. 15.)

[Illustration: FIG. 15--A SECTION OF AN INFLAMED LUNG SHOWING THE
EXUDATE WITHIN THE AIR SPACES. Compare this with Fig 6. Fig 15 is from
the human lung, in which the air spaces are much larger than in the
mouse.]

The microscopic examination of any normal tissue of the body shows
within it a variable number of cells which have no intimate
association with the structure of the part and do not seem to
participate in its function. They are found in situations which
indicate that these cells have power of active independent motion. In
the inflamed tissue a greatly increased number of these cells is
found, but they do not appear until the height of the process has
passed, usually not before thirty-six or forty-eight hours after the
injury has been received. 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.

[Illustrat