Hearing. 



406. THE ORGAN OF HEARING. Stimulation of the Auditory Nerve. 

 The normal manner in which the auditory nerve is excited, is by means of 

 sonorous vibrations, which set in motion the end-organs of the acoustic nerve, 

 which lie ini the endolymph of the labyrinth of the inner ear, on membranous 

 expansions of the cochlea 

 and semicircular canals. 

 Hence, the sonorous vi- 

 brations are first trans- 

 mitted to the fluid in 

 the labyrinth, and this, 

 in turn, is thrown into 

 waves, which set the end- 

 organs into vibration. 

 Thus, the excitement of 

 the auditory nerves is 

 brought about by the 

 mechanical stimulation 

 of the wave-motion of the 

 lymph of the labyrinth. 



The fluid or lymph 

 of the labyrinth is sur- 

 rounded by the exceed- 

 ingly hard osseous mass 

 of the temporal bone (fig. F - 57 ^ 



577). Only at one small Scheme of the organ of hearing. AG, external auditory meatus ; 



roundish and slightly T, tympanic membrane; K, malleus with its head (h), short 

 process (kf), and handle (m) ; a, incus, its short process (x), and 

 its long process united to the stapes (s) by means of the Sylvian 

 ossicle (z) ; P, middle ear ; o, fenestra ovalis ; r, fenestra rotunda ; 

 x, beginning of the lamina spiralis of the cochlea ; pt, scala 

 tympani, and vt, scala vestibuli; V, vestibule; S, saccule; U, 

 utricle ; H, semicircular canals ; TE, Eustachian tube. The 

 long arrow indicates the line of traction of the tensor tympani ; 

 the short curved one, that of the stapedius. 



triangular area, the fe- 

 nestra rotunda (r), the 

 fluid is bounded by a 

 delicate yielding mem- 

 brane, which is in con- 

 tact with the air in the 

 middle ear or tympanum 

 (P). Not far from the fenestra rotunda is the fenestra ovalis (o), in which the 

 base of the stapes (s) is fixed by means of a yielding membranous ring. The outer 

 surface of this also is in contact with the air in the middle ear. As the perilymph 

 of the inner ear is in contact at these two places with a yielding boundary, it is 

 clear that the lymph itself may exhibit oscillatory movements, as it must follow the 

 movements of the yielding boundaries. 



