HEARING 



253 



gathering shell-like external car to a membrane, the ear 

 drum or tympanum, which entirely shuts off the inner 

 end of the auditory canal from the next cavity, the 

 middle ear. The tympanum has a diameter of about a 



Auditory 

 nerve 



Eustachian tubt 



FIG. 137. Diagram of a section through the ear, showing the arrangement of 

 its various parts. 



quarter of an inch, is thin, and receives the vibrations 

 of the air in the canal. 



Middle ear. Imbedded in the tympanum is one end 

 of a tiny bone, the hammer, which is the first of a train 

 of three bones crossing the cavity of the middle ear. 

 These bones, the hammer, anvil and stirrup, are called the- 

 ear bones. The innermost one, the stirrup, lies against 

 a thin membrane which closes a small opening, the so- 

 called oval window. This window opens into a bony 

 case which contains the real or inner ear. The vibrations 

 of the tympanum are thus transmitted by the ear bones 

 to the inner ear. 



