Cerebral arterial circle (Circle of Willis). The brain has been removed to demonstrate an intact cerebral arterial circle that is formed by anastomotic connections between the basilar and internal carotid arteries. The basilar artery, formed by the union of the right and left vertebral arteries, terminates with the posterior cerebral arteries. These vessels connect to the internal carotid arteries via the posterior communicating arteries. The anterior cerebral arteries, terminal branches of the internal carotid system, enter the longitudinal fissure and are connected by the short anterior communicating artery. The oculomotor nerves emerge from the midbrain and pass between the superior cerebellar and posterior cerebral arteries before penetrating dura at the oculomotor triangle. The trochlear nerves enter the cavernous sinus and course forward between the oculomotor and ophthalmic nerves to enter the orbit together via the superior orbital fissure. The abducens nerves penetrate the dura over the clivus and course superolaterally toward the petrous apex. Here, they pass beneath the petrosphenoid ligament (Gruber's ligament) to enter the cavernous sinus. The optic nerves are shown entering the optic canals, while the olfactory bulbs and tracts (separated by the crista galli) convey olfactory information from the nasal epithelium to the olfactory cortex. (Image courtesy of PA Rubino)