740 SPECIAL SENSES 



forcibly when relaxed than when it is tense. In certain cases of facial 

 palsy, in which it is probable that the branch of the facial going to the 

 tensor tympani was affected, the ear has been found painfully sensitive 

 to powerful impressions of sound. This probably has no relation to 

 pitch, and most sounds that are painfully loud are comparatively grave. 

 Artillerists are in danger of rupture of the membrana tympani from 

 sudden concussions. To guard against this injury, it is recommended 

 to stop the ear, draw the shoulder up against the ear most in danger, 

 and particularly to innate the middle ear after Valsalva's method. 

 " This method consists in making a powerful expiration, with the mouth 

 and nostrils closed " (Troltsch). 



Mechanism of the Ossicles of the Ear. The ossicles of the middle 

 ear, in connection with the muscles, have a twofold office : First, by the 

 action of the muscles the membrana tympani may be brought to differ- 

 ent degrees of tension. Second, the angular 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 processes 

 which is a conical projection at the root of the handle the attachment 

 is looser and there is even an incomplete joint-space. The long process 

 is attached closely to the Glasserian fissure of the temporal bone. 



The malleus is articulated with the incus by a peculiar joint. This 

 is so arranged, presenting a sort of cog, that the handle can rotate only 

 outward ; and when a force is applied that 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 

 that are fitted together and allow the key to turn in one direction, but 

 are separated so that only the upper portion 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 

 tympanum. Its articulation with the malleus has just been indicated. 

 By the extremity of its long process, it also is 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 fibre-cartilage that 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 



