These may be classified into: 1. The Active Constituents. 2. The Inert Constituents.

The latter are the cellulose, wood, and other structural parts of the drug, and in some instances starch, albumen, fat, wax, coloring-matter, and other substances which have no distinct pharmacologic action, though their presence in a preparation may have a modifying effect on the absorbability and activity of the active pharmacologic constituents.

The active constituents may be active in two different ways, viz.: pharmacologically active, i. e., having an action on living animal tissues, and pharmaceutically active, i. e., capable of causing precipitation or otherwise notable chemic changesin a prescription or preparation. Both kinds are found in cinchona bark, which contains not only quinine and other alkaloids upon which its pharmacologic activity depends, but also tannic acid, an astringent drug. In an ordinary dose of cinchona the tannic acid is too little in amount to have any important astringent effect, and is, therefore, not pharmacologically active; yet if the cinchona preparation is mixed with a preparation of iron, the tannic acid becomes pharmaceutically active and changes the iron salt into ink. Again, the pharmacologically active principles of digitalis are not readily soluble in water, so an aqueous preparation, such as the infusion, would not represent the activity of digitalis were it not for the fact that digitalis also contains a body which possesses the peculiar property of rendering the active medicinal principles soluble in water. This body (digitonin) is, therefore, pharmaceutically active, and as such is important.

A constituent is called an active principle when to it may be attributed, either wholly or in part, the physiologic action of the drug.

The active constituents of organic drugs may be either: a. Single chemic bodies, or b. Mixtures of such a nature that separation into their components is not advantageous.

The classes of active constituents are:

A. The Single Chemicals.

1. Plant acids and their salts.

2. Alkaloids.

3. Neutral principles.

4. Toxalbumins.

5. Ferments.

6. Sugars, starches, and gums.

7. Tannins.

B. The Mixtures.

1. Fixed oils, fats, and waxes.

2. Volatile oils.

3. Resins.

4. Oleoresins.

5. Gum-resins.

6. Balsams.

The last three are natural exudations from plants.