THE SENSES 131 



is attached to the margin of the hole by a membrane, the 

 latter by a membrane. The posterior wall shows openings 

 into the mastoid cells and presents a small bony projection 

 which transmits the stapedius muscle. The anterior wall 

 has above a bony canal carrying the tensor tympani muscle, 

 and below this the canal of the Eustachian tube which com- 

 municates with the posterior nares (Fig. 68, EnT.). 



In the tympanic cavity are three ossicles the malleus (m.), 

 incus (i.), and stapes (s.), forming a chain between the mem- 

 brana tympani and the fenestra ovalis. The handle of the 

 malleus is attached to the membrana tympani, and each time 

 a wave of condensation hits the membrane, it drives in the 

 handle of the malleus. This, by a small process, pushes 

 inwards the long process of the incus which thrusts the stapes 

 into the fenestra ovalis, and thus increases the pressure in the 

 enclosed fluid of the internal ear. The fenestra rotunda with 

 its membrane acts as a safety valve. The bones rotate 

 round an antero-posterior axis passing through the heads of 

 the malleus and incus. They thus form a lever with the 

 arm to which the power is applied the handle of the malleus 

 longer than the other arm. The advantage of this is that, 

 while the range of movement of the stapes in the fenestra 

 ovalis is reduced, its force is proportionately increased. 



The range of movement is still further controlled by the 

 stapedius muscle which twists the stapes in the fenestra. 

 This muscle seems to act when loud sounds fall on the ear, 

 and when its nerve supply, derived from the facial nerve, is 

 paralysed, such sounds are heard with painful intensity. 



If the membrana tympani is violently forced outwards by 

 closing the nose and mouth and forcing air up the Eustachian 

 tube, the incus and stapes do not accompany the malleus and 

 membrane, since the malleo-incal articulation becomes un- 

 locked. 



The membrana tympani is so loosely slung that it has no 

 proper note of its own, and responds to a very large range 

 of vibrations. By the attachment to it of the handle of the 

 malleus it is well damped, and stops vibrating as soon as 

 waves of condensation and rarefaction have ceased to fall 

 upon it. The tensor tympani muscle, supplied by the 

 fifth cranial nerve, has some action in favouring the vibration 



