A simple incised wound of the skin and superficial tissues made by a knife which is chemically and bacteri ologically clean inflicts an uncomplicated mechanical injury on a limited number of cells' in the epidermis and dermis. This is followed after a few hours by a familiar reaction : the injured part feels full and uncomfortable and is swollen, red and pain ' ful. These are the outward signs of an inflammatory reaction, which consists intimately in a dilatation of the blood vessels, an increased flow of blood to the neighbourhood and an alteration in the walls of the capillary vessels which enables liquid plasma and leucocytes to pass through from the blood to the tissue spaces. The cells killed by the injury undergo autolysis and in the course of a few days they are replaced by fresh cells derived from the adjacent epithelial cells of the skin and from the connective tissue of the dermis, as is appropriate.
The result in the end is that the wound is healed and the part restored to normal. It is impossible to suppose that this reaction is started by the absence of the cells which were originally de stroyed : the actual stimulus is no doubt provided by the soluble products of their autolysis, which can be shown outside the body to have a stimulating action on cell-growth and by suitable ex periments to alter the permeability of capillary vessels. The proc ess is self-regulated and the inflammatory reaction is accessory to the restorative. growth. By increasing the local circulation it raises the temperature of the part and so promotes all cellular activities; it provides plasma, which helps to wash away the soluble remains of the dead cells, and leucocytes, which eat the more resistant remnants of the cells; the discomfort which attends the hyperaemia and swelling is of value in securing rest to the injured part and freedom from further mechanical disturbance, conditions which help cell-growth and healing. The process is also automatically regulated quantitatively for the amount of auto lysate, i.e., the amount of stimulus, depends on the extent of the original injury. Its effect varies with the responsivity of different tissues and, allowing for that, is determined by its concentration. If this is very high, which occurs in the ordinary way only with exceptionally massive destruction of tissue, it injures and kills cells and is responsible for the fatal condition known as second ary shock which may follow extensive injuries. Lower concentra tions stimulate growth, as often happens with cellular poisons. And if the products of autolysis are present only in concentra tions below the threshold value for stimulation, they have no action on the tissues. Hence in the example under consideration the response is limited to the immediate neighbourhood of the injury and the growth of cells takes place where it is needed and not in all the parts of the body to which traces of autolysate are carried in the blood. The local reaction to a bullet or a piece of glass buried in the tissues illustrates the same regulative re action. The response consists in a growth of the connective tissue next to the foreign body without any obvious destruction of cells, due to the irritants nowhere reaching an actually poisonous concentration owing to their small solubility in the tissue fluids.
The history of an abscess illustrates a similar kind of injury stimulus and response, complicated by the fact that the infliction of the injury is itself a process rather than an event. Abscesses can be produced experimentally by unorganized irri tants, but in the natural course they are caused by bacteria which are capable of multiplication and produce poisonous substances which are effective agents in killing and injuring cells, and as stimuli have, in a general way and apart from special actions, the same effect as the products of autolysis. At first the organisms are present only in small numbers and do relatively little damage, but as they increase in numbers the amount and concentration of their poisons increase in parallel until a substantial piece of tissue is destroyed. This undergoes autolysis and the stimulus of the autolysate is added to that of the bacterial toxins. (See INFLAM MATION AND ITS SEQUELS.) That specially sensitive cells may, however, be injured is shown by the fever and feeling of illness which an abscess may cause and by the functional impairment of the secretory cells of the kidney and stomach and other delicate tissues. We also find evi dence of the reception of stimuli in distant parts of the body in the growth of leucocytes in the bone-marrow. Any good-sized abscess contains in its pus more leucocytes than are present at any one time in the circulating blood, from which it follows that the formation of an abscess leads to the production of fresh leucocytes and that the cells which develop into leucocytes must be excep tionally responsive to the small amounts of stimulating sub stances which reach them. This deduction is confirmed by direct examination of the bone-marrow where leucocytopoiesis is found going on actively, often with a vigour which results in there being an obvious excess of these cells in the circulating blood. The purposive and reparative nature of this reaction needs no further elaboration beyond indicating that the passage of plasma out of the inflamed vessels brings antibodies (see IMMUNITY) to bear on the bacteria to which the leucocytes are also directly inimical and that the general, as contrasted with the local, rise of tern perature promotes the multiplication of leucocytes at places dis tant from the abscess itself. The reaction does not always succeed: the bacteria may generate such a concentration of poisons through out the body as is fatal to essential organs such as the brain and heart. On the other hand, an abscess may cure itself by destroying so much tissue that the pus finds an exit to the surface of the body and the absorption of poisons ceases because it becomes mechan ically easier for them to pass outwards with the stream of plasma than inwards, a process of natural cure imitated and anticipated by surgical interference. The same result is gained if the bac teria die out or are killed off.