Dorsocraniad of the fenestra vestibuli is a large fossa (_d_) which contains the tensor tympani muscle. Dorsocaudad of this, partly bounded by the squamous portion of the temporal, is another large fossa (_e_), the cranial end of which is occupied by the incus, while its caudal end is occupied by the head of the malleus.
Nearly caudad of this fossa and separated from it by an oblique bony septum is a third fossa (_f_) which is narrow and curved. It is continuous with a notch in the mastoid portion of the bone. When the tympanic is articulated the notch is converted into a foramen (=stylomastoid foramen=, Fig. 22, _j_) for the exit of the seventh nerve. The fossa gives passage to the seventh nerve and also lodges the stapedius muscle. A groove may be traced from the stylomastoid foramen to the caudal border of the fossa for the tensor tympani muscle, where it passes into a canal (_g_). The groove and canal are parts of the =canalis facialis= or =facial canal= (aqueductus Fallopii) for the passage of the seventh nerve through the petrous bone.
The medial surface (Fig. 23, _b_) of the petrous portion shows near its middle a fossa, the =internal auditory meatus= (_n_). This is divided by a partition of bone into a dorsal and ventral part. The dorsal portion is the beginning of the facial canal (aqueductus Fallopii) by which the seventh nerve passes through the petrous bone to emerge at the stylomastoid foramen. The ventral portion shows at its bottom several small foramina for the auditory nerve.
Dorsocaudad of the internal auditory meatus is a deep fossa (_o_) for a small lobe, the so-called appendicular lobe, of the cerebellum. This may be called the appendicular fossa.
The dorsal surface is triangular and presents near its apex a foramen--the =hiatus facialis= (_p_), the opening of a canal which joins the canalis facialis and transmits the superficial petrosal branch of the nerve of the pterygoid canal (Vidian nerve). That part of the dorsal surface which lies caudad of the hiatus facialis is known as the =tegmen tympani=.
The base of the petrous is attached to the mastoid portion (Fig. 23, _e_).
(For an account of the structures within the petrous bone and the tympanic cavity, see the description of the internal and middle ear.)