These causes may all be included in the condition known as irritation of the tissues, resulting from irritants of various kinds, the action of which is immediate in producing the inflammatory condition. Exciting causes may be divided into external, which are easily recognized, and internal, which are more obscure, and are assisted by some predisposition of the organism.

Cold is a frequent cause of inflammation, and its effects are due to sudden changes in the constitution of the blood from an arrest of the function of the tissues, temporary in its nature, which interferes with the emunctory action, whereby effete and irritant materials which should be eliminated are retained, and poison the blood. Heat is also a cause of inflammation, its effects varying from a slight redness, denoting transient hyperemia, to vesication, either superficial or deep. When death of tissue results from such a cause, suppuration ensues on the separation of the eschar; and when there is a loss of cicatricial power, repair by granulation and suppuration (second intention) is prevented. The inflammation resulting from simple burns and scalds, provided no eschar is formed, and air is excluded, soon subsides.

Mechanical violence excites inflammation; yet, under favorable circumstances, this condition resulting from an incised wound is soon arrested, owing to its benign form, by the particles of lacerated tissue being carried away by the blood and the subsequent liquid exudation, while those that remain undergo liquefaction and absorption by the lymphatics; hence, when the cut surfaces are brought and retained in proper apposition, speedy union, by the "first intention" results. Union by "first intention "is induced by such changes as cell-proliferation, the formation of new capillaries, and the generation of cicatricial tissue. It is only when the vitality of the tissue is destroyed and foreign matters left in the wound, especially a punctured one, caused by a rough or rusty instrument, that there are complications. The tissues themselves, when they lose their vitality, become irritants, even when the dead matter is very minute. A boil furnishes an example, the core of which is composed mainly of yellow, elastic fibres with some leucocytes or pus cells in the meshes. This mass becomes dead, and while the white fibrous element liquefies and mingles with the pus, the yellow fibres remain unchanged and constitute the irritant body, to the presence of which is due the suppurative inflammation. The presence of a clot of blood in a wound may prevent union, and cause pus formation.

Chemical irritants excite inflammation by first causing increased redness, which steadily extends and becomes more intense until a considerable diameter is attained. There is also increased heat and fullness of the part affected, and an eschar forms as the result of the action of the chemical agent upon the epidermis in the case of the skin, or the corium in the case of mucous membrane, the depth of the action depending upon the nature of the chemical irritant. After one or two days the narrow circle of redness disappears, and, after one or two weeks, the eschar separates, disclosing an area of smooth cicatricial tissue. When the action of such irritants is slight, repair soon follows, without the formation of an eschar; but when their action is severe enough to devitalize the tissue, the sloughs are thrown off without suppuration, if the parts have been protected from the air, or antiseptic applications have been made.

Mineral irritants, such as mercury and arsenic, for example, cause inflammation by a process different from that of chemical irritants. The inflammatory action of mercury and arsenious acid is developed only after the poison has entered the circulation, and a certain amount has been received by the stomach, when active inflammation of the mouth with salivation (mercurial stomatitis) supervenes, if the agent is mercury, or active gastric hyperemia with vomiting, if the agent is arsenic. There is a specific poisonous action brought about by such irritants, on account of the tissues of the mouth and stomach being more sensitive to the influence of these mineral poisons. The cause of this peculiar susceptibility is as yet obscure.

Micro-organisms are capable of exciting inflammation by direct contact with tissues, when the latter are exposed by injury.

Many of these low forms of life are indestructible by the most extreme heat and cold, and also by the strongest chemical agents. And whenever the oxygen is prevented from entering a wound these micro-organisms generate with great rapidity, and are nourished by the fluids and granulating surfaces which surround them. The chemical and vital changes which these animal materials undergo bring about putrefaction through the agency of fermentation, and certain poisonous combinations are thus formed. These micro-organisms, therefore, acting as a poison, decompose the materials generated for repair, and thus prevent the constructive process; they also act as a putrefactive ferment, producing septic poisons destructive in their action. It has been definitely determined that these organisms, although present in every destructive inflammation, "do not occur in the blood nor in the tissues of the healthy living body of man or of the lower animals." Diminished vitality, whatever may be the cause, favors the invasion and development of micro-organisms in the form of parasites.

While some of the common parasites cause injurious effects by inciting inflammation in tissues and organs, they only act as foreign bodies, while the microscopic fungi and their germs, acting as invisible particles of organized matter, cause the greatest injury. If they are protected from the influence of oxygen, these micro-organisms germinate very rapidly, and derive their sustenance from the fluids of the inflamed tissue and the granulating surface. The form known as vibrio septica generate putrefaction in animal matter by bringing about a process of fermentation - dental caries affords an example. The chemico-vital changes accompanying the process of putrefactive fermentation give rise to septic poisons, which, being absorbed in the circulation, result in septicemia and pyemia. Dr. Koch affirms that "bacteria do not occur in the blood nor in the tissues of the healthy living body, either of man or of the lower animals," and the same investigator also says: "When introduced by inoculation into animal tissues, they multiply and excrete soluble substances, which get into the surrounding tissues by diffusion."

The tissues, by the contact of the micrococci, lose their vitality, and the parasite growth multiplies and spreads rapidly in the dead tissues, advancing directly towards the parts in which vitality yet remains.