G, The trigeminal, oculomotor, and trochlear nerves have been removed and the abducens nerve preserved. The dura forming the carotid collar has been folded upward after dividing the collar beginning below at the level of the lower dural ring and extending upward to just below the upper ring. The inferolateral trunk, a branch of the intracavernous carotid, passes above the abducens nerve. The tentorial artery arises from the inferolateral trunk, and the inferior hypophyseal artery arises directly from the internal carotid artery rather than from the meningohypophyseal artery, which is absent in this case. H, The bone and mucosa in the lateral wall of the sphenoid sinus have been removed to expose the dura that forms the medial wall of the cavernous sinus. The optic canal and optic sheath are located in the superolateral margin of the sphenoid sinus. The optic strut forms the floor of the optic canal and separates the lower margin of the optic canal from the upper margin of the superior orbital fissure. The maxillary nerve crosses along the midportion of the lateral wall of the sphenoid sinus and is separated from the superior orbital fissure by the maxillary strut, a narrow bridge of bone between the fissure and the foramen rotundum. The medial aspect of the superior orbital fissure produces a bulge in the lateral wall of the sphenoid sinus between the optic and maxillary struts. The intracavernous carotid can be seen through the dura in the area behind the optic strut. The medial side of the carotid collar, which extends around the medial side of the artery in the area behind the optic strut, is not as well defined and is wider than on the lateral side of the artery. The lower margin of the collar on the medial side of the clinoid segment is located at approximately the level of the broken line extending from the lower margin of the optic strut to the floor of the sella. The small venous channels within the collar can be seen through the dura. The anterior intracavernous sinus courses at the posterior margin of the upper dural ring. The lateral wing of the sphenoid sinus extends below the maxillary nerve and below the middle fossa floor. I, The optic strut has been removed. A small opening in the dura admits a probe that has been passed forward between the dura and the internal carotid artery along the lower margin of the carotid collar. The dura that lines the upper surface of the optic strut and the lower part of the optic sheath is continuous posteriorly with the upper ring. The lower margin of the collar is located at approximately the level of the broken line extending from the lower margin of the optic strut to the floor of the sella. J, The segment of the optic nerve coursing above the optic strut has been removed, but the segment passing through the annular tendon, from which the rectus muscles arise, has been preserved. The dura has been opened along the margin of the lower ring to expose the venous spaces within the cavernous sinus which extend upward inside the carotid collar. The venous spaces inside the envelope of the cavernous sinus increase in size in the area below the collar. K, The bone in the lateral wall of the sphenoid sinus has been removed to expose the medial aspect of the optic canal, superior orbital fissure, the prominence over the maxillary nerve, and the middle fossa dura. The dura in the medial wall of the cavernous sinus has been removed to expose the intercavernous carotid and the abducens nerve. The lower margin of the cavernous sinus is marked with yellow arrows and is located below the intracavernous segment of the carotid and the abducens nerve. The optic canal and opticocarotid recess are located above the superior orbital fissure and anterior bend of the cavernous carotid. The prominence of the superior orbital fissure is located below the opticocarotid recess. Another recess extends laterally into the maxillary strut, the bridge of bone that separates the superior orbital fissure and the foramen rotundum. The maxillary nerve produces a prominence in the lateral wall of the sphenoid sinus below which is located the lateral wing of the sphenoid, which extends below the floor of the middle fossa in the region of the anterolateral triangle. The basilar sinus is the largest connection across the midline between the cavernous sinuses. L, The middle fossa dura, lateral to the sphenoid sinus and between the superior orbital fissure and maxillary nerve, has been opened to expose the medial aspect of the temporal lobe. Bone has been removed below the maxillary nerve to expose the floor of the middle fossa below the temporal lobe. The optic, oculomotor, and trochlear nerves and the internal carotid artery are exposed above the sella. (Images courtesy of AL Rhoton, Jr.)