ORGANS OF HEARING. 225 



others. The external coverings, namely, the skin and muscles, are 

 continued over the organs of hearing as well as over the whole skull, 

 so that the sonorous vibrations propagated in the \vater have no spe- 

 cial or direct access to the auditory apparatus. Still, however, 'we 

 may regard as external meatuses or conductors of sound certain 

 membranous intervals in the skull resembling fontanelles, and situa- 

 ted upon the crown of the head, e. g., Silurus, Cobitis, Clupea, &c. 

 It is very rare to find, as in Lepidoleprus, an infundibuliform canal, 

 comparable to the external auditory meatus, and invariably situated 

 above the external branchial fissure, where it opens upon the sides 

 of the occipital bone and is there closed by integument. 



The Membranous Labyrinth consists, 1st. Of the simple vestibule ; 

 this is a transparent sac, provided with nervous expansions, and of 

 very varied form, which is loosely attached to the skull in the vicin- 

 ity of the cerebellum, and receives the ampullae of the semicircular 

 canals. 2d. Of the auditory sac (saccus vestibuli), which, lying 

 close upon the vestibule, is externally more or less detached, and 

 internally always separated from the vestibule by a partition. Oc- 

 casionally, however, it is united to the vestibule by a membranous 

 commissure, lies further posteriorly, and is usually divided into a 

 pair of chambers, which, like the vestibule, contain mostly auditory 

 ossicles or calcareous parts, surrounded by the fluid of the labyrinth. 

 3d. Of the arciform or semicircular canals ; of these an anterior and 

 posterior canal are constantly found, both standing perpendicularly, 

 and two of their crura usually opening together ; lastly, an external 

 horizontal canal. Toward the vestibule the canals are dilated into 

 ampullae, and their arched portions are not unfrequently received 

 partly into the adjacent bones, to which they are fastened by cellu- 

 lar ligaments. The structure is similar in the true Cartilaginous 

 Fishes (Plagiostomi) ; the labyrinth, however, is in them completely 

 separated from the cranial cavity, and completely imbedded in a 

 mass of cartilage, which is much harder where it is in contact with 

 the membranous labyrinth. The sac and vestibule do not appear to 

 be separate in these Fishes, though the labyrinth is elongated to form 

 a flask-shaped duct leading upward and outward upon the middle of 

 the occipital bone. In this situation are also placed either two or 

 four openings, closed by membrane, and approximated to each other, 

 which correspond to the two fenestras, and have the external integ- 

 ument continued over them. 



In the Rays, and some species of Sharks, we find four openings 

 upon the skull, two for each ear ; the posterior conduct only into the 



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