624 



THE SENSES 



mation 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 tym- 

 panum. The long process of the malleus receives the undulations of the 

 membrana tympani, 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 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 have, how- 

 ever, 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 communi- 

 cated 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 ac- 

 complished by the contractions of the tensor 

 tympani muscle. The exact influence of the 

 stapedius muscle in the act of hearing is un- 

 known. 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 

 nose and mouth be closed and the cheeks blown out, a sense of pressure is 

 produced in both ears the moment we swallow. This is due, doubtless, to 

 the bulging out of the tympanic membrane by the compressed air, which at 

 that moment enters the Eustachian tube. The principal office of the Eusta- 

 chian tube has relation to the prevention of the effects of increased tension of 

 the membrana tympani. Its existence and openness will provide for the 

 maintenance of the equilibrium between the air within the tympanum and 

 the external air, so as to prevent the inordinate tension of the membrana 

 tympani which would be produced by too great or too little pressure on either 

 side. While discharging this office it serves as an outlet for mucus. If the 



FIG. 440. Diagram to Illus- 

 trate the Action of the Ossicles 

 of the Middle Ear in the Conduc- 

 tion of Sound to the Internal 

 Ear. 



