306 THE SENSORIAL FUNCTIONS. 



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 ro- 

 tunda, 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 be- 

 hind 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.t 



Great uncertainty prevails with regard to the real func- 

 tions performed by the several parts of this very complex 

 apparatus. It is most probable, however, that the sonorous 

 vibrations of the air w r hich reach the external ear, are di- 

 rected down the meatus, and striking against the ear-drum 

 which closes the passage, throw that membrane into vibra- 

 tions of the same frequency; to which the action of its mus- 

 cles, which appear intended to regulate its tension, may also 

 contribute. The vibrations of the ear-drum, no doubt, ex- 

 cite corresponding motions in the air contained in the cavity 

 of the tympanum; which, again, communicates them to the 

 membrane of the fenestra rotunda; while, on the other hand, 

 the membrane closing the fenestra ovalis, receives similar 

 impressions from the stapes, conveyed through the chain of 

 tympanic ossicula, which appear to serve as solid conductors 

 of the same vibrations. Thus, the perilymph, or fluid con- 

 tained in the labyrinth, is affected by each external sound, 

 both through the medium of the air in the tympanum, and 

 by means of the ossicula: the undulations thus excited pro- 



* Scala tympani. 



f In Fig. 396, the anterior trunk of the auditory nerve is seen (at G) dis- 

 tributing branches to the ampullse (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 calcareous body (o) and a second branch 

 (K) which is distributed over the cochlea. (D) is the nerve called the por- 

 tia 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 enter- 

 ing at the back of the vestibule. 



