THE SKULL THE CRANIUM 205 



as long as the surface itself. Posterior to this point is a nearly circular 

 opening called the fenestra rotunda, or round window, which faces 

 outward, backward, and upward and opens into the base of the cochlea. 

 The promontory is crossed just in front of its highest convexity by a 

 faint, oblique groove which is directed transversely to the long axis 

 of the skull. The highest part of the promontory, behind this groove, 

 marks the large first whorl of the cochlea ; the lower, smaller part in 

 front of it contains the two smaller whorls. 



The region of the inferior surface within and behind the promon- 

 tory, beginning at the notch for the Vidian nerve on the anterior 

 border and extending as a strip along the inner border, is slightly 

 concave transversely and is applied to the upper surface of the intro- 

 verted plate of the entotympanic, which forms part of the roof of the 

 larger medial chamber of the bulla. The promontory itself displays 

 along its long axis a slight flattening, which marks the place where it 

 rests on the septum of the bulla. On its outer face, which forms part 

 of the roof of the outer or smaller chamber of the bulla, the true 

 tympanum, are grooves for the tympanic plexus of vessels and nerves. 

 Only a portion of the petrous is seen as the roof of the inner or medial 

 chamber of the bulla, and this is the inner and posterior part of the 

 promontory. 



The remaining part of the inferior surface of the petrous lies on 

 the outer side of the promontory, and enters into the formation of the 

 roof of the tympanum. It is triangular, bounded in front by the 

 basal portion of the anterior border, which joins the alisphenoid, and 

 behind by the basal border, which joins the squamous. Occupying 

 the anterior half is a deep, oblong, oblique fossa, which contains 

 the tensor tympani muscle. An arcuate, transverse, elevated line 

 separates this fossa behind from three features, a foramen, a bony 

 bar, and a fossa. 



The foramen lies to the medial side on the outer wall of the 

 promontory, and is called from its shape the fenestra ovalis, or oval 

 window. Its long axis is directed from before backward, and it faces 

 almost directly outward. In the macerated skull it opens into the 

 vestibule of the inner ear, but in the natural state it is occupied by the 

 base of the stapes, one of the ossicles of the ear. 



On the outer side is a somewhat crescentic and slightly concave 

 area, which, with the concavity on the under surface of the squamous, 



