SENSE OF HEARING. 651 



tympani, the neck corresponding to the upper border of this membrane, 

 while the head projects above it, lying comparatively free in the cavity 

 of the tympanum. It is, however, maintained more or less closely in 

 its position by thin ligamentous bands arising from the bony wall of the 

 tympanic cavity and inserted into its head and neck, and by the tendon 

 of the internal muscle of the malleus or " tensor tympani," which, com- 

 ing from a direction anterior and internal to the bone, is inserted into 

 the upper extremity of its handle. The action of this muscle is to 

 draw the handle of the malleus inward, tightening the membrana tym- 

 pani, and rotating the head of the malleus slightly outward. The prin- 

 cipal movement of the malleus is therefore a rocking, to and fro move- 

 ment, about a nearly horizontal axis situated at the junction of the 

 handle and the neck. 



The head of the malleus is articulated with the body of the incus by 

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

 the surfaces are so different in their inclination, one being very steep, 

 the other but slightly oblique, that when the handle of the malleus is 

 drawn inward, the two articular surfaces lock together, and the incus 

 follows the movement of the malleus ; but when the latter bone is 

 drawn outward, the surfaces may glide upon each other, without the 

 incus necessarily moving at the same time. 



The third bone of the middle ear, the stapes, has in its form the most 

 exact resemblance to its namesake, an ordinary metallic stirrup. It is 

 articulated by its angular extremity to the lower end of the long arm 

 of the incus in such a manner as to be nearly horizontal in position, its 



Fig. 210. 



RIGHT TEMPORAL BONE of the new-born infant, seen from its inner side; showing 

 the internal surface of the membrana tympani and the chain of bones in their natural posi- 

 tion. (Riidinger.) 



two arms being placed, one anteriorly the other posteriorly. Its oval 

 base corresponds in form, and nearly in size, with the fenestra ovalis 

 of the bony labyrinth, in which it is inserted; being adherent by its 

 surface and its edges to the internal periosteum of the labyrinth. 



