754 SPECIAL SENSES. 



lower end. Near the short process which is a little, conical projection at 

 the root of the handle the attachment is looser and there is even an incom- 

 plete 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. 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 compared 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 (Helmholtz). 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. 



The body of the incus is attached to the posterior bony wall of the tym- 

 panic 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-cartilage, which is closely united to the bony wall of the 

 labyrinth, by an extension of the periosteum. 



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 considerable traction can be exerted 

 upon 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 increasing 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 prac- 

 tically a solid and continuous, angular, bony 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 important physical point, however, 

 which has been the subject of much discussion, 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 them- 



