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At night the pupil dilates more than in the day-time, and hence vision with the extramacular or peripheral portions of the retina is correspondingly better. It is, therefore, a symptom of serious retinal disease.
In retinitis pigmentosa the peripheral or extramacular portions of the retina are subject to a pigmentary degeneration that renders them insensitive to light, and patients so afflicted are consequently incapable of seeing at night as well as others. They stumble and run against objects easily seen by the normal eye.
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