Inflammation is the protective reaction of irritated and damaged tissues which still retain vitality.
The causes of inflammation may be divided into mechanical, chemical, and vital, or infectious and non-infectious.
Traumatism of any nature, such as a blow or the action of chemicals, can give rise to an inflammatory reaction and be non-infectious.
The common cause, however, is the action of bacteria upon the tissues. The great majority, therefore, of inflammations are infectious or vital in variety.
A non-infectious one may become infectious through a secondary deposit of bacteria.
An infectious inflammation is distinguished by the fact that it is likely to be progressive, is capable of indefinite increase, and may also be transmitted from one individual to another.
Before taking up the pathologic changes of the circulation it will be necessary to first consider the normal differences in the blood-current in arteries, veins, and capillaries.
In arteries the stream is not constant; it is regularly intermittent on account of the rhythmic contractions of the heart. It is more rapid than in the veins; the red cells cannot be distinguished at the height of systole, but at the end of the heart's action the current slows sufficiently for them to be seen. The corpuscles occupy the entire lumen, except that at the end of the pulse-wave they momentarily withdraw from the wall of the blood-vessel.
In veins the stream is constant and is regular in speed. Instead of cells and plasma being uniformly mixed there are two zones present: an axial or central zone, composed of blood-cells, and a peripheral one, made up of the blood-plasma. In this latter there are occasionally a few leukocytes, but no erythrocytes found.
In capillaries the current is neither constant nor regularly intermittent. It is constant during the flow.
The changes in the circulation in inflammation are as follows:
1. A momentary contraction of the blood-vessel following the introduction of the irritant. This is followed by:
2. A marked dilatation and relaxation of the vessel with at first an increase in the rapidity of the flow. Arterioles are first affected, then veins and capillaries.
3. Further increase in dilatation with slowing of the current. Instead of the cells being unrecognizable in the arteries, they now become distinctly visible. Marked changes now occur, particularly in the venous circulation. The plasmatic zone, which at first contained only a few leukocytes, shows an increase in their number until it is entirely filled with them.
Fig. 21. - Acute Inflammation (Mallory).
Peripheral arrangement of polymorphonuclear leukocytes in vein.
Fig. 22. - Inflammation of the Mesentery, Showing Overfilling of the Blood-vessels, with Emigration of Leukocytes and Dia-pedesis of Red Corpuscles (Ziegler).
Subsequent to this there takes place an exudation of fluid and blood-cells from the vessels.