Morphology of Virchow Robin and perivascular spaces. Delineated by basal membranes of glia, pia and endothelium, the Virchow Robin space (VRS) depicts the space surrounding vessels penetrating into the parenchyma. The VRS is obliterated at the capillaries where the basement membranes of glia and endothelium join. The complex pial architecture may be understood as an invagination of both cortical and vessel pia into the VRS. The pial funnel is not a regular finding. The pial sheath around arteries extends into the VRS, but becomes more fenestrated and eventually disappears at the precapillary section of the vessel. Unlike arteries (as shown in this figure), veins do not possess a pial sheath inside the VRS. ISF may drain by way of an intramural pathway along the basement membranes of capillaries and arterioles into the lymphatics at the base of the skull (green arrows). It should be noted that the figure does not depict the recently suggested periarterial flow from the SAS into the parenchyma and an outward flow into the cervical lymphatics along the veins (for discussion see text "Current research"). Also, it is still a matter of debate whether the Virchow Robin space, extending between the outer basement membrane of the vessel and the glia, represents a fluid-filled open space (see text). VRS: Virchow Robin space, SAS: subarachnoid space.