vni THE AUDITORY OSSICLES 371 



above, articulated together in such a manner that they 

 may practically be considered as forming one bone which 

 turns upon a horizontal axis. This axis passes through 

 the horizontal process of the incus and the slender process 

 of the malleus, and its ends rest in the walls of the tym- 

 panum. Its general direction is represented by the line 

 a 6 in Fig. 122, or by a line perpendicular to the plane of 

 the paper, passing through the head of the malleus in 

 Fig. 119. 



The two bones may be roughly compared to two spokes 

 of a wheel, of which the axle is represented by the axis 

 just described ; it should be added, however, that one 

 spoke, the incus, is sliorter than the other, and that the 

 movement of the two spokes is limited to a very small arc 

 of a circle. 



When the membrane of the drum, thrown into vibration 

 by some sound, moves inwards and outwards in its vibra- 

 tions, it necessarily carries with it, in each inward and 

 outward movement, the handle of the malleus which is 

 attached to it. But with each inward and outward move- 

 ment of the handle of the malleus, the long process of the 

 incus also moves inward and outward, carrying with it the 

 stapes which is attached to its end. Hence each vibration, 

 each inward thrust, and each outward or backward return 

 of the membrane of the drum, produces by means of the 

 chain of ossicles a corresponding vibration of the mem- 

 brane of the fenestra ovalis to which the stapes is attached. 



(iv) The Muscles of the Tympanuin. — The char- 

 acters of the vibration of a membrane, and the readiness 

 with which it takes up or responds to, aerial vibrations 

 reaching it, are largely modified by its degree of tension ; 

 the membrane acts differently when it is tightly stretched 

 from what it does when it is loose. Now, within the 

 cavity of the tympanum are two small, but relatively 

 strong muscles. One, called the stapedius, passes from 

 the floor of the tympanum to the foot of the stapes and 

 the orbicular bone, the other, the tensor tympani. from 



B B 2 



