The numbers present depend much upon the character of the agent which has produced the injury, and they may be more numerous than the ordinary leucocytes which migrate from the blood vessels.
All these changes which an injured part undergoes are found when closely analyzed to be purposeful; that is, they are in accord with the conditions under which the living matter acts, and they seem to facilitate the operation of these conditions.

It has been said that the life of the organism depends upon the cooerdinated activity of the living units or cells of which it is composed.

The cells receive from the blood material for the purpose of function, for cell repair and renewal, and the products of waste must be removed.

In the injury which has been produced in the tissue all the cells have suffered, some possibly displaced from their connections, others may have been completely destroyed, others have sustained varying degrees of injury.
If the injury be of an infectious character, that is, produced by bacteria, these may be present in the part and continue to exert injury by the poisonous substances which they produce, or if the injury has been produced by the action of some other sort of poison, this may be present in concentrated form, or the injury may have been the result of the presence of a foreign body in the part.

Under these conditions, since the usual activities of the cells in the injured part will not suffice to restore the integrity of the tissue, repair and cell formation must be more active than usual, any injurious substances must be removed or such changes must take place in the tissue that the cell life adapts itself to new conditions.
[Illustration: FIG.