Mechanism of Vomiting. Different opinions have been held as to the precise agency concerned in the act of vomiting. Some have maintained that it is effected exclusively by the contraction of the stomach; others that this organ is entirely passive, and that the diaphragm and abdominal muscles alone are concerned; others, again, that both act conjointly; and the last opinion is the one, I believe, now generally adopted. That the abdominal muscles and the diaphragm contract in the effort, is obvious to every one, acquainted with the anatomy of the parts, who has ever vomited himself, or seen another vomit. As to the stomach, it has been seen to contract at the moment its contents were discharged; and I am quite confident that I have felt it, in my own person, spasmodically contracting under the influence of an emetic. The combination of actions seems to be as follows; all occurring simultaneously. The rima glottidis is closed, so as to prevent expiration; the cardiac orifice relaxes, so as to permit the upward passage of liquids from the stomach; the pylorus contracts, and thus obstructs the passage downward; and, the diaphragm descending, and the abdominal muscles pressing inward, while the stomach itself contracts correspondingly, it necessarily follows that the contents of the stomach are driven upward, as in this direction is the only outlet. it is true that bile is sometimes thrown up, and even the contents of the small intestines, in certain rare instances; but, in these cases, the constriction which prevents the downward movement of the liquids, instead of being at the pylorus, is in the small intestine, at some point below that from which the inverted movement takes place. The closure of the glottis is necessary; as otherwise the contraction of the abdominal muscles would be in part expended in the expulsion of the air from the lungs, and the stomach would feel little of the pressure. There is no occasion for the intervention of an inverted peristaltic action, which some have imagined. The result is explicable in accordance with the most obvious physical laws. A cavity whose capacity is variable is compressed; and its liquid contents escape through the only existing outlet.*

* The case, as reported by Dr. Peacock, is contained in the Med. Times and Gaz. (Jan. 1865, p. 23). The patient was a young woman, who, while on her way in a carriage to an infirmary, on account of disease of the heart, but without any specially urgent symptoms, was suddenly seized with violent vomiting, followed immediately by complete collapse, with great dyspnoea, faintness, and an absent pulse. On reaching the hospital, she appeared as if in the dying state, from which, however, the system slightly reacted. But she soon relapsed, and died in a few hours. On examination, considerable valvular disease was discovered, with a complete detachment of the tendinous cords connecting one of the folds of the mitral valve, which thus floated loosely, and allowed free regurgitation. (Note to the third edition.)

* The above view of the mechanism of vomiting is, in a great degree, confirmed by the following case, reported, on the part of M. Patry, by M. Sappey, at a meeting of the Paris Academy of Medicine, June 16, 1863. A boy of 11 was gored by a bull, so that the abdomen was laid open, and the stomach, the spleen, and a part of the bowels were rendered visible. As vomiting occurred in the course of the case, an opportunity was given of witnessing the part which each organ performed in the process. The phenomena of vomiting took place in the following order; contraction of the diaphragm, vermicular contraction of the stomach, beginning at the pylorus and extending to the cardia; propulsion of the gastric contents towards the oesophagean extremity; energetic contraction of the oesophagus; retraction of the stomach at each effort; dilatation of the cardiac orifice through the agency of the longitudinal fibres of the oesophagus; and, finally, the filling of the tube by the liquids of the stomach, and vomiting. The agency by which, as here stated, the cardiac orifice is opened, appears to me doubtful. The longitudinal fibres may contract, but in which may accidentally enter, or morbidly accumulate in the stomach. Hence, the presence of acrid bile often provokes vomiting, as in cholera morbus; and acid matters have the same effect, as in sick-headache. Even spontaneous irritation or inflammation equally excites the nervous centres, and causes nausea and vomiting. irritant emetics are no doubt capable of operating in the same way.

Effective Cause of Vomiting. The muscular movements above explained are the mere mechanical agency by which vomiting is produced. The influence which calls the muscles into joint action is the real and effective cause. This cannot be the simple impression of the emetic upon the gastric mucous membrane; for, though such an impression might excite the peristaltic movement of the stomach by a direct transmission to its muscular coat, the movement would not produce vomiting. The cardia being closed, if any effect were produced, it would be to expel the contents of the stomach downward. it could not possibly call into harmonious co-operation all the agencies concerned in vomiting. The impression, therefore, must be conveyed to some common centre, capable of uniting and harmonizing all these movements. This centre is in the nervous system. it is probably in the medulla oblongata, and the neighbouring parts of the encephalon. it will be remembered that the expelling movements are not the whole of the phenomena. These are preceded by the sensation of nausea, which is an important part in the ordinary process of emetic action. An impression, therefore, is made on the stomach; this is followed by an impression on the nervous centres, probably in the medulla oblongata, and at the base of the brain; this excites the sensation of nausea, which is referred to the stomach, as any other sensation, a touch, for instance, is referred to the point first acted on; as a consequence of this disturbance in the nervous centres, an influence is sent forth through the efferent nerves, to the stomach, the diaphragm, the abdominal muscles, the muscles of the larynx, and the sphincter fibres of the cardiac and pyloric orifices, which brings them all into simultaneous and concordant operation. That this nervous communication is essential is proved by two facts. if the par vagum on both sides be divided, the stomach will not act; and it is well known that, when the brain is rendered quite insensible by opium, so as not to be able to feel the impression made by the emetic substance, no amount of it will produce vomiting.