Finally, take the famous passage in the eighth chapter, _De copiâ sanguinis transeuntis per cor e venis in arterias, et de circulari motu sanguinis_:-- "And now, as for the great quantity and forward movement of this blood on its way, when I shall have said what things remain to be said--though they are well worth considering, yet they are so new and strange that I not only fear harm from the envy of certain men, but am afraid lest I make all men my enemies; so does custom, or a doctrine once imbibed and fixed down by deep roots, like second nature, hold good among all men, and reverence for antiquity constrains them. Be that as it may, the die is cast now: my hope is in the love of truth, and the candour of learned minds. I bethought me how great was the quantity of this blood. Both from the dissection of living animals for the sake of experiment, with opening of the arteries, with observations manifold; and from the symmetry of the size of the ventricles, and of the vessels entering and leaving the heart--because Nature, doing nothing in vain, cannot in vain have given such size to these vessels above the rest--and from the harmonious and happy device of the valves and fibres, and all other fabric of the heart; and from many other things--when I had again and again carefully considered it all, and had turned it over in my mind many times--I mean the great quantity of the blood passing through, and the swiftness of its passage--and I did not see how the juices of the food in the stomach could help the veins from being emptied and drained dry, and the arteries contrariwise from being ruptured by the excessive flow of blood into them, unless blood were always getting round from the arteries into the veins, and so back to the right ventricle--I began to think to myself whether the blood had a certain movement, as in a circle--_coepi egomet mecum cogitare, an motionem quandam quasi in circulo haberet_--which afterward I found was true." This vehement passage, which goes with a rush like that of the blood itself, is a good example of the width and depth of Harvey's work--how he used all methods that were open to him. He lived to fourscore years; "an old man," he says, "far advanced in years, and occupied with other cares": and, near the end of his life, he told the Hon. Robert Boyle that the arrangement of the valves of the veins had given him his first idea of the circulation of the blood:--