204 



MAMMALIAN ANATOMY 



foramen. The posterior inner angle is rounded, and abuts against the 

 exoccipital. The posterior outer boundary of this surface is arcuate, 

 and is united with the mastoid and squamous. The union with the 



FIG. 139. 



Apex. 



Posterior Border. 



Promontory. 

 Fenestra Rotunda. 



Anterior Border. 



r TENSOR TYMPANI. 



Part of Fossa for Malleus and Incus, 

 Fenestra Ovalisjust beyond. 



Over External Semicircular Canal. 



With Mastoid. 



Jugular Notch. 

 PETROUS PORTION OF TEMPORAL BONE. INFERIOR SURFACE. 



mastoid is short, but complete ; that with the squamous embraces 

 fully the outer half of the border and can be easily broken, leaving 

 a nearly straight, sometimes even slightly emarginate, edge. The 

 posterior outer angle is almost a right angle, and is slightly rounded. 



The anterior external boundary is the anterior border of the bone. 

 This is directed from without obliquely inward and forward to meet 

 the posterior border at the apex. It presents three nearly equal 

 parts. The first of these is the sharp outer third ; it articulates with 

 the squamous and extends from the angle to the tip of the spine 

 already mentioned as visible on the front edge of the anterior 

 surface. Internal to this portion is a middle third, nearly straight, 

 rounded and inverted, and directed slightly more transversely to the 

 axis of the bone ; it articulates with the alisphenoid. A decided 

 notch for the passage of the Vidian nerve separates this middle third 

 from the inner third, which is again sharp and takes the same 

 direction as the outer third, and articulates with the basisphenoid. 



The inferior surface is convex in both directions in such manner 

 that, while the inner part faces almost directly downward, the outer 

 part faces outward and downward, and, while the anterior part faces 

 downward and slightly forward, the posterior outer part faces down- 

 ward and backward. The most striking feature on this surface is the 

 prominent central swelling, the promontory, caused by that part of 

 the internal ear called the cochlea. The promontory is ovoid, about 

 twice as long as it is wide ; its long diameter lies slightly to the inner 

 side of the long axis of the surface and parallel to it, and is about half 



