In many cases the causes of inflammation are obscure, and this applies mainly to internal organs whose actions are greatly hidden from our view. Inflammation is producible artificially; and if we study the modes of production we shall find that agents are used to which the general name of Irritants is applied.

The inflammation is produced, it is said, by irritating the part in some way. The name irritant is apt to be misleading, as it embodies the conception of a stimulating action: whereas the so-called irritants, in their nature and action, possess characters rather of a deadening than of a stimulating kind. They are agents which, when acting strongly, are calculated to kill the tissues, and acting less strongly they may be supposed to damage them; it is in this latter case that they produce inflammation. A certain degree of heat or cold will kill, a less degree will lead, if sufficiently prolonged, to inflammation. Such agents as croton oil, nitrate of silver, chloride of zinc, have a damaging action on the tissues, and they are some of the commonest in use when inflammation is to be produced. Again, traumatic causes produce inflammation; a direct injury inflicted may kill, but acting less vigorously it may produce inflammation. The agents, however, although their direct action is deleterious, may yet induce a reaction in the tissues, and in this sense stimulate them.

In inflammations of internal organs it often happens that no such definite cause is to be discovered, but even in them the direct action of an irritant is sometimes demonstrable. In one of the hospitals in Glasgow a medical man prescribed a table-spoonful of cough mixture; the nurse gave a table-spoonful of carbolic acid instead. The result was an acute pneumonia presenting the usual physical signs and symptoms. In this case there was a deleterious agent introduced into the stomach and so into the circulation, and it acted on the tissue of the lungs (perhaps mainly on the blood-vessels), possibly during the process of excretion by these organs.

Again, when large blisters are applied, it sometimes happens that the active chemical principle is absorbed, and, passing into the blood, produces inflammation of the kidneys probably during the process of excretion. And so in all inflammations we are to infer the existence of some deleterious agent, although it is very often difficult to tell whence it has come and what is its nature.

At this point it may properly be inquired whether the Nervous system has anything to do with the causation of inflammation. There are not a few facts which suggest, at least, some connection. In some ■of the instances the connection is clearly an indirect one. For instance, when inflammation of the eye results from division of the fifth nerve, the inflammation is really traumatic in its origin. By the severance of the nerve the reflex action of winking is abolished as well as the secretion of tears, and so the organ is exposed to injuries and unable to get rid of foreign bodies. The inflammation is the result of irritation from without, and, if the eye be carefully protected (say by stitching the lids together) the inflammation does not occur. For similar reasons anaesthetic parts in general are specially liable to inflammation, and the inflammation is apt to be exceptionally severe.

But there are cases where nervous action has a very important and apparently direct relation to the cause of the inflammation. A man with a stone impacted in his urethra may have an inflammation of the testicle. Or, after the passage of a bougie through a urethra with a partially healed wound, there may be a rigor with immediate suppression of urjne and an acute inflammation of the kidneys. These results can only be effected by reflex action, and there are various ways in which we may suppose them to occur. Reflex irritation may produce, through the vasomotor nerves, a contraction of the arteries of an organ, and in the case of the kidney this contraction may be sufficient virtually to bring the circulation to a standstill. The immediate suppression of urine resulting on the passage of a bougie can scarcely be accounted for except on the supposition of such a spasmodic contraction amounting almost to occlusion of the renal arteries. If such a stagnation of the blood continues long enough the vessels will be so damaged that when the circulation is restored inflammation will result.

There are, however, cases of internal inflammations which are greatly under nervous influences, but in which the connection cannot be explained merely on the supposition of a reflex stimulation of the vasomotor nerves. The manifest relief which many inflammations undergo in consequence of counter-irritation has been pointed out by Lister as indicating the influence of the nervous system in producing inflammation. The application of the actual cautery to the skin over an inflamed and painful joint will often relieve the pain and inflammation instantaneously. It is as if there was in the nerves of the part a state of over-action which affected injuriously the tissues, and increased the liability to an intensity of inflammation. This over-action being relieved by a still greater stimulation of nerves connected reflexly, the inflammation ' subsides.

The nervous system itself can scarcely act as an irritant, but it may so alter the tissues that agents which normally would produce no such effect will irritate and produce inflammation. The case of Urticaria may here be cited. This condition is primarily an affection of the nervous system, but the more definite inflammatory changes are brought about by friction, by the clothes or otherwise, so that a normal stimulus produces under the abnormal nervous conditions a pathological result.

As an example of how local conditions may determine the occurrence of inflammation, the fact may be cited that a mild inflammation of the bone-marrow produced by caustics may be converted into an intense septic inflammation by causing the animal to eat putrid food. Here an existing damaged state makes the tissues unable to resist the attack of a further damaging agent which normally they are able to withstand. It may be similar in the case of many internal inflammations produced apparently by exposure of the surface of the body to cold. Such exposure may produce, by reflex action, a state of the nervous system rendering the organ peculiarly liable to damage from conditions of the blood which otherwise would not produce any such effect. In all these cases, also, it is to be borne in mind that the reflex action may affect specially the vascular system.

It may be added that besides conditions brought about through the nervous system there may be Individual peculiarities, hereditary or acquired, rendering different persons variously liable to the action of irritants, and even in the same person the various organs of the body may show different degrees of resistance. The character of the inflammation will also, to some extent, be influenced not merely by the nature of the agent causing it, but also by the state of the individual.

In studying individual cases of inflammation it will be important to consider by what path the irritant has reached the part which has become inflamed. In many cases it reaches it by the blood, and as the blood is distributed in every part of the organ, the inflammation will not probably show any special localization. And so, when we find two symmetrical organs, both of which are attacked in every region, we infer that the agent causing the inflammation has come by the blood, or, at least, by a path common to both. Of course, there may be local differences in the organ itself of such a character that every part will not be equally susceptible to the action of an agent calculated to produce inflammation, and so the disease may develop more in one part of the organ than in another, although the general character of its distribution will generally still suggest the path by which the agent has come.