This appears as an exaggerated form of the general sense of relaxation, weakness and languor which follows on prolonged violent exertion in a hot atmosphere. There is more or less impairment of the vaso-motor nerve centre in the medulla, relaxation of the capillary system, and flagging of the heart’s action, which loses its customary stimulus, by reason of the defective supply of blood returned by the veins. This may become so extreme that the patient dies by syncope. In other cases the paresis is mainly shown in the vaso-motor system, and its centres in the medulla, the blood is delayed in the distended capillaries and veins, it becomes overcharged with carbon dioxide, the heart’s action is accelerated and feeble, the pulse rapid, weak and fluttering, perspiration breaks out on the skin, and the temperature is normal or subnormal. The muscular weakness, the flaccid condition of the facial muscles, and general depression suggest a state of collapse. This condition is not necessarily due to exposure to the intensity of the sun’s rays, but may come on in animals subjected for a length of time to artificial heat, and especially if the air is impure, and if the subject has to undergo severe physical exertion. _Treatment._ In slight cases of this kind a stimulant is usually desirable and ammonium carbonate in bolus or solution will usually serve a good purpose. In its absence alcohol or spirits of nitrous ether may be given. Digitalis is of great value in sustaining the flagging action of the heart and has the advantage that as digitalin it can be given hypodermically when it is impossible to give ammonia, alcohol or ether by the mouth. For the same reason nitro-glycerine may be resorted to, or even atropia as a vaso-motor stimulant. Active friction of the body and limbs will aid circulation and indirectly stimulate the heart, and in case of subnormal temperature it may be supplemented by a warm bath in the smaller animals, kept up until the normal temperature in the rectum has been restored. Thermic Fever. Sun-stroke. This is readily distinguished from _heat exhaustion_ by the predominance of the hyperthermia. While in =heat exhaustion= the temperature is usually subnormal, in =sun-stroke= it is excessive, (108°–113° F.). _Causes._ The immediate cause of sun-stroke is exposure to undue heat, but this need not be the heat of the sun’s rays direct. A large proportion of cases in the human subject are attacked during the night, and again at sea where an attack in a passenger is practically unknown, it is terribly common among stokers working in a close atmosphere of 100° to 150° F.