THE SKULL THE CRANIUM. 



209 



rises. It is obtusely rounded below and is closely applied to the bulla, 

 from which, however, it is separated in front by the stylomastoid fora- 

 men. Its end lies just over the pit in the bulla for the tympanohyal. 

 The external surface is Qjently concave from before backward, and faces 



c? / 



outward and slightly backward. It is roughened, and gives origin for 

 most of its extent to the rectus capitis lateralis muscle. 



The posterior surface of the mastoid (Fig. 142) is of small extent 

 and limited to the upper half of the bone. It is triangular, excavated, 

 and roughened, faces inward, downward, and backward, and artic- 

 ulates with the lower part of the lateral border of the occipital. Its 

 lower outer angle is continued down as the thin posterior border of the 

 mastoid process, separating the external and internal surfaces. The 

 upper angle is continued upward as a roughened border of varying 

 extent and width, which articulates with the thinner part of the 

 lateral border of the occipital. The inner lower angle is continuous 

 with the posterior border of the petrous. The outer border is almost 



FIG. 142. 



Squamosal. with Parietal. 



Zygomatic Process. 



On Roof of Tympanum. 



Ridge marking inner edge of 

 portion of Petrous covered 

 by Tentorium. 



Fossa for Append icnlar Lobe 



of Cerebellum. 

 Over Superior Semicircular 



Canal. 



Over Posterior Semicircu- 

 lar Canal. 



of Middle Cerebral Fossa. 

 AUsjihenoid. 



With <>cci]>/t:i.'. 



Mastoid. 

 Pit for Durn. 



Aquxductus Cochlece. 



Internal Auditory ifeatus. ' 

 Outer Margin of Middle Lacerated Foramen. 



Post-glenoid Process. 



Opening in roof of Tympanum. 



Groove for Eustachian Tube. 



With Basisphenoid. 



Apex of Petrous. 



Beginning of Aquxductus Fallopii. 



Bulla. 

 LEFT TEMPORAL BONE, INNER ASPECT. 



straight, and is directed upward and inward ; the lower border is 

 emarginate, and is directed inward and slightly upward ; the inner 

 border is irregular, and is directed upward and outward. 



The inner surface of the mastoid consists of three parts : (1) A 

 small upper part (Fig. 142), the only portion which appears in the 

 cranial cavity. It is a narrow strip running upward and backward 



14 



