irculating through it, corresponding to the changes in
the temperature outside of the body. These variations are of great
importance, having to do with the

*Maintenance of the Normal Temperature.*--It is necessary to the
continuance of life that the temperature of the body be kept at a nearly
uniform degree, called the _normal temperature_, which is about 98.6 deg. F.
The maintenance of the normal temperature depends mainly upon four
conditions: the chemical changes at the cells, the circulation of the
blood, the nervous system, and _the skin_. The chemical changes produce
the heat, the blood in its circulation distributes the heat over the body,
and the nervous system controls the heat-producing and distributing
processes (page 320). The skin is the chief means by which the body gets
rid of an excess of heat and, by so doing, avoids overheating. (91)

*How the Skin cools the Body.*--The skin is a means of ridding the body of
an excess of heat in at least two ways:

1. _By the conduction and radiation of heat from its surface_ as from a
stove. This goes on all the time, but varies with the amount of heat
brought to the surface by the blood.

2. _By the evaporation of the perspiration._ It is a well-established and
easily demonstrated principle that liquids in evaporating use up heat.(See
Practical Work.) It is also a matter of everyday experience that the
perspiration has a cooling effect upon the body and that its flow
increases with the amount of heat to be gotten rid of. The quantity of
perspiration secreted, and of heat disposed of through its evaporation,
also varies with the amount of blood circulating through the skin.

*Temperature Regulation by the Skin.*--Variations in the quantity of blood
circulating through the skin enable this organ to throw off just the right
amount of heat for keeping the body at the normal temperature. If it is
necessary for the body to rid itself of an excess of heat, the quantity of
blood circulating in the skin is increased. This brings the blood near the
surface, where more heat can be radiated and where it may cause an
increase in the perspiration. On the other hand, if the body is in danger
of losing too much heat, the circulation diminishes in the skin and
increases in the internal organs. This stops the rapid loss of heat from
the surface. The skin in this work is of course made to cooperate with
other parts of the body. That it is not the only organ concerned in
regulating