In the _lungs_ the pleuræ and subpleural and interlobular tissue are extensively infiltrated and thickened by a profuse yellowish serous or sero-sanguineous exudate, so that the appearance may closely resemble that of lung plague. The lung tissue is consolidated, hepatized and dark red, with at some points emphysema. The pleural sac is usually filled by a serous or bloody effusion (2 to 25 quarts) and there is often extensive implication of the pericardium. The tracheo-bronchial mucosa and bronchial glands show extensive infiltration and thickening. In the _abdomen_ are found extensive infiltrations and blood extravasations in the mucosa and submucosa of the stomachs and intestines, softening and shedding of the epithelium, infiltrations of the peritoneum, diaphragm, and sublumbar adipose tissue, and softening and degeneration of the liver and kidneys. The intestinal gastric and mesenteric glands are usually infiltrated, softened and blood-stained. Engorgement of the spleen is exceptional. The _blood_ is very black but not usually materially changed in consistency nor coagulability. Petechiation of the different serosæ and other tissues is a prominent feature. In _chronic cases_ the lesions are mostly shown in the lungs and lymph glands. The lungs show circumscribed lobular islets of congestion, induration or caseation, offering a suggestion of tubercle, which is all the more deceptive when cretifaction has set in. The caseous centres may vary in size from a pea to a walnut, and some may have ruptured to form a vomica discharging into a bronchium. Bronchia leading to affected lobules are blocked with muco-purulent matter, yellowish, thick and tenacious, and their mucosa is thickened and puckered. The enlarged lymph glands are especially those of the bronchia, trachea, mediastinum, bowels, mesentery and sublumbar region. The bacillus is present in the exudate but is especially abundant in the blood, and in the chronic cases in the bronchial mucous. _Diagnosis._ From _anthrax_ (gloss-anthrax) this affection is easily distinguished by the absence from the blood and exudates of the large, square ended anthrax bacillus, by the absence of enlargement and blood engorgement of the spleen, and of the softness and diffluence of the blood clot which characterize anthrax. Swine which are with difficulty inoculated with anthrax are very susceptible to hæmorrhagic septicæmia. Sheep which are very receptive to anthrax are somewhat refractory to the disease now in hand. Pigeons resist anthrax but readily contract septicæmia hæmorrhagica. From _black quarter_ it is readily distinguished by the absence of emphysema and crepitation and of a secondary cooling in the external swellings, by the presence of the germ in abundance in the blood, by its smaller size, its bipolar staining, and its lack of motility and of spores. Inoculation with black quarter bacillus kills the guinea pig, but spares the pigeon.