THE SKULL THE CEANIUM 203 



in outline, the anterior and posterior borders being straight and 

 parallel. The outer border is slightly arcuate, and the outer anterior 

 angle rounded. Just in front of this angle, "on the anterior border, 

 is a more or less prominent spine or hook, which marks the anterior 

 limit of the articulation of the petrous with the squamous, and also 

 the outer boundary of a vacuity in the roof of the tympanum between 

 the petrous, the ectotympanic, and the entotynipanic, which is ex- 

 posed by the removal of the overlying bony tentorium (Fig. 135). 

 This region of the surface lies directly over the tympanic cavity, and 

 especially that part which contains the tensor tympani muscle, and is 

 known as the tegmen l tympani. 



(b) The inner, small, triangular portion of the anterior surface 

 is depressed and concave from before backward and from without 



FIG. 138. 



Spine on Anterior Border. 

 Tegmen Tympani. 



Superior Border- f -ttia.tu* FaUopii. 



PETROUS PORTION OF TEMPORAL BONE. ANTERIOR SURFACE. 



inward. Its base is at the hiatus Fallopii ; its sharp upturned apex 

 is the apex of the entire bone. Its anterior border is arcuate and 

 sharp, but posteriorly it passes without break into the posterior 

 surface. It supports the Gasserian ganglion on the fifth cranial 

 nerve, and is the only part of the anterior surface visible in the 

 cranial cavity, as it is not covered by the tentorium. 



The inferior surface of the petrous (Figs. 137, 139) is wider and 

 shorter than the posterior surface. It is triangular or pear-shaped. 

 The inner boundary is the posterior border of the bone. This tends 

 almost directlv from before backward, and turns outward at the 



i/ 



posterior end. Its anterior end is covered by the basisphenoid. For 

 its anterior two-thirds it is gently sinuate, and lies near the outer 

 edge of the basilar plate of the occipital. The posterior third is 

 emarginate, often very deeply, to form with the occipital the jugular 



1 A roof. 



