foramen magnum
In anatomy, in the occipital bone, the foramen magnum is one of the several oval or circular apertures in the base of the skull, through which the medulla oblongata (an extension of the spinal cord) enters and exits the skull vault. Apart from the transmission of the medulla oblongata and its membranes, the foramen magnum transmits the Spinal Accessory nerve, vertebral arteries, the anterior and posterior spinal arteries, the membrana tectoria and alar ligaments. [WP,unvetted].