The posterior margin of the inner plate has the Constrictor arising from its lower half ; above this the pharyngeal end of the cartilaginous Eustachian tube rests against the border just external to the pharyngeal opening, and is fastened here by a strong fascia ; the small Eustachian spine on the border marks the junction of the tubal and constrictor regions.
A projection, more or less prominent, on the posterior edge of the outer plate a little way down, may mark the anterior attachment of a " pterygo-spinous " ligament extending to the base of the sphenoidal spine. The ligament may be short, fastened higher up on the plate, or two ligaments, long and short, may be present : ossification may extend some way into these, so that a bony bar may be present here. The nerves issuing from the foramen ovale have varying relations with these bands, which are probably modified fibres of the external pterygoid muscle.
The font border of the inner plate, thick below, is in articulation with the vertical plate of the palate bone ; it turns in under the body of the sphenoid, and here it is interrupted by a groove that marks the ptcrygo-palatine canal. The inner side of the plate here has the projection of the vaginal process, which receives, between it and the body, the ala of the vomer (Fig. 183).
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 lacerum. 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 sphenomaxillary 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.