496 



HEARING, VOICE AND SPEECH 



When the tympanic membrane moves in and out in response to the at- 

 mospheric vibrations the handle of the hammer naturally moves with 

 it; but the head of the hammer moves in the opposite direction. Too 

 great an excursion of the handle outward is prevented by the external 

 ligament. 



The incus articulates with the head of the malleus by means of a peculiar 

 saddle-shaped joint, the physiological significance of which has been pointed 

 out by Helmholtz. This joint is provided with ratchet teeth, which, as will 

 be evident from inspection of Fig. 198, engage each other in such a way that 

 the incus is carried along with every movement of the manubrium inward, 

 while they are disengaged when the manubrium moves outward. In this way 



the danger of tearing the stapes from its 

 fastening in the foramen ovalis when the 

 tympanic membrane is for any reason pushed 

 outward, is diminished. Helmholtz estimates 

 that the hammer can be rotated five degrees 

 outward without carrying the anvil with it. 

 If we imagine the hammer and anvil 

 locked together by their ratchet teeth so that 

 the two move inward as one solid body, the 

 system formed by the two ossicles can be 

 regarded as a one-armed lever, the fulcrum 

 of which lies where the apex of the short 

 process of the anvil is supported against the 

 wall of the tympanic cavity. The tip of 

 the manubrium constitutes the point where 

 the power is applied, the apex of the long 

 process of the anvil the point where the load 

 i. e., the stapes is acted upon. These 

 three points lie almost exactly in a straight 

 line, the joint between the stapes and anvil 

 lying only a little inside the line joining 



the other two. The lever a a (Fig. 198) is about 9.5 mm. long, the short arm 

 between the two apices of the anvil being about 6.3 mm. i. e., just two-thirds 

 of the long arm. 



It follows that when the hammer and anvil are firmly engaged, the excur- 

 sion of the incus-stapes joint is only two-thirds that of the apex of the manu- 

 brium, but the pressure which the stapes exerts on the oval window is one 

 and one-half times as great as that which acts upon the apex of the manu- 

 brium (Helmholtz). 



The top of the stapes is attached by a strong ligament to the long process 

 of the incus, and its base is fastened into the fenestra ovalis by means of a 

 thin membrane. The stapes must accompany all the movements of the long 

 process of the incus, so that when the tympanic membrane moves inward, the 

 base of the stapes is pressed into the labyrinth. At the same time there takes 

 place a slight rotation of the stapes around the long axis of its base. A limit 

 is set to the movement of the base inward by the resistance of the membrane 

 holding it in the oval window. 



FIG. 198. Hammer and anvil, after 

 Helmholtz. PE, processus Folia- 

 nus ; Tt, tensor tympani ; 6, ratchet 

 tooth of the anvil. 



