The various muscular and nervous actions, going on constantly throughout life, derive the force necessary for their production from the oxidation of the various articles contained in our food, and as the oxidation of all its digestible constituents is really nearly as complete as if they had been subjected to combustion, the body gains nearly as much heat from the consumption of its food as if the latter were actually burned. In climates where the temperature of the air is less than that of the blood, a good deal of heat is conducted away from the body by its contact with the air; but where, as in hot climates, the difference is but small, or the temperature of the air may even exceed that of the blood, it is obvious that some further mechanism is required if the temperature of the body is to be kept at the normal level of health. This requirement is met by evaporation from the surface of the body, the amount of which is regulated automatically by a special set of nerves which are known as the vaso-motor system, whose function it is to regulate the calibre of the blood-vessels throughout the body, according to the varying nutritive necessities of the several organs to which they are distributed. We are all familiar with the fact that either extreme heat or violent exertion will alike bring about free perspiration, and in both cases the object is the same, viz., to cool down the body which tends to become overheated; in the one case by the warmth of the surroundings, and in the other by the activity of the chemical changes going on within the body to provide the force required for the various muscular and nervous actions involved in the work performed. On the other hand, under the influence of cold the skin becomes bloodless and dry, very little blood being allowed to circulate at the actual surface, the bulk of it being kept to the deeper parts of the body, well beneath the protective coating of fat which lies immediately below the skin. Buried in this fat, and opening by delicate tubes on the surface, are enormous numbers of small glandular bodies--the sweat-glands--each of which, when sufficiently freely supplied with blood, pours out a fluid consisting mainly of water, but containing also a little common salt and minute quantities of other mineral constituents, as well as a trifling amount of organic or animal matter, which has served its purpose in the organism and is thrown off in this way as being of no further value.