206 THE EAR 



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. Ttiis bone, a flattened stirrup arch, 

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

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

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

 ligament. 



Muscles of 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 in the fenestra ovalis. Its line of 

 traction (Fig. 44), which is almost parallel to the long axis of the 

 oval window, causes the footplate to move on the posterior annul* 

 ligament as on a hinge. Contraction of this muscle thus draw! 

 the anterior end of the footplate outwards. 



These two muscles are therefore antagonistic ; simultaneous 

 contraction balances the ossicles and regulates the degree t< 

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

 the tension of the two muscles prevents a slack engagement 

 between the ossicles. If the bearings were not kept together witl 

 sufficient force, slipping, knocking and loss of power would ensue. 

 This state of equilibrium is absolutely necessary if the system oi 

 membranes and ossicles is to move in immediate response to the 

 slightest alteration in air pressure. 



Before going into the mode of action of the ear bones, a pressui 

 equalising device comes up for consideration. As has just beei 

 said, perfect equilibrium of vibrating parts is necessary for perfe( 



