The Periclinoidal Region. A, The carotid artery enters the cranial cavity by passing along the medial side of the anterior clinoid process and below the optic nerve. The dura lining the upper surface of the anterior clinoid process extends medially in 2 directions: the upper extension passes above the optic nerve to line the anterior root of the lesser wing of the sphenoid bone and form the falciform ligament; and the lower extension passes slightly downward to line the upper margin of the optic strut and form the anterior part of the distal dural ring. B, The dura lining the roof of the optic canal and anterior clinoid has been removed. The falciform ligament extends along the posterior edge of the anterior root of the lesser wing and above the optic nerve to blend medially into the dura mater covering the planum sphenoidale. C, The optic nerve and chiasm have been elevated to expose the pituitary stalk, ophthalmic artery, and a superior hypophyseal artery. The anterior clinoid process has been removed to expose the carotid oculomotor membrane formed by the dura lining the lower margin of the clinoid that separates the clinoid from the oculomotor nerve and extends medially to form the proximal dural ring. The dura that extends medially off the upper surface of the anterior clinoid process to line the upper surface of the optic strut also forms the anterior part of the distal dural ring, which defines the upper edge of the clinoid segment of the ICA. This dura forming the proximal ring slopes downward as it proceeds medially, so that the medial part of the distal dural ring actually lies at the level of the lower rather than the upper surface of the anterior clinoid. The lateral part of the distal ring near the origin of the ophthalmic artery tightly adheres to the lateral wall of the ICA. D, The clinoid segment of the ICA, located between the proximal and distal dural rings, has been exposed by removing the anterior clinoid. The ICA between the proximal and distal ring is enclosed in a thin layer of dura referred to as the carotid collar. The proximal ring is loosely applied to the clinoid segment and allows the clinoid venous plexus, a thin venous plexus that courses inside the carotid collar and outside the carotid wall, to communicate inside the ring with the anterior part of the cavernous sinus. E, The right ICA has been retracted laterally to expose the carotid cave. The dura along the posterior edge of the carotid cave contains the anterior intercavernous sinus. F, The clinoid segment has been retracted anteriorly to expose the part of the cave adjacent the diaphragm sellae. G, Oblique anterior superior view. The roof of the sphenoid sinus has been removed to expose the medial side of the right ICA. A green piece has been inserted into the carotid cave. The cave, the short downward directed pouch inside the carotid collar, extends below the level of the distal dural ring between the arterial wall and the carotid collar. H, The distal dural ring and the carotid collar have been divided and the dural flaps retracted with white silk to expose the carotid cave, the space between the carotid collar and the outer carotid wall that opens upward into the intradural space. (Images courtesy of AL Rhoton, Jr.)