688 HANDBOOK OF PHYSIOLOGY. 



the arrows, nearly perpendicular to itself. From the long process of 

 the malleus they are propagated to its head (b) : thence into the incus (c), 

 the long process of which is parallel with the long process of the malleus. 

 From the long process of the incus the undulations are communicated 

 to the stapes (<^), which is united to the incus at right angles. The 

 several changes in the direction of the chain of bones hare, however, no 

 influence in changing the character of the undulations, which remain the 

 same as in the meatus externus. From the long process of the malleus, 

 the undulations are communicated by the stapes to the fenestra ovalis 

 in a perpendicular direction. 



Increasing tension of the rnembrana tympani diminishes the facility 

 of transmission of sonorous undulations from the air to it. 



The dry mernbrana tympani, on the approach of a body emits a loud 

 sound, rejects particles of sand strewn upon it more strongly when lax 

 than when very tense; and it has been inferred, therefore, that hearing 

 is rendered less acute by increasing the tension of the membrana tym- 

 pani. 



The pharyngeal orifice of the Eustachian tube is usually shut ; dur- 

 ing swallowing, however, it is opened; this may be shown as follows: 

 If the nose and mouth be closed and the cheeks blown out, a sense of 

 pressure is produced in both ears the moment we swallow; this is due, 

 doubtless, to the bulging out of the tympanic membrane by the com- 

 pressed air, which at that moment enters the Eustachian tube. 



Similarly the tympanic membrane may be 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. 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. 



The principal office of the Eustachian tube has relation to the pre- 

 vention of these effects of increased tension of the membrana tympani. 

 Its existence and openness will provide for the maintenance of the equi- 

 librium 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 the apertures in violins do; to supply the tympanum with 

 air; and to bean outlet for mucus. If the 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 prop- 

 agated up the tube to the tympanum by means of a catheter inserted 

 into the pharyngeal orifice of the Eustachian tube. 



