2. Soluble mineral substances.—Acids, Alkalies, and Salts, may pass off by any of the fluid secretions. The chief of these secretions are the sweat, the urine, and the secretion of the bowels. The sweat is only fluid when the skin is kept covered and very warm. In other cases the choice lies between the kidneys and the bowels. The kidneys are the grand purifiers of the blood, so that most of the substances which stimulate the other glands may occasionally pass off in the urine. And soluble minerals, which require some amount of water for proper solution, would tend for this reason towards the most fluid of the secretions.
But there are some other things which counter-balance this tendency. The constitution of the urine is such that it cannot safely be disturbed to a very great extent. If there be much excess in it either of acid or of alkali, a deposit is occasioned. Dr. Bence Jones found that Sulphuric acid never passed out in the urine in any quantity. We find too that when a certain quantity of a saline is administered, too great to pass off readily by the kidneys, it prefers to act on the bowels. It is more likely to be diuretic if much diluted with water; but this is not the only directing cause, for a very large quantity of a saline will prove purgative even when largely diluted. (Vide Prop. II.) Thus the general rule is this: soluble minerals are in small doses Diuretic, in large quantity Purgative. This applies more especially to salts; for mineral acids act as astringents on the bowels,—and mineral alkalies, being corrosive, cannot be given in large doses.
3. Ammonia and volatile oil.—The laws of endosmosis favour the passage of soluble substances through to a liquid secretion. They are dissolved and carried away on the other side of the membrane through which they pass. In the same way it appears that volatile substances may dissolve in, and be carried away by, air. Thus they have a tendency towards the aeriform secretions. These are,—the expired air, or the secretion of the air-cells of the lungs,—and the ordinary cutaneous transpiration. While passing through the air-cells, these matters stimulate the secretion of the lining mucous membrane. They cause a morbid secretion of this surface to be replaced by a more natural one. Thus Ammonia and volatile oils are Expectorant and Diaphoretic. They may all pass out in the urine, but do not especially tend to do so, with the exception of some that are acrid, and approach to the nature of the resinous group. Turpentine, Juniper, and Copaiba, are Diuretic.
4. Acrid fixed oils.—Such are Castor and Croton Oils; they are Purgatives, passing off by the bowels.
5. Resinous and neutral acrid principles.—These are soluble in alkalies, and thus partly absorbed in the intestinal canal. Most of them are Cathartic, whether introduced into the stomach or injected into the veins. Such are the resins of Scammony, Jalap, and Gamboge, and the principles of Colocynth and Elaterium. Some few of them are Diaphoretic, as Guaiacum and Mezereon. Some again are Diuretic, especially those which are liquid or associated with a volatile oil, which is the case with Copaiba, Cubebs, Cantharides, and others.
6. Vegetable principles soluble in water.—Most of these are Diuretic; as are the Vegetable Acids, and the Alkaloids of Digitalis, Tobacco, and Colchicum. So also are the principles of Broom and Sarsaparilla. The Emetine in Ipecacuanha, and the Morphia in Opium, act on the skin. Aloesin, the principle of Aloes, is purgative. So is Cathartin, the soluble acrid principle of Senna.
Having thus briefly sketched out the particular tendencies in the operation of the groups of medicines which act on the glands, it remains for me to say that in very many cases these medicines have actually been proved to pass out of the body by the glands whose secretions they tend to increase. Whenever we are in a position to inquire into the facts by chemical or other means, we find that the Eliminative medicine is itself contained in the secretion which is augmented by its action, and that when the secretion upon which it usually acts is not augmented, the medicine has passed off by some other secretion instead. There are doubtless many cases in which no inquiry of the kind has been yet made; but it will be seen when we consider separately the Eliminative orders, that all that is known on the subject is in favour of the above statement.
Assuming, then, the third and fourth minor propositions together, we conclude that Eliminative medicines, which must pass out of the blood, tend to pass out by some glands more than by others, and that the result of their passage through a gland is to increase its secretion.
They do not exert a blood-influence, nor do they act on the nervous functions; but they operate on those obscure vital forces by which secretion is directed and controlled.