The most common dyspeptic trouble, and probably the most common human ailment, consists of the group of symptoms usually denominated indigestion or dyspepsia. No account of the diseases of the alimentary canal would be complete without separate consideration of them. These symptoms are abnormal sensations of different kinds, such as a sense of weight, fullness, sinking, shooting, aching, burning, or boring referred to the region of the stomach, the chest, or the back, especially between the shoulders. This discomfort arises in some cases when the stomach is empty and disappears when food is eaten; or it comes on after food and lasts a time varying from a few minutes to several hours; in other cases the discomfort is almost continuous. There is generally some flatulence and eructation of gas. The flatulence is often associated with the abnormal sensations; the eructation, or belching of gas, frequently gives considerable relief to the painful feelings or discomfort, and it is often noisy and uncontrollable. The belching may be accompanied by the regurgitation of food, or of a fluid having a sweet, sour, or neutral taste (water brash, pyrosis) varying in quantity from a teaspoonful to a pint. Vomiting, preceded by nausea, is sometimes a symptom. The appetite may be normal or deficient; the tongue clean and healthy, or more or less thickly coated; there may be constipation or diarrhea; the urine may be normal, or scanty, highly coloured, and giving a deposit. There are secondary phenomena, such as headache, giddiness, vertigo, restlessness, sleeplessness, mental depression, melancholia, palpitation or irregularity of the heart, etc. It must, however, be distinctly understood that, while dyspepsia is a name applied to this group of symptoms, there is no discoverable local manifestation in the stomach or constitutional effect which gives a warrant for the establishment of the group as a separate and distinct disease. As a matter of fact, "indigestion" or "dyspepsia" includes all the functional derangements of the stomach, and most of those of the bowels, as well as the liver. It occurs in the course of very many morbid states of the body which are independent of the stomach, and may have a constitutional origin or an origin in some part of the body quite remote from the stomach. Consequently, as pathological knowledge increased the term became limited in its applicability, while the diseases of the stomach which present the same symptoms were more definitely understood.

Nevertheless the stomach becomes deranged from many causes connected with the food itself, or with the conditions and circumstances under which it is consumed. Thus, if the food is not properly masticated, either because of undue haste in eating, defects or absence of teeth, soreness or ulcers of the tongue or mouth, or even from toughness of the food, there must necessarily be a delay in the disintegration of such foods in the stomach and consequently a delay in their passage into the bowels. Imperfect mastication is often one of the causes of indigestion; it may be the sole cause. In such cases disorder arises because the process of dissolution of the food is unduly prolonged; or because the gastric juice is unable to disintegrate the masses which have been swallowed, and consequently they are either vomited or discharged into the duodenum with pain and difficulty. This delay of food in the stomach is of importance, as will be seen later on; it means that the stomach is always at work, never empty; another meal being taken before the previous one is disposed of. The continued presence of food in the stomach is a source of irritation, it causes catarrh of the mucous membrane, an excessive secretion of gastric juice or of hydrochloric acid, and in the end may lead to dilatation of the stomach, or to atrophy of the gastric glands and a deficiency of the gastric juice.

An excess of food is injurious to the stomach in a similar manner. It throws a larger amount of work on the stomach in manipulating and disintegrating the food and passing it on to the duodenum. If large meals are very often taken, they not only disturb the stomach by keeping it constantly at work, but they cause an excessive secretion of gastric juice or else hyperacidity. Many people continue to take an excessive amount of food with impunity for years; but there are very few people who can always continue to do so: an excess of food does little harm to children or young adults, but in middle life it is a factor in the production of dyspepsia, for, like other overworked organs, the stomach becomes unable to do its duty; it loses its muscular tone; it ultimately becomes unable to secrete enough gastric juice, or either the pepsin or hydrochloric acid becomes deficient.

The excess may consist of some particular class of food. A large quantity of meat eaten regularly day by day is a great tax on the stomach, since the action of gastric juice is necessary for the digestion, which is completed by the pancreatic juice. The excess of meat is often accompanied by the consumption of an unusually small amount of bread, potatoes, and other vegetables; consequently, the body has to draw upon its store of fat for its supply of heat and energy, and the person becomes thin. At the same time extra work is thrown on the liver to metabolize the waste materials from the meat, and on the kidneys, which have to secrete them, and both liver and kidneys may ultimately suffer in consequence.

The consumption of too much fat is also likely to interfere with digestion. In the first place, it delays the disintegration and dissolution of the food which is necessary for its digestion; secondly, it checks the secretion of gastric juice; and, thirdly, a large quantity of fat leaves the stomach with difficulty, if at all, and may undergo decomposition.

The excessive consumption of carbohydrates, especially ordinary sugar (cane-sugar) and sweet foods, will cause catarrh of the mucous membrane of the stomach; and, by coating the food with mucus, prevent the access of the gastric juice to it. An excess of sweet cakes, and even ordinary bread, causes many people to have heartburn, acidity, flatulence, palpitation, and other disagreeable symptoms. If bread, toast, and similar foods are not thoroughly masticated, they do not undergo that transformation into dextrin and maltose (malt sugar) which is the ordinary result of salivary digestion. They therefore enter the stomach in the form of insoluble starch, which mechanically interferes with the action of the gastric juice; digestion is thereby delayed, the food is detained a long time in the stomach, and fermentation occurs, with the development of gas and irritating acids.

The presence of bran in wholemeal bread or coarse oatmeal and of fibre (cellulose) in vegetable foods is another cause of the indigestion of foods. Cellulose is the substance which gives firmness to vegetable structures: it is to them what gristle and bone is to animals. Therefore, one may easily understand the difficulty of digesting it. Moreover, it gets tougher and harder as it increases in age. Whereas the soft cartilages of a young animal become ossified, the cellular tissues of the vegetable become lignified - that is, they are turned into wood. This is observable by comparison of a young and an old carrot; a young carrot may be as tender as a potato, an old one as hard as match-wood. A similar comparison may be drawn between young and old kidney beans: the young pods are so tender that they snap readily, whence the Americans call them "snap beans" ; but old ones are stringy, the fibres becoming coated with a material which makes them as tough as horn. We can easily understand how it is that pain and even sickness may arise from the consumption of radishes, onions, carrots, turnips, cabbages, and various other vegetables, when we consider that the older they are the more lignin is deposited on the cells which form the fibres, and coats them with a substance like cork or wood, and renders them inaccessible to the digestive juices.

There is no doubt that condiments and spices are also detrimental when taken in excess. The agreeable sensation of warmth following the use of mustard, pepper, horseradish, is due to these substances causing the mucous membrane to be flushed with blood: the glands are thereby excited to secrete gastric juice and mucus; but the stimulation may become a chronic irritation, with hypersecretion or catarrh. A certain amount of vinegar is a useful addition to cabbage, green peas, kidney beans, and other vegetables: acid fruits are also useful to the body; but an excess of such acids results in a derangement of the digestive functions, which is easily observable when persons persistently eat lemons or take an immoderate quantity of lemon juice or lime juice. An excess of tea or coffee, especially if taken with meat or when digestion is at its height, is equally injurious. Foods which are too hot or too cold are also injurious, and are very liable to cause a catarrh of the stomach : whence arise the injurious effects of ices, ice-creams. Alcohol causes or aggravates every form of indigestion.