There are two forms in which gangrene is observed: dry and moist.
Dry gangrene, or mummification, is a condition which occurs in consequence of a gradual diminution and final cessation of the blood supply, with the venous outflow intact. In this way, aided by evaporation and the venous return, there is a gradual drying of the parts. Diseases of the arteries and increasing pressure upon them from growing tumors, causes this variety.
Moist gangrene is due to the sudden arrest of the arterial supply, or a similar obstruction to the venous return.
This is the variety commonly met with from crushing or cutting accidents; from the effects of carbolic and other acids; from cold; and from thrombosis and embolism.
A thrombus is a blood clot occluding the lumen of a vessel. An embolus is a loosened part of a thrombus or any other foreign substance, free in the blood stream, such as a drop of fat, an air globule, or a detached particle of tissue from growths in the heart or vessels. Any one of these may find lodgment in a terminal vessel, and plug it.
Moist gangrene therefore differs from dry gangrene in that the arrest of circulation takes place more or less suddenly when the tissues are suffused with blood.
The dry form of gangrene does not occur regularly in the diseases in which it might be expected, and though a true wet gangrene is not found, neither is the typical mummification.
Moist gangrene may occur in diabetes, in senility and in Reynaud’s disease, and probably assumes this form on account of the sudden onset of inflammation in the part from some slight abrasion, or from weak heart action.