These conditions are distinct. Sun-stroke, which is comparatively rare, is due to the direct action of the sun on the brain and spinal cord. Heat-stroke, on the other hand, would appear to be due to the lack of escape of heat from the body, owing to insufficient evaporation from the skin, and to the effect of muscular fatigue. As a result, poisonous substances accumulate and act detrimentally upon the nerve cells. There is also a deficient supply of oxygen to the blood. High relative humidity plays a very important part in producing attacks of heat-stroke. The milder forms are known as Heat exhaustion and Heat prostration; the severe form is often associated with true sun-stroke. _Symptoms._—Heat exhaustion is really a form of faintness, and recovery soon takes place after a rest. The symptoms of heat prostration are giddiness, often associated with nausea. The patient is bathed in a clammy sweat, his pulse is thready, his breathing shallow, and, it may be, sighing. The condition may pass into unconsciousness, but the temperature is not raised, and death rarely results. There are two kinds of true Heat-stroke; one a form with high temperature, the other what is called Heat cramp, which is common amongst ships’ firemen in the tropics. An early warning sign of heat-stroke is a desire for frequent micturition, and other premonitory symptoms are a dry skin, giddiness, drowsiness, headache, and intolerance of light. The pulse becomes quick and irregular, the skin is hot and dry, and the temperature elevated. The patient may become comatose, or exhibit delirium or convulsions. There is an asphyxial type, in which the face becomes cyanosed and the breathing is in abeyance. _Prophylaxis._—Avoid severe exercise in a hot sun. Wear suitable clothing, which should be loose and easy, and protect the head, and especially the nape of the neck, from the sun’s rays. Dark or tinted glasses are useful. Water should be taken freely, and the bowels kept open; but alcohol must be avoided. Sniffing a mixture of water and vinegar and damping the face are helpful measures.