The inferior medullary velum (posterior medullary velum) is a thin layer of white substance, prolonged from the white center of the cerebellum, above and on either side of the nodule; it forms the infero-posterior part of the fourth ventricle.

Somewhat semilunar in shape, its convex edge is continuous with the white substance of the cerebellum, while its thin concave margin is apparently free; in reality, however, it is continuous with the epithelium of the ventricle, which is prolonged downward from the posterior medullary velum to the taeniae.

Fm: Foramen of Magendie; imv: inferior medullary velum; icp: inferior cerebellar peduncle; pica: posterior inferior cerebellar artery (third and forth segments) tc: tela chorioidea; tvj: telovelar junction ( yellow discontinous line).

Blake's pouch cyst is a cystic appearing structure that represents posterior ballooning of the inferior medullary velum into the cisterna magna