Action of Medicines upon the System.
We might arrange all remedial agents in three classes, according to the effect they exert upon the system, whether it is physical, chemical or vital. All agents act in one of these three ways, though it is impossible sometimes to distinguish between them, as an agent may first affect the body by its physical properties, and secondly by both a chemical and vital change in some of its constituents.
Physical Effects of Remedies.—The action of remedies may be owing to their form; in this case we say the action is physical; but this is the case with but very few agents, and these are the anthelmintics: thus the hairs of the pods of mucuna pruriens, or the stanni pulvis of the pharmacopoeia, are said to act in this manner. These agents are supposed to act mechanically in destroying worms. All other agents that have been said to act in this manner, would produce no effect at all if they did not act in a living body, hence we may name such agents physico-vital.
Chemical Effects of Remedies.—According to Pereira, in consequence of the mutual affinities which exist between some medicines and the constituents of the tissues and of the blood, numerous and important chemical effects are produced in the animal economy. The halogenous bodies, some of the combustible metalloids, the acids, the alkalis, metallic salts, tannin, creosote and alcohol act in this way.
1. The halogenous bodies (chlorine, bromine and iodine) abstract hydrogen and unite with bases. Indirectly they become oxydizere by taking hydrogen from water and setting free the oxygen. In some cases they may, perhaps, combine directly with organic substances.
2. The non-metallic combustibles (sulphur and phosphorus) combine with both oxygen and hydrogen.
3. The acids (sulphuric, nitric, hydrochloric, phosphoric and acetic) combine with bases, decompose many salts, and unite with or decompose the organic constituents of the body.
4. The alkalis unite with acids, decompose some salts, and combine with or decompose the organic constituents of the body.
5. Most metallic salts react chemically on the organic tissues, and give rise to the formation of new compounds.
6. Tannic and gallic acids.—Tannic acid, in the impure state called tannin, acts on the animal tissues in virtue of its affinity for their constituents. It forms with albumen and gelatine compounds which are insoluble in water; and it also combines with fibrine. When taken into the stomach, it unites with the constituents of the epithelium, and of the mucous membrane of the alimentary canal. It becomes absorbed, and is evacuated from the system in the urine.
7. Creosote, Alcohol and Ether.—Both creosote and alcohol cause the coagulation of albumen.
In addition to the classes of agents named, there are but very few but what exert some chemical influence on the system. As a general rule, however, the beneficial effect of a remedy is not entirely dependent upon its chemical effect, but upon some change in the vital force of the system, so that we might justly call the action of this class of agents chemico-vital.
Vital Effects of Medicines.—The influence of all remedies upon the system must for the most part be vital in its nature, though it may depend in some part of its action on its physical condition, or on its chemical properties; its major action is such only as could be exerted in a living body. The class of vegetable remedies exert the least physical or chemical effect upon the system; they appear to act directly upon the vital force of the entire system, or on some particular organ or parts.
These remedies either stimulate or depress the functions of the different secreting and excreting organs, increase or diminish digestion and sanguification, stimulate or depress the nervous system, etc. They might be divided into three classes according to their effects upon the system, though such a division would but generalize the subject. The first of these classes would include all those agents which highten or augment vital action, and would hence be called stimulants; the second those that depress nervous action, or sedatives; and the third those that alter either the nature or quality of vital action, being neither stimulant nor sedative; this class might be called alteratives.