A simple sac or cavity with smooth walls and containing a clear, slightly sticky fluid interposed between two moving surfaces of the body to reduce friction. Subcutaneous bursae are found where the skin stretches around the greater curvature of a joint, as in the elbow or knee, and considerable chafing may occur; they may be single or multiple sacs. These bursae may enlarge as a result of continuous excessive irritation, as in housemaid's knee or miner's elbow.