Endonasal Dissection of the Sella and Parasellar Structures. A, The bone over the internal carotid arteries, sella, and optic nerves has been drilled and the dura mater exposed. B, The dura mater covering the carotid arteries has been partially removed. Below and lateral to the point where the sellar prominence meets the internal carotid artery superiorly (medial opticocarotid point, green arrowheads), an indentation can be seen bilaterally (yellow arrows). This is the dural impression produced by the middle clinoid, and it is located in the upper middle level of the sellar prominence. C, The right middle clinoid base is located at the center of the C-shaped anterior bend of the cavernous carotid artery. The middle clinoid is covered by the periosteal layer of the dura mater (yellow arrow). The middle clinoid protrudes inside the cavernous sinus with its base just below the projection level of the clinoid segment of the internal carotid artery according to the intracranial dissections. The proximal dural ring is not identifiable from an endonasal view on the medial side of the artery. D, The dura mater over the internal carotid artery has been completely removed, and blue silicon filling the cavernous sinus is seen at the site of the middle clinoid process. (Images courtesy of AL Rhoton, Jr.)