Stepwise Dissection Showing the Relationship Between Exocranial Landmarks and Endocranial Structures. E, The right half of the lower clivus has been opened. The border between the middle and lower clivus approximates the level of the anterior edge of the pharyngeal tubercle. The glossopharyngeal and vagal nerves are exposed above the hypoglossal canal. F, The left half of the lower clivus has also been opened, and the medulla has been exposed. The intracranial demarcation between the middle and lower clivus is just above the glossopharyngeal nerve at the level of the pontomedullary sulcus. G, The bone and dura of the right half of the upper clivus has been opened. The vertical segment of the internal carotid artery, which ascends along the lateral limit of the posterior wall of the sphenoid sinus, is referred to as the paraclival segment. The border between the upper and middle clivus is located at the same level of the lower limit of the paraclival segment of the internal carotid artery. H, Removing the left half of the upper clivus and opening the dura exposes the upper part of the pons. The lacerum segment of the petrous carotid is positioned at the anterior genu below the paraclival segment. I, The right half of the middle clivus has been opened. A thin dural bridge has been preserved at the junction of the upper and middle clivus, and a thin bony bridge has been preserved at the junction of the middle and lower clivus. J, The left half of the middle clivus has been opened to expose the intradural structures in the upper, middle, and lower clival regions. The middle clival opening exposes the lower pons. (Images courtesy of AL Rhoton, Jr.)