7.1 Divisions of the Skeletal System

Functions and Divisions of the Skeletal System

The skeletal system is the body's structural framework. It provides support and protection, enables movement, and carries out vital metabolic functions like mineral storage and blood cell production. It's divided into two major divisions: the axial skeleton (your central axis) and the appendicular skeleton (your limbs and the girdles that attach them to the axis).

Functions of the Skeletal System

The skeleton does far more than hold you upright. There are four major functions you need to know:

• Support and movement — Bones provide the structural framework for the body and serve as attachment points for muscles, tendons, and ligaments. Without this scaffolding, muscles would have nothing to pull against.
• Protection — Bone surrounds and shields vital organs. The cranium protects the brain, the vertebral column protects the spinal cord, and the rib cage protects the heart and lungs.
• Mineral storage — Bone matrix stores calcium and phosphate. These minerals can be released into the bloodstream as needed to maintain calcium homeostasis, which is critical for nerve signaling, muscle contraction, and blood clotting.
• Hematopoiesis — Red bone marrow inside certain bones produces blood cells: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

Axial vs. Appendicular Skeleton

The 206 bones of the adult skeleton are split into two divisions based on location and primary function.

A helpful way to remember: axial shares a root with axis, meaning the central line of the body. Everything that hangs off that axis is appendicular.

Components of the Axial Skeleton

Skull — Divided into two groups of bones:

• Cranium (brain case): 8 bones that enclose and protect the brain — frontal, parietal (2), temporal (2), occipital, sphenoid, and ethmoid.
• Facial bones: 14 bones that form the structure of the face — maxillae (2), zygomatic (2), nasal (2), lacrimal (2), palatine (2), inferior nasal conchae (2), vomer (1), and the mandible (1). The mandible (lower jaw) is the only freely movable bone of the skull.

Vertebral column — 33 vertebrae arranged in five regions:

1. Cervical — 7 vertebrae (neck region)
2. Thoracic — 12 vertebrae (mid-back; each articulates with a pair of ribs)
3. Lumbar — 5 vertebrae (lower back; largest and strongest)
4. Sacral — 5 fused vertebrae forming the sacrum
5. Coccygeal — 4 fused vertebrae forming the coccyx (tailbone)

Intervertebral discs sit between adjacent vertebrae (from C2 down to the sacrum). These discs are made of fibrocartilage and act as shock absorbers while allowing slight movement between vertebrae.

Rib cage — 12 pairs of ribs, classified by how they attach to the sternum:

• True ribs (pairs 1–7): Attach directly to the sternum via their own costal cartilage.
• False ribs (pairs 8–10): Attach indirectly to the sternum through the costal cartilage of the rib above.
• Floating ribs (pairs 11–12): Have no anterior attachment to the sternum at all.

Sternum (breastbone) — Composed of three parts from superior to inferior: the manubrium, the body, and the xiphoid process. The manubrium articulates with the clavicles and the first pair of ribs.

Components of the Appendicular Skeleton

Pectoral (shoulder) girdle — Connects the upper limbs to the axial skeleton. It consists of the clavicle (collarbone) and the scapula (shoulder blade) on each side.

Upper limbs — Moving from proximal to distal:

1. Humerus (upper arm)
2. Radius and ulna (forearm)
3. Carpals (8 wrist bones)
4. Metacarpals (5 hand bones)
5. Phalanges (14 finger bones)

Pelvic girdle — Connects the lower limbs to the axial skeleton. Each hip bone (coxal bone) is formed by the fusion of three bones: the ilium, ischium, and pubis. The sacrum and coccyx are technically axial bones, but they articulate with the pelvic girdle and function as part of the pelvis.

Lower limbs — Moving from proximal to distal:

1. Femur (thigh bone; longest and strongest bone in the body)
2. Patella (kneecap; a sesamoid bone embedded in the quadriceps tendon)
3. Tibia and fibula (leg bones)
4. Tarsals (7 ankle bones)
5. Metatarsals (5 foot bones)
6. Phalanges (14 toe bones)

Skeletal System Components and Development

Beyond the bones themselves, several other tissues and structures make up the skeletal system:

• Bone tissue (osseous tissue) — The hard connective tissue that provides structure and strength. It contains an organic matrix (collagen for flexibility) and inorganic mineral salts (calcium phosphate for hardness).
• Joints (articulations) — Where two or more bones meet. Joints vary widely in how much movement they allow, from immovable (skull sutures) to freely movable (shoulder joint).
• Ligaments — Dense bands of connective tissue that bind bone to bone at joints, providing stability.
• Cartilage — Flexible connective tissue found at joint surfaces, in the rib cage, between vertebrae, and in other areas. It cushions bones and reduces friction.
• Ossification — The process by which new bone tissue forms. During development, most bones form through endochondral ossification (replacing a cartilage model with bone), while some flat bones like those of the skull form through intramembranous ossification (bone develops directly from mesenchymal tissue without a cartilage stage).