Now if the vena cava be taken up with forceps or the thumb and finger, and the course of the blood intercepted for some distance below the heart, you will at once see it almost emptied between the fingers and the heart, the blood being exhausted by the heart's pulsation, the heart at the same time becoming much paler even in its dilatation, smaller in size, owing to the deficiency of blood, and at length languid in pulsation, as if about to die.

On the other hand, when you release the vein the heart immediately regains its color and dimensions.

After that, if you leave the vein free and tie and compress the arteries at some distance from the heart, you will see, on the contrary, their included portion grow excessively turgid, the heart becoming so beyond measure, assuming a dark-red color, even to lividity, and at length so overloaded with blood as to seem in danger of suffocation; but when the obstruction is removed it returns to its normal condition, in size, color, and movement."(4) This conclusive demonstration that the veins return the blood to the heart must have been most impressive to Harvey, who had been taught to believe that the blood current in the veins pursued an opposite course, and must have tended to shake his faith in all existing doctrines of the day.
His next step was the natural one of demonstrating that the blood passes from the arteries to the veins.

He demonstrated conclusively that this did occur, but for once his rejection of the ancient writers and one modern one was a mistake.

For Galen had taught, and had attempted to demonstrate, that there are sets of minute vessels connecting the arteries and the veins; and Servetus had shown that there must be such vessels, at least in the lungs.
However, the little flaw in the otherwise complete demonstration of Harvey detracts nothing from the main issue at stake.