472 



THE SPECIAL SENSES 



estra ovalis there is strung across the cavity a chain of 

 tluee bones called from their shape the hammer (malleus), 

 the anvil (incus), and the stirrup (stapes) bones. One end 

 of the malleus rests against the tympanic membrane, while 

 its other end rests against the incus. The incus, in turn, 

 articulates, at its farther end with the stapes, whose interior 

 end rests against the membrane of the fenestra ovalis. 

 These bones, together with the air, permit vibrations of 

 the tympanic membrane to be transmitted across the 

 middle ear to the membrane of the fenestra ovalis. The 

 Eustachian tubes play a part in this transmission in per- 

 mitting the escape of air into the throat when the pres- 

 sure becomes too great, and by admitting it return the 

 membrane to position. If it were not for this outlet a 

 sudden increase of pressure on the tympanum might burst 

 the drum or one of the inner membranes. Its action also 

 increases the sensitiveness of the ear drum to vibrations, 



as is seen in the deafness 

 that results when this tube 

 is clogged, as by mucus in 

 a cold. The middle ear is 

 called the tympanum. 



The inner ear. — The 

 fenestra ovalis and fenestra 

 rotunda are the openings 

 by which the tympanum 

 communicates with an ir- 

 regular cavity of the tem- 

 poral bone called the 

 This labvrinth and its contents 



Posterior 

 Canal 



Anterior 

 Canal 



Fenestra Ovalis 



Fenestra 

 Jtotunda 



Fig. 224 — Bony labyrinth. 



osseous, or bony, labyrinth. 

 constitute the inner ear. 

 The entire labyrinth is lined with a membrane which 



