Burns of Fourth Degree—­Total Destruction of the Skin.—­These follow the more prolonged action of any form of intense heat.  Large, black, dry eschars are formed, surrounded by a zone of intense congestion.  Pain is less severe, and is referred to the parts that have been burned to a less degree.  Infection is liable to occur and to lead to wide destruction of the surrounding skin.  The amount of granulation tissue necessary to fill the gap is therefore great; and as the epithelial covering can only be derived from the margins—­the skin glands being completely destroyed—­the healing process is slow.  The resulting scars are irregular, deep and puckered, and show a great tendency to contract.  Keloid frequently develops in such cicatrices.  When situated in the region of the face, neck, or flexures of joints, much deformity and impairment of function may result (Fig. 63).

[Illustration:  FIG. 63.—­Cicatricial Contraction following Severe Burn.]

In burns of the fifth degree the lesion extends through the subcutaneous tissue and involves the muscles; while in those of the sixth degree it passes still more deeply and implicates the bones.  These burns are comparatively limited in area, as they are usually produced by prolonged contact with hot metal or caustics.  Burns of the fifth and sixth degrees are met with in epileptics or intoxicated persons who fall into the fire.  Large blood vessels, nerve-trunks, joints, or serous cavities may be implicated.

#General Phenomena.#—­It is customary to divide the clinical history of a severe burn into three periods; but it is to be observed that the features characteristic of the periods have been greatly modified since burns have been treated on the same lines as other wounds.

The first period lasts for from thirty-six to forty-eight hours, during which time the patient remains in a more or less profound state of shock, and there is a remarkable absence of pain.  When shock is absent or little marked, however, the amount of suffering may be great.  When the injury proves fatal during this period, death is due to shock, probably aggravated by the absorption of poisonous substances produced in the burned tissues.  In fatal cases there is often evidence of cerebral congestion and oedema.

The second period begins when the shock passes off, and lasts till the sloughs separate.  The outstanding feature of this period is toxaemia, manifested by fever, the temperature rising to 102, 103, or 104 F., and congestive or inflammatory conditions of internal organs, giving rise to such clinical complications as bronchitis, broncho-pneumonia, or pleurisy—­especially in burns of the thorax; or meningitis and cerebritis, when the neck or head is the seat of the burn.  Intestinal catarrh associated with diarrhoea is not uncommon; and ulceration of the duodenum leading to perforation has been met with in a few cases.