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.
One of three events must occur when a medicine is applied with effect to any part of the body. Either it must enter the circulation, and be carried with the blood throughout the system, acting upon such parts as may be susceptible to its influence; or the local impression, first produced by it, must be conveyed through nervous communication to the parts, more or less distant, in which its effects are to be displayed; or, finally, it must act exclusively in the vicinity of its application. Each of these modes of operation requires to be considered.