If the bacteria gain the upper hand, this process of liquefaction which is characteristic of suppuration, extends into the surrounding tissues, the protective barrier of leucocytes is broken down, and the suppurative process spreads.  A fresh accession of leucocytes, however, forms a new barrier, and eventually the spread is arrested, and the collection of pus so hemmed in constitutes an abscess.

Owing to the swelling and condensation of the parts around, the pus thus formed is under considerable pressure, and this causes it to burrow along the lines of least resistance.  In the case of a subcutaneous abscess the pus usually works its way towards the surface, and “points,” as it is called.  Where it approaches the surface the skin becomes soft and thin, and eventually sloughs, allowing the pus to escape.

An abscess forming in the deeper planes is prevented from pointing directly to the surface by the firm fasciae and other fibrous structures.  The pus therefore tends to burrow along the line of the blood vessels and in the connective-tissue septa, till it either finds a weak spot or causes a portion of fascia to undergo necrosis and so reaches the surface.  Accordingly, many abscess cavities resulting from deep-seated suppuration are of irregular shape, with pouches and loculi in various directions—­an arrangement which interferes with their successful treatment by incision and drainage.

The relief of tension which follows the bursting of an abscess, the removal of irritation by the escape of pus, and the casting off of bacteria and toxins, allow the tissues once more to assert themselves, and a process of repair sets in.  The walls of the abscess fall in; granulation tissue grows into the space and gradually fills it; and later this is replaced by cicatricial tissue.  As a result of the subsequent contraction of the cicatricial tissue, the scar is usually depressed below the level of the surrounding skin surface.

If an abscess is prevented from healing—­for example, by the presence of a foreign body or a piece of necrosed bone—­a sinus results, and from it pus escapes until the foreign body is removed.

#Clinical Features of an Acute Circumscribed Abscess.#—­In the initial stages the usual symptoms of inflammation are present.  Increased elevation of temperature, with or without a rigor, progressive leucocytosis, and sweating, mark the transition between inflammation and suppuration.  An increasing leucocytosis is evidence that a suppurative process is spreading.

The local symptoms vary with the seat of the abscess.  When it is situated superficially—­for example, in the breast tissue—­the affected area is hot, the redness of inflammation gives place to a dusky purple colour, with a pale, sometimes yellow, spot where the pus is near the surface.  The swelling increases in size, the firm brawny centre becomes soft, projects as a cone beyond the level of the rest of the swollen area, and is usually surrounded by a zone of induration.