FUNCTIONS OF THE MIDDLE EAR. 539 



The long process of the malleus receives the undulations of 

 the membrana tympani (Fig. 201, a, a) and of 

 the air in a direction indicated by the arrows, FlG - 201 - 



nearly perpendicular to itself. From the 

 long process of the malleus they are propa- 

 gated to its head (6) ; thence into the incus 

 (e), 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 (d), which is 

 united to the incus at right angles. The 

 several changes in the direction of the chain 

 of bones have, however, no influence on that 

 of the undulations, which remains the same 

 as it was in the meatus externus and long 

 process of the malleus, so that the undula- 

 tions are communicated by the stapes to the 

 fenestra ovalis in a perpendicular direction. 



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 tension of the membrana tympani. 

 Mu'ller has confirmed this by experiments with small mem- 

 branes arranged so as to imitate the membrana tympani ; and 

 it may be confirmed also by observations on one's self. For 

 the membrana 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 nos- 

 trils are closed. In the first case, the compressed air is forced 

 with a whizzing sound into the tympanum, the membrana tym- 

 pani is made tense, and immediately hearing becomes indis- 

 tinct. The same temporary imperfection of hearing is pro- 

 duced by rendering the membrana tympani tense, and convex 

 towards the interior, by the effort of inspiration. The imper- 

 fection 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 in- 

 duced collapse of the walls of the Eustachian tube, which pre- 

 vents the restoration of equilibrium of pressure between the air 

 within the tympanum 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. 



