Sex Change for Vitruvian Man

From Draw Like da Vinci, p. 100: Human Proportions: The length of a man's outspread arms is equal to his height. From the tip of the longest finger of the hand to the shoulder joint is four hands, or, if you will, four faces. Leonardo took his idea for Vitruvian Man from the texts of Vitruvius, a Roman architect in the 1st century BC who wrote about the proporations of the body in relation to the square and the circle. Leonardo improved on the crude concepts of Vitruvius by placing the navel at the centre of the circle and the root of the penic at the centre of the square, and divided the body parts according to the gold section. As you see in Sex Change for Vitruvian Man, the base of Leonardo's square is a tangent to the circle at the feet. The outstretched feet contact the circle above the square, demonstrating the reduction in a person's height when their legs are in motion. The head and hand are a useful measure to assess proportion. Observe how many heads fit into the length of a figure – Leonardo's Vitruvian Man is eight heads high. The average height of a person is between seven and eight head-lengths. 

Horizontal hatching in the background emphasises the contours of the body and gives depth to the form. It also makes the figure appear lighter in tone than the paper itself. Leonardo's Vitruvian Man has both an erect and a flaccid penis.

Read more: 'Study of the proportions of the human body, known as the Vitruvian Man', Gallerie dell'Accademia di Venezia