178 OSTEOLOGY. 



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 of the squamous part of that bone. Posteriorly this surface is bounded 

 by the superior nuchal or curved line, in the centre of which is placed the projecting 

 external occipital protuberance. Laterally the squamous part of occipital bone is 

 separated from the mastoid portion of the temporal bone by the occipito-mastoid 

 outure, which curves backwards and laterally, from the extremity of the jugular 

 process in front, around the base of the mastoid process behind. In front and in 

 the median plane this plate of bone is pierced by the foramen occipitale magnum, the 

 anterior half of which has been already seen to lie 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 medulla, is lodged within 

 the foramen, together with the meninges which cover it, whilst the vertebral 

 arteries and the spinal portions of the 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 inferior surface of the 

 base of the skull. 



If a line be drawn on either side from the incisive foramen in front, through the stylo- 

 mas toid 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 greater and lesser palatine foramina. It will then pass 

 between the hamulus and the lateral pterygoid lamina, overlying the foramen ovale, the foramen 

 spinosum, the opening of the osseous part of the auditory tube and the angular spine of the 

 sphenoid ; behind this it will cut through the root of the styloid process and define laterally the 

 limits of the jugular fossa. After passing through the stylo-mastoid foramen, if the line be 

 prolonged backwards it will usually be found 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 acoustic meatus to the other. This will be found to pass through the 

 root of the styloid process, the jugular foramen, the hypoglossal canal ; 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 greater palatine foramen of the opposite side. This will be seen to overlie, from behind 

 forwards, the lateral part of the jugular foramen and the inferior opening of the carotid canal. 

 The line indicates the direction of the carotid canal, and cuts the foramen lacerum anteriorly ; in 

 front of this it usually corresponds to the position of the posterior aperture of the pharyngeal canal. 



Mandible and Atlas in Position. The examination of the base of the skull 

 is incomplete unless the student examines it with the mandible and atlas in 

 position. The relation of the ramus of the mandible to the infra-temporal fossa 

 has been already sufficiently studied (p. 169); one or two points, however, may be 

 emphasised. The angular spine of the sphenoid lies just medial to the condyle of the 

 mandible when that structure is in position in the articular part of the mandibular 

 fossa, and it is noteworthy that immediately to the medial side of the angular 

 spine is the commencement of the osseous part of the auditory 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 mandible 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 medial surface of each side of the body of 

 the mandible is traversed by the mylo-hyoid line, which commences posteriorly 

 just behind the root of the last molar tooth and runs downwards and forwards 

 towards the symphysis in front. 



When the atlas 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 medial to and immediately below 

 and slightly in front of the tips of the mastoid processes. They can thus be easily felt 

 in the living subject. Anteriorly they are separated by a short interval from 

 the styloid processes, and the stylo-mastoid foramina lie immediately in front and 



