The importance of Harvey's discovery of the circulation of the blood can only be properly estimated by bearing in mind what was done by his predecessors in the same field of inquiry. Aristotle had taught that in man and in the higher brutes the blood was elaborated from the food in the liver, conveyed to the heart, and thence distributed by it through the veins to the whole body. Erasistratus and Herophilus held that, while the veins carried blood from the heart to the members, the arteries carried a subtle kind of air or spirit. Galen discovered that the arteries were not merely air-pipes, but that they contained blood as well as vital air or spirit. Sylvius, the teacher of Vesalius, was aware of the presence of valves in the veins; and Fabricius, Harvey's teacher at Padua, described them much more accurately than Sylvius had done; but neither of these men had a true idea of the significance of the structures of which they wrote. Servetus, the friend and contemporary of Vesalius, writing in 1533, correctly described the course of the lesser circulation in the following words: "This communication (i.e. between the right and left sides of the heart) does not take place through the partition of the heart, as is generally believed; but by another admirable contrivance, whereby from the right ventricle the subtle blood is agitated in a lengthened course through the lungs, wherein prepared, it becomes of a crimson colour, and from the vena arterialis (pulmonary artery) is transferred into the arteria venalis (pulmonary vein). Mingled with the inspired air in the arteria venalis, freed by respiration from fuliginous matter, and become a suitable home of the vital spirit, it is attracted at length into the left ventricle of the heart by the diastole of the organ." But when Servetus comes to speak of the systemic circulation, what he has to say is as old as Galen.
The opinions, therefore, on the subject of the blood and its distribution which were prevalent at the end of the sixteenth century prove—
(1) That although the blood was not regarded as stagnant, yet its circulation, such as is nowadays recognized, was unknown;
(2) That one kind of blood was thought to flow from the liver to the right ventricle, and thence to the lungs and general system by the veins, while another kind flowed from the left ventricle to the lungs and general system by the arteries;
(3) That the septum of the heart was regarded as admitting of the passage of blood directly from the right to the left side;
(4) That there was no conception of the functions of the heart as the motor power of the movement of the blood, for biologists of that day doubted whether the substance of the heart were really muscular; they supposed the pulsations to be due to expansion of the spirits it contained; they believed the only dynamic effect which it had on the blood to be that of sucking it in during its active diastole, and they supposed the chief use of its constant movements to be the due mixture of blood and spirits.
This was the state of knowledge before Harvey's time. By his great work he established—
(1) That the blood flows continuously in a circuit through the whole body, the force propelling it in this unwearied round being the rhythmical contractions of the muscular walls of the heart;
(2) That a portion only of the blood is expended in nutrition each time that it circulates;
(3) That the blood conveyed in the systemic arteries communicates heat as well as nourishment throughout the body, instead of exerting a cooling influence, as was vulgarly supposed; and