Vomiting is a reflex act produced by many causes. It is a very complex performance, and it is governed by one of the nerve "centres" in the medulla. The vomiting centre is easily excited by various things, such as tickling the throat, an irritation of the stomach, liver, bowels, or peritoneum, and even of some parts of the body which have no direct connexion with the stomach. It will be as well to give a list of the chief causes of vomiting other than gastric diseases. Mechanical causes, such as tickling the throat, or violent coughing after a meal, are very liable to produce vomiting. Irritation of the organs of special sense by offensive odours, horrible sights, or disagreeable sounds, will cause vomiting. These modes of irritation do not have the same effect on everybody; thus, one person may be caused to vomit by the odour of apples, and another by some sort of perfume which would not be disagreeable to the average individual. Severe pain arising from the passage of a gallstone, renal calculus, dysmenorrhea, or from an injury to some part of the body, is liable to cause vomiting. It occurs in inflammation of the kidneys, and especially in uraemic poisoning. It is commonly a symptom of meningitis, tumours of the brain, neurasthenia, hysteria, and functional derangements of the nervous system. It likewise occurs at the onset of various infectious diseases, especially in children. Pregnancy is a common cause, and it may be the result of various drugs. Some emetics, such as tartar emetic and antimony wine, are carried in the blood stream and act by directly irritating the vomiting centre; indeed, they will produce vomiting when injected into the blood; others, like mustard, act indirectly by irritating the mucous membrane of the stomach; and still others by their nauseous taste or odour act upon the vomiting centre indirectly, i.e. through the higher nerve centres or organs of special sense.

When a person is about to vomit there is usually a profuse flow of saliva, the skin becomes pale and moist with perspiration. The actual process of vomiting is attended by a full inspiration, a contraction of the diaphragm, and a simultaneous spasm of the muscles of the abdomen; at the same time there is a relaxation of the cardiac sphincter and some contraction of the stomach itself, and the combined pressure produces an evacuation of its contents.

When vomiting occurs in dyspepsia, using this word in an extended sense, it usually occurs soon after a meal, that is within half an hour, but it may be delayed an hour or two. It does not occur in every dyspeptic patient, and in some patients it occurs only at long and irregular intervals. The vomited matters necessarily vary in appearance with the time which has elapsed since the consumption of the food. In some cases the food is very little altered, and the description usually given is that "it came back just as she took it". In other cases the vomiting does not occur until long after the meal is eaten; the material brought up has probably undergone fermentation, has a fermented or a sour smell, the odour of rotten eggs, of putrefaction, or of faeces; it presents a brown frothy scum and more or less sediment when it has been standing. The quantity of material brought up varies considerably. It may be little more than regurgitation or "repeating of the food," which is very similar to the normal rumination of cattle and other animals who "chew the cud"; or it may be an eructation of a little sweetish or sour fluid and wind. There is every variation in the quantity between this and the enormous volumes which are brought up from a badly dilated stomach. When vomiting occurs while the stomach is empty, that is without food, the material which comes up consists chiefly of saliva and ropy mucus or, perhaps, bile; in some cases it consists of an acid fluid, sometimes gastric juice sour enough to set the teeth on edge. The vomiting in these various cases should be distinguished from each other more clearly.

In acute gastritis vomiting is frequently the first symptom complained of. It occurs repeatedly and becomes painful and distressing to the patient. The vomit consists first of food, and any attempt at feeding induces it. But it goes on even when the stomach is empty, and continues, attended by nausea, violent retching, and painful hiccup, until mucus comes up streaked with blood or mixed with bile.

In chronic gastritis the vomiting is different. It is a common but not a constant symptom. It nearly always has reference to the food, and seldom occurs when the stomach is empty. It usually comes on after meals, and may occur two or three times a day or only at intervals of months. It is not usually succeeded by retching and hiccup, but by complete relief of the pain and other disagreeable symptoms which the food may have induced. The vomiting, however, may be due to chronic gastro-duodenal catarrh, and is then liable to take the form of "morning sickness," especially in alcoholic subjects. It occurs before food is taken, and the material brought up consists of saliva which was swallowed during the night, possibly some remnants of the last meal, and mucus. The latter varies in amount from a few pellets of glairy substance floating in a scanty fluid to an abundance of ropy material whose character is best observed by pouring it from one vessel into another. If there is much food in the "vomit before food" it is an indication of considerable delay in the digestive process, and enfeeblement of the motor power of the stomach, and possibly more or less dilatation.

In dilatation of the stomach the vomiting is again different. The typical vomiting of dilatation is very easy and free; it is not usually preceded by nausea, but there is a good deal of discomfort and possibly some pain in the region of the umbilicus. The vomiting does not occur every day as a rule; as a matter of fact it does not occur for a week or two together in many cases of dilatation, but when it does happen the amount brought up is very abundant, varying from two to eight pints, and comes up so easily as to give the impression of its being "pumped up". The vomit is often exceedingly irritating to the throat and mouth, and causes a burning sensation in the chest and throat and sets the teeth on edge. It consists of a variety of materials, and includes undigested articles of food eaten perhaps a week before; it is sour, fermented, and offensive to the smell. But the relief which is experienced by the removal of the accumulated food, secretions, and products of fermentation is so great that patients sometimes voluntarily provoke vomiting by irritating their throat with their fingers or a feather.