Lateral view of the ambient cistern. The ambient cistern lies lateral to the midbrain and posterior to the thalami, connecting the quadrigeminal cistern dorsally with the interpeduncular cistern ventrally. Within the ambient cistern lies the superior cerebellar artery, which divides into two major branches to irrigate the superior cerebellum. The oculomotor nerve traverses the interpeduncular cistern between the posterior cerebral arteries and the superior cerebellar arteries. The trochlear nerve courses within the ambient cistern from the dorsal midbrain along the edge of the tentorium to the cavernous sinus. The trigeminal nerve exits the ventrolateral pons and has a close relationship with the superior cerebellar artery, which can be pathologic in some patients with trigeminal neuralgia. Pontine circumflex arteries arise directly from the basilar artery and irrigate the ventral and lateral pons. (Image courtesy of AL Rhoton, Jr.)