992 CONDUCTION THROUGH TYMPANUM. [BOOK in. 



thrown into vibrations by a sound, the curves described by the 

 style indicate that the chain of bones moves with every vibra- 

 tion of the membrane. On the other hand, the comparatively 

 loose attachments of the several ossicles is an obstacle to the 

 molecular transmission of sonorous vibrations through them. 

 Moreover, sonorous vibrations can only be transmitted to or pass 

 along such bodies as either are very long compared to the length 

 of the sound-waves, or, as in the case of membranes and strings, 

 have one dimension very much smaller than the others. Now 

 the bones in question are not only not especially thin in any 

 one dimension, but are in all their dimensions exceedingly small 

 compared with the wave-lengths of the vibrations of even the 

 shrillest sounds we are capable of hearing ; hence they must be 

 useless for the molecular propagation of vibrations. We may 

 conclude then that when waves of sound throw the tympanic 

 membrane into vibrations, each inward excursion of the mem- 

 brane is followed by a corresponding impulse given by the foot 

 of the stapes to the perilymph. As we have seen the space 

 through which the end of the incus moves is less than that 

 through which the handle of the malleus moves, and the move- 

 ments of the stapes are in addition restricted by the manner of 

 its attachment to the rim of the fenestra ovalis ; but the energy 

 with which the end of the incus and hence the stapes moves is 

 proportionately increased, so that we might almost speak of the 

 gentle swingings of the tympanic membrane being converted 

 into smart taps on the perilymph of the labyrinth. 



The impulses thus given to the perilymph at the fenestra 

 ovalis travel along the intricate passages of the perilymph 

 spaces, and finally break upon the fenestra rotunda; if the 

 membrane which closes this orifice be watched it may be ob- 

 served to pulsate in sequence with the pulsations of the fenestra 

 ovalis. During their passage these impulses are communicated 

 to the endolymph and in some way or other affect the endings 

 of the auditory nerve. How they do this we shall presently 

 study; but we may here call attention to the fact that the 

 waves of sound which fall on the tympanic membrane are for 

 the most part not simple in character but complex, and in many 

 cases exceedingly so. This complexity is carried on into the 

 vibrations of the tympanic membrane and so into the impulses 

 given to the perilymph; the waves which sweep past the end- 

 ings of the auditory nerve are, so to speak, reproductions of the 

 complex aerial waves passing down the meatus. 



616. By far the greater number of sounds which we hear 

 reach the tympanic membrane by passing through the air down 

 the meatus. One great use of the long external passage is prob- 

 ably to protect the delicate tympanic membrane from the acci- 

 dents to which it would be subject were it freely exposed on the 

 surface of the body ; but it has also a use in transmitting to the 



