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The main function of the skeletal system is to provide a solid framework for muscles and to act as support and protection for internal organs. Bones form the basis of the skeleton, but there are other parts of the skeletal system that enable freedom of movement: joints, cartilage, ligaments and tendons. On their own, bones are fixed and can only move at joints. The other types of connective tissue interact with the muscular system and other bones of the skeletal system to provide a solid and flexible framework. Bones consist of a variable lattice of calcium to provide the most support possible, are considered hard connective tissue, and are the basic parts of the skeletal system.
Bones in the skeleton connect to one another at joints, joined by ligaments and cushioned by cartilage. Several types of joints exist, each allowing a different type of movement. Hinge joints, such as the knee and elbow, allow the joint to move along one axis. Ball-and socket joints, such as the shoulder or hip, allow full rotation. Gliding and pivot joints such as in the neck and wrists, meanwhile, allow rotation around a fixed position. Joints and cartilage are coated in synovial fluid, which adds a lubricating effect to the cartilage.
Cartilage is one of the parts of the skeletal system that cushions the ends of bones against grinding due to friction. The sections between joints are where cartilage exists most prominently, but the ear, nose and bronchial tubes also contain it. This substance is more flexible than bone but not as much as muscle; its elasticity cushions the bones of joints against sudden shock. Another of the defining characteristics of cartilage is its lack of blood vessels, which causes it to heal from injury slowly compared to other parts of the skeletal system.
Ligaments are tough, elastic bands of tissue that attach to the ends of bones at the joints. They keep joints stable so that joints cannot exceed their designed range of motion. Although ligaments are elastic, this property only persists when the ligament is below a certain length; extending the ligament past this length is called hyperextension and takes months to heal.
Tendons have a similar structure to ligaments. The main function of a tendon is to provide the pull necessary to move a bone; the tendon attaches to a muscle and the contraction of the muscle pulls on the tendon. In turn, the other end of the tendon moves the opposite member of a joint. In skeletal muscle pairs, tendons are located on opposite sides of the joint to produce muscle extension and contraction.
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