THE SKULL. 35 



for the exit of the seventh nerve. The fossa gives passage to 

 the seventh nerve and also lodges the stapedius muscle. A 

 groove may be traced from the stylomastoid foramen to the 

 caudal border of the fossa for the tensor tympani muscle, where 

 it passes into a canal (g}. The groove and canal are parts of 

 the canalis facialis or facial canal (aqueductus Fallopii) for 

 the passage of the seventh nerve through the petrous bone. 



The medial surface (Fig. 23, ^) of the petrous portion 

 shows near its middle a fossa, the internal auditory meatus (). 

 This is divided by a partition of bone into a dorsal and ventral 

 part. The dorsal portion is the beginning of the facial canal 

 (aqueductus Fallopii) by which the seventh nerve passes 

 through the petrous bone to emerge at the stylomastoid 

 foramen. The ventral portion shows at its bottom several 

 small foramina for the auditory nerve. 



Dorsocaudad of the internal auditory meatus is a deep 

 fossa (0) for a small lobe, the so-called appendicular lobe, of 

 the cerebellum. This may be called the appendicular fossa. 



The dorsal surface is triangular and presents near its apex 

 a foramen the hiatus facialis (/), the opening of a canal 

 which joins the canalis facialis and transmits the superficial 

 petrosal branch of the nerve of the pterygoid canal (Vidian 

 nerve). That part of the dorsal surface which lies caudad of 

 the hiatus facialis is known as the tegmen tympani. 



The base of the petrous is attached to the mastoid portion 

 (Fig. 23, e). 



(For an account of the structures within the petrous bone 

 and the tympanic cavity, see the description of the internal 

 and middle ear.) 



The mastoid portion (Figs. 22 and 23, e) is attached by its 

 base to the pyramidal petrous portion, with which it forms an 

 angle of about 120 degrees. It appears in the lateral wall of 

 the skull between the parietal bone and the occipital (Fig. 

 40, d\ The lambdoidal ridge is continued on its outer surface 

 to the caudal border of the external auditory meatus. Caudad 

 of the stylomastoid foramen it forms a slight nipple-like 

 eminence, the mastoid process (Fig. 22, /). Its inner face 

 looks into the cranial cavity. 



