BASE OF THE SKULL. 177 



condition the fissure is filled up with cartilage. The inferior surface of the 

 petrous bone included between these two fissures is rough and irregular, and 

 affords attachments near its apex to two small muscles, the levator veli palatini 

 and the tensor tympani. Immediately behind the angular spine the petrous 

 part of the temporal is pierced by a circular hole, the inferior opening of the 

 carotid canal. This passes upwards, and then turns medially and forwards towards 

 the apex of the bone, where it may again be seen opening into the lateral and upper 

 side of the foramen lacerum. Laterally the wall of the vertical part of this canal, 

 which is usually very thin, separates it from the cavity of the tympanum, as may be 

 seen by holding the skull up to the light and looking into the external acoustic 

 meatus. The carotid canal transmits the internal carotid artery, together with 

 the sympathetic plexus around it. It is noteworthy that the two carotid canals 

 he in line with the anterior edges of the two external acoustic meatuses. 



The jugular foramen is an opening of irregular shape and variable size placed 

 between the petrous part of the temporal in front and the jugular process of 

 the occipital bone behind. The former is excavated into a hollow called the 

 jugular fossa, which forms a roof to the upper and lateral part of the space, whilst 

 the latter, by a curved edge, either rounded or sharp, constitutes its posterior 

 border. There is often considerable difference in the size of the jugular foramina ; 

 that on the right side (with the skull in its normal position) is usually the larger. 

 The foramen is occasionally subdivided into two by spicules of bone which bridge 

 across it. Lodged within the fossa is the bulb of the internal jugular vein, in front 

 of which the inferior petrosal sinus passes down to join the internal jugular vein 

 below the foramen. Effecting an exit between the two veins, in order from 

 before backwards, are the glosso-pharyngeal, vagus, and accessory nerves. Small 

 meningeal branches from the ascending pharyngeal and occipital arteries also 

 enter the foramen. The two jugular foramina lie in line with a line drawn through 

 the centres of the two external acoustic meatuses. Following the direction of 

 a line connecting the angular spine of the sphenoid and the mastoid process of the 

 temporal, and placed immediately lateral to the apertures of the carotid canal 

 and jugular foramen, is the vaginal process of the tympanic plate of the temporal 

 bone, the edge of which is sharp and thin, and serves to separate the inferior surface 

 of the petrous part of the temporal from the non-articular part of the mandibular 

 fossa. Springing from this crest immediately lateral to the jugular fossa, and 

 in line with the middle of the external acoustic meatus, is the styloid process 

 of the temporal bone. Its relation to the jugular foramen is of great importance 

 as the internal jugular vein lies close to its medial side. 



Immediately behind the root of the styloid process, medial to and in line with 



; ; the front of the mastoid process, is the stylo-mastoid foramen, which is the inferior 

 aperture of the canalis facialis. Through it the facial nerve passes out and the 

 stylo-mastoid branch of the posterior auricular artery passes in. The medial 

 surface of the mastoid process is deeply grooved at its base for the origin of the 



I posterior belly of the digastric muscle. Medial to this, and running along, just 

 wide of the occipito-mastoid suture, is a shallow groove in which the occipital 



>;: artery is lodged. Just medial to the stylo-mastoid foramen is the synchondrosis 



J between the extremity of the jugular process of the occipital bone and the petrous 



part of the temporal. The jugular process is a bar of bone which limits the jugular 



I fossa posteriorly and abuts on the occipital condyles medially ; its inferior surface is 



y convex from before backwards and affords attachment to the rectus capitis lateralis 

 muscle. The occipital condyles are placed between the jugular processes and the 

 foramen magnum. Limited in front by a rounded thickening which becomes 

 confluent with the anterior border of the foramen magnum, they form by their 

 medial sides the lateral boundaries of that aperture on its anterior half. Laterally 

 they are continuous with the jugular processes, in front of which they overhang 

 a fossa which is pierced behind by the 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 fossae are situated just behind the posterior extremities of 

 the condyles. Not infrequently the floor of each is pierced by the condyloid canal, 



12 



