Other causes of vomiting are more obscure in their action. Association of ideas is a mental operation; yet a very frequent and certain cause of vomiting is, the recollection of objects connected with the evacuation of the stomach at a former period. Motion in a circle, in a ship, or in an unaccustomed direction, has the same effect. The motion of a wheel carriage, especially if the windows are closed, or the person sits in the back seat of a coach, will often produce vomiting. This effect, as connected with the changes in the common sensorium, must remain in obscurity.

The principle on which emetics act is not readily explained. It has been said that they are constantly sedatives; and, as plausibly, that they are always stimulant. Very powerful emetics belong to each class, yet, perhaps, a different principle influences their operation. The affection of the stomach is apparently increased action; but, in medicine, increased action is sometimes owing to a defect. It is, more obviously, irregular action; and we might thus attribute vomiting to the principle we have already endeavoured to establish, that irregular action is connected with a diminution of tone. We certainly, in this way, approach, at least, very near the truth; and the facts will in general support it. In every instance, however, except where vomiting is owing to an affection of the brain, there appears to be a substance inimical to the constitution, which the stomach, influenced by the vires medicatrices, attempts to discharge; and the necessary motions are consequently excited. Yet we must keep in our view, that languor and faintness, from any cause, will produce the 4 H same effect; and]we thus see why causes of extreme debility will equally excite this organ, independent even of the presence of any medicine, certainly by the intervention of any violent commotion. In this way may, probably, be explained the experiments of those who have excited vomiting by injections of emetic medicines into the veins. In fact, every foreign substance in the blood vessels excites such commotions, with faintings and convulsions; nor is it surprising that the stomach should equally suffer. In general, then, the most active emetics are the most powerful sedatives; and the whole class of poisons, particularly the narcotic cathartics, are violently emetic.

The motions of the stomach during the operation of emetics are, as we have said, inverted. This has been proved by ocular observation; and it is equally certain, that the action of the muscular fibres of the oesophagus is equally inverted. A nauseous draught, the repetition of an emetic, will sometimes excite the action of the oesophagus only; and we once saw it so permanently excited by a crystal of emetic tartar sticking in it, that the mildest fluids could not, for a long time, pass into the stomach. The action of the fibres of the stomach surrounding the cardia is, in some instances, exclusively excited: as in those who discharge wind, a small portion of acid, of oil, or any substance swimming on the surface of the contents of the stomach, and producing cardialgia. The more violent exertions of this organ alone discharge its whole contents; and such exertions must be strong and long continued before they are communicated to the duodenum. These are not facts merely of curiosity, but of great importance in the exhibition of vomits. It is in vain to expect benefit from them, if only the slight ineffectual discharge of a little of the tea, which has been drunk, takes place. The strain, such as arises from the action of the greater curvature, is necessary, if any viscid mucus is to be evacuated; if any effect on the liver can be expected. The evacuation of bile appears towards the end of the operation, sometimes after the interval of two or three hours; frequently on taking in the first draught of negus, or a similar cordial. The expediency of the remedy is then triumphantly pointed out; but, in reality, the bile was the effect, and was not previously in the stomach. The assisting actions of the diaphragm and abdominal viscera are sufficiently felt during the operation, if the facts were not ascertained by the experiments of Mr. Haighton.

These observations are of some importance in the administration of emetics. If the medicine is not for some time in the stomach previous to the vomiting, the whole organ is seldom excited. It has been usual to direct that the emetic shall be first discharged, probably from its apprehension of doing some injury. The practice is, however, proper, from its thus exciting every portion; but, as the vomiting, without some contents, is painful, on the first oppearancc of sickness a little camomile tea may be allowed. In the whole operation, however, if more than a half pint of any fluid is contained in the stomach atone time, the greater is the probability of its acting incompletely. In cases of poisons the vomiting is extremely violent, and we then only want to dilute, and to render the action as easy as is consistent with the discharge. The dry vomits, as they are called, where all drinking is precluded, are painful remedies, but of great importance in assisting the bronchial discharges, or in relieving visceral obstructions.

Opposed to the severity of dry vomits, are the milder nauseating doses of antimonials or squills. These assist, in some degree, the discharge from the bronchia, but not so effectually as full vomiting. Their chief advantages are in the earlier state of fevers; in which they, in some degree, contribute to relieve the dryness of the skin, and to mitigate, by this effect, the great heat.

The use of emetics is very extensive. In fevers of every kind they are most powerful remedies. In in-termittents, the vomiting, sometimes excited on the accession by nature, has taught us to lessen the violence of the paroxysm by emetics; and occasionally to prevent it, by their previous exhibition, and continuing to support the perspiration they excite. In every intermittent, and remittents also, we find bilious congestions, which active vomiting contributes to relieve. By this means the paroxysms of each gradually become milder; and there have been many instances where no other remedy was required.