Dorsal aspect of the midbrain. The midbrain has been sectioned transversely. Lateral to this are the temporal lobes (retracted). The trochlear nerves originate from the dorsal surface or tectum of the midbrain near the midline cerebral aqueduct, which connects third and fourth ventricles. These nerves wrap around the cerebral peduncles within the subarachnoid space. On either side of the midbrain are branches of the superior cerebellar and posterior cerebral arteries. They arise from the basilar artery. The superior cerebellar arteries course in a subtentorial direction to the superior surface of the cerebellum while the posterior cerebral arteries course in a supratentorial direction and supply the occipital and posteromedial temporal lobes. The pineal gland is located in the midline of the brain posterior to the third ventricle, between the corpus callosum and superior colliculi. It projects into the quadrigeminal cistern. The great cerebral vein (great vein of Galen) also lies within the quadrigeminal cistern and is formed by union of the two internal cerebral veins and basal veins of Rosenthal. It curves backward and upward to empty at the confluence of the inferior sagittal sinus and straight sinus. (Image courtesy of PA Rubino)