34 THE SKELETON OF THE CAT. 
mastoid portion (Figs. 22 and 23, e¢), which is flattened and 
triangular and is attached by its base to the base of the 
pyramid. 
The petrous portion may be described as having a base and 
three sides, lateral, dorsal, and medial. It completes the 
medial wall of the tympanic bulla, so 
that it is not possible to see it from the 
exterior of a skull except through the 
auditory meatus (Fig. 22, 2). When the 
bones of the skull are articulated its dor- 
sal surface is covered by the tentorium 
and alisphenoid. Its lateral face looks 
Fic. 25.—Petrous Bonz or into the tympanic cavity, while the 
RIGHT SpE, LATERAL medial face looks into the cranial cavity. 
SURFACE, ENLARGED. : i 
a, fenestra cochlese; 4, pro- Its lateral face (Fig. 25) (medial 
“apie tone ae te wall of the tympanic cavity) presents 
tympani muscle; ¢, fossa for Just ventrad of the middle of its base a 
incus and malleus; /, fossa large circular foramen, the fenestra 
continuous with stylomastoid 
foramen; g, foramen leading cochlee (a) (or fenestra rotunda), which 
sosiscralieanalls looks caudolaterad; it leads into the 
cochlea. The fenestra cochlew lies at the summit of a nipple- 
like elevation, the promontory (4), which is continued toward 
the apex of the bone as a gradually diminishing semicylindrical 
ridge, due to the presence within it of the bony cochlea. 
Dorsad of the fenestra cochleez is the much smaller fenestra 
vestibuli (c) (or ovalis) which leads into the vestibule. It is 
occupied in the natural condition by the base of the stapes. 
Dorsocraniad of the fenestra vestibuli is a large fossa (@) 
which contains the tensor tympani muscle. Dorsocaudad of 
this, partly bounded by the squamous portion of the temporal, 
is another large fossa (¢), the cranial end of which is occupied 
by the incus, while its caudal end is occupied by the head of 
the malleus. 
Nearly caudad of this fossa and separated from it by an 
oblique bony septum is a third fossa (/) which is narrow and 
curved. It is continuous with a notch in the mastoid portion 
of the bone. When the tympanic is articulated the notch is 
converted into a foramen (stylomastoid foramen, Fig. 22, 7) 
