Skull and Hyoid 195 



surface of petrous, and lower part of squama. A petro-squamous suture runs back 

 from the angle between the petrous and squama. 



Where the great wing forms part of the floor of the fossa it is in a nearly horizontal 

 plane, and its under surface is the inferior, basal, or pterygoid surface already seen 

 below : in front it turns upward and is separated from the lesser wing by the sphenoidal 

 fissure, and this turned-up part carries on its front aspect the orbital surface of the wing. 

 Laterally it turns up on the side wall of the skull, and here its outer surface constitutes 

 the temporal surface of the wing. 



These three surfaces, which we have already inspected, are more or less flat areas, 

 looking in different directions, and therefore separated from each other by sharply- 

 marked borders, but on the cerebral aspect the angles between the corresponding 

 districts are raised and rounded off, making the wing a thick bony mass hollowed to 

 receive the temporo-sphenoidal lobe of the brain. 



The back border of the small wing overhangs the greater wing to a small extent, 

 so that one must look under the former to see the extent of the sphenoidal fissure : 

 the fissure opens into the cavity of the orbit, being between the two wings, which are 

 both concerned in forming part of the orbital walls. 



The anterior fossa is floored mainly by the orbital plates of the frontal, but behind 

 these are the lesser wings with which they articulate, and between them is the cribriform 

 plate of the ethmoid so termed because it is pierced by many foramina for the passage 

 of the olfactory nerve fibres. This upper surface of the ethmoid articulates behind 

 with a raised part of the sphenoid connecting the two lesser wings and frequently 

 termed the jugum sphenoidale, which is also in the floor of the anterior fossa. 



As soon as the arrangement of the bones in the fossae is seen and mastered, the 

 student can proceed to the recognition of some of the more important points visible 

 (see Fig. 162) on this aspect of the base. The crista galli is a blunt process of bone on 

 the ethmoid for attachment of the " falx cerebri " : this is the sagittal sheet of dura 

 mater that lies between the two cerebral hemispheres. Immediately behind the jugum 

 sphenoidale is the optic groove, leading on each side to the optic foramen, through which 

 the optic nerve and ophthalmic artery pass to the orbit. The olivary eminence is 

 between this groove and the pituitary fossa. The groove, foramina, eminence, and 

 fossa are all in the middle primary fossa. 



The upper opening of the foramen lacerum is seen just in front of the apex of the 

 petrous : here the internal carotid artery emerges from the apex and runs forward 

 on to the side of the body of the sphenoid, which accordingly shows a carotid groove 

 here. 



The internal auditory meatus, for the passage of the facial and auditory nerves, 

 is easily recognised on the inner side of the petrous, and the jugular foramen is situated 

 below it, between the petrous bone and the ex-occipital : large grooves are seen leading 

 to this foramen, which lodge the venous sinuses that form the internal jugular vein. 

 Internal to this again, and on a lower level, near the margin of the foramen magnum, 

 is another canal that ends below in front of and external to the condyle, and is therefore 

 termed the anterior condylar foramen (Fig. 162). 



Behind the foramen magnum is a median internal occipital crest, ending above 

 in the internal occipital protuberance. 



Grooves for the lateral sinuses run horizontally forward on each side from the 

 protuberance and turn down on the petro-mastoid to reach the jugular foramina. 

 The tentorium cerebelli, the partial sloping diaphragm of dura mater that separates 

 the cerebrum and cerebellum, is attached along the horizontal parts of the grooves 

 and then along the prominent upper edges of the petrous bones to the projecting 



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