540 THE SENSE OF HEARING. 



If the pressure of the external air or atmosphere be very 

 great, while on account of collapse of the walls of the Eusta- 

 chian tube, the air in the interior of the tympanum fails to 

 exert an equal counter-pressure, the membrana tympani will 

 of course be forced inwards, and imperfect deafness be pro- 

 duced. Thus it may be explained why, in a diving-bell, voices 

 sound faintly. In all cases, the effect of the increased tension 

 of the membrana tympani is not to render both grave and 

 acute sounds equally fainter than before. On the contrary, as 

 observed by Dr. Wollaston, the increased tension of the mem- 

 brana tympani, produced by exhausting the cavity of the 

 tympanum, makes one deaf to grave sounds only. 



The principal office of the Eustachian tube, in Muller's 

 opinion, has relation to the prevention of these effects of in- 

 creased tension of the membrana tympani. Its existence and 

 openness will provide for the maintenance of the equilibrium 

 between the air within the tympanum and the external air, so 

 as to prevent the inordinate tension of the membrana tympani 

 which would be produced by too great or too little pressure on 

 either side. While discharging this office, however, it will 

 serve to render sounds clearer, as (Henle suggests) the aper- 

 tures in violins do ; to supply the tympanum with air ; and to 

 be an outlet for mucus : and the ill effects of its obstruction 

 may be referred to the hindrance of all these its offices, as well 

 as of that one ascribed to it as its principal use. 



The influence of the tensor tympani muscle in modifying 

 hearing may also be probably explained in connection with 

 the regulation of the tension of the membrana tympani. If, 

 through reflex nervous action, it can be excited to contraction 

 by a very loud sound, just as the iris and orbicularis palpe- 

 brarum muscle are by a very intense light, then it is manifest 

 that a very intense sound would, through the action of this 

 muscle, induce a deafening or muffling of the ears. In favor 

 of this supposition we have the fact that a loud sound excites, 

 by reflection, nervous action, winking of the eyelids, and, in 

 persons of irritable nervous system, a sudden contraction of 

 many muscles. 



The influence of the stapedius muscle in hearing is unknown. 

 It acts upon the stapes in such a manner as to make it rest 

 obliquely in the fenestra ovalis, depressing that side of it on 

 which it acts, and elevating the other side to the same extent. 



When the fenestra ovalis and fenestra rotunda exist together 

 with a tympanum, the sound is transmitted to the fluid of the 

 internal ear in two ways, namely, by solid bodies and by 

 membrane ; by both of which conducting media sonorous vibra- 

 tions are communicated to water with considerable intensity. 



