210 SPECIAL SENSES. 



tion 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 ham- 

 mer inward, and with it the membrana tympani. At the 

 same time it pulls upon the axis-band of the hammer, draw- 

 ing it inward and putting it upon the stretch. Another ef- 

 fect, as we have shown, is to draw the head of the hammer 

 away from the tympano-incudal joint, to tighten all the liga- 

 ments 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 posi- 

 tion where the cogs of the malleo-incudal joint fit into one 

 another the tightest. Finally, the long process of the anvil 

 is compelled to perform a rotation inward in company with 

 the handle of the hammer ; in so doing, as we shall see fur- 

 ther 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 re- 

 markable. By the contraction of the single mass of elastic 

 fibres constituting the tensor tympani (whose tension, besides, 

 is variable and may be adapted to the wants of the ear) all 

 the inelastic tendinous ligaments of the ossicles are simulta- 

 neously put upon the stretch." 1 



The body of the incus is attached to the posterior bony 

 wall of the tympanic cavity. Its articulation with the mal- 

 leus 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, 



1 HELMHOLTZ, Mechanism of the Ossicles of the Ear and Membrana Tympani, 

 New York, 1873, p. 40. 



