THE SKELETON. 45 



process is the glenoid cavity for the articulation of the 

 condyle of the mandible. Immediately caudad of this 

 cavity is the postglenoid process. The mastoid portion 

 of the bone is somewhat triangular in shape, about two 

 centimeters long, and lies caudad of the external meatus. 



The tympanic portion appears on the base of the skull 

 as the auditory bulla. Its cavity is divided into two 

 unequal chambers by a bony septum rising from the 

 floor and reaching almost to the roof. The cephalic 

 or true tympanic chamber, sometimes called the middle 

 ear, is the smaller, and has on its lateral wall a horseshoe- 

 shaped prominence known as the tympanic ring, to 

 which the tympanic membrane of the drum of the ear 

 is attached. There are four apertures in the walls of 

 the cephalic tympanic chamber: The meatus auditorius 

 externus, or the external auditory canal; the Eustachian 

 tube, leading from the cephalic dorsal angle of the 

 chamber to the pharynx; the fenestra ovalis and the 

 fenestra rotundum, opening into the internal ear. The 

 caudal or mesal chamber of the bulla is larger than the 

 cephalic, but has no special features worthy of consider- 

 ation. 



The petrous portion of the temporal bone is not visible on 

 the external surface of the skull, but may be seen by look- 

 ing into the external auditory meatus. It forms the mesal 

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

 part is the fenestra ovalis. In a sagittally bisected skull the 

 petrous (Fig. 17) may be recognized by the internal audi- 

 tory 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, con- 

 tains the foramen rotundum opening into the base of 

 the cochlea. The apex of the promontory, mesad of 

 the fenestra rotundum, forms the wall of the first whorl 



