The Special Senses 



269 



of sand, called otoliths, or ear stones. Every movement of 

 the fluid itself throws these grains from side to side. 

 ,-v 424. The Auditory Nerve. The auditory nerve, or nerve 

 of hearing, passes to the inner ear through a passage in 

 the solid bone of the skull. Its minute filaments spread at 

 last over the inner walls of the membranous labyrinth in 

 two branches. One goes to the vestibule and the ampullae 

 at the ends of the semicircular canals ; the other leads to 

 the cochlea. 



3 425. Mechanism of Hearing. Waves of sound reach the 

 ear and are directed by the concha to the external passage, 

 at the end of which they reach 

 the tympanic membrane. 

 When the sound waves beat 

 upon this thin membrane it is 

 thrown into vibration, repro- 

 ducing in its movements the 

 character of the air vibrations 

 that have fallen upon it. 



The vibrations of the tym- 

 panic membrane are now passed 

 along the chain of bones 

 attached to its inner surface 

 and reach the stirrup bone. 

 The stirrup now performs a to- 



FIG. 144. Bony Internal Ear of 

 the Right Side. (Magnified ; 

 the upper figure of the natural 

 size.) 



and-fro movement at the oval A > l window , (foram f : ale) ; *' ? 



>, semicircular canals ; * represents 



the bulging part (ampulla) of each 

 canal ; E, F, G, cochlea ; //, round 

 window (foramen rotundum). 



window, passing the auditory 

 impulse inwards to the internal 

 ear. 



Every time the stirrup bone is pushed in and drawn out 

 of the oval window the watery fluid (the perilymph) in the 

 inner ear is set in motion more or less violently, accord- 

 ing to the intensity of the sound. The membranous 



