"Anatomy Lesson of Dr. Jan Deijman (detail)", 1656 perhaps by Rembrandt van Rijn (1606 - 1669) Historic Museum, Amsterdam, Netherlands
The exposure of the Rotula (rotating or movable sesamoid bone) in front of the knee or "knee cap" (and slightly like a cup). Sesamoid bones often form within a muscle, in this instance the quadriceps or anterior thigh muscle.