A, Superior view of the cranial base in the region of the cavernous sinus. The cavernous sinus extends from the superior orbital fissure anteriorly, to the petrous apex posteriorly, and it is bordered by the sella medially and the middle fossa laterally. It fills the posterior margin of the superior orbital fissure, which is located below the anterior clinoid process and its posterior wall, extends from the lateral edge of the dorsum sellae to the medial margin of the trigeminal impression and Meckel’s cave. Numerous venous channels open into the cavernous sinus. These include the basilar, anterior and posterior intercavernous, and the superior and inferior petrosal sinuses; the sylvian and ophthalmic veins, and the veins exiting the foramen ovale, rotundum, and spinosum; and the carotid canal and the sphenoidal emissary foramen. Each structure is shown by colored arrows. The basilar sinus is the largest communicating channel between the cavernous sinuses. B, Superior view of the roof of the cavernous sinus. The oculomotor triangle, through which the oculomotor enters the roof of the cavernous sinus, is located between the anterior and posterior petroclinoid and the interclinoid dural folds. The interclinoid fold ex-tends from the anterior to the posterior clinoid process. The anterior petroclinoid fold extends from the petrous apex to the anterior clinoid process and the posterior petroclinoid fold extends from the petrous apex to the posterior clinoid process. The internal carotid artery has been divided at the point it exits the roof of the cavernous sinus. A probe has been inserted under the falciform dura fold, which extends medially from the anterior clinoid process and above the optic canal to the chiasmatic sulcus. C, The dura has been removed from the roof and lateral wall of another cavernous sinus. The anterior clinoid process has been preserved. The abducens nerve passes around the lateral surface of the internal carotid artery. The oculomotor and ophthalmic nerves have been divided to expose the floor of the cavernous sinus, which is located at the level of the lower edge of the carotid sulcus on the sphenoid body below the intracavernous carotid. (Images courtesy of AL Rhoton, Jr.)