Human Skeleton: Structure, Function, and Divisions

Posted on Mar 27, 2025 in Human Biology

The Human Skeleton: Structure, Function, and Divisions

The skeleton performs several crucial functions:

  1. Support and Shape: It supports and shapes the body.
  2. Protection: It protects vital organs.
  3. Movement: It allows movement through muscle attachments.

Bone Classification

Bones are classified based on their size and shape:

  • Long Bones: These bones, such as the humerus and femur, have a length that exceeds their width and thickness.
  • Flat Bones: Flat bones, like the shoulder blade and ribs, have length and width that dominate over their thickness. They protect organs such as the brain, heart, and lungs.
  • Short Bones: Short bones, such as the vertebrae, have similar width, length, and thickness.

The distribution of compact and spongy bone depends on the bone’s shape. Compact bone is dense and hard, while spongy bone is softer and more fragile.

Divisions of the Skeleton

  • Axial Skeleton: Includes the skull, rib cage, and spine.
  • Appendicular Skeleton: Includes the shoulder girdle, upper extremities, pelvic girdle, and lower extremities.

Head (Skull)

The skull consists of:

  • Unpaired Bones: Frontal, occipital, sphenoid, ethmoid
  • Paired Bones: Temporal and parietal

Face

The face consists of:

  • Paired Bones: Palate, zygomatic (cheek), nasal, lacrimal, maxilla
  • Unpaired Bone: Vomer
  • Mandible: Lower jaw

Rib Cage

The rib cage includes the sternum and ribs:

  • Ribs 1-7: True ribs
  • Ribs 8-10: False ribs
  • Ribs 11-12: Floating ribs

Spine (Vertebral Column)

The spine consists of vertebrae:

  • 7 Cervical vertebrae
  • 12 Thoracic vertebrae
  • 5 Lumbar vertebrae
  • 5 Sacral vertebrae
  • 4-5 Coccygeal vertebrae
Cervical Vertebrae

Features of cervical vertebrae:

  • Small, quadrilateral vertebral body
  • Bifid transverse process with a transverse foramen for the vertebral artery
  • Large, triangular vertebral foramen
  • Bifid spinous process
Thoracic Vertebrae

Features of thoracic vertebrae:

  • Cylindrical vertebral body with articular facets for the ribs

Appendicular Skeleton

The appendicular skeleton includes the upper and lower extremities.

Upper Extremity

  • Pectoral Girdle: Scapula and clavicle
  • Arm: Humerus
  • Forearm: Radius and ulna
  • Hand: Carpals, metacarpals, phalanges

Lower Extremity

  • Pelvic Girdle: Two innominate bones
  • Thigh: Femur
  • Leg: Tibia, fibula, patella
  • Foot: Tarsals (calcaneus, talus, cuboid, 3 cuneiforms), metatarsals, phalanges

Girdles

  • Clavicle: An S-shaped bone that articulates with the scapula to form the shoulder skeleton.
  • Scapula: A triangular flat bone that articulates with the clavicle through the spinous process.

Humerus

The humerus is the bone of the arm. It articulates with the bones of the forearm at the bottom.

Forearm

The forearm is formed by the ulna and radius, which are involved in flexion and extension. They articulate with the hand.

Joints

Joints are the connections between two bones.

  • Immobile Joints: Such as the skull bones, are joined by sutures (dentate, squamous, lace).
  • Semi-Mobile Joints: Involve vertebral bodies with a meniscus between them and ligaments for support.
  • Mobile Joints (Synovial Joints): Involve bone surfaces, articular cartilage, ligaments, and fibrocartilage (meniscus). Synovial fluid lubricates and allows movement.

Muscles

  • Wide Muscles: Low force and movement, located in the head and neck.
  • Short Muscles: Very strong with little movement, located in the spine.
  • Large Muscles: High force and motion, located in the upper and lower limbs.

Muscle Functions

  • Motion
  • Joint stability
  • Protection
  • Posture maintenance
  • Power generation

Muscle Properties

  • Contraction: Shortens fibers
  • Elasticity: Recovers shape after stretching
  • Excitability: Responds to stimuli

Muscles facilitate flexion and extension, enabling movement of different body segments and the spine.