THE SKULL THE CEANIUM 245 



is the mastoid foramen, which transmits a vein to the lateral sinus 

 (Fig. 176), and behind the fossa is a smaller furrow for the occipital 

 artery. The inner surface of the mastoid is deeply concave, and 

 lodges the lateral sinus. 



The petrous portion is relatively longer than the petrous in the 

 cat ; its apex reaches beyond the anterior margin of the glenoid cavity. 

 It is a four-sided pyramid ; the outer side is covered by the squamous 

 and tympanic portions except at the apex. 



The posterior surface (Fig. 176) is much narrower than the corre- 

 sponding surface on the petrous of the cat ; the superior border is 

 marked externally by a groove for the superior petrosal sinus and 

 internally depressed for the passage of the fifth cranial nerve. The 

 internal auditory meatus is oblong, and the canal is directed outward 

 and backward. The opening of the aquseductus vestibuli is in the 

 posterior surface of the petrous, behind the auditory meatus. A rudi- 

 mentary appendicular (floccular) fossa is sometimes present, below 

 the groove for the petrosal sinus. 



The anterior surface is wider than the posterior ; its apex, which 

 overhangs the carotid canal, is impressed by the Gasserian ganglion 

 on the fifth nerve. The hiatus Fallopii is small, and lies to the inner 

 side of a small opening for the lesser petrosal nerve. The position of 

 the superior semicircular canal is indicated by a swelling, the eminentia 

 arcuata, external to which is the thin part of the roof of the tym- 

 panum distinguished as the tegmen tympani. A petro-squamous 

 suture often persists between this surface and the squamous. 



The inferior surface (Fig. 177) presents at the outer posterior part 

 the stylo-mastoid foramen, in front of which is the styloid process 

 embraced on the outside by the produced lower margin of the tym- 

 panic plate known as the vaginal process. Lateral to the foramen is 

 the tympano-mastoid suture, or auricular fissure, through which issues 

 the auricular branch (Arnold's) of the tenth cranial nerve (Fig. 178). 

 Medial to the foramen is a quadrate roughened area for articulation 

 with the occipital bone. In front of this area is a deep transversely 

 oval jugular fossa, which contains the beginning of the jugular vein. 

 Its outer wall is pierced by a small foramen, the auricular canaliculus, 

 for the auricular branch (Arnold's) of the tenth cranial nerve. Its 

 inner margin is prolonged as the jugular spine. 



The jugular fossa is separated from the posterior opening of the 



