Placement of Foramen Magnum

The foramen magnum (from the Latin, meaning “great hole”) is the large opening in the base of the skull through which the spinal cord exits the cranial vault. The foramen magnum is situated in the occipital bone, and forms around the base of the brainstem (the medulla oblongata), separating the brain above from the spinal cord below. In humans the foramen magnum is anteriorly positioned, with its anterior portion lying on the bitympanic line (a line that connects the inferolateral points of the right and left tympanic plates), and is inferiorly oriented (opening directly downward). The portion of the occipital bone that lies anterior to the foramen magnum (the basioccipital or basiocciput) is relatively short in humans. In apes the foramen magnum lies well behind (posterior) of the bitympanic line, posterior of a relatively long basioccipital. In addition to being more posteriorly positioned, the foramen magnum in apes is more vertically oriented (opening backwards and downwards, rather than directly downwards). Human/ape differences in the position and orientation of the foramen magnum reflect differences in habitual body posture and mode of locomotion. In orthograde (upright), bipedal humans the cranium rests atop the vertebral column: the anterior position of the foramen magnum helps to balance the mass of the head above the vertebrae, and its inferior orientation reflects the anatomical relationship between the cranium and vertebral column. In pronograde (with a body more parallel to the ground), quadrupeal apes, the cranium projects anterior of the vertebral column, and thus the foramen magnum is positioned and oriented posteriorly. Accordingly the position and orientation of the foramen magnum is considered to be a reliable reflection of mode of locomotion in fossil hominins. The configuration of the foramen magnum in humans may also be related to expansion of the cerebral cortex, particularly the frontal and occipital lobes. Expansion of brain volume during development results in increased flexion of the cranial base (around a transverse axis passing through the pituitary fossa), resulting in an increasingly anterior and inferior position of the foramen magnum (during postnatal development, humans experience about 30° of rotation in the position of the foramen magnum as the brain grows).