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. 
