THE AUDITORY APPARATUS 



body is provided with an articular surface, which looks forwards and 

 articulates with the head of the malleus. The short cms is directed 

 backwards, and its extremity is attached, by ligaments, to the posterior 

 wall of the tympanum, below the opening into the tympanic antrum. The 

 long crus proceeds downwards and medially, in a direction nearly parallel 



Articular surface for 

 head of malleus 



Crus breve 



Body 



Crus_ 

 longum 



Processus 

 lenticularis 



FIG. 216. The Left Incus. (Howden. ) 

 A. Anterior aspect. B. Medial aspect. 



to that of the manubrium of the malleus, but more medial, and on a plane 

 posterior to the manubrium. On its inferior extremity, which is bent 

 medially, there is a small knob of bone, called the processus lenticularis, 

 which articulates with the head of the stapes. 



The malleus and incus move together on an axis which is formed by 

 the processus anterior of the malleus 'and the crus breve of the incus. 

 The articular surfaces of the two bones are provided with peculiar catch- 

 teeth which interlock when the bones are performing their ordinary 

 movements. When, however, force is applied to the medial surface of 

 the membrana tympani, as, for instance, when 

 the tympanum is inflated through the auditory 

 tube, the incudo-malleolar joint gapes and the 

 malleus moves by itself. Traction upon the 

 -Cms posterior attachments of the stapes, through the incus, is 

 thus avoided. 



The stapes is shaped like a stirrup, and 

 presents a head or lateral extremity separated 

 by a slightly constricted neck from two crura 



FIG. 217. Left Stapes, which join a medial plate called the basis 

 (Howden.) stapedis. The head is excavated by an articular 



cup for the processus lenticularis of the incus. 



The crura are grooved longitudinally on their concave sides (sulcus 

 stapedis). The posterior crus is more sharply curved than the anterior 

 crus. The base fits into the fenestra vestibuli and corresponds in its 

 outline with that aperture. Its lower border is straight, whilst its upper 

 border is curved. 



Ligaments of the Auditory Ossicles. In addition to the delicate 

 articular capsules, which surround the joints between the auditory ossicles, 

 there are certain bands which connect the bones to the walls of the tym- 

 panum and serve to restrain their movements. 



In connection with the malleus there are (i) an anterior ligament > 



Head 



Neck 

 rus anterior 



Foot-plate 



