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Ibn al-Nafis’s theory on the circulation of blood was first written in Sharh tashrih al-qanum li’ Ibn Sina or Commentary on Anatomy in the Canon of Ibn Sina. The translation of his book was performed by Max Meyerhof. His paper included a critique on Galen’s theory on the pores of the heart. Based on animal dissection, Galen hypothesized that the muscle separating the two sides of the heart had to be porous in order for blood to travel within the heart. He further added that this transit was facilitated by the lungs. However, Galen could not observe pores on the heart and believed that the pores were too small to see. Al-Nafis rejected Galen’s pore theory with the following passage:

“The blood, after it has been refined in [the right] cavity, must be transmitted to the left cavity where the (vital) spirit is generated. But there is no passage between these cavities, for the substance of the heart is solid in this region and has neither a visible passage…”

He posited that the “pores” of the heart are closed and the substance of the heart is thick. Instead, he theorized that the blood rose into the lungs via the arterious vein and then circulated into the the left cavity of the heart.