Posterior view of the Foramen of Magendie and fourth ventricle.  In this dissection, the cerebellar tonsils and vermis have been retracted to expose the inferior medullary velum, which forms the posterior roof of the fourth ventricle.  The velum consists largely of tela choroidea, a thin membrane lined by pia mater on its outer surface and ependyma on its inner surface.   A tuft of choroid plexus develops from the tela choroidea and suspends into the roof into the IV ventricle. The Foramen of Magendie, a midline opening in the inferior medullary velum, is one of three openings (the other two are the paired lateral foramina of Luschke) that drain CSF from the IV ventricle into the foramen magnum.  The posterior inferior cerebellar artery (PICA), seen prominently on the left side of the image, is a branch of the vertebral artery. PICA supplies the dorsolateral medulla and then reflects onto the posterior and inferior surface of the cerebellum. (Image courtesy of AL Rhoton, Jr.)