A, Superior view. The osseous structures, which nearly encircle the clinoid segment of the internal carotid artery, include the anterior clinoid laterally, the optic strut anteriorly, and the carotid sulcus medially. The carotid sulcus begins lateral to the dorsum sellae at the intracranial end of the carotid canal, extends forward just below the sellar floor, and turns upward along the posterior surface of the optic strut. The anterior clinoid process projects backward from the lesser wing of the sphenoid bone, often overlapping the lateral edge of the carotid sulcus. The anterior root of the lesser sphenoid wing extends medially to form the roof of the optic canal. The posterior root of the lesser wing, referred to as the optic strut, extends from the inferomedial aspect of the anterior clinoid to the sphenoid body. The bony collar around the carotid artery formed by the anterior clinoid, optic strut, and carotid sulcus is inclined downward as it slopes medially from the upper surface of the anterior clinoid to the carotid sulcus. Another small prominence, the middle clinoid process, situated on the medial side of the carotid sulcus at the level of the tip of the anterior clinoid process, projects upward and laterally. In some cases, there is an osseous bridge extending from the tip of the middle clinoid to the tip of the anterior clinoid. In well-pneumatized sphenoid bones, the carotid sulcus is seen as a prominence in the lateral wall of the sphenoid sinus just below the floor of the sella. B, Posterior view of the optic strut, optic canal, and the superior orbital fissure. The optic strut separates the optic canal and superior orbital fissure and forms the floor of the optic canal and the superomedial part of the roof of the superior orbital fissure. The posterior surface of the strut is shaped to accommodate the anterior wall of the clinoid segment. The artery courses along and may groove the medial half of the lower aspect of the anterior clinoid before turning upward along the medial edge of the clinoid. The air cells in the sphenoid sinus may extend into the optic strut and anterior clinoid. In this case, the sphenoid sinus has pneumatized to a degree that bone is absent over the anterior part of the carotid sulcus, just medial to where the optic strut attaches to the body of the sphenoid bone. The maxillary strut is the bridge of bone separating the superior orbital fissure from the foramen rotundum. C, Oblique posterior view of the right optic strut. The lateral part of the bony collar around the clinoid segment is formed by the anterior clinoid, the anterior part is formed by the posterior surface of the optic strut and the part of the carotid sulcus located medial to the anterior clinoid process. The posterior surface of the optic strut is wider medially adjacent to the carotid sulcus than it is laterally at the site of attachment to the anterior clinoid process. The optic strut slopes downward from its lateral end so that the medial part of the bony collar is located below the level of the part of the collar joining the anterior clinoid. The inferomedial aspect of the right anterior clinoid is grooved by the artery. D, Superior view of specimen with bilateral caroticoclinoidal foramen and interclinoidal osseous bridges. An osseous bridge connects the tips of the anterior and middle clinoid processes bilaterally, thus creating a bony ring around the artery, called a caroticoclinoidal foramen, on each side. There is also an interclinoidal osseous bridge connecting the anterior and posterior clinoid processes on both sides. E, Superior view of another specimen, in which the lesser sphenoid wings and the base of the anterior clinoids and roof of the optic canals have been removed. The remaining part of the anterior clinoid is held in place by its attachment to the optic strut. The medial side of the anterior clinoid is grooved to accommodate the clinoid segment. F, Enlarged view of the left half of E. The posterior face of the optic strut is shaped to accommodate the anterior surface, and the medial aspect of the anterior clinoid is grooved to accommodate the lateral surface of the clinoid segment. The tip of the anterior clinoid process is the site of a small bony projection directed toward the middle clinoid process, with the anterior and middle clinoids nearly completing a ring around the clinoid segment at the level of the cavernous sinus roof. (Images courtesy of AL Rhoton, Jr.)