a d is a head’s length, c b is a head’s length. The four smaller toes are all equally thick from the nail at the top to the bottom, and are 1/13 of the foot.
[Footnote: See Pl. XIV, No. 1, a drawing of a foot with the text in three lines below it.]
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.