Vasculature surrounding the midbrain. The midbrain has been sectioned transversely. Lateral to this are the temporal lobes (retracted). The cerebral peduncles are prominent on the anterior midbrain. Dorsal to these are the darkly pigmented substantia nigra, the largest nuclei in the midbrain. The oculomotor nerves can be seen leaving the midbrain within the interpeduncular fossa passing between the superior cerebellar and terminal posterior cerebral arteries, branches of the basilar artery. The trochlear nerves originate from the dorsal surface of the midbrain near the midline cerebral aqueduct and wrap around the cerebral peduncles within the subarachnoid space. Anteriorly, the intradural portion of the internal carotid artery is exposed. It gives rise to the posterior communicating arteries to the posterior cerebral arteries and, slightly more distally, to the anterior choroidal arteries. The latter vessels usually course along the lateral aspect of the optic tracts to the lateral geniculate bodies. The ICA divides into its terminal anterior and middle cerebral arteries. The anterior cerebral arteries course medially around the optic nerves to enter the longitudinal interhemispheric fissure. The larger middle cerebral arteries enter the Sylvian (lateral) fissures. The pituitary stalk is located posterior to the optic chiasm between it and the mammillary bodies of the diencephalon. 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)