protective barrier which tends to prevent the spread of the organisms
and to restrict their field of action. Within the area thus
circumscribed the struggle between the bacteria and the phagocytes takes
place, and in the process toxins are formed by the organisms, a certain
number of the leucocytes succumb, and, becoming degenerated, set free
certain proteolytic enzymes or ferments. The toxins cause
coagulation-necrosis of the tissue cells with which they come in
contact, the ferments liquefy the exudate and other albuminous
substances, and in this way _pus_ is formed.

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 fasciæ 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