The ultrastructure of the thoracic duct and cisterna chyli was studied
in the Formosan rock monkey. This duct is approximately 40% of the cr
own-rump length. Bicuspid valves ensure one-way drainage of the lymph
and prevent reflux of lymph. Ultrastructural variations between the 2
sides of the valves and the functional significance of the different t
ypes of valve are discussed. The muscular organisation in the tunica m
edia is similar to that of the human thoracic duct. The abdominal and
thoracic parts of the duct contain smooth muscle and probably exhibit
active peristalsis, while the cervical part is less muscular and funct
ions like a vein.