We all know that common
caustic burns us wherever it touches; and it burns the tongue only in a
somewhat more marked manner. Nitric or sulphuric acid attacks the
fingers each after its own kind. A mustard plaster makes us tingle
almost immediately; and the action of mustard on the tongue hardly
differs, except in being more instantaneous and more discriminative.
Cantharides work in just the same way. If you cut a red pepper in two
and rub it on your neck, it will sting just as it does when put into
soup (this experiment, however, is best tried upon one's younger
brother; if made personally, it hardly repays the trouble and
annoyance). Even vinegar and other acids, rubbed into the skin, are
followed by a slight tingling; while the effect of brandy, applied,
say, to the arms, is gently stimulating and pleasurable, somewhat in the
same way as when normally swallowed in conjunction with the habitual
seltzer. In short, most things which give rise to distinct tastes when
applied to the tip of the tongue give rise to fainter sensations when
applied to the skin generally. And one hardly needs to be reminded that
pepper or vinegar placed (accidentally as a rule) on the inner surface
of the eyelids produces a very distinct and unpleasant smart.
The fact is, the liability to be chemically affected by pungent or acid
bodies is common to every part of the skin; but it is least felt where
the tough outer skin is thickest, and most felt where that skin is
thinnest, and the nerves are most plentifully distributed near the
surface.
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