468 MAMMALIAN ANATOMY 



lary branch of the trigeminal nerve, beyond which the base of the 

 cranium slopes upward and forward, passing into the posterior wall of 

 the orbital cavity. On the outer side of the foramen ovale the surface 

 is continuous over the lower part of the squamo-sphenoidal suture, 

 with the wide transverse glenoid cavity on the posterior root of the 

 zygomatic arch. 



The posterior edge of the glenoid cavity is elevated internally into 

 the postglenoid process, behind which is seen the variable post- 

 glen oid venous foramen. 



The major portion of the remaining inferior surface of the skull 

 is occupied by the two large auditory bullae. These prominent, ovoid, 

 bony sacs are situated on each side at the outer posterior part, leaving 

 a flattened oblong space between them. The outline of the bulla is 

 oval, but slightly wider in front than behind. The long axis is con- 

 tained four times in the total length of the skull, and is a fourth 

 greater than the transverse axis. It is directed from behind obliquely 

 inward and forward. The outer wall of the bulla is pierced for the 

 external auditory meatus, above and exterior to which is' seen the 

 narrow under surface of the posterior root of the zygoma. At the 

 anterior end, on the inner side, is the Eustachian opening, on the 

 inner wall of which is the minute opening of the carotid canal, and on 

 the outer side are the Glaserian fissure and the opening of the canal 

 of Huguier. A little behind the middle of the inner margin of the 

 bulla and between it and the basioccipital plate is the small posterior 

 opening of the canal for the rudimentary internal carotid artery. 

 Behind this, again, is the large triangular jugular foramen, at the 

 lower end of the posterior wall whereof is seen the anterior opening 

 of the anterior condyloid foramen. The stylo-hyoid muscle takes 

 origin from the posterior margin of the bulla. Behind the bulla 

 is the transverse lower edge of the paroccipital process, the outer 

 end of which is roughened to afford attachment to the digastric muscle. 



On the outer side of the bulla, behind the auditory meatus and 

 directed downward and forward, is the mastoid process. To its lower 

 edge are attached the stylo-giossus and cleido-mastoid muscles. In 

 front of the mastoid process is the stylo-mastoid foramen, through 

 which the facial nerve gains exit from the skull. Just within the tip 

 of the mastoid process is the pit for the insertion of the tympano-hyal 

 element of the hyoid apparatus. 



