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BONES. 123

of the temporal bone at the external boundary of this foramen, while
the Vidian canal commences below at its anterior margin.
Tlie hiatus Fallopii is a small canal, the opening of which is situated
on the anterior surface of the petrous portion of the temporal bone
just external to the termination of the carotid canal. This canal trans-
mits the Vidian nerve, Mhicli is a branch of the seventh or facial nerve,
and goes to the spheno-palatine ganglion. On the apex of the petrous
portion of the temporal bone is an irregular depression for the lodg-
ment of the ganglion of Gasser (semilunar ganglion). This is the
laro-e o-ano-lion of the fifth nerve.
Three divisions are thrown off from this ganglion within the middle
fossa, and pass outwardly through its walls or floor.
The middle cerebral lobes of the brain rest upon the floor of the
middle fossae of the skull.
The Posterior Foshcb of the brain-case are in great part formed l)y
the occipital bone. This bone, in conjunction with a portion of the
body of the sphenoid bone, forms the floors; their anterior boundary
is formed by the posterior surface of the petrous portion of the tem-
poral bone, while the mastoid portion of the temporal bone and a small
portion of the parietal bones complete the sides.
These fossse are deeper and larger than the others. Their central
portions anteriorly are marked by the posterior clinoid processes.
The elongated concave surface (^f bone between the posterior clinoid
processes and the foramen magnum is composed of the dorsum sellse of
the sphenoid bone anteriorly and the basilar process of the occipital
bone posteriorly. This surface lodges the medulla oblongata and the
basilar artery.
The posterior surface of the petrous portion of the temporal bone is
marked by a large opening, the internal auditory meatus. The seventh
and eighth nerves pass into this opening, the seventh going to the face,
while the eighth passes to the internal ear.
The posterior lacerated foramen (foramen lacerum posterius) is below
the internal auditory meatus, between the petrous portion of the tem-
poral bone and the basilar process of the occipital bone. It is a large,
irregular, twisted, wedge- or pear-shaped aperture, the base rounded and
directed to the posterior and distal portion of the base of the skull, the
axis of its external opening being toward the mastoid process of the
temporal bone. This base is rounded, being formed by the jugular
fossa of the occipital and temporal bones. It is Iiere, within this fora-
men, that the internal jugular vein is formed by the termination of the
lateral sinuses.^ The apex of the posterior lacerated foramen is gener-
ally separated into two divisions by the intrajugular processes of bone.
The posterior division transmits from the brain-case the ninth (glosso-
pharyngeal), the tenth (pneumogastric), and the eleventh (spinal acces-
sory) nerves, wdiile the anterior division gives passage to the inferior
petrosal sinus.
The deep groove for the accommodation of the lateral sinus termi-
* The sinuses of the brain-case are membranous for the passage of venous blood.
They resemble veins, diflering from them in that they lack the fibrous and muscuLar
coats of these vessels.