ABSCESS, a collection of pus localized in solid tissues. When situated in a cavity special names are applied; e.g., empyema (pleural cavities), hypopyon (aqueous chamber of eye), pyosal pinx (Fallopian tube). The usual method of formation is as follows. When staphylococci or other pyogenic organisms have reached some spot, being carried thither by the blood stream or directly introduced, they begin to multiply and form their specific toxin. This causes death of tissue cells in the neigh bourhood and the resulting proteid material serves as nutriment for further multiplication of the organisms. The irritant action of the toxins leads to local inflammatory changes in the tissues whereby extension of bacterial growth and effects are limited. The inflammatory fluid poured out by the congested blood vessels (see INFLAMMATION) coagulates (hence the early firmness to touch), but the coagulum and the dead tissue cells are soon liquefied by ferments produced by the bacteria (hence the fluidity of the contents of an abscess).
Throughout the whole process multitudes of leucocytes have left the blood vessels and collected at the seat of inflammation. These give to the abscess fluid its characteristic creamy appear ance. Were it not for the zone of surrounding inflammation the irritant and destructive action of the micro-organisms would extend indefinitely, and something akin to this occurs if the bacteria be particularly virulent or the resisting powers of the individual unusually low. The fluid in an abscess "points" in the direction of least resistance. Frequently this is towards the surface, or the pus may track along the muscle. When an abscess opens on the skin, the repair tissue in its walls usually brings about healing through the formation of scar tissue ; when it opens into a serous cavity (see CoELom) it leads to a generalized suppuration probably fatal unless surgically treated with success. (W. S. L.-B.) ABSCISSA. In Cartesian co-ordinates (see CO-ORDINATES) the abscissa of a point (P) is the part (OP') of the x axis lying between the origin (0) and the point (P') where a line (PP') parallel to the y axis cuts the x axis. In each of the two figures, the line-segment PP" is also called the abscissa of P. The axis of x (OX) is called the axis of ab scissas. The term was used in its techni cal sense by Pietro Mengoli in 1659.