It has seemed best that the various references to Harvey's Latin text should be made to that of the easily accessible _Opera Omnia_ rather than to that of the rarer first editions of the several treatises. In the case of the passages quoted from the treatise _De Motu Cordis et Sanguinis in Animalibus_ and from the treatise _De Generatione Animalium_, the Latin of the _Opera Omnia_ has been collated by the present writer with that of the first editions. The first editions of the Exercises to Riolanus and of the various letters have not been accessible to him. Much use has here been made of Harvey's private lecture notes, first published in 1886 by the Royal College of Physicians of London. All the passages (except those from the Scriptures) quoted in the present paper from writers other than Harvey have been translated into English by the present writer directly from the original Greek text or the original Latin text, as the case may be. JOHN G. CURTIS. EDITORIAL NOTE Professor Curtis, to whom I am indebted for much kindly help extended during a warm friendship of nearly thirty years, died September 20, 1913. One of his final requests was that his younger colleague arrange for the publication of the present paper, upon which its writer had been engaged for a period of several years and which happily was practically completed. This request, coming to me after the death of my friend, could be considered only as a command. It has, therefore, fallen to me to make a careful study of his text, to fill in with my own words occasional slight gaps, to make occasional verbal changes, to certify to the correctness of his numerous references, and to make the manuscript, written and in places rewritten many times with his own hand, ready for the press. This I have done with affection for his memory and with appreciation of his scholarly attainments. Dr. Curtis's work represents a more profound study of Harvey's ideas and comparison of them with those of the most important of Harvey's predecessors than has heretofore appeared. It is the work of one who from the background of the physiological science of to-day delighted in mastering the ideas of the fathers of modern physiology. If his work is to be summarized in a single sentence, it may be said that he has shown Harvey to be a disciple more of Aristotle than of Galen. Although Harvey had the courage and the originality to break away from him whose ideas had prevailed for fourteen centuries, and to find the truth in regard to the movement of the blood, he found much to approve in the master who had lived five hundred years before Galen. Harvey's true position in the world of physiological thought has not before been made known. Herein lies Professor Curtis's contribution to the history of his science. FREDERIC S. LEE. COLUMBIA UNIVERSITY, June 1, 1915. CONTENTS PAGE PREFATORY NOTE v