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.
Strong bottles, jugs, and jars, if they can be securely stoppered, may be used sometimes instead of hot water bags. The same precautions are necessary. Bricks, flat irons, or thick flannel bags containing salt or sand may be heated in the oven and used in the same way. Salt and sand retain heat for a long time, but are correspondingly slow to heat; therefore one bag should be heating in the oven while the other is in use. Their effect on the skin must be no less carefully watched than the effects of other hot applications.
Hot dry flannel may be used without fear of burning a patient, and it sometimes yields sufficient
warmth to relieve pain, particularly abdominal pain of babies. After it has been heated on a radiator or in an oven, it should be applied quickly and covered closely with another flannel to prevent escape of heat.
Dry heat can be applied conveniently by an electric pad. The part to be heated may be wrapped in flannel or placed directly above or below the pad. The pad should be carefully watched to see that the switch is not accidentally turned, as it is possible for the pad to become hot enough to burn the patient or to set fire to the bed covers.