THE SKULL THE CEANIUM 215 



The internal surface is thickened in a narrow band round the inner 

 rim of the external auditory meatus. To the sharp, everted, lower 

 edge of this rim the drum of the ear, the membrana tympani, is 

 attached. 



The inner chamber of the bulla is much larger than the outer 

 chamber, and extends further back. It is larger behind than in front. 

 A transverse section made perpendicular to the septum is cresceutic. 



The superior surface (Fig. 147) of the bulla is formed by the 

 turning of the upper edge of the external surface inward in all di- 

 rections toward the centre of the bulla. The parts belonging to the 



FIG. 147. 



Eustachian Groove. 



Ectotympanic Portion. 



-Anterior Angle. 



-Enters Notch in Basinphenoid. 



f* f'i / -/&%? -f&r w : ^m. 



Groove for Chorda Tympani.- 



Attached to Squamous. I 

 Notch for 



SUPERIOR SURFACE OF LEFT AUDITORY BULLA. 



entotympanic and ectotympanic are separated behind by a notch 

 forming part of the pit for the tympano-hyal, and in front by a 

 groove, the lower wall of the Eustachian canal. 



The inner, entotympanic portion is crescentic, the emargination 

 facing outward and forward. In general, this surface is convex trans- 

 versely and is wide and flat in front, and narrow and more convex 

 behind. The anterior border of the entotympanic portion of the supe- 

 rior surface of the bulla is the sharp edge of the Eustachian groove. 

 It is straight and directed obliquely forward and inward, and the inner 

 angle is produced into a sharp process. The emarginate outer border 

 is sharp and notched near the middle. The arcuate inner border is 

 parallel with the outer border behind, but straight in front. It is not 

 clearly defined from the general external surface of the bulla. On the 

 flat part in front, just behind the anterior border and parallel with 

 it, is an elevated ridge which is prolonged inward and forward to a 

 point. The inner surface of this ridge is fastened to the outer surface 



