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.
By the term mucilage is here meant a solution, or an apparently liquid state of gummy matter. There are two sets of substances belonging to this category; one, those in which the gummy matter has been isolated, and is kept in that state, as gum arabic and tragacanth; the other, in which it is still contained in the plants or parts of the plants affording* it, and from which it must be extracted for use by water, as flaxseed and slippery elm. it must be understood, too, that the gummy matter is not identical in the different medicinal substances consisting of or containing it, but that it is chemically more or less distinct in all. I shall treat first of the isolated gums, and afterwards of the others, but without further distinction.