Harvey not only demonstrated the presence of these valves, but showed conclusively, by simple experiments, what their function was, thus completing his demonstration of the phenomena of the circulation.
The final ocular demonstration of the passage of the blood from the arteries to the veins was not to be made until four years after Harvey's death.

This process, which can be observed easily in the web of a frog's foot by the aid of a low-power lens, was first demonstrated by Marcello Malpighi (1628-1694) in 1661.

By the aid of a lens he first saw the small "capillary" vessels connecting the veins and arteries in a piece of dried lung.

Taking his cue from this, he examined the lung of a turtle, and was able to see in it the passage of the corpuscles through these minute vessels, making their way along these previously unknown channels from the arteries into the veins on their journey back to the heart.

Thus the work of Harvey, all but complete, was made absolutely entire by the great Italian.