240.  Nature and Properties of Sweat.  The sweat is a turbid, saltish fluid with a feeble but characteristic odor due to certain volatile fatty acids.  Urea is always present in small quantities, and its proportion may be largely increased when there is deficiency of elimination by the kidneys.  Thus it is often observed that the sweat is more abundant when the kidneys are inactive, and the reverse is true.  This explains the increased excretion of the kidneys in cold weather.  Of the inorganic constituents of sweat, common salt is the largest and most important.  Some carbon dioxid passes out through the skin, but not more than 1/50 as much as escapes by the lungs.

The sweat ordinarily passes off as vapor.  If there is no obvious perspiration we must not infer that the skin is inactive, since sweat is continually passing from the surface, though often it may not be apparent.  On an average from 1-1/2 to 4 pounds of sweat are eliminated daily from the skin in the form of vapor.  This is double the amount excreted by the lungs, and averages about 1/67 of the weight of the body.

The visible sweat, or sensible perspiration, becomes abundant during active exercise, after copious drinking of cold water, on taking certain drugs, and when the body is exposed to excessive warmth.  Forming more rapidly than it evaporates it collects in drops on the surface.  The disagreeable sensations produced by humid weather result from the fact that the atmosphere is so loaded with vapor that the moisture of the skin is slowly removed by evaporation.

  Experiment 124.  Study the openings of the sweat glands with the aid
  of a strong magnifying glass.  They are conveniently examined on the
  palms.

A man’s weight may be considerably reduced within a short time by loss through the perspiration alone.  This may explain to some extent the weakening effect of profuse perspiration, as from night sweats of consumption, convalescence from typhoid fever, or the artificial sweating from taking certain drugs.

241.  The Skin as a Regulator of the Temperature of the Body.  We thus learn that the skin covers and protects the more delicate structures beneath it; and that it also serves as an important organ of excretion.  By means of the sweat the skin performs a third and a most important function, viz., that of regulating the temperature of the body.

The blood-vessels of the skin, like those of other parts of the body, are under the control of the nervous system, which regulates their diameter.  If the nervous control be relaxed, the blood-vessels dilate, more blood flows through them, and more material is brought to the glands of the skin to be acted upon.  External warmth relaxes the skin and its blood-vessels.  There results an increased flow of blood to the skin, with increased perspiration.  External cold, on the other hand, contracts the skin and its blood-vessels, producing a diminished supply of blood and a diminished amount of sweat.