There are many superstitions prevalent among uneducated people as
to "second sight," recovery of vision, etc., which render their
reports of such things untrustworthy. The real explanations of
such cases are too varied for discussion here.
Nyctalopia etymologically means night blindness, but the general
usage, making the term mean night-vision, is so strongly
intrenched that it is useless and confusing to attempt any
reinstatement of the old significance. The condition in which one
sees better by night, relatively speaking, than by day is due to
some lesion of the macular region, rendering it blind. 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. All night-prowling animals have widely dilatable
pupils, and in addition to this they have in the retina a special
organ called the tapetum lucidum, the function of which is to
reflect to a focus in front of them the relatively few rays of
light that enter the widely-dilated pupil and thus enable them
the better to see their way. Hence the luminous appearance of the
eyes of such animals in the dark.
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