in.] THE CRANIAL SKELETON. 89 



Between the outer side of the petrous bone and the adja- 

 cent margin of the ala of the sphenoid is a groove for the 

 reception of the Eustachian tube. 



Behind the foramen magnum is the great occipital expanse. 



On each side of the base of the skull we find the zygoma 

 (arching but little outwards), the glenoid surface, the Eternal 

 auditory meatus, and the mastoid process. 



Looking at the side of the skull, we see a fissure not yet 

 noticed. This is the ptery go-maxillary, which runs up be- 

 tween the posterior border of the maxilla and the adjacent 

 pterygoid process. At its summit it meets the inner end 

 of another fissure, namely the spheno-maxillary, which runs 

 forwards and outwards between the inferior margin of the 

 great ala of the sphenoid and the upper part of the maxilla. 



These two fissures by their junction form an angular 

 depression called the spheno-maxillary fossa, and into it 

 open the foramen rotundum, the vidian and spheno-pala- 

 tine foramina, and the upper end of the posterior palatine 

 canal. 



15. In the INSIDE OF THE SKULL, as seen from above 

 when a horizontal section is made and its top removed, we 

 note the following structures. In the middle, in front, the 

 crista galli and cribriform plate, bounded on each side by the 

 orbital plates of the frontal (which are but slightly convex), 

 and behind by the anterior part of the sphenoid and orbital 

 wings, each pierced at its base by the optic foramen, and 

 having a sharp posterior margin marking off the hinder limit 

 of what is called the anterior fossa of the skull. 



The middle fossa includes, in its centre, the basilar part 

 'of the sphenoid (with the pituitary fossa and posterior clinoid 

 process), and on each side, the great ala of the sphenoid 

 (pierced towards its root by the round and oval foramina, 

 and leaving the sphenoidal fissure between it and the orbital 

 wing), the squamous part of the temporal bone, and the an- 

 terior part of its petrous portion at the inner end of which is 

 seen the anterior end of the carotid canal opening immedi- 

 ately above the foramen lacerum medius. 



A ridge running forwards and inwards along the petrous 

 bone defines its anterior part from its posterior, and at the 

 same time the anterior margin of the posterior fossa of the 

 skull. This ridge has attached to it a membrane called 

 the tentorium, which divides the cerebellum from the cerebral 

 part of the brain. The posterior fossa includes the hinder 

 part of the petrous bone with its internal auditory meatus, 



