Endoscopic Borders of the Internal Carotid Artery Segments. A, 45-degree reference view of the left ICA showing the anatomic borders of each segment described. B, 0-degree view of the parapharyngeal ICA that extends from the common carotid bifurcation to the external orifice of the ICA canal. The Eustachian tube, the basopharyngeal fascia, the mucosa and the superior constrictor muscle at the posterior wall of the nasopharynx, and the underlying prevertebral muscles have been removed. The inferior clivus and the anterior arch of C1 have been exposed. The capsule of the atlanto-occipital joint has been opened. The pharyngeal tubercle, the site of attachment of the pharyngeal raphe, is identified. The supracondylar groove, an osseous depression superior to the condyle, is the site of attachment of the rectus capitus anterior muscle, the atlanto-occipital membrane, and the joint capsule. Following the groove laterally leads to the exocranial orifice of the hypoglossal canal. C and D, 45-degree view of the left parapharyngeal ICA as it enters the ICA canal. The carotid nerve, a direct prolongation of the superior cervical ganglion, accompanies the carotid as it ascends through the cranial base. The hypoglossal nerve is seen exiting the hypoglossal canal medial to the jugular foramen where the accessory and vagus nerves exit. As the vagus nerve passes by the lateral aspect of the hypoglossal canal, it is joined by the hypoglossal nerve that travels with it in the carotid sheath until about the level of the transverse process of C1 where the hypoglossal courses laterally and anteriorly to the parapharyngeal ICA to reach the muscles of the tongue (not shown). The accessory nerve exits the jugular foramen and descends posterolaterally between the parapharyngeal ICA and the jugular vein toward its muscles. D, The supracondylar groove has been drilled, and posterior to it, the hypoglossal canal is identified. Transcondylar and transjugular tubercle approaches, inferior and superior, respectively, to the supracondylar groove have been initiated. Both approaches are considered extensions to the inferior transclival approach and are directed medially to the parapharyngeal segment of the ICA. (Images courtesy of AL Rhoton, Jr.)