ORGAN OF HEARING. 



183 



and otoconieSj according as they are of a hard or a soft consistence. 

 The small square figures (Fig. 70), represent their size and appearance 

 in the dog and the hare. 



Fig. 73. 



Fig. 74. 



Auditory Nerve taken out of the Cochlea. 



1, 1, 1. Trunk of the nerve. 2, 2. Its filaments in the 

 zona ossea of the lamina spiralis. 3, 3. Its anastomoses 

 in the zona vesicularis. 



Papillae of the Auditory Nerve, on a 

 segment of the spiral lamina of the 

 cochlea of a young Mouse. 



The lower portion is the osseous, and 

 the higher the membranous part of the 

 lamina. Magnified 300 times. 



It is in the cavities of the internal ear, and on the different parts of 

 the membranous labyrinth, that the auditory or acoustic nerve is dis- 

 tributed. This nerve is the portio mollis of the seventh pair, of most 

 anatomists. It arises, like other nerves of the senses, from the medulla 

 oblongata ; and near the anterior paries of the fourth ventricle. Thence 

 it passes obliquely outwards, forwards, and upwards, and enters the 

 meatus auditorius internus, the foramen of which is situate on the pos- 

 terior surface of the pars petrosa. The base of this meatus corresponds 

 to the inner surface of the vestibule, and to the base of the cochlea. 

 Through the first foramen, near the base of the meatus, the portio dura 

 of the seventh pair or facial nerve passes to gain the aqueduct of 

 Fallopius; along which it proceeds, giving off filaments to different 

 parts of the middle ear, and ultimately issuing by the stylo-mastoid 

 foramen to be lost on the muscles of the face. Below the part of the 

 meatus, where the facial nerve emerges, are several other foramina, 

 through which filaments of the auditory nerve attain the labyrinth. 

 These are distributed to the vestibule, semicircular canals, and cochlea; 

 and terminate, by very delicate ramifications, in the tissue and at the 

 surface of the membrane that lines the labyrinth. The precise mode 

 in which the ramifications terminate has been a matter of dispute : 

 some affirming, that they end in papillae, as in the marginal figure from 

 Treviranus (Fig. 74) ; others, that the fibres return by loops. The 

 arrangement is probably analogous to that which prevails in the retina. 1 



Such is the apparatus concerned in the function of audition. Before 

 proceeding to the physiology of these different parts, and the assistance 



1 Carpenter's Human Physiology, 352, Lond., 1842. 



