This is a class of medicines not hitherto introduced into the systems of therapeutics, though described, we apprehend, by Dr. G. Pearson, in his course of the Materia Medica, under the appellation of acentropoctics, from α, a privative, and Inirritantia 4571 stimulus, a term we might have adopted, had it occurred to us in an earlier stage of the work. The great doubt which remains is, whether this be not properly a subdivision of sedatives. We think that strictly it is so; and we noticed these medicines in the article Anodynes, q. v., but thought it would be useful to the younger student to bring the whole subject into one view, as its application is extensive, and utility considerable.

The sources of irritation in the human body are numerous. Of this kind are external stimuli; acrimony in the first passages, or the secreted fluids; inflammatory stimulus, particularly of the mucous membranes; scirrhi, or other indurations; extraneous substances lodged in the cellular membrane, or among the fibres of the muscles; worms, ossifications, or extravasated blood. The remedies of these irritations occur under their proper heads; and it is rather the object of this article to speak of the nervous irritations more generally, whose source is less obvious.

Nervous excitement often arises from a particular state of the nervous power, or, as we may be allowed to style it, the nervous fluid. Whatever be the state which causes animation, the increase of that energy is irritation. This irritation, according to its different circumstances, is allayed by cold, by heat, by exhausting the nervous power, or, more directly, destroying its activity; by diluting, and thus diminishing the activity of its cause; by sheathing the nerves from its action, or discharging it.

Cold we have already spoken of; and the sedative power of this remedy, either by its continued effect or its repetition, has been already explained. It properly belongs to a subsequent head, but it is distinguished in this place in consequence of its application. It is particularly adapted to the increased excitements which produce a more active circulation, either in general or in particular organs; in general, chiefly in haemorrhages, and locally, in those inflammations which rapidly destroy the texture of the part, or produce atony, from excess of stimulus. It is thus one of the most ready and powerful inirritants that we employ.

Heat. The regulation of temperature, in a different way, is often effectual in lessening irritation, viz. by the continuation of a degree somewhat lower than that of the body, and much lower than that of the affected part. Thus water of the heat of 92° to 95° gradually sooths the irritated nerve, and lessens its excitement. Air acts more slowly, and, of course, less effectually; for the application even of the water must be long continued before it produces any effect. To their warmth a variety of demulcent remedies owe their efficacy, but often to their other qualities, which we shall soon notice.

Exhausting the nervous power, by stimulants, somewhat below in their effects those of the irritating cause, is often effectual. By this our object is to continue the excitement, not in a morbid degree, but by diminishing its power to exhaust safely the irritability of the nerve. Thus volatile alkali and eau de luce succeed in lessening the fatal effects of the viper's poison; alcohol and turpentine relieve burns; mercury sometimes lessens the irritation of the hydrophobic poison, and perhaps partly in this way of the lues venerea. We were long since taught to prevent the trismus expected to arise from the puncture of a nerve, by applying ethereal spirit of turpentine. Many similar remedies are employed, and this is one of the most successful refinements of modern practice.

In a similar way we destroy the activity of the nervous power by sedatives; by tonics, which lessen irritability; and by narcotics. We lessen irritation by opium and by vinegar; by bark and other vegetable astringents; by lead, copper, zinc, and silver. The narcotics we employ are tobacco, belladonna, hemlock, and digitalis. Each is useful in this way.

We sometimes lessen irritation by diluting the stimulus, and this is necessary when acrid poisons are carried to the excretories, as cantharides to the bladder, or when saline acrimony abounds in the blood. Dilution is, however, most often necessary when acrimony abounds in the primae viae; a more frequent occurrence than any other of this kind.

The diminution of irritation by sheathing the parts, and thus defending them from acrimony, includes the class of demulcents. This class is of considerable extent, and the medicines we shortly enumerated in that article. They are the oils and fats in all their variety, including spermaceti and bees wax; the pure mucilages, including the gums and althaea; the farinaceous mucilages, as the lint and hempseed, the quince and fenu-greek seeds; the fecula of wheat, and some miscellaneous vegetables, as the branca ursina, the melilot, the white lily, & c. We can easily conceive that these can sheath the fauces, the epiglottis, the stomach, and intestines; but it is more difficult to suppose that they can be carried into the blood,"and again acton the excretory vessels. This is, however, undoubtedly true, and we see it certainly in the urinary organs, probably in the lungs. These demulcents, like warm water externally, seem to sooth irritation beyond the part to which they are applied; for such is the consent of the small vessels on every portion of the surface with each other, that changes produced in one part are, by sympathy, communicated to the whole.

The medicines which discharge the acrid matter can scarcely be enumerated among these; for puncturing a furunculus cannot be styled an inirritant. It was mentioned, however, to connect the whole, and to suggest that a source of irritation in distant parts is often productive of great inconvenience. When violent symptoms of irritation, therefore, appear, of which the immediate cause is not perceived, it will be necessary to extend our views to every part of the body; and we may thus be able to discover and discharge substances which have been unnoticed, and produced, without suspicion of the cause, the greatest inconveniences.

We have confined, in this view, the action of inirri-tants to cases of excitement; yet we shall find other sources of irritation from privations. Thus, hunger produces symptoms of irritation; the want of the usual distention in any of the cavities, and fatigue, have a similar effect. The only remedy in common to both these causes of irritation is warmth, or particularly warm water: but it is unnecessary to enlarge at present on this subject; since to add the remedies of this kind would render the class less natural, and we should anticipate what will occur under another article. See Irritation.