3rd nerve compression due to uncal herniation: This view is a close-up showing herniating uncus on left side of the slide and compression of the third nerve lying adjacent to it. The close proximity of the third nerve to the edge of the tentorium and to the medial temporal lobe subjects it to compression early during the development of uncal herniation, accounting for the early clinical feature of pupillary dilatation on the side of the herniation. (For orientation, the ventral surface of the midbrain and interpeduncular fossa lie along the lower margin of the slide. The two arteries cut across are the posterior cerebral arteries.)