THE SPECIAL SENSES 407 



Across the inner end of this tube is stretched a thin 

 membrane known as the car drum. 



Fig. 187. Diagram to illustrate the structure of the ear. 



2. Beyond the ear drum is a small cavity in the 

 temporal bone. This is the middle ear. It is connected 

 with the pharynx by the Eustachian tube. Through 

 this, air- enters the middle ear and presses on the inside 

 of the ear drum, balancing the pressure of the air on 

 the outside of the drum. A sudden very loud noise 

 might otherwise burst the ear drum because of the 

 great pressure outside. There are three bones in the 

 middle ear; the hammer, the anvil, and the stirrup. 

 These small bones are arranged in a chain across the 

 cavity of the middle ear. The first is connected with 

 the ear drum and the third is fastened to a membrane 

 over an opening into the internal ear. 



