USES OF DIFFERENT PARTS OF THE AUDITORY APPARATUS. 841 



Mechanism of the Ossicles of the Ear. The ossicles of the middle ear, in connection 

 with the muscles, have a twofold function : First, by the action of the muscles, the 

 membrana tympani may be brought to different degrees of tension. Second, the chain 

 of bones serves to conduct sonorous vibrations to the labyrinth. It must be remembered 

 that the handle of the malleus is closely attached to the membrana tympani, especially 

 near its lower end. Near the short process, the attachment is looser and there is even 

 an incomplete joint-space at this point. The long process is attached closely to the 

 Glasserian fissure of the temporal bone. 



The malleus is articulated with the incus by a very peculiar joint, which has been 

 accurately described by Helmholtz. This joint is so arranged, presenting a sort of cog, 

 that the handle of the malleus can rotate only outward ; and, when a force is applied 

 which would have a tendency to produce a rotation inward, the malleus must carry the 

 incus with it. This mechanism has been aptly compared by Helmholtz to that of a 

 watch-key with cogs which are fitted together and allow the whole key to turn in one 

 direction, but are separated so that only the upper portion of the key turns when the force 

 is applied in the opposite direction. In the articulation between the malleus and the 

 incus, the only difference is that there is but one cog ; but this is sufficient to prevent an 

 independent rotation of the malleus inward. This enables us to understand the action 

 of the tensor tympani muscle. By the contraction of this muscle, " all the bands which 

 give firmness to the position of the ossicles are rendered tense. This muscle, in the first 

 place, draws the handle of the hammer inward, and with it the membrana tympani. At 

 the same time it pulls upon the axis-band of the hammer, drawing it inward and putting 

 it upon the stretch. Another effect, as we have shown, is to draw the head of the ham- 

 mer away from the tympano-incudal joint, to tighten all the ligaments of the anvil, those 

 toward the hammer as well as those at the end of its short process, and to lift the latter 

 up from its bony bed. In this way the anvil is brought into the position where the cogs 

 of the malleo-incudal joint fit into one another the tightest. Finally, the long process 

 of the anvil is compelled to form a rotation inward in company with the handle of the 

 hammer ; in so doing, as we shall see further on, it presses upon the stirrup and drives 

 it into the oval window against the fluid of the labyrinth. 



"In this respect the construction of the ear is very remarkable. By the contraction 

 of the single mass of elastic fibres constituting the tensor tyinpani (whose tension, besides, 

 is variable and may be adapted to the wants of the ear) all the inelastic tendinous liga- 

 ments of the ossicles are simultaneously put upon the stretch." (Helmholtz.) 



The body of the incus is attached to the posterior bony wall of the tympanic cavity. 

 Its articulation with the malleus has just been indicated. By the extremity of its long 

 process, it is also articulated with the stapes, which completes the chain. In situ, the 

 stapes forms nearly a right angle with the long process of the incus. 



The stapes is articulated with the incus, as indicated above, and its oval base is applied 

 to the fenestra ovalis. Surrounding the base of the stapes, is a ring of elastic fibro-carti- 

 lage, which is closely united to the bony wall of the labyrinth, by an extension of the 

 periosteum over the base of the stapes. 



"The relation of the stirrup to the anvil is such that, if the handle of the hammer be 

 drawn inward, the long process of.the anvil presses firmly against the knob of the stirrup; 

 the same takes places if the capsular ligament between both be cut through." (Helm- 

 holtz.) 



The articulations between the malleus and the incus and between 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 cavity, there is no danger of tearing the stapes from its 

 attachment to the fenestra ovalis ; for, when the handle of the malleus is drawn outward, 

 the cog-joint between the malleus and the incus is loosened and no great traction can be 

 exerted upon the stapes. 



Although experiments have demonstrated pretty conclusively the mechanism of the 



