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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



slender one downwards. The latter is bent up at its tip, and connected by 

 a minute disc (3), the os orbicutare, with (4) the stirrup-shaped stapes, the 

 oval base of which fits into the fenestra ovalis. The Eustachian tube enters 

 the tympanic cavity below and in front by a very short canal, formed by 

 the apposition of a groove in the tympanic to the surface of the periotic. 

 The par-occipital and mastoid processes overlap the tympanic behind, and 

 hold it in place. 



The cavity of the brain-case closely corresponds to the shape 

 of the brain, and is divided into a small olfactory fossa in front, 

 ill-marked off from the large cerebral fossa, which extends back 

 to the anterior limit of the periotics and supra-occipital, where a 

 sharp ridge separates it from the cerebellar fossa. 



NERVE-EXITS. 



