HEARING 



439 



-CochleoL 



..jltxstadhian tube 



Explain from the diagram, how we hear ? 



Placed directly against the tympanic membrane, and connecting 

 it with another membrane which separates the middle from the 

 inner ear, is a chain of three tiny bones, the smallest of the body. 

 They are held in ,± 



place by very small g*?™** S^? e 22?* Served 

 muscles which are 

 delicately adjusted 

 so as to tighten or 

 relax the mem- 

 branes guarding the 

 middle and inner 

 ear. 



The inner ear. 

 The inner ear is one 

 of the most com- 

 plicated, as well as 

 one of the most 

 delicate, organs of the body. Deep within the temporal bone 

 there are found two parts, one of which is called, collectively, the 

 semicircular canals, the other the cochlea (kok'le-d). 



It has been discovered by experimenting with fish, in which 

 the semicircular canal region forms the chief part of the ear, 

 that this region has to do with the equilibrium or balancing of the 

 body. 



That part of the ear which receives sound waves is known as the 

 cochlea (Lat., snail shell) because of its shape. This complicated 

 organ is lined with sensory cells provided with cilia, and its cavity 

 is filled with a fluid. It is believed that somewhat as a stone 

 thrown into water causes ripples to emanate from the spot where 

 it strikes, so sound waves are transmitted by means of the fluid 

 filling the cavity to the sensory cells of the cochlea and thence 

 to the brain by means of the auditory nerve. 



The character of sound. When vibrations which are received 

 by the ear follow one another at regular intervals, the sound is said 

 to be musical. If the vibrations come irregularly, we call the 

 sound a noise. If the vibrations come slowly, the pitch of the 

 sound is low ; if they come rapidly, the pitch is high. The ear 



