30 ELEMENTS OF MAMMALIAN ANATOMY. 
and has on its lateral wall a horseshoe-shaped prominence 
known as the tympanic ring, to which the tympanic mem- 
brane of the drum of the ear is attached. There are four 
apertures in the walls of the cranial tympanic chamber: The 
meatus anditorius externus, or the external auditory canal; 
the Eustachian tube, leading from the cranial dorsal angle 
of the chamber to the pharynx; the fenestra ovalis and the 
fenestra rotundum, opening into the internal ear. The 
caudal or medial chamber of the bulla is larger than the 
cranial, but has no special features worthy of consideration. 
The petrous portion of the temporal bones is not visible 
on the external surface of the skull, but may be seen by 
looking into the external auditory meatus. It forms the 
medial wall of the tympanic cavity. The foramen seen in 
its dorsal part is the fenestra ovalis. In a sagittally bi- 
sected skull the petrous (Fig. 17) may be recognized by the 
internal auditory meatus entering it. By removing the 
bulla, a large portion of the petrous bone may be seen from 
its ventral aspect. A central prominence, the promontory, 
contains the fenestra rotundum opening into the base of the 
cochlea. The apex of the promontory, mediad of the 
fenestra rotundum, forms the wall of the first whorl of the 
cochlea. Dorsal to the fenestra rotundum is the fenestra 
ovalis, opening into the vestibule of the internal ear. In 
the recent state this opening is closed by a membrane in 
which is imbedded the foot of the stapes. The cochlea in 
the interior of the bone may be displayed by cutting away 
the bony rim of the fenestra rotundum and then chipping 
off a crust of bone in a line from this foramen to the junc- 
ture of the basioccipital and basisphenoid bones. 
The petrous bone, viewed dorsally in a bisected skull, 
appears in the floor of the brain cavity ventrad of the ten- 
torium, the plate of bone partly separating the cerebrum 
from the cerebellum. Its surface is pierced by the internal 
