194 Anatomy of Skeleton 



In front of the line of pterygoid processes are placed the bones of the face : the 

 pterygoid processes form a part of the support of the facial skeleton. Owing to the 

 presence of the face it is not easy to follow the proper cranial base, but with a little 

 trouble a fair notion of its arrangement can be obtained even with the face bones 

 in situ, and this can be supplemented by an examination of a skull in which they have 

 been removed. 



Thus, looking into the orbits, the roof is seen to be considerably higher than the 

 base of the skull we have so far examined, yet it is formed by the orbital plates of the 

 cranial frontal bone ; at the back part, however, it may be possible to see that each 

 orbital plate articulates with a narrow-pointed process of bone, the lesser wing of the 

 sphenoid, which is separated below by the sphenoidal fissure from the orbital surface 

 of the great wing. 



At its lower border the orbital surface of the great wing becomes continuous with 

 the under or basal surface which we have already examined, so that the base is com- 

 pleted in this region ; examination of this part from the side will show the continuity 

 of the cranial base. But in the middle there is an interval between the orbital plates 

 of the frontal ; a finger in one orbit and thumb in the other will grasp between them a 

 delicate bony mass, the ethmoid, concerned in the formation of the upper part of the 

 nasal cavities, and the upper plate of this mass fills up the interval between the orbital 

 plates in the floor of the cranial cavity. The back surface of the ethmoid comes up 

 against the front surface of the body of the sphenoid, and thus the base is completed 

 centrally. 



The relations to each other of these bones in the base of the cranium can be grasped 

 more completely if this part of the skull is examined from above, the top of the skull 

 being removed (Fig. 162). The upper or cerebral aspect of the base is divisible into 

 three large (primary) fosses, anterior, middle and posterior, each extending from one 

 side to the other. 



The posterior fossa is seen to be floored by the occipital, and the three basal parts 

 of this bone are easily recognised arranged round the foramen magnum. At the sides 

 in front there are obliquely set prominent bones which are directed forwards and inwards 

 and divide the posterior from the middle fossa. Examine these bones and compare 

 their relations with the under surface of the base, and it becomes apparent that these 

 are simply the petrous bones standing up in the cavity of the skull : their inner and 

 posterior aspects form part of the wall of the posterior fossa and have the jugular 

 foramina between them and the occipital, while their upper and front aspects form 

 part of the floor of the middle fossa. External to each petrous bone the squama can 

 be recognised as forming a part of the side wall of the middle fossa. 



The sphenoid can be easily examined from this aspect ; its central portion or body 

 is continuous through the basi-sphenoid with the basi-occiput, and the floor of the 

 posterior fossa can be traced forward on to the sloping back of the sphenoid. The 

 body presents a deep pituitary fossa for the pituitary body, which connects the two 

 lateral portions of the middle fossa across the mid-line, and is overhung from behind 

 by the edge of the back part of the bone. The upper aspect of the body bears a fancied 

 resemblance to a Turkish saddle,* and the back portion, which constitutes the front 

 end of the posterior fossa, is hence frequently termed the dorsum settee. 



On each side of the body the great wings are seen to form a large part of the floor 

 of the middle fossa. Each great wing extends back, as seen on the lower surface, to 

 fit into the angle between the petrous and squamous parts of the temporal : thus the 

 floor of each half of the middle fossa is made up of sphenoid, upper, outer, and front 



* The upper hollow surface is often termed the sclla turcica. 



