MIDDLE EAR 603 



long process of the incus moves inwards. The stapes, or stirrup bone, is fixed 

 in the fenestra ovalis of the internal ear, in the inner surface of the tympanum, 



FIG. 305. To show the relations of the malleus and incus to one another. The 

 shaded area between the two bones shows the articular surfaces which connect 

 them. The overlapping of the two bones at the lower part of these surfaces 

 is well shown. It is this arrangement which causes motion to be conveyed 

 from one to the other. 



by the annular ligament. It is placed almost at right angles to the long 

 process of the incus, and therefore is pressed into the foramen ovale when this 

 process moves inwards. 



THE MUSCLES found in the tympanum 'are the tensor tympani, which 

 is attached to the handle of the malleus, and the stapedius attached to the base 

 of the stapes. The tensor is innervated by a motor branch of the th cranial 

 nerve, and when it is stimulated it draws the handle of the malleus inwards 

 and so increases the tension of the tympanic membrane. At the same time 

 the plunger of the stapes is displaced into the oval window, thus putting 

 compression on the contents of the internal ear. The contraction of the 

 tensor has been supposed to have a protective function and has been com- 

 pared to the sphincter pupillse (Helmholtz). Others hold that it modifies 

 the response to low and medium tones, but even here there is a divergence of 

 opinion, because while some hold (I think correctly) that the tensor by its 

 contraction decreases the natural period of the drum and thus enables it to 

 respond to rapid changes of phase and high tones, others have held the 

 opposite view. Observation shows that contraction occurs when sounds 

 (particularly tones of high pitch) fall on the drum, and that the contraction 

 is bilateral even if the stimulus be only unilateral. The reflex therefore 

 travels via the auditory nerve to the motor centre of the 5th nerve. 



Since the tensor tympani is uncontracted when no sounds are falling on 



