Realdus Columbus, the associate of Dr. Caius at Padua, had in 1559 published a treatise containing some advanced views, showing that the blood once having entered the right ventricle from the vena cava, cannot return in consequence of the opposition of the tricuspid valves, and he further perceived the effect of the pulmonary valves; but he still held the idea that the blood had to be converted in the lungs into a kind of spirit, and looked upon the liver as the fountain-head of the blood. Finally, he denied the muscular structure of the heart. Cæsalpinus added to this some more complete idea of the greater circulation, but he knew nothing of the valves in the veins, and held to the belief that there were two kinds of blood, one for the growth, another for the nourishment of the body. He imagined that it was only during sleep that the veins become distended while the pulsations of the arteries become moderated. He had no idea of the connection between the emptying of the arteries and the filling of the veins, nor of the heart being the cause of the blood’s movement. Fabricius, Harvey’s teacher of anatomy, had made such a distinct step in advance in discovering the valves of the veins and the effect they must have, that it is quite astonishing that he should not have proceeded farther. But the fact is, that without the microscope as developed in after years,[5] it was impossible to solve a multitude of questions satisfactorily, and we may rather marvel that Harvey was able to achieve so much with the means at his disposal. The principal means he employed to this end was undoubtedly the vivisection of animals. Chapter i. of his celebrated treatise on the Motion of the Heart and the Blood (Frankfort, 1628) begins emphatically, “When I first gave my mind to vivisections, as a means of discovering the motions and uses of the heart, and sought to discover these from actual inspection, and not from the writings of others, I found the task so truly arduous, so full of difficulties, that I was almost tempted to think, with Fracastorius, that the motion of the heart was only to be comprehended by God.[6] “At length, and by using greater and daily diligence, having frequent recourse to vivisections, employing a variety of animals for the purpose, ... I thought ... that I had discovered what I so much desired, with the motion and the use of the heart and arteries....