334 ANATOMY AND PHYSIOLOGY 



SUMMARY 



The function of the external ear is to gather and direct the sound 

 waves to the membrane of the tympanum. (The ceruminous 

 glands and hairs of the auditory canal protect the membrane from 

 foreign bodies floating in air currents.) 



The function of the middle ear or tympanum is to transmit the 

 vibrations thus caused, by the chain of bones to the oval window. 



The function of the internal ear is to receive and transmit these 

 impressions to the brain (the perilymph and endolymph modify 

 the force of the vibrations as well as transmit them) . If they are 

 received by the cochlear nerve, they are conducted to the auditory 

 area in the temporal lobe of the cerebrum. If by the vestibular 

 nerve, they are conducted to the cerebellum. 



Associated nerves and functions. Many coordinated move- 

 ments are associated with the sense of hearing. 



Listening causes a local increase of tension in the muscles which 

 affect the position of the eyes and head; very intent listening is 

 accompanied by a steadying or stiffening of the skeletal muscle 

 system, and also an instinctive slowing or stopping of respiration. 

 A sudden noise at one side causes an involuntary turning of the 

 head toward the location of the sound. 



The associations of the vestibular nerves are very wide. They 

 include not only the nerves of muscles which move the face and 

 head, but the pneumogastric or vagus, and centers in the anterior 

 and lateral tracts of the cord in its whole length, whereby the 

 muscle-sense nerves of the body are all referred to the cerebellum. 

 In this manner, many different muscle groups are associated and 

 coordinated in harmonious body movements which we are under 

 constant necessity to perform in the common activities of life. 



