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 rent Motion of the Blood: Still all this was only Opinion and Belief: It was rational, and became Men of their Genius's; but, not having as yet been made evident by Experiments, it might as easily be denied as affirmed. Servetus first saw that the Blood passes through the Lungs; Columbus went further and shew'd the Uses of the Valves or Trap-doors of the Heart, which let the Blood in and out of their Respective Vessels, but not the self same Road: Thus the Way was just open when Dr. Harvey came, who built upon the First Foundations; to make his Work yet the easier, the Valves of the Veins which were discovered by F. Paul the Venetian, had not long before been explained by Fabricius ab Aqua pendente, whence the Circulation was yet more clearly demonstrated.

There was one thing still wanting to compleat this Theory, and that was the Knowledge how the Veins received that Blood which the Arteries discharged; first it was believed that the Mouths of each sort of Vessels joined into one another; that Opinion was soon laid aside, because it was found that the capillary Vessels were so extremely small, that it was impossible with the naked Eye to trace them. This put them upon ima-