IDS 



THE SKULL. 



[CHAP. 



through the auditory bulla (see Fig. 56) it is seen to be a 

 simple thin-walled bony capsule, imperfect above, where it 

 fits on to the petrosal and squamosal bones, and prolonged 

 externally into the much thickened spout-like floor of the 

 meatus externus. At the inner extremity of this floor is a 

 freely projecting oval lip (/), which gives attachment to the 

 membrana tympani, and which is the original and first ossified 

 ring-like portion of the tympanic bone. In the front of the 



FIG. 56. Section of the left auditory bulla and surrounding bones of a Bear (Ursus 

 ferox). Sq squamosal ; T tympanic ; BO basioccipital ; am external auditory 

 meatus ; t tympanic ring ; e Eustachian canal ; Car carotid canal. (From Proc. 

 Zool. Soc. 1869.) 



floor of the bulla is the groove for the Eustachian canal (e) ; 

 between this and the inferior part of the tympanic ring, a 

 low and thin ridge of bone with a concave free margin rises 

 from the floor of the cavity. This is the only indication of 

 any septum or division of the cavity of the bulla. 



Behind the bulla, the prominent and tuberous paroccipital 

 process projects downwards, outwards and backwards, stand- 



