THE SKELETON 43 



of the tympanic cavity. The foramen seen in its dorsal part 

 is the fenestra vestibuli. In a sagittally dissected skull the 

 petrous (Fig. i8) may be recognized by the internal acoustic 

 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 

 cochleae opening into the base of the cochlea. The apex of 

 the promontory, mediad of the fenestra cochleae forms the wall 

 of the first whorl of the cochlea. Dorsal to the fenestra 

 cochleae is the fenestra vestibuli, 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 cochleae 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, 

 the plate of bone partly separating the cerebrum from the 

 cerebellum. Its surface is pierced by the internal acoustic 

 meatus, which gives passage to the acoustic nerve. Close 

 examination reveals a division of the canal into two parts, a 

 ventral for the eighth nerve and a dorsal, the canalis facialis, 

 for the facial nerve. This canal twists through the petrous 

 laterad, and thence between the petrous, squamosal, and mas- 

 toid to the stylomastoid foramen (Fig. i8, sf). 



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 of the 

 brain. The plate of bone projecting obliquely craniad from 

 the caudal border of the parietal is the tentorium, an ossifica- 

 tion of the dura mater separating the cerebrum from the 

 cerebellum. 



