578 HANDBOOK OF PHYSIOLOGY. 



jects 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 

 intensity. 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 pro- 

 pagated 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 

 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. 



The long process of the malleus receives the undula- 

 tions of the membrana tympani (Fig. 389, a, a) and of 

 the air in a direction indicated by 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 (d) which is united to the incus at right angles. 

 The several changes in the direction of the chain of 

 FIG. 389. 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 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- 

 brana tympani. Miiller has confirmed this by experiments with small 

 membranes arranged so as to imitate the membrana tympani; and it may 

 be confirmed also by observations on one's self. 



The pharyngeal orifice of the Eustachian tube is usually shut; during 

 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 pres- 

 sure is produced in both ears the moment we swallow; this is due, doubt- 

 less, to the bulging out of the tympanic membrane by the compressed 

 air, which at that moment enters ths Eustachian tube. 



