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.