[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, thus causing the serious condition known as blood poisoning. [Illustration: 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.