458 Dynamic Theory. 



tympanic membrane which receives the first agitation from the air or 

 outside world ; it is stretched across the inner end of the auditory canal 

 in a diagonal or oblique direction. The agitation of this membrane is 

 conveyed across the inner part of the drum to the fenestra ovalis by 

 means of a chain of small bones, four in number, and peculiarly arranged. 



FIG. 193. Bones of the Ear. 

 A. Incus or anvil. 

 H. Hammer or Malleus. 



Hammer Handle attached to Tympanum. 

 .Long process. 

 . Os Orbiculare. 

 S. Stapes, attached to Fenestra Ovalis. 



The one next the tympanic membrane and per- 

 manently attached to it is the malleus or 

 hammer. When agitated it jars upon the one 

 next it which is called the incus or anvil. 

 Next to the incus and separating it from the 

 Fia. 193. last bone which is called the stapes or stirrup, 



is the very small, round bone called the os-orbiculare. It is frequently 

 fused with the anvil bone. The head or top part of the stirrup is con- 

 nected with the orbiculare while its foot is planted directly flat on the 

 fenestra ovalis. The tympanum is an organ of noise, but not of music. 

 If it had a single fundamental note we could get no knowledge of any 

 noise except that of one particular pitch. But the contact of the ham- 

 mer handle across its disc gives it an uneven and complicated tension, 

 so that it is jarred by all sounds within certain limits. 



FIG. 194. Human Labyrinth and Ear bones 

 magnified. 



a.-^Stapes (stirrup) covering the fenestra 

 ovalis. 



6. Fenestra rotunda ( round window ). 



c. The horizontal canal. 



d. e. The perpendicular canals. 



/.Cochlea (snail). 



a. Incus ( anvil ). 



ft. The long process of hammer. 



i. Head of Malleus (hammer). 



i. Manubrium ( handle ). 



K Os-orbiculare (the smallest bone in the 



/.Junction of canals. 

 m m m. Ampullae. 



The cavity of the drum is thus en- 

 closed by the tympanic membrane on 

 the outside and bounded by the mem- 

 branes of the two fenestras, or win- 

 dows leading into the two internal sacs, 

 on the opposite or inner side. It has one opening, however. A canal 

 called the Eustachian tube leads from the front part of the drum, from a 

 point between the tympanic membrane and the fenestra rotunda, to the 

 pharynx (fig. 191, 7^ and 192, Et\ This tube allows air to pass from 

 the throat to the drum of the ear, and in cases of diseases of the tym- 



FlG. 194. 



