CHAP, in.] SKELETON OF THE HEAD AND TRUNK. 83 



^In the middle, in front, is a small fossa bounded anteriorly by the 

 cribriform plate, and on each side by the orbital plates of the frontal. 

 This is the olfactory, or rMnencephaUc, fossa, which shelters the 

 olfactory lobes of the brain. The hinder part of its floor is formed 

 by the pre-sphenoid. 



Behind the olfactory fossa is the great middle fossa of the cranium, 

 bounded posteriorly by the ossified tentorium, which is attached to 

 the parietals and petrosals. In the middle of the floor of this great 

 chamber is the pituitary fossa, or sella turcica (Fig. 49), situated on 

 the upper surface of the basi-sphenoicl. On each side of this (placed 

 successively in series running backwards and outwards) are (1) the 

 optic foramen (piercing the orbito-sphenoid) ; (2) the sphenoidal 

 fissure (placed beside the anterior wall of the pituitary fossa) ; 

 (3) the foramen rotundum ; and lastly (4) the oval foramen ; the 

 latter being on a line with the hinder part of the pituitary fossa. 



External to this line of foramina on each side, there is a depression 

 (on the upper surface of the ali-sphenoid and squamcsal) which is 

 called the internal temporal fossa, because it shelters the temporal 

 lobe of the brain. 



Behind the pituitary fossa is the clinoid plate, and behind and 

 external to this on each side is an opening between the petrosal, 

 the ali-sphenoid, and the anterior end of the tentorium, which 

 opening communicates with the cavity of the auditory bulla and with 

 the foramen lacerum anterius. 



Behind the clinoid plate and the tentorium, is the posterior fossa of 

 the cranium called also the cerebellar fossa, because it shelters that 

 part of the brain called the cerebellum. The floor of this fossa is formed 

 by the basi- occipital, and its sides by the ex-occipitals and petrosals. 



Between the basi-occipital and the adjacent petrosal there runs 

 forwards on each side a very minute canal destined for the carotid 

 artery. The surface of the petrosal exhibits the conspicuous 

 opening to the internal ear the meatus auditorius internus with 

 the fossa above it for a process of the cerebellum (Fig. 35, B, cb), 

 above and behind which again is the prominence indicating the 

 anterior vertical semi-circular canal. 



Behind each foramen lacerum posterius, and separated from it 

 by a narrow bridge of bone, is the anterior condyloid foramen. The 

 inner surface of the sides and roof of the cranium is marked by wide 

 and shallow depressions corresponding with eminences on the surface 

 of the brain, and also with narrow meandering grooves indicating tbe 

 course of the blood-vessels. 



The skull VERTICALLY BISECTED, in the direction of the sagittal 

 suture, exposes the larger size of the brain cavity than of the part 

 which forms the face ; also the inclined condition of the cribriform 

 plate (Fig. 48, cp) and the more nearly vertical position of the foramen 

 magnum at quite the hinder end of the skull. 



The basilar parts of the occipital and sphenoid bones (Fig. 49, bo, bs, 

 and ps,) are seen to become thicker as we go forwards ; they form the 

 true axis of the skull, and a line drawn from the anterior margin of 



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