Mortification is not an unfrequent occurrence in the skin; it arises from congestion and inflammation, and takes place more readily the more insuperable the mechanical interruption to the circulation, and the greater the exhaustion of nervous power either in the system generally, or - in consequence of extreme severity of the inflammation or unfavorable external circumstances - in the part itself. Sometimes it takes the form of moist, sometimes of dry gangrene, - sphacelus - mummification. In the former, the epidermis is raised in vesicles of various size, which are filled with a discolored sanguineo-sanious fluid, and the tissue of the skin degenerates to a loose, pulpy, and offensively smelling mass of a brownish, brownish-green, or blackish color: in the latter the cutaneous tissue changes to a black, pretty firm, dry eschar, which is frequently puffed out with gas, developed in the subcutaneous tissues, when they are affected with moist gangrene.

The mortification may extend from the skin to the subcutaneous tissues or vice versâ, or again it may attack both structures, however heterogeneous, together.

Examples of primary gangrene, as well as of the secondary proceeding from inflammation, are furnished by gangrena senilis, by the bluish-red congestion and gangrene that occur, often at several spots, on paralyzed limbs, by the sphacelus accompanying or following typhus and typhoid fevers, by bed-sloughs, anthrax (pustula maligna), cancrum oris (noma), by the mortification that takes place from various internal or external causes in inflamed skin, in ulcers, and in wounds, and by hospital gangrene.

Besides these two forms of mortification, a third is sometimes observed, which has been termed white gangrene (Mayo, Aschersohn). The skin becomes converted into a dirty yellowish-white, or grayish-white, friable eschar. A similar form of necrosis of cellular tissue has been pointed out already, and a similar eschar on serous and mucous membranes. It may arise from the stretching - which is equivalent to compression - of the tissue, or rather of the capillary vessels by an inflammatory swelling, or from the vessels being destroyed in necrosis of the subcutaneous cellular tissue (pseudoerysipelas).