166 OSTEOLOGY. 
runs across it; the posterior and external rounded part of the foramen is occupied 
by the lateral sinus, which here joins the internal jugular vein. A meningeal 
branch from the ascending pharyngeal or occipital artery also enters the skull 
through this compartment. The fore and internal part of the foramen is confluent 
with the groove for the inferior petrosal sinus, which turns downwards in front of 
the spicule above referred to. The interval between the portions of the foramen 
occupied by the two veins allows of the transmission of the glosso-pharyngeal, 
vagus, and spinal accessory nerves in order from before backwards. About a quarter 
of an inch above and to the outer side of the fore part of the foramen jugulare the 
posterior surface of the petrous portion of the temporal bone is pierced by the 
internal auditory meatus, through which the facial and auditory nerves, together 
with the pars intermedia of Wrisberg, and the auditory branch of the basilar artery 
leave the cranial cavity. Behind the jugular foramen and close to the margin of 
the foramen magnum the opening of the posterior condylic foramen, when present, 
may be seen. This gives passage to a vein which joins the vertebral vein 
inferiorly. The internal aperture of the mastoid foramen is noticed opening into 
the groove for the lateral sinus, a little below the level of the superior border of the 
petrous temporal. Through it passes an emissary vein which joins the occipital 
vein externally; the mastoid branch of the occipital artery also enters the cranial 
cavity through this foramen. 
The posterior fossa is divided into two halves posteriorly by the internal occipital 
crest, to which the falx cerebelli is attached, the floors of the hollows on either side 
of which are often exceedingly thin and are for the lodgment of the lateral lobes of 
the cerebellum. The grooves for the following blood sinuses are usually distinct— 
the superior petrosal running along the superior border of the petrous temporal ; 
the inferior petrosal lying along the line of suture between the petrous temporal 
and basilar process of the oceipital bone; the occipital sinus grooving the internal 
occipital crest; and the lateral sinus curving forwards and outwards from the 
internal occipital protuberance, across the ‘internal surface of the squamous 
occipital, to reach the posterior inferior angle of the parietal bone, in front of which 
it turns downwards and inwards to reach the jugular foramen, describing a sigmoid 
curve, and grooving deeply the inner surface of the mastoid and posterior aspect of 
the petrous portions of the temporal bone. Before it terminates at the jugular 
foramen it again reaches the occipital bone and channels the upper surface of the 
jugular process of that bone. Sleht grooves for meningeal arteries are also seen— 
some pass upwards, whilst others turn downwards and are occupied by branches 
from the posterior offsets of the middle meningeal arteries. 
MESIAL SAGITTAL SECTION OF THE SKULL. 
Such a section should be made a little to one or other side of the mesial plane, 
so as to pass through the nasal fosse lateral to the septum; one half will then 
display the nasal septum in position, whilst in the other the outer wall of the 
nasal fossa of that side will be exposed. 
The form of the cranial cavity is of course subject to many variations dependent 
on individual and racial peculiarities. The following details are, however, worthy 
of note. The hinder border of the foramen magnum (opisthion), and consequently 
the floor of the posterior cranial fossa occupies the same horizontal plane as the 
hard palate. The anterior border of the foramen magnum (basion) les a little 
higher, so that the plane of the foramen is, in the higher 1 races at least, oblique, and 
is directed downwards and slightly forwards. From the basion, a line passing 
upwards and forwards to reach the suture between the sphenoid and ethmoid 
passes through the basicranial axis formed by the basioccipital, the basisphenoid, 
and the presphenoid. The basicranial axis is wedge-shaped on section posteriorly, 
whilst anteriorly it is of considerable width, and has within it the large sphenoidal 
air sinus. Its upper surface leads upwards and forwards with a varying degree of 
obliquity from the basion to the overhanging edge of the dorsum selle, in front of 
which the pituitary fossa, the floor of which is quite thin, is well seen in the 
section. 
