Count the adult skull without the tiny ear bones and you get 22: eight plates around the brain and fourteen in the face. Seams called sutures lock most of them in place.
They do not start that rigid. In infancy the plates are separate so the head can pass through birth and the brain can grow. The seams close gradually through childhood.
A helmet that looks like one piece is a joined set. The cracks you can still trace on a skull are the old borders.
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