46 ELEMENTS OF MAMMALIAN ANATOMY. 



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 juncture 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 

 tentorium. Its surface is pierced by the internal auditory 

 meatus, which gives passage to the auditory nerve. 

 Close examination reveals a division of the canal into 

 two parts, a ventral for the eighth nerve and a dorsal, 

 the aqueductus Fallopii, for the facial nerve. This 

 aqueduct twists through the petrous laterad, and thence 

 between the petrous, squamosal, and mastoid to the 

 stylomastoid foramen. 



The parietal bone is paired and joins its fellow in the 

 median line, forming the caudal half of the sagittal 

 suture. Its point of greatest convexity is the parietal 

 eminence. Its cerebral or internal surface presents 

 slight arborescent grooves which in the recent state 

 sheltered the meningeal artery. The plate of bone pro- 

 jecting obliquely cephalad from the caudal border of 

 the parietal is the tentorium, an ossification of the dura 

 mater separating the cerebrum from the cerebellum. 



The interparietal is a triangular bone situated at the 

 junction of the two parietals and occipital bones. Its 

 sutures are usually obliterated quite early. 



The occipital (Figs. 16, 17, and 18) is a single bone 

 surrounding the foramen magnum and articulating with 

 the interparietal, parietals, temporals, and sphenoid. In 



