572 THE HUMAN BODY. 



another, is almost completely removed by " damping" it; i.e. 

 placing in contact with it something comparatively heavy and 

 which has to. be moved when the membrane vibrates. This 

 is effected by the tympanic bones, fixed to the drum-membrane 

 by the handle of the malleus. Another advantage is gained 

 by the damping; once a stretched membrane is set vibrating it 

 continues so doing for some time; but if loaded its movements 

 cease almost as soon as the moving impulses. The dampers 

 of a piano are for this purpose; and violin-players have to 

 "damp" with the fingers the strings they have used when 

 they wish the note to cease. The tympanic bones act as 

 dampers. 



Functions of the Auditory Ossicles. When the air in 

 the external auditory meatus is condensed it pushes in the 

 handle of the malleus. This bone then slightly rotates on 

 the axial ligament and, locking into the incus where the 

 two bones articulate, causes the long process (Jl, Fig. 163) 

 of the latter to move inwards. The incus thus pushes-in the 

 stapes ; the reverse occurs when air in the auditory passage is 

 rarefied. Aerial vibrations thus set the chain of bones swing- 

 ing, and push in and pull out the base of the stapes, which 

 sets up waves in the perilymph of the labyrinth, and these 

 are transmitted through the membranous labyrinth to the 

 endolymph. These liquids being chiefly water, and practi- 

 cally incompressible, the end of the stapes could not work in 

 and out at the oval foramen, were the labyrinth elsewhere 

 completely surrounded by bone : but the membrane covering 

 the round foramen bulges out when the base of the stapes is 

 pushed in, and vice versa ; and so allows of waves being set 

 up in the labyrinthic liquids. These correspond in period 

 and form to those in the auditory meatus; their amplitude is 

 determined by the extent of the vibrations of the drum mem- 

 brane. 



The form of the tympanic membrane causes it to transmit 

 to its centre, where the malleus is attached, vibrations of its 

 lateral parts in diminished amplitude but increased power; so 

 that the tympanic bones are pushed only a little way but with 

 considerable force. Its area, too, is about twenty times as 

 great as that of the oval foramen, so that force collected on 

 the large area is, by pushing the tympanic bones, all concen- 

 trated on the smaller. The ossicles also form a bent lever 

 (Fig. 163) of which the fulcrum is at the axial ligament and 



