Illustration of Galen's Theory of the blood, 1292. Among Galen's major contributions to medicine was his work on the circulatory system. He was the first to recognize that there are distinct differences between venous (dark) and arterial (bright) blood. Although his anatomical experiments on animal models led him to a more complete understanding of the circulatory system, nervous system, respiratory system, and other structures, his work contained scientific errors. He believed the circulatory system to consist of two separate one-way systems of distribution, rather than a single unified system of circulation. He believed venous blood to be generated in the liver, from where it was distributed and consumed by all organs of the body. He posited that arterial blood originated in the heart, from where it was distributed and consumed by all organs of the body. The blood was then regenerated in either the liver or the heart, completing the cycle.