ORGAN OF IIEAraNG IN FISHES. 343 



eacli of wlucli dilates, at. its origin, into an ' ampulla,' wliicli lias 

 some processes from its inner surface. The two canals again 

 communicate with the vestibule, where they cross each other : the 

 two divisions of the acoustic nerve first surround the ampulhe 

 before they spread over the rest of the labyrinth. The acoustic 

 communicates with the cranial cavity by two openings : the ini'crior 

 and larger is oval and closed by membrane : the superior gi\'e8 

 passage to the acoustic nerve. 



In all other Fishes the memln'anous labyrinth, fig. 229, «, o, 

 consists of a vestibule, ib. «, and three semicircular canals, o ; the 

 vestibule dilating into one or more ' sacculi,' separated by a con- 

 striction, or by a narrow canal, from the ' alveus communis,' and 

 containing, besides the iiuid called ' endolymph,' two or more 

 masses of carbonate of lime, called ' otolites.' ' These are com])act 

 and crystalline in Osseous Fishes. The largest, fig. 81, ig", is an 

 oval or round flattened body, striated and indented at the margins ; 

 convex, and sometimes grooved {Ejildypus), on one side, more or 

 less excavated on the other. The smaller otolite is less regular in 

 its shape: there are often two of these. Each semicircular canal 

 rises by an ampulliform end, fig. 229, e,f,g, ivoxn the 'alveus 

 communis,' a, and communicates, by the opposite end, either with 

 another canal, or with the vestibule separately, without previous 

 dilatation: two of the canals are subvertical in their course, and 

 are anterior, e, and j^ostcrior, g, in relative position : the third 

 canal, /^, is external and horizontal. A septum is continued across 

 the amjjulla from the line where the division of the acoustic nerve 

 enters : a large proportion of the nerve expands upon the sac of 

 the otolites. In some fishes this communicates with the vestilralc 

 by a narrow canal. All the j^arts of the laljyrinth are of large 

 size ; yet the compartments of the otocrane which the semicircular 

 canals, fig. 229, e,f,g, traverse, 'are much too wide for them, 

 and they are supported in these passages by a very fine cellular 

 membrane.'^ In the Pike {Esox lucius) a pyriform membranous 

 sac, lodged in the commencement of the spinal canal, ojtens into 

 the vestibule near the entrance of the posterior semicircular canal. 



The Plectognaths, Lophobranchs, Holocephali, and Sturgeons 

 resemble the bony fishes in the form and position of the labyrinth ; 

 but the otolites are represented by cretaceous particles ; and in 

 the Chimtera the communication between the cranium and otocrane 

 begins to contract. The otolites are a hard chalky substance in 



' Figures of these bodies will be found in xx. vol. iii, pi. 35; in LXvni., lxxi., and 

 in Lxxii., with microscopic figures of the crystals. 

 " Hunter, vii. iii. p. 101. 



