192 



PHYSIOLOGY 



CHAP. 



the organ of hearing, and penetrate to and stimulate the endings 

 of the auditory nerve, and arouse the sensations of tones and 

 noises in the sensory centres. 



I. Of the three parts into which the organ of hearing is 

 divided (Fig. 72) the internal ear alone contains in the cochlea 

 the terminal sense-organ that is excited by sound vibrations ; the 

 outer and middle ear are mere complementary physical parts of 

 the apparatus, which serve to promote and facilitate the conduction 

 of sound-waves to the cochlea. 



..J3 



6' 



FIG. 72. Diagram of the human ear as a whole. (After Debierre.) 1, pinna or auricle ; 2, external 

 auditory meatus ; 3, tympanic membrane ; 4, stapes attached to fenestra ovalis (vestibuli) ; 

 5, bony portion of Eustachian tube ; (5, cartilaginous portion ; 6', its internal orifice ; 7, cavity 

 of vestibule tilled with perilymph ; 8, semicircular canals with utricle ; {), promontory ; 

 10, fenestra rotunda (cochleae) with arrow indicating tympanic orifice of cochlea ; 11, tym- 

 panic cavity filled with air; 12, cochlear canal filled with endolymph, united to sacculus 

 vestibuli by small canal; 13, scala vestibula; 14, scala tympani terminating in fenestra 

 rotunda ; 15, apex of cochlear canal, where the two scalae unite at 15' ; 16, cochlear aque- 

 duct ; 17, vestibular aqueduct ; 18, endolymphatic sac ; 19, parotid region. 



The external ear consists of the pinna, and the external 

 auditory canal or meatus, closed at the end by the tympanic 

 membrane. 



The middle ear, or tympanic cavity, is an irregular, hollow 

 chamber with bony walls, filled with air. It contains the chain of 

 auditory ossicles the malleus, incus, and stapes, with their two 

 short muscles the internal aperture of the Eustachian tube 

 which opens into the pharynx, the fenestra ovalis (vestibuli) 

 closed by a membrane, to which the base of the stapes is inserted, 



