These are substances which increase the appetite and aid gastric digestion.

From observations made on the stomach in persons or animals where a gastric fistula has been present, it has been found that in the normal condition, when the stomach is empty and quiet, the mucous membrane is of a pale rose colour. When stimulated mechanically, by rubbing it gently with a feather or glass rod, the mucous membrane becomes redder, and such abundant secretion of gastric juice occurs that it runs down in drops along the walls of the stomach.

1 Jolyet, Gaz. Med. de Paris, 1877.

When the irritation is greater - as, for example, when the mucous membrane is rubbed roughly instead of gently - an opposite effect is produced. The vessels then contract, the mucous membrane becomes pale, and the secretion of gastric juice stops, secretion of mucus commences, and if the irritation be carried still further, vomiting occurs.

Almost all substances which, when applied to the skin, act as irritants, as arsenic and salts of copper, silver, or zinc, and those also which, without irritating the skin, irritate the nerves of taste, as bitters, produce a feeling of appetite in the stomach, but they only do this in certain conditions of the stomach, and in certain quantities. The appetite appears to be associated with gentle stimulation of the gastric walls; stronger stimulation destroys the appetite, still greater irritation causes nausea and, lastly, vomiting.

Fig. 127.   Diagram to illustrate the supposed nervous connections of the stomach. A gentle stimulus applied to the walls of the stomach is transmitted by the afferent nerves, A, to a nerve centre, B, and thence along the vaso dilating nerves, c, and the secreting nerves, D,to the vessels of the mucous membrane and the cells of the gastric follicles.

Fig. 127. - Diagram to illustrate the supposed nervous connections of the stomach. A gentle stimulus applied to the walls of the stomach is transmitted by the afferent nerves, A, to a nerve-centre, B, and thence along the vaso-dilating nerves, c, and the secreting nerves, D,to the vessels of the mucous membrane and the cells of the gastric follicles. A stronger stimulus is transmitted up to the nerve-centre, E, and thence along the vaso-constricting fibres, F, and the secreting fibres, D, of the mucous follicles. A still stronger stimulus is transmitted to h, and thence along the motor nerves to the abdominal walls, K k, causing them to contract and produce retching or vomiting.

In cases of atonic dyspepsia, where the stomach is below par, as, for instance, in anaemia and debility, slight stimulants or irritants produce appetite.

In such cases, where the tongue is usually smooth and flabby, bitters and metallic salts are useful. But when the stomach is already too irritable, and the tongue is red with enlarged papillae, such substances are likely to irritate still more, and thus, instead of increasing the appetite, to diminish it, and produce nausea. The increased irritability of the stomach which precedes a bilious attack is often signalised by an unusually good appetite, which continues during the meal, so that food is eaten with relish. A still greater irritability is characterised by a great appetite before meals, which disappears, giving place to anorexia as soon as a few mouthfuls have been swallowed, and the gastric irritation heightened by the increased circulation consequent on the introduction of the food. In such cases, bitters are likely to do harm, and gastric sedatives, such as bismuth, are required.

The stomach has not merely to receive food, it has to digest it, and in the process of digestion there are three factors : 1st, secretion of the gastric juice which is to render the food capable of absorption and of assimilation; 2ndly, movements of the stomach to break up the food and mix it thoroughly with the solvent juice; and 3rdly, absorption of the products of digestion.