Recto: a study of the trunk of a man, in profile to the right, with the right arm extended, indicating the surface muscles; the skeleton of a left foot; the same, seen from below; six diagrammatic drawings of the skeleton of the big toe; the bones of a left foot, in profile to the left and the corresponding leg bones, drawn upside down; with notes on the drawings. Verso: five studies of the skeleton of the right upper limb, showing pronation and supination; a diagram demonstrating pronation; notes on the drawings. Leonardo examines the rotation of the forearm, showing the role of the biceps, its double origin and its point of insertion on the radius. He notes that the arm shortens slightly on rotation because the radius and ulna cross over, as explained by the geometrical diagram in the right margin with a note that 'a line loses depth the more obliquely it is placed.' This subtle observation of physical reality shows how far Leonardo's investigation of the human form had developed in the twenty years since his campaign of proportion studies. RL 19000v Text adapted from 'Leonardo da Vinci: the Divine and the Grotesque'