much blood may come that the vessels
further on are unable to carry it away fast enough. Some of the fluid
part of the blood is then forced out into the tissues, and the part
becomes swollen. Distension of the tissues and pressure on the nerve
endings cause pain, and the injured part now exhibits the characteristic
symptoms of inflammation.

[Illustration: FIG. 21.--"THE HISTORY OF A BOIL." This figure represents
a cross-section of normal skin. Note the surface layer, or cuticle, and
the "true skin," or cutis. In the cutis one sees that the blood
capillaries are just wide enough for the blood-cells to pass through "in
single file." The skin has just been pricked by a dirty pin. On the
point of this pin were several poisonous germs which were deposited at
_a_. (_From Emerson's "Essentials of Medicine."_)]

[Illustration: FIG. 22.--"THE HISTORY OF A BOIL" (continued). The poison
from these germs diffuses through the cutis. The capillaries dilate. The
leucocytes force their way through the walls of the capillaries and
travel towards these germs. Note the dumb-bell shape of the leucocytes
as they pass through the minute holes in the capillary walls, and their
pseudopods as they travel towards their common destination, attracted by
the poison from the germs. The skin in this region is now swollen, red,
hot, and painful. (_From Emerson's "Essentials of Medicine."_)]

At this point, if the injury begins to heal or the bacterial infection
to yield, the extra blood supply is gradually carried off, the blood
vessels resume their normal size, and the tissues return to their usual
condition. If, however, the infection does not yield so quickly, more
and more white blood corpuscles assemble and pass through the walls of
the tiny blood vessels into the tissues. Here the struggle continues.
Some bacteria and some white blood corpuscles are killed, and substances
are formed which liquify these dead cells and also some cells of the
surrounding tissues. The resulting fluid is called pus or matter, and in
the case of a boil we then say it has come to a head. If the infection
occurs near a cavity or near the surface of the body, the pus may escape
by breaking through at the point of least resistance, and may carry most
of the poisons along with it. If the pus finds no outlet it may be
gradually absorbed by the blood stream, and healing may result without
discharging. On the other hand, the germs may make their way into the
circulation, t