THE MIDDLE BAE 471 



exactly alike, a description of one applies in every particu- 

 lar to the other. 



The external ear. — The oval, folded, and roughly funnel- 

 shaped external ear consists, as has been mentioned, of a 

 projecting structm-e of cartilage covered with a layer of 

 skin and blood vessels, and bound to the side of the head by 

 bands of muscle. These muscles in some individuals and 

 in many of the lower animals are capable of voluntary 

 movement. The irregular outre surface of this ear con- 

 verges near the center to form a fuimel-shaped canal about 

 an inch long. This canal is lined with skin whose sebaceous 

 glands secrete a peculiar form of fluid which on exposure 

 loses its water and thickens into a bitter yellow wax {ear 

 wax). This wax serves to keep moist a membrane stretched 

 across the bottom of the canal (called the tympanic mem- 

 brane) . This membrane is covered externally by a thin 

 layer of skin, and is extremely flexible. The external flap 

 collects the sound waves, acting in this respect like the horn 

 of an ear trumpet, and concentrates them upon this ear 

 drum which is thus set in vibration. 



The middle ear. — The tympanic membrane forms the 

 external covering of a small cavity called the middle ear, 

 or tympaniom. This cavity is lined with mucous mem- 

 brane and filled with air. A narrow tube {Eustachian 

 tube) connects it with the pharynx. In swimming, water 

 often passes from the pharynx through this tube into the 

 middle ear and produces ringing in the ears, until it is 

 allowed to run out. On the inner surface of the cavity 

 are two small openings covered with membrane. One is 

 called, from its shape, the round window, or fenestra rotunda. 

 The other which is oval in shape is called the fenestra 

 ovalis. Between the tympanic membrane and the fen- 



