The human skull is the bony structure of the head that provides the frame for the face and forms a protective enclosure for the brain. This framework is an assembly of several distinct, tightly interconnected bones. These bones are classified into two main groups: the bones that form the braincase (cranium) and the bones that constitute the facial skeleton. The skull serves as the uppermost part of the axial skeleton, supporting the head.
The Eight Cranial Bones
The part of the skull that directly encases the brain is known as the cranium, or neurocranium. It is formed by eight bones joined together by immovable, fibrous joints called sutures, which provide a fixed, protective structure. These cranial bones are divided into unpaired bones and paired bones.
Unpaired Cranial Bones
Four cranial bones are unpaired. The frontal bone makes up the forehead and the upper part of the eye sockets. The occipital bone forms the posterior portion of the skull and features the foramen magnum, a large opening where the spinal cord connects to the brain. The sphenoid bone is a complex, butterfly-shaped bone spanning the width of the skull’s base. The ethmoid bone is a small bone located between the eyes, contributing to the nasal cavity roof and the eye socket wall.
Paired Cranial Bones
The remaining four bones consist of two pairs. The two parietal bones are flat bones that form the curved roof and sides of the skull, articulating along the midline. The two temporal bones are situated on the sides and base of the skull, surrounding the ear canal and housing the structures of the inner ear.
Primary Functions of the Cranium
The primary role of the cranium is providing a secure shell for the brain. The dense, hard bone of the neurocranium is designed to absorb and distribute mechanical force from impacts. This protective function extends to the brainstem and cerebellum, which are housed within the cranial cavity.
Beyond protection, the cranium provides numerous surfaces for muscle attachment. These include muscles responsible for moving the head and neck, and the muscles of mastication used for chewing. The cranial bones also contribute to the bony enclosures that support major sensory organs, such as the eyes, by forming the orbits.
Specific cranial bones, like the frontal and sphenoid, contain paranasal sinuses. These sinuses lessen the overall weight of the skull and contribute to voice resonance. The base of the cranium is also perforated by numerous openings, or foramina, which allow cranial nerves and blood vessels to pass between the brain and the body.
Cranial Versus Facial Bones
The entire skull consists of more bones than the eight that comprise the cranium. The skull is composed of the eight cranial bones, which form the braincase, and an additional fourteen bones that make up the facial skeleton. This means the complete adult skull structure contains twenty-two bones in total.
The fundamental distinction lies in their anatomical roles: cranial bones form the protective vault (neurocranium), while facial bones form the structure of the face (viscerocranium). Facial bones shape the eye sockets, nasal cavity, and mouth, providing a framework for soft tissues. Examples include the two maxillae (upper jaw) and the mandible (lower jaw), which is the only movable bone of the skull.
The facial bones provide structure for the initial parts of the digestive and respiratory systems. For instance, the nasal bones form the bridge of the nose, and the zygomatic bones create the prominent cheekbones. Unlike the fixed cranial bones, the facial bones are structured to support functions like breathing, chewing, and facial expression.