HEARING. 293 



torial) rests on the tops of the hair cells, and by rubbing on them 

 when they move, this membrane augments the action of the 

 basilar membrane. 



We must now consider how the sound waves are brought 

 from the outside to the oval window. The pinna of the ear col- 

 lects the sound waves from the outside and directs them into the 

 external auditory canal, at the inner end of which they strike 

 the drum of the ear or tympanic membrane. This membrane is 

 stretched loosely in an oblique direction, across the canal and is 

 composed partly of fibers which radiate to the edge of the 

 membrane from the handle of the malleus, a process of one of 

 the auditory ossicles, to which it is attached. Because of these 

 properties, the tympanic membrane, unlike an ordinary drum, 

 is capable of vibrating to a great variety of notes, and the 

 vibrations cause the handle of the malleus to move in and out. 

 Between the tympanic membrane and the cochlea is the middle 

 ear or tympanum consisting of a cavity across which stretches 

 the auditory ossicles composed of three small bones, the malleus, 

 the incus and the stapes. Besides the long process or handle 

 already described, the malleus consists of a rounded head sit- 

 uated above and forming a saddle-shaped articulation with the 

 head of the incus and a short process which runs from just be- 

 low the head to the anterior wall of the tympanum. The incus 

 is somewhat like a bicuspid tooth, the malleus articulating with 

 the crown, and having two fangs, a short one passing backward 

 and a long one vertically downwards. This process, at its lower 

 end, suddenly bends inwards to form a ball and socket joint 

 with a stirrup-shaped bone (the stapes), the foot piece of which 

 is oval in shape and fits into the oval window already mentioned. 



The ossicles act together as a bent lever, the axis of rotation 

 passing through the short process of the malleus in front and the 

 short process of the incus behind. If perpendiculars be drawn 

 from this axis to the tips of the handle of the malleus and the 

 long process of the incus, it will be found that the latter is only 

 two-thirds the length of the former (Fig. 58). The amplitude 

 of movement at the stapes will therefore be only two-thirds of 

 that at the center of the tympanic membrane, but one and one- 



