The term hydrotherapy will be used here in its broadest sense, and may be defined as the hygienic and systematic use of water, both externally and internally, for the preservation and restoration of health and the prevention of disease. The hygienic and therapeutic value of the systematic use of water is just beginning to be appreciated by the medical profession. When this newly acquired knowledge is put to practical use by the great masses of the people, there will be a greatly diminished necessity for the use of drugs. Indeed, water has been pronounced by a high medical authority to be, and probably is, more nearly a panacea for all human ills than any other known agent. The bath is generally considered merely as a cleansing procedure, whereas this is only one of its beneficial effects. There is, in addition, the stimulation of all the functions and organs of the body obtained through the temperature of the water, and the mechanical stimulation which is obtained by the mode of application. Again, the skin is not a mere covering for the body, but one of its most important organs, with well-defined functions; so that, in order to obtain a clear understanding of the subject, it is necessary first to consider briefly the functions of the skin; and, secondly, the physiologic action of water. =Description of the Skin.=—The skin is a very sensitive and complex organ, and upon the condition of the skin and the vicissitudes to which it is exposed the health of the individual is dependent to a very great degree. [Illustration: Fig. 1.—Vertical section of skin. _sbg_, Sebaceous glands; _ep_, epidermis; _h_, hair; _d_, derma (Fox).] The skin is composed of three distinct layers—the epidermis, the corium or true skin, and the subcutaneous connective tissue. The appendages of the skin are the hair, nails, the sebaceous and sweat-glands. This complicated structure is supplied with blood-vessels, lymphatics, and nerves. _The Epidermis._—The outer layer of this is the horny layer; when a blister is formed, its fluid raises the entire epidermis from the true skin. The flat scales forming the horny layer are continually being thrown off; this process of desquamation is increased by the friction of the clothes, of bathing, massage, and so forth, and is as constantly being replaced by new cells from underneath. The _corium_, or true skin, is the most important part of the integument. This is a thick, felt-like tissue which is pierced in all directions for the passage of the blood-vessels, lymphatics, sweat-ducts, and nerves, and affords lodgment for the hair follicles and sebaceous glands. The tension of the skin is produced by its muscular structure and elastic network, and is subject to temperature changes. This power of contractility is known as the tone of the skin.