Endoscopic Borders of the Internal Carotid Artery Segments. E and F, 45-degree and 0-degree views, respectively, focusing on the left petrous apex. The petrous segment of the ICA extends from the exocranial orifice of the carotid canal up to the posterolateral aspect of the exocranial surface of foramen lacerum. The external orifice of the carotid canal is bordered laterally by the vaginal process of the tympanic part of the temporal bone (not shown). The carotid ridge of the petrous bone, as it extends from anterior to posterior, forms the medial border of the external orifice and separates it from the jugular foramen. Connecting the vaginal process laterally to the carotid ridge medially is a narrow ridge along the rough quadrilateral surface of the petrous apex; we term it the anterior crest as it forms the anterior border of the exocranial orifice of the carotid canal. The vidian nerve is seen coursing toward its posterior opening at the lateral edge of foramen lacerum. Lateral to that, the V3 nerve is seen exiting the cranial base at a 90-degree angle through the foramen ovale. G and H, 30-degree views directed at the petrous segment of the left ICA in 2 different specimens. The carotid ridge, the anterior crest, and the vaginal process of the temporal bone border the ICA entrance to the petrous bone, from medial to lateral, respectively. The ICA canal is bordered laterally by the sulcus tubae, which is located at the exocranial surface of the sphenopetrosal fissure. This sulcus runs parallel to the canal and serves as an attachment site for the cartilaginous part of the Eustachian tube. The horizontal aspect of the petroclival fissure forms the medial border of the carotid canal. The inferior petroclival vein has been removed; the horizontal part of the petroclival fissure and the inferior petrosal sinus, which course on its dorsal surface, run parallel to the carotid canal. This segment of the petroclival fissure connects the posterior edge of the foramen lacerum anteriorly with the anterior margin of the jugular foramen posteriorly. The supracondylar groove can be followed laterally from the pharyngeal tubercle toward the exit of the hypoglossal nerve from its canal. (Images courtesy of AL Rhoton, Jr.)