sentences:  They emerge from the foramina on the posterior surfaces of the vertebral bodies.They communicate through small openings on the front and sides of the vertebral bodies with the anterior external vertebral plexuses, and converge behind to the principal canal, which is sometimes double toward its posterior part, and open by valved orifices into the transverse branches which unite the anterior internal vertebral plexuses.The basivertebral veins become greatly enlarged in advanced age.