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A bursa (plural bursae or bursas; Latin: Bursa synovialis) is a small fluid-filled sac located at the point where a muscle or tendon slides across bone. Bursae serve to reduce friction between the two moving surfaces. There are hundreds of them throughout the body. Bursa is Latin for purse, which is what a bursa resembles.
Infection or irritation of a bursa leads to bursitis (inflammation of a bursa). The general term for disease of bursae is "bursopathy". But basically, current medical studies have no specific knowledge of the entire bursae system. The medical profession can treat the system by reducing inflammation but they do not understand the cause.
Everybody tells us it's a fluid filled sac, but what material is the sac made of, and what material is the fluid made of?