The occipital bone is an unpaired trapezoidal-shaped bone that forms the cranial base. It is the most posteriorly located bone of the skull, and it surrounds the foramen magnum (Read more!) - an opening that transmits the brainstem (more precisely, the medulla oblongata), spinal branch of the accessory nerve (CN XI), vertebral arteries, anterior spinal artery and posterior spinal artery, alar ligaments, and tectorial membrane.