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.
Now, Tartar Emetic and Ipecacuanha do not act in this way. When injected into the blood elsewhere, in sufficient quantity, they are found to produce vomiting. They have also special actions on the heart and lungs, which are not possessed by merely irritant emetics. They seem to me to act specifically on the Vagus Nerve, which is supplied to these organs as well as to the stomach, and to cause vomiting by deranging its functions. By this action on the Vagus while in the blood, they excite, in a special way, the same reflex contractions which are produced, in the case of an irritant emetic, by irritation of the extremity of that nerve in the mucous membrane. They are thus Neurotics, or nerve-medicines. They are not gland-medicines; or, at least, there is no proof that they are excreted by the stomach, and thus they do not come under my definition of Eliminatives. All substances which touch the surface of the stomach cause it to pour out its secretion.