The chemical changes which take place throughout the system during the process of waste and repair, are accompanied by the production of heat, and consequently render the temperature of the body higher as a rule, than that of the surrounding air. On the other hand, radiation, evaporation (principally from the lungs and skin), and conduction of heat by contact with objects of lower temperature, reduce this heat. The balance between these respective processes of heating and cooling is not left to chance, but is maintained within narrow limits by the nervous system. Although the temperature of the surface of the body is subject to extensive fluctuations, that of the interior cannot be kept higher than say 1040 F. or lower than 96° F. for even a few hours without grave disturbance of health ensuing. The power of the regulating action of the nervous system on the temperature of the body is by no means absolute; for we find that by prolonged exposure to great atmospheric heat in the tropics, the nervous system may lose its power of control, and heat apoplexy may ensue with a sudden and abnormally high rise (say, over 1070 F.) of temperature. As the rapidity of evaporation, in circumstances of equal atmospheric temperature, varies in inverse proportion to the quantity of moisture in the air; heat apoplexy occurs among horses as a rule, only in hot damp climates, like that of Calcutta for instance. We should here note that the evaporation of perspiration and not the mere pouring out of that fluid on the skin by the sweat glands, is an effective means for cooling the surface. The glare of a tropical sun seems to exert a very disturbing influence on the heat-controlling power of the nervous system; for we find that a thick shade to the eyes (preferably made of a bad conductor of heat, like pith or wood for example) affords great protection from sunstroke to animals which work under the rays of a tropical sun. In this respect, it is much more effective than pith sun-protectors placed over the top of the head, poll, and loins. We should bear in mind that the connection between the brain and the eyes by the optic nerve and its expansion, the retina, is short and direct. Most persons who have lived in the tropics must have felt the cooling effect on the entire system produced by the wearing of blue, green, or neutral-tinted glasses in the open on a very hot sunshiny day. As evaporation implies loss of fluid, a full supply of drinking water is an effective means for cooling the body of an animal exposed to great heat.

Exercise raises the temperature by increasing tissue change, as we learn from the fact that it causes the lungs to give off much more carbonic acid than they do during rest. At the same time, it stimulates the evaporation of moisture from the lungs and skin; and the nerves preserve the balance of healthy temperature.

When the influence of any cooling process, such as that of external cold, is greater than the influence of normal heat production, there will be undue bodily waste owing to increased chemical decomposition being necessary to obtain the extra heat. Consequently, when horses are "turned out" in cold weather, they will require more food to keep them in "condition" than if they were comfortably housed.