Endoscopic View of the Internal Carotid Artery Segments. I, 30-degree view looking superiorly at the left petrous ICA. The inferior petrous apex has been drilled. After entering the carotid canal, the ICA rises vertically first before it turns horizontally to follow a posterior-to-anterior and a lateral-to-medial trajectory toward the posterior border of the exocranial surface of the foramen lacerum. The vidian nerve is followed posteriorly from the pterygopalatine ganglion toward the foramen lacerum and is a landmark for identifying the anterior aspect of the petrous ICA. J and K, 0-degree view. The paraclival segment extends from the posterior aspect of the exocranial surface of the foramen lacerum up to the superior limit of the medial petrous apex and the upper edge of the upper/vertical portion of the petroclival fissure, immediately inferior to the sellar floor. K, The bone around the ICA has been drilled and the periosteal dura opened to expose the vessel and the sympathetic chain that accompanies it. The lower segment of the gasserian ganglion giving rise to V2 and V3 nerves is seen lateral to the paraclival ICA. The maxillary strut, which forms part of the roof of the rotundum canal and floor of the superior orbital fissure, is also observed. L, the parasellar segment extends from the superior limit of the medial petrous apex and the upper edge of the petroclival fissure up to the level of the proximal dural ring of the ICA. Medial to the parasellar ICA is the medial venous compartment of the cavernous sinus, which forms as the artery deviates away from the periosteal dura superior to the upper edge of the petroclival fissure. Lateral to the parasellar ICA is the narrow lateral compartment of the cavernous sinus where the abducens nerve passes and is adherent to the inferolateral surface of the vessel. Further laterally, the lateral wall of the sinus where the ophthalmic, trochlear, and oculomotor nerves are seen coursing toward the superior orbital fissure is identified. M, 0-degree view. The paraclinoid ICA extends from the proximal to the distal dural ring of the ICA. The proximal dural ring is an incomplete and incompetent ring that does not fuse tightly with the artery’s adventitia. The anterior aspect of this ring is formed by the dura lining the inferior surface of the optic strut and extending medially toward the carotid sulcus. The lateral margin of the ring is an extension of the dura covering the inferior surface of the anterior clinoid and is in continuation with the periosteal dura of the roof of the superior orbital fissure. The anteromedial portion of the proximal ring is formed by the dura lining the carotid sulcus at this level and extending medially toward the face of the sella. The distal dural ring is a complete and competent ring that tightly fuses with the artery’s adventitia. Immediately distal to the distal dural ring the ophthalmic artery is seen after it arises from the intradural ICA segment. (Images courtesy of AL Rhoton, Jr.)