The internal vertebral venous plexuses (intraspinal veins) lie within the vertebral canal in the epidural space, and receive tributaries from the bones and from the spinal cord. They form a closer network than the external plexuses, and, running mainly in a vertical direction, form four longitudinal veins, two in front and two behind; they therefore may be divided into anterior and posterior groups. The anterior and posterior plexuses communicate freely with one another by a series of venous rings (retia venosa vertebrarum), one opposite each vertebra.