We include here the lesions produced by the application of excessive heat to the skin, whether the heated substance applied be in the solid, liquid, or gaseous form. The heat acting as an irritant damages or kills the tissue, and so we have signs of inflammation, frequently associated with necrosis.

Burns have been distinguished, according to their intensity, into three degrees, characterized by the three phenomena - hyperemia, vesication, sloughing.

In the First degree the heat is only sufficient to produce a hyperaemia of the skin. Exposure to the rays of the sun, or to hot water of a temperature of 100° F., and upwards, causes such a relaxation of the arteries as to induce considerable congestion. The inflammatory nature of this hyperemia is shown by its continuance for some time after removal of the cause, and by the occasional occurrence of some cedematous swelling of the skin. After a time there may be desquamation.

The Second degree is produced by the action of a temperature of 140° to 185° F., or by the evanescent action of a still higher temperature. The effect is to cause an inflammatory exudation from the papillary layer of the skin, which accumulates in the epidermis so as to form a vesicle. The changes in the rete Malpighii are similar to those already described as occurring in the formation of the small-pox vesicle, but they are more rapidly produced and the layer of cells is more homogeneously affected. The inflammation soon subsides,.and from the remaining cells of the rete Malpighii new epidermis is reproduced; there may be already a complete coating of new epidermis before the vesicle bursts. But sometimes the vesicle bursts early, and the inflamed surface of the skin is exposed; or nearly the whole of the rete Malpighii may have been destroyed, even the interpapillary processes. In these cases the covering-over of the surface with epidermis may be delayed, and the surface may even become like a granulating wound, discharging pus.

In the Third degree there is necrosis not only of the epidermis but of the true skin. The temperature is over 140° F., and if very high may have acted for a very short time. As a general rule the necrosed tissue lies as a dry crust on the surface. An inflammatory process occurs around it, with exudation, going on usually to suppuration. In this way the slough is separated, and a granulating wound is left, which heals in the usual way, leaving a cicatrix, the character of which varies with the depth of the slough.

Reference has already been made to the fact that Hemoglobinuria sometimes follows burns, and also that reduction of temperature may occur. These, along with reduction of blood-pressure from dilatation of arteries, probably have to do with the occasional rapid occurrence of death from extensive burns.

If the person survive for some time after the burning, inflammations of internal organs, the lungs, kidneys, serous membranes, are frequently found; also fatty degeneration of the heart and of the liver. These are to be ascribed with great probability to the alteration of the blood.

It is much more difficult to account for the occurrence of the Duodenal ulcer after burns. When several weeks elapse before death, there is found, in about 20 per cent, of the cases, one or more ulcers, usually situated in the duodenum near the pylorus, but sometimes in the pyloric region of the stomach. It may be a superficial hsemorrhagic erosion or a deeply penetrating ulcer.

(B) Frost-Bite And Chilblain

These conditions are produced by the action of excessively low temperatures. We can distinguish three degrees here also.

The common Chilblain or Pernio is an example of the First degree. If portions of the body are frequently exposed to a low temperature and then heated, a chronic or subacute inflammation is induced. This occurs especially in the fingers and toes. There is passive hypera-mia sometimes amounting almost to stasis, so that the part has livid hue. There is also serous exudation, so that the part is swollen and cedematous; there may be red corpuscles as well as leucocytes in the exudation. Sometimes ulceration occurs.

The Second degree includes the Less severe cases of frost-bite. The skin has been frozen and too rapidly thawed, the result being such an injury to the tissue as to cause an acute inflammation The epidermis is raised in blisters, which often contain a bloody serum. There may even be sloughing of the superficial layers of the cutis, and by the separation of the slough an indolent ulcer forms.

In the Third degree, forming the More severe cases of frost-bite, there has been prolonged exposure to very intense cold. The consequence is the complete freezing of part of the body, especially in regions removed from the heart, as the fingers and toes. The tissues are stiffened by freezing, and the blood coagulated, so that sometimes parts can be broken off like glass. When the parts are thawed gangrene sets in. The whole frozen part does not die, but there is partial recovery with inflammation. The gangrenous part is separated in the usual way, generally with decomposition (moist gangrene).

(C) Wounds And Excoriations

The phenomena connected with the healing of wounds and the formation of granulations have been considered in previous pages. When merely the epidermis is removed by an injury, forming an Excoriation, it is restored with remarkable celerity. The epidermic cells at the margin and those of the deeper layers divide and form new cells which restore the surface.

In the case of a deeper wound the epidermis also shows great vigour, and it sometimes advances beyond the line of the other healing processes. Dr. Macewen has pointed out to the author that the new formed epidermis may advance over a blood-clot filling up a wound. The epidermis here has a remarkably transparent hyaline appearance and is continuous, not with the Malpighian layer, but with the layers superficial to that. The character of the cells here and their transparency suggest that they may be derived from the stratum lucidum. It may, indeed, be suggested that this layer in the normal skin is composed of young epidermis, and that the loss •on the surface is supplied by new-formation here and not in the Malpighian layer.