The Da Vinci Notebooks at sacred-texts.com
The proportions varyat different ages (362-367).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.