148 Anatomy of the Rabbit. 



margin of the tympanic bulla encloses a roughly triangular area, 

 into the ventral part of which the petrous portion of the petro- 

 mastoid projects as a smooth, white, convex ridge, the pro- 

 montory (promontorium). Above and behind the promontory the 

 tympanic cavity is extended toward the mastoid portion of the bone 

 as the tympanic or mastoid antrum (antrum tympanicum), and 

 the interior of the mastoid portion is partly occupied by small 

 extensions of the tympanic antrum, termed the mastoid cells 

 (cellulae mastoideae). At the anteroventral angle of the area 

 already described, a deep notch indicates the point of entrance of 

 the auditory tube. The exposed surface of the petromastoid 

 presents two apertures, one of which, situated posteroventrally, 

 is open in the dried skull, and is the cochlear fenestra (fenestra 

 cochleae). In the natural condition it is closed by a thin membrane 

 which separates the tympanic cavity from the perilymphatic 

 space containing the membranous labyrinth. The second aper- 

 ture, the vestibular fenestra (fenestra vestibuli), lies above and 

 in front of that just described. It is closed by the base of the 

 stapes. 



The auditory ossicles (ossicula auditus) comprise three ele- 

 ments, namely, the malleus, incus, and Stapes, which bridge the 

 space intervening between the tympanic membrane and the open- 

 ing to the internal ear as represented by the vestibular fenestra. 

 They occupy the dorsal angle of the triangular area already de- 

 scribed, and lie immediately above the promontory. The malleus 

 is the lateral element. The main portion, termed the head, is 

 concealed by the projecting edge of the external acoustic meatus. 

 It bears a stout vertical process, the manubrium mallei, which 

 in the natural condition lies in contact with the tympanic mem- 

 brane. The incus is the intermediate element; it is directly 

 articulated with the malleus, and bears a downwardly-directed 

 long limb (crus longum), for articulation with the minute head 

 of the stapes. The latter element is a small stirrup-shaped bone, 

 occupying an almost transverse position, and articulated at its base 

 with the margin of the vestibular fenestra. 



