Anatomy instructor George Bridgman (1865-1943) was famous for drawing directly on the bodies of the models who posed for his figure drawing classes at the Art Students League in New York.
According to Norman Rockwell, Bridgman would dig a piece of soft red chalk out of his shirt pocket, and then would "walk to the model stand and draw the muscles of the stomach and the line of the rib cage right on the model with his chalk. The models disliked this. They say it gave them a queasy, squirmy sort of feeling to have their muscles marked on their skin in soft red chalk. And then there was no place at the League where they could wash properly and they'd have to go home with their muscles outlined in red."
From the book: Norman Rockwell: My Adventures As an Illustrator