274 SKELETON OF THE BABBIT 



which projects into the cranial cavity, and, 

 together with a similar but smaller ridge on 

 the supra-occipital, separates the cerebral and 

 cerebellar fossse from each other. 



On the outer surface of the petrous portion 

 of the periotic, and seen only after removal of 

 the tympanic bone, are two small apertures. 

 Of these the anterior and more conspicuous 

 one is the fenestra ovalis, in which the stapes 

 often remains wedged ; while the posterior and 

 more irregular shaped one is the fenestra 

 rotunda, which is close to the posterior edge 

 of the bone, and overhung by the mastoid 

 process. Immediately below the fenestra 

 ovalis is a rounded sweUing, the promontory, 

 within which the cochlea lies, 

 ii. The tympanies are a pair of somewhat flask- 

 shaped bones, imperfect on their imier sides, 

 and closely applied to the outer surfaces of the 

 periotics. The expanded body of the flask, 

 or bulla, encloses the tympanic cavity ; and 

 the upwardly directed neck, or meatus audi- 

 torius extemus, supports the cartilage of the 

 large external ear. Within the bulla, at the 

 base of the meatus, is an incomplete bony 

 ring, across which the tympanic membrane is 

 stretched during life. 



The Eustachian tube leaves the tympanic 

 cavity by an aperture at the antero-inferior 

 angle of the tympanic bone, between it and the 

 periotic ; and passes forwards to the pharynx. 



The bony canal by which the facial nerve 

 leaves the skull, runs along the posterior 

 border of the tympanic, between it and the 

 mastoid process of the periotic, and opens below 

 at the stylomastoid foramen. 



On the ventral surface of the tympanic 

 bone, near its inner border, is a conspicuous 



