ANIMAL HEAT, nearly all animals pos sess a heat-regulating mechanism by which they maintain a temperature necessary for the con tinuance of life processes. In many cold blooded animals this sustains a temperature only slightly above that of the surrounding media, and thus in winter they relapse into a torpid state. Some few, however — bees being an ex ample — have a higher temperature and are not torpid. In warm-blooded animals, especially those high in the evolutionary scale, a high constant temperature is usually sustained. Some warm-blooded animals occupy an intermediary position. In summer the temperature is high and constant, in winter they hibernate and the temperature is low and dependent upon that of the surrounding medium. Some cold-blooded animals living in the tropics may really show very high degrees of temperature, thus the terms warm-blooded and cold-blooded are rela tive only. The mean average temperature in man is 36.97° C. (98.4° F.) in the mouth, C. F.) in the axilla, and 372° C. F.) in the rectum. There are slight daily variations, the lowest temperature usually being between midnight and early morning during sleep. Certain warm-blooded animals show in teresting average temperatures. Thus, the horse is 99-100° F., ox 100-101° F., cow 101 102° F., sheep 104-105° F., dog F., cat 101° F., pig 101-103° F., rabbit 101-107° F., rhesus monkey 101° F., duckbill platypus 76° F., hen 106-109° F., duck 107-110° F., sparrow 110° F. The higher the standard of heat, the less able is the animal to bear a reduced tem perature. A F. drop causes vital changes in a bird or animal, and death finally ensues. Fish and frogs, however, have been found to survive after having been enclosed in ice. In cold-bloodel animals the temperature, as has been noted, varies widely. The study of the temperature of bees is of much interest in this connection.
Several conditions modify the regulation of the animal heat; day and night, age, muscular work, sleep, sex, race, pregnancy, idiosyncracies, surrounding temperature, season of the year, baths and certain drugs, all have a distinct influence on the heat regulatory apparatus. The variations in temperature in man compatible with life are wide; a range of less than 2° F. is normal, but variations from 75° to 112° F. have been recorded and the patients recovered. Temperatures below F. and above 106° F. are dangerous.
The chief sources of animal heat are the chemical processes of the body and they are dependent on the food supply. Every kind of food has its definite percentage of heat-pro ducing elements measured in units or calories. Thus 1 gm. (15 grains) of the white of egg has 4,896 calories; the same amount of cow's milk 5,733 calories, of fat 9,600 calories, etc. These are purely physical values, but they have their physiological equivalents. The chief sources of heat production in the human body are the muscles, the heart contraction being a very important one, and the glands (intestines, liver, etc.). Loss of heat takes place through the skin by radiation and conduction, by evapora tion, from the respiration, and from the dejecta.
Regulation of these many factors is in the province of the nervous system. The vaso motor system controls the heat loss by regu lating the amount of blood in the deep and superficial portions of the body, the respiratory centre regulates the amount of respiration and the cerebral cortex regulates the amount of muscular activity that is the main source of the heat production (see FEVER). Consult Kirkes, (Textbook of Physiology> (Philadelphia 1907).