The Physical Forces, Mechanical, Chemical, Thermal, And Electric, often directly affect the tissues. Their effect is to damage the parts on which they act. The mechanical forces cause wounds, bruises, fractures, etc.; the chemical forces produce disintegration or less severe injury to the tissues; excessive heat and cold kill the tissues, and, acting less strongly, do a certain amount of damage. In all these cases the immediate effects are local ones, the living structures acted upon are injured. But as the tissues are living tissues, certain phenomena ensue which are the expression of the physiological processes, altered by the damage done to the tissue. We have a series of phenomena generally grouped under the term Inflammation, which arise more or less directly from the damage done to the living structures, and more particularly to the blood-vessels. The phenomena of inflammation are essentially local, having their seat in the damaged part; they are the expression of its own action. To some extent these phenomena are also the result of an attempt in the part to recover from the damage inflicted, or, as it is called, of a reaction of the living tissues against the injury done.

But even in the case of purely physical causes, the phenomena do not always remain purely local. As the blood-vessels of the injured part are in communication with the general circulation, the products of the changes occurring in the tissues may be carried throughout the body, and produce changes in the blood and tissues as a whole. An example of this is furnished by some cases of extensive burns of the skin. The blood in the parts exposed to the excessive heat may be damaged; the red corpuscles are sometimes killed. The dead corpuscles are disintegrated and their products are carried with the general circulation, and may do damage, especially while in the process of elimination by the kidneys. Again, in almost all inflammations the fluid of the blood passes out, to some extent, amongst the tissues or on to surfaces. In these situations it may undergo putrefactive or other changes, the products of which may pass back into the blood by way of the lymphatics or veins, and so be carried throughout the body. The result of this is a different series of phenomena, included under the term Fever or Pyrexia. Hence fever is a frequent accompaniment of inflammation, even when the latter is produced by physical causes.