The Notebooks of Leonardo Da Vinci

Compiled and edited from the original manuscripts by Jean Paul Richer in Two Volumes

Physiology

849

The waters return with constant motion from the lowest depth of the sea to the utmost heights of the mountains; not obeying the nature of heavenly bodies; and in this they resemble the blood of animated beings which always moves from the sea of the heart and towards the top of the head; and here may be a burst vein, as may be seen when a vein bursts in a nose; all the blood rises from below to the level of the burst vein. When water rushes out from the burst vein in the earth, it obeys the law of other bodies that are heavier than air since it always seeks low places.[1]

850

The blood that returns when the heart opens up again is not the same as that which closes the valves of the heart.

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[1] From this passage it is quite plain that Leonardo had not merely a general suspicion of the circulation of the blood , but a very clear conception of it.