63 indeed, it seems probable, was practically answered even within Harvey's lifetime, what was left for Harvey to discover was nothing less than the circulation itself. His predecessors had but impinged, and that by guesswork, upon different segments of the circle, and then gone off at a tangent into outer darkness, whilst he worked and proved and demonstrated round its entire periphery. JSis_demon stratio ns a nd direct proofs w ere all new, and his indirect arguments nearly all new. Whenever he made use of any- thing already known, he most punctually acknowledged it. Of his demonstration in the way of injection I have already spoken ; of his demonstration of the use of the valves liTlhe veins, and his proof that they are similar in function to the arterial, a fact previously unsuspected (see p. 65 I c), the thirteenth chapter of the treatise De Motu speaks with figures ; of his indirect, but irre- fragable argument, in the eighth chapter, from the quantity of blood thrown out by the heart at each pulsation, an argument which a mathematician such as Harriott, or Warner, might have hit upon, but, so far as I have
