1. Long Bones

Each long bone has:

• An elongated shaft → diaphysis.
• Two expanded ends → epiphyses.
• Epiphyses are smooth and articular.
• Shaft typically has:
  • 3 surfaces separated by 3 borders.
  • Central medullary cavity.
  • Nutrient foramen directed away from the growing end.

Types of Long Bones

2. Short Bones

• Shape is usually:
• Cuboid → like a cube.
• Scaphoid → boat-shaped.
• Examples:
• Tarsal bones.
• Carpal bones.
• They are pierced by blood vessels.

3. Flat Bones

• Resemble shallow plates.
• Form boundaries of certain body cavities.
• Examples:
• Bones in vault of skull.
• Sternum.
• Ribs.
• Scapula.

4. Irregular Bones

Examples:

• Hip bone.
• Bones in base of skull, e.g.:
• Sphenoid.
• First cervical vertebra.
• Second cervical vertebra.

5. Pneumatic Bones

• Certain irregular bones contain large air spaces lined by epithelium.
• Examples:
• Maxilla.
• Sphenoid.
• Ethmoid.
• etc.

Functions

• Make the skull light in weight.
• Help in resonance of voice.
• Act as air-conditioning chambers for inspired air.
• Improve timbre (quality) of the voice.

6. Sesamoid Bones

• These are bony nodules embedded in tendons or joint capsules.
• They have no periosteum and ossify after birth.
• They are related to an articular or nonarticular bony surface.
• Surfaces of contact are covered with hyaline cartilage and lubricated by a bursa or synovial membrane.

Examples

• Patella → in the tendon of quadriceps femoris.
• Pisiform → in the tendon of flexor carpi ulnaris.
• Fabella → in the tendon of lateral head of gastrocnemius.
• Bone developed in the tendon of adductor longus in professional riders.

Functions of Sesamoid Bones

7. Accessory (Supernumerary) Bones

• Not always present.
• May occur as ununited epiphyses developed from extra centres of ossification.

Examples

• Sutural bones of skull.
• Cervical ribs.
• Lumbar ribs.

Sutural / Wormian Bone

• Mostly seen in the region of lambda.
• Can also be present at asterion and pterion.
• Such bones are common in hydrocephalic skulls.

1. Membrane (Dermal) Bones

• Ossify in membrane:
• Intramembranous or mesenchymal ossification.
• Thus, they are derived from mesenchymal condensations.

Examples

• Bones of vault of skull → frontal, parietal.
• Facial bones → maxilla.

Cartilaginous Bones

• Ossify in cartilage:
• Intracartilaginous or endochondral ossification.
• Thus, they are derived by replacement of preformed cartilaginous models.

Examples

• Bones of limbs → humerus, femur.
• Vertebral column.
• Thoracic cage.

Membrano-cartilaginous Bones

• Ossify partly in membrane and partly in cartilage.

Examples

• Clavicle:
  • Sternal end → endochondral ossification.
  • Rest of bone → intramembranous ossification.
• Mandible.
• Occipital.
• Temporal.
• Sphenoid.

2. Somatic and Visceral Bones

1. Compact Bone

• Dense in texture like ivory, but extremely porous.
• Best developed in the cortex of long bones.
• Adaptation to:
• Bending forces.
• Twisting forces.
• These forces are a combination of:
• Compression.
• Tension.
• Shear.

2. Cancellous / Spongy / Trabecular Bone

• Open in texture.
• Made up of a meshwork of trabeculae (rods and plates).
• Between trabeculae are marrow-containing spaces.

Types of Trabecular Meshwork

1. Meshwork of rods.
2. Meshwork of rods and plates.
3. Meshwork of plates.

• Cancellous bone is an adaptation to compressive forces.
• Bones are constructed to combine:
• Strength.
• Elasticity.
• Lightness in weight.
• Architecture of bone may be modified by mechanical forces.
• Form of bone is primarily determined by heredity.

Comparison of Compact and Cancellous Bone

Lamellae and Calcar Femorale

• Compression lamellae from upper part of head resist compression force.
• Tension lamellae from lower part of the head resist bending forces in the neck.
• Calcar femorale resists shearing stresses between the neck and shaft.
• Compression lamellae resist shearing stresses due to pull of muscles attached to the greater trochanter.

1. Lamellar Bone

• Most mature human bones, whether compact or cancellous, are composed of thin layers of bone tissue called lamellae.
• In cancellous bone, lamellae are arranged as branching curved plates.
• In compact bone, they are arranged concentrically as:
• Haversian system.
• Secondary osteon.

2. Woven Bone

Seen in:

• Fetal bone.
• Fracture repair.
• Cancer of bone.
• Collagen fibres and bone crystals are arranged randomly.

3. Fibrous Bone

• Found in young foetal bones.
• Common in:
• Reptiles.
• Amphibia.

1. Young Long Bones

Arterial supply of a long bone is derived from the following sources:

1. Nutrient artery.
2. Periosteal arteries.
3. Epiphysial arteries.
4. Metaphysial arteries.

a. Nutrient Artery

• Enters the shaft through the nutrient foramen.
• Runs through the cortex.
• Divides into:
• Ascending branch.
• Descending branch.
• These branches turn down to form hairpin bends.
• Each branch divides into a number of small parallel channels.
• These terminate in the adult metaphysis by anastomosing with:
• Epiphysial arteries.
• Metaphysial arteries.
• Periosteal arteries.

Distribution

• Medullary cavity.
• Inner 2/3rd of cortex.
• Metaphysis.

Growing Ends of Bones

Direction of Nutrient Foramen

• Nutrient foramen is directed away from the growing end of the bone.

b. Periosteal Arteries

• Especially numerous beneath:
• Muscular attachments.
• Ligamentous attachments.
• Ramify beneath the periosteum.
• Enter the Volkmann’s canals.
• Supply the outer 1/3rd of the cortex.

c. Epiphysial Arteries

• Derived from periarticular vascular arcades (circulus vasculosus) found on the nonarticular bony surface.
• Out of numerous vascular foramina in this region:
• Only a few admit the arteries → epiphysial and metaphysial.
• Rest are venous exits.

d. Metaphysial Arteries

• These are neighbouring systemic vessels.
• Pass directly into the metaphysis.
• Reinforce the metaphysial branches from the primary nutrient artery.

2. Miniature Long Bones

• In miniature long bones, e.g. metacarpals:
• Infection begins in the middle of the shaft rather than at the metaphysis.
• The nutrient artery breaks up into a plexus immediately upon reaching the medullary cavity.
• In adults, chances of infection are minimized because the nutrient artery is mostly replaced by periosteal vessels.

3. Long Short Bones

• Nutrient artery enters the middle of the shaft.
• Divides to form a plexus.
• Periosteal artery supplies the major part of bone.
• Periosteal artery may replace the nutrient artery.

4. Short Bones

• Supplied by numerous periosteal vessels.
• These enter their nonarticular surfaces.

5. Vertebra

• Body is supplied by:
• Anterior vessels.
• Posterior vessels.
• Vertebral arch is supplied by large vessels entering the bases of transverse processes.
• Red marrow is drained by two large basivertebral veins.
• These foramina lie on the posterior aspect of the body of vertebra.

6. Rib

A rib is supplied by:

1. Nutrient artery → enters just beyond the tubercle.
2. Periosteal arteries.