the opening through which the spinal cord exits the cranial cavity
The foramen magnum (Latin: great hole) is a large opening in the occipital bone of the human skull. It is one of the several oval or circular openings (foramina) in the base of the skull.The spinal cord, an extension of the medulla, passes through the foramen magnum as it exits the cranial vault.he opisthion is the midpoint on the posterior margin of the foramen magnum and is a cephalometric landmark. Another landmark is the basion located at the midpoint on the anterior margin of the foramen magnum. The foramen magnum is a very important feature in bipedal mammals.