162 OSTEOLOGY. 
The line indicates the direction of the carotid canal, and cuts the foramen lacerum medium 
anteriorly ; in front of this it usually corresponds to the position of the posterior aperture of the 
pterygo-palatine canal. 
The examination of the base of the skull is incomplete unless the student examines 
it with the lower jaw and atlas vertebra in position. The relation of the ramus of 
the lower jaw to the zygomatic fossa has been already sufficiently studied (p. 153) ; 
one or two points, however, may be emphasised. The alar spine of the sphenoid 
lies just internal to the condyle of the jaw when that structure is in position in the 
articular part of the glenoid fossa, and it is noteworthy that immediately to the 
inner side of the alar spine is the commencement of the osseous Eustachian tube. 
The root of the styloid process occupies the centre of the interval between the 
mandibular ramus and the front of the mastoid process. 
Anteriorly the arcade formed by the body of the lower jaw adds greatly to the 
depth of the hard palate. In this space are lodged the tongue and the structures 
which form the floor of the mouth. The inner surface of each side of the body of 
the mandible is traversed by the internal oblique line (linea mylo-hyoidea), which 
commences posteriorly just behind the root of the last molar tooth and runs down- 
wards and forwards towards the symphysis in front. 
When the atlas vertebra is in articulation with the occipital bone it is well to 
recognise the relation of its transverse processes to the surrounding structures. The 
extremities of these processes lie in line with the ends of the jugular processes of 
the occipital bone, and thus come to be placed just internal to and immediately below 
and slightly in front of the tips of the mastoid processes. They can thus be easily felt 
in the living. Anteriorly they are separated by a short interval from the styloid 
processes, and the stylo-mastoid foramina lie immediately in front and slightly to 
the outer side of their extremities. The student will note that there is no hole in 
the jugular process of the occipital bone corresponding to the arterial foramen in the 
transverse process of the atlas through which the vertebral artery passes. The 
course of this vessel over the upper surface of the posterior arch behind the 
superior articular processes of the atlas will be seen to coincide with the posterior 
condylic fosse and the margins of the foramen magnum inmediately internal 
thereto, where a slight grooving of the edge often indicates the course of the artery. 
In front, the anterior tubercle of the atlas falls in line with the pharyngeal tubercle 
on the under surface of the basioccipital, and the student must not overlook the 
fact that the anterior surface of the cervical column does not coincide with the 
anterior margin of the foramen magnum, but lies nearly half an inch in front of 
that, in a coronal plane passing immediately in front of the external auditory 
meatus. Behind, the upper surtace of the posterior arch of the atlas overlaps the 
hinder margin of the foramen magnum, and it is by the apposition of these 
two surfaces that extension is checked at the occipito-atlantal articulation, 
THE SKULL IN SECTION, 
By the removal of the skull-cap the interior of the cranial cavity is exposed. 
The deep surface of the cranial vault is grooved mesially for the superior longitudinal 
sinus, on either side of which are seen numerous depressions for the lodgement of 
Pacchionian bodies. On holding the bone up to the light, the floor of these little 
hollows is oftentimes seen to be very thin. A short distance in front of the 
lambda, and on either side of the sagittal suture, are the internal openings of the 
parietal foramina. ‘The inner tables of the frontal and parietal bones are grooved 
for the meningeal arteries. The principal branch of the middle meningeal runs more 
or less parallel to and at a variable distance behind the line of the coronal suture. 
Along the bottom of these grooves small foramina may be seen for the passage of 
nutrient arteries to the bone, and the floor of the longitudinal sinus is likewise 
pierced by small apertures for the transmission of veins. 
THE UPPER SURFACE OF THE BASE OF THE SKULL. 
Cranial Fosse.—The upper surface of the base of the skull is divided into 
