Views of The Circulation of Blood Before Harvey
R.A. Young wrote: "Wiberg suggests that the early Greeks knew of the circulation, and quotes a passage from one of the Hippocratic writings which would bear that interpretation."
At the time of Harvey's publication, Galen had been an influential medical authority for several centuries. Galen believed that blood passed between the ventricles by means of invisible pores. According to Galen's views, the venous system was quite separate from the arterial system, except when they came in contact through the unseen pores. Arabic scholar Ibn al-Nafis had disputed aspects of Galen's views, providing a model that seems to imply a form of pulmonary circulation in his Commentary on Anatomy in Avicenna's Canon (1242). Al-Nafis stated that blood moved from the heart to the lungs, where it mixed with air, and then back to the heart, from which it spread to the rest of the body. Harvey's discoveries inevitably and historically came into conflict with Galen's teachings and the publication of his treatise De Motu Cordis incited considerable controversy within the medical community. Some doctors affirmed they would "rather err with Galen than proclaim the truth with Harvey." Galen incompletely perceived the function of the heart, believing it a "productor of heat", while the function of its affluents, the arteries, was that of cooling the blood as the lungs "...fanned and cooled the heart itself". Galen thought that during dilation the arteries sucked in air, while during their contraction they discharged vapours through pores in the flesh and skin.
Until the 17th century, two separate systems were thought to be involved in blood circulation: the natural system, containing venous blood which had its origin in the liver, and the vital system, containing arterial blood and the 'spirits' which flowed from the heart, distributing heat and life to all parts. Like bellows, the lungs fanned and cooled this vital blood.
Independently from Ibn Al-Nafis, Michael Servetus identified pulmonary circulation, but this discovery did not reach the public because it was written down for the first time in the Manuscript of Paris in 1546. It was later published in the theological work which caused his execution in 1553, almost all copies of which were destroyed. Pulmonary circulation was described by Renaldus Columbus, Andrea Cesalpino and Vesalius, before Harvey would provide a refined and complete description of the circulatory system.
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