HEARING. 30.) 



leaving an aperture by which tlie two portions of 

 the tube communicate togetlier. In Fig. 395, a 

 bristle (b, b) is passed through this aperture. The 

 central pillar, round which these tubes take two 

 and a half circular turns, is termed the modiolus. 

 Its apex is seen at m. One of these passages is 

 distinguished by the name of the vestibular tube* 

 in consequence of its arising from the cavity of the 

 vestibule ; and the other by that of the tympanic 

 tube,^ because it begins from the inner side of 

 the membrane which closes the fenestra rotunda, 

 and forms the only separation between the interior 

 of that tube, and the cavity of the tympanum. 

 The trunk of the auditory nerve occupies a hollow 

 space immediately behind the ventricle ; and its 

 branches pass through minute holes in the bony 

 plate which forms the wall of that cavity ; being 

 finally expanded on the different parts of the mem- 

 branous labyrinth. I 



Great uncertainty prevails with regard to the 

 real functions performed by the several parts of 

 this very complex apparatus. It is most probable, 

 however, that the sonorous vibrations of the air 

 w hich reach the external ear, after being collected 

 and reinforced by the grooves in that organ, are 



* Scala vestibuli. f Scala tympani. 



I In Fig. 396, the anterior trunk of the auditory nerve is seen 

 (at g) distributing branches to the ampullaj (a, a), the utricle (u), 

 and the calcareous body it contains ; while the posterior trunk (n) 

 divides into a branch, which supplies the sacculus (s) and its cal- 

 careous body (o), and a second branch (k) which is distributed 

 over the cochlea, (d) is the nerve called the portio dura, which 

 merely accompanies the auditory nerve, but has no relation to the 

 sense of hearing. In Fig. 390, the auditory nerve (n) is seen en- 

 tering at the back of the vestibule. 



VOL. II. C C 



