382 ELEMENTARY PHYSIOLOGY LESa 



Thus, there is reason to believe that when the tym- 

 panic membrane is set vibrating, it causes the process of 

 the malleus, which is fixed to it, to swing at the same 

 rate ; the head of the malleus consequently turns through 

 a small arc on its pivot, the slender process. But, as 

 stated on p. 371, the turning of the head of the malleus 

 involves the simultaneous turning of the head of the 

 incus upon its pivot, the short process. In consequence 

 the long process of the incus also swings at the same 

 rate. The length of the long process of the incus, 

 measured from the axis, on which the two bones turn, 

 is less than that of the handle of the malleus ; hence the 

 end of it moves through a smaller space. The arc through 

 which it moves has been estimated as being equal to about 

 two-thirds of that described by the handle of the malleus. 

 The extent of the push is thereby somewhat diminished, 

 but the force of the push is proportionately increased ; 

 in so confined a space this change is advantageous. The 

 long process of the incus, however, is so fixed to the 

 stapes, and the stapes so attached to the membrane of 

 tlie fenestra ovalis, that the incus cannot vibrate without 

 throwing into vibrations, to a corresponding extent and 

 at the same rate, the membrane of the fenestra ovalis. 

 But every vibration, every pull and push, imparts a 

 corresponding set of shakes to the perilymph, which fills 

 the vestibule, the scala vestibuli and the scala tympani. 

 These shakes are communicated to the endolymph in tlie 

 canalis cochlearis, and, in some way, stimulate the delicate 

 endings of the cochlear division of the auditory nerve. 

 Probably the vibrations of the basilar membrane play an 

 important part. 



(x) Tlie Conversion of Sonorous Vibrations into 

 Sensations of Sound. — We do not at present know 

 what kind of changes the vibrations of the endolymph 

 give rise to in the cochlea ; nor do we at present know the 

 exact way in which the changes thus set up are able to 

 excite the terminal filaments of the auditory nerve. But 



