For the purpose of curing disease the medical practitioner makes use of many substances of animal, vegetable and mineral origin, as well as an increasing number of substances prepared by the chemist synthetically. The substances that are so used are known inclusively and collectively as the "materia Medica." Any substance administered to a patient for the purpose of curing or alleviating disease may be termed a "Drug." But not all substances that have been used by man as medicines are still in common use in civilized lands to-day, and many of the newer remedies, though highly lauded by their discoverers have not, and in many cases will not prove to be of sufficient merit to come into common favour. In consequence of this and as a guide to the physician and especially as an aid to his allies the pharmacists, most modern governments have caused to be prepared and published, books known as "Pharmacopoeias ." Such a pharmacopoeia contains the correct legal or "official" names both in Latin and in the vulgar tongue of such substances of the materia medica as are judged by those who compile the pharmacopoeia to be in common use in the country and to be of value to the physician. Further for the guidance of those who purchase crude drugs and prepare them for the patient's use the pharmacopoeia contains accurate descriptions of the physical and chemical characteristics of the drugs and of the methods by which they are prepared for administration. The term "official" may be applied only to such drugs, preparations, methods and doses as are included in the British Pharmacopoeia. This term must be carefully distinguished from the more inclusive term "officinal" which may be applied to any drug, etc., whether included in the Pharmacopoeia or not, so long as it is in common use.

"Pharmacognosy" is the science of the source and characteristics of the substances of the materia medica. This includes a knowledge of the natural history of all the plant, animal, and mineral products in the materia medica, as well as a knowledge of the methods of chemical preparation of those drugs that are produced synthetically, and a knowledge of the chemical and physical characteristics of all drugs. Some of the more important facts of the pharmacognosy of the drugs reviewed in this book will be referred to when they are individually considered.

"Pharmacy" is the art of the proper preparation of the substances of the materia medica for use (exhibition) and administration as medicines. This science may be divided into three branches. Firstly, Chemical Pharmacy, or the preparation of substances of definite chemical composition, such as salts, acids, alkaloids, etc. This branch has now passed entirely out of the hands of the practising physician and almost entirely out of those of the practising pharmacist. Secondly, Galenical Pharmacy, or the preparation for administration in the form of medicine of drugs of indefinite chemical composition, which are, as a rule, products of plant or animal life and usually intimate mixtures of many chemical substances. Galenical pharmacy has now been almost entirely abandoned by the physician and only some of the simplest procedures are now carried out by him. The practising pharmacist as a rule no longer carries out the more complex galenical procedures but purchases many of his stock of galenicals from the larger pharmaceutical houses. Thirdly, Dispensing, Magistral Pharmacy, or the preparing and putting up in suitable form for the patient the drugs or their galenical preparations ordered by the physician.

"Posology" is the branch of medical science that deals with the doses of drugs and their preparations. The knowledge of this subject is of the utmost importance for the physician.

"Pharmacology" is the science that deals with the action of drugs upon the animal body. This science is often termed "Pharmacodynamics"; the term "Pharmacology" being then used in a broader sense to include pharmacy, pharmacology, pharmacognosy, and posology. "Therapeutics" is the art of applying the knowledge of these four sciences to the treatment of disease.

The Pharmacopoeia also prescribes the systems of weights and measures, which are to be used in the operations of pharmacy. The older system, the Imperial System, is still almost exclusively used for Magistral Pharmacy, in spite of its obvious disadvantages, and in it alone the doses are given in the Pharmacopoeia Britannica.