Cerebellar tonsillar herniation: In tonsillar herniation the displaced cerebellar tissue lies closely adjacent to the posterior and lateral surfaces of the medulla oblongata. The tissue that has herniated is delimited by a well-defined groove that is created by the rim of the foramen magnum. Cerebellar tonsillar herniation may occur with increased pressure in the posterior fossa or may be secondary to increased pressure in supratentorial structures.