Now, it is a law of physics that the change from liquid to vapor involves a loss of heat.  A few drops of ether or of any volatile liquid placed on the skin, produce a marked sense of coldness, because the heat necessary to change the liquid into vapor has been drawn rapidly from the skin.  This principle holds good for every particle of sweat that reaches the mouth of a sweat gland.  As the sweat evaporates, it absorbs a certain amount of heat, and cools the body to that extent.

242.  How the Action of the Skin may be Modified.  After profuse sweating we feel chilly from the evaporation of a large amount of moisture, which rapidly cools the surface.  When the weather is very warm the evaporation tends to prevent the bodily temperature from rising.  On the other hand, if the weather be cold, much less sweat is produced, the loss of heat from the body is greatly lessened, and its temperature prevented from falling.  Thus it is plain why medicine is given and other efforts are made to sweat the fever patient.  The increased activity of the skin helps to reduce the bodily heat.

The sweat glands are under the control of certain nerve fibers originating in the spinal cord, and are not necessarily excited to action by an increased flow of blood through the skin.  In other words, the sweat glands may be stimulated to increased action both by an increased flow of blood, and also by reflex action upon the vaso-dilator nerves of the parts.  These two agencies, while working in harmony through the vaso-dilators, produce phenomena which are essentially independent of each other.  Thus a strong emotion, like fear, may cause a profuse sweat to break out, with cold, pallid skin.  During a fever the skin may be hot, and its vessels full of blood, and yet there may be no perspiration.

[Illustration:  Fig. 103.—­Papillae of the Skin of the Palm of the Hand.

In each papilla are seen vascular loops (dark lines) running up from the vascular network below, the tactile corpuscles with their nerve branches (white lines) which supply the papillae.]

The skin may have important uses with which we are not yet acquainted.  Death ensues when the heat of the body has been reduced to about 70 degrees F., and suppression of the action of the skin always produces a lowering of the temperature.  Warm-blooded animals usually die when more than half of the general surface has been varnished.  Superficial burns which involve a large part of the surface of the body, generally have a fatal result due to shock.

If the skin be covered with some air-tight substance like a coating of varnish, its functions are completely arrested.  The bodily heat falls very rapidly.  Symptoms of blood-poisoning arise, and death soon ensues.  The reason is not clearly known, unless it be from the sudden retention of poisonous exhalations.