NERVE 169 



If the membrana tjmpani is forced violently outwards by 

 closing the nose and mouth and forcing air up the Eustachian 

 tube, as in one clinical method of overcoming obstruction of 

 the Eustachian tube, the incus and stapes do not accompany 

 the malleus and the membrane, because the malleo-incal 

 articulation becomes unlocked, and the head of the malleus 

 slides on the incus. 



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. 



Two slender muscles are attached to the ossicles — 



(1) The stapedius is carried from the posterior wall of the 

 cavity and is inserted into the orbicular process of the stapes. 

 It tends to twist the stapes in the oval window and so to 

 limit its range of movement inwards and to favour its out- 

 ward movement. It is supplied by the seventh cranial nerve, 

 and in paralysis of this loud sounds may be heard with 

 painful intensity. 



(2) The tensor tympani passes from the inner walls of the 

 cavity outwards to the base of the handles of the malleus. It 

 tends to pull the tympanic membrane inwards and to thrust 

 the head of the stapes further into the oval window. It is 

 supplied by the fifth cranial nerve, and when this is paralysed 

 hearing is impaired. 



These two muscles are thus antagonistic to one another 

 and, as in the case of other antagonists, their action is prob- 

 ably co-ordinated. They must exercise a balancing action 

 on the movement of the stapes inwards during the positive 

 phase of a sound wave and outwards during the succeeding 

 negative phase. 



The Eustachian tube has a double function. It allows the 

 escape of mucus from the middle ear, and it allows the 

 entrance of air, so that the pressure is kept equal on both 

 sides of the membrana tympani. Its lower part is generally 

 closed, but opens in the act of swallowing. This part is 

 surrounded by an arch of cartilage to one side of which fibres 



