The Axial Skeleton: Structure, Function, and Components

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

The Axial Skeleton

Overview and Functions

The axial skeleton forms the central axis of the human body and is essential for protection, support, and movement. It consists of the skull, vertebral column, thoracic cage, auditory ossicles, and hyoid bone.

Protection: Shields the brain, spinal cord, and vital organs within the thorax.
Support: Provides the main framework for the body, supporting the head and trunk.
Muscle Attachment: Serves as attachment points for muscles involved in head, neck, and trunk movements, as well as respiration.
Stabilization: Stabilizes portions of the appendicular skeleton.

Posterior view of the human axial skeleton Labeled anterior and posterior views of the axial skeleton

Major Components of the Axial Skeleton

Skull: 8 cranial bones and 14 facial bones
Auditory Ossicles: 6 small bones in the middle ear
Hyoid Bone: Supports the tongue and larynx
Vertebral Column: 24 vertebrae, sacrum, and coccyx
Thoracic Cage: Sternum and 24 ribs

The Skull

Cranial Bones

The cranium (braincase) is composed of 8 bones that enclose and protect the brain within the cranial cavity.

Occipital bone
Frontal bone
Sphenoid bone
Ethmoid bone
Left and right parietal bones
Left and right temporal bones

Inferior view of the cranial floor with labeled foramina and bones Lateral view of the skull with labeled bones and features Superior view of the skull showing sutures

Facial Bones

The 14 facial bones support entrances to the digestive and respiratory tracts and form the structure of the face.

Paired maxillae, lacrimal, nasal, zygomatic, palatine, and inferior nasal conchae
Vomer (single)
Mandible (single)

Key Skull Features

Sinuses: Air-filled chambers that lighten the skull and produce mucus.
Sutures: Immovable joints connecting skull bones (e.g., coronal, sagittal, lambdoid).

Selected Cranial Bones and Features

Occipital Bone

Forms the posterior and inferior surfaces of the cranium.

External occipital protuberance: Midline projection for ligament attachment.
Occipital condyles: Articulate with the atlas (C1 vertebra).
Foramen magnum: Large opening for the spinal cord.
Jugular foramen: Passage for internal jugular veins.
Hypoglossal canals: Passage for hypoglossal nerves.

Inferior view of the occipital bone with labeled features

Parietal Bones

Form the lateral and superior portions of the cranium.

Temporal lines: Attachment points for the temporalis muscle.
Grooves: Indicate the path of blood vessels inside the skull.

Parietal bone with labeled temporal lines

Frontal Bone

Forms the anterior cranium and superior portion of the orbits (eye sockets).

Frontal sinuses: Help flush the nasal cavities.
Supra-orbital margin: Protects the eyes.
Lacrimal fossa: Houses the lacrimal gland.
Supra-orbital foramen/notch: Passage for nerves and blood vessels.

Frontal bone with labeled features Sectioned frontal bone showing sinuses and foramina

Temporal Bones

Form part of the lateral walls of the cranium and house structures of the ear.

Zygomatic process: Articulates with the zygomatic bone.
Mastoid process: Muscle attachment; contains air cells.
Styloid process: Ligament and tendon attachment.
Petrous part: Contains inner ear structures.
Carotid canal, foramen lacerum, external/internal acoustic meatus: Passageways for nerves and vessels.

Lateral view of the temporal bone with labeled features Temporal bone with labeled squamous, petrous, and mastoid parts Temporal bone with labeled external acoustic meatus and processes

Sphenoid Bone

Forms part of the cranial floor, connects cranial and facial bones, and strengthens the skull.

Sphenoid sinus: Cleanses the nasal cavity.
Sella turcica: Houses the pituitary gland.
Lesser and greater wings: Form parts of the cranial floor and orbit.
Pterygoid processes: Muscle attachment sites.
Optic canal, foramen rotundum, ovale, spinosum: Passageways for nerves and vessels.

Sphenoid bone with labeled foramina and wings Sphenoid bone with labeled processes and sinuses

Ethmoid Bone

Forms the anterolateral floor of the cranium, roof of the nasal cavity, and part of the nasal septum and orbit.

Ethmoid sinuses: Network of air cells opening into the nasal cavities.
Cribriform plate: Contains olfactory foramina for nerve passage.
Crista galli: Attachment for the falx cerebri (dura mater).
Lateral masses: Contain superior and middle nasal conchae.
Perpendicular plate: Forms part of the nasal septum.

Ethmoid bone with labeled cribriform plate and crista galli Ethmoid bone with labeled conchae and perpendicular plate

Selected Facial Bones

Maxillae

Support the upper teeth, form the upper jaw and hard palate, and contribute to the orbit and nasal cavity.

Maxillary sinuses: Produce mucus for the nasal cavity.
Alveolar processes: Support upper teeth.
Palatine processes: Form the anterior hard palate.
Infra-orbital foramen: Passage for nerves and vessels.

