Upper clivus-anterior brainstem interface. Portions of the cerebellum, midbrain, pons, and inferior surface of the cerebrum are visible. Numerous vessels supplying these structures can be identified. The basilar artery gives rise to the superior cerebellar and posterior cerebral arteries. The oculomotor and trochlear nerves leave the midbrain between these two vessels. The posterior cerebral arteries usually receive the posterior communicating arteries from the internal carotid arteries as part of the arterial circle of Willis. The internal carotid terminates by dividing into the larger middle and, smaller, anterior cerebral arteries. The anterior choroidal artery arises from the posterior wall of the internal carotid between the origin of the posterior communicating artery and its termination. The anterior cerebral arteries course anteromedially to pass anterior to the genu of the corpus callosum to supply the medial surfaces of the cerebral hemispheres. Its first part (A1 segment) ends at the origin of the anterior communicating artery connecting the right and left anterior cerebral arteries. The second part (A2 segment) extends from this point anterior to the lamina terminalis and terminates either at the genu of the corpus callosum or the origin of the callosomarginal artery. The optic nerve and internal carotid artery can be seen medial to the anterior clinoid process. Here, the optic nerve enters the optic canal to the orbit. Lateral to the body of the sphenoid is the cavernous sinus. Several cranial nerves are contained within its lateral wall. From superior to inferior are the oculomotor, trochlear, and ophthalmic nerves. These course forward to enter the orbit through the superior orbital fissure. The abducens nerve can be seen penetrating the dura mater of the clivus and will enter the cavernous sinus and course along the lateral surface of the internal carotid artery before passing through the superior orbital fissure to the orbit. The trigeminal nerve crosses over the apex of the petrous bone to enter Meckel's cave where it expands as the Gasserian (sensory) ganglion. Here, it gives rise to its three divisions or branches. The ophthalmic nerve travels to the orbit. The maxillary nerve passes through the foramen rotundum to enter the pterygopalatine fossa. The mandibular nerve passes through the foramen ovale to enter the infratemporal fossa. Nearby is the foramen spinosum transmitting the middle meningeal artery. Near the lateral pontomedullary junction arise the facial and vestibulocochlear nerves. They enter the internal auditory canal in the petrous temporal bone. Most of this has been drilled away. The vestibulocochlear nerve ends in the inner ear; the labyrinthine segment of the facial nerve ends at the geniculate (sensory) ganglion. Here arises the greater superficial petrosal nerve to the pterygopalatine fossa providing parasympathetic fibers to the pterygopalatine ganglion. The tympanic segment of the facial nerve continues within the medial wall of the middle ear cavity. The transverse sinus is exposed at the interface between the cerebellum and occipital lobes of the cerebrum where it is contained within the tentorium cerebelli. (Image courtesy of M Nunez)