Major Bones of the Human Skeleton

Axial Skeleton

Skull

• Bony framework encasing the brain; composed of 22 individual bones (8 cranial + 14 facial).

• Protects the brain, houses sensory organs (eyes, ears, nose, tongue), and provides attachment points for facial muscles.

• Cranial vault distributes impact forces, reducing risk of brain injury.

• Openings (foramina) allow passage of nerves and blood vessels—e.g.

  • $\text{Foramen magnum}$ for spinal cord.

  • $\text{Optic canal}$ for optic nerve (CN II).

• Ethically relevant in neurosurgery and forensic identification.

Rib Cage

• 12 pairs of ribs + sternum form thoracic cage.

• Primary roles:

  • Protects heart and lungs.

  • Supports respiratory mechanics—expansion/contraction change intrathoracic volume.

• Rib classification:

  • True ribs (1–7): attach directly to sternum via costal cartilage.

  • False ribs (8–10): attach indirectly.

  • Floating ribs (11–12): no anterior attachment.

• Clinical note: Rib fractures can compromise ventilation or puncture pleura, causing pneumothorax.

Sternum

• Flat bone at anterior midline of thorax; parts: manubrium, body, xiphoid process.

• Landmark for CPR—compressions performed over lower half of body segment.

• Ossifies fully in adulthood; xiphoid often cartilaginous in youth.

Spine (Vertebral Column)

• 33 vertebrae in early life (24 remain distinct in adults: 7 cervical, 12 thoracic, 5 lumbar; plus fused sacrum & coccyx).

• Functions:

  • Protect spinal cord.

  • Provide axial support and flexibility.

  • Acts as attachment for ribs and back muscles.

• Curvatures distribute mechanical stress:

  • Cervical and lumbar lordoses (concave posteriorly).

  • Thoracic and sacral kyphoses (concave anteriorly).

Pectoral Girdle & Upper Limb

Clavicle

• S-shaped long bone, collarbone; braces scapula, transferring weight of limb to axial skeleton.

• Only long bone ossified intramembranously; first to start ossifying, last epiphysis to fuse (>25 yr).

• Common fracture site at middle third due to fall on out-stretched hand (FOOSH).

Humerus

• Single long bone of upper arm; articulates proximally with scapula (glenoid fossa) and distally with radius & ulna (elbow).

• Landmarks: greater/lesser tubercles (muscle attachment), surgical neck (fracture risk, axillary nerve injury), olecranon fossa.

Radius

• Lateral forearm bone (thumb side) in anatomical position.

• Head articulates with capitulum of humerus; distal end forms wrist joint with carpal bones.

• Allows pronation/supination by crossing over ulna.

Ulna

• Medial forearm bone; larger proximal end forms olecranon (elbow tip).

• Stable axis for forearm rotation; trochlear notch articulates with humerus.

Pelvic Girdle & Lower Limb

Pelvis (Os Coxae + Sacrum + Coccyx)

• Two hip bones (ilium, ischium, pubis) fuse during adolescence.

• Functions: bear body weight, protect pelvic organs, provide attachment for lower limbs.

• Pelvic inlet dimensions critical in obstetrics—$\text{Conjugate diameter} \approx 11\,\text{cm}$ required for vaginal delivery.

Femur

• Longest, heaviest, strongest bone; $\approx 1/4$ of height.

• Proximal head articulates with acetabulum; distal condyles with tibia.

• Angle of inclination (125°) optimizes bipedal gait; deviation → coxa vara/valga.

• Fracture of neck common in elderly (osteoporosis) → risk of avascular necrosis of head (damaged medial circumflex femoral artery).

Patella

• Sesamoid bone embedded in quadriceps tendon; increases leverage of knee extension.

• Protects anterior knee joint; guides tendon.

• Dislocation more common laterally due to Q-angle; prevented by vastus medialis obliquus muscle tone.

Tibia

• Medial, weight-bearing bone of leg; second largest bone.

• Proximal plateau forms knee joint; distal medial malleolus forms part of ankle mortise.

• Frequent site of compound fractures due to subcutaneous placement along anterior border.

Fibula

• Lateral, slender bone; bears minimal weight.

• Provides lateral ankle stability (lateral malleolus) and muscle attachment.

• Serves as graft donor site (vascularized fibular graft) without major locomotor deficit.

Numerical & Statistical Context

• Adult human skeleton: $206$ named bones.

  • Axial skeleton: $80$ bones.

  • Appendicular skeleton: $126$ bones.

• Long bones typically ossify from $1$ primary + $2$ secondary centers (rule of thumb; clavicle is exception).

Real-World & Clinical Connections

• Orthopedics, physical therapy, sports medicine rely on understanding these bones for diagnosis and rehabilitation.

• Forensic anthropology determines age, sex, stature from skeletal remains (e.g., femur length → height approximation: $\text{Height(cm)} = 2.32\times\text{Femur length(cm)} + 65$).

• Ethical considerations in bone donation, cadaver use, and surgical interventions.

Integrative Remarks

• Bones interact with muscular, nervous, and vascular systems to enable movement, protect organs, and maintain mineral homeostasis (Ca$^{2+}$, PO$_4^{3-}$).

• Developmentally, endochondral vs intramembranous ossification underpin evolutionary adaptations such as bipedalism (pelvis, femur angles).

• Injuries to these listed bones constitute majority of trauma admissions—emphasizing foundational relevance for allied health curricula.