Enlarged Anterior Views of the Midline Cranial Base Before and After Opening the Clivus. A thin dural bridge separates the upper and middle clivus, and a thin bony bridge separates the middle and lower clivus. The border between the upper and middle clivus is located at the junction of the paraclival and lacerum segments of the petrous carotid (yellow dotted line), which is approximately 5 mm above the posterior end of the vidian canal. The border between the middle and lower clivus is situated approximately 4 mm above the top of the pharyngeal tubercle. Opening the upper, middle, and lower clivus exposes the anterior surface of the upper half of the pons, the lower half of the pons, and the medulla, respectively. When followed from its anterior to posterior end, the vidian canal courses in a slight lateral angle to the sagittal plane and almost parallel to or at a slight downward angle in relation to the plane of the hard palate. The posterior end of the vidian canal is located inferolateral to the anterior genu, or the lacerum segment, of the internal carotid artery. (Images courtesy of AL Rhoton, Jr.)