The third element in the action of cold is more purely mechanical, and
this, though in a sense secondary, is of immense import. When any
body, capable of expansion by heat, that is to say, by radiant motion
of its own particles, is reduced in temperature, it loses volume,
contracts, or shrinks. The animal body is no exception to this rule; a
ring that will fit tightly to the warm finger will fall off the same
finger after exposure to cold. The whole of the soft parts shrink, and
the vessels contract and empty themselves of their blood. Cold applied
to the skin in an extreme degree blanches the skin, and renders it
insensible and bloodless, so that if you prick it it does not bleed,
neither does it feel. In cases where the body altogether is exposed to
extreme cold this shrinking of the external parts is universal; the
whole surface becomes pale and insensible; the blood in the small
vessels superficially placed is forced inward upon the heart and
vessels of the interior organs; the brain is oppressed with blood;
sleep, or coma, as it is technically called, follows, and at last life
is suspended.
In exposure to the lowest wave of temperature in this country these
extreme effects are not commonly developed; but minor effects are
brought out which are most significant. In particular, the effect on
the lungs is strongly marked. The capillary vessels of the lungs,
making up that fine network which plays over the computed six hundred
millions of air vesicles, undergo paralysis when the cold air enters,
and in proportion as such obstruction from this cause is decisive, the
blood that should be brought to the air vesicles is impeded, and the
process of oxidation is mechanically as well as chemically suppressed.
The same contraction is also exerted on the vessels of the skin,
driving the blood into the interior and better protected organs. Hence
the reason why on leaving a warm room to enter a cold frosty air there
is an immediate action of the visceral organs from pressure of blood
on them, and not unfrequently a tendency to diarrhoea from temporary
congestion of the digestive tract. Three factors are at work, in fact,
whenever the low wave of temperature affects the animal body;
abstraction of heat from the body, beyond what is natural; arrest of
chemical action and of combustion; paralysis of the minute vessels
exposed to the cold.