Transverse view of midbrain at tentorial notch. The edge of the tentorium cerebelli remains intact and forms the anterolateral boundary of the tentorial notch or incisura. A portion has been out away to expose the superior surface of the cerebellum. The midbrain has been sectioned transversely. The darker pigmented substantia nigra, the largest nucleus in the midbrain, is seen dorsal to the left cerebral peduncle. Dorsal and slightly medial to it is the red nucleus. The left oculomotor nerve can be seen leaving the midbrain within the interpeduncular fossa passing between the superior cerebellar and posterior cerebral arteries. The trochlear nerve originates from the dorsal surface of the midbrain and wraps around the cerebral peduncle to enter the subarachnoid space. Anteriorly, the intradural portion of the internal carotid artery is exposed. It gives rise to its terminal anterior and middle cerebral arteries. The anterior cerebral artery courses medially around the optic nerve to enter the longitudinal interhemispheric fissure. The larger middle cerebral artery will enter the Sylvian (lateral) fissure. The optic chiasm and tract are also visible. (Image courtesy of PA Rubino)