270 Life and Health 



labyrinth occupies the central portion of the vestibule and 

 the passages leading from it. When, therefore, the peri- 

 lymph is shaken it communicates the impulse to the fluid 

 (endolymph) contained in the inner membranous sac. 



The endolymph is now driven against the sides of the 

 membranous sac and so strikes the ends of the nerves of 

 hearing, which transmit the auditory impulses to the seat 

 of sensation in the brain. 



It is in the seat .of sensation in the brain that the vari- 

 ous auditory impulses received from different parts of the 

 inner ear are interpreted as sounds. It is the extent of 

 the vibrations that determines the loud ness of the sound ; 

 the rapidity of them that determines the pitch. 



NOTE. The otoliths and the semicircular canals probably have 

 nothing to do with the sense of hearing. They enable us to feel the 

 position of the head, and thus help us to preserve our equilibrium. 



' Experiment 122. Hold a ticking watch between the teeth, or touch 

 the upper incisors with a vibrating tuning fork ; close both ears and 

 observe that the ticking or vibration is heard louder. Unstop one 

 ear and observe that the ticking or vibration is heard loudest in the 

 stopped ear. 



Experiment 123. Hold a vibrating tuning fork on the incisor teeth 

 until it can no longer be heard. Close one or both ears, and it will 

 be heard again. 



Experiment 124. Listen to a ticking watch or a tuning fork kept 

 vibrating electrically. Close the mouth and nostrils and take either 

 a deep inspiration or deep expiration, so as to alter the tension of the 

 air in the tympanun ; in both cases the sound is diminished. 



- Experiment 125. With a blindfolded person test his sense of the 

 direction of sound, e.g., by clicking two coins together. It is very 

 imperfect. Let a person press both auricles against the side of the 

 head and hold both hands vertically in front of each meatus. If a 

 person makes a sound in front, the observed person will refer it to a 

 position behind him. 



