The Skull: Structure, Bones, and Key Features

The Skull: Structure and Overview

Introduction to the Skull

The skull is the most complex bony structure in the human body, composed of cranial and facial bones. It serves to protect the brain, support sensory structures, and provide attachment points for muscles involved in facial expression and mastication.

• Cranial bones (cranium): Enclose and protect the brain, provide muscle attachment sites.

• Facial bones: Form the framework of the face, house cavities for special senses, provide openings for air and food, secure teeth, and anchor facial muscles.

• Most skull bones are flat bones joined by immovable sutures, except the mandible, which is connected by a movable joint.

Major Sutures of the Skull

• Coronal suture: Between frontal and parietal bones

• Sagittal suture: Between right and left parietal bones

• Lambdoid suture: Between parietal and occipital bones

• Squamous suture: Between parietal and temporal bones

Geography of the Skull

Cranial Vault and Base

The cranium is divided into the cranial vault (calvaria) and the cranial base:

• Cranial vault: Forms the superior, lateral, and posterior aspects of the skull, including the forehead.

• Cranial base: Forms the inferior aspect, divided into anterior, middle, and posterior cranial fossae, which cradle the brain.

• The cranial cavity is the space enclosed by the vault and base, housing the brain.

Other Cavities and Openings

• Additional cavities: Middle and internal ear cavities, nasal cavity, orbits (eye sockets).

• Several bones contain paranasal sinuses that lighten the skull and enhance resonance of the voice.

• Numerous foramina, canals, and fissures allow passage of nerves and blood vessels.

Cranial Bones

Overview of Cranial Bones

The cranium consists of eight bones: paired parietal and temporal bones, and unpaired frontal, occipital, sphenoid, and ethmoid bones. These bones form a protective 'helmet' for the brain.

Frontal Bone

• Forms the forehead, superior wall of the orbits, and most of the anterior cranial fossa.

• Features: Supraorbital margins (under eyebrows), supraorbital foramen (passage for artery and nerve), glabella (smooth area between orbits), and frontal sinuses.

Parietal Bones and Major Sutures

• Form most of the superior and lateral aspects of the skull.

• Articulate with other cranial bones at the four major sutures: coronal, sagittal, lambdoid, and squamous.

Occipital Bone

• Forms the posterior wall and base of the skull.

• Features: Foramen magnum (passage for spinal cord), occipital condyles (articulate with first vertebra), external occipital protuberance, and nuchal lines (muscle attachment).

Temporal Bones

• Form the inferolateral aspects of the skull and part of the cranial base.

• Three major parts: Squamous (zygomatic process, mandibular fossa), tympanic (external acoustic meatus), and petrous (houses middle and inner ear, mastoid and styloid processes).

• Important foramina: Jugular foramen, carotid canal, foramen lacerum, internal acoustic meatus.

Sphenoid Bone

• Keystone bone of the cranium, articulates with all other cranial bones.

• Features: Body, greater and lesser wings, pterygoid processes, sella turcica (houses pituitary gland), sphenoidal sinuses.

• Openings: Optic canal, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum.

Ethmoid Bone

• Most deeply situated cranial bone, forms the bony area between the nasal cavity and orbits.

• Features: Cribriform plates (with cribriform foramina for olfactory nerves), crista galli (attachment for brain covering), perpendicular plate (part of nasal septum), ethmoidal air cells, superior and middle nasal conchae.

Sutural Bones

• Tiny, irregular bones found within sutures, most often in the lambdoid suture. Their significance is unknown.

Facial Bones

Overview of Facial Bones

The facial skeleton consists of 14 bones: mandible, vomer (unpaired), maxillae, zygomatics, nasals, lacrimals, palatines, and inferior nasal conchae (paired). The maxillae are the keystone bones of the face.

Mandible

• Largest, strongest facial bone; forms the lower jaw.

• Features: Body, rami, mandibular angle, coronoid process, condylar process (forms temporomandibular joint), alveolar process (anchors teeth), mandibular and mental foramina.

Maxillary Bones (Maxillae)

• Form the upper jaw and central portion of the facial skeleton; all facial bones except the mandible articulate with the maxillae.

