ANIMAL HEAT. Heat generated in animal bodies by certain of the changes constantly taking place within them. A certain amount of heat is necessary to the proper performance of the functions of the body, and any material increase or decrease of it from the standard endangers health. The air and other objects surrounding the body being in almost all cases colder than it, are constantly stealing part of its warmth; but within the system there are processes constantly going on which produce more heat. When the heat thus generated is not dissipated fast enough, so that the body tends to become warmer than the due degree, perspiration results, the evapora tion of which carries off the excess. The power of producing heat is in relation to the climate in which the animal is accustomed to live. It is weaker in warm climates than in cold, and consequently when an animal is removed from a warm to a cold climate it frequently pines and dies. In most fish and reptiles, commonly termed "cold-blooded animals," the temperature differs but little from that of the water or air in which they live: the same is the case with hiber nating animals dur:ng, the later part of their torpid condition. it may thus occur that the degree of temperature of "cold-blooded" animals may be higher than that of man.
Man has the power, to a greater degree than other warm-blooded animals, of adapting himself to changes of surrounding temperature. His average standard of heat is about 98.0° F. (30.S° C.), varying with circumstances, being slightly higher after exercise or a hearty meal. and at noonday than at midnight. It differs slightly in various parts of the body, the interior being from %° F. to 1%° F. higher than the exterior. It also varies in diseased conditions of the body, rising to 106° F., or even 111° F. to 113° F., in a fever or sunstroke or heatstroke, and falling as low as 90' F. in cholera. A temperature of 105° F.. if maintained for several hours, is al most inevitably fatal. But if the body be in
a healthy condition, the standard of heat is maintained, even when the person is exposed to intense heat, as in the case of men attending furnaces; one can for a short time lie exposed to 350° F. of dry heat without materially raising the temperature of his own body, although he will lose weight by the copious perspiration induced.
Throughout the animal kingdom the power of generating heat bears a close relation to the activity or sluggishness of the animal. Thus, many birds which are perpetually in action have the highest temperature (100° F. to 112° F.) : and the swallow and quick-ffighted birds higher than the fowls which keep to the ground. The higher the standard of animal hent.the less able is the animal to bear a reduction of its tempera ture; if that of a bird or mammal be reduced 30° F. the vital changes become slower, more languid, and death ensues. Fish and frogs, on the other hand, may be inclosed in ice and still survive.
The sources of animal heat in the living body are the chemical and physical changes contin ually taking place. The chemical changes are those occurring in respiration. digestion. nutri tion, secretion, and muscular and nervous action. It has been shown experimentally that when those functions are performed there is an in crease of temperature. it is probable that mus cular action is the most important item in heat production. The ultimate sources of heat are (1) the energy locked up in the food consumed and (2) in the oxygen inhaled in respiration. The food, in the processes of digestion. is split up into its constituent parts: these are absorbed, and may become parts of the textures and fluids of the body for a time; and these textures, in the performance of their functions, disintegrate, become redissolved, and are then eliminated by various channels from the body; all of these pro cesses generate heat.