3 The Skeletal System

Roles of the skeleton

The skeletal system is the body's supporting framework. It includes bones, cartilage, joints, and ligaments. It gives the body shape and support, protects organs, works with muscles to produce movement, stores minerals, and houses marrow that produces blood cells.

Bone structure and renewal

Bone is living connective tissue. Its flexible collagen framework is hardened by mineral salts, chiefly calcium phosphate. Two forms of bone tissue work together: compact bone is dense and strong, forming the outer layer of most bones, while spongy bone is a lighter network of bony struts with spaces between them. Compact bone is organized into cylindrical units called osteons; the spaces in spongy bone often contain red bone marrow, where blood cells are made.

A typical long bone, such as the femur, has a central shaft called the diaphysis and expanded ends called epiphyses. The shaft surrounds a medullary cavity, which commonly contains yellow marrow in adults. A thin outer membrane, the periosteum, carries blood vessels and nerves and provides sites for tendon and ligament attachment. The inner surfaces are lined by the endosteum. At the ends of the bone, smooth articular cartilage covers the surfaces that meet at joints, helping reduce friction and absorb impact.

Bone is continually renewed. Osteoblasts build bone matrix, osteoclasts break it down, and osteocytes—mature bone cells embedded in the matrix—help maintain it. This remodeling repairs small areas of damage and adjusts bone to changing demands.

Bone formation and growth

Bone forms through two processes, together called ossification:

• Intramembranous ossification forms bone directly from embryonic connective tissue. It produces many flat bones of the skull and parts of the clavicles.

• Endochondral ossification replaces a cartilage model with bone. It forms most bones, including the long bones of the limbs.

In a growing long bone, cartilage at the epiphyseal plate is gradually replaced by bone, lengthening the bone. When growth ends, the plate becomes an epiphyseal line. Bones can continue to remodel after they stop growing in length.

The skeleton's divisions

The adult skeleton is commonly described as having 206 bones and is divided into the axial and appendicular skeletons.

The axial skeleton forms the body's central axis and contains 80 bones. It includes the skull, auditory ossicles, hyoid bone, vertebral column, ribs, and sternum. These bones support the head and trunk and protect the brain, spinal cord, heart, and lungs. For example, the vertebrae surround the spinal cord, while the ribs and sternum form a protective cage around organs in the chest.

The appendicular skeleton contains 126 bones: the bones of the upper and lower limbs and the girdles that attach them to the axial skeleton. The pectoral girdle, made up of the clavicles and scapulae, connects the arms to the trunk. The pelvic girdle connects the legs to the trunk and provides a strong base for bearing body weight. Limb bones and their joints support actions ranging from grasping a cup to walking and running.

Joint structure and movement

A joint, or articulation, is where bones meet or where bone meets cartilage. Joints can be classified by their structure or by how much movement they allow.

Structurally, joints are fibrous, cartilaginous, or synovial. Fibrous joints connect bones with dense connective tissue; skull sutures are an example. Cartilaginous joints connect bones with cartilage; the joints between vertebral bodies are an example. Synovial joints have a fluid-filled cavity between the articulating bones and are generally the most mobile. Their joint surfaces are covered with articular cartilage, a surrounding capsule encloses the joint, and synovial fluid lubricates the moving surfaces.

Functionally, joints are described as synarthroses (immobile or nearly immobile), amphiarthroses (slightly movable), or diarthroses (freely movable). All synovial joints are diarthroses. Common synovial joint shapes help determine possible movements: the elbow is a hinge joint, the joint between the atlas and axis vertebrae allows rotation, and the shoulder and hip are ball-and-socket joints that permit movement in several directions.

Ligaments connect bones and help stabilize joints. Tendons connect muscles to bones.

How skeletal functions work together

The skeleton's functions are interrelated:

• Support: Bones provide a framework that supports the body and gives it shape.

• Protection: The skull, vertebral column, rib cage, and pelvis shield delicate organs.

• Movement: Bones act as levers, joints provide sites of motion, and skeletal muscles pull on bones.

• Blood-cell production: Red bone marrow produces blood cells.

• Mineral storage: Bone stores minerals, especially calcium and phosphate, and can release them as the body requires.

• Energy storage: Yellow bone marrow stores fat.

Bone is living, mineralized connective tissue that is built and renewed throughout life. Bones form directly from connective tissue or replace a cartilage model. Joints connect bones and range from immobile to freely movable. Together, bones and joints support and protect the body, enable movement, store minerals and energy, and support blood-cell production.