The skin should be viewed as a complex organ with a number of distinct functions. In fact no other organ in the body has so many and varied duties to perform and none is more constantly exposed to so many injurious influences. To a certain extent the skin and its functions are essential to the life of the individual. If these functions be long suspended, or any considerable portion of the skin be destroyed, health is impossible and life doubtful; whereas one may enjoy a long period of comparative health after the loss of an extremity, an eye or a kidney. The intestinal tract, kidneys, lungs, and skin comprise the four emunctories of the body. To some extent partial suppression of the excretory action of any one of these four is equalized by extra activity on the part of the other three. A full appreciation of this fact may impress upon the student the importance of the excretory function of the skin which, strange as it may seem, is little appreciated. The physiological functions to be considered are those of protection, secretion, heat-regulation, respiration and sensation.
Protection may be styled the chief passive function of the skin and is easily explained when the nature of the epidermis, corium, subcutaneous tissue and their varied contents are noted. The epidermis, because of its thickness and impermeability and the insensibility of the corneous layer, serves to protect the body from the injurious effects of extreme temperature and from the absorption of many poisonous substances. The oily glandular secretion partially aids in this respect by preventing too great evaporation of water from within. The corium through its firmness, flexibility and elasticity, and the subcutaneous tissue with its loosely bound bundles of fibrous tissue and fat serve well to protect the muscles, nerves and blood-vessels from all external traumatic agencies. The weakest points in the line of defence offered by the skin are the glandular orifices. However, in the case of the sweat and in the pilosebaceous system the secretion which is decidedly pasty and fatty is somewhat defensive. The abundant hairy growth on the head serves as an additional protection for the brain.
The sebaceous and sweat-glands are the secretory organs of the skin. They furnish oil and moisture to make the skin soft and pliable, and assist in the regulation of the body temperature and in giving off waste material.
Sweat is secreted in the coil or gland proper and finds its way through a duct to the surface. Ordinarily it is rapidly evaporated from the surface in the form of vapor so that its presence is not perceived (insensible perspiration). If, on the other hand, evaporation is retarded or the secretion markedly increased, sweat accumulates on the surface in drops (sensible perspiration). In health sweating is more pronounced in some parts of the body than in others, such as the palms, soles, face, neck, axillae and genitocrural regions, due either to more numerous and larger glands or to a normal increase of heat in the parts specified. The quantity of water vapor normally given off by the skin varies from one to two pounds daily, and is nearly double that eliminated by the lungs. The nervous system is an undoubted factor in the production of sweat, because its secretion is dependent upon nerves, the centers of which are situated in the spinal cord as far up as the medulla oblongata, which probably contains the general center. These centers may be directly or reflexly stimulated, and act, through the local nerve-fibers, directly on the epithelia of the sweat-glands. Sweat is increased by external heat, changes in the blood, and by the ingestion of such drugs as camphor, pilocarpin and strychnine. Pilocarpin and some other alkaloids are believed to stimulate the secretion of sweat by acting directly on the peripheral nerves. The secretion of sweat is diminished by cooling the skin, by suspension of the blood supply and by such drugs as atropine, morphine, etc. A normal increase of sweat is attended with increased activity of the local circulation, but in abnormal conditions the perspiratory nerves may act independently of the vasomotor system and a free secretion of sweat occur when the skin is pale and cold (cold sweat).