On the Motion of the HeartWilliam Harvey

Harvey's short Latin treatise of 1628, seventy-two pages in its first edition, that overturned a theory of blood which had stood for fourteen centuries. Galen had taught that venous and arterial blood were distinct substances, continuously produced by the liver and the heart and consumed by the tissues, passing through invisible pores in the septum between the ventricles. Harvey demonstrates, through anatomical dissection, vivisection of cold-blooded animals whose hearts beat slowly enough to be watched, and a decisive quantitative argument, that the blood instead moves in a closed circuit: out from the left ventricle through the arteries, back through the veins to the right ventricle, through the lungs to the left side of the heart again.

The quantitative argument is the hinge. Harvey estimates the volume expelled in each heartbeat and multiplies by the number of beats in an hour: the result vastly exceeds anything the liver could plausibly produce or the tissues absorb. The blood must therefore return. Ligature experiments on the arm confirm the direction of flow. The chapters on the motion of the heart itself establish that its contraction, not its dilation, is the active phase.

On the Motion of the Heart founds experimental physiology. It combines careful observation with rigorous calculation in a way that prefigures the mathematical natural philosophy Newton would bring to the physical world.