will tend to cool the body. On the other hand, when by the same
mechanism the cutaneous vessels are constricted, there will be a smaller
flow of blood through the skin, which will serve to check the loss of heat
from the body (secs. 195 and 270).

Again, the special nerves of perspiration act directly as regulators
of temperature. They increase the loss of heat when they promote the
secretion of the skin, and diminish the loss when they cease to promote
it.

The practical working of this heat-regulating mechanism is well shown by
exercise. The bodily temperature rarely rises so much as a degree during
vigorous exercise. The respiration is increased, the cutaneous capillaries
become dilated from the quickened circulation, and a larger amount of
blood is circulating through the skin. Besides this, the skin perspires
freely. A large amount of heat is thus lost to the body, sufficient to
offset the addition caused by the muscular contractions.

It is owing to the wonderful elasticity of the sweat-secreting mechanism,
and to the increase in respiratory activity, and the consequent increase
in the amount of watery vapor given off by the lungs, that men are able to
endure for days an atmosphere warmer than the blood, and even for a short
time at a temperature above that of boiling water. The temperature of a
Turkish bath may be as high as 150 degrees to 175 degrees F. But an
atmospheric temperature may be considerably below this, and yet if long
continued becomes dangerous to life. In August, 1896, for instance,
hundreds of persons died in this country, within a few days, from the
effects of the excessive heat.

A much higher temperature may be borne in dry air than in humid air, or
that which is saturated with watery vapor. Thus, a shade temperature of
100 degrees F. in the dry air of a high plain may be quite tolerable,
while a temperature of 80 degrees F. in the moisture-laden atmosphere of
less elevated regions, is oppressive. The reason is that in dry air the
sweat evaporates freely, and cools the skin. In saturated air at the
bodily temperature there is little loss of heat by perspiration, or by
evaporation from the bodily surface.

This topic is again discussed in the description of the skin as a
regulator of the bodily temperature (sec. 241).

227. Voluntary Means of Regulating the Temperature. The voluntary
factor, as a means of regulating the heat loss in man, is one of great
importance. Clothing retards the l