302 ■ THE SKELETON OF THE EABBIT. 



the cerebellum. The lower one is divided by a 

 ridge of bone into two, of which the anterior 

 one is the opening of the aquseductus Fallopii, 

 transroitting the facial nerve ; and the posterior 

 one is that of the meatus aaditorius internus, 

 through which the auditory nerve enters the 

 internal ear. 



The anterior border of the periotic is pro- 

 duced inwards into a prominent crest of bone 

 which projects into the cranial cavity, and, 

 together with a similar but smaller ridge on 

 the supra-occipital, separates the cerebral and 

 cerebellar fossffi 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 o.valis, in which the stapes 

 often remains wedged ; while the posterior 

 and more irregularly 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 swelling, the promontory, 

 within which the cochlea lies, 

 ii. The tympanies are a pair of somewhat flask- 

 shaped bones, imperfect on their inner 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 externus, 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 Ufe. 



The Eustachian tube leSves the tympanic 

 cavity by an aperture at the antero-inferior 



