Irritants are substances which, when applied to the skin, cause a greater or less degree of vascular excitement or inflammation. They are employed for the sake of their local action, to produce increased circulation in the part to which they are applied, and thus to remove abnormal conditions already present in it.

When irritants are employed for the purpose of affecting reflexly a part remote from the seat of application they are named Counter-irritants.

Irritants are subdivided, according to the amount of effect produced, into rubefacients, vesicants, pustulants, and escharotics.

Rubefacients produce simply congestion and redness, which may be merely temporary, passing off in a few minutes, or may be more permanent, remaining for several days.

When more powerful, so as to cause exudation between the true skin and epidermis, giving rise to vesicles, they are called vesicants, or epispastics.

When they do not affect the whole skin alike, but do so unequally, and irritate isolated parts in it, such as the orifices of the sudoriferous glands, so powerfully as to give rise to pustules, they are called pustulants.

When they destroy the tissues altogether, forming a slough, they are called caustics or escharotics.

The difference between these sub-classes is chiefly one of degree, and not of kind. The weaker ones produce the higher degrees of action when applied for a long time, and the stronger ones produce the slighter kinds of action when applied for a short time.

It must be remembered that, although inflammation is usually associated with increased circulation, the two things are essentially different.

Inflammation is the injury to the tissue; the increased circulation is the attempt to repair it.

Increased circulation occurs wherever we have increased functional activity, whether this be for the purpose of performing a normal function, as in glands during the process of secreting, and in muscles during contraction, or for the purpose of repair. When repair is going on slowly, the process may be frequently quickened by increasing the supply of blood to the part, and this is the reason for using friction, and liniments and blisters of various kinds, in cases of chronic inflammation in joints or in ulcers.

Sometimes irritation fails to cause absorption, from being too weak. In a case of rheumatic gout which I saw some years ago, irritating liniments had been applied for some time in vain, until, by mistake on the patient's part, so much iodine liniment was put on at once as to cause vesication over the whole back of the hand, when recovery began immediately.

In acute inflammation, however, the greatly increased circulation, along with the heightened sensibility of the sensory nerves in the inflamed part, causes much pain, and this is relieved when the tension of the blood in the inflamed part is lessened. We notice this very clearly when the finger is inflamed in consequence of a prick from a thorn, a bruise, or other injury. When it is allowed to hang by the side, the throbs of pain, coincident with every pulse-beat, become excruciating, while, if raised above the head, so that the pressure of blood in the vessels is less, the pain becomes greatly diminished. The tension in the vessels may be relieved likewise by causing contraction of the arteries leading to the part by a cold compress around the arm (Fig. 120), or by dipping the finger in cold water; but relief is also afforded by a warm poultice applied to the finger. At first sight it seems strange that heat and cold should both relieve the pain, but a little consideration will show that they both relieve the tension in the vessels of the inflamed part. Cold does so by causing a reflex contraction of the afferent arteries, and thus diminishing the quantity of blood going to the inflamed part. Warmth, on the other hand, dilates the capillaries of the collateral circulation, and thus diverts the current away from the inflamed vessels.

Fig. 120. - Tracings from the radial artery at the wrist : A before and B after the application of a cloth dipped in cold water round the arm. (After Winternitz.)

The use of counter-irritation as a remedial measure depends on the fact that similar alterations to those produced by heat and cold on the finger may be produced on the circulation in internal organs reflexly through the nervous system.

When an irritant is applied to any part of the skin, it causes a local dilatation of the vessels and redness of that part, but contraction of the vessels in other parts of the body. Probably this contraction takes place with the greatest force in certain organs having a definite nervous relation to that part of the surface diagram is supposed to represent the end of the finger. The small star indicates the point of irritation, e.g. a prick by a thorn. The line in the centre of each figure is intended to represent the nerve going to the injured part; and at the side of each figure is an artery and vein connected by a capillary network. In a the capillary network around the seat of irritation is seen to be much congested; the nerve-filaments are thus pressed upon and pain is occasioned, b represents the condition of the finger after the application of cold to the arm or hand. In consequence of the contraction of the afferent arteries the finger becomes anaemic; no pressure is exerted on the nervous filaments, and pain is alleviated. c represents the finger after it has been encased in a warm poultice; the capillary network at the surface of the finger is dilated, and the blood is thus drawn away from the seat of irritation and the pain therefore relieved.

which is irritated. Zulzer found that when cantharides-collodion was painted repeatedly over the back of a rabbit for fourteen days, the vessels underneath the skin, and the superficial layers of muscles, were congested. The deeper layers of the muscles, the thoracic wall, and even the lung itself, were much paler and more anaemic than those of the other side.