The waters return with constant motion from the lowest depths of the sea to the utmost height of the mountains, not obeying the nature of heavier bodies; and in this they resemble the blood of animated beings which always moves from the sea of the heart and flows towards the top of the head; and here it may burst a vein, as may be seen when a vein bursts in the nose; all the blood rises from below to the level of the burst vein. When the water rushes out from the burst vein in the earth, it obeys the law of other bodies that are heavier than the air since it always seeks low places. [Footnote: 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. Leonardo's studies on the muscles of the heart are to be found in the MS. W. An. III. but no information about them has hitherto been made public. The limits of my plan in this work exclude all purely anatomical writings, therefore only a very brief excerpt from this note book can be given here. WILLIAM HARVEY (born 1578 and Professor of Anatomy at Cambridge from 1615) is always considered to have been the discoverer of the circulation of the blood. He studied medicine at Padua in 1598, and in 1628 brought out his memorable and important work: _De motu cordis et sanguinis_.] 850. That the blood which returns when the heart opens again is not the same as that which closes the valves of the heart. Some notes on medicine (851-855). 851. Make them give you the definition and remedies for the case ... and you will see that men are selected to be doctors for diseases they do not know. 852. A remedy for scratches taught me by the Herald to the King of France. 4 ounces of virgin wax, 4 ounces of colophony, 2 ounces of incense. Keep each thing separate; and melt the wax, and then put in the incense and then the colophony, make a mixture of it and put it on the sore place. 853. Medicine is the restoration of discordant elements; sickness is the discord of the elements infused into the living body. 854. Those who are annoyed by sickness at sea should drink extract of wormwood. 855. To keep in health, this rule is wise: Eat only when you want and relish food. Chew thoroughly that it may do you good. Have it well cooked, unspiced and undisguised. He who takes medicine is ill advised. [Footnote: This appears to be a sketch for a poem.] 856. I teach you to preserve your health; and in this you will succed better in proportion as you shun physicians, because their medicines are the work of alchemists. [Footnote: This passage is written on the back of the drawing Pl. CVIII. Compare also No. 1184.] _XV_. _Astronomy_.