The colour of pus also varies:  when due to one or other of the varieties of the bacillus pyocyaneus, it is usually of a blue or green colour; when mixed with bile derivatives or altered blood pigment, it may be of a bright orange colour.  In wounds inflicted with rough iron implements from which rust is deposited, the pus often presents the same colour.

The pus may form and collect within a circumscribed area, constituting a localised abscess; or it may infiltrate the tissues over a wide area—­diffuse suppuration.

ACUTE CIRCUMSCRIBED ABSCESS

Any tissue of the body may be the seat of an acute abscess, and there are many routes by which the bacteria may gain access to the affected area.  For example:  an abscess in the integument or subcutaneous cellular tissue usually results from infection by organisms which have entered through a wound or abrasion of the surface, or along the ducts 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.