{{U}} (31) {{/U}}with the human eye, a cat's{{U}} (32) {{/U}}have more rods than cones,{{U}} (33) {{/U}}helps the cat see in the dark. Cats also have elliptical pupils that open and close faster and can become larger than our round{{U}} (34) {{/U}}. In addition, cats and some{{U}} (35) {{/U}}nocturnal animals have a mirror like membrane on the back of their eyes, which reflects the light passing through the rods back through them in the, opposite direction, This "double exposure" allows cats to{{U}} (36) {{/U}}well in near darkness.
The human eye changes gradually{{U}} (37) {{/U}}age,{{U}} (38) {{/U}}the changes are critical. In darkness, eyes adapt{{U}} (39) {{/U}}widening the pupils to let in as much light as possible. {{U}}(40) {{/U}}you get older, these muscles weaken and do not respond as well{{U}} (41) {{/U}}the need to let in more light.
The result is a small{{U}} (42) {{/U}}when you try to see in poor light. It's{{U}} (43) {{/U}}if your eyes were still young but you were wearing sunglasses at night. There is also evidence that as we age we lose more rods{{U}} (44) {{/U}}cones.
In the young eye, rods{{U}} (45) {{/U}}cones by nine to one in the part of the retina called the macula. But an autopsy study of older adults found that while the cones{{U}} (46) {{/U}}intact, almost a third of the rods in the macula had been lost.
The less responsive muscles in the iris also affect the eye's ability to adjust when the intensity of light changes,{{U}} (47) {{/U}}as when a car with its headlights on approaches and then passes.
In older eyes, this phenomenon,{{U}} (48) {{/U}}dark adaptation, takes longer, which{{U}} (49) {{/U}}you see less well in the dark after being in the light, and vice versa. The diminished number of rods may be a factor, but in addition, the light-sensitive pigment in the rods regenerates more slowly in{{U}} (50) {{/U}}eyes.