Death of portions of the skin occurs under various circumstances. Perhaps the commonest case is that of Bedsores (Decubitus), in which a piece of skin which has been exposed to pressure whilst lying, dies and is discharged as a slough. This occurs chiefly in weak and debilitated persons. It is met with frequently in insane persons and paralytics, and it is matter of controversy how far the affection of trophic nerves may have to do with the necrosis (see under Necrosis).

In all forms of Cutaneous ulcers there is necrosis, which may be in the molecular or in the more massive form. The margin of the ulcer is inflamed and infiltrated with inflammatory products. Enlargement takes place by a gradual molecular necrosis of the inflamed structures, and generally implies the continuous action of an irritant. The irritant is mostly an infective virus, and the majority of the ulcers will be again referred to under subsequent headings.

In the case of such Infective ulcers there is usually a granulating surface which is gradually destroyed by the action of the virus. When the virus has been destroyed, then the ulcer proceeds to heal like a granulating wound.

The Varicose ulcer, arising in connection with varicose veins, owes its origin to a prolonged venous hypersemia, leading to oedema and infiltration of the skin. The nutrition of the skin is seriously compromised, and a trivial external injury may cause the epidermis to give way. The exposed and inflamed skin forms granulations, but these also, from the persistent venous engorgement, are readily destroyed by slight injuries, and the ulcer advances, while the inflammatory infiltration of the skin outside extends. The granulations will only proceed to healing in the usual way when the venous hyperemia is removed.

The Perforating ulcer of the foot is a peculiar form which is believed by some to be due to a trophic lesion of the nerves. There have been found in some cases inflammatory lesions of the nerves, and the disease is sometimes associated with nervous affections, especially locomotor ataxia and more rarely with diabetes. It is questionable how far mere anaesthesia is sufficient to account for the formation of the ulcer, as seems to be the case in anaesthetic leprosy. The ulcer is characterized by the formation of an aperture on the surface of the foot, which leads to penetrating sinuses in its substance, often attacking the bones.