The corrosive and narcotico-acrid poisons may produce by this local action a degree of irritation sufficient to cause death. In the case of the first, some erosion of the mucous surface may occur. By both kinds violent vomiting and purging is apt to be produced, and succeeded by symptoms of collapse. These last, however, are not _therapeutic_ agents, when in such doses. Antidotes, employed to counteract these poisons, are remedies which are given to act locally in extraordinary cases. There are three chief kinds of them; Demulcents, to sheath the irritated surface, and protect it from further injury; Emetics and Purgatives, to get rid of the poison; and chemical antidotes, to neutralize it or render it insoluble while in the stomach. With this last object, acids are given in alkaline, and alkalies in acid poisoning. The soluble salts of Lead may be precipitated and rendered insoluble by sulphuric acid or sulphates. Those of Mercury, Copper, and Zinc, by albumen. Tannic acid precipitates the vegetable alkaloids. There are some other special antidotes of the same kind. Let us now consider the three kinds of remedial agents in ordinary use, which are employed for the purpose of producing a local effect on the mucous surfaces, before absorption, or without absorption. _a. Irritant emetics._--Two kinds of medicines are employed to produce Vomiting,--specific emetics, and irritant emetics. The former act from the blood; the latter, by local irritation. In the same way that irritation of the external surface of the body will sometimes cause at the same time the _direct_ contraction of a neighbouring muscle, and the _reflex_ contraction of others at a distance, so does local irritation operate on the surface of the stomach. On the one hand the muscle of the stomach itself is caused to contract, so that, the pylorus being at the same time forcibly closed, it tends to expel its contents in the wrong direction. On the other hand, a large set of distant muscles is thrown into sudden action. First, a quick deep breath is taken by means of the inspiratory muscles. Then the aperture of the glottis is spasmodically closed, so that, the lungs being full, the diaphragm cannot be pushed upwards. Then immediately the abdominal muscles contract, and being unable to act on the diaphragm, they press on the stomach, emptying it forcibly of its contents. All this is by reflex action, and follows sympathetically the contractions of the stomach, co-operating with it, and resulting, like it, from irritation of the sensitive mucous surface.[27] Such is the action of an irritant emetic.