![]() 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. 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. 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. Human/ape differences in the position and orientation of the foramen magnum reflect differences in habitual body posture and mode of locomotion. 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. Pongo have the most anteriorly situated foramen magnum of the apes, but the most vertically oriented. In addition to being more posteriorly positioned, the foramen magnum in apes is more vertically oriented (opening backwards and downwards, rather than directly downwards). In apes the foramen magnum lies well behind (posterior) of the bitympanic line, posterior of a relatively long basioccipital. The portion of the occipital bone that lies anterior to the foramen magnum (the basioccipital or basiocciput) is relatively short in humans. ![]() 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 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). 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 (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. ![]()
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