The mixed and powdered dried ripe seeds of Sinapis alba and Brassica nigra, grown in temperate regions.
It is a pale-yellow or colorless fluid, of intensely pungent and penetrating odor, burning taste, and a blistering and corrosive action on the skin.
White mustard seeds do not possess this volatile oil, but contain a rubefacient principle which resembles it, and which is set free in the same way on the addition of water.
Applied externally, as in poultices, baths, etc., mustard is a nerve stimulant, rubefacient, and vesicant, causing redness, heat, and severe burning pain, and, if long applied, blistering. Relief of previous pain and loss of sensibility to other impressions follow the action of mustard. The heart, blood-pressure, respirations, and nerve centres generally are first stimulated, then quieted, and, if vesication has resulted, depressed, even seriously. In baths, mustard dilates the vessels of the skin and relieves the blood-pressure in congested organs.
The principles of counter-irritation are, first: that, by causing dilatation of the vessels of that part to which application is made, there is contraction of the vessels in other parts of the body, especially if there is nervous connection between the two, and following this principle, counter-irritants are usually applied at some distance from the congested part, as in the use of footbaths; second, that, by increasing the activity of the circulation, counter-irritants promote the reabsorption of inflammatory products, and for this purpose they are sometimes applied directly over the affected region.
In making mustard paste, the addition of the white of an egg modifies the local irritant action, making it more easily borne by the skin, and less liable to injure its structure, while it does not interfere with the physiological effect. The paste should be mixed only with tepid water, as hot water dissipates the volatile oil, vinegar destroys it, and alcohol prevents its formation. Internally, mustard in small amount, as taken with food, stimulates the appetite and gastric circulation. In large doses it is a rapidly acting stimulant emetic, leaving little or no depression.
The action of mustard externally must always be specially watched with comatose patients, as injury to the skin does not show at first in a state of sluggish circulation, and may become serious before it is noticed. There is an official mustard plaster, Emplastrum Sinapis, which is to be moistened thoroughly with tepid water before applying. It reddens the skin within five minutes.
Average emetic dose, ʒ iiss.-10 Gm.