ANIMAL HEAT is that generated in animal bodies by certain of the changes con stantly 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 healthy standard endangers life. 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 incessantly 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, the accumulation finds vent in perspiration, the evaporation of which carries off the excess. The power of producing heat is iu 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,"ahe temperature differs but little from that of the water or air in which they live; the same is the case with hibernating animals during the latter part of their torpid condition.
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 beat is about 98.4° F., varying with circumstances, being slightly higher after exercise or a hearty meal, and at noonday than at midnight. It also varies in diseased conditions of the body, rising to 100° in a fever, and falling as low as in cholera. 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 be exposed to 350° of heat without materially raising the temperature of his own body, although he will lose weight by the copious perspiration necessary for the evaporation.
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°-112°); and the swallow and quick-flighted birds, higher than the fowls which keep to the ground. The higher the standard of A.
H. the less able is the animal to bear a reduction of its temperature; if that of a bird or mammal be reduced 30°, the vital changes become slower, more languid, and death ensues. Fish and frogs, on the other hand, may be inclosed in ice and yet survive.
The Sources of animal heat in the living body arc the chemical and physical changes continually taking place. The chemical changes are those occurring in respiration, digestion, nutrition, secretion, and muscular and nervous action. It has been shown experimentally that when those functions are performed there is an increase of tempera ture. Heat is, no doubt, also produced by any movements causing friction. The ulti mate 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, disin tegrate, become redissolved, and are then eliminated by various channels from the body: all of these processes generate heat. On the other hand, the oxygen of the air, by intu ing, in the process of respiration, with the carbon or hydrogen of certain of the tissues or of the food, produces carbonic acid and water, and thus also heat is generated. If we estimated the potential energy of the food consumed and of the oxygen inhaled in respiration as so much heat, and also estimated, as near accuracy as possible, the amount of heat produced in the various processes above referred to, it would be found that this latter amount of heat would be less than that derivable from the food and oxygen. This deficiency is accounted for by the work done by the body, partly as internal mechanical work, such as the movements of heart and lungs, etc., and partly as external mechanical work, such as the movements of the body in the performance of the daily activities of life. This view of A. H., which is now universally adopted, was first put forward by J. R. Mayer, of Heilbronn, in 1842-1845, and numerous applications of it have since been made to many physiological and pathological phenomena.