Classification and Structure of Bones

Classification of Bones

• Bones are classified into four groups based on shape: long, flat, short, and irregular.
• Long bones: Longer than they are wide, mostly compact bone with spongy bone at the ends, examples include femur and humerus.
• Flat bones: Thin, flattened, and curved, two thin layers of compact bone sandwiching a layer of spongy bone, examples include most skull bones, ribs, and sternum.
• Short bones: Generally cube-shaped, mostly spongy bone with an outer layer of compact bone, examples include carpals and tarsals.
• Irregular bones: Do not fit into other bone classification categories, made up of unique shapes, examples include vertebrae and hip bones.

' Structure of a Long Bone

• Diaphysis: The shaft of the bone, made of compact bone.
• Periosteum: Fibrous connective tissue membrane covering the diaphysis, secures to the underlying bone using perforating (Sharpey’s) fibers.
• Epiphyses: The ends of the bone, composed of spongy bone enclosed by a thin layer of compact bone.
• Articular cartilage: Covers the external surfaces of the epiphyses, made of hyali ne cartilage, reduces friction at joint surfaces.
• Epiphyseal Plate: Flat plate of hyaline cartilage found in young, growing bone, responsible for lengthwise growth of the bone. Epiphyseal line is the remnant of this plate in adults.
• Endosteum: Connective tissue lining the inner surface of the shaft of the bone.
• Medullary cavity: Space inside the shaft, contains yellow marrow (mostly fat) in adults. Contains red marrow for blood cell formation until age 6 or 7.

Bone Markings

• Sites of attachment for muscles, tendons, and ligaments.
• Passages for nerves and blood vessels.
• Projections or processes: Grow out from the bone surface (all projections start with "T").
• Depressions or cavities: Indentations in the bone (All depressions start with "F" except facet).

Vertebral Column (Spine)

• Axial support extending from skull to pelvis.
• Composed of 26 vertebral bones separated by intervertebral discs.
• 7 cervical vertebrae: Neck region.
• 12 thoracic vertebrae: Chest region.
• 5 lumbar vertebrae: Lower back region.
• Sacrum: Fusion of 5 vertebrae.
• Coccyx: Fusion of 3-5 vertebrae.

Common Features of Vertebrae

• Body (centrum): The main part of the vertebra.
• Vertebral arch: Forms the posterior opening for the spinal cord.
  • Pedicle: Connects the body to the lamina.
  • Lamina: Forms the posterior part of the arch.
• Vertebral foramen: Opening for the spinal cord.
• Transverse processes: Project laterally from the arch.
• Spinous process: Projects posteriorly from the arch.
• Superior and inferior articular processes: Articulate with adjacent vertebrae.

Thoracic Cage

• Bony thorax, or thoracic cage, protects organs of the thoracic cavity.
• Consists of:
  • Sternum: Breastbone.
  • Ribs:
    • True ribs (pairs 1-7): Directly attach to the sternum.
    • False ribs (pairs 8-12): Attach indirectly to the sternum through cartilage.
    • Floating ribs (pairs 11-12): Do not connect to the sternum.
  • Thoracic vertebrae: The 12 vertebrae of the chest region.

Appendicular Skeleton

• Composed of 126 bones, including:
  • Limbs (appendages): Arms and legs.
  • Pectoral girdle: Attaches the upper limbs to the axial skeleton (clavicle and scapula).
  • Pelvic girdle: Attaches the lower limbs to the axial skeleton (hip bones).

Bones of the Shoulder Girdle (Pectoral Girdle)

• Composed of two bones:
  • Clavicle: Collarbone.
  • Scapula: Shoulder blade.
• Light and poorly reinforced girdle, allowing for exceptional upper limb flexibility. L

Bones of the Upper Limbs

• Humerus: Upper arm bone.
  • Proximal end articulates with the scapula at the glenoid cavity.
  • Distal end articulates with the forearm bones (ulna and radius) at the trochlea and capitulum.
• Ulna: Medial bone of the forearm in anatomical position.
  • Proximal end articulates with the humerus.
• Radius: Lateral bone of the forearm in anatomical position.
  • Proximal end articulates with the humerus.