MOVEMENTS OF THE STAPES 279 



The stapedius muscle pulls the capitulum in the inferior-posterior 

 direction, thus sinking the inferior and posterior edges of the footplate 

 into the oval window to a depth determined by the length of its annular 

 ligaments. This pull of the stapedius tensor places the posterior crus of 

 the stapes under compression and the anterior crus under tension. An 

 unsymmetrical, pestle-like rolling motion (Stuhlman [1937]) applied to 

 the capitulum by the lenticular process of the incus can therefore be 

 duplicated by the footplate as it rests under the tension of the stapedius 

 tensor in its oval window. 



The structure of the stapes is such that it can readily conform to the 

 two modes of vibration attributed to it by Bekesy [1936]. One of these 

 he observed to be a rocking motion about a major axis of the footplate 

 at high acoustic pressures, which was observed suddenly to change to a 

 rocking motion around the posterior end of the footplate at low acoustic 

 pressures. 



The kidney-shaped footplate of the stapes is set in an annular liga- 

 ment about 3 mm thick which connects its margin with the adjacent wall 

 of the oval window (fenestra vestibuli). The elastic ligaments bridging 

 the gap, according to Wrightson and Keith, have the average dimensions 

 shown in Fig. VII-14. Of special significance is the fact that the 

 anterior ligaments are about ten times longer than the less active poste- 

 rior ligaments. 



Movements of the Stapes 



In its rest position, with no tension on the stapedius tensor, the foot- 

 plate is held by its ligaments in a nearly vertical plane with the convex 

 edge of the plate in the superior position. Under contractile action of 

 the stapedius tensor the inferior and posterior margins are pushed into 

 the window, while the superior and anterior margins are pulled out to the 

 limits set by the width of the annular ligament. These movements are 

 illustrated in Fig. VII-15. The resultant diagonal motion of the capitu- 

 lum carries the attached lenticular process of the incus with it and 

 pushes the posterior end of the short crus of the incus into a shallow 

 depression (fossa incudis) where a fan-shaped ligament anchors this end 

 of the incus to the fossa in the posterior wall. This motion puts the 

 ligaments of the malleus-incus articulation under tension, producing the 

 previously discussed loose-coupled malleus-incus joint. 



On inward motion of the lenticular process, the capitulum is put 

 under compression, raising and pushing the stapes in the anterior direc- 

 tion. This compression pushes the superior and anterior edge of the 

 footplate into the oval window. To resist the stresses of this complex 

 motion, the inner surface of the footplate has developed, where it makes 



