Having previously endeavoured to explain the various modes in which medicines are absorbed and pass into the blood, and having now defined the action of some few before absorption, the greater part of our investigation remains still to be accomplished. The actions of medicines in the blood, and their various and complicated operations in the cure of diseases, have to be traced out, and, if possible, accounted for. The remaining six Propositions concern the behaviour of medicines after their passage into the blood. The first two of them are comparatively unimportant. The Fifth is merely an extension of the First Proposition,--in which the same rule is applied to the blood which was there proved of a surface,--and indeed follows in part from the latter. The Sixth Proposition defines three kinds of changes which certain medicines are liable to undergo during their stay in the system. PROP. V.--_That the medicine, when in the blood, must permeate the mass of the circulation, so far as may be required to reach the parts on which it tends to act._ _That there are two possible exceptions to this rule:_ _a. The production of sensation or pain at a distant point._ _b. The production of muscular contraction at a distant point._ We might already have concluded that, as a general rule, it is impossible for medicines to exert their primary action on a remote part by nervous or any other agency, but that they must actually reach the part which they affect by means of the circulation. The experiments, already quoted, of Magendie, Blake, Ehbert, and others, show that even those medicines and poisons which tend most powerfully to influence the nervous centres, cannot act by nervous connexion, or without being allowed to pass on in the blood. They must actually reach the brain, before they can act upon it. The circulation of the blood is sufficiently quick to allow of this. The action of nerve-medicines when applied to a part, being similar to that which follows their absorption, would alone render it highly probable that in the latter case they reached the part in the blood. Thus Morphia, Hydrocyanic Acid, Chloroform, and Aconite, benumb the superficial nerves: Belladonna dilates the pupil; and Strychnia augments muscular irritability; whether locally applied, or administered through the stomach. Neurotic medicines have even been detected after death in the parts and organs which they influence. Thus Alcohol has been found in the brain, and Lead in the spinal cord and muscles. From these various facts we may conclude that however near these remedies may be brought to that part of the nervous system over which their power extends, whether it be centre or periphery, they do not in general affect it, unless they are allowed to reach it.