3 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 auditorius 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 



