The Human Skull

The human skeleton provides the framework and the joints that muscles pull against, and the skull is one of its most distinctive parts. The cranium (braincase) is built of several separate flat bones — the frontal, the two parietals, the two temporals, the occipital, the sphenoid and the ethmoid — that interlock at immovable, jagged joints called sutures; for example, the coronal suture joins the frontal bone to the two parietal bones, the sagittal suture runs between the two parietal bones along the midline, and the lambdoid suture joins the parietal bones to the occipital bone at the back of the skull. Sitting centrally at the base of the cranium, the sphenoid bone touches and helps lock together nearly every other cranial bone, much as a keystone locks the stones of an arch in place — which is why it is called the “keystone bone” of the cranium.

The skull rests on the vertebral column through two rounded knobs on the occipital bone, called occipital condyles, which fit into matching depressions on the first neck vertebra (the atlas). Because there are two condyles rather than one, the human skull is described as dicondylic; this two-point articulation gives a stable, largely fixed connection (unlike the single-condyle, more mobile joint of reptiles and birds) while still allowing controlled nodding movements at the atlanto-occipital joint. …