=Abscess.=--An abscess is a _circumscribed collection of pus_. The term is used in contradistinction to _purulent infiltration_, in which the collection is not circumscribed, but is exceedingly diffuse and extends itself in various directions, the amount at any spot being almost inappreciable. Purulent infiltration is regarded as the more serious of the two conditions, as it is more difficult for pus to escape under these circumstances than when it can be evacuated through a single opening. The term _phlegmon_ is one now generally used to indicate a suppurative process, usually of the general character of purulent infiltration rather than of abrupt abscess, but generally employed to include both conditions. The adjective _phlegmonous_ is coupled with the names of other surgical infectious diseases to indicate that it is complicated by suppuration, _e. g._, phlegmonous erysipelas. _Pus_ is a product of bacterial activity usually formed rapidly rather than otherwise, and abscess formation or phlegmonous activity of any kind is a question of but a few days. _Empyema_ means a collection of pus in a preëxisting cavity. The significance of this condition is well described in the story of inflammation and suppuration, to paraphrase Sutton, read zoölogically, as though it were the story of a battle: The leukocytes (phagocytes) are the defending army, the vessels its lines of communication, the leukocytes being, in effect, the standing army maintained by every composite organism. When this body is invaded by bacteria or other irritants, information of the invasion is telegraphed by means of the vasomotor nerves, and leukocytes are pushed to the front, reinforcements being rapidly furnished, so that the standing army of white corpuscles may be increased to thirty or forty times the normal standard. In this conflict cells die, and often are eaten by their companions. Frequently the slaughter is so great that the tissues become burdened by the dead bodies of the soldiers in the form of pus, the activity of the cells being proved by the fact that their protoplasm often contains bacilli in various stages of destruction. These dead cells, like the corpses of soldiers who fall in battle, later become hurtful to the organism which, during their lives, it was their duty to protect, for they are fertile sources of septicemia and pyemia. This illustration may seem romantic, but is warranted by the facts.