Osseous, Dural, and Cranial Nerve Relationships. A, Posterior superior view. The dura on the right half of the floor of the posterior fossa has been removed while preserving the CNs. CNs V through XII pierce the posterior fossa dura. The upper clivus and middle clivus are separated at the axial plane of the dural pori of the abducens nerve formed where the abducens nerves penetrate the dura. The middle clivus and lower clivus are separated by the axial plane of the dural glossopharyngeal meati, through which the glossopharyngeal nerves pass to enter the jugular foramen. A red dotted line shows the demarcation between the upper and middle clivus, and a yellow dotted line shows the demarcation between the middle and lower clivus. The intradural space behind the upper clivus includes CNs III and V; behind the middle clivus includes CNs VI, VII, and VIII; and behind the lower clivus includes CNs IX, X, XI, and XII. B, Posterior oblique view, left side. Meckel’s cave, through which the trigeminal nerve passes, is located superolateral to the dural porus of the abducens nerve. The dura over the jugular foramen has 2 perforations: a glossopharyngeal meatus through which the glossopharyngeal nerve passes and a vagal meatus through which the vagus and accessory nerves pass. The glossopharyngeal meatus is located superior to the vagal meatus. C, Posterior oblique view after removing the dura on the right side. The inferior petrosal sinus courses along the intracranial surface of the petroclival fissure. D, Enlarged view focusing on the dural porus of the abducens nerve and Dorello’s canal. Gruber’s ligament extends from the lower part of the lateral edge of the dorsum sellae to the upper edge of the petrous apex. Dorello’s canal (white dotted line) is a narrow, interdural, triangular space bounded by Gruber’s ligament superiorly, the petrous apex inferolaterally, and the clivus inferomedially. After piercing the dura, the abducens nerve courses beneath Gruber’s ligament and through a large venous confluence at the junction of the cavernous, basilar, and the superior and inferior petrosal sinuses. The dural porus of the abducens nerve is located just below the upper edge of the petrosal apex. E, Enlarged view focusing on the jugular foramen. The glossopharyngeal, vagus, and accessory nerves enter the intrajugular part of the jugular foramen, which is located between the petrosal and sigmoid parts. The glossopharyngeal nerve, after reaching the bony notch at the upper border of the jugular foramen, courses downward through the jugular foramen separate from the vagus and accessory nerve. F, The demarcation between the upper and middle clivus corresponds to the level 3.4 mm below the upper edge of the petrous apex. The demarcation between the middle and lower clivus corresponds to the level of the notch for the glossopharyngeal nerve at the upper medial edge of the jugular foramen. (Images courtesy of AL Rhoton, Jr.)