The Axial Skeleton

Overview and Divisions

The axial skeleton forms the longitudinal axis of the human body and is essential for support, protection, and movement. It consists of 80 bones, including the skull, thoracic cage, and vertebral column.

• Axial skeleton: Includes bones of the head and trunk.

• Appendicular skeleton: Includes bones of the limbs and girdles.

Components of the Axial Skeleton

• Skull: 8 cranial bones, 14 facial bones

• Associated bones: 6 auditory ossicles, 1 hyoid bone

• Thoracic cage: Sternum and 24 ribs

• Vertebral column: 24 vertebrae, sacrum, coccyx

Functions of the Axial Skeleton

• Support and protection: Brain, spinal cord, thoracic and abdominal organs

• Attachment points: Muscles for head, neck, trunk movement, breathing, and stabilization of appendicular skeleton

The Skull

Cranial and Facial Bones

The skull is composed of 22 bones: 8 cranial bones that protect the brain and 14 facial bones that support entrances to the digestive and respiratory tracts.

• Cranial bones: Occipital, parietal (2), frontal, temporal (2), sphenoid, ethmoid

• Facial bones: Maxillae (2), palatine (2), nasal (2), vomer, inferior nasal conchae (2), zygomatic (2), lacrimal (2), mandible

Associated Bones

• Auditory ossicles: Three tiny bones in each ear (malleus, incus, stapes) for hearing

• Hyoid bone: Supports the larynx and attaches muscles of the tongue and pharynx

Major Sutures of the Skull

Sutures are immovable joints connecting skull bones.

• Lambdoid suture: Occipital to parietal bones

• Coronal suture: Frontal to parietal bones

• Sagittal suture: Between parietal bones

• Squamous suture: Temporal to parietal bones

Bone Markings and Foramina

Bone markings include foramina (openings for nerves and blood vessels), fissures, and sinuses.

• Sinuses: Air-filled chambers that lighten bone, moisten air, and act as resonating chambers for speech

• Foramina: Passageways for nerves and vessels (e.g., supra-orbital foramen, optic canal, foramen magnum)

Sectional Anatomy of the Skull

Sectional views reveal the spatial relationships of cranial and facial bones, sinuses, and foramina.

Occipital Bone

• Location: Posterior and inferior surfaces of cranium

• Markings: External occipital protuberance, occipital condyles, nuchal lines

• Foramina: Foramen magnum, jugular foramen, hypoglossal canals

Parietal Bones

• Location: Superior and lateral surfaces of cranium

• Markings: Superior and inferior temporal lines, grooves for cranial blood vessels

Frontal Bone

• Location: Anterior cranium, roof of eye sockets

• Markings: Forehead, supra-orbital margin, glabella, lacrimal fossa, frontal sinuses

• Foramina: Supra-orbital foramen/notch

Facial Bones

Maxillae

• Functions: Support upper teeth, form orbital rims, upper jaw, hard palate, contain maxillary sinuses

• Markings: Orbital rim, anterior nasal spine, alveolar process, palatine processes, nasolacrimal canal

• Foramina: Infra-orbital foramen, inferior orbital fissure

Palatine Bones

• Functions: Form posterior hard palate, contribute to orbit floors

• Markings: Horizontal plate, perpendicular plate

• Foramina: Lateral foramina for nerves and vessels

Nasal Bones

• Functions: Support bridge of nose, connect to nasal cartilage

Vomer

• Function: Forms inferior portion of bony nasal septum

Inferior Nasal Conchae

• Functions: Slow inhaled air, create turbulence, increase surface area for warming/humidifying air

Zygomatic Bones

• Functions: Form cheekbones, rims/lateral walls of orbits, zygomatic arches

• Markings: Temporal process

• Foramina: Zygomaticofacial foramen

Lacrimal Bones

• Function: Form medial walls of orbits

• Markings: Lacrimal sulcus

Mandible

• Function: Forms lower jaw

• Markings: Body, alveolar part, mental protuberance, mylohyoid line, ramus, condylar process, coronoid process, mandibular notch

• Foramina: Mental foramina, mandibular foramen

Hyoid Bone

• Functions: Supports larynx, attaches muscles of larynx, pharynx, tongue

• Markings: Body, greater horns, lesser horns

Orbital and Nasal Complexes

Orbital Complex

The orbital complex consists of seven bones forming the bony sockets for the eyes.

• Bones: Frontal, maxilla, palatine, lacrimal, ethmoid, sphenoid, zygomatic

Nasal Complex and Paranasal Sinuses

• Bones: Frontal, sphenoid, ethmoid, maxillae, lacrimal, inferior nasal conchae, nasal bones

• Paranasal sinuses: Frontal, ethmoidal, sphenoidal, maxillary sinuses

• Function: Lighten skull, moisten/clean air, resonance for speech

Fontanelles and Skull Development

Infant Skull and Fontanelles

• Infant skull: Many ossification centers, bones connected by fibrous tissue

• Fontanelles: Large fibrous areas allowing for brain growth and flexibility during birth

• Types: Sphenoidal, mastoid, anterior, posterior fontanelles

Vertebral Column

Curves of the Vertebral Column

The vertebral column consists of 26 bones and four curves: cervical, thoracic, lumbar, and sacral.

• Primary curves: Thoracic and sacral (present at birth, accommodate organs)

• Secondary curves: Cervical and lumbar (develop after birth, shift weight for upright posture)

Clinical Note: Spinal Curvature Disorders

• Kyphosis: Excessive posterior thoracic curvature

• Lordosis: Excessive anterior lumbar curvature

• Scoliosis: Lateral curvature of the spine

Vertebral Regions and Anatomy

• Cervical: 7 vertebrae (smallest, most flexible)

• Thoracic: 12 vertebrae (articulate with ribs)

• Lumbar: 5 vertebrae (largest, support most weight)

• Sacral: Sacrum (5 fused vertebrae)

• Coccygeal: Coccyx (3-5 fused vertebrae)

The Thoracic Cage

Structure and Function

The thoracic cage consists of thoracic vertebrae, ribs, costal cartilages, and sternum.

• Functions: Protects thoracic organs, provides muscle attachment for breathing and movement

Ribs

• True ribs: First 7 pairs, attach directly to sternum

• False ribs: Pairs 8-12, do not attach directly to sternum

• Floating ribs: Pairs 11-12, no sternum attachment

Sternum

• Parts: Manubrium, body, xiphoid process

• Functions: Articulates with clavicles and ribs, provides muscle attachment

Summary Table: Axial Skeleton Components