G, The dura between the upper and lower dural ring that forms the carotid collar has been separated from the internal carotid artery and folded upward to expose the thin venous plexus extending inside the carotid collar. H, The venous space surrounding the carotid artery and the trigeminal nerve has been cleared. This exposes the full course of the intracavernous carotid, which is quite tortuous and bulges upward to elevate the roof of the cavernous sinus. The upper dural ring, which is tightly adherent to the artery, extends below the optic nerve and across the upper surface of the optic strut. I, Enlarged view of the intravenous carotid artery. The ophthalmic nerve has been depressed to expose the abducens nerve as it passes around the lateral margin of the carotid artery. The inferolateral trunk arises from the intracavernous carotid, passes above the VIth nerve, and gives rise to a branch that passes forward toward the foramen rotundum. The meningohypophyseal artery has been distorted by the tortuous course of the internal carotid artery. The dorsal meningeal branch of the meningohypophyseal trunk passes backward toward Dorello’s canal and the clivus. The petrosphenoid ligament (Gruber’s ligament) roofs Dorello’s canal through which the abducens nerve enters the lower margin of the sinus. J, The lower margin of the cavernous sinus is marked with a yellow line, and Meckel’s cave, the arachnoid cistern around the trigeminal nerve, is outlined with a green line. The lower edge of the cavernous sinus is located medial to the junction of the upper and middle third of Meckel’s cave. The bone in the anteromedial and anterolateral triangles has been opened to expose the lateral wing of the sphenoid sinus extending laterally below the trigeminal nerve. K, The trigeminal ganglion and the adjacent part of the posterior root and division have been removed to expose the petrous segment of the internal carotid artery and the lateral wing of the sphenoid sinus. L, Bone has been removed in the floor of the sphenoid sinus to “unroof” the vidian canal in which the vidian nerve courses. The vidian nerve is formed by the union of the greater petrosal nerve and the deep petrosal nerve, the latter arising from the periarterial carotid plexus. The two roots join in the region of the foramen lacerum and enter the vidian canal to reach the pterygopalatine ganglion in the pterygopalatine fossa. (Images courtesy of AL Rhoton, Jr.)