THE SENSES. 579 



Similarly the tympanic membrane maybe pressed in by rarefying the 

 air in the tympanum. This can be readily accomplished by closing the 

 mouth and nose, and making an inspiratory effort and at the same time 

 swallowing (Yalsalva). In both cases the sense of hearing is temporarily 

 dulled; proving that equality of pressure on both sides of the tympanic 

 membrane is necessary for its full efficiency. 



Functions of Eustachian Tube. The principal office of the Eus- 

 tachian tube, in Muller'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 discharging 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. If the Eustachian tube were permanently open, 

 the sound of one's own voice would probably be greatly intensified, a 

 condition which would of course interfere with the perception of other 

 sounds. At any rate, it is certain that sonorous vibrations can be propa- 

 gated up the Eustachian tube to the tympanum by means of a tube in- 

 serted into the pharyngeal orifice of the Eustachian tube. 



Action of the Tensor Tympani. The influence of the tensor tym- 

 pani muscle in modifying hearing may also be probably explained in con- 

 nection 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 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 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. 



Action of the Stapedius. 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. It pre- 

 Tents too great a movement of the bone. 



Functions of the Fluid of the Labyrinth. The fluid of the laby- 

 rinth is the most general and constant of the acoustic provisions of the 

 labyrinth. In all forms of organs of hearing, the sonorous vibrations 

 affect the auditory nerve through the medium of liquid the most con- 

 venient medium, on many accounts, for such a purpose. 



The crystalline pulverulent masses (otoliths) in the labyrinth would 

 reinforce the sonorous vibrations by their resonance, even if they did not 



