THE SENSES. 189 



dispersed by the solid walls of the tube. The vibrations of the mem- 

 brana tympani are transmitted, therefore, by the chain of ossicula to the 

 fenestra ovalis and fluid of the labyrinth, their dispersion in the tym- 

 panum being prevented by the difficulty of the transition of vibrations 

 from solid to gaseous bodies. 



The necessity of the presence of air on the inner side of the membrana 

 tympani, in order to enable it and the ossicula auditus to fulfil the objects 

 just described, is obvious. Without this provision, neither would the 

 vibrations of the membrane be free, nor the chain of bones isolated, so as 

 to propagate the sonorous undulations with concentration of their inten- 

 sity. But while the oscillations of the membrana tympani are readily 

 communicated to the air in the cavity of the tympanum, those of the solid 

 ossicula will not be conducted away by the air, but will be propagated 

 to the labyrinth without being dispersed in the tympanum. 



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 vibration of their particles, or, most probably, by both 

 these kinds of motion. 



Movements of the ossicula. E. Weber has 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 toward the 

 labyrinth, the membrane of the fenestra rotunda may, by 

 the pressure communicated through the fluid of the laby- 

 rinth, be pressed toward the cavity of the tympanum. 



The long process of the malleus receives the undula- 

 tions of the membrana tympani (Fig. 364, 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 (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 (d), which is united to the incus at right angles. 

 The several changes in the direction of the chain of 

 bones ha^ve, however, no influence on that of the undu- Flo 364 



lations, which remain the same as it was in the meatus 

 externus and long process of the malleus, so that the undulations are 

 communicated by the stapes to the fenestra ovalis in a perpendicular 

 direction. 



Increasing tension of the membrana tympani diminishes the facility 

 of transmission of sonorous undulations from the air to it. 



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 mem- 



