586 THE SENSE OF HEARING. 



in the tympanum. Equally necessary is the communica- 

 tion of the air in the tympanum with the external air, 

 through the medium of the Eustachian tube, for the main- 

 tenance of the equilibrium of pressure and temperature 

 between them. 



The propagation of sound through the ossicula of the 

 tympanum to the labyrinth must be effected either by 

 oscillations of the bones, or by a kind of molecular vibra- 

 tion of their particles, or, most probably, by both these 

 kinds of motion. * 



The long process of the malleus receives the undulations 

 of the membrana tympani (fig. 20 1 , 

 a, a) and of the air in a direction 

 indicated by the arrows, nearly per- 

 pendicular to itself. From the long 

 process of the malleus they are 

 propagated to its head (I) ; thence 

 into the incus (c), the long proceses 

 of which is parallel with the long 

 process of the malleus. From the 

 long process of the incus the undu- 

 lations 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 undu- 

 lations, which -remains the same as it was in the meatus 

 externus and long process of the malleus, so that the undu- 

 lations are communicated by the stapes to the fenestra 

 ovalis in a perpendicular direction. 



* Edouard Weber lias shown that the existence of the membrane over 

 the fenestra rotunda will permit approximation and removal of the stapes 

 to and from the labyrinth. When by the stapes the membrane of the 

 fenestra ovalis is pressed towards the labyrinth, the membrane of the 

 fenestra rotunda may, by the pressure communicated through the fluid 

 of the labyrinth, be pressed towards the cavity of the tympanum. 



