768 THE SENSE OF HEARING 



sented in the adjoining diagram (Fig. 385). When combined into one 

 single mass, these bones act upon the axis a-b, the manubrium c 

 and stapes d then pursuing precisely the same course, inward as well as 

 outward (Fig. 386). This system is rendered especially sensitive by 

 the fact that a large part of the total mass of the malleus and incus 

 comes to lie above their axis of rotation a-b, so that their upper por- 

 tions are made to act as a counterpoise for the parts situated below 

 this axis. The latter constitute the real lever, sensitized, as has just 

 been stated, by this counterpoising weight. It should be noted, how- 

 ever, that the oscillations of the stapes possess a smaller amplitude 

 than those of the eardrum, the relationship between them being as 



FIG. 385. FIG. 386. 



FIG. 385. To ILLUSTRATE THE LEVER ACTION OF THE EAR BONES. 

 M, the malleus; e, the incus; a-b, the axis of rotation; a, short process of incus abut- 

 ting against the tympanic wall; a-p, the power arm; a-r, the load arm of the lever. 



(McKendrick.) 



FIG. 386. SCHEMA TO ILLUSTRATE THE WAY IN WHICH THE EAR OSSICLES ACT TO- 

 GETHER AS A BENT LEVER IN TRANSMITTING THE MOVEMENTS or 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; 0-6, the 

 axis of rotation. The arrows indicate a movement inward of the tympanic membrane. 

 (Howett.) 



0.04 mm. to 0.4 mm. The force, however, with which they strike 

 against the fenestra ovalis, is increased in the proportion of 2 to 3, 

 because the length of the arms of the lever formed, on the one hand, by 

 the manubrium, and, on the other, by the long process of the incus, is 

 as 3 to 2. Furthermore, since the area of the eardrum is about twenty 

 times as large as that of the membrane closing the foramen ovale, 

 the initial energy is concentrated in this way upon an area twenty 

 times smaller than that exposed to the sound waves. Consequently, 

 the force of these waves is augmented % X 20 = 30 times, when pro- 

 jected against the fenestra ovalis. 



It is also of importance to remember that this system is not given 

 to after-vibration, because it is made to act under a considerable 



