THE EAR, HEARING, TASTE AND SMELL 197 



other advantage is gained by the damping; once a stretched mem- 

 brane 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 ex- 

 ternal auditory meatus is condensed it pushes in the tympanic 

 membrane which carries with it 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. 70) of the latter to move inwards. The incus 

 thus pushes in the stapes; the reverse occurs when air in the au- 

 ditory passage is rarefied. Aerial vibrations thus set the chain of 

 bones swinging, 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 endo- 

 lymph. These liquids being chiefly water, and practically incom- 

 pressible, 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 membrane. 



The form of the tympanic membrane causes it to transmit to its 

 center, 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 push- 

 ing the tympanic bones, all concentrated on the smaller. The 

 ossicles also form a bent lever (Fig. 70) of which the fulcrum is at 

 the axial ligament and the effective outer arm of this lever is about 

 half as long again as the inner, and so the movements transmitted 

 by the drum-membrane to the handle of the malleus are com- 

 municated with diminished range, but increased power, to the base 

 of the stapes. 



