[Illustration: FIG. 24.--"THE HISTORY OF A BOIL" (concluded). The boil has finally ruptured. The liquid pus has escaped carrying with it the germs and most of their poisons; the migration of leucocytes has stopped; the capillaries are returning to normal size and now new tissue will grow and fill up this hole. (_From Emerson's "Essentials of Medicine."_)] Cold acts in just the opposite way. It decreases the size of the blood vessels so that less blood comes to cause pain and swelling; at the same time it diminishes the number of white blood corpuscles and the nutritive substance brought by the blood. The nature and location of the infection determine whether heat or cold is to be preferred. Counter-irritants, of which mustard is an example, have a complicated action. A counter-irritant affects the blood circulation of the place to which it is applied, and at the same time it irritates the superficial nerves, which in turn stimulate other more distant nerves. The latter nerves control the circulation in tissues not adjoining those to which the counter-irritant is applied, and thus it is possible for a mustard paste, for example, if applied at one point to bring about changes in the blood supply of another part of the body. The mechanism by which counter-irritation is brought about is an intricate nervous process called reflex action. HOT APPLICATIONS In applying either moist or dry heat the danger of burning or scalding a patient must be constantly kept in mind. This danger is always great, but it is especially great when the skin is tender like that of babies, children, and old people, or when the vitality is low as in cases of chronic or exhausting illness. Unfortunately accidents in applying heat are not uncommon; a moment's carelessness may cause serious injury and prolonged suffering. DRY HEAT.--Hot water bags are used to apply dry heat. They should be filled not more than two-thirds full of hot water, but the water must not be so hot that there is the slightest possibility of scalding the patient if the bag should leak. Boiling water should never be used. Before the stopper is screwed on, expel the air by squeezing the bag or by resting it upon a flat surface until the water reaches the top. After closing the bag make sure that both bag and stopper are in order, by noting whether leakage occurs when the bag is inverted and pressed moderately. Before it is placed near the patient the bag should be dried and entirely covered with a towel or canton flannel bag.