CONDUCTION THROUGH THE TYMPANUM. 601 



of course, greatest at the point at which it is drawn upon, and 

 gradually decreasing towards the margin. Since the existence 

 of a tone of a definite pitch depends upon a certain degree of 

 tightness of the membrane, if no two parts of the membrane 

 have exactly the same degree of tightness, then of course no one 

 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 forwards 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 continu- 

 ance of any special vibration in the membrane of the drum. 



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

 wards 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 open- 

 ing leading into the vestibule. The stirrup bone acts as a kind 

 of adaptable extremity to this inner arm of the lever, being ad- 

 herent 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 backwards 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 



