658 



THE SENSES 



tory ossicles to fulfil the objects just described, is obvious. Without this pro- 

 vision, neither would the vibrations of the membrane be free nor the chain 

 of bones isolated, so as to propagate the sonorous undulations with con- 

 centration 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 ossicles 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 audi- 

 tory ossicles to the labyrinth must be effected by 

 oscillations of the bones as a whole. 



The existence of the membrane over the 

 fenestra rotunda permits approximation and 

 removal of the stapes to and from the labyrinth. 

 When the membrane of the fenestra ovalis is 

 pressed toward the labyrinth by the stapes, the 

 membrane of the fenestra rotunda may, by the 

 pressure communicated through the fluid of the 

 labyrinth, be pressed toward the cavity of the 

 tympanum. The long process of the malleus 

 receives the undulations of the membrana tym- 

 pani, figure 440, 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 

 the Action of the are propagated to its head, b; thence into the 

 cus, 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 communi- 



cated 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 in changing the character of the undulations, which remain the 

 same as in the meatus externus. From the long process of the malleus 

 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. It has been inferred, therefore, that hearing is rendered less 

 acute by increasing the tension of the membrana tympani. This is 

 accomplished by the contractions 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 



FIG. 440. Diagram to Il- 

 lustrate 



the Internal Ear. 



