Most of the vegetable acids of a sour taste have a certain resemblance in effect, which serves to group them together as a subordinate division of the class of arterial sedatives or refrigerants, to which they undoubtedly belong.

Effects on the System

These acids are locally excitant, but, in their general influence on the system, are sedative to the circulation. in small quantities, and largely diluted with water, they are generally grateful to the palate, and acceptable to the stomach, which they moderately stimulate, as shown by their sharpening effect on the appetite. After a time, they render the pulse somewhat slower and softer, and diminish the general temperature; and, if the system is excited, have an agreeable and refreshing effect upon it. They often also moderately promote the secretions, especially that of the kidneys; but this influence on the secretory functions is not generally very obvious, unless there has previously been a diminution of the functions from an over-excitement of the organ, as when the skin is dry and heated, or the liver inactive from congestion. Under such circumstances they appear to act by a sedative influence on the capillaries, the irritation of which is reduced to a point at which secretion can take place. Hence the moisture of surface, and the gentle flow of bile, which occasionally follow their administration in the conditions just referred to. in the same way they seem occasionally to relax the bowels, and to promote pulmonary exhalation.

When taken too largely, and too long, they are apt to produce irritation of the gastro - intestinal mucous membrane, to lessen the appetite and disturb digestion, and, partly through this effect, and partly, in all probability, through a direct depression of the circulatory function, to impair the organic processes generally, and thus occasion paleness, general weakness, and emaciation.

In large doses, in a concentrated state, they are capable of acting as poisons; and one of them at least, the oxalic acid, is extremely noxious. They appear to operate poisonously in two ways; first, by inflaming, if not corroding, the mucous membrane of the stomach and bowels; and secondly, by a powerfully depressing influence on the heart, exerted either directly, or through the altered blood.

Mode of Operating

The local irritant effect of the acids results from their contact with the surface affected; their sedative operation upon the circulation takes place through absorption. Whether they enter the blood as acids is not certainly known, as they have not been detected in that fluid in the free state; but the probability is that they generally do so, and always, when they do not meet a free or carbonated alkali in the stomach or bowels, in sufficient quantity to saturate them. After entrance, however, they certainly do not remain free, but probably instantly combine with the alkaline matter of the blood, and operate through the compounds thus formed. The similarity, we might almost say the identity, of their sedative action with that of the refrigerant salts, may thus be readily understood. it is the saline combination formed in the blood, and not the acid, that acts. Hence, too, it happens that they cannot be detected in the free state in the secretions. it is true that they are capable of rendering the urine acid, as shown by the experiments of Dr. H. Bence Jones with lemon-juice (Lond. Med. Times and Gaz., Oct. 1854, p. 408); but this acidity has not been shown to depend on the presence of the acid used, and is probably merely an increase of the normal acidity, which has been ascribed to uric acid, but the source of which has not, perhaps, been satisfactorily demonstrated. This much has been determined, that the quantity of uric acid set free in the urine is increased by the copious use of the vegetable acids; and it is highly probable that the latter, on entering the circulation, possess themselves of the alkaline base with which the former is there combined, and thus cause its elimination. Sometimes the saline compound formed by the acid after exhibition may be detected in the urine.

The only acids belonging to the present category, which are used medicinally to any considerable extent, in the uncombined state, are the citric and acetic, which are generally taken in the form of lemon-juice and vinegar. The tartaric is sometimes employed, though generally as a chemical agent. Oxalic acid, in small doses, is said to have similar properties; but it is so liable, in consequence of the resemblance of its crystals to those of Epsom salt, to be taken for it through mistake, and is so fearfully poisonous in over-doses, that no good which could be derived from its employment as a medicine would compensate for the danger.