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.
Most medicines probably produce their peculiar effects by operating on the vital properties of the system, or of the part affected. Of the nature of this action we are quite ignorant, and must remain so until the nature of life itself is better understood. In the present state of our knowledge, all that we can say is, that living structure is endowed with certain susceptibilities, and medicines with certain properties, or powers, through which, when the two are brought together, certain changes of condition or action take place in the former. The medicine is said to act, and the living structure to be acted on, because it is in the latter that the changes are most obvious, and most interesting. This kind of operation is called physiological, or vital, in reference to the character of the effects produced in the system, and dynamic, in reference to the supposed possession of an active power by the medicine. We may speculatively ascribe the results to physical or chemical reaction; but this explanation is, in the great majority of instances, destitute of the shadow of proof, and is indeed altogether insufficient, with our existing knowledge of physics and chemistry, to account for many of the phenomena. Until, therefore, further light is obtained, it is safest to be content with the mere statement of the fact.*
When medicines are introduced into the system, they generally show a tendency to act on some one part, or set of parts, preferably to others, and not unfrequently act on these parts exclusively. Thus, some medicines operate more especially on the brain, others on the heart, others on the stomach and bowels, and others again on some one or more of the secretory organs, though equally in contact with the parts unaffected as with those upon which they act. This tendency is ascribable to the peculiar constitution of the several parts of the system, giving them peculiar susceptibilities; just as the eye alone is sensible to light, and the organs of smell and taste to odours and sapid substances. A medicine, therefore, entering the circulation, and reaching all parts of the system, on such parts only as are so constituted as to be susceptible to its influence; as light, though falling on the whole surface of the body, produces its characteristic impression only upon the eye. The chemists explain this peculiar direction of medicines by the supposition, that it is only in the organs affected that they find affinities capable of being disturbed by their presence.
* Much has been said of late of another mode of action of medicines, based on their influence upon the function of nutrition; some being supposed to act. by in-ovearing that function, others by diminishing it. Of course, medicines which stimulate or depress the functions generally must stimulate or depress nutrition, as they affect other functions; and there may be some whose special tendency is to act upon nutrition; but the resulting changes are effects of the medicines and not their method of action. The question arises, how do they increase or diminish nutrition? and we are then brought back to the chemical or physiological theory. (Note to the third edition).
It is not impossible that some medicines may act upon the blood through its vital properties, modifying the condition of the living corpuscles and fibrin, in the same manner as others act upon the living solid tissues. Such medicines may operate exclusively on the blood, producing effects upon the various functions simply through the change effected in that fluid; or they may operate at the same time directly upon the organs. The probability, however, is, that, in the great majority of instances, medicines are merely conveyed by the blood to the part in which their effects are experienced; for otherwise these effects would be more frequently universal, as any change in the blood itself must be felt more or less in all the organs and functions.