PISCES FISHES. 527 



sides with the same kind of oily or mucilaginous fluid, which filte 

 up the wide interspace that exists between the brain and the dura 

 mater lining the inner surface of the skull. 



Fig. 233. 



As in all other Vertebrata, there are three semicircular canals, 

 disposed nearly as in the human ear, and each dilated in like man- 

 ner into an ampulla which receives the filaments of the acoustic 

 nerve. Two of the semicircular canals coalesce before they open 

 into the vestibule, so that there are only five orifices whereby the 

 three semicircular canals communicate with the vestibular cavity. 



The membranous vestibule (supported in the figure by two pins), 

 is of variable shape, and its walls are very delicate. Its cavity, as 

 well as the interior of the semicircular canals, is filled with a trans- 

 parent glairy fluid ; and it moreover encloses certain hard bodies 

 (otolitkt*), generally three in number, suspended by delicate fila- 

 ments in its interior. 



The otolithes of osseous fishes are of a stony hardness, resembling 

 shells, and their structure is nothing at all like that of bone. 

 Their shape varies in different species, but, nevertheless, is so con- 

 stantly the same in fishes of the same kind, that the forms of these 

 pieces might be employed as an important zoological character. 



In the cartilaginous fishes the otolithes are quite soft, resembling 

 starch : in both classes they are composed principally of chalk, and 

 effervesce strongly when dissolved in acids. 



The auditory nerve gives a filament to each of the semicircular 

 canals, which penetrates into the ampulla of the canal to which it is 

 destined, and there spreads out ; but the larger portion of the nerve 



