The human skull is a complex bony structure composed of 22 separate bones that provide the structural framework for the head and face. These bones are divided into the facial skeleton (viscerocranium) and the cranium (neurocranium). The facial skeleton includes 14 bones that form the face, such as the jawbones and cheekbones. The cranium is the bony enclosure that surrounds the brain and consists of eight distinct bones.
The Core Function of the Cranium
The cranium’s primary purpose is to create a robust, protective vault for the brain and its associated structures. This bony case acts as a shield against physical trauma, safeguarding the central nervous system. The thick, curved bones of the cranial vault are strong, absorbing and dissipating impacts that could cause severe neurological injury.
Protection extends beyond the brain to include specialized sensory organs. The cranial bones form the orbital cavities, which house and protect the eyeballs and musculature. The bones of the cranial base also enclose the inner and middle ear structures, preserving the mechanisms for hearing and balance.
Beyond protection, the cranium provides attachment sites for muscles of the head and neck. These muscular attachments allow for a wide range of movements, including the rotation and stabilization of the head. The cranium’s base articulates with the first cervical vertebra, the atlas, which supports the head atop the spine.
The Eight Major Cranial Bones
The eight bones of the cranium are a combination of four single, unpaired bones and two sets of paired bones, which together form the protective shell around the brain. The unpaired bones include the frontal, occipital, ethmoid, and sphenoid bones. The paired bones are the two temporal bones and the two parietal bones.
The frontal bone is the large, single bone forming the forehead and the roof of the orbital cavities, extending backward to the coronal suture. The two parietal bones form the superior and lateral walls of the cranium, meeting along the sagittal suture. The single occipital bone, located at the back and base of the skull, features the foramen magnum, allowing the spinal cord to connect to the brain.
The temporal bones are the two paired bones situated on the sides of the head, below the parietal bones and above the ear canals. These complex bones house the inner ear apparatus and contribute to the temporomandibular joint where the jaw articulates. The single sphenoid bone is an irregular, butterfly-shaped bone that spans the width of the skull base, serving as a structural link between the cranium and the facial bones.
The sphenoid bone connects with nearly every other bone of the skull, earning it the description of the “keystone” of the cranium. The single ethmoid bone is a small, delicate bone located deep within the skull, anterior to the sphenoid bone. It contributes to the medial wall of the orbit and the roof of the nasal cavity. This bone is characterized by its sieve-like plate that allows olfactory nerves to pass into the brain.
Structural Connections: Sutures and Fontanelles
The eight cranial bones are held together by specialized joints known as sutures, which are unique to the skull. These connections are fibrous, interlocking joints that appear as jagged, irregular lines where the bone edges meet. In adults, sutures are immovable, creating a rigid structure that reinforces the protective function of the cranium.
The major sutures are named for their location, such as the coronal suture, which lies between the frontal and parietal bones, and the lambdoid suture, which separates the occipital bone from the parietal bones. These joints do not fully fuse until late adolescence or early adulthood, allowing the skull to continue expanding over time. This delayed fusion is necessary to accommodate the growth of the brain.
In infancy, the sutures are much more flexible, and the points where multiple sutures meet are wider, forming membranous gaps called fontanelles, or “soft spots.” Fontanelles are a temporary adaptation that serves two purposes. First, they allow the bony plates of the skull to slightly overlap during childbirth, helping the infant’s head pass through the narrow birth canal.
Second, the fontanelles and flexible sutures permit the rapid growth of the brain throughout the first two years of life. The largest is the diamond-shaped anterior fontanelle, which typically closes between 7 and 19 months of age. The smaller posterior fontanelle, found toward the back of the head, usually closes much earlier, often between one and two months after birth.