NORMA BASALIS OF THE SKULL. 161 
(p. 114). Its relation to the jugular foramen is of great importance, as the internal 
jugular vein lies immediately to its inner side. 
Immediately behind the root of the styloid process, internal to and in line with 
the front of the mastoid process, is the stylo-mastoid foramen (foramen stylo- 
mastoideum), the lower aperture of the aqueeductus Fallopii through which the facial 
nerve passes out and the stylo-mastoid branch of the posterior auricular artery 
passes in. The inner surface of the mastoid process is deeply grooved at its base 
for the origin of the posterior belly of the digastric muscle. Internal to this, and 
running along just wide of the occipito-mastoid suture, is a shallow groove in which 
the occipital artery is lodged. Immediately internal to the stylo-mastoid foramen 
is the synchondrosis between the extremity of the jugular process (processus 
jugularis) of the occipital bone and the petrous temporal. The former is a bar of 
bone which limits the jugular fossa posteriorly and abuts on the occipital condyles 
internally ; its under surface is convex from before backwards and affords attachment 
to the rectus lateralis muscle. The occipital condyles (condyli occipitales) are 
placed between the jugular processes and the foramen magnum. Ending in front 
by a rounded thickening which becomes confluent with the anterior border of the 
foramen magnum, they form by their inner sides the lateral boundaries of that 
aperture on its anterior half. Externally they are confluent with the jugular 
processes, in front of which they overhang a fossa which is pierced behind by the 
anterior condylic foramen (canalis hypoglossi), through which passes the hypoglossal 
nerve, together with a small vein and occasionally a small meningeal branch 
derived from the ascending pharyngeal artery. 
The posterior condylic fosse are situated just behind the posterior extremities of 
the condyles. Not unfrequently their floor is pierced by the posterior condylic 
foramen (canalis condyloideus), through which the posterior condylic vein emerges. 
The base of the skull behind the jugular processes and condyles of the occipital 
bone is formed by the nuchal surface (planum nuchale) of the tabular plate of that 
bone. Posteriorly this surface is bounded by the superior curved line, in the centre 
of which is placed the projecting external occipital protuberance. Laterally the 
tabular plate is separated from the mastoid portion of the temporal bone by the 
occipito-mastoid suture, which curves backwards and outwards from the extremity 
of the jugular process in front around the base of the mastoid process behind. In 
front and in the middle line this plate of bone is pierced by the foramen magnum, 
the anterior half of which has been already seen to he between the occipital 
condyles. Usually of oval form, though in some cases it tends to approach the 
circular, the plane of this opening is inclined downwards and slightly forwards. 
The extreme anterior edge of the foramen is sometimes called the basion, whilst the 
extreme posterior margin is termed the opisthion. The lower border of the medulla 
oblongata, where it becomes continuous with the spinal cord, is lodged within the 
foramen, together with the meninges which cover it, whilst the vertebral arteries 
and the spinal portions of the spinal accessory nerves pass upwards through it. 
The anterior and posterior spinal arteries, some small veins, and the roots of the 
first cervical nerves, also traverse it from above downwards. 
The student will no doubt experience considerable difficulty in bearing in mind the relative 
positions of the various foramina and processes which he has studied on the under surface of the 
base of the skull. 
If a line be drawn on either side from the anterior palatine canal in front, through the stylo- 
mastoid foramina posteriorly, it will be found to cut or pass near to the following objects :—On 
the hard palate it will lie close to the posterior and accessory palatine canals. It will then pass 
between the hamular process and the external pterygoid plate overlying the foramen ovale, the 
foramen spinosum, the opening of the osseous Eustachian canal and the spine of the sphenoid ; 
behind this it will cut through the root of the styloid process and define externally the limits of 
the jugular fossa. After passing through the stylo-mastoid foramen, if the line be prolonged 
backwards it will usually be tound to pass over the mastoid foramen in the occipito-mastoid 
suture. Another line of much value is one drawn across the base of the skull from the centre 
of one external auditory meatus to the other. This will be found to pass through the root of 
the styloid process, the jugular foramen, the anterior condylic foramen ; it then crosses the front 
of the occipital condyles, and corresponds with the anterior edge of the foramen magnum. 
A line which may be found useful is one drawn from the stylo-mastoid foramen of one side to 
the posterior palatine canal of the opposite side. This will be seen to overlie, from behind 
forwards, the outer part of the jugular foramen and the inferior opening of the carotid canal. 
11 
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