TYMPANIC OSSICLES 2.):i 



rotating on any other axis than a horizontal one whose line 

 passes through the head of the malleus and the anterior ligament 

 (Fig. 64). The posterior surface of the head of the hanniier fits 

 into the saddle-like hollow in the anterior surface of the body of the 

 anvil-bone or incus. This is a larger bone than the malleus, weigh- 

 ing on the aN-erage 25 mg. The body of the incus is drawn out 

 on its posterior side to a process — the short process, which is 

 attached by a ligament to the posterior wall of the tympanic 

 cavity, forming one end of the malleo-incal axis mentioned above. 

 Almost at right angles to the short process, the inferior surface of 

 the incus tapers down to the knob-like os orbiculare, forming the 

 long process. 



The OS orbiculare articulates with the knob on the top of the 

 stirrup bone or stapes. This bone, a flattened stirrup arch, 

 weighing only about 3 mg., is set almost at right angles to the 

 long process of the incus. Its oval footplate is attached to the 

 margin of the fenestra ovalis by a short stiff membrane, the annular 

 ligament. 



Muscles oJ middle ear. 



Two slender muscles are attached to the ossicles : 



(i.) The stapedius is inserted into the knob at the head of the 



stapes and is attached to the posterior wall of the tympanic cavity. 

 (ii.) The tensor tympani arises from the inner wall of the cavity, 



passes outwards and upwards above the Eustachian tube, to be 



inserted in the upper part of the handle of the malleus. 



Function of the muscles. 



The tensor, on contraction, draws the handle of the malleus 

 inwards, and so, as its name implies, increases the tension on the 

 tympanic membrane. This decreases the natural period of 

 vibration of the drum, and this makes it more sensitive to high 

 tones, and better fitted to adjust its vibrations to rapid changes 

 of phase. Paralysis of this muscle impairs hearing. 



The stapedius prevents the footplate of the stapes from having 

 purely a piston-like action on the fenestra ovalis. Its line of 

 traction (Fig. 64), which is almost parallel to the long axis of the 

 oval window, causes the footplate to move on the posterior annular 

 ligament as on a hinge. Contraction of this muscle thus draws 

 the anterior end of the footplate outwards. 



These two muscles are therefore antagonistic ; simultaneous 

 contraction balances the ossicles and regulates the degree to 

 which the perilymph of the internal ear is displaced. Further, 

 the tension of the two nmscles prevents a slack engagement 



