FUNCTION OF THE EXTERNAL AND MIDDLE EARS 



709 



dispersion in the tympanum 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 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 undula- 

 tions with concentration of their intensity. While the oscillations of the 

 membrana tympani are readily communicated to the air in the cavity of 

 the tympanum, those of the solid ossicles will not be conducted away by 

 the air, but will be propagated to the labyrinth without being dispersed in 

 the tympanum. The existence of the membrane 

 over the fenestra rotunda permits vibration of 

 the stapes to and from the labyrinth. When 

 the membrane of the fenestra ovalis is pressed 

 toward the labyrinth by the stapes, the pressure 

 is communicated through the fluid of the laby- 

 rinth toward the cavity of the tympanum at 

 the fenestra rotunda which yields. 



The propagation of sound through the 

 auditory ossicles to the labyrinth must be 

 effected by oscillations of the bones as a whole. 

 The long process of the malleus receives the 

 undulations of the membrana tympani, figure 

 440, a, a, in a direction indicated by the arrows 

 and nearly perpendicular to itself. The vibra- 

 tions of the long process of the malleus sets the 

 chain of bones vibrating about the axis fixed by 

 the attachment of the anterior ligament of the 

 malleus and the ligaments of the incus, see 

 figure 432. From the long process of the incus, which is parallel with the 

 long process of the malleus, the undulations are communicated to the 

 stapes and by the stapes to the fenestra ovalis in a perpendicular direc- 

 tion. Increasing tension of the membrana tympani diminishes the facility 

 of transmission of sonorous undulations from the air to it. It has been 

 inferred, therefore, that hearing is rendered less acute by increasing the 

 tension of the membrana tympani. This is accomplished by the contrac- 

 tions of the tensor tympani muscle. The exact influence of the stapedius 

 muscle in the act of hearing is unknown. It acts upon the stapes in such 

 a manner as to make it rest obliquely in the fenestra ovalis, depressing 

 that side of the stapes on which it is attached and elevating the other 

 side to the same extent. It seems to prevent too great a movement of 

 the bone. 



The pharyngeal orifice of the Eustachian tube is usually shut. During 

 swallowing, however, it is opened; which may be shown as follows: If the 



FIG. 440. Diagram to Il- 

 lustrate the Action of the 

 Ossicles of the Middle Ear in 

 the Conduction of Sound to 

 the Internal Ear. 



