Drug remedies are known collectively as the "materia medica," or medical materials. The science which deals with the properties of drugs is called materia medica or, more correctly, pharmacology. It is a term that is employed in a broad sense to include everything relating to drugs.

In connection with drugs, there are several great fields of work, the most important being:

1. Pharmacognosy - the study of the physical properties of crude drugs. The pharmacognosist studies the methods by which drugs are collected, their appearance on the market, the characters by which they may be identified and their quality estimated, their adulterants in the whole and in the powdered state, etc.

2. Pharmacy - the art of preparing drugs for use. Manufacturing pharmacy is the art of manufacturing drugs into forms suitable for use in medicine. Dispensing pharmacy is the art of making up prescriptions. The pharmacist makes his knowledge tell on the manufacture of preparations and their combination into prescriptions. He studies weights and measures, solubilities, incompatibilities, keeping qualities, chemic reactions, the extraction of active principles, and the making of preparations suitable for use in the practice of medicine.

3. Pharmaceutic chemistry - the study of the chemistry of drugs and preparations of drugs.

4. Pharmacodynamics or pharmacology (in its restricted sense) - the study of the action of drugs. The pharmacologist studies the action of drugs on the tissues and structures of living things.

The practising physician does not require a knowledge of pharmacognosy, and he needs only such knowledge of pharmacy as may prove helpful to him in prescribing the drugs he desires his patient to have. But his knowledge of pharmacology should be extensive.

Drugs are either: (1) Pure chemicals, such as sodium bicarbonate or potassium iodide; (2) mixed mineral products, such as petroleum oil, vaseline, or ichthyol; or (3) certain animal or plant parts or products. Of animal nature or origin are musk, canthar-ides, adrenaline, lard, honey; and of plant nature or origin are herbs, barks, roots, leaves, fruits, seeds, resins, alkaloids, etc.

"Crude drugs" are the commercial forms of the natural animal or plant drugs as they are brought to the market. Their employment in medicine is due to the fact that they contain or yield more or less definite chemic bodies of medicinal value. These bodies are known as the "active constituents." In some cases these constituents are found in all parts of a plant, so that the whole plant is marketed as the crude drug; but mostly they occur in one part only, such as the leaf or root, or are stored in greatest abundance in one part, so that that part is selected for the market and is the crude drug. Sometimes, as in the case of asafetida, an exudate contains the active constituents and is the crude drug, no structural part of the plant being marketed at all. The crude drug of digitalis is the dry leaf, the leaf of the digitalis plant being the chief depository of the peculiar constituents on which digitalis depends for its medicinal activity; the crude drug of rhubarb is the dried root; of peppermint, the leaves and flowering tops; of cascara, the bark; of opium, the dried milk juice; of Spanish fly, the whole dried insect.