Skull and Hyoid 225 



The front of the external plate widens in its upper part ; this is the part that lies 

 behind the upper jaw and forms the back wall of the spheno-maxillary fossa of which 

 the vertical plate of the palate, articulating with the inner pterygoid plate, forms the 

 inner wall, and the tuberosity of the palate, fitting in between the pterygoid plates 

 behind and the jaw in front, makes the lower boundary. This spheno-maxillary 

 surface of the external plate is continuous with the area already noticed immediately 

 below the lower edge of the orbital surface of the great wing, on to which the foramen 

 rotundum opens. 



The two pterygoid processes are completely separate in the embryonic skull, 

 and vessels and nerves pass between them below the base of the skull ; later, as the 

 processes are formed and fuse together, this vasculo-nervous bundle is enclosed in a 

 canal, the Vidian canal or foramen, which passes between the two plates high up, just 

 below the junction of body and great wing. The anterior opening of this canal is thus 

 into the spheno-maxillary fossa and its posterior opening is in the thickness of the 

 fused mass of great wing, body, and pterygoid plates, that is, it is in the lower part of 

 the front wall of the foramen lacerum, and just above and internal to the scaphoid 

 fossa and below the commencement of the carotid groove. 



We can now examine with greater care the region of the foramen lacerum and cavernous 

 sinus, choosing a skull, if possible, in which the foramen is of fair size, not closed in too much by 

 petrous ossification (Fig. 182). The carotid artery is directed forwards, inwards and upwards 

 from the apex of the petrous, but on reaching the sphenoid it loses the inward inclination, runs 

 upwards and forwards in the groove on the bone, and then turns sharply inwards and upwards 

 under the overhanging anterior clinoid process, to pierce the dura mater internal to this by turning 

 upwards to the brain. It has the carotid plexus on its outer side as it leaves the petrous and lies 

 in the foramen Jacerum. The great superficial petrosal nerve, leaving the hiatus Fallopii, lies in a 

 groove that is directed downwards and forwards and inwards to the foramen lacerum, so that 

 the nerve enters the foramen above and outside the issuing artery ; but, because it is directed 

 downwards while the artery is running upwards, the nerve is very quickly below the level of the 

 artery, and when the artery turns forwards on to the sphenoid it lies above the nerve. The nerve 

 has run downwards and inwards to the lower part of the front wall of the foramen, in a straight 

 line for the Vidian canal, which opens here, and in doing so it runs obliquely along the outer side 

 of the artery, in contact with it and the carotid plexus, and receives a short branch from this 

 plexus, the great deep petrosal, so that the combined fibres run on as the Vidian nerve to the spheno- 

 maxillary fossa and ganglion. In addition to these fibres are some backward-running ones from 

 the ganglion that complete the nerve in the canal. 



The sixth nerve runs nearly horizontally forward from the apex of the petrous to the lower 

 part of the sphenoidal fissure. It pierces the dura a little distance behind and below the apex, 

 and reaches this by passing outside the inferior petrosal sinus and below the superior petrosal sinus 

 and petro-clinoid ligament. Running forward from this it must cross obliquely above the carotid 

 artery in the foramen and lie above and outside the artery when this vessel turns forward on the 

 sphenoid ; after this the artery is ascending continuously while the nerve goes forward, so that 

 the nerve is lying below and outside the vessel when this turns under the clinoid projection. 



If now we look on the cavernous sinus as formed posteriorly by the junction of the two petrosal 

 sinuses, it is apparent that the artery and the sixth nerve must lie altogether deep to the back part 

 of the sinus, for the superior petrosal sinus comes above the nerve and the inferior one runs up along 

 its inner side, between it and the posterior clinoid process, to join the upper sinus ; further forward, 

 however, the artery comes into relation with the inner wall and roof of the sinus while the nerve 

 comes against the outer wall, for this is directed inwards as well as forwards. 



The cavernous sinus lies outside and below the interclinoid band that joins the anterior and 

 posterior processes, and is thus roofed in by the triangular area of dura mater (Fig. 181) that 

 lies here and is made by the edges of the tentorium cerebelli. The third nerve pierces the dura 

 in this situation and the fourth nerve a little further back, so these nerves lie in the roof of the 

 sinus. A glance at the bone will show that the ophthalmic and maxillary divisions of the fifth 

 nerve must lie against its outer wall, and the ganglion itself is a lateral relation of the posterior 

 part of the sinus. 

 F.A. 15 



