SENSE OF HEARING. 617 



control of the will to any extent, although some individuals can 

 produce a slight to-and-fro motion of the auricle. When the 

 hearing is imperfect the hand is sometimes applied to the ear in 

 such manner as to increase its projection from the side of the head 

 and to supplement it, so as to collect more sound-waves than 

 would otherwise enter the meatus. 



In order that the membrana tympani should respond to the 

 many tones and shades of tones which reach it, it is important that 

 the pressure upon the internal and external surfaces should be the 

 same, and this is effected by the passage of air through the Eus- 

 tachian tube, so that in going from an atmosphere of one density 

 into that of another the equilibrium is thus maintained. The 

 pharyngeal aperture of the tube, ordinarily closed, is opened by 

 the action of the tensor palati at each act of swallowing. 



When the membrana tympani moves inward, the manubrium 

 of the malleus moves inward also, and with it the incus, the 

 articulation between these two ossicles being such that in tjiis in- 

 ward movement they move as one. In speaking of this articula- 

 tion Helinholtz says : " In its action it may be compared with the 

 joints of the well-known Breguet watch-keys, which have rows of 

 interlocking teeth, offering scarcely any resistance to revolution in 

 one direction, but allowing no revolution whatever in the other/' 

 When the membrana tympani is forced strongly outward, the manu- 

 brium glides in the joint, and the incus follows it for but a short 

 distance ; if this was not so, in such movements of the membrana 

 tympani the stapes would be pulled out of the fenestra ovalis, and 

 severe, if not irretrievable, injury would result. This extreme 

 outward movement of the membrana tympani might result from 

 increased pressure within the tympanum or diminished pressure 

 in the external -auditory meatus. These movements of the drum- 

 membrane and the stapes are at most but limited ; the maximum 

 for the membrane being only from y 1 ^ to ^ mm., and of the stapes 

 from about ^ to ^ mm. The amplitude of movement of the 

 latter may be only Y^STF mm - 



Thus is accomplished the conversion of sound-waves into 

 waves of perilymph. Helmholtz says : " The mechanical problem 

 which the apparatus within the drum of the ear had to solve was 

 to transform a motion of great amplitude and little force, such as 

 impinges on the drum-skin, into a motion of small amplitude and 

 great force, such as had to be communicated to the fluid in the 

 labyrinth." A study of the ossicles (Fig. 396) shows that their 

 movement is around the axis of rotation, a-x in Fig. 397. If the 

 distance from this axis to the tip of the manubrium is measured, 

 it will be found to be one and one-half times that from the axis 

 to the end of the long process of the incus, with which the stapes 

 articulates, so that the amplitude of the movement of the stapes 



