A, Neural structures in the quadrigeminal cistern. The anterior wall of the quadrigeminal cistern is formed by the pulvinar, superior, and inferior colliculi and the superior cerebellar peduncles. The cistern extends downward between the cerebellum and midbrain into the cerebellomesencephalic fissure. The roof of the third ventricle, anterior to the pineal, has been opened. The striae medullaris thalami extend forward along the lateral wall of the third ventricle, beginning posteriorly at the habenular commissure. The right temporal horn, uncus, and cerebral peduncle have been exposed. B, The internal cerebral and basal veins join in the quadrigeminal cistern to form the vein of Galen. The posterior cerebral arteries enter the upper part of the quadrigeminal cistern and the superior cerebellar arteries enter the lower part. The trochlear nerve courses between the superior cerebellar and posterior cerebral arteries. C, Infratentorial exposure of the venous complex in the supracerebellar area. The basal, internal cerebral, anterior calcarine, and superior vermian veins converge on the vein of Galen. The left posterior cerebral artery gives rise to a branch that enters the lower surface of the tentorium. D, Another specimen. The internal cerebral, basal, and anterior calcarine veins converge on the vein of Galen. E and F, occipital transtentorial exposure. E, The occipital lobe has been retracted and the tentorium divided adjoining the straight sinus to expose the quadrigeminal cistern. F, Enlarged view. The exposure extends forward to the margin of the cerebral peduncle, uncus, and the crural cistern. The basal vein passes around the brainstem on the medial side of the temporal lobe to reach the quadrigeminal cistern. The internal cerebral veins exit the roof of the third ventricle and empty into the vein of Galen. A combined supra- and infratentorial exposure can be obtained by dividing the transverse sinus and tentorium, but should only be considered if there is a nondominant transverse sinus on the side of the exposure. (Images courtesy of AL Rhoton, Jr.)