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 - an opening that transmits the brainstem (more precisely, the medulla oblongata), spinal branch of the accessory nerve (CN XI), vertebral arteries, anterior and posterior spinal arteries, alar ligaments, and tectorial membrane.