BASE OF THE SKULL. 179 



a little internal to both ; it varies in size in different individuals, and is often 

 absent; when present, it transmits a small vein. It opens below in the ptery- 

 goid fossa, just at the outer side of the scaphoid depression. Behind and ex- 

 ternal to the latter opening is the foramen ovale, which transmits the third 

 division of the fifth or inferior maxillary nerve, the small meningeal artery, 

 and the small petrosal nerve. On the outer side of the foramen ovale is the 

 foramen spinosum, for the passage of the middle meningeal artery ; and on the 

 inner side of the foramen ovale, the foramen lacerum medium. The lower 

 part of this aperture is filled up with cartilage in the recent state. On the 

 anterior surface of the petrous portion of the temporal bone is seen, from 

 without inwards, the eminence caused by the projection of the superior semi- 

 circular canal, the groove leading to the hiatus Fallopii, for the transmission of 

 the petrosal branch of the Vidian nerve ; beneath it, the smaller groove, for 

 the passage of the smaller petrosal nerve ; and, near the apex of the bone, the 

 depression for the Casserian ganglion, and the orifice of the carotid canal, for 

 the passage of the internal carotid artery and carotid plexus of nerves. 



The Posterior Fossa, deeply concave, is the largest of the three, and situated 

 on a lower level than either of the preceding. It is formed by the occipital, 

 the petrous, and mastoid portions of the temporal, and the posterior inferior 

 angle of the parietal bone; is crossed by three sutures, the petro-occipital, 

 masto-occipital, and masto-parietal ; and lodges the cerebellum, pons Varolii, 

 and medulla oblongata. It is separated from the middle fossa in the median 

 line by the basilar suture, and on each side by the superior border of the 

 petrous portion of the temporal bone. This serves for the attachment of the 

 tentorium cerebelli, is grooved externally for the superior petrosal sinus, and 

 at its inner extremity presents a notch, upon which rests the fifth nerve. Its 

 circumference is bounded posteriorly by the grooves for the lateral sinuses. 

 In the centre of this fossa is the foramen magnum, bounded on either side by 

 a rough tubercle, which gives attachment to the odontoid ligaments; and a 

 little above these are seen the internal openings of the anterior condyloid 

 foramina. In front of the foramen magnum is the basilar process, grooved for 

 the support of the medulla oblongata and pons Yarolii, and articulating -on 

 each side with the petrous portion of the temporal bone, forming the petro- 

 occipital suture, the anterior half of which is grooved for the inferior petrosal 

 sinus, the posterior half being encroached upon by the foramen lacerum pos- 

 terius, or jugular foramen. This foramen 'is partially subdivided into two 

 parts; the posterior and larger division transmitting the internal jugular vein, 

 the anterior the eighth pair of nerves. Above the jugular foramen is the 

 internal auditory foramen, for the auditory and facial nerves and auditory 

 artery; behind and external to this is the slit-like opening leading into the 

 aquaeductus vestibuli ; whilst between the two latter, and near the superior 

 border of the petrous portion, is a small triangular depression, which lodges a 

 process of the dura mater, and occasionally transmits a small vein into the 

 substance of the bone. Behind the foramen magnum are the inferior occipital 

 fossa3, which lodge the lateral lobes of the cerebellum, separated from one 

 another by the internal occipital crest, which serves for the attachment of the 

 falx cerebelli, and lodges the occipital sinuses. The posterior fossae are sur- 

 mounted, above, by the deep transverse grooves for the lodgment of the lateral 

 sinuses. These channels, in their passage outwards, groove the occipital bone, 

 the posterior inferior angle of the parietal, the mastoid portion of the temporal, 

 and the occipital just behind the jugular foramen, at the back part of which 

 they terminate. Where this sinus grooves the mastoid part of the temporal 

 bone, the orifice of the mastoid foramen may be seen ; and, just previous to its 

 termination, it has opening into it the posterior condyloid foramen. 



The External Surface of the base of the skull (Fig. 130) is extremely 

 irregular. It is bounded in front by the incisor teeth in the upper jaws; 

 behind, by the superior curved lines of the occipital bone; and laterally, by 



