Structure of a Long Bone

![Diagram showing a detailed cross-section of a long bone, highlighting periosteum, compact bone, spongy bone, yellow marrow, red marrow, blood vessels, and epiphyseal plate/line]

Key Components of a Long Bone

1. Diaphysis (Shaft)

• The long, cylindrical main body of the bone
• Made primarily of compact (cortical) bone
• Contains the medullary cavity filled with yellow marrow
• Surrounded by periosteum (outer membrane)

2. Epiphyses (Ends)

• The expanded ends of the long bone
• Covered with articular cartilage
• Contains red marrow within spongy bone
• Usually where joints form with other bones

Remember that epiphyses is plural (multiple ends) while epiphysis is singular (one end)!

3. Bone Structure Layers

Outer Layer (Periosteum)

• Double-layered membrane covering the bone
• Contains blood vessels and nerves
• Essential for bone growth and repair
• Anchors tendons and ligaments

Compact Bone

• Dense, solid bone tissue
• Organized in osteons (Haversian systems)
• Provides strength and protection
• Contains small channels for blood vessels and nerves

Spongy Bone (Trabecular)

• Lightweight, honeycomb structure
• Found mainly in epiphyses
• Contains red marrow between trabeculae
• Helps absorb shock and distribute forces

The arrangement of trabeculae in spongy bone follows lines of stress, making the bone stronger where it needs to be!

4. Bone Cavities

Medullary Cavity

• Runs through the center of the diaphysis
• Contains yellow marrow (mainly fat)
• Surrounded by endosteum (inner membrane)
• Helps reduce overall bone weight while maintaining strength

Marrow Spaces

• Red marrow in epiphyses (blood cell production)
• Yellow marrow in diaphysis (energy storage)

Don't confuse the medullary cavity with marrow spaces in spongy bone - they serve different purposes!

5. Growth Areas

Epiphyseal Plate/Line

• Growth plate in children and adolescents
• Becomes epiphyseal line in adults
• Located between epiphysis and diaphysis
• Site of bone lengthening during growth

When looking at a bone cross-section, imagine it like a tree trunk with rings - each layer has its own specific function and purpose.

Think of a long bone like a chicken drumstick:

• The hard outer shell is compact bone

• The spongy ends are the epiphyses

• The hollow center contained marrow

• The joint end had smooth cartilage