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 coördinated 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. 16.--PHAGOCYTOSIS. _a_, _b_, _c_ are the
microphages or the bacterial phagocytes. (_a_) Contains a number of
round bacteria, and (_b_) similar bacteria arranged in chains, and