Cranial Bones

1. Overview

The cranial bones are a group of eight bones that collectively form the neurocranium, which encases and protects the brain. These bones also contribute to the formation of the cranial cavity, provide attachment sites for head and neck muscles, and house structures such as the meninges, cranial nerves, and blood vessels. Unlike facial bones, cranial bones primarily function in protection rather than movement or facial expression.

2. Location

Cranial bones are located in the superior and posterior portion of the skull, forming the cranial vault and cranial base:

• Superiorly and laterally: Parietal and frontal bones form the dome-like roof of the skull.

• Posteriorly: Occipital bone forms the back and base of the skull.

• Anteriorly: Frontal bone forms the forehead and part of the orbital cavity.

• Inferiorly: Sphenoid and ethmoid bones contribute to the cranial base and nasal cavities.

• Laterally: Temporal bones house the structures of the ears and form part of the cranial base and sides.

3. Structure

The human skull contains eight cranial bones:

• Frontal bone (1): Forms the forehead, superior part of the orbit, and anterior cranial fossa.

• Parietal bones (2): Form the sides and roof of the cranial vault.

• Temporal bones (2): Contain the external auditory canal and form part of the middle and posterior cranial fossae.

• Occipital bone (1): Forms the posterior skull and contains the foramen magnum for passage of the spinal cord.

• Sphenoid bone (1): A butterfly-shaped bone forming the central base of the skull; articulates with all other cranial bones.

• Ethmoid bone (1): Located between the orbits; forms part of the nasal cavity and medial orbital walls.

These bones are joined together by sutures (immovable fibrous joints), such as the sagittal, coronal, lambdoid, and squamous sutures.

4. Function

The cranial bones serve several essential functions:

• Protection: Encase and safeguard the brain from mechanical injury.

• Support: Provide structural support to the head and anchor soft tissues such as the meninges.

• Attachment sites: Serve as origins for muscles involved in chewing, head movement, and facial expressions.

• Pathway housing: Contain foramina, canals, and fissures that allow passage of cranial nerves and blood vessels.

5. Physiological role(s)

Beyond mechanical protection, cranial bones play critical physiological roles:

• Brain homeostasis: Protects neural tissue from temperature extremes, mechanical impact, and infectious agents.

• Hematopoiesis: In infants and children, the diploë (spongy bone between outer and inner compact bone) may contain red bone marrow involved in blood cell production.

• Growth and development: Fontanelles and sutures allow for skull expansion during brain growth in infancy and early childhood.

• Sensory integration: Houses structures for vision, hearing, smell, and balance (e.g., optic canal, petrous part of temporal bone).

6. Clinical Significance

Cranial bones are involved in a variety of clinical and developmental conditions:

• Fractures:

  • Common in trauma; basilar skull fractures may cause cerebrospinal fluid (CSF) leaks, raccoon eyes, or Battle’s sign.

• Craniostenosis (Craniosynostosis):

  • Premature fusion of one or more cranial sutures, leading to abnormal head shape and possibly increased intracranial pressure.

• Hydrocephalus:

  • Excess CSF can cause separation of sutures and bulging fontanelles in infants; treated with shunting procedures.

• Paget’s disease of bone:

  • Can cause thickening of cranial bones, leading to nerve compression and symptoms like hearing loss.

• Tumors:

  • Metastatic lesions, meningiomas, or multiple myeloma may involve cranial bones, requiring imaging and biopsy.

• Surgical relevance:

  • Understanding cranial bone anatomy is essential for neurosurgery, craniotomy, and procedures involving sinuses and orbital decompression.