Skull Base within the Parasellar Region. A and B, A complete caroticoclinoid ring (3% of cases) joins the anterior clinoid process with the middle clinoid process forming a ring around the internal carotid artery with its base at the center of the anterior bend of the internal carotid artery. The chiasmatic sulcus has an anterior limbus that separates it from the planum anteriorly and a posterior limbus with the tuberculum that limits it posteriorly. When a middle clinoid is present, the tuberculum ends laterally in a horizontal Y shape (red dotted line), which is continuous superiorly with the opticocarotid elevation directed to the optic strut, and inferiorly with the middle clinoid process or in this case, the caroticoclinoid ring. C and D, The “kissing clinoids” or incomplete caroticoclinoid ring, the anterior and middle clinoids almost touch each other. E and F, Prominent middle clinoid processes. Note the alpha angle measured in dry crania; for the middle clinoid orientation in the midsagittal plane (blue dotted vertical line), the alpha angle average was 116.25 degrees. G and H, Small middle clinoid processes of less than 1.5 mm that were considered not clinically relevant in surgical procedures. (Images courtesy of AL Rhoton, Jr.)