In the mouth they cause a peculiar taste, and a feeling of roughness in the teeth. They cause an increased flow of saliva from the parotid, and of the thin saliva which the submaxillary secretes when the chorda tympani is irritated, but have no effect on the sympathetic saliva. They are therefore given to allay thirst in fever, the increased secretion of saliva which they provoke keeping the fauces moist (p. 357).

Acids stimulate the secretion of the alkaline saliva and intestinal juice, and excite the expulsion of bile from the gall-bladder. They are supposed generally to stimulate those glands whose secretions are alkaline. On the other hand alkalis stimulate the secretion of gastric juice, which is acid; and they are supposed to stimulate in general those glands whose secretion is acid. Professor Ringer supposes that the converse is also the case, and that acids and alkalis severally hinder the secretions of a like character. This supposition may be correct, and no doubt when an acid is present - e.g. in the stomach - it will neutralise any alkali which may be taken, and either retard its stimulant action on the gland or prevent it altogether, according to the relative quantities of acid present and of alkali employed. The presence of much alkali will also hinder the action of an acid stimulus in the same manner, but whether acids and alkalis have any further effect in hindering secretion than that just mentioned is uncertain.

Acids are partly neutralised by the saliva, and partly act as astringents on the mouth and fauces. They are thus used in congestion of the throat. As they corrode the teeth, they are generally given through a glass tube or quill, and the teeth should be rubbed with chalk afterwards.

Digestion in the stomach is accomplished by the action of pepsin along with dilute hydrochloric acid (.2 per cent. in man). This ferment only acts in presence of free acid; but the amount of acid necessary is different in different animals, being greatest in the carnivora (.3 per cent. HC1 in the dog) and least in the herbivora. Pepsin seems able to go on dissolving fibrin almost without a limit, but fresh acid must always be added. If the secretion is deficient, digestion goes on slowly and fermentation of the food takes place, causing the formation of other acids and liberation of gases.

The secretion of gastric juice may be stimulated by alkalis given just before meals; but if the stomach is so much out of order as not to respond to the stimulus, hydrochloric or phosphoric acid may be given after meals, alone, or with pepsin. In febrile conditions there is a deficiency of free acid in the stomach, although pepsin is present in plenty. In chronic gastric catarrh, especially when accompanied by dilatation, the free acid is greatly diminished, and in carcinoma of the stomach it would seem to be wanting in the great majority of cases. In such conditions, therefore, the administration of diluted hydrochloric acid is indicated.

For acid eructations and heartburn depending on excessive acidity of the gastric juice, acids should be given before meals (Ringer).

Some persons are troubled by eructations of sulphuretted hydrogen with a taste of rotten eggs. These persons have generally oxalic acid in the urine, and frequently suffer from depression of spirits. Such patients are benefited by acids, especially nitro-hydrochloric acid. Persons who suffer from dyspepsia and depression of spirits with oxaluria are also benefited by mineral acids, even when no sulphuretted hydrogen is present in the intestines.

When the use of acids is long continued they lessen the secretion of gastric juice, and produce a catarrhal condition of the mucous membrane of the stomach. They should therefore not be given for more than a week or two at a time. They should then be left off for a short time, or alternated with alkalis. Constant use of acid wines has a similar tendency to produce catarrh. Vinegar is sometimes drunk in order to lessen obesity or even plumpness. It has this effect by inducing gastrointestinal catarrh, but sometimes the derangement of the digestion occasioned by it has been so great as to cause death.

Acids stimulate the expulsion of bile from the gall-bladder, and the secretion of intestinal juice. As they will be rapidly neutralised by the bile and pancreatic juice, and absorbed in the duodenum, they can hardly reach the lower and middle parts of the alimentary canal as acids. Their action in relieving diarrhoea is difficult to explain.

When absorbed from the intestine they must pass through the liver before they can reach the general circulation (p. 399 et seq.). It is probable that during their passage through the portal system they alter the processes of tissue-change which go on in the liver, and check the formation of urea. The reason for this supposition is that acids are excreted in the urine chiefly in the form of ammoniacal salts. In the normal condition ammonia is readily converted into urea in the organism, and when given internally it appears in the urine in the form of urea, and not of ammoniacal salts. The appearance of these salts in the urine after the administration of acids shows that the normal process of conversion into urea has been diminished. Possibly it is to such alterations in the tissue-change in the liver that the so-called tonic action of acids is due (p. 410), as well as the marked benefit obtained in hepatic disorders from the administration of nitric and nitro-hydrochloric acids. Although acids appear in the urine in combination with ammonia and other bases, yet their free administration increases the acidity of the urine. They are therefore used to prevent the deposits of phosphatic calculi which are apt to occur in alkaline urine.

Poisoning by Acids. - The symptoms of poisoning by acids, and the antidotes to be employed, have already been described (pp. 395, 397, and 486). In cases of acute poisoning where death has not occurred too quickly, much albumen, haematin, and indican have appeared in the urine, and fatty degeneration of the liver, muscles, and kidneys has been found. In the kidneys the cloudy swelling and fatty degeneration of the cells were accompanied by evidences of inflammation in the connective tissue also, as it exhibited proliferation of nuclei, especially along the course of the vessels.