Axial Skeleton: Skull, Vertebral Column, and Thoracic Cage

Complete anatomy of the axial skeleton - cranial and facial bones, vertebrae, sacrum, coccyx, ribs, and sternum. Includes foramina, curvatures, and landmark features.

Skeletal System 4 min read
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The axial skeleton forms the central axis of the body and includes the skull, vertebral column, and thoracic cage. It protects the brain, spinal cord, heart, and lungs.

Skull (Cranium)

The skull consists of 22 bones: 8 cranial bones and 14 facial bones (not including the ear ossicles).

Cranial Bones (Neurocranium)

Bone Count Key Features
Frontal 1 Frontal sinuses, supraorbital foramen/notch, glabella
Parietal 2 Sagittal suture (midline), coronal suture (anterior)
Temporal 2 External auditory meatus, mastoid process, styloid process, petrous part
Occipital 1 Foramen magnum, occipital condyles, external occipital protuberance
Sphenoid 1 Sella turcica (pituitary fossa), greater/lesser wings, pterygoid processes
Ethmoid 1 Cribriform plate, perpendicular plate, superior/middle conchae

Sutures

Suture Between Closure Age
Coronal Frontal + parietal ~30 years
Sagittal Parietal + parietal ~30 years
Lambdoid Parietal + occipital ~30 years
Squamous Parietal + temporal Variable

Cranial Fossae

Fossa Bones Contents
Anterior cranial fossa Frontal, ethmoid, sphenoid (lesser wing) Frontal lobes, olfactory bulbs
Middle cranial fossa Sphenoid, temporal Temporal lobes, pituitary gland, cavernous sinuses
Posterior cranial fossa Temporal (petrous), occipital Brainstem, cerebellum, fourth ventricle

Facial Bones (Viscerocranium)

Bone Count Key Features
Nasal 2 Bridge of nose
Maxilla 2 Alveolar processes (upper teeth), maxillary sinuses, palatine processes
Zygomatic 2 Cheek bones, zygomatic arch (with temporal)
Mandible 1 Body, ramus, angle, coronoid process, condylar process, alveolar processes (lower teeth)
Palatine 2 Hard palate (posterior), orbital floor
Lacrimal 2 Lacrimal groove (tear drainage)
Inferior nasal concha 2 Turbinate in nasal cavity
Vomer 1 Nasal septum (posterior)

Skull Foramina and Contents

Foramen Bone Contents
Cribriform plate Ethmoid CN I (olfactory)
Optic canal Sphenoid (lesser wing) CN II (optic), ophthalmic artery
Superior orbital fissure Sphenoid CN III, IV, V1, VI
Foramen rotundum Sphenoid CN V2 (maxillary)
Foramen ovale Sphenoid CN V3 (mandibular)
Foramen spinosum Sphenoid Middle meningeal artery
Internal acoustic meatus Temporal (petrous) CN VII, VIII
Jugular foramen Temporal + occipital CN IX, X, XI, internal jugular vein
Hypoglossal canal Occipital CN XII
Foramen magnum Occipital Medulla, vertebral arteries, CN XI
Stylomastoid foramen Temporal CN VII (facial)
Carotid canal Temporal Internal carotid artery

Paranasal Sinuses

Sinus Location Drainage
Frontal Frontal bone Middle meatus
Maxillary Maxilla Middle meatus
Ethmoid (anterior) Ethmoid bone Middle meatus
Ethmoid (posterior) Ethmoid bone Superior meatus
Sphenoid Sphenoid bone Sphenoethmoidal recess

Vertebral Column

General Structure

Segment Vertebrae Region Special Features
Cervical 7 (C1-C7) Neck Transverse foramina, bifid spinous processes (C2-C6)
Thoracic 12 (T1-T12) Chest Costal facets (for ribs), long spinous processes
Lumbar 5 (L1-L5) Lower back Large bodies, mammillary processes
Sacrum 5 (fused) Sacral Sacral foramina, sacral canal
Coccyx 4 (fused) Tailbone Small triangular bone

Curvatures

Curvature Direction Region
Cervical Lordosis (anterior) Neck
Thoracic Kyphosis (posterior) Upper back
Lumbar Lordosis (anterior) Lower back
Sacral Kyphosis (posterior) Sacrum

Typical Vertebra

Feature Description
Body (centrum) Anterior weight-bearing portion
Vertebral arch Posterior, formed by pedicles + laminae
Vertebral foramen Opening for spinal cord
Transverse processes Lateral projections (2)
Spinous process Posterior projection (1)
Superior/inferior articular processes Facet joints with adjacent vertebrae
Intervertebral foramina Between vertebrae (nerve root exit)

Cervical Vertebrae (C1-C7)

Vertebra Unique Features
Atlas (C1) No body, ring-shaped, articulates with occipital condyles (yes/no movement)
Axis (C2) Odontoid process (dens) for C1 rotation (no/yes movement)
C3-C6 Typical cervical: small body, transverse foramina, bifid spinous processes
C7 Vertebra prominens: long spinous process (palpable)

Thoracic Vertebrae (T1-T12)

  • Heart-shaped body
  • Costal facets for rib articulation (on body and transverse processes)
  • Long, downward-pointing spinous processes
  • Narrow vertebral canal

Lumbar Vertebrae (L1-L5)

  • Large, kidney-shaped body
  • Short, thick pedicles
  • Flat, rectangular spinous processes
  • Large vertebral canal

Sacrum

  • 5 fused vertebrae
  • Sacral promontory (anterior ridge)
  • Median sacral crest (fused spinous processes)
  • Sacral foramina (4 pairs, anterior and posterior)
  • Sacral hiatus (entrance to sacral canal)

Thoracic Cage

Sternum

Part Description
Manubrium Upper portion, articulates with clavicles and rib 1
Body (gladiolus) Middle portion, articulates with ribs 2-7
Xiphoid process Inferior tip (cartilaginous, ossifies with age)

Joints:

  • Sternal angle (angle of Louis): Manubriosternal joint (T4 level)
  • Xiphisternal joint (T9 level)

Ribs

Type Count Description
True ribs 7 (1-7) Attach directly to sternum via costal cartilage
False ribs 3 (8-10) Attach indirectly to sternum (via costal cartilage of rib 7)
Floating ribs 2 (11-12) No anterior attachment

Typical rib structure:

  • Head: Articulates with vertebral body (two facets)
  • Neck: Between head and tubercle
  • Tubercle: Articulates with transverse process
  • Shaft: Body of rib
  • Costal groove: Inferior margin (intercostal vessels, nerve)

Intercostal Spaces

Layer Muscle Direction Innervation
External intercostal Oblique (superomedial to inferolateral) Anterior Intercostal nerves
Internal intercostal Oblique (inferomedial to superolateral) Posterior Intercostal nerves
Innermost intercostal Deep layer Variable Intercostal nerves

Contents (superior to inferior): Intercostal vein, artery, nerve (VAN)