4. OSSEOUS SYSTEM

Introduction

The Osseous System, also known as the Skeletal System, is the framework that supports our body. It is made up of bones and cartilages, which together protect vital organs, give shape, and allow movement when muscles contract.

Despite their hardness, bones are living tissues — they have their own blood vessels, nerves, and the ability to grow, repair, and even remodel themselves in response to stress and strain.

Structure and Composition of Bone

A bone is made up of both organic and inorganic materials.

  • Organic part (one-third) – connective tissue (mainly collagen fibers), which gives flexibility and toughness.
  • Inorganic part (two-thirds) – calcium salts like calcium phosphate and calcium carbonate, which make it hard and strong.

In strength, bone can be compared to iron and steel, yet it remains light and resilient.

Major Functions of Bones

  1. Give shape, strength, and support to the body.
  2. Provide attachment sites for muscles, tendons, and ligaments.
  3. Act as levers for movement.
  4. Protect delicate organs — skull protects the brain, ribs protect the heart and lungs, and vertebral column protects the spinal cord.
  5. Bone marrow produces blood cells.
  6. Store 97% of body calcium and phosphorus.
  7. Contain immune cells (reticuloendothelial cells) for defense.
  8. Air-filled bones in the skull (paranasal sinuses) help in voice resonance and reduce skull weight.

Classification of Bones

A. According to Shape

  1. Long Bones: Long shaft with two ends (e.g., humerus, femur, tibia).
  2. Short Bones: Cube-shaped, found in wrists and ankles (e.g., carpals, tarsals).
  3. Flat Bones: Thin and protective (e.g., skull, ribs, sternum).
  4. Irregular Bones: Complex shapes (e.g., vertebrae, hip bone).
  5. Pneumatic Bones: Contain air spaces (e.g., maxilla, sphenoid).
  6. Sesamoid Bones: Small bones in tendons (e.g., patella).
  7. Accessory Bones: Extra bones due to additional ossification centers (e.g., sutural bones).
  8. Heterotopic Bones: Develop in soft tissues due to strain (e.g., rider’s bone).

B. Developmental Classification

  1. Membrane (Dermal) Bones: Form directly in membranes (e.g., skull bones).
  2. Cartilaginous Bones: Develop from cartilage (e.g., limb bones, vertebrae).
  3. Membrano-cartilaginous Bones: Partly from membrane, partly from cartilage (e.g., clavicle, mandible).
  4. Somatic Bones: Form body framework.
  5. Visceral Bones: Derived from pharyngeal arches (e.g., hyoid, ear ossicles).

C. Regional Classification

  • Axial Skeleton: Skull, vertebral column, and thoracic cage.
  • Appendicular Skeleton: Bones of limbs.

D. Structural Classification

I. Macroscopic Structure

  • Compact Bone: Dense and solid, found in outer shafts of long bones; strong against bending and twisting.
  • Cancellous (Spongy) Bone: Porous, found at bone ends, helps bear compressive forces.

According to Wolff’s Law, bone structure adapts to stress and strain — becoming thicker and stronger where force is greater.

II. Microscopic Structure

  1. Lamellar Bone: Mature bone with concentric rings (Haversian systems).
  2. Woven Bone: Found in fetal bones or fracture repair sites.
  3. Fibrous Bone: Found in early fetal stages and some lower animals.
  4. Dentine and Cement: Found in teeth.

Gross Structure of a Long Bone

1. Shaft (Diaphysis)

  • Periosteum: Outer covering rich in nerves and blood supply; helps in growth and repair.
  • Cortex: Hard compact bone giving strength.
  • Medullary Cavity: Central cavity filled with bone marrow (red in children, yellow in adults).

2. Bone Ends (Epiphyses)

  • Made of spongy bone covered by smooth hyaline cartilage for joint movement.

Parts of a Young Bone

  1. Epiphysis: Ends of bones formed from secondary centers.
    • Pressure Epiphysis: Bears weight (e.g., head of femur).
    • Traction Epiphysis: Site of tendon attachment (e.g., trochanters).
    • Atavistic Epiphysis: Remnant of ancestral bones (e.g., coracoid).
    • Aberrant Epiphysis: Occasionally present (e.g., head of 1st metacarpal).
  2. Diaphysis: The main shaft formed from a primary ossification center.
  3. Metaphysis: Zone between diaphysis and epiphysis; major site of bone growth.
  4. Epiphysial Plate: Growth plate allowing bone length increase until adulthood.

Blood Supply of Bones

1. Long Bones

  • Nutrient Artery: Supplies medullary cavity and inner cortex.
  • Periosteal Arteries: Supply outer cortex.
  • Epiphysial and Metaphysial Arteries: Supply bone ends.

Mnemonic for nutrient foramen direction:
“To the elbow I go, from the knee I flee”
(Shows the direction of nutrient arteries.)

2. Short and Irregular Bones

Supplied by numerous small periosteal and nutrient vessels.

Nerve Supply of Bones

  • Sympathetic nerves: Control blood flow (vasomotor).
  • Sensory nerves: Present in periosteum and articular ends — responsible for bone pain during injury.

Development and Ossification

Bone development starts from mesodermal tissue in two ways:

  1. Intramembranous (Mesenchymal) Ossification: Direct bone formation from connective tissue (e.g., skull).
  2. Endochondral (Cartilaginous) Ossification: Bone replaces cartilage (e.g., limb bones).
  • Primary centers appear before birth (around 8th week).
  • Secondary centers appear after birth, usually at puberty.
  • Bone growth continues until about 25 years, after which fusion is complete.

Law of Ossification:
Centers that appear first are the last to unite — the delayed end is the growing end.

Growth of Long Bones

  1. Lengthwise Growth: Due to cell multiplication in the epiphyseal plate.
  2. Thickness Growth: Occurs beneath the periosteum.
  3. Remodeling: Continuous process of bone deposition (by osteoblasts) and removal (by osteoclasts) to maintain shape and strength.

Detailed Notes:

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