The adult human skull is not a single bone but a complex, articulated structure composed of multiple separate bones that interlock tightly. This bony casing, known as the cranium, forms a protective housing for the brain and the framework of the face. The cranium functions primarily to safeguard delicate neural tissue while providing attachment points for the muscles responsible for facial expression and chewing.
Bone Count and Skull Divisions
The typical adult skull is comprised of 22 bones, excluding the auditory ossicles and the hyoid bone. These bones are divided into two main groups. The first is the neurocranium, or braincase, which consists of eight bones that surround the brain. These include the unpaired frontal, occipital, and sphenoid bones, along with the paired parietal and temporal bones.
The second group is the facial skeleton, or viscerocranium, which provides the underlying structure for the face and contains 14 bones. These bones form the eye sockets, the nasal and oral cavities, and the jaw. Examples include the maxilla, the zygomatic bones (cheeks), and the mandible, which is the only movable bone of the skull.
The Connecting Joints: Understanding Sutures
The bones of the adult skull are united by specialized joints called sutures, which are unique to the cranium. These joints are classified as fibrous joints, held together by dense connective tissue. In adults, sutures are rigid and immobile, providing strength and protection for the brain.
The edges of the bones meet in an intricate, interlocking, jagged configuration resembling a serrated line. This design creates a strong seam that distributes mechanical stress across the skull. Major examples include the coronal suture (separating the frontal and parietal bones) and the sagittal suture (running along the midline between the parietal bones). Over time, the fibrous tissue within these sutures may ossify, or turn completely into bone, further fusing the cranial plates.
Structural Differences in the Infant Skull
The developmental phase of the skull requires initial flexibility. At birth, the skull bones are not yet fully fused, allowing for temporary, flexible connective tissue spaces where the sutures intersect. These membranous gaps are known as fontanelles, or soft spots, and are a temporary characteristic of the infant skull.
The fontanelles serve two primary biological functions. The first is allowing the bony plates to overlap slightly during childbirth, which permits the head to pass through the narrow birth canal. The second is accommodating the rapid growth of the brain during infancy, acting as expansion joints for the cranium. The posterior fontanelle typically closes within the first two months after birth, while the larger anterior fontanelle closes later, usually between 9 and 18 months of age.