The thoracic duct was dissected in 30 fresh cadavers, and the anatomy and histology of the duct, at various levels, described. The lymph flow mechanism is correlated with the anatomical findings. The abdominal and lower two-thirds of the thoracic part are more muscular and probably have active peristalsis. The cervical part is less muscular and functions like a vein, passively conveying the lymph it receives. The ostial valve at the lympho-venous junction is bicuspid and has special characteristics and functions. The flow of lymph through the lympho-venous junction, although continuous, mainly occurs during expiration. The ostial valve functions when there is a sudden rise in venous tension causing stretching of the venous wall and the ostial cusps.