OSSICLES AND LIGAMENTS. 675 



plate, which is convex at one margin and almost straight at the other, cor- 

 responding with the shape of the f'enestra ovalis : the surface turned to the 

 vestibule is convex, while the opposite is excavated. The head (a) is 

 marked by a superficial depression which receives the orbicular process of 

 the incus ; and below it is a constricted part (6), the neck of the bone. 

 The crura (e) extend from the neck to the base, and are grooved on the 

 inner surface : the anterior crus is shorter and straighter than the other. 



Fig. 241. 



THE THREE OSSICLES OF THE TYMPANUM : THE CENTRAL BONK is THE MALLEUS, THE LEFT-HAND 



BONE INCUS, AND THE RIGHT-HAND ONE STAPES. 



Malleus : a. head ; 6, neck ; c, long handle ; d, long, and e, short process. 

 Incus : a, head ; 6, body ; c, short process ; d, long process ; e, orbicular process. 

 Stapes: a, head ; 6, neck ; c, crus ; d, base. 



Position of the ossicles (fig. 242). The malleus (d) is placed vertically 

 in the tympanum, with the head upwards, and the joint surface turned 

 backwards to articulate with the incus. Its handle is inserted externally 

 into the membrana tympani between the mucous and fibrous layers ; and 

 its long process is directed forwards into the Glaserian fissure. 



The incus (e) is also upright, so that the long process is vertical, and 

 the short one horizontal. Externally it is united with the malleus ; the 

 short process is received into the large aperture of the mastoid cells ; and 

 the long one descends, like the handle of the malleus, but rather posterior 

 to it and nearer the inner wall of the cavity, to join inferiorly with the 

 stapes. 



The stirrup bone (/) has a horizontal position, with the crura directed 

 forwards and backwards : its base is fixed over the fenestra ovalis, and its 

 head is united with the long process of the incus. 



Ligaments of the ossicles. The small bones of the tympanic cavity are 

 united into one chain by joints, and are further kept in position by liga- 

 ments fixing them to the surrounding wall. 



Joints of the bones. Where the ossicles touch they are connected together 

 by articulations corresponding with the joints of larger bones ; for the osse- 

 ous surfaces are covered with cartilage, are surrounded by a thin capsular 

 ligament of fibrous tissue, and lubricated by a synovial sac. One articu- 

 lation of the nature above described exists between the heads of the mal- 

 leus and incus, and a second between the extremity of the long process of 

 the incus and the head of the stapes. 



Union of the bones to the wall. The bones are kept in place by the 

 reflection of the mucous membrane over them, and by special ligaments. 

 From the head of the malleus a short suspensory band of fibres is directed 

 upwards to the roof of the tynpanum. Another ligamentous band (poste- 

 rior) passes backwards from the incus, near the end of its short process, 



