Foramen Magnum Placement
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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. The foramen magnum in apes and humans is formed by the fusion of the four individual parts of the occipital (pars squama, left and right pars lateralis, and pars basilaris). 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). Pongo have the most anteriorly situated foramen magnum of the apes, but the most vertically oriented. During growth of the occipital bone, human have proportionately less anteroposterior growth in the basioccipital than in the pars lateralis and pars squama posterior to the foramen magnum, whereas the pattern of growth is reversed in non-human primates with proportionately more growth in the basiocciptal.
Human/ape differences in the position and orientation of the foramen magnum reflect differences in habitual body posture and mode of locomotion. However, recent work suggests that foramen magnum orientation is not a good indicator of the orientation of the neck during habitual locomotion because foramen magnum orientation is not significantly correlated with basicranial flexion, orbital axis orientation, the orientation of the head relative to the neck, or the size of the cerebellum relative to the posterior basicranium. In orthograde (upright), bipedal humans have a cranium that 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 have a cranium that projects anterior of the vertebral column, and thus the foramen magnum is positioned and oriented posteriorly. The difference in placement of the foramen magnum has implications for infant mobility and how a mother holds her young infant because the foramen magnum lies close to the center of the basicranium in human neonates and more posteriorly in nonhuman primates. As human infants grow, their foramen magnum remains in the center of the skull, while posterior drift of the foramen magnum occurs in nonhuman primates through the resorption at the posterior end of the cranial base and deposition at the anterior end.
Accordingly the position and orientation of the foramen magnum is considered to be a reliable reflection of mode of locomotion in fossil hominins. The foramen magnum position has been cited as evidence of bipedal locomotion (a hominin characteristics), so fossil hominins are classified based on evidence of an anteriorly placed foramen magnum, as is the case for Ardipithecus and Sahelanthropus. The positions of biporion (line between porion points at the midpoint of the external margin of the external auditory/acoustic meatus) and bicarotid (line between carotids points at the center of the carotid foramen at the intersection of the maximum anteroposterior and mediolateral diameters) relative to basion (point where anterior margin of the foramen magnum is intersected by the mid-sagittal plane) have been used as a measure for distinguishing hominins from non-hominins. There is a considerable amount of overlap in the measures between Pan and modern humans, but the basion to bicarotid measure when used alone or bivariately with the basion to biporion measure can be used to identify distinguish Plio-Pleistocene hominins. The basion to biporion measure alone does not distinguish Pan from Plio-Pleisotcene hominins and modern humans. Other cranial base measurements have not been able to differentiate hominins from non-hominin apes. Based on these measures, Ardipithecus is clearly a hominin, but the hominin status of Sahelanthropus is unclear.
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).
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