CHAP, iv.] HEARING. 991 



tin-own into vibration much more readily by a particular sound 

 than by any others, that sound would be dominant in all our 

 hearing; and unless, as in vision, psychical experience inter- 

 vened to correct the mere sensation, we should be misled in our 

 judgments as to what was taking place around us. 



This general usefulness of the tympanic membrane is secured 

 partly by features proper to itself, partly by the fact that it is 

 'damped' by the attachment to it of the chain of ossicles. 

 Without attempting to enter into a discussion of a matter which 

 is in many ways complex, we may say that the following feat- 

 ures contribute to make the tympanic membrane sensitive to a 

 large variety of sounds. In the first place its dimensions are 

 relatively small. In the second place the material of which it 

 is composed is peculiar, so that it is in a special way unyielding 

 and rigid; it retains its form when cut away from its bony 

 attachments by a circular incision, and the malleus, including 

 its handle, may be removed from it without distorting it. In 

 the third place, its remarkable form, that of a shallow funnel 

 with sides gently convex towards the ineatus, has a marked 

 effect upon its capabilities of vibration. The chain of ossicles, 

 attached at its far end, to the membrane of the fenestra ovalis 

 has a ' damping ' effect, similar to that, familiar to eveiy one, of 

 lessening or stopping the sound of a vibrating empty wine-glass 

 or tumbler by pressing the finger on it, and this 'damping' 

 while it diminishes to a certain extent all the vibrations of the 

 membrane is especially effective in the case of excessive vibra- 

 tions, such as those which might be produced by the sound 

 which is the fundamental note of the membrane. 



615. The vibrations thus set going in the tympanic mem- 

 brane are transmitted from it to the chain of Ossicles. The 

 transmission might take place in two ways. In the first place 

 the vibrations, the alternate bendings inwards and outwards of 

 the membrane, might, by carrying with it the attached handle 

 of the malleus, work the chain of ossicles as a lever, in the 

 manner described in 613, so that each inward flexion of the 

 tympanic membrane led to the membrane of the fenestra ovalis 

 pushing the perilymph of the labyrinth inwards, while the return 

 outwards again of the one led to a like retuni of the other. 

 In the second place the transversal vibrations of the tympanic 

 membrane might set up longitudinal vibrations in the substance 

 of the malleus, which would travel as longitudinal vibrations 

 through the chain, and so reach the perilymph. In the one 

 case the whole chain of ossicles swings to and fro, in the other 

 case the sound is propagated by molecular movement. That 

 the ossicles do move en masse has been proved by recording 

 their movements in the usual graphic method. A very light 

 style attached to the end of the incus or to the stapes is made to 

 write on a travelling surface ; when the tympanic membrane is 



