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, thus causing the serious condition known as blood poisoning.
Fig. 23.—"The History of a Boil" (continued). The migration of leucocytes has continued until now they form a dense mass surrounding the germs. The poison of the germs has killed all the leucocytes and also all the cutis immediately around them, and now digestive fluids from the dead leucocytes is turning the whole dead mass into liquid pus. The boil has "come to a head." There is a little lump on the skin and through its thin covering of cuticle can be seen the yellow pus. (From Emerson's "Essentials of Medicine.")
Inflammation may be treated by means of hot applications, cold applications, or counter-irritants. The effect of heat is to dilate the vessels and hence to increase the flow of blood to the injured part. This increased blood supply makes
the reparative process go on more vigorously, and also makes it possible for the accumulated fluid to be more rapidly carried away. Moist heat softens the tissues so that pus, if formed, can escape more easily.
Fig. 24.—"The History of a Boil" (concluded). The boil has finally ruptured. The liquid pus has escaped carrying with it the germs and most of their poisons; the migration of leucocytes has stopped; the capillaries are returning to normal size and now new tissue will grow and fill up this hole. (From Emerson's "Essentials of Medicine.")
Cold acts in just the opposite way. It decreases the size of the blood vessels so that less blood comes to cause pain and swelling; at the same time it diminishes the number of white blood corpuscles and the nutritive substance brought by the blood. The nature and location of the infection determine whether heat or cold is to be preferred.