7.2 The Skull

The skull is a complex structure that protects the brain and houses the major sensory organs (eyes, ears, nose). It consists of two main parts: the brain case (neurocranium) and the facial skeleton (viscerocranium). The bones connect at immovable joints called sutures, and the interior is divided into compartments that hold different regions of the brain, the orbits for the eyes, the nasal cavity, and air-filled paranasal sinuses.

Bones and Sutures of the Skull

Bones of the skull and face

The neurocranium (brain case) encloses and protects the brain. It's made of 8 bones:

• Frontal bone forms the forehead and the superior portion of the orbits (eye sockets).
• Parietal bones (2) form the superior and lateral walls of the cranial cavity. They meet each other at the midline.
• Temporal bones (2) form the inferior lateral walls of the cranial cavity. They also house the structures of the middle and inner ear.
• Occipital bone forms the posterior and inferior portions of the cranial cavity. It contains the foramen magnum, the large opening where the spinal cord connects to the brain.
• Sphenoid bone spans the middle of the cranial floor and has greater and lesser wings that contribute to the orbit walls. Think of it as a butterfly-shaped bone that touches almost every other cranial bone.
• Ethmoid bone forms part of the anterior cranial fossa, the medial walls of the orbits, and the superior portion of the nasal septum. Its cribriform plate has tiny holes that allow olfactory nerves to pass through.

The viscerocranium (facial skeleton) forms the structure of the face. It's made of 14 bones:

• Maxillae (2) form the upper jaw and the anterior portion of the hard palate. They also contribute to the floor of the orbits.
• Palatine bones (2) form the posterior portion of the hard palate. If you run your tongue along the roof of your mouth toward the back, you're feeling the palatine bones.
• Zygomatic bones (2) form the cheekbones and contribute to the lateral walls of the orbits.
• Lacrimal bones (2) are tiny bones forming part of the medial walls of the orbits. They contain grooves for the nasolacrimal ducts, which drain tears from the eyes into the nasal cavity.
• Nasal bones (2) form the bridge of the nose.
• Vomer forms the inferior portion of the nasal septum.
• Inferior nasal conchae (2) are thin, curved bones that project into the nasal cavity and help direct airflow.
• Mandible is the lower jaw and the only freely movable bone of the skull. It articulates with the temporal bones at the temporomandibular joint (TMJ).

Suture lines and adjacent bones

Sutures are fibrous joints that bind the skull bones together. They allow very slight movement and, in younger individuals, permit growth. The major sutures to know:

• Coronal suture runs transversely (side to side) between the frontal bone and the two parietal bones.
• Sagittal suture runs along the midline between the two parietal bones, from the coronal suture to the lambdoid suture.
• Lambdoid suture runs posteriorly between the parietal bones and the occipital bone. It's named for its resemblance to the Greek letter lambda (λ).
• Squamous sutures are located on the lateral sides of the skull, between each parietal bone and the corresponding temporal bone.
• Occipitomastoid sutures run between the occipital bone and the mastoid portion of each temporal bone, located posteriorly.

In infants, fontanelles ("soft spots") are membrane-covered gaps where sutures intersect. They allow the skull to compress slightly during birth and accommodate rapid brain growth. The anterior fontanelle (at the junction of the coronal and sagittal sutures) typically closes by about 18 months of age.

Skull structure and development

• The calvaria (skullcap) is the dome-like superior portion of the skull, formed by the frontal, parietal, and occipital bones.
• Diploe is the layer of spongy (cancellous) bone sandwiched between two layers of compact bone in the flat bones of the skull. This layered structure makes the skull both lightweight and strong.
• Craniometry refers to the measurement of skull dimensions. The cephalic index (ratio of skull width to length) is one such measurement used to classify skull shapes.

Cranial Fossae, Temporal Regions, and Paranasal Sinuses

Cranial fossae and temporal regions

The interior base of the skull is divided into three step-like depressions, each housing specific brain regions. They descend from anterior to posterior like a staircase.

