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124 A^UTOMY.
nates at the posterior boundary of this foramen. From this point it
extends outwardly over the junction of the petrous M^th the mastoid
portion of the temporal bone, curves backward over the posterior infe-
rior angle of the parietal bone, and thence inward over the occipital
bone to the torcular Herophili, which is situated at the internal occipital
protuberance, and formed by the confluence of all the sinuses of the brain
excepting those transmitted through the petrosal sinuses. The posterior
condyloid and mastoid foramina open into this groove. In the region
of these sinuses the bones are generally extremely thick.
The central portion of the floor of the posterior fossse of the cranium
is pierced by the foramen magnum. This foramen transmits the spinal
cord and ith n embranes, the vertebral arteries, and the roots of the
spinal accessory nerves. The anterior condyloid foramen opens into
the anterior border of the foramen magnum. It transmits the twelfth
cranial nerve (hypoglossal).
The CerebeUar Fossce form that portion of the posterior fossse situated
between the lateral sinuses and the foramen magnum ; they lodge the
lobes of the cerebellum.

The External Surface of the Brain-case.
The external surface of the brain-case (Fig. 64) is divided into
five regions—a superior, inferior, anterior or facial, and two lateral
regions.
The Superior Bee/ion extends longitudinally from the supraorbital
arches anteriorly to the superior curved line on the occipital bone pos-
teriorly, and from the right to the left temporal ridges. It is in shape
an elongated dome, its length extending antero-posteriorly. It is flat-
tened in front to form the forehead, and projects behind. It is marked
by four eminences, two frontal and two parietal.
The greatest width of the superior region is generally from one pari-
etal eminence to that of another. No muscles arise or are attached in
this region, but it is well marked by fine pits for the attachment of the
pericranium, over whicli })lay the fibres and aponeuroses of the occipito-
frontalis muscle.
Base of the Brain-case.—When the fiicial bones are removed
from the anterior portion of the skull, the under surface of the brain-
case corresponds in great measure with the floor of its internal surface,
like which it is separated into three divisions—anterior or facial, middle
or cervical, and posterior or occipital ; and these divisions are situated
directly under corres])onding ones internally. This surfiice, however,
is rougher and the jjrojcctions of bones are nnich more prominent than
on the internal surfiu-e.
The Anterior Portion is bounded anteriorly by the supraorbital arches,
with their notches or foramina, and the rough articulating surface for
the nasal bones and the nasal ))r()cesses of the sujx'rior maxillary bones;
laterally by the external angular processes of the frontal bone and the
anterior border of the great wing of the s])licnoid ; and posteriorly by
the inferior border of the great wing of tlie sphenoid bone, and a line
drawn from the base of the anterior surface of the pterygoid process on