OSSICLES OF THE EAR 741 



the incus and the stapes are so arranged that when the membrana 

 tympani is forced outward as it may be by inflation of the tympanic 

 cav ity there is no danger of tearing the stapes from its attachment to 

 the fenestra ovalis ; for when the handle of the malleus is drawn out- 

 ward, the cog-joint between the malleus and the incus is loosened and 

 no considerable traction can be exerted on the stapes. 



The tensor tympani is by far the more important of the two muscles 

 of the middle ear. Its action is to tighten the cog-like joint between 

 the malleus and the incus, to tighten, also, all the ligaments of the 

 incus, to draw the long process of the malleus inward, thereby increas- 

 ing the tension of the membrana tympani, and to press the base of the 

 stapes against the fenestra ovalis. By the action of this muscle the 

 chain of ossicles becomes practically a solid and continuous angular 

 rod. 



Although experiments have demonstrated the mechanism of the 

 ossicles and the action of the tensor tympani, both as regards the chain 

 of bones and the membrana tympani, direct observations are wanting 

 to show the exact relations of these different conditions of the ossicles 

 and of the membrane to the physiology of audition. One very impor- 

 tant physical point, however, which has been the subject of much dis- 

 cussion, is settled. The chain of bones acts as a single solid body in 

 conducting vibrations to the labyrinth. It is a matter of physical 

 demonstration that vibrations of the bones themselves would be incon- 

 ceivably rapid as compared with the highest tones that can be appre- 

 ciated by the ear, if it were possible to induce in these bones regular 

 vibrations. Practically, then, the ossicles have no independent vibra- 

 tions that can be appreciated. This being the fact, the ossicles simply 

 conduct to the labyrinth vibrations induced in the membrana tympani by 

 sound-waves ; and their arrangement is such that these vibrations lose 

 but little in intensity. While it has been shown experimentally that 

 the amplitude of vibration in the membrana tympani and the ossicles 

 diminishes as the tension of the membrane is increased, it would seem 

 that when the tensor tympani contracts, it must render the conduction 

 of sound-waves to the labyrinth more delicate than when the auditory 

 apparatus is in a relaxed condition, which may be compared with the 

 " indolent " condition of accommodation of the eye. When the mem- 

 brana tympani is relaxed and the cog-like articulation between the 

 malleus and the incus is loosened, the vibrations of the membrane and 

 of the malleus may have a greater amplitude ; but when the malleo- 

 incudal joint is tightened and the stapes is pressed against the fenestra 

 ovalis, the loss of intensity of vibration, in conduction through the bones 

 to the labyrinth, must be reduced to the minimum. With this view, 



