AUDITORY OSSICLES 557 



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

 of the tympanum, near the opening into the tympanic antrum. 

 The long crus proceeds downwards and medially, in a direction 

 nearly parallel to that of the manubrium of the malleus, but 

 more medial, and on a plane posterior to that process. On 

 its inferior extremity, which is bent medially, there is a small 

 knob of bone called the processus lenticularis. This 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 inner surface of 

 the membrana tympani, as, for instance, when the tympanum is inflated 

 through the auditory tube, the incudp - malleolar joint gapes and the 

 malleus moves by itself. Traction upon the 

 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 which join a medial 

 plate, the basis stapedis. The head is 2 m~ den 



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 capsular ligaments, which surround the joints between 

 the auditory ossicles, there are certain bands which connect 

 the bones to the walls of the tympanum and serve to restrain 

 their movements. 



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

 passes from its anterior part, at the 'root of the processus anterior, to the 

 anterior wall of the tympanum in the neighbourhood of the petro-tympanic 

 fissure ; (2) a lateral ligament which extends from its lateral process to 

 the margin of the notch of Rivinus ; and (3) a superior ligament which 

 connects the head with the roof of the tympanum. 



The ligament of the incus binds the extremity of its short crus to the 

 posterior wall of the tympanum, whilst the annular ligament of the stapes 

 connects the margin of its base to the circumference of the fenestra vestibuli. 



