In the mouth they neutralise any acid present. They may thus relieve toothache due to irritation of the exposed nerve in a carious tooth or of the roots of the teeth close to the gums by acid secretions. A dilute solution of sodium bi-carbonate as a wash to the mouth frequently relieves soreness of the teeth, or headache depending on dental irritation, and prevents injury from acid tonics. Alkalis are used in the shape of borax to heal aphthae in the mouth and as soap for cleaning the teeth.

In the stomach they increase the amount of gastric juice secreted; and where this is deficient and the food lies heavy and is digested slowly and with difficulty, they should be given before a meal or just at its commencement, either in the form of a medicinal mixture or as aerated potash or soda water. The amount of acid secreted by the stomach after their introduction is sufficient to neutralise them pretty rapidly, and probably only the caustic alkalies which act very rapidly have time to produce any local action before they are neutralised, unless large quantities have been ingested. Where there is a large amount of mucus on the surface of the stomach it will both hinder the exit of the gastric juice from the follicles and the entrance of the peptones from the stomach into the blood. Caustic alkalies have a great power of dissolving mucus. They probably do this to some extent before they are neutralised, and this may be the reason why we occasionally find that they are of great service when a corresponding amount of their carbonates does little or no good. From the effect they produce on the secretion of gastric juice, alkalis in small doses are said to act as gastric stimulants (p. 363).

When the amount of acid in the stomach is too great, either because too great a proportion of it has been present in the gastric juice, or because it has been generated by the decomposition of food, digestion goes on slowly, and burning acid eructations take place after meals. In such cases we give alkalis to neutralise the excess and to restore the proportion of acid in the stomach to its normal. They are then said to act as antacids (p. 369).

Alkalis are serviceable as antidotes in poisoning by acids, metals, and alkaloids. They neutralise the acids, they precipitate the metals as insoluble oxides, and they render alkaloids less soluble by taking away the acid with which they are generally combined. They thus retard their absorption and afford time for the use of other means.

The chyme from the stomach is normally acid, and will therefore act as a stimulus to the expulsion of bile from the gall-bladder. It is partly neutralised by the bile and pancreatic juice, but generally remains acid throughout the small intestines and will act as a stimulus to the secretion of intestinal juice. If it be neutralised by alkalis in the stomach, this stimulus will be removed and digestion consequently impaired. Many substances will thus pass through the intestinal canal undigested, which amounts to the same thing as if less food had been taken.

Through this derangement of the digestion the blood will become poorer in solids, the person will become emaciated, the fat will naturally be first absorbed, and, along with this, perhaps pathological formations may also disappear.

The excessive use of alkalis or their carbonates is thus injurious, and their employment to reduce obesity may, unless carefully watched, be followed by serious consequences, like the use of acids for a similar purpose (p. 569).

Caustic alkalis injected directly into the blood cause death in a few minutes, probably from formation of alkali-albuminate in the blood and its consequent coagulation. Shortly after death the blood is found coagulated. Smaller amounts taken in from the stomach will to some extent increase the alkalinity of the blood, but are rapidly separated by the kidneys. They cause thirst, and probably the larger amount of water drunk in consequence is one cause of the diuresis they produce. From their power of dissolving fibrin outside the body, they have been given in acute rheumatism to prevent fibrinous deposits on the heart. It is not certain that the amount we can introduce into the blood without injury to the patient has this effect.

After small doses of liquor potassae the urea and sulphuric acid in the urine are increased, and Parkes therefore thinks that the tissue-change of the albuminous substances is increased. Alkalis are therefore classed as alteratives (p. 414).

They are used both to increase the amount of water passed and to diminish its acidity if this be excessive. They are therefore classed amongst diuretics (p. 432), and remote antacids (p. 370).

General Action of the Group of Chlorides. - Chloride of sodium is not only one of the most abundant saline constituents of the animal body, but it is one of the most important solvents of albuminous substances. Water will dissolve albumins proper, but globulins are insoluble in it, and are precipitated by it from solutions. Dilute solutions of chloride of sodium on the contrary dissolve both albumins and globulins. From this action of water on albuminous substances it is very irritating when applied to a cut surface, or to the delicate mucous membrane of the nose, while muscles dipped in it swell up, and pass into a state of rigor. Weak solutions of chloride of sodium, on the other hand, have no irritating action, and may be applied to cut surfaces or mucous membranes without causing pain, and to muscle and nerve without producing any injurious effect. A solution of the strength of 0.65 per cent. is the one usually employed in physiological experiments as a basis for the nutritive fluid in artificial circulation through the frog's heart or vessels, and as a solvent for alkaloids which are to be injected into the lymph-sac of the frog, in order to avoid the local irritation which the injection of a watery solution would produce. A solution of this strength is often called 'normal salt solution' in physiological treatises.