The application of counter-irritation as an expedient of therapeutics has never failed of appreciation, how vague soever have been the notions as to its mode of action, and how much these notions have varied from age to age. Before physiology had obtained the data on which a proper theory could be founded, the dogma of some master was submissively accepted by his followers, until a more specious theory was brought forward to displace it.

When the work of Magendie, the pioneer, and of Bichat, the interpreter and clinician, laid the foundations of physiological therapeutics, a true conception of the methodus medendi was dimly foreshadowed. Then the relation of the nervous centers to certain disorders of nutrition—e. g., the myopathies of spinal origin—and the alterations of the structure of nerve-centers induced by pathological states of the peripheral nerves, became established facts, and, in consequence, the influence of counter-irritation began to be rationally interpreted. Already the doctrine of reflexes had been universally established, and now the condition of the vessels as regulated by a system of dilator and constrictor organic muscular fibers, which are influenced by sensory impressions at distant points, has become a well-recognized fact of physiology. These data given, the principles of counter-irritation are no longer obscure, and no dogma is needed to give them concrete expression.

When the skin is irritated, by a mustard-plaster, for example, the superficial vessels of the part dilate, and an increased amount of blood is present in them. For a short distance around the part irritated, also, more or less dilatation of the vessels takes place. In this way a small amount of blood may be temporarily imprisoned. The influence which the retention in an external part of so small an amount of blood has on the general circulation, must be very slight. The obvious relief often afforded by a mustard-plaster can hardly, therefore, be ascribed to this limited withdrawal of blood.

An irritation established in the neighborhood of a part in which a morbid action is proceeding may, by reason of the contiguity of the tissues, affect the vascular supply to the diseased textures. Ubi irri-tatio, ibi fluxus; but, in order that the fluxion shall modify diseased action, it is necessary that there be a continuity of the vascular connections. The method of Furneux Jordan, which consists in the application of the counter-irritant to the neighboring vascular area, is based on this principle.

An irritation which consists in a local fluxion, and a state of altered sensibility in the nerves of the part, may affect the functions of distant organs. Counter-irritation applied to a considerable surface increases the action of the heart, raises the temperature of the body, and exalts the irritability of the nervous system. These are the general or systemic effects. Distinctly localized results are also produced. When one hand is immersed in cold water, a positive fall of temperature takes place in the other. Irritation of the lumbar region, as Brown-Séquard has shown, is followed by contraction of the vessels of the kidneys. Extensive injury to the surface of the body, by burning or scalding, may excite ulcerative action in the duodenum, or may set up a pneumonia. Injury to a motor-nerve trunk may be followed by ascending neuritis and serious atrophic changes in the multipolar ganglion-cells of the anterior columns. It follows from these facts that an irritation of the surface which involves the end-organs of the nervous system will affect the caliber of the arterioles and modify the functions of the trophic nerves. In these results we find a rational explanation of the methodus medendi of counter-irritation.

Certain other physiological laws deserve attentive consideration in this connection. An irritation which first produces a tetanic state of the vaso-motor nervous system may, if too long continued, exhaust the irritability of the organic muscular fiber, and cause paresis. Moderate irritation will exalt the functional power of the trophic centers; but excessive and long-continued injury to the surface may set up atrophic changes, of which there are numerous examples. In these physiological facts also we find a rational explanation of the injury not infre-quently done by too powerful or too protracted counter-irritation.

Vesicants, in addition to the effects of counter-irritants sketched above, cause an exudation of serum. This exudation may have a twofold effect: 1. To lessen the gross amount of the blood-serum, and thus diminish the blood-pressure; and, 2. To remove toxic or pathological materials from the tissues and fluids of the inflamed part. More powerful systemic effects are produced, and vaso-motor paresis and trophic changes are more quickly induced, by blisters than by rubefacients.