This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
These are medicines which cause or facilitate the discharge of secreted liquids from the lungs, including the larynx, trachea, bronchia, and pulmonary air-vesicles. They operate in several distinct methods. To understand their effects, it is necessary to bear in mind that, in the air-passages, as in the skin, there are probably two distinct extravasations, one of which is a true secretion produced by the epithelial cells of the mucous membrane, and constitutes the bronchial mucus, the other a mere exhalation, partly liquid and partly vapour, through the coats of the capillaries and the basement membrane.