602 MANUAL OF PHYSIOLOGY. 



tone can be more conspicuous than another. This is the case 

 with the tympanic membrane. 



The independent vibrations of the membrane are further pre- 

 vented by the tympanic ossicles. These little bones do not really 

 vibrate, but move back and forward in time to the sound vibra- 

 tion. If a body not capable of vibrating with the membrane of 

 a common drum be attached to it, the drum would not sound. 

 A touch of the finger of the musician to the membrane suffices 

 to check the sound produced by a drum. The handle of the 

 malleus, which is joined to the other bones, being fixed to the 

 membrane, acts in this way as a damper, and checks the con- 

 tinuance of any special vibration in the membrane of the 

 drum. 



The small muscle attached to the malleus so as to draw it to- 

 ward the cavity of the drum is called the tensor tympani. 



CONDUCTION OF SOUND VIBRATIONS THROUGH THE TYMPANUM. 



The motions occurring in the membrane of the drum are con- 

 veyed across the tympanic cavity by means of the three small 

 bones known as the malleus, the incus, and the stapes. Together, 

 these ossicles form an angular or two-armed lever, one end of 

 which (the handle of the malleus) is attached to the centre of 

 the tympanic membrane, and the other (the long limb of the 

 incus), which is the shorter arm of the lever, pushes the stapes 

 against the little secondary tympanic membrane, which fits the 

 oval opening leading into the vestibule. The stirrup bone acts 

 as a kind of adaptable extremity to this inner arm of the lever, 

 being adherent to the membrane of the vestibule and jointed to 

 the long arm of the incus. This little angular lever works round 

 an axis which passes from before backward through the head of 

 the malleus, and lies above the membrane of the drum ; the two 

 points which act as the bearings or pivots of the motion being 

 the slender process of the malleus in front, and the short limb of 

 the incus behind. 



When the tympanic membrane vibrates in response to the 

 sound waves of the air, it moves in and out, and the handle of 

 the hammer bone must move in and out with it. The body of 



