"The median prosencephalic vein (of Markowski) is a precursor to the vein of Galen; typically before birth, its anterior portion regresses with formation of the internal cerebral veins and its posterior portion persists as the vein of Galen. If an arterio-venous fistula exists, it balloons out to become the 'aneurysmal' vein of Galen malformation. This occurs because the vein is thin walled, lies free within the subarachnoid space, and is unable to accommodate the arterial flow."