Brain artery. Coloured scanning electron micrograph (SEM) of a freeze-fractured artery from the brain. The fracture plane has passed through the artery, showing the central space (lumen) that carries the blood. The arterial tube runs to upper left, passing under the brain's arachnoid mater membrane (green, across top). The internal surface of a blood vessel is lined with endothelial cells. Magnification unknown.