When a part, as a toe, a whole foot, leg, or arm dies, while the rest of the body lives, it is said to mortify, slough, or suffer gangrene. Once in a while the feet of an old person may undergo slow and dry gangrene. When an artery, as that of an arm, is plugged up by a clot, the arm is apt to mortify in consequence. Frozen feet or toes often die and slough off. Sometimes, especially in ill-ventilated hospitals, stumps of amputated limbs, and wounds of various kinds, slough instead of healing (hospital gangrene). Quite rarely, sore mouth in children may become gangrenous ; and even a lung, or a portion of it, may become: the seat of gangrene. In the last case, the patient is almost sure to die.

Mortification of a part is always more or less dangerous to the life of the whole body in two ways. First, the sloughing process may extend gradually from the part affected towards the centre of the body; and thus, involving vital parts, it may become fatal. Or dead matter from the gangrenous portion may be absorbed by the vessels, and so poison the blood (septicaemia) in a manner seldom recovered from.

When mortification is confined to a small part of the body, as a frozen toe or finger, the rest of the system being in a healthy state, a line of demarcation naturally forms, separating the dead from the living tissues. In some cases, a surgeon will then consider it best to hasten and complete the process ; removing the sloughing part, by an operation. In other instances, the dead parts will drop off, leaving' a surface which will gradually heal.