xvni THE SPECIAL SENSES AND SENSATIONS 263 



sometimes from swinging in a hammock, is due to the 

 same cause. 



180. The Course of a Sound Wave. The cartilaginous 

 funnel or concha of the external ear catches sound 

 waves and directs them into the auditory canal, which 

 conveys them to the drumhead. The sensitive membrane 

 of the drumhead responds to the waves and vibrates 

 like the diaphragm of the telephone. These vibrations 

 are transmitted to the chain of bones of the middle ear, 

 which modify, intensify, and focus the motion upon the 

 membrane stretched across the oval window of the 

 internal ear, into which the base of the stapes fits. 

 The vibrations of the oval window set in motion both 

 the perilymph of the bony labyrinth and the endolymph 

 within the membranous sac. The waves enter the 

 vestibule, pass through the semicircular canals and the 

 entire cochlea, and back to the vestibule, where they pass 

 into the middle ear at the round window. The motion 

 of the liquids causes the membranous sac to vibrate, 

 and this vibration irritates the delicate nerve endings. 

 These nerve impressions are carried to the proper cen- 

 ters in the brain, and we hear. What a complicated 

 process hearing is ! 



1. Hold a watch between the lips for a moment, and then close 

 the teeth down on it and notice how loud it seems to tick. 



2. Cover both ears and then uncover one and notice the result. 

 Repeat several times. Explain. 



3. Hold a vibrating tuning fork between the lips until it can no 

 longer be heard. Close the teeth down on it and it will be heard 

 again. When it can no longer be heard the second time, cover one 

 or both ears and it will be heard a third time. Explain. 



