Water as the blood of the earth
The microcosm analogy: water rising from sea to mountain like sap in a vine and blood in the body
Two passages develop Leonardo's microcosm theory, in which water is the vital humor of the arid earth and the same cause that drives it against the natural course of heavy things also moves the humors in every animate body. Water rises from the deepest sea to the highest mountain summits, pours down through broken veins, and returns to climb again in continuous revolution through the earth's passages. The second passage reinforces the idea with the analogy of sap rising in a cut vine and blood surging through the broken veins of the forehead. A small circular sketch sits at the upper left of f.236v, and both densely written leaves carry further writing beyond the two transcribed passages.
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Water lifted from the deep sea to the mountain summits
Water rises from the lowest depth of the sea to the highest summits of the mountains, then, pouring through broken veins, returns to the low sea only to surge up again with speed. Leonardo describes this as a continuous revolution, sometimes rising by accidental motion and sometimes ruining down by natural course, wheeling through the terrestrial passages.
Water as the vital humor of the earth's body
Water is presented as that which is dedicated to be the vital humor of the arid earth, moving through its ramifying veins against the natural course of heavy things. This same cause, moving the earth's waters from within to the exterior and from the lowest to the highest, is identified as the very cause that moves the humors in all species of animate bodies.
The vine and the veins of the forehead
The second passage grounds the analogy in living bodies: as water rises from the lower part of the vine to its cut branches, falls back on its roots, penetrates them and rises again, so does the water of the earth. Likewise, as low blood rises high and pours through the broken veins of the forehead, so from the lowest sea the water raises itself to the mountain summits.