But many of the great anatomists of the sixteenth century were unable to make this demonstration, even when it had been brought to their attention by such an authority as Vesalius.

Fallopius, writing to Vesalius on the subject in 1562, declared that he was unable to find such valves.

Others, however, such as Eustachius and Fabricius (1537-1619), were more successful, and found and described these structures.

But the purpose served by these valves was entirely misinterpreted.

That they act in preventing the backward flow of the blood in the veins on its way to the heart, just as the valves of the heart itself prevent regurgitation, has been known since the time of Harvey; but the best interpretation that could be given at that time, even by such a man as Fabricius, was that they acted in retarding the flow of the blood as it comes from the heart, and thus prevent its too rapid distribution throughout the body.
The fact that the blood might have been going towards the heart, instead of coming from it, seems never to have been considered seriously until demonstrated so conclusively by Harvey.
Of this important and remarkable controversy over the valves in veins, Withington has this to say: "This is truly a marvellous story.