• Features: Alveolar processes (anchor upper teeth), palatine processes (form anterior hard palate), maxillary sinuses, infraorbital foramen.

Zygomatic Bones

• Form the cheekbones and part of the inferolateral margins of the orbits.

Nasal Bones

• Fused medially to form the bridge of the nose.

Lacrimal Bones

• Contribute to the medial walls of each orbit; contain a groove for the lacrimal sac (tear drainage).

Palatine Bones

• L-shaped bones forming the posterior part of the hard palate and part of the nasal cavity and orbits.

Vomer

• Plow-shaped bone forming part of the nasal septum.

Inferior Nasal Conchae

• Thin, curved bones projecting into the nasal cavity, forming part of its lateral walls.

The Hyoid Bone

Structure and Function

• Located in the anterior neck, inferior to the mandible.

• Unique: does not articulate directly with any other bone; anchored by ligaments to the styloid processes of the temporal bones.

• Acts as a movable base for the tongue and attachment point for muscles involved in swallowing and speech.

Special Features of the Orbits and Nasal Cavity

The Orbits

• Bony cavities housing the eyes, cushioned by fatty tissue.

• Formed by parts of seven bones: frontal, sphenoid, zygomatic, maxillary, palatine, lacrimal, and ethmoid.

• Contain openings for nerves and blood vessels (superior/inferior orbital fissures, optic canal).

The Nasal Cavity

• Constructed of bone and hyaline cartilage.

• Roof: cribriform plates of ethmoid; lateral walls: conchae and perpendicular plates; floor: palatine processes of maxillae and palatine bones.

• Divided by the nasal septum (vomer and perpendicular plate of ethmoid, plus septal cartilage).

• Lined with mucosa to warm, moisten, and filter air; conchae increase turbulence for air filtration.

Paranasal Sinuses

Structure and Function

• Located in the frontal, sphenoid, ethmoid, and maxillary bones.

• Air-filled, mucosa-lined spaces that lighten the skull, warm and humidify air, and enhance voice resonance.

Self-Assessment and Clinical Connections

Key Questions

• Which bones are the keystone bones of the facial skeleton? Maxillae.

• What bone forms the bulk of the orbit floor? Maxilla. The sense organ found in the orbit is the eye.

• What is the only freely movable joint in the skull? Temporomandibular joint (mandible).

• What is the likely cause of death if the hyoid bone is fractured? Strangulation.

• What process forms most skull bones in the embryo? Intramembranous ossification from mesenchymal connective tissue.

Clinical Note

• Mastoiditis: Infection of the mastoid air cells, which can spread to the brain due to thin bony separation.

Summary Table: Major Cranial and Facial Bones and Markings

Bone	Key Markings	Main Functions
Frontal	Supraorbital foramen, glabella, frontal sinus	Forehead, roof of orbits, anterior cranial fossa
Parietal	Coronal, sagittal, lambdoid, squamous sutures	Superior/lateral skull
Occipital	Foramen magnum, occipital condyles, nuchal lines	Posterior skull, cranial base
Temporal	Zygomatic process, mastoid/styloid process, external acoustic meatus	Inferolateral skull, cranial base
Sphenoid	Sella turcica, optic canal, pterygoid processes	Keystone of cranium
Ethmoid	Cribriform plate, crista galli, perpendicular plate	Medial orbit, nasal cavity, septum
Mandible	Body, ramus, alveolar process, mental foramen	Lower jaw, teeth anchor
Maxilla	Alveolar/palatine processes, infraorbital foramen	Upper jaw, hard palate
Zygomatic	Temporal/frontal/maxillary processes	Cheekbones, orbit margin
Nasal	Articulates with frontal, maxilla, ethmoid	Bridge of nose
Lacrimal	Lacrimal fossa	Medial orbit wall, tear drainage
Palatine	Horizontal/perpendicular plates	Posterior hard palate, nasal cavity
Vomer	Plow-shaped	Nasal septum
Inferior Nasal Concha	Curved bone	Lateral nasal cavity