====== Median prosencephalic vein ======

The median prosencephalic vein is a precursor to the [[vein of Galen]], and usually regresses before birth. If an arterio-venous fistula exists, it balloons out to become the 'aneurysmal' [[vein of Galen malformation]]. This occurs since the vein is thin-walled, lies free within the subarachnoid space, and is unable to accommodate the arterial flow.