Composed of many layers, the retina of humans and most vertebrates contains a stratum of rods and cones, the photosensitive cells of the eye. Rods are elongate and are responsible for perceiving the basics of a visual image, such as size, shape, and intensity. Cones, on the other hand, are shorter than rods and can perceive color and subtle detail. They are also less sensitive to light than rod cells and are significantly less numerous.