Cause.—The cause of anthrax is a microscopic organism known as the anthrax bacillus. (See [Pl. XXVIII], fig. 7.) In form it is cylindrical or rodlike, measuring 1/5000 to 1/2500 inch in length and 1/25000 inch in diameter. Like all bacteria, these rodlike bodies have the power of indefinite multiplication, and in the bodies of infected animals they produce death by rapidly increasing in numbers and producing substances which poison the body. In the blood they multiply in number by becoming elongated and then dividing into two, each new organism continuing the same process indefinitely. Outside the body, however, they multiply in a different way when under conditions unfavorable to growth. Oval bodies, which are called spores, appear within the rods, and remain alive and capable of germination after years of drying. They also resist heat to a remarkable degree, so that boiling water is necessary to destroy them. The bacilli themselves, on the other hand, show only very little resistance to heat and drying. It has long been known that the anthrax virus thrives best under certain conditions of the soil and on territories subject to floods and inundations. The particular kinds of soil upon which the disease is observed are black, loose, warm, humous soils; also those containing lime, marl, and clay, finally peaty, swampy soils resting upon strata which hold the water, or, in other words, are impervious. Hence fields containing stagnant pools may be the source of infection. The infection may be limited to certain farms, or even to restricted areas on such farms. Even in the Alps, more than 3,000 feet above sea level, where such conditions prevail in secluded valleys, anthrax persists among herds.