bur·sa fa·bri·c'i·i. the bursa of Fabricius in poultry, a blind saclike structure located on the posterodorsal wall of the cloaca; it performs a thymuslike function. Synonym(s): bursa of Fabricius. Lymphoid tissue at the junction of the cloaca and the gut in birds, which gives rise to B lymphocytes.arge bursa on the summit of the calcaneus where the superficial digital flexor muscle tendon is partly inserted as it passes distally to the foot; in the horse there may be an additional small subcutaneous bursa over the tendon at this site; its inflammation causes 'capped hock'. cloacal bursa. bursa of Fabricius.