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BONES. 59
tyrapani, is a thin layer of bone which forms the roof of the tympanum
and the bony portion of the Eustachian tube. It is bounded anteriorly
by the petro-squamous fissure, which commences internally at the angle
between the squamous and petrous portions of the bone, and extends
outwardly to the masto-parietal suture: internally it extends downward
and backward, forming the glenoid fissure (fissure of Glasserius).
The Posterior Surface looks backward and inward ; it is less oblique
than the anterior, and forms, in great part, the anterior border of the
posterior fossa of the brain-case. Near its centre is a large orifice
leading into a short canal. The canal is directed outward, and is called
the internal auditory meatus. It transmits the seventh (facial) and
eighth (auditory) nerves and the auditory artery.
The meatus is about four lines in depth, and terminates in a thin
plate of bone, the lamina cribrosa, in the lower portion of which are
several small openings for the transmission of the divisions of the audi-
tory nerve ; in the upper portion is tlie aqueduct of Fallopius, for the
passage of the facial nerve. This canal has a tortuous course through
the petrous portion of the temporal bone, passing at first outward for
a short distance between the cochlea and vestibule to the inner wall of
the tympanum ; then backward over the fenestra ovalis, the ear, and then
downward, terminating at the stylo-mastoid foramen. External to the
internal auditory meatus, and between it and the posterior fossa, is a
slit-like opening, quite indistinct in some cases, which leads to a canal,
tlie aqueductus vestibuli. This canal transmits venous blood from the
internal ear. The superior border, which divides the anterior from the
posterior surface, is grooved for the superior petrosal sinus, but it never
extends to the apex of the bone. That portion of the border internal
to the meatus is depressed for the reception of a thick fold of dura mater,
under which the third, fourth, fifth, and sixth nerves pass.
The Inferior Surface of the petrous portion is rough and uneven.
From within outwardly, or from the apex to a large foramen situ-
ated about midway of this surface, is a rough triangular space which
gives attachment to the levator palati and tensor tympani muscles. The
large round foramen is the external o])ening to the canal for the internal
carotid artery. It first passes upward, then horizontally forward and
inward to the apex of the bone, from which point the vessel enters the
brain-case.
A plexus of the sympathetic nerve accompanies the artery in its
course through the canal. External to and a little above this foramen
is a smooth, deep depression, the jugular fossa, which varies in size in
different skulls, and when articulated with the jugular notch in tlie
occipital bone the two form the jugular foramen. Just back of the
jugular fossa, at the commencement of the border of the mastoid por-
tion of the bone, is an irregular, rough surface, the jugular facet, which
articulates by synchondrosis with the transverse process of the occipital
bone.
Several small foramina are situated in this portion of the bone. In
the ascending portion of the carotid canal is a small foramen f )r the
tympanic branch of the internal carotid artery, and between the jugular
fossa and the opening for the carotid canal will be found a foramen for