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and they mutually receive from each other blood and spirits through certain invisible and extremely small vessels." It is difficult to understand how Galen missed the circulation of the blood.

He knew that the valves of the heart determined the direction of the blood that entered and left the organ, but he did not appreciate that it was a pump for distributing the blood, regarding it rather as a fireplace from which the innate heat of the body was derived.

He knew that the pulsatile force was resident in the walls of the heart and in the arteries, and he knew that the expansion, or diastole, drew blood into its cavities, and that the systole forced blood out.