HEARING 1031 



Can be conducted through them to the labyrinth as a molecular 

 vibration; for when they are anchylosed, and the foot of the stapes 

 fixed immovably in the foramen ovale, as sometimes occurs in 

 disease, hearing is greatly impaired. 



Of course, every vibration of the tympanic membrane must cause 

 a corresponding condensation and rarefaction of the air in the 

 middle ear; and this may act on the membrane closing the fenestra 

 rotunda, and set up oscillations in the pcrilymph of the scala tym- 

 pani. That this is a possible method of conduction of sound is 

 shown by the fact that, even after closure of the oval foramen, a 

 slight power of hearing may remain. But under ordinary con- 

 ditions by far the most important part of the conduction takes 

 place via the ossicles. And when it is remembered that the tym- 

 panic membrane is about thirty times larger than that which fills 

 the oval foramen, it will be seen that the force acting on unit area 

 of the foot of the stapes may be much greater than that acting on 

 unit area of the membrana tympani, and that the mode of trans- 

 mission by the ossicles is a very advantageous method of trans- 

 forming the feeble but comparatively large excursions of the tym- 

 panic membrane into the smaller but more powerful movements of 

 the stapes. The average excursion of the membrane of the oval 

 foramen does not at most amount to more than 0-04 millimetre. 

 Even the so-called cranial conduction of sound when a tuning-fork 

 is held between the teeth or put in contact with the head, which 

 was at one time supposed to be due solely to direct transmission 

 of the vibrations through the bones of the skull to the liquids of 

 the labyrinth or the end-organs of the auditory nerve, has been 

 shown to take place, in great part, through the membrana tympani 

 and ossicles; the vibrations travel through the bones to the tym- 

 panic membrane, and set it oscillating. So that this test, when 

 applied to distinguish deafness caused by disease of the middle ear 

 from deafness due to disease of the labyrinth or the central nervous 

 system, may easily mislead, although it enables us to say whether 

 the auditory meatus is blocked by wax, e.g. beyond the tym- 

 panic membrane. 



A membrane like a drum-head has a note of its own, which it gives 

 out when struck, and it vibrates more readily to this note than to any 

 other. It would evidently be a serious disadvantage if the tympanic 

 membrane, whose office it is to receive all kinds of vibrations, and 

 respond to all, had a marked fundamental tone which would be con- 

 tinually obtruding itself among other notes. The difficulty is obviated 

 by the damping action of the ossicles and the liquids of the labyrinth 

 on the movements of the membrane, which in addition is not stretched, 

 but lies slackly in its bony frame, so that when the handle of the malleus 

 is detached from it, it retains its shape and position. 



The tensor tympani, when it contracts, pulls inwards the handle of 

 the malleus, and thus increases the tension of the tympanic membrane. 

 The precise object of this is obscure. It has been suggested that damp- 



