THE ENTIRE SKULL 481 



side in the solid substance of the petrous portion of the temporal bone, 

 while the other two, the tympani or middle ears, lie on each side 

 below them between the base of the skull and the auditory bulla. 

 They contain the small auditory ossicles, communicate medially with 

 the labyrinths, open to the exterior through the Eustachian opening 

 into the pharynx, and, when the membranous drum of the ear is 

 removed, through the external auditory meatus into the cartilaginous 

 external ear. 



CRANIAL CAVITY. 



The Cranial Cavity occupies that greater part of the skull which 

 lies above and behind the orbital cavities. Inasmuch as its walls are 

 nearly everywhere of uniform thinness, the size and general shape of 

 the cavity are well exhibited by the exterior form. The cranial cavity 

 is an oval chamber about twice as long as high and one-third longer 

 than wide. From the floor, which is for the most part flat, the concave 

 sides rise without a dividing line and curve over above to form the 

 vaulted roof. The posterior wall is straight and nearly vertical ; it 

 is pierced at its lower part by the median foramen magnum. The 

 anterior wall is concave from side to side and slopes forward as well 

 as upward. It is prolonged forward as the sides of a narrow quadrate 

 olfactory fossa which is closed in front by the cribriform plate of the 

 ethmoid, the anterior boundary of the entire cranial cavity. 



The cranial cavity is enclosed by the frontals, the parietals, the 

 interparietal, the occipital, the temporals, and the sphenoid. These 

 bones are so arranged that the floor is formed by the sphenoid and the 

 occipital ; the anterior wall by the frontal ; the sides and roof by the 

 frontals, the parietals, and the interparietals ; and the posterior wall 

 by the occipital only. 



The cranial walls exhibit constrictions, projections, and variations 

 in direction which divide the cavity into the olfactory and the anterior, 

 middle, and posterior cranial fossse. The posterior fossa is separated 

 from the middle fossa by the sloping imperfect partition formed of 

 the tentorial processes of the parietals ; the middle fossa is well defined 

 from the anterior fossa in front by the elevation of the floor and the 

 change in its inclination and by lateral ridges beginning behind the 

 optic foramina and continuing for some distance upward on the side 

 walls. The anterior fossa is defined in front from the olfactory fossa 



by the abrupt constriction of the cranial walls. 



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