688 THE SENSE OF HEARING. 



Increasing tension of the membrana tympani diminishes 

 the facility of transition of sonorous undulations from the 

 air to it. Mr. Savart observed that the dry membrana 

 tympani, on the approach of a body emitting a loud sound, 

 rejected particles of sand strewn upon it more strongly 

 when lax than when very tense ; and inferred, therefore, 

 that hearing is rendered less acute by increasing the ten- 

 sion of the membrana tympani. Miiller has confirmed 

 this by experiments with small membranes arranged so 

 as to imitate the membrana tympani ; and it may be con- 

 firmed also by observations on one's self. For the mem- 

 brana tympani on one's own person may be rendered tense 

 at will in two ways, namely, by a strong and continued 

 effort of expiration or of inspiration -while the mouth and 

 nostrils are closed. In the first case, the compressed air 

 is forced with a whizzing sound into the tympanum, the 

 membrana tympani is made tense, and immediately hear- 

 ing becomes indistinct. The same temporary imperfection 

 of hearing is produced by rendering the membrana tym- 

 pani tense, and convex towards the interior, by the effort 

 of inspiration. The imperfection of hearing, produced by 

 the last-mentioned method, may continue for a time even 

 after the mouth is opened, in consequence of the previous 

 effort at inspiration having induced collapse of the walls 

 of the Eustachian tubes, which prevents the restoration of 

 equilibrium of pressure between the air within the tym- 

 panum and that without : hence we have the opportunity 

 of observing that even our own voice is heard with less 

 intensity when the tension of the membrana tympani is 

 great. 



If the pressure of the external air or atmosphere be 

 very great, while on account of collapse of the walls of 

 the Eustachian tube, the air in the interior of the tym- 

 panum fails to exert an equal counter pressure, the mem- 

 brana tympani will of course be forced inwards, and 

 imperfect deafness be produced. Thus it may be explained 



