Endoscopic Approach to the Lower Clivus. M, The remainder of the lower clivus has been drilled while preserving the hypoglossal canals. The exposure includes the anterior aspect of the medulla; the glossopharyngeal, vagus, accessory, hypoglossal, and C1 nerves; and the vertebral, posterior inferior cerebellar, and anterior spinal arteries. N, Closer view of the right side. The hypoglossal rootlets, which arise along the preolivary sulcus, usually pass posterior to the vertebral artery. The posterior inferior cerebellar artery arises near the junction of the hypoglossal rootlets with the medulla. The glossopharyngeal, vagus, and accessory nerves arise behind the hypoglossal rootlets, as a line of rootlets originating from the retro-olivary sulcus, a shallow groove between the olive and the posterolateral surface of the medulla. The rostral part of the accessory nerve is hidden by the hypoglossal canal and vertebral artery. O and P, Another lower clival region. O, The lower clivus and hypoglossal canals have been drilled. P, Closer view of the right side. (Images courtesy of AL Rhoton, Jr.)