: 



CHAP, iv.] % HEARING. 1331 



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 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 ex- 

 ceedingly 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 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 movements 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, 

 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 eridolymph 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 



F. 85 



