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.
And what is proved of nerve-medicines holds good still more obviously with medicines that act on the blood, and on blood disorders. Nearly all of these have been found to exist in the blood, and to pervade the whole mass of the circulation, wherever at first introduced.
The glands of the body form a third case in which we require proof of actual local access. This matter will be discussed when we have to consider the subject of Eliminative medicines, when I shall attempt to show that the majority of those medicines actually pass through and are excreted by the glands which they affect. When Mercury is chemically detected in the secretions of the liver and bowels; Sulphur in that of the skin; Turpentine and Copaiba in that of the kidneys; it is evident that these substances must have reached bodily the glandular organs to which their action is directed.