688 THE SENSE OF HEARING. 



why, in a diving-bell, voices sound faintly. In all cases, 

 the effect of the increased tension of the membrana tym- 

 pani 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 membrana tympani, 

 produced by exhausting the cavity of the tympanum, makes 

 one deaf to grave sounds only. 



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

 opinion, has relation to the prevention of these effects of 

 increased 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 dis- 

 charging this office, however, it will serve to render sounds 

 clearer, as (Henle suggests) the apertures 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 tym- 

 pani. If, through reflex nervous action, it can be excited 

 to contraction by a very loud sound, just as the iris and 

 orbicularis palpebrarum 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 favour of this supposition we 

 have the fact that a loud sound excites, by reflexion, 

 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 



