At the level of C1 (atlas), the vertebral foramen is an ovoid opening, which is narrowed in its anterior half by the lateral masses and bounded by the anterior and posterior arches.  The anterior part of the vertebral foramen houses the dens of the C2 (axis), the posterior part contains the spinal cord and its associated meninges, the spinal root of the accessory nerve and the anterior  posterior spinal arteries.