This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
These are medicines which, with the simple tonic powers characteristic of the pure bitters, possess others also, which modify the tonic action, and give them an influence on the system more or less peculiar.
This peculiarity may be owing either to the distinctive character of the bitter principle itself, or to some other principle or principles which may be associated with it. Thus, Peruvian bark owes its characteristic remedial virtues to the peculiarity of its bitter principles; while serpen-taria and wild-cherry bark have other active constituents besides the bitter; the additional constituent being, in the one, hydrocyanic acid which is sedative, and in the other, a stimulating volatile oil.