What is the Cranial Bone?

The cranial bones are eight bones that form the top part of the skull, which encloses the brain. They consist of the paired temporal and parietal bones and the single frontal, sphenoid, ethmoid, and occipital bones. The cranial bones are joined by a special type of fixed joint called a suture. The sutures appear as jagged lines on the surface of the skull. Together the cranial bones form the skullcap, which is also called the calvaria.

The frontal bone forms the forehead and the tops of the eye sockets. Over each eye is a ridge that forms the upper margin of the orbit. The frontal bone contains the frontal sinuses and also forms a part of the structure that holds the frontal lobes of the brain.

The two parietal bones make up much of the vault, or top of the skull. They form the major joints of the skullcap with each other, the frontal bone, and the occipital bone at the rear of the skull. They are joined with the frontal bone at the coronal suture, which runs from left to right over the skull. The parietal bones interlock with each other at the sagittal suture at the crest of the head, which runs from front to back. At the rear of the head they form the lambdoid suture with the occipital bone and also the two squamous sutures, where they meet the temporal bones below them on either side of the head.

The temporal bones are located opposite each other below the parietal bones. They lie underneath the temples and contain the external ear canal, where sounds are carried to the inner ear. The temporal bones meet the facial bones at the ridge that runs from the cheek toward the ear. They also form the two movable jaw joints, where they meet the lower jawbone.

Viewed from the sides, the sphenoid bone appears to be two bones, but is actually one bone that spans completely across the front of the skull. Sometimes described as bat-shaped, it has a pair of features called the greater and lesser wings. The greater wings are located directly in front of the temporal bones, while the lesser wings form part of the orbit of the eye. The sphenoid bone forms a joint with each of the other cranial bones, except the occipital bone.

The ethmoid bone is a thin bone situated within the skull in front of and somewhat above the sphenoid bone. It forms a part of the nasal septum, as well as part of the roof and sides of the nasal cavities. The ethmoid bone is the smallest and most delicate of the cranial bones.