386 



THE SPECIAL SENSES. 



tion of this axis is represented by the Hne a-h in Fig. 171. This hne 

 passes through the neck of the malleus ; so that as the handle moves 

 in the head of the malleus and the upper part of the incus move in 

 the opposite direction, — while the long process of the incus together 

 with the stapes, being below the axis, moves in the same direction 

 as the handle (see Fig. 171^). The chain of bones, therefore, acts 

 like a bent lever whose fulcrum is at a, the power arm being repre- 



Fig. 171.— To illustrate the lever 

 action of the ear bones (McKendrick) : 

 M, The malleus; e, the incus; a-h, the 

 axis of rotation: a, short process of 

 incus abutting against the tympanic 

 wall ; a-p, the power arm ; a-r, the 

 load arm of the lever. 



Fig. 171 A. — Schema to illustrate the 

 way in which the ear ossicles act to- 

 gether as a bent lever in transmitting the 

 movements of the tympanic membrane 

 to the membrane of the fenestra ovalis. 

 1, The handle of the malleus; 2, the long- 

 process of the incus; 3, the stapes ; a-b, 

 the axis of rotation. The airows indi- 

 cate a movement inward of the tympanic 

 membrane. 



sented by the line p-a and the load arm by the line r-a. According 

 to Helmholtz,* the distance p-a is equal to 9.5 mms., while r-a 

 is 6.3 mms. The movement at r, therefore, or the movement 

 of the stapes, will have only two-thirds of the amplitude of the 

 movement at p, but will have a correspondingly greater force 

 (one and one-half times). The mechanisms of the tympanic 

 membrane and the ear bones combine, therefore, to convert the 

 vibratory movements of the tympanic membrane into smaller 

 but more intense movements of the membrane of the fenestra 

 ovalis. It should be borne in mind, however, that the amplitude 

 of these movements under normal conditions is very minute. 

 That of the base of the stirrup is estimated at about 0.04 

 mm., while the amplitude at the tip of the manubrium, though 

 relatively much larger, is still less than a millimeter (0.2 to 

 0.7 mm.). The minute but relatively intense movements of the 

 stapes set into vibration the perilymph in the internal ear, and 

 through these movements the sensory nerve cells in the cochlea 



* Helmholtz, "Die Lehre von den Tonempfindungen, etc.," fifth edition, 

 1896. See also EngUsh translation by Ellis. 



