OF PAINTING.
Indicate which are the muscles, and which the tendons, which become prominent or retreat in the different movements of each limb; or which do neither [but are passive]. And remember that these indications of action are of the first importance and necessity in any painter or sculptor who professes to be a master &c.
And indicate the same in a child, and from birth to decrepitude at every stage of its life; as infancy, childhood, boyhood, youth &c.
And in each express the alterations in the limbs and joints, which swell and which grow thinner.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.