1 156 THE EAR. 



structure and mode of action. The posterior surface of the rounded 

 head of the hammer fits into the somewhat saddle-shaped concavity of 

 the anterior surface of the anvil. At the lower margin of the joint 

 the prominent edge of the malleus fits on the lower edge of the 

 articular surface of the incus in such a way, that with every move- 

 ment of the edge of the malleus inwards the incus is carried inwards 

 at the same time ; whereas, when the malleus moves outwards, the 

 incus behind is not compelled to follow the malleus to the full extent 

 of its movements. 



The short process of the incus fits into a slight depression on the 

 posterior wall of the tympanic cavity, close to the entrance to the 

 mastoid cells, where it is fixed by a ligament. The tip of the short 

 process is covered with cartilage, and a certain degree of movement 

 is possible at the point of contact with the temporal bone. 



The remaining connection of the incus is that of its long process 

 with the stapes. This presents the usual features of a joint. Its 

 capsule contains a considerable amount of elastic fibre, and it is some- 

 what firmer and stronger on its inferior aspect. The movements of the 

 incus, under the influence of sound-waves, are similar to those of the 

 malleus, but less in extent. The bone rotates round an axis, which 

 passes from the tip of the short process through its body, and which, if 

 continued, would pass through the neck of the malleus, and just above 

 the cog-like edge of the articular surface. Thus, as has already been 

 pointed out, when the handle of the malleus moves inwards, the greater 

 part of the body of the incus and of the head of the malleus move 

 outwards together, and the long process of the incus moves inwards. 

 The result of this lever-like mechanism is that, when the membrana 

 tympani and the handle of the malleus move inwards, the base of the 

 stapes is pushed into the oval window. 



The movements of the stapes. — This bone is fixed in the fenestra 

 ovalis by the annular ligament. It is placed almost at a right angle 

 to the long process of the incus, and one would at first sight imagine 

 that it must slightly follow the to-and-fro movements of that process. 

 It does not, however, move exactly in this way, because the fibres of the 

 annular ligament are more tense below than above the foot-plate of the 

 bone, and hence movement is more free along the upper edge of the foot- 

 plate than along the lower edge. 



Before we discuss the movements of the ossicles, as constituting a 

 chain or lever passing from the membrana tympani to the fenestra 

 ovalis, it is necessary to describe the action of the intrinsic muscles of 

 the ear. 



The tensor tympani. — When this muscle contracts, the membrana 

 tympani becomes more tense. A careful consideration of the exact 

 position of its insertion into the bone shows that the tendon is attached 

 to the anterior surface of the inner edge of the manubrium, and not to 

 its posterior surface ; contraction of the muscle must tend to rotate the 

 malleus on its vertical axis. Further, the point of insertion is much 

 nearer the axis of rotation than the attachment of the radial fibres of 

 the tympanic membrane. Thus the mechanical arrangement is that 

 of a lever of the third order, in which the power (contracting force of 

 the muscle) is placed at a short distance from the fulcrum, while the 

 weight (resistance or tension of the fibres of the membrane) is at a 

 considerable distance. Hence the force with which the muscle renders 



