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

Astringents are medicines which from the very nature of their action—apparently a chemical one—cannot operate at all without touching the muscular fibre, which they cause to become contracted.

But in laying down this rule of the necessity of local access for the production of the primary[30] effect of a medicine, we must be careful that we do not make it too absolute. In saying that no action at all can be propagated by the agency of the nervous system, we do not make proper allowance for the vital properties of nerve-fibre. The vital nature of common nerve-fibre is such that two actions can be conducted along it;—an impulse producing sensation, or an impulse producing muscular action, at a distant point. It is known that an impression on the terminal extremity of a sensory nerve is capable of producing either sensation or motion at a distance, by what is called reflex nervous action. Though this impression must pass through the brain or spinal cord, yet these centres are not appreciably affected by it. Now it is possible, though it does not often happen, that the action of a medicine on the extremity of a nerve may cause this distant action, without reaching the part at which it is manifested. It may be positively affirmed that in no other case can the direct action of a medicine be conducted along a nerve.

Having made allowance for the physiological characters of nerve, we must also take notice of the vital properties of muscular fibre. We sometimes find, especially in the case of unstriped muscle, that when one part of a muscular organ is caused to contract, a wave of contraction is propagated along the fibres; and this action may even be extended to a neighbouring muscle, either by contact or sympathy. In one or two cases it seems that muscular contraction may occur in this way as the result of the action of a medicine.

We will now consider separately the two exceptional cases.

a. A medicine may occasionally produce pain or sensation at a distant part, without reaching that part. We often find that a morbid action at one part of the body is capable of producing pain or uneasiness at another distant part by a reflex nervous action. We are familiar with instances of this among the symptoms of disease. The pain in the knee which occurs in disease of the hip-joint; in the left arm, in some cases of heart disease; and in the right shoulder, in disorders of the liver; are examples. Certain impressions on the surface of the stomach may cause such a reflex pain. Swallowing a piece of ice will sometimes produce pain over the brow; and it is likely that the headache which follows over-eating, or a large dose of a tonic medicine, may result in a similar way from mere irritation of the stomach. Any irritant, as a solution of Iron, Arsenic, or Zinc, will do the same. The action of a violent purgative will cause headache while it lasts. Although we cannot deny the possibility of other actions of the same kind, yet there are so few medicines which, in ordinary and safe doses, are capable of producing pain in any way, that it becomes difficult or impossible to adduce a satisfactory example of an agent which operates on distant parts in this manner when in the blood.

b. A medicine may occasionally produce muscular contraction at a distant part, without reaching that part. This may be done in two ways; either by a reflex nervous action, or by a propagation of the contraction from one muscle to another in its neighbourhood. The first of these never results from the action of a medicine while in the blood; but it may follow an impression on a surface. Thus we have already seen that the irritation of the mucous membrane of the stomach, and probably of the filaments of the Vagus nerve distributed in it, which precedes the act of vomiting, may cause the contraction of the muscles of the abdomen.

The propagation of contraction from one muscular organ to another appears to take place in the case of violent purgatives, particularly some, as Aloes and Savine, which act on the lowest portion of the large intestine, exciting it to a peristaltic contraction,—whose action may thus be extended to the contiguous Uterus in the female, causing it also to contract. This renders the employment of such medicines dangerous in cases of pregnancy, as tending to produce abortion. In other cases these agents may be useful; for by the irritation and congestion which follows this action on the Uterus, they may cause the appearance of the menstrual secretion when deficient or retained. This also is the result of an action on a surface.

Now, though in such cases the action of a medicine seems to be continued from the part where it is situated to a distant point, there are certainly no authenticated examples of such an action by a remedy in the blood. Yet it would not be reasonable to deny the bare possibility of such a thing. But such instances are at all events rare, and, being cases of the manifestation of their vital properties by nerve and muscle, rather than of the proper action of a medicine, must not be considered to invalidate the above proposition, which, as a general rule, is of considerable importance.