The Skull: Structure, Bones, and Cavities

Module 7.2: The Skull

Overview of Skull Structure

The skull is the most complex bony structure in the human body, consisting of 22 bones (excluding the auditory ossicles). It is divided into two main groups: cranial bones, which encase the brain, and facial bones, which form the framework of the face. These bones also contribute to the protection and housing of special sensory organs.

• Cranial bones (8): Frontal, parietal (2), temporal (2), occipital, sphenoid, ethmoid

• Facial bones (14): Maxilla (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal concha (2), vomer, mandible

• All skull bones except the mandible are joined by immovable joints called sutures.

Cranial Cavity and Skull Regions

The cranial bones form the cranial cavity, which houses and protects the brain. The cranial cavity is divided into the cranial vault (calvaria) and the cranial base. The cranial base is further subdivided into three fossae: anterior, middle, and posterior, each accommodating different regions of the brain.

• Cranial vault: Superior portion, forms the "roof" of the cranial cavity.

• Cranial base: Inferior portion, forms the "floor" of the cranial cavity.

• Cranial fossae: Depressions in the cranial base for brain regions.

Major Cavities of the Skull

Besides the cranial cavity, the skull contains several smaller cavities that house sensory organs and other structures:

• Orbits: Contain the eyeballs and associated structures.

• Nasal cavity: Houses sensory receptors for smell and is part of the respiratory tract.

• Oral cavity: Surrounds the teeth and tongue, forming the entry to the digestive tract.

• Paranasal sinuses: Air-filled spaces in certain skull bones that lighten the skull and enhance voice resonance.

Internal Structure of the Skull

The internal view of the skull reveals the arrangement of cranial fossae and the passageways for nerves and blood vessels. The superior view shows the anterior, middle, and posterior cranial fossae, each formed by different cranial bones.

• Anterior cranial fossa: Houses the frontal lobes of the brain.

• Middle cranial fossa: Accommodates the temporal lobes.

• Posterior cranial fossa: Contains the cerebellum.

Disarticulated Skull

Understanding the skull's complexity is aided by viewing it as a disarticulated structure, where each bone is separated. This "exploded" view helps visualize how the bones fit together, especially complex ones like the ethmoid and sphenoid.

Cavities of the Skull

The Orbit

The orbit is a bony cavity that houses the eyeball, associated muscles, nerves, blood vessels, and the lacrimal gland. It is formed by seven bones: frontal, maxilla, zygomatic, sphenoid, ethmoid, lacrimal, and palatine. The complexity of the orbit makes injuries challenging to treat.

Nasal Cavity and Paranasal Sinuses

The nasal cavity is the entryway to the respiratory tract, formed by several bones and lined with mucous membranes. The paranasal sinuses are air-filled spaces in the frontal, sphenoid, ethmoid, and maxillary bones, connected to the nasal cavity. They function to filter, warm, and humidify air, lighten the skull, and enhance voice resonance.

• Posterior wall: Sphenoid body and pterygoid processes

• Lateral walls: Ethmoid, palatine, inferior nasal conchae, maxilla

• Roof: Cribriform plate of ethmoid

• Floor: Hard palate (palatine bones and maxillae)

• Nasal septum: Hyaline cartilage (anterior), perpendicular plate of ethmoid and vomer (posterior)

Clinical note: Infections can spread easily between the nasal cavity and sinuses, leading to sinusitis.

Oral Cavity

The oral cavity contains the teeth, tongue, and salivary glands. Its roof is the hard palate, and its walls are formed by the maxillae and mandible. The floor and posterior wall are made of soft tissues.

Fetal and Infant Skull

Fontanels and Skull Flexibility

Infant skulls have "soft spots" called fontanels, which are membranous areas where ossification is incomplete. These allow for flexibility during birth and accommodate brain growth. Major fontanels include the anterior, posterior, sphenoid, and mastoid fontanels. Sutures are also unfused in infants, providing additional flexibility.

Clinical note: The degree of suture fusion can help estimate age in forensic investigations.

Sex Differences in the Skull

Male and female skulls have distinct features. Males typically have a sloped forehead, prominent supraorbital ridge, a mandibular angle near 90 degrees, and a larger mastoid process. Females have a straighter forehead, less prominent ridge, a wider mandibular angle, and a smaller mastoid process.

The Hyoid Bone

Structure and Function

The hyoid bone is a small, C-shaped bone in the superior neck. It does not articulate with any other bone but is suspended by muscles and ligaments. It serves as an attachment point for muscles involved in swallowing and speech. In forensic science, a fractured hyoid bone may indicate strangulation.

Table: Major Cranial and Facial Bones

Bone	Location	Main Function(s)
Frontal	Forehead, roof of orbits	Protects brain, forms upper face
Parietal	Superior and lateral skull	Protects brain
Temporal	Inferior lateral skull	Protects brain, houses ear structures
Occipital	Posterior skull	Protects brain, contains foramen magnum
Sphenoid	Base of skull	Supports brain, forms part of orbits
Ethmoid	Medial orbit, nasal cavity	Supports nasal cavity, forms part of orbits
Maxilla	Upper jaw	Holds upper teeth, forms part of orbit and palate
Mandible	Lower jaw	Holds lower teeth, only movable skull bone
Zygomatic	Cheek	Forms cheekbone, part of orbit
Nasal	Bridge of nose	Supports nose cartilage
Lacrimal	Medial orbit	Houses lacrimal sac
Palatine	Posterior hard palate	Forms part of palate and nasal cavity
Inferior nasal concha	Lateral nasal cavity	Filters and humidifies air
Vomer	Nasal septum	Divides nasal cavity