Although the cranium—the largest section of the skull—might appear to be one solid bone, there are actually 22 bones that encase the brain. Twenty-one of those pieces are fused together by sutures, which are nearly rigid, fibrous joints found only in the skull. Similar connective fibers are found in teeth and the spine.
These sutures give the appearance of cracks or fissures. These are easiest to see on the side of the head where the temporal bone meets the parietal bone, and in the back where the occipital bone adjoins the temporal and parietal bones.
The skull consists of the cranium and the mandible, or jawbone. It is the semi-circular bone at the bottom of the skull and attached to the cranium at the jaw. The mandible is responsible for the up and down motion of the jaw.
The mandible also holds the bottom row of teeth; the maxilla holds the upper row. Mammals, including humans, develop two sets of teeth during their lives.