48 ELEMENTARY PHYSIOLOGY. 



is followed by the stapes or stirrup bone (12), the 

 last span in the little bridge. This rests against a 

 small window-like membrane which is stretched over 

 an opening in the inner side of the drum. At the 

 bottom of the ear-drum or middle ear is an opening 

 into a tube which leads from the ear to the throat 

 This is called the Eustachian tube. Its object is to 

 supply the ear-drum with air, for without air inside 

 to balance the pressure of the air on the outside 

 of the drum-head, the action of the latter would be 

 very imperfect and our hearing, in consequence, very 

 dull. We frequently experience the truth of this 

 statement; for whenever the Eustachian tube be- 

 comes clogged, as in the case of a very bad cold, 

 our hearing is very much impaired, 



The inner ear is carefully hidden in a 



Ear InnCr hU w P lace in tne s li<i bon e- In that 

 part of it which lies next to the mid- 

 dle ear, is a little hall-way, or vestibule, about as 

 large as a grain of wheat. This leads, on one side, 

 into the arched or semi-circular hall-ways which are 

 called the semi-circular canals (13, 14, 15). On 

 the other side the vestibule opens into the cochlea 

 (16, 17), which is shaped like a snail shell or a tiny 

 winding stair. Here the auditory nerve, or nerve 

 of hearing, takes up the impression of a sound and 

 transmits it to the brain. 



All sounds are produced by the vibra- 



Hear. tions of bodies. To make this plainer, 



when a bell is struck its particles are 



