Keep these functions in mind when you study the structure of the bones. The shoulder girdle and hip girdle attach the limbs to the axial skeleton. The cranial bones are mostly flat bones, but a few are irregular bones. The cranium encloses and protects the brain. Its flat bones also provide a site for the attachment of head and neck muscles.
Fontanels allow the skull to be compressed as it passes through the birth canal during delivery.
The fontanels also permit rapid growth of the brain during infancy. The spaces between the cranial bones eventually ossify and the fontanels close by the 24th month. The mandible is connected to the rest of the skull by a freely movable joint. The sutures have a serrated appearance and look like seams between the bones of the skull. Together, these bones form a kind of bony helmet around the brain.