AUDITOKY APPAKATUS OF FISHES. 



6(51 



space that exists between the brain and the dura mater lining the inner 

 surface of the skull. 



Fig. 323. 



Auditory apparatus of the Skate. 



(1822.) As in all other Vertebrata, there are three semicircular 

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

 manner 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 semi- 

 circular canals communicate with the vestibular cavity. 



(1823.) 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 (oto- 

 liths), generally three in number, suspended by delicate filaments in its 

 interior. 



(1824.) The otoliths of osseous fishes are of a stony hardness, resem- 

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



(1825.) Their shape varies in different species, but, nevertheless, is 

 so constantly the same in fishes of the same kind, that the forms of these 

 pieces might be employed as an important zoological character. 



(1826.) In the cartilaginous fishes the otoliths are quite soft, resem- 

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

 and effervesce strongly when dissolved in acids. 



(1827.) The auditory nerve gives a filament to each of the semicir- 

 cular 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 

 is distributed over the vestibular sacculus, where it forms a beautiful 

 network. 



(1828.) There is no cochlea, although some writers imagine that they 

 can distinguish a rudiment of this part of the ear in a slight projection 

 from the walls of the vestibule. 



