Following the local action of extreme heat a condition known as a burn results, in which there is relaxation of the blood-vessels, exudation of serum, and possibly of blood. The extent of the injury to the tissues depends on the degree of heat and its time of action. According to the extent, burns may be divided into four classes: (i) Hyperemia of the exposed surface; (2) extravasation of serum and liquefaction of certain cells, thus forming vesicles; (3) coagulation of the cellular protoplasm with resulting necrosis and extension into the deeper tissues; (4) charring of the tissues and extensive, deep involvement.

Death may result from burns, either immediately from shock or later from exhaustion, from a perforating ulcer of the duodenum, or from toxic substances formed either within the body or absorbed from the skin. There may be marked alterations within the blood and their action may prevent the kidneys from carrying on their functions.

The cause of the duodenal ulcer is not clear, but may depend upon thrombosis of some small vessel and subsequent digestion by the gastric juice. As a rule, a burn, even of the first degree, will prove fatal if it involves one-third the surface of the body.

If the exposure has been general, the result will vary according to the cause, whether steam, dry air, or sun, etc. Exposure to dry air or sun may cause heatstroke or heat exhaustion. In the first there occur symptoms of heart failure, dyspnea, and coma, if severe. Usually the pulse is full and rapid, face flushed, very high temperature, dry skin, labored breathing, unconsciousness, and muscular relaxation. In heat exhaustion the skin is moist, cool, pale, pulse small and soft, unconsciousness unusual, and temperature may be subnormal.

Extreme cold will bring about conditions very similar to those resulting from heat, and will have various symptoms, according to whether the effects have been superficial or deep. In the former the tissue may completely recover, but in the latter the blood-vessels may be involved and gangrene follow.

The primary effect of cold is to bring about a contraction of the superficial blood-vessels. This, however, gives way to a paralytic dilatation, on account of which more blood enters the chilled part and the entire body will be affected.

If the tissue should freeze during the stage of contraction, the part would appear pale; if during that of dilatation, it would be swollen and dusky in color.

Electricity causes destruction of tissue either by the heat generated or-by the resistance of the body to its passage.

Death may result instantly from disturbance of the nervous system or there may be extensive and destructive burns. Sometimes there is involvement of internal organs.

X-rays when applied too closely or for too great a length of time occasionally give rise to a dermatitis or even to burns of the first, second, or third degrees. As a sequel to the dermatitis, squamous-celled carcinoma has quite frequently developed and gone on to a fatal termination.

Barometric pressure may cause disturbances if it be either greatly increased, as in deep-sea divers or caisson-workers, or diminished, as in mountain-climbers and in persons ascending in balloons. In the latter the blood shows an increase in the number of red corpuscles, in their specific gravity, and in their hemoglobin content.

Season also has a distinct effect upon disease; pneumonia and bronchitis being most common in winter, typhoid fever and malaria in spring, yellow fever and enteric disorders in the summer. In cold weather certain diseases become more frequent on account of the crowding of the people.