THE BASE OF THE SKULL. 211 



on the inner side of the foramen ovale, the foramen lacerum medium. This 

 aperture is filled up with fibrous tissue in the recent state. The Yidian nerve 

 and a meningeal branch from the ascending pharyngeal artery pierce this 

 cartilage. On the anterior surface of the petrous portion of the temporal bone is 

 seen, from without inward, the eminence caused by the projection of the superior 

 semicircular canal : outside this a depression corresponding to the roof of the 

 tympanum : the groove leading to the hiatus Fallopii, for the transmission of the 

 petrosal branch of the Yidian nerve and the petrosal branch of the middle 

 meningeal artery ; beneath it, the smaller groove, for the passage of the lesser 

 petrosal nerve : and. near the apex of the bone, the depression for the Gasserian 

 ganglion : and the orifice of the carotid canal, for the passage of the internal carotid 

 arterv 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 posterior third 

 of the superior surface of the body of the sphenoid, by the occipital, the petrous 

 and rnastoid portions of the temporal, and the posterior inferior angle of the 

 parietal bone : it is crossed by four sutures, the petro-occipital. the masto-occipital. 

 the rnasto-parietal. and the basilar : and lodges the cerebellum, pons Yarolii, and 

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

 the dorsum ephippii. and on each side by the superior border of the petrous portion 

 of the temporal bone. This border serves for the attachment of the tentorium 

 cerebelli. is grooved for the superior petrosal sinus, and at its inner extremity 

 presents a notch, upon which rests the fifth nerve. The circumference of the 

 fossa is bounded posteriorly by the grooves for the lateral sinuses. In the centre 

 of this fossa is thefor<i)/i>.-/i magnum, bounded on either side by a rough tubercle, 

 which gives attachment to the odontoid or check ligaments ; and a little above 

 these are seen the internal openings of the anterior condyloid foramina, through 

 which pass the hypoglossal nerve and a meningeal branch from the ascending 

 pharvngeal artery. In front of the foramen magnum is a grooved surface, formed 

 by the basilar process of the occipital bone and by the posterior third of the superior 

 surface of the body of the sphenoid, which supports the medulla oblongata and 

 pons Yarolii. and articulates 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 fae foramen 



im pott. ://'?/.*. or jugular foramen. This foramen presents three compartments : 

 through the anterior passes the inferior petrosal sinus : through the posterior, the 

 lateral sinus and some meningeal branches from the occipital and ascending 

 phai-yngeal arteries : and through the middle, the glosso-pharyngeal, pneumo- 

 gastric, and spinal accessory nerves. Above the jugular foramen is the internal 

 awl.itorii in cat n*. for the facial and auditory 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, 



-mall, 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 rV/.?.*,/-. which lodge the hemispheres 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 sinus. The 

 3e are surmounted, above, by the deep transverse grooves for the 

 lodgment of the lateral sinuses. These channels, in their passage outward, 

 groove the occipital bone, the posterior inferior angle of the parietal, the rnastoid 

 portion of the temporal, and the jugular process of the occipital, and terminate at 

 the back part of the jugular foramen. Where this sinus grooves the mastoid portion 

 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. 

 Neither foramen is constant. 



The External Surface of the Base of the Skull (Fig. 173) is extremely irregular. 

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



