[UPDATED] The basivertebral foramen is an opening  in the posterior aspect of each vertebra (click on the picture for a larger image) which allows the exit of the basivertebral veins. These foramina can be single (as seen in the image) or multiple. The basivertebral veins represent a communication that allows drainage of the vertebral body venous sinuses into the extensive complex venous network of the internal venous plexuses that surrounds the spinal cord.

Photography by D.M.Klein