ALTHOUGH bicuspid venous valves in the internal jugular and subclavian veins at the thoracic inlet in man were identified by sixteenth century anatomists' and are included in modern textbooks of anatomy,2 many physicians are unaware of their existence. The function and competence of these thoracic inlet venous valves have been controversial for several hundred years. In 1628, Harvey wrote, "the edges of the valves in the jugular veins hang downwards, and are so contrived that they prevent blood from rising upwards."3 Nearly 3 centuries later, Mackenzie4 reviewed the evidence and could not reach any firm conclusion about the competence of the internal jugular vein valve. In patients with tricuspid regurgitation, Muller and Shillingford found sudden decreases in systolic pressure (v) waves as the catheter was withdrawn from the superior vena cava past the subclavian vein valves.5 Wood claimed that in tricuspid regurgitation, the venous valves take on the function of the tricuspid valve.6 Keith,7 however, concluded that in man, the internal jugular vein valves were the weakest set of venous valves separating the thorax and abdomen from the extremities because they were the only set of From the Peter

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