While death due to medullary compression associated with herniation of the cerebellar tonsils has long been known, relatively little attention has been paid to the sequela of compression of the brain stem caused by herniation of the stem through the tentorial opening. In 1920 Meyer called attention to medial displacement and herniation of the hippocampal gyrus into the tentorial incisura in cases of space-consuming intracranial lesions. The subject was later elaborated by van Gehuchten,1Jefferson,2Reid and Cone,3Moore and Stern4and Smyth and Henderson.5All these contributors have amply demonstrated that space-consuming lesions may cause appreciable herniation of the hippocampal gyrus of the temporal lobe into the potential space that lies directly above and between the free edges of the tentorium cerebelli and the brain stem. The same type of herniation has been recently observed by Schwarz and Rosner6in 100 cases of