Cranium

The cranium is the part of the skull that encloses and protects the brain. It forms the upper portion of the skull and provides attachment points for muscles of the head and neck. While the term skull refers to the entire bony framework of the head (including the mandible), the cranium specifically excludes the lower jawbone.

Structure

The cranium consists of 22 bones, with 8 forming the neurocranium and 14 forming the viscerocranium. These bones are joined by immovable sutures. The neurocranium forms a protective case around the brain and includes the frontal, parietal, temporal, occipital, sphenoid, and ethmoid bones. The viscerocranium comprises the facial bones, excluding the mandible.

Location

The cranium is located at the upper part of the skull, above the neck, and encases the brain. It rests superior to the cervical spine and forms the upper portion of the head, articulating with the mandible below and the vertebral column at the foramen magnum.

Divisions of the Cranium

The cranium is divided into two main parts:

• Neurocranium: The protective case around the brain.
• Viscerocranium: The bones that form the face.

The neurocranium develops primarily from the paraxial mesoderm and forms the cranial cavity. The viscerocranium, largely derived from neural crest cells, supports the facial structures and houses the openings for the respiratory and digestive systems.

Bones of the Cranium

Neurocranium (8 bones)

• Frontal bone – forms the forehead and superior part of the orbit.
• Parietal bones (2) – form the superior and lateral walls of the cranium.
• Temporal bones (2) – house the structures of the ear.
• Occipital bone – forms the back and base of the skull; contains the foramen magnum.
• Sphenoid bone – a central bone that articulates with almost all other cranial bones.
• Ethmoid bone – located between the eyes; forms part of the nasal cavity and orbit.

Viscerocranium (14 bones)

• Maxilla (2) – form the upper jaw and parts of the nasal cavity and orbit.
• Zygomatic (2) – cheekbones.
• Nasal (2) – bridge of the nose.
• Lacrimal (2) – small bones in the medial orbit.
• Palatine (2) – part of the hard palate and nasal cavity.
• Inferior nasal conchae (2) – project into the nasal cavity.
• Vomer – forms the lower part of the nasal septum.
• Mandible – lower jaw; although part of the skull, it is not considered part of the cranium.

Sutures of the Cranium

Sutures are fibrous joints that connect the bones of the cranium. In infants, these areas are flexible and allow for brain growth.

• Coronal suture – between the frontal and parietal bones.
• Sagittal suture – between the two parietal bones.
• Lambdoid suture – between the parietal and occipital bones.
• Squamous suture – between the temporal and parietal bones.

Fontanelles are soft membranous gaps in infants where sutures meet, such as the anterior and posterior fontanelles. They usually close by 18-24 months of age.

Cranial Fossae

The base of the cranial cavity is divided into three distinct depressions that support different parts of the brain:

• Anterior cranial fossa – holds the frontal lobes of the brain.
• Middle cranial fossa – contains the temporal lobes and the pituitary gland.
• Posterior cranial fossa – houses the cerebellum and brainstem.

Foramina and Canals

The cranium contains numerous foramina (holes) and canals through which blood vessels and cranial nerves pass:

• Foramen magnum – spinal cord, vertebral arteries.
• Optic canal – optic nerve (CN II).
• Jugular foramen – internal jugular vein, cranial nerves IX, X, XI.
• Superior orbital fissure – cranial nerves III, IV, V1, VI.
• Internal acoustic meatus – facial and vestibulocochlear nerves (CN VII and VIII).

Sinuses and Cavities

The cranium contains air-filled spaces called paranasal sinuses, which lighten the skull and help with voice resonance.

• Frontal sinus
• Sphenoidal sinus
• Ethmoidal air cells
• Maxillary sinus

Additionally, mastoid air cells are located in the temporal bone. Infections can spread from the middle ear to these spaces, causing mastoiditis.

Function

• Protection: Shields the brain from mechanical injury.
• Support: Provides a rigid structure for the brain, eyes, and auditory organs.
• Attachment: Serves as a site for muscle attachments involved in facial expression, chewing, and head movement.
• Passage: Contains foramina and canals for cranial nerves and blood vessels.

Clinical Correlations

• Skull fractures – especially at the pterion can cause epidural hematomas due to rupture of the middle meningeal artery.
• Basilar skull fracture – may cause leakage of cerebrospinal fluid (CSF) from the nose or ears.
• Craniosynostosis – premature closure of cranial sutures, leading to abnormal head shapes.
• Hydrocephalus – increased intracranial pressure due to CSF accumulation, which may cause skull expansion in infants.

Development and Ossification

The cranium develops from mesenchymal tissue by two types of ossification:

• Intramembranous ossification – for most flat bones of the skull (e.g., frontal, parietal).
• Endochondral ossification – for bones at the base of the skull (e.g., sphenoid, occipital).

Fontanelles and sutures allow for postnatal brain growth. The anterior fontanelle is the largest and closes by 18–24 months.

Anatomical Variations and Surface Landmarks

Several external landmarks on the cranium are clinically important:

• Bregma – intersection of coronal and sagittal sutures.
• Lambda – junction of sagittal and lambdoid sutures.
• Pterion – thinnest region; junction of frontal, parietal, sphenoid, and temporal bones.
• Inion – most prominent point of the external occipital protuberance.
• Glabella – smooth area between the eyebrows.

These landmarks are often used in neurosurgery and radiological assessments.