a congenital deformity in which the medulla oblongata and cerebellum protrude through the foramen magnum (see foramen), so that the cerebellum overlaps the top of the spinal cord. Hydrocephalus and meningomyelocele are commonly associated with the different types of the deformity. [Julius Arnold (1835–1915), German pathologist; Hans Chiari (1851–1916), Austrian pathologist]