842 A MANUAL OF PHYSIOLOGY 



the sound-waves to the liquids of the labyrinth ; and obser- 

 vation and experiment fully confirm this idea. Tracings of 

 the movements of the ossicles have been obtained by attach- 

 ing very small levers to them, and their movements have 

 been directly observed with the microscope. Even in man 

 it may be shown, by viewing the membrane through a series 

 of slits in a rapidly-revolving disc (stroboscope), that it 

 vibrates when sound-waves fall on it. 



When the handle of the malleus moves inwards, the joint 

 between that bone and the incus is locked, on account of 

 the shape of the articular surfaces, and the stapes is pressed 

 into the oval foramen. When the tympanic membrane 

 passes outwards, the handle of the malleus and foot of the 

 stapes do the same. But the joint now unlocks, and exces- 

 sive outward movement of the stapes, which might result in 

 its being torn from its orbicular attachment, is prevented. 

 The ossicles vibrate en masse. It is only to a trifling 

 extent that sound 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 mem- 

 brane closing the fenestra rotunda, and set up oscillations in 

 the perilymph of the scala tympani. 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 conditions by far 

 the most important part of the conduction takes place via 

 the ossicles. And when it is remembered that the tympanic 

 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 transmission by the ossicles is a very advantageous 

 method of transforming the feeble but comparatively large 

 excursion of the tympanic membrane into the smaller but 



