243.  The Skin and the Kidneys.  There is a close relationship between the skin and the kidneys, as both excrete organic and saline matter.  In hot weather, or in conditions producing great activity of the skin, the amount of water excreted by the kidneys is diminished.  This is shown in the case of firemen, stokers, bakers, and others who are exposed to great heat, and drink heavily and sweat profusely, but do not have a relative increase in the functions of the kidneys.  In cool weather, when the skin is less active, a large amount of water is excreted by the kidneys, as is shown by the experience of those who drive a long distance in severe weather, or who have caught a sudden cold.

[Illustration:  Fig. 104.—­Magnified View of a Sweat Gland with its Duct.

The convoluted gland is seen surrounded with big fat-cells, and may be traced through the dermis to its outlet in the horny layers of the epidermis.]

244.  Absorbent Powers of the Skin.  The skin serves to some extent as an organ for absorption.  It is capable of absorbing certain substances to which it is freely exposed.  Ointments rubbed in, are absorbed by the lymphatics in those parts where the skin is thin, as in the bend of the elbow or knee, and in the armpits.  Physicians use medicated ointments in this way, when they wish to secure prompt and efficient results.  Feeble infants often grow more vigorous by having their skin rubbed vigorously daily with olive oil.

A slight amount of water is absorbed in bathing.  Sailors deprived of fresh water have been able to allay partially their intense thirst by soaking their clothing in salt water.  The extent to which absorption occurs through the healthy skin is, however, quite limited.  If the outer skin be removed from parts of the body, the exposed surface absorbs rapidly.  Various substances may thus be absorbed, and rapidly passed into the blood.  When the physician wishes remedies to act through the skin, he sometimes raises a small blister, and dusts over the surface some drug, a fine powder, like morphine.

The part played by the skin as an organ of touch will be considered in sections 314 and 315.

Experiment 125. To illustrate the sense of temperature.  Ask the person to close his eyes.  Use two test tubes, one filled with cold and the other with hot water, or two spoons, one hot and one cold.  Apply each to different parts of the surface, and ask the person whether the touching body is hot or cold.  Test roughly the sensibility of different parts of the body with cold and warm metallic-pointed rods.

  Experiment 126.  Touch fur, wood, and metal.  The metal feels
  coldest, although all the objects are at the same temperature.  Why?