CHAP, iv.] HEARING. 193 



the stapes is made to write on a travelling surface ; when the 

 tympanic membrane is thrown into vibrations by a sound, the 

 curves described by the style indicate that the chain of bones 

 moves with every vibration 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 trans= 

 mitted to or pass along such bodies as either are very long com- 

 pared to the length of the sound-waves, or, as in the casa 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 dimen- 

 sions 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 vibra- 

 tions. We may conclude then that when waves of sound throw 

 the tympanic membrane into vibrations, each inward excursion of 

 the membrane 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 propor- 

 tionately 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, 

 the details of which we shall presently study, and finally break 

 upon the fenestra rotunda; if the membrane which closes this 

 orifice be watched it may be observed 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 mem- 

 brane 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 endings of the auditory nerve are, so to speak, reproductions 

 of the complex aerial waves passing down the meatus. 



818. By far the greater number of the 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 probably 

 to protect the delicate tympanic membrane from the accidents to 

 which it would be subject were it freely exposed on the surface of 



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