THE SENSE OF HEARING 737 



the long process of the incus. Since this process is united at almost a 

 right angle to the stapes bone, the latter is forced toward and into the 

 foramen ovale, thus producing a pressure on the perilymph. With the 

 cessation of the pressure the elastic forces of the membrane and of the liga- 

 ments return the handle of the malleus to its former position; by the unlock- 

 ing of the malleo-incudal joint the entire chain also returns to its former posi- 

 tion without exerting undue traction on the basal attachment of the stapes. 



As the long process of the incus is shorter than the handle of the malleus, 

 and as the movement between them takes place around an axis from be fore 

 backward, it follows that the excursion of the incus and stapes will be less 

 than that of the malleus, while the force will be greater. Hence as the 

 vibrations are transferred from the tympanic membrane of large area to the 

 base of the stapes of small area (20 to 1.5), they lose in amplitude but in- 

 crease is force. Their pressure on the perilymph is therefore 13.3 times 

 greater than on the membrana tympani. In addition to its function as a 

 transmitter of vibrations, the chain of bones serves as a point of attachment 

 'for muscles which regulate the tension of the tympanic membrane and 

 the pressure on the labyrinth. 



The Function of the Stapedius Muscle. The function of thestapedius 

 muscle is a subject of much discussion. According to Henle, its function 

 is so to adjust the stapes bone that it will be prevented from exerting an undue 

 pressure'on the perilymph during the inward excursions of the incus process. 

 According to Toynbee, its function is to press the posterior part of the stapes 

 inward, make it a fixed point, and place the anterior part in such a position 

 that it will vibrate freely and accurately. 



The Function of the Eustachian Tube. In order that the tympanic 

 membrane may vibrate freely it is essential that the air pressure on both 

 sides shall be equal at all times. This is made possible by the Eustachian 

 tube. Were it not for this passageway, with each inward swing of the mem- 

 brane the air in the tympanic cavity would be condensed and its pressure 

 raised, in consequence of which the movement of the membrane would be 

 retarded; with each outward swing, the air would be rarefied and its pressure 

 lowered below that of the atmosphere, and in consequence the movement 

 outward would be retarded; the maximum response, therefore, of the mem- 

 brane to a given vibration could not be attained and the resulting sound 

 would be muffled and indistinct. But as with each vibration of the 

 membrane the air can pass into and out of the tympanum through this 

 partially closed tube, inequalities of pressure are prevented and a free 

 vibration permitted. 



The impairment in the acuteness of hearing which is caused by either 

 a rise or fall of pressure in the middle ear can be shown 



1. By closing the mouth and nose and then forcing air from the lungs 



through the Eustachian tube into the tympanum, thus increasing the 

 pressure. 



2. By closing the mouth and nose and then making an effort of deglutition. 



As this act is attended by an opening of the pharyngeal end of the 

 Eustachian tube, the air in the tympanum is partly withdrawn and the 

 pressure lowered. In each instance hearing is impaired. After 

 either experiment the normal condition is restored by swallowing with 

 the nasal passages open. 



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