Arnold-Chiari (Chiari type II) malformation; posterior view: An important cause of hydrocephalus is the Arnold-Chiari malformation. A poster view of the cerebellum and brain stem in this malformation reveals that caudal medulla oblongata (including part of the fourth ventricle) lies within the spinal canal below the foramen magnum. (The foramen magnum is approximately at the level of the upper free end of the spinal dura in this slide.) A portion of the inferior cerebellum, usually cerebellar tonsils, commonly is displaced caudally into the spinal canal along with the medulla, as exemplified here.