Leonardo's initial anatomy studies of 1489 focused on human skulls. He started with a skull that had been sawed in half, top to bottom (fig. 60). Then the front of the left half was sawed off. His groundbreaking technique of drawing the two halves together made it easy to see how the inner cavities were positioned relative to the face. For example, the frontal sinus, which Leonardo is the first person to correctly depict, is shown to rest just behind the eyebrow.