Maxilla with labeled processes and foramen

Palatine Bones

Form the posterior hard palate and contribute to the orbital floor.

Horizontal plate: Posterior hard palate.
Perpendicular plate: Extends vertically, contributing to the nasal cavity and orbit.

Palatine bone with labeled plates and processes

Nasal Bones, Vomer, and Inferior Nasal Conchae

The nasal bones support the bridge of the nose, the vomer forms the inferior nasal septum, and the inferior nasal conchae create air turbulence in the nasal cavity.

Zygomatic and Lacrimal Bones

The zygomatic bones form the cheek and part of the orbit, while the lacrimal bones form part of the medial orbital wall and house the lacrimal sac.

Mandible

The mandible forms the lower jaw and is the only movable skull bone.

Alveolar process: Supports lower teeth.
Body and ramus: Main horizontal and ascending portions.
Condylar process: Articulates with the temporal bone (TMJ).
Coronoid process: Muscle attachment.
Mandibular foramen: Entrance to the mandibular canal.

Mandible with labeled features

Infant Skull

The infant skull contains fontanelles, which are fibrous areas between cranial bones that allow for skull distortion during birth and brain growth.

Superior view of infant skull with fontanelles Lateral view of infant skull with fontanelles

Vertebral Column

Structure and Function

The vertebral column consists of 24 vertebrae, the sacrum, and the coccyx. It supports the head, neck, and trunk, protects the spinal cord, and transfers weight to the lower limbs.

Primary curves: Thoracic and sacral, present at birth.
Secondary curves: Cervical and lumbar, develop after birth.
Intervertebral discs: Pads of fibrocartilage between vertebrae for shock absorption.

Lateral view of vertebral column showing spinal curves and regions Vertebral column with labeled vertebrae

Cervical Vertebrae

There are seven cervical vertebrae (C1–C7). The atlas (C1) and axis (C2) are specialized for head movement.

Atlas (C1): Supports the skull; allows "yes" movement.
Axis (C2): Has the dens (odontoid process); allows "no" movement.
Vertebra prominens (C7): Has a long spinous process.

Atlas and axis articulation with labeled features Typical cervical vertebra superior view Typical cervical vertebra lateral view

Thoracic Vertebrae

Twelve thoracic vertebrae (T1–T12) articulate with the ribs via costal facets.

T1–T9: Superior and inferior costal facets.
T10–T12: Single costal facet.
T1–T10: Transverse costal facets for rib articulation.

Thoracic vertebra superior view Thoracic vertebra lateral view

Lumbar Vertebrae

Five lumbar vertebrae (L1–L5) are large, lack costal facets, and have spatulated spinous processes and triangular vertebral foramina.

Lumbar vertebra lateral view Lumbar vertebra superior view

Sacrum

The sacrum consists of five fused vertebrae and protects pelvic organs. It provides passage for nerves and extensive muscle attachment.

Sacral canal: Passage for sacral nerves.
Sacral foramina: Exit points for nerves.
Auricular surface: Articulates with pelvic girdle.
Sacral promontory: Anterior bulge at the base.

Sacrum posterior view with labeled features Sacrum anterior view with labeled features Sacrum lateral view with labeled features

Coccyx

The coccyx (tailbone) forms from the fusion of 3–5 coccygeal vertebrae and serves as an attachment for ligaments and muscles of the pelvic floor.

Coccyx with labeled features

Thoracic Cage

Ribs

The ribs are elongated, curved bones that articulate posteriorly with thoracic vertebrae and protect thoracic organs.

True ribs (1–7): Attach directly to the sternum via costal cartilage.
False ribs (8–12): Do not attach directly to the sternum.
Vertebrochondral ribs (8–10): Attach to cartilage of rib 7.
Floating ribs (11–12): No anterior attachment.

Thoracic cage with labeled ribs and sternum

Rib and Vertebrae Articulations

Articulation of rib with thoracic vertebra

Sternum

The sternum (breastbone) forms the anterior midline of the thoracic wall and consists of three parts:

Manubrium: Articulates with clavicles and first ribs.
Body: Attaches to manubrium, xiphoid process, and costal cartilages.
Xiphoid process: Last to ossify; attaches to diaphragm and rectus abdominis.

Sternum with labeled features

Hyoid Bone

The hyoid bone supports the larynx and serves as an attachment for muscles of the tongue, pharynx, and larynx. It does not articulate with any other bone.

Greater horns: Muscle attachment for tongue movement.
Lesser horns: Attach to stylohyoid ligaments.

Hyoid bone with labeled features

References

Clemente, Carmine D. Anatomy: A Regional Atlas of the Human Body. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins Health, 2011.
Martini, Frederic, Nath, Judi, and Bartholomew. Fundamentals of Anatomy and Physiology. Boston, MA: Benjamin Cummings, 2012.
Mescher, Anthony L., and Luiz Carlos Uchôa Junqueira. Junqueira's Basic Histology: Text and Atlas.