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.
I do not include in this division, in reference at least to their peculiar and characteristic properties, the general remedies which may sometimes be made to act locally by confining them to a particular part; as opium, for example, and belladonna, both of which are sometimes applied to the surface, with the view of affecting exclusively the neighbourhood of their application. The division includes only remedies which either have a special direction to some particular organ or part of the body, to whatever portion of it they may be applied, or which, if possessed of general powers, are employed locally for some effect different from the general; as when potassa, for instance, is used as an escha-rotic, cantharides for blistering, and mustard as a rubefacient, which effects are not incident to their internal use as medicines.
With a few exceptions, all the local remedies are more or less stimulant; and the possession, therefore, of this property, or that of depression, does not constitute a sufficient ground of distinction between them. Consequently, some other basis of classification must be sought for; and it has appeared to me that they might be most conveniently arranged, according as they are employed to affect the functions, or to change the organization, or to act merely as mechanical agents.