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114 THROUGH NORWAY WITII A KNAPSACK.
injury, and are therefore almost insensible to all but
the most violent mechanical attacks.
If we compare the eyes and the teeth, in their
relations to heat and cold, we shall find that the conditions
are reversed. The eye is the most hardy, while the
teeth are the most delicately sensitive organs of the
body. For what end is this ordained ? The teeth are
bad conductors of heat, and are coated with a glossy
enamel; therefore, if they are suddenly heated or cooled,
the surface will expand or contract before the heat is
communicated throughout the substance of the tooth;
and thus the enamel will be cracked, just as thick glass
will crack, or as the enamel of thick porcelain will crack,
if similarly treated: we have all seen old mugs, and
thick cups and saucers, with a network of fine cracks
upon the enamel. Many of us persist in drinking
scalding tea and coffee out of such cups, in spite of
the double warning of the cracked enamel and aching
teeth ; and by constant repetition of this folly the
sensibility of the teeth becomes destroyed, and their
substance soon follows their sensibility.
Now let us examine the eye in its relation to heat
and cold. When, in spite of wrappers and overcoats,
we are chilled to the skin—when the fingers and toes
are numbed, and the nose with all the warm breath
that passes through it, is threatening most painfully to
freeze and mortify—the wet, uncovered eyes remain
almost insensible to the freezing air: the cold can do
them no injury, and therefore exposure to it inflicts no
pain. The advantages of this are obvious. We may
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