NEURO-MUSCULAR MECHANISM 141 



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 unlocked. 



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 con- 

 densation 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 of the 

 membrane, and its paralysis diminishes the FlG - 69. Transverse 

 acuteness of hearing. ^ion . ^^ 



(Jiirtilagiuous lower 



The Eustaehian tube has a double function. pa rt of Eustachian 

 It allows the escape of mucus from the middle Tube to show the 

 ear, and it allows the entrance of air, so that Cartilaginous Arch 



^, . , , i , i i n cut across and the 



the pressure is kept equal on both sides ot way in which it ig 

 the membrana tympani. Its lower part is pulled down and the 

 generally closed, but opens in the act of tuheopenedinswal- 

 swallowing. It is surrounded by an arch of 

 cartilage, to one side of which fibres of the tensor palati are 

 attached, so that when this muscle acts in swallowing, the arch 

 of cartilage is drawn down and flattened, and the tube opened 

 up (fig. 69). 



When the Eustachian tube gets occluded, as a result of 

 catarrh of the pharynx, the oxygen in the middle ear is 

 absorbed by the tissues, and the pressure falls. As a result, 

 the membrane is driven inwards by the atmospheric pressure, 

 and does not readily vibrate, and hearing is impaired. 



4. Internal Ear 



The internal ear is a somewhat complex cavity in the petrous 

 part of the temporal bone, the osseous labyrinth. It is filled 

 with fluid, the perilymph. It consists of a central space, the 

 vestibule ( V.), into which the f enestra ovalis opens. From the 



