Relationship of structures at the tentorial notch. The cerebrum has been dissected away and the brainstem preserved in situ in this specimen. The anterior and posterior petroclinoid ligaments represent, respectively, the attached and free edges of the tentorium cerebelli, the second largest dural fold, that divides the cranial cavity into supratentorial and infratentorial compartments. The anterior petroclinoid ligament is an extension of the tentorium, which attaches laterally to the petrous ridge of the temporal bone and posteriorly to the transverse groove of the occipital bone. The posterior petroclinoid ligaments arise from the medial extensions of the tentorial notch. The tentorial notch or incisura is a U-shaped space that curves around the junction of the midbrain and pons to accommodate passage of the brainstem into the posterior fossa. The hyper-pigmented substantia nigra lies within the tegmentum of the midbrain posterior to the cerebral peduncles. The oculomotor (CNIII) nerves can be seen leaving the ventral midbrain, passing through the interpeduncular fossa toward the lateral wall of the cavernous sinus. They penetrate the dura between the anterior and posterior petroclinoid ligaments lateral to the posterior clinoid processes. In their course they pass caudal to the posterior cerebral artery, between this and the superior cerebellar artery, both branches of the midline basilar artery. The trochlear (CNIV) nerves leave the dorsal surface of the midbrain and curve around the cerebral peduncles to gain the lateral wall of the cavernous sinus. They also pass between these two arteries and penetrate the dura between the free and attached edges of the tentorium cerebelli. The basilar artery terminates as the posterior cerebral arteries, which receive the posterior communicating arteries from the internal carotid arteries to complete the posterior portion of the circle of Willis before curling around the cerebral peduncles above the tentorium. They ultimately supply the occipital lobes and posteromedial portions of the temporal lobes. Ventral to the midbrain is the body of the sphenoid featuring the sella turcica, the midline depression containing the pituitary gland and distal pituitary stalk through the opening of the diaphragma sellae. The sella is bounded anteriorly by the tuberculum sellae and posteriorly by the dorsum sellae and its posterior clinoid processes. In this view, medial to the anterior clinoid processes are the optic nerves (CNII) emerging from the optic canals and the supraclinoid internal carotid arteries. (Image courtesy of PA Rubino)