364 



COORDINATION AND SENSATION 



rows of rod-like bodies, called the rods of Corti. These, 

 by leaning toward each other, form a kind of tunnel be- 

 neath. They are exceedingly numerous, numbering more 

 than 6000, and form a continuous series along the margin 

 of the membrane. 



How We Hear. The sound waves which originate in 

 vibrating bodies are transmitted by the air to the external 

 ear. Passing through the auditory canal, the waves strike 



against the membrana 

 tympani, setting it into 

 vibration. By the bridge 

 of bones and the air 

 within the middle ear 

 the vibrations are carried 

 to and concentrated upon 

 the liquid in the internal 

 ear (Fig. 154). From 

 here the vibrations pass 

 through the channels of 

 the cochlea and set into vibration the contents of the scala 

 media and different portions of the basilar membrane. 

 This serves as a stimulus to the fibers of the auditory 

 nerve, causing them to transmit impulses which, on passing 

 to the brain, produce the sensation of hearing. 



Much of the peculiar structure of the cochlea is not understood. Its 

 minute size and its location in the temporal bone make its study extremely 

 difficult. The connection of the scala vestibula with the scala tympani, 

 and this with the middle ear, is necessary for the passage of vibrations 

 through the internal ear. Its liquids, being practically incompressible 

 and surrounded on all sides by bones, could not otherwise yield to the 

 movements of the stapes. (See Practical Work.) The rods of Corti 

 are thought to act as dampers on the basilar membrane, to prevent 

 the continuance of vibrations when once they are started. 



Detection of Pitch. The method of detecting tones of different pitch 



FIG. 154. Diagram illustrating pas- 

 sage of sound waves through the ear. 



