The skull is the bony framework of the head, forming the topmost part of the human skeleton. This intricate structure is a complex arrangement of multiple individual bones joined together. It is a specialized component of the axial skeleton, providing the foundation for the head’s shape and serving as protective housing for the brain.
Dividing the Skull into Cranium and Facial Skeleton
The skull is anatomically organized into two primary sections: the neurocranium, often called the braincase, and the viscerocranium, which forms the facial skeleton. In most adults, the entire skull is composed of 22 bones. Eight bones make up the protective neurocranium, and 14 bones constitute the viscerocranium.
The neurocranium is the larger, dome-shaped component that surrounds the brain, brainstem, and cerebellum. Its structure is formed by bones like the frontal bone, two parietal bones, and the occipital bone, which create a robust vault. Its primary function is providing a rigid cavity for the central nervous system structures. The neurocranium also forms the base of the skull, which contains passages for nerves and blood vessels to enter and exit the brain.
The viscerocranium is the anterior portion of the skull, which provides the underlying structure for the face. It includes bones such as the maxilla (upper jaw) and the mandible (lower jaw). This facial skeleton supports the soft tissues of the face and gives an individual their characteristic appearance. It also provides the framework for the orbits, which house the eyes, and the nasal and oral cavities, which begin the respiratory and digestive tracts.
Essential Roles of the Skull
The skull’s primary biological role is to offer protection for the brain tissue housed within the cranial cavity. The thick, curved bones of the neurocranium are designed to absorb and deflect external forces, reducing the likelihood of injury to the brain. This bony shell surrounds the brain, which controls all bodily functions. The skull’s protective function extends to the brainstem, which connects the brain to the spinal cord and manages involuntary actions like breathing and heart rate.
Beyond safeguarding the nervous system, the skull acts as a housing unit for several sensory organs. The bony orbits are deep sockets that fully enclose and protect the eyes from external impact. Similarly, the temporal bones contain the delicate structures of the inner and middle ear, which are responsible for hearing and maintaining balance.
The facial bones of the viscerocranium provide support and attachment points for various muscles. Muscles responsible for chewing, known as the muscles of mastication, anchor firmly onto the skull bones, particularly the mandible and parts of the cranium. The skull also provides the foundation for the numerous small muscles of facial expression. The skull supports the initial segments of the respiratory system by structuring the nasal passages and sinuses, which warm and moisten inhaled air.
How Skull Bones Connect and Mature
The bones of the skull are primarily connected by sutures, which are fibrous joints unique to the cranium. These connections appear as jagged, interlocking lines, providing strength and structural rigidity to the adult skull. Unlike the movable joints found in the limbs, sutures are synarthrodial joints, meaning they are fixed and permit little to no movement, with the exception of the joint connecting the lower jaw.
In infants, the structure of the skull is significantly different to accommodate birth and early development. At birth, the cranial bones are separated by wider regions of dense connective tissue called fontanelles, commonly known as soft spots. These fontanelles allow the bony plates to slightly overlap and change shape during passage through the birth canal, which minimizes pressure on the baby’s head.
The presence of fontanelles also allows for the rapid growth of the brain during the first two years of life. The posterior fontanelle, located toward the back of the head, typically closes within the first two months after birth. The larger anterior fontanelle, found on the top-front of the head, remains open longer, often closing between seven and 19 months of age. As a child matures, the fibrous tissue of the fontanelles and sutures gradually ossifies, or turns to bone, creating the fused, immovable skull of the adult.