THE SKULL THE CEANIUM 



217 



is firmly attached to an area on the under surface of the squamous. 

 The outer arcuate border is rounded. The plane of this surface faces 

 outward, forward, and upward. It is applied to the lower surface of 

 the alisphenoid and to the squamous of the temporal, without touch- 

 ing at all points ; it thus forms the lower wall of the Glaserian fissure ; 

 the deep groove for the chorda tympani becomes closed to form the 

 canal of Huguier. 



The Tympanum. The outer chamber of the auditory bulla, lying 

 between the base of the skull and the ectotympanic, is known as the 

 tympanum, or middle ear (Figs. 145, 148) . It is a lens-shaped cavity ; 



FIG. 148. 



Parietal. 



Arrow through 

 Middle Lace- 

 rated Foramen. 



Root of Tentorium. 

 Ectotympanic 

 Hook. 



Rim f External 

 Auditory Meatus. 



Post-glenoid Foramen. 



Outer Chamber of Bulla, 

 or Tympanum. 



Ectotympanic. 



Inner Chamber of Bulla. Entotympanic. 



THE SKULL. TRANSVERSE SECTION THROUGH EXTERNAL AUDITORY MEATUS, SEEN 



FROM BEHIND. 



when seen from the side its outline is almost circular, but seen from 

 above or below it appears as a long oval. The vertical and longi- 

 tudinal diameters are, therefore, almost equal, and are twice as great 

 as the transverse diameter. 



The cavity does not lie parallel, however, to the median longi- 

 tudinal vertical plane of the skull, but is so placed that its antero- 

 posterior diameter passes from in front backward and outward and its 

 vertical axis is directed from above downward and inward. The cavity 

 is not perfectly regular ; its walls are marked by the eminences and 

 depressions on the investing bones. It is widest below the middle. 



