_Definition._ An acute bacteridian disease of domestic and wild mammals, and of man, manifested by doughy, painful and often crepitating swelling in the vicinity of the affected part, and proving fatal in many cases in twenty-four to forty-eight hours. _Cause._ The essential cause is the bacillus of malignant œdema, the septic vibrio of Pasteur, bacillus septicus gangrenæ of Arloing, the bacillus of septicæmia gangrenosa of others. _Morphology._ This is a staff-shaped microbe 3–4 μ. long by 1 μ. broad, often united in chains of three or more to form long flexuous filaments. They are furnished with numerous flagella by which they are rendered very actively motile. The movements are tardy or simply flexuous in the filaments. Spores form in the isolated bacilli (not in the filaments) in suitable culture media and at a temperature of from 20° to 38° C. They occupy a place near the centre of the bacilli, not the ends as in the bacillus of emphysematous anthrax. The bacilli are anærobic and die quickly in air, but the spores are unaffected by oxygen. The spores are similarly resistant to most disinfectants. They will grow readily in ordinary culture media if oxygen is excluded, for example under an atmosphere of hydrogen, nitrogen or carbon dioxide, and liquefy gelatine. Even the oxygen present in the circulating blood is highly inimical to them, so that they are rarely found in the blood during life, but rapidly invade both it and the tissues after death, and the suspension of respiration. In peptonised and glucosed gelatine the colonies are characterized as globules of liquefaction usually combined with gas. The germ is widely distributed in soils in general and not confined to limited areas like the bacilli of anthrax and black quarter. _Animals susceptible._ The bacillus attacks man, horse, ass, goat, sheep, pig, mouse, Guinea pig, rabbit, white rat, cat, dog, chicken, pigeon, and duck. The mature ox is immune, but calves suffer. Dogs are often immune having already suffered from the disease. A first attack gives immunity from a second. _Infection Channels._ Inoculation on an abrasion of the skin or surface sore is not usually infecting, the oxygen of the air destroying the germ. If, however, it is inserted deeply in the connective tissue, subcutem, it grows readily in a susceptible animal. Hence the danger of infection in deep wounds the recesses of which are not exposed to the air, and in such it becomes a most redoubtable surgical complication. If such wounds are the seat of active inflammation, with abundant exudate and more or less exclusion of the air-bearing blood, and in cases of blood stasis the field is specially inviting to the bacillus.