PHYSIOLOGY OF THE EAR. 



CHAPTER XX. 



THE EAR AS AN ORGAN FOR SOUND SENSATIONS. 



In discussing the physiology of the ear it is necessary to consider 

 the functional importance of its various parts, the external ear 

 consisting of the lobe or pinna, the external auditory meatus, and 

 the tympanic membrane; the middle ear, with its chain of ossicles, 

 its muscles and ligaments, and the Eustachian tube ; and the internal 

 ear, with its cochlea, vestibule (utriculus and sacculus), and semi- 

 circular canals. The eighth cranial or so-called auditory nerve is 



Fig. 157. Semidiagrammatic section through the right ear (Czermak) : G, External 

 auditory meatus; T, membrana tympani; P, tympanic cavity; o, fenestra ovalis; r, fen- 

 estra rotunda; B, semicircular canal; *S, cochlea; Vt, scala vestibuli; Pt, scala tympani; 

 E, Eustachian tube. 



distributed entirely within the internal ear; the fibers of the coch- 

 lear branch, which alone perhaps are concerned with hearing, end 

 among the sensory nerve cells of the cochlea, while the vestibular 

 branch supplies similar sense cells situated in the utriculus, sacculus, 

 and the ampulhe of the semicircular canals. We may consider 

 first the functions of the ear in respect to the sensations of sound. 

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