Saccades are the raoid movements your eye makes over a scene as you take in the details. They are needed because a small portion at the center of the retina, called the fovea, contans the majority of your light senisng cells. The saccades allows you to build up an overall detailed view rather like taking lots of photographs and then stitching them together to form a panorama.
Used in Chap. 22: pages 347, 348