THE SKULL THE CEANIUM 207 



lates with the squamous. The inner part of the border is sharp and 

 arcuate, overlaps the cranial surface of the basisphenoid, and joins the 

 other two borders of the petrous at the apex. 



The posterior border (Fig. 139) has the same general direction as 

 the other borders, but is more nearly longitudinal. It presents an 

 upper and a lower sharp edge, and with the contiguous border of the 

 occipital forms a groove for the lower petrosal sinus. It is arcuate, 

 especially in front. Behind it is notched to assist in forming the 

 jugular foramen. A short distance in front of the jugular notch is the 

 small slit-like opening of the aquseductus cochleae, which leads into 

 the internal ear. The border lies against the lateral border of the 

 basioccipital, the cranial surface of which it slightly overlaps in front. 



The superior border of the petrous (Fig. 135) is seen in the cranial 



FIG. 140. 



OUTER VIEW. INNER VIEW. 



POSITION OF MASTOID. 



cavity of the skull in its completeness when the tentorium, which 

 covers its outer two-thirds, is broken away. It is directed from without 

 obliquely inward, forward, and downward. In the outer two-thirds it 

 is elevated and arcuate forward, and separates definitely the anterior 

 and posterior surfaces. The inner third is low and concave, permitting 

 the two surfaces to pass into each other. The superior border meets 

 the posterior and anterior borders at the apex of the bone, and, behind, 

 forms an acute angle with the basal border of the anterior surface and 

 almost a right angle with the corresponding border of the posterior 

 surface. The superior border gives attachment to the dura mater, in 

 which is included the superior petrosal sinus. 



THE MASTOID PORTION. 



The mastoid lies at the back of the temporal bone, behind and 

 below the squamous, behind and lateral to the petrous, and above and 



