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, 0) of the petrous portion 
shows near its middle a fossa, the internal auditory meatus (vz). 
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, @). 
(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, ¢) 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, 2). 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. 
