960 A MANUAL OF PHYSIOLOGY 



tuting the second portion of the bent lever, passes inwards, 

 carrying with it the stapes, which is attached to it by an almost 

 rigid joint, and the stapes is pressed into the oval foramen. 

 Since the long process of the incus is about one-third shorter 

 than the handle of the malleus, the excursion of the point of the 

 former is correspondingly smaller than that of the latter, but at 

 the same time more powerful. 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 excessive 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 con- 

 ducted 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 perilymph of 

 the scala tympani. That this is a possible method of conduc- 

 tion 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 excursions of; the tympanic m'embrane 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 tympanic 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 



