrae, 

 and there is a tympanic cavity with a columella. The cochlea is a very slightly 

 bent canal, divided by a membranous septum into two passages, scala vestibuli 

 and scala tympani. 



In Mammalia, the general characters and structure of the organ of hearing 

 closely resemble those of man. 



VOL. II. F 



