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  3. (31) with the human eye, a cat’s (32) have more rods than cones, (33) helps the cat see in the dark. Cats also have elliptical p

(31) with the human eye, a cat’s (32) have more rods than cones, (33) helps the cat see in the dark. Cats also have elliptical p

admin2009-06-24  53
问题     (31) with the human eye, a cat’s (32) have more rods than cones, (33) helps the cat see in the dark. Cats also have elliptical pupils that open and close faster and can become larger than our round (34). In addition, cats and some (35) 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 (36) well in near darkness.
    The human eye changes gradually (37) age, (38) the changes are critical. In darkness, eyes adapt (39) widening the pupils to let in as much light as possible. (40) you get older, these muscles weaken and do not respond as well (41) the need to let in more light.
    The result is a small (42) when you try to see in poor light. It’s (43) 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 (44) cones.
    In the young eye, rods (45) 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 (46) 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, (47) as when a car with its headlights on approaches and then passes.
    In older eyes, this phenomenon, (48) dark adaptation, takes longer, which (49) 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 (50) eyes.

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