The foramen magnum is a large opening in the floor of the posterior cranial fossa.  The lateral parts of the occipital bone surround it laterally, the squamous part posteriorly and the basilar part anteriorly, where the opening is narrowed by the occipital condyles.  

The anterior and posterior atlanto-occipital membranes attach to the anterior and posterior margins, respectively.  The foramen magnum transmits the medulla oblongata and meninges, spinal rami of the accessory nerve, vertebral arteries and veins and associated sympathetic plexus, and anterior and posterior spinal arteries.