Systematic quantitative investigations were performed in the coronary arteries of 102 hearts of 36 mammal species with an overall more than tenthousandfold difference of their heart weight. After postmortem coronary angiography with a filling pressure of 100 mm Hg x-rays were taken, and the widest diameters of the coronary artery stems were determined. We found a nearly linear correlation between diameter of a standardized coronary artery and virtual diameter of heart, but the increase in diameter of coronary arteries exceeded somewhat that of the diameter of heart especially for heart weights surmounting 100 g. Perhaps relative enlargement of coronary arteries in the bigger hearts contributes to the prevention of large increase of blood flow velocity.

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