Venous drainage of the spine and the spinal cord is arranged like arteries in a reverse configuration. The vessels have very thin walls and are frequently without valves. The intraspinal longitudinal system is composed of the anterior and posterior spinal veins, extending longitudinally along the middle area of the spinal cord fed by the radiculomedullary or segmental radicular veins. In the intervertebral foramen, the radicular veins join the internal vertebral venous plexus (epidural venous plexus), which have segmental communications with the external vertebral venous plexus. Blood passes to the vertebral, intercostal posterior, and lumbar veins through the hemiazygos and azygos systems to the caval system.