344 



ANATOMY OF VERTEBRATES. 



228 



the Lepidosiren, in which, as well as in the Plagiostomes, the 

 whole labyrinth is buried in the thick basi-lateral walls of the 

 cranium. In Plagiostomes the capsule conforms more closely in 



size and configuration to the membranous 



o 



labyrinth ; its passages and compartments 

 are lined by a delicate perichondrium, from 

 which filaments are detached to support 

 the semicircular canal. The vestibule is 

 divided in the Skate and Tope into three 

 compartments, the ( alveus communis,' 

 fig. 228, a ; the sac, ib. b, and the cysticule, 

 ib. c ; and it has also a small crecal append- 

 age, called the ( utricule,' ib. d : the otolitic 

 contents are like soft chalk, and are dis- 

 posed in two masses ; one very large, occu- 



Organ of hearing, Skate. LXVIII. . , -i ,, i , i , i , i 



pymg the sac and the cysticule, the other 



small, and lodged in the utricule. A canal extends in Sharks 

 from the vestibular capsule to a foramen at the upper part of 

 the occiput, which is closed by the skin. In the Rays, besides 

 this ( fcnestra capsuloe,' ib. v, a membranous canal, ib. o, p, is pro- 

 duced from the vestibule itself, and, as Hunter well describes, 

 s from the union of the two perpendicular canals, fig. 228, p ; 

 which is the case with all the Ray kind, the external orifice 

 being small, and placed on the upper flat surface of the head.' 

 So minute and approximated are these ( outer ears,' 1 that Scarpa 

 may be pardoned for overlooking them, though scarcely for 

 the warmth with which he repudiates their existence. 2 The 

 "' meatus vestibuli' is provided at its bent extremity, fig. 228, o, 

 with a special muscle, ib. w. 



A true tympanic cavity and membrane, together with a cochlea, 

 are absent in all Fishes. But in many osseous species a com- 

 munication is established, either by tubular prolongations, or by 

 chains of ossicles, between the acoustic labyrinth and the air- 

 bladder. Weber 3 discovered the latter interesting structure in 

 the Carp, Loach, and Sheat-fish. A canal is sent from the sac of 

 each vestibule, fig. 229, &, to a common ( sinus impar,' ib. //, in 

 the substance of the basi-occipital : this communicates on each 



1 xcu. p. 296. Hunter's original memoir 'On the Organ of Hearing in Fishes ' was 

 printed in the volume of the Philosophical Transactions for 1782, not, as Breschet 

 states, in the year 1786. (LVIII. p. 53.) 



2 'Hunterum autcm atque Monroum vehementer super hue re sibi hallucinates 

 fuisse.' (LX. pp. 1,2.) 



LXXHI. 



