802 



MECHANISM OF THE AUDITORY OSSICLES. 



Besides this, the malleus is fixed by ligaments which prescribe the direction of its 

 movements. Two ligaments the lig. mallei anticum (passing from the processus 

 Folianus) and the posticum (from a small crest on the neck) together form a com- 

 mon axial band (v. Helmholtz), which acts in the direction from behind forwards, i.e., 

 paralleHo the surface of the tympanic membrane. The neck of the malleus lies 

 between the insertions of both ligaments. The united ligament determines the 



" axis of rotation " of the 

 movement of the malleus. 



When the handle of the 



malleus is drawn inwards, of 



course its head moves in the 



opposite direction, or outwards. 



vrtL. The incus, a, is only partially 



's*g fixed by a ligament, which at- 



M 



Fig. 585. Fig. 584. 



Fig. 584. The auditory ossicles (right). Cm, head; C, neck; Pbr, short process ; Prl, long 

 process ; M, handle of the malleus ; Ci, body ; O, articular surface ; h, short, and v, long 

 process of the incus ; O.S, so-called lenticular ossicle ; C.s, head ; a, anterior, and p, poste- 

 rior limb ; P, plate of the stapes. Fig. 585. Tympanum and auditory ossicles (left) mag- 

 nified. A.G., external meatus ; M, membrana tympani, which is attached to the handle of 

 the malleus, n, and near it the short process, p ; h, head of the malleus ; a, incus ; k, its 

 short process with its ligament ; I, long process ; s, Sylvian ossicle ; S, stapes ; Ax, Ax. 

 is the axis of rotation of the ossicles, it is shown in perspective, and must be imagined to 

 penetrate the plane of the paper ; t, line of traction of the tensor tympani. The other 

 arrows indicate the movement of the ossicles when the tensor contracts. 

 taches its short process to the wall of the tympanic cavity, in front of the entrance 

 to the mastoid cells, k. The not very tense articulation joining it to the head 

 of the malleus, h, which lies with its saddle-shaped articular surface in the hollow 

 of the incus, is important. The lower margin of the incus (fig 584, S) acts like 

 a tooth of a cog-wheel. Thus, when the handle of the malleus moves inwards 

 to the tympanic cavity, the incus, and its long process, b, which is parallel to 

 the handle of the malleus, also pass inwards. The incus forms almost a right 

 angle with the stapes, S, through the intervention of the Sylvian ossicle, s. If, 

 however, as by condensation of the air in the tympanum, the membrana tympani 

 and the handle of the malleus move outwards, the long process of the incus does 

 not make a similar movement, as the malleus moves away from this margin of the 

 incus. Hence, the stapes is not liable to be torn from its socket. The malleus and 

 incus form an angular lever, which moves round a common axis (fig. 581 and fig. 

 585, Ax, Ax). In the inward movement, the malleus follows the incus, as if both 

 formed one piece. The common axis (fig. 580) is not, however, the axial ligament 

 of the malleus, but it is formed anteriorly by the processus Folianus, IF, directed 

 forwards, and posteriorly by the short process of the incus directed backwards. 

 The rotation of both ossicles around this axis occurs in a plane vertical to the plane 



