The Action of Medicines in the System / Or, on the mode in which therapeutic agents introduced into the stomach produce their peculiar effects on the animal economy

by Frederick William Headland

3. That others again, called Sedatives, act so as to depress nervous force, in general or in particular.

Having investigated at some length the action of Blood-medicines, we now commence the consideration of another class of remedies, which differ very widely from the last in their mode of operation.

The action of Hæmatics is slow, but more or less durable, because it is evidenced in the blood; and a change in the blood produced by this action continues for a longer or a shorter time.

The action of Neurotics, or Nerve-medicines, is rapid; but it is transient, and is soon over. A Neurotic medicine does not cause any change in the blood; and it cannot remain in it, but soon passes out. It acts by contact with nerve, apparently producing no lasting change even in nerve-fibre; and as the cause of the action cannot remain, the effect also soon passes away. Whereas Hæmatics, durable agents in the blood, are used to counteract the causes of inveterate and chronic disorders; these Neurotics, which produce a transitory, but more or less forcible impression on the nervous system, are employed to rouse it when torpid, or to depress it when over-excited. Rarely of use in chronic blood-disorders, they are given mostly in the temporary emergencies of acute diseases. But it is obvious that even a temporary agent may prove of permanent efficacy by remedying a temporary emergency.

It was feasible to attempt some explanation of the actions of medicines in the blood, occasionally more or less analogous to known chemical influences. But the agency of nerve-medicines is of a far more incomprehensible kind. When we consider that little or nothing is known, or can be known, about the ultimate causes of sensation, or motion, or nervous excitement, there is no need for wonder that we find ourselves at a loss to explain the operation of medicines that influence these conditions.

Thus I must chiefly limit my remarks on Neurotics to defining what their action is, finding it impossible to state with certainty how they act. And the field of inquiry being so limited, it follows that there is much less to be said about them than had to be said of Hæmatics.

Nearly all the powerful poisons that act after passage into the blood, belong to this class. Their action in most points of view is such as completely to exceed our means of comprehension.

Sudden death may be produced by it. But there is no apparent cause for this. We find no erosion or perforation of the coats of the stomach or intestines; no mechanical disorganization of the tissues, or chemical change in them; no hæmorrhage, or vascular disease; no rupture of nerve-fibres. Whence, then, could death have arisen? How could the mere presence of a few atoms in the blood,—half a grain, one-twentieth, or even one-fiftieth of a grain—how could this apparently contemptible influence have produced so essential a derangement of the vital functions as to stop them altogether? It is impossible to answer.

The action of such remedies in the sudden causation or alleviation of nervous symptoms, when applied in the cure of disease, is equally wonderful. How are we to account for their different actions on different nerves? How is it that Opium contracts the pupil, and Belladonna dilates it?—that Digitalis affects the heart, and Stramonium the respiration?—that Prussic acid will cause convulsions, and Hyoscyamus delirium? In what way are these various operations brought to pass? It is impossible to answer.

Though it is, I say, quite impossible to frame for any one of these questions a certain or satisfactory reply, on account of the manifest insufficiency of our acquaintance with the details of such actions as these, yet I must now venture to repeat an idea which I have already referred to at the commencement of this Essay (p. 46,) and state my belief in the bare possibility of the operations of Neurotic agents being explicable upon mechanical grounds. It is generally believed among scientific men that each particle of a compound body is made up of a number of indivisible atoms, each of which is inconceivably minute in size. And as these compound bodies have each a peculiar chemical constitution, so must each of their ultimate parts be composed of a peculiar arrangement of simpler atoms, and thus have a certain shape of its own more or less different from the shape of every other compound atom. Both the substance of a nerve, and the active part of a nerve-medicine, consist of a number of definite compound atoms. And it is possible that the atom of a stimulant medicine may be of such a shape as that it shall be unable to coincide with, or to fit into, the series of atoms forming the sensitive surface of the nerve, and thus irritate this when brought into contact with it; and that the compound atoms of a sedative may so arrange with these nerve particles as to fit among and extinguish their salient points, and annihilate their natural sensibility. We learn from the phenomena of the senses that the nerves are very much under the influence of mechanical impulses of all kinds, and particularly minute and inappreciable impulses of this description. Another fact which gives additional credibility to such an idea is, that those Neurotic substances which are chemically alike are in general alike also in their influence on nerve.