THE SPECIAL SENSES 



347 



but it is only a drum skin. Three little bones stretch across 

 the true drum, which is the middle ear, to a small film 

 separating the middle ear from the internal ear. These 

 three little bones are called the hammer, anvil > and stir- 

 rup. The middle ear communicates 

 with the pharynx by a narrow tube 

 called the Eustachian tiibe. This 

 tube is for the purpose of admit- 

 ting air to the middle ear, so as to 

 equalize the air pressure on each 

 side of the membrane and prevent 

 straining it. Sometimes blowing 

 the nose may press the air up into 

 the middle ear and press the walls FIG. 224. Diagram of Left Ear. 

 of the Eustachian tube together and close it. This causes 

 slight deafness for the time. The pressure may be relieved 

 by holding the nose closed and swal- 

 lowing, thus opening the passage to 

 the middle ear. One end of the 

 hammer is attached to the inner sur- 

 face of the drum skin ; the other end 

 is attached to the anvil ; and one 

 prong of the anvil is attached to the 

 stirrup, which in. turn is fastened by 

 base to the small film stretched 

 across a round hole in the bone, open- 

 ing into the inner ear, or labyrinth. 

 The inner ear consists of several 

 cavities containing a liquid in which 

 rest the endings of the auditory nerve. The cavities are 

 the snail-shell and the three loops, or semicircular canals. 

 627. Sound waves, entering by the meatus, set the drum 

 skin to shaking ; the vibrations are conveyed by the 

 chain of bones across the middle ear to the liquid of the 

 inner ear. The wave travels through air in the outer ear, 



FIG. 225. Bones of Right 

 Ear, enlarged. 



c, hammer; d, anvil; e,f t 

 stirrup. 



