By these we mean high heat, great cold, dampness, sudden changes and partial exposures of the body to either extreme, or electrical influences; these last being very little understood.

Sunstroke is a familiar accident in warm climates. Cold-stroke is less common, but I have known it to be almost as sudden as the opposite. Continued heat predisposes to disorders of the liver, stomach and bowels. Cold, with dampness, promotes affections of the lungs and other organs within the chest.

Catching cold : what is it ? For example; one comes in warm from exercise on a spring or autumn day, takes off his coat, and sits down near a window to "cool off." His skin is relaxed and moist with perspiration, whose evaporation, under the window-breeze, goes on rapidly. Suppose the breeze to blow on his back, between his shoulders. That part is cooled more than the rest of his body. Its blood-vessels and skin-pores contract under the cooling process, detaining the perspiration and driving the blood inward from the surface. Some of the waste matter which the skin would have thrown off by sweating, but for this chilling, is now kept in the blood.

The result may be made more serious than a mere cold. If there be a weak or susceptible part within the chest (bronchial tubes, lungs, pleura, or heart) it suffers from overloading with blood and waste material; and we have a bronchitis, a pneumonia, a pleurisy, or an inflammation of the heart. Among these, the first is the most frequent, and the last the least so ; but even it does sometimes happen, especially in a rheumatic person.