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How Do Joints Work?


The junction between two or more bones is termed a joint or articulation.

Joints are classified as "movable" and "immovable." The latter occur where the bones are completely interlocked or fixed as in the skull and face; a layer of cartilage or fibrous tissue intervenes between the bones and firmly binds them together. The former occur where very free movement is necessary and their structure is more complex and varied.

A typical movable joint is made up of the ends of two or more bones capped with articular cartilage in close apposition, enwrapped by a thin layer of connective tissue called the " synovral membrane."

A natural joint lubricant, the synovial fluid, is produced by this membrane.

The joint is also enclosed in a sheath of fibrous tissue, the capsule, which is strengthened and reinforced by ligaments-bands of tissue which stretch from bone to bone-and by the tendons of various muscles which act around the joint.

Several varieties of movable joints are described according to the nature of the movement they allow. The ball-and-socket joint gives a very wide range of movement in any direction and is found in the shoulder and hip. A to and fro movement taking place round one axis is found in the hinge joint exemplified in the elbow and knee. Where the surfaces of the opposing bones merely glide over each other and there is very limited movement, as in the wrist and ankle, a gliding joint is described.

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