CHAPTER XX 

 THE EAR AND THE SENSE OF HEARING 



JUST as in the sense of sight the waves of light produce their 

 impression on the delicate processes of certain cells connected 

 with the optic nerve, so in the sense of hearing the vibrations 

 of sound act on the processes of certain cells connected with the 

 auditory nerve. These cells are part of a delicate membrane 

 of complicated structure lodged in spaces in the hard or 

 petrous portion of the temporal bone. This part of the 

 organ of hearing is called the internal ear. The sound 

 waves are received by the external ear, that which we 

 commonly call the "ear," and are conducted to the internal 

 ear by the structures in the middle ear. 



The External Ear. The external ear consists of a plate 

 of elastic cartilage of complicated shape, covered by connec- 

 tive tissue containing numerous blood-vessels, and then by the 

 skin. Some small muscles are present passing to the ear 

 from neighbouring parts. The external ear surrounds a more 

 or less funnel-shaped opening, the beginning of a passage 

 rather more than an inch long, the external auditory 

 canal, which leads to the middle ear, from which it is shut 

 off by a membraneous partition stretched across, called the 

 tympanic membrane. 



The Middle Ear. The middle ear is a flattened drum- 

 like cavity in the temporal bone, from the inner side of 

 which a tube about i inch long, called the Eustachian 

 tube, leads to the upper part of the pharynx. The cavity, 

 therefore, has access to the air in the pharynx ; if the tym- 

 panic membrane did not exist there would be a passage from 

 the opening of the ear through the middle ear into the 



