[Illustration:  Fig. 96.—­Diagrammatic Scheme to illustrate in a very General Way Absorption and Excretion.

  A, represents the alimentary canal;
  L, the pulmonary surface;
  K, the surface of the renal epithelium;
  S, the skin;
  o, oxygen;
  h, hydrogen,;
  n, nitrogen.
]

233.  The Skin.  The skin is an important and unique organ of the body.  It is a blood-purifying organ as truly as are the lungs and the kidneys, while it also performs other and complex duties.  It is not merely a protective covering for the surface of the body.  This is indeed the most apparent, but in some respectes, the lest important, of its functions.  This protective duty is necessary and efficient, as is proved by the familiar experience of the pain when a portion of the outer skin has been removed.

The skin, being richly supplied with nerves, is an important organ of sensibility and touch.  In some parts it is closely attached to the structures beneath, while in others it is less firmly adherent and rests upon a variable amount of fatty tissue.  It thus assists in relieving the abrupt projections and depressions of the general surface, and in giving roundness and symmetry to the entire body.  The thickness of the skin varies in different parts of the body.  Where exposed to pressure and friction, as on the soles of the feet and in the palms of the hands, it is much thickened.

The true skin is 1/12 to 1/8 of an inch in thickness, but in certain parts, as in the lips and ear passages, it is often not more than 1/100 of an inch thick.  At the orifices of the body, as at the mouth, ears, and nose, the skin gradually passes into mucous membrane, the structure of the two being practically identical.  As the skin is an outside covering, so is the mucous membrane a more delicate inside lining for all cavities into which the apertures open, as the alimentary canal and the lungs.

[Illustration:  Fig. 97.—­A Layer of the Cuticle from the Palm of the Hand.  (Detached by maceration.)]

The skin ranks as an important organ of excretion, its product being sweat, excreted by the sweat glands.  The amount of this excretion evaporated from the general surface is very considerable, and is modified as becomes necessary from the varied conditions of the temperature.  The skin also plays an important part in regulating the bodily temperature(sec. 241).

234.  The Cutis Vera, or True Skin.  The skin is remarkably complex in its structure, and is divided into two distinct layers, which may be readily separated:  the deeper layer,—­the true skin, dermis, or corium; and the superficial layer, or outer skin,—­the epidermis, cuticle, or scarf skin.

The true skin consists of elastic and white fibrous tissue, the bundles of which interlace in every direction.  Throughout this feltwork structure which gradually passes into areolar tissue are numerous muscular fibers, as about the hair-follicles and the oil glands.  When these tiny muscles contract from cold or by mental emotion, the follicles project upon the surface, producing what is called “goose flesh.”