of the skin; an abscess in the breast from organisms which have passed
along the milk ducts opening on the nipple, or along the lymphatics
which accompany these. An abscess in a lymph gland is usually due to
infection passing by way of the lymph channels from the area of skin or
mucous membrane drained by them. Abscesses in internal organs, such as
the kidney, liver, or brain, usually result from organisms carried in
the blood-stream from some focus of infection elsewhere in the body.

A knowledge of the possible avenues of infection is of clinical
importance, as it may enable the source of a given abscess to be traced
and dealt with. In suppuration in the Fallopian tube (pyosalpynx), for
example, the fact that the most common origin of the infection is in the
genital passage, leads to examination for vaginal discharge; and if none
is present, the abscess is probably due to infection carried in the
blood-stream from some primary focus about the mouth, such as a gumboil
or an infective sore throat.

The exact location of an abscess also may furnish a key to its source;
in axillary abscess, for example, if the suppuration is in the lymph
glands the infection has come through the afferent lymphatics; if in the
cellular tissue, it has spread from the neck or chest wall; if in the
hair follicles, it is a local infection through the skin.

#Formation of an Abscess.#--When pyogenic bacteria are introduced into
the tissue there ensues an inflammatory reaction, which is characterised
by dilatation of the blood vessels, exudation of large numbers of
leucocytes, and proliferation of connective-tissue cells. These
wandering cells soon accumulate round the focus of infection, and form a
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