Synovial fluid is a thick sticky fluid secreted by synovial membranes into the synovial cavity. Though the presence of synovial fluid is one of the reasons behind the mobility of the joints, but the most accurate reason is the arrangement of the bones at the joint. The spheroidal ball-like end of one bone articulates with the cup-shaped depression in another which allows the bone with the ball head to be moved freely in many planes. Shoulder joints and hip joints are the ball-and-socket joints.

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