Cranial Bones

The cranial bones are the bones that form the protective case around the brain, known as the cranium or neurocranium. These bones enclose the cranial cavity and provide structural support and protection for the brain, meninges, cranial nerves, and blood vessels. In the adult human skull, there are eight cranial bones, which are fused together by immovable joints called sutures. Understanding these bones is fundamental to the study of human anatomy, especially for students in medicine, dentistry, and allied health sciences.

Structure

The cranial bones are composed of dense cortical bone externally and a thin layer of cancellous (spongy) bone in between, known as the diploë. These bones are lightweight yet strong, providing both protection and space for the attachment of muscles and passage of blood vessels. They are joined by fibrous sutures, which allow for skull expansion during growth and fuse over time.

Location

The cranial bones are located in the upper part of the skull, surrounding and enclosing the brain. They form the roof, walls, and base of the cranial cavity and articulate with the bones of the facial skeleton (viscerocranium) at the front and base.

Functions

• Protection: Encases and protects the brain and associated structures.
• Support: Forms the framework for the skull and head shape.
• Attachment: Serves as an anchoring site for muscles involved in head movement, chewing, and facial expression.
• Passage: Provides foramina and canals for cranial nerves and blood vessels to pass through.

List of Cranial Bones (8 Total)

The eight cranial bones include:

• Frontal bone (1)
• Parietal bones (2)
• Temporal bones (2)
• Occipital bone (1)
• Sphenoid bone (1)
• Ethmoid bone (1)

Anatomy of Individual Cranial Bones

1. Frontal Bone

The frontal bone forms the forehead and the roof of the orbits. It contains the frontal sinuses and contributes to the anterior cranial fossa.

• Key features: Supraorbital foramen, glabella, frontal sinuses.
• Articulations: Parietal, sphenoid, ethmoid, nasal, zygomatic, and maxilla bones.

2. Parietal Bones (2)

The parietal bones form the superior and lateral aspects of the cranial vault. These paired bones are bordered by the coronal, sagittal, squamous, and lambdoid sutures.

• Key features: Parietal eminence, temporal lines.
• Articulations: Frontal, temporal, occipital, sphenoid bones.

3. Temporal Bones (2)

These paired bones form the sides and base of the skull. They house the structures of the inner and middle ear.

• Key parts: Squamous, petrous, mastoid, tympanic, and styloid parts.
• Key features: External acoustic meatus, mastoid process, styloid process, zygomatic process.
• Houses: Auditory ossicles and vestibulocochlear apparatus.

4. Occipital Bone

The occipital bone forms the posterior and inferior part of the skull. It contains the foramen magnum, through which the spinal cord passes.

• Key features: Foramen magnum, external occipital protuberance, occipital condyles, hypoglossal canal.
• Articulations: Parietal, temporal, sphenoid bones and the atlas (C1 vertebra).

5. Sphenoid Bone

The sphenoid is a complex, butterfly-shaped bone located at the base of the skull, centrally between all other cranial bones. It connects to nearly every bone in the skull.

• Key features: Sella turcica (holds the pituitary gland), greater and lesser wings, pterygoid processes, optic canals, superior orbital fissure, foramen rotundum/ovale/spinosum.
• Function: Structural keystone of the skull.

6. Ethmoid Bone

The ethmoid bone is located between the orbits and forms part of the anterior cranial base, nasal septum, and medial wall of the orbit.

• Key features: Cribriform plate (CN I fibers), crista galli, perpendicular plate, ethmoidal air cells.
• Clinical relevance: A fragile bone prone to fracture in nasal or cranial trauma.

Development of Cranial Bones

Most cranial bones develop via intramembranous ossification, meaning they form directly from mesenchymal tissue without a cartilage model. Some bones, like the sphenoid and occipital, also undergo endochondral ossification at their base, where they begin as cartilage and are replaced by bone.

During infancy, the skull bones are not fully fused, allowing flexibility for birth and brain growth. These gaps are called fontanelles, which gradually close over the first few years of life:

• Anterior fontanelle: Closes by 18–24 months.
• Posterior fontanelle: Closes by 2–3 months.

Surface Landmarks Based on Cranial Bones

• Bregma: Intersection of coronal and sagittal sutures (frontal + parietal bones).
• Lambda: Intersection of sagittal and lambdoid sutures (parietal + occipital).
• Pterion: Junction of frontal, parietal, temporal, and sphenoid bones; thinnest part of the skull.
• Asterion: Junction of parietal, occipital, and temporal bones.

Clinical Significance

1. Skull Fractures

Understanding cranial bones is critical in trauma. A blow to the side of the head (pterion) can fracture the sphenoid and rupture the middle meningeal artery, leading to an epidural hematoma.

2. Craniosynostosis

Premature fusion of cranial sutures can affect the shape of the skull and brain development. For example, sagittal suture fusion leads to a long narrow skull (scaphocephaly).

3. Surgical Approaches

Neurosurgical access to brain regions is often guided by cranial bone landmarks (e.g., burr holes through parietal or frontal bones).

4. Sinus Infections

Frontal, ethmoidal, and sphenoidal bones contain air sinuses that may become infected (sinusitis), which can spread into the cranial cavity if severe.

5. Growth and Development Assessment

Pediatricians monitor fontanelle closure and head shape to evaluate developmental milestones and detect abnormalities like hydrocephalus.