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.
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 Andreas Vesalius, before Harvey would provide a refined and complete description of the circulatory system.
Harvey's other major work was Exercitationes de generatione animalium, published in 1651.
The book starts with a description of development of the hen's egg. The major part is theoretical, dealing with Aristotle's theories and the work of the physicians following Galen and up to Fabricius. Finally he deals with embryogenesis in viviparous animals especially hinds and does. The treatment is generally Aristotelian and limited by use of a simple magnifying lens.
Needham claims the following achievements for this work.
His doctrine of omne vivum ex ovo (all life comes from the egg) was the first definite statement against the idea of spontaneous generation. He denied the possibility of generation from excrement and from mud, and pointed out that even worms have eggs.
He identified the citricula as the point in the yolk from which the embryo develops and the blastoderm surrounding the embryo.
He destroyed once and for all the Aristotelian (semen-blood) and Epicurean (semen-semen) theories of early embryogeny.
He settled the long controversy about which parts of the egg were nutritive and which was formative, by demonstrating the unreality of the distinction.
eBook The Visitations of the County of Oxford Taken in the Years