To summarize--until the seventeenth century there were believed to be two closed systems in the circulation, (1) the natural, containing venous blood, had its origin in the liver from which, as from a fountain, the blood continually ebbed and flowed for the nourishment of the body; (2) the vital, containing another blood and the spirits, ebbed and flowed from the heart, distributing heat and life to all parts. Like a bellows the lungs fanned and cooled this vital blood. Here and there we find glimmering conceptions of a communication between these systems, but practically all teachers believed that the only one of importance was through small pores in the wall separating the two sides of the heart. Observation--merely looking at and thinking about things--had done all that was possible, and further progress had to await the introduction of a new method, viz., experiment. Galen, it is true, had used this means to show that the arteries of the body contained blood and not air. The day had come when men were no longer content with accurate description and with finely spun theories and dreams. It was reserved for the immortal Harvey to put into practice the experimental method by which he demonstrated conclusively that the blood moved in a circle. The "De Motu Cordis" marks the final break of the modern spirit with the old traditions. It took long for men to realize the value of this "inventum mirabile" used so effectively by the Alexandrians--by Galen--indeed, its full value has only been appreciated within the past century. Let me quote a paragraph from my Harveian Oration.(33) "To the age of the hearer, in which men had heard and heard only, had succeeded the age of the eye in which men had seen and had been content only to see. But at last came the age of the hand--the thinking, devising, planning hand, the hand as an instrument of the mind, now re-introduced into the world in a modest little monograph from which we may date the beginning of experimental medicine." (33) Osler: An Alabama Student, etc., pp. 329-330.