THE SKULL THE CEANIUM 201 



band winds spirally around the wall of the auditory pit and terminates 

 in the circular plate in front. This band constitutes the spiral crib- 

 riform tract and transmits nerves to the cochlea. The facial canal 

 begins as a deep groove in the anterior wall of the auditory meatus, 

 and runs upward and backward and enters the petrous by a wide 

 opening under the overhanging upper auditory rim. At this point 

 is another deep pit, the porous floor of which constitutes the superior 

 cribriform spot, seen in the fovea hemielliptica of the vestibule. The 

 progress of the facial canal through the bone will be traced hereafter. 



Near the upper basal angle of the surface, and separated from 

 the internal auditory meatus by a rounded oblique bony ridge, is 

 a large deep fossa, which receives the appendicular lobe of the 

 cerebellum. It is irregularly triangular. Its anterior boundary is 

 the upper part of the sharp superior border. Its posterior and 

 upper wall is formed by a prominent oblong swelling, which marks 

 the position within the bone of the superior semicircular canal. 

 Behind and above this swelling, occupying the region as far as the 

 upper edge of the surface, is often found a shallow groove. The 

 continuation backward of the convex surface lying between the 

 appendicular fossa and the internal auditory meatus marks the 

 position within the bone of the posterior semicircular canal. Where 

 this ends, on the posterior border is a cleft, which penetrates the 

 bone deeply, affords attachment to a fold of the dura mater, and 

 leads into the internal ear through the aquseductus vestibuli. The 

 remaining portion of this surface is concave from above downward 

 in front and behind the auditory meatus, and slightly convex from 

 above downward along the posterior border. 



The anterior surface of the petrous (Fig. 138) is also triangular, 

 but much narrower than the other two surfaces. It is at least three 

 times as long as it is wide at the widest place, near the outer basal 

 border. It consists of two well-defined parts : (a) an outer and 

 posterior prominent quadrate portion, and (b) an inner, anterior, 

 depressed triangle. Where the two regions join is seen a small 

 slit-like or rounded foramen, known as the hiatus Fallopii, which 

 leads into the facial canal or aquseductus Fallopii. 



(a) The outer, prominent part of the surface is convex from 

 without inward, and marked by a faint arborescent groove running 

 up from the hiatus Fallopii near the anterior border. It is quadrate 



