bursa (bûrˈsə) [key], closed fibrous sac lined with a smooth membrane, producing a viscous lubricant known as synovial fluid. Bursas are found in regions where muscles or tendons rub against other muscles, tendons, or bones. The bursas function in two ways, lubricating points of friction, and dissipating force by distributing it through a fluid medium. Normally, the bursas produce just enough synovial fluid to reduce friction. However, constant irritation may lead to oversecretion and consequent enlargement of the bursa, a condition known as bursitis. In the hand and foot, the bursa assumes a tubular form. Called the synovial sheath, the structure encloses the tendons along their entire length.