• Anterior cranial fossa is the most superior (shallowest) step. It's bounded by the frontal bone anteriorly, the lesser wings of the sphenoid laterally, and the cribriform plate of the ethmoid posteriorly. It supports the frontal lobes of the brain.
• Middle cranial fossa sits one step lower, posterior to the anterior fossa. It's bounded by the greater wings of the sphenoid laterally and the temporal bones inferolaterally. It houses the temporal lobes and the pituitary gland (which sits in the sella turcica of the sphenoid).
• Posterior cranial fossa is the deepest and most inferior. It's bounded primarily by the occipital bone and contains the cerebellum, pons, and medulla oblongata. The foramen magnum opens from this fossa.

Two additional fossae on the external skull are worth knowing:

• Infratemporal fossa lies deep to the zygomatic arch on the lateral skull. It contains the muscles of mastication (temporalis, masseter, medial and lateral pterygoids), the maxillary artery, and the mandibular nerve (V3).
• Pterygopalatine fossa is a small, wedge-shaped space between the maxilla and the pterygoid process of the sphenoid. It serves as a major crossroads for nerves and vessels, containing the pterygopalatine ganglion, the maxillary nerve (V2), and terminal branches of the maxillary artery.

Paranasal sinuses: location and function

The paranasal sinuses are air-filled cavities within certain skull bones that all communicate with the nasal cavity. Each sinus is named for the bone it occupies:

• Frontal sinuses sit within the frontal bone, superior to the orbits. They drain into the middle meatus.
• Maxillary sinuses are the largest paranasal sinuses, located within the maxillae inferior to the orbits. They also drain into the middle meatus. Because their drainage opening is near the top of the sinus (not the bottom), they're prone to fluid accumulation and infection.
• Ethmoid air cells are clusters of small sinuses within the ethmoid bone, between the orbits. They drain into the superior and middle meati.
• Sphenoid sinuses are located within the body of the sphenoid bone, posterior to the ethmoid air cells. They drain into the sphenoethmoidal recess.

Functions of the paranasal sinuses:

1. Lighten the skull by reducing overall bone mass.
2. Humidify and warm inspired air as it passes through the nasal cavity.
3. Act as resonating chambers for voice production, contributing to the quality and timbre of your voice.
4. Produce mucus that keeps the nasal cavity moist and traps inhaled particles and pathogens.

Orbits, Nasal Cavity, and Foramina

Orbit structure and contents

Each orbit is a cone-shaped bony socket that protects the eye. Seven bones contribute to its walls:

• Roof: frontal bone (and lesser wing of the sphenoid posteriorly)
• Floor: maxilla and zygomatic bone
• Medial wall: ethmoid and lacrimal bones
• Lateral wall: greater wing of the sphenoid and zygomatic bone

Three major openings connect the orbit to the rest of the skull:

1. Optic canal transmits the optic nerve (CN II) and the ophthalmic artery.
2. Superior orbital fissure transmits the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves; the ophthalmic division of the trigeminal nerve (V1); and the ophthalmic veins.
3. Inferior orbital fissure transmits the maxillary nerve (V2) and the infraorbital vessels.

The orbit contains the eyeball, six extraocular muscles (superior, inferior, medial, and lateral rectus; superior and inferior oblique), and the nerves and vessels that supply these structures.

Anatomy of the nasal cavity

The nasal septum is the midline partition dividing the nasal cavity into right and left halves. It's formed by two bones: the perpendicular plate of the ethmoid superiorly and the vomer inferiorly. A piece of septal cartilage completes the anterior portion.

The nasal conchae (turbinates) are scroll-like bony projections from the lateral walls of the nasal cavity. They create turbulence in airflow, which increases contact between air and the moist mucosal lining. This helps warm, humidify, and filter the air you breathe.

• Superior nasal concha is the smallest, located most superiorly (part of the ethmoid bone).
• Middle nasal concha is larger, located inferior to the superior concha (also part of the ethmoid bone).
• Inferior nasal concha is the largest and sits lowest in the nasal cavity. It's the only concha that is its own separate bone.

The spaces beneath each concha are called meati (singular: meatus), and they receive drainage from the paranasal sinuses.

Foramina and openings in the skull

The skull has numerous foramina (openings) that allow nerves and blood vessels to pass through. Here are the most important ones:

A helpful way to remember the three sphenoid foramina: moving from anterior to posterior, they go rotundum → ovale → spinosum. Rotundum carries V2, ovale carries V3, and spinosum carries the middle meningeal artery.