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by 1788 as an English word in physiology, shortened from medieval Latin bursa mucosa "mucus pouch," from Medieval Latin bursa "bag, purse," from Late Latin bursa, variant of byrsa "hide," from Greek byrsa "hide, skin, wineskin, drum," of unknown origin; cf. purse (n.).
bursa bur·sa (bûr'sə)
n. pl. bur·sas or bur·sae (-sē)
A sac or saclike bodily cavity, especially one containing a viscous lubricating fluid and located between a tendon and a bone or at points of friction between moving structures.
within the mammalian body, any small pouch or sac between tendons, muscles, or skin and bony prominences at points of friction or stress. The bursas are classified by type as adventitious, subcutaneous, or synovial. Adventitious, or accidental, bursas arise in soft tissues as a result of repeated subjections to unusual shearing stresses, particularly over bony prominences. Subcutaneous bursas ordinarily are ill-defined clefts at the junction of subcutaneous tissue and deep fasciae (sheets of fibrous tissue); these bursas acquire a distinct wall only when they become abnormal, and they are sometimes classified as adventitious. Synovial bursas are thin-walled sacs that are interposed between tissues such as tendons, muscles, and bones and are lined with synovial membrane. In humans a majority of synovial bursas are located near the large joints of the arms and legs.