The broad meaning of medicine (Medicina) is "the science and art of healing and curing the sick" (Gould); but aside from this meaning the word is used in a restricted sense, to signify a drug used for the cure or relief of disease. The word drug means "a substance, simple or compound, natural or prepared, single or mixed with other substances, used as a medicine" (Gould); and "Materia Medica" covers the entire list of such substances, with their whole history.

On beginning the study of Materia Medica a general knowledge should be acquired of the classification of drugs considered from three standpoints: I. Their source of derivation. II. Their physiological actions.

III. Their ultimate forms and appearance as prepared in the pharmacy by definite, standard formulae, for administration.

I. Both the organic and the inorganic worlds furnish material useful for medicine, and in the former both the animal and the vegetable kingdoms are represented. The class of inorganic drugs is large, and comprises alkalies, alkaline earths, acids, metals, and non-metals. Among them all are many familiar elements, as lead, iron, etc.

The animal kingdom furnishes but a small quota. The drugs of vegetable origin are by far the most numerous, and are obtained from green and flowering plants, both fresh and dry, fungi, and lichens. The whole plant may be represented, or a part only, as the flowers, seeds, fruit, stems, or roots. The constituents of vegetable drugs are many and varied in character, some of them being of great potency. They are extracted from the plant and isolated in a pure form by elaborate chemical processes, and by means of especially constructed appliances.

The principal ones are as follows: aromatic, odorous, and bitter principles, albuminous bodies, starches and sugars, glucosides, oils, gums, resins and oleo-resins, and alkaloids. Gums are exudations from the stems of plants. Resins are solid, brittle, non-volatile substances, insoluble in water; and oleo-resins may be broken up into resins and volatile oils.

The alkaloids are the most important, forming as they do a class of poisons of marked characteristics and great intensity. They are nitrogenous compounds, many of them of deadly power, and are spoken of as the "active principles" of those drugs in which they are found, and to which they lend their own distinctive properties. The name alkaloid is given to them from their similarity in many ways to alkalies.

The other constituents of vegetable drugs are relatively unimportant.