556 



THE NERVOUS SYSTEM. 



ward. When in movement, ' the malleus oscillates about a nearly hori- 

 zontal axis situated at the junction of its handle and neck. 



The head of the malleus is articulated with the incus by a capsular 

 joint with double-inclined surfaces. As Helmholtz has shown, these 

 surfaces have such an inclination, that when the handle of the malleus 

 is drawn inward, they lock together, and the incus follows the move- 

 ment of the malleus ; but when the latter bone is drawn outward, they 

 may glide upon each other, without necessarily moving the incus. 



The third bone of the middle ear, the stapes, has a close resemblance 

 in form to its namesake, a metallic stirrup. It is articulated by its 

 angular extremity to the end of the long arm of the incus in a nearly 

 horizontal position. Its oval base corresponds in form, and nearly in 

 size, with the fenestra ovalis of the bony labyrinth, in which it is 

 FIG. 149. inserted ; being adherent by 



its surface and its edges to 

 the internal periosteum of 

 the labyrinth. 



The stapes, accordingly, 

 forms a kind of movable lid 

 or piston-head occupying the 

 fenestra ovalis, and capable 

 of transmitting to the fluid 

 of the labyrinth the impulses 

 received from the membrana 

 tympani: The extent of in- 

 ward and outward movement 

 of the base of the stapes has 

 been determined by Helm- 

 holtz in the following man- 

 ner : The cavity of the tym- 

 panum and that of the vesti- 

 bule having been opened from 

 above, the point of a fine sewing-needle was inserted into the fibrous 

 covering of the base of the stapes on the side of the vestibule, and 

 the needle allowed to rest, near its insertion, upon an adjacent edge of 

 bone. It thus formed a kind of index-lever, which would indicate by 

 its movement very slight displacements of the stapes. The stapes 

 was then pressed inward and outward, as freely as its attachments 

 would allow, either by direct pressure or by condensing and rarefy- 

 ing the air in the external auditory meatus ; the force, in the latter 

 case, being transmitted through the membrana tympani and chain of 

 bones. The movements were also estimated by opening the superior 

 semicircular canal of the labyrinth, and inserting into it a slender glass 

 tube of known calibre, a portion of which, as well as the vestibule, was 

 filled with water; any inward pressure being indicated by a corre- 

 sponding rise of the water-level in the tube. The movement of the 

 stapes, in these experiments, varied from .025 to .072 millimetre. 



RIGHT TEMPORAL BONE of the new-born infant, seen 

 from its inner side ; showing the membrana tympani 

 and chain of bones in their natural position. (Rii- 

 dinger.) 



