408 A YEAR IN SCIENCE 



3. The internal ear is far more complex than the 

 other parts. It is an irregular cavity in the temporal 

 bone lined with a membranous sac. Lying next to the 

 middle ear is a tube coiled like a snail shell and called 

 the cochlea. Next to this there is a central portion 

 back of which there are three semicircular canals lying 

 in three planes and placed perpendicular to each other. 

 All of these tubes are filled with a watery fluid. In 

 the cochlea is a thin membrane in which the auditory 

 nerves terminate. 



Action of ear. The external ear gathers the sound 

 waves so that they pass down the auditory canal to the 

 ear drum. This then starts to vibrate. These vibrations 

 pass through the three bones of the middle ear to the 

 membrane separating it from the internal ear. From 

 this membrane the liquids in the middle ear are set in 

 motion. They in turn cause the membrane in the 

 cochlea to vibrate and thus stimulate the nerves from 

 which impulses are sent to the brain. 



It is generally supposed that different parts of this 

 membrane respond to different rates of vibration and 

 this makes it possible for us to distinguish between 

 different pitches. The range of hearing is very great. 

 We can hear sounds corresponding to vibrations from 

 about 30 to about 40,000 per second. Between these 

 limits as many as 6,000 variations of pitch can be 

 perceived. 



The semicircular canals are not used for hearing. 

 They are used in maintaining the. balance, or equilib- 



