A, Lateral view of the paraclinoidal area. The anterior clinoid process projects backward from its base on the lesser wing. The carotid artery passes upward, and the optic nerve enters the optic canal on the medial side of the anterior clinoid. The origin of the ophthalmic artery bulges upward, below the optic nerve. The oculomotor nerve enters the roof of the cavernous sinus and passes along the lower margin of the anterior clinoid process. B, The anterior clinoid and the bone forming the roof and lateral wall of the optic canal have been removed. The clinoid segment of the internal carotid artery is located in the medial wall of the space created by removal of the anterior clinoid. The dura that covers the medial and lower surfaces of the anterior clinoid has been preserved. The layer of dura that covers the medial side of the anterior clinoid forms the lateral part of the carotid collar. The upper surface of the optic strut forms the floor of the optic canal. The anterior wall of the clinoid segment rests against the posterior surface of the optic strut. The dura lining the upper and lower surfaces of the optic strut extends backward to form the anterior part of the upper and lower dural rings and the dura lining the posterior surface of the strut forms the anterior part of the carotid collar. The falciform ligament is a dural fold, which extends above the optic nerve at the posterior edge of the optic canal. C, Enlarged view of the lateral aspect of the carotid collar and optic strut. The anterior surface of the clinoid segment rests against the posterior surface of the optic strut. This strut is triangular in cross section. The upper surface of the optic strut, which forms of the lower margin of the optic canal, slopes forward and downward from its intracranial edge. The lower surface of the optic strut forms part of the upper margin of the superior orbital fissure. The posterior surface of the optic strut is contoured to accommodate the anterior wall of the clinoid segment. The dura that lines the lower margin of the anterior clinoid process and extends medially above the oculomotor nerve to surround the internal carotid artery and form the lower dural ring is referred to as the carotidoculomotor membrane. The lower ring does not tightly adhere to the surface of the artery, as does the upper ring. The lower ring admits venous tributaries of the cavernous sinus, referred to as the clinoid venous plexus, which course between the clinoid segment and the carotid collar and can be seen through the collar. Thus, the clinoid segment of the internal carotid artery, which was once considered to lie outside the cavernous sinus, is partially surrounded by the venous tributaries of the cavernous sinus. D, The optic strut has been removed. The layer of dura that lines the posterior surface of the optic strut forms the anterior part of the carotid collar. The medial end of the strut attaches to the body of the sphenoid bone immediately in front of the carotid sulcus. In some cases, the interior of the optic strut is aerated by a lateral extension of the sphenoid sinus, which may also extend through the strut into the anterior clinoid. The dura lining the lower margin of the anterior clinoid, which forms the carotidoculomotor membrane, also forms the most anterior portion of the roof of the cavernous sinus. The venous channels of the cavernous sinus extend upward between the carotid collar and the clinoid segment. The layers of dura lining the upper and lower surfaces of the clinoid process come together at the posterior tip of the anterior clinoid. E, The outer layer of dura in the lateral wall of the cavernous sinus has been removed to expose the trigeminal, oculomotor, and trochlear nerves, which are enmeshed in the inner layer of dura. The maxillary strut, the bridge of bone located between the superior orbital fissure and the foramen rotundum, has been removed. The oculomotor and trochlear nerves course on the lower surface of the carotid-oculomotor membrane. A tentorial artery passes above the ophthalmic nerve. The openings created by removal of the optic and maxillary struts and the bone below the maxillary nerve open into the sphenoid sinus. F, The segment of the optic nerve that courses above the optic strut has been removed. The carotidoculomotor membrane has been separated from the oculomotor nerve and folded upward to expose the tributaries of the cavernous sinus, which course between the carotid collar and the clinoid segment, and extend upward to near the upper ring. (Images courtesy of AL Rhoton, Jr.)