64 



OSTEOLOGY 



THE PETROMASTOID PORTION. This consists of the 

 mastoid portion, the thick, conical process of bone 

 behind the external opening of the ear, and a pyram- 

 idal portion called the petrous portion, forms part of 

 the floor of the skull. The mastoid portion contains on 



FIG. 29 



Glenoid fossa. 



% Mastoid 

 foramen.. 



DIGASTRIC. 



Left temporal bone: outer surface. (Gray.) 



section a number of cellular spaces communicating 

 with one another, called the mastoid cells. The 

 mastoid cells open into the middle ear, and contain 

 air. They are lined with a prolongation of mucous 

 membrane from the tympanum, which extends into 

 them through an opening, by which they communicate 



