Animal Heat

ANIMAL HEAT. Under this heading is discussed the physiology of the temperature of the animal body.

The higher animals have within their bodies certain sources of heat, and also some mechanism by means of which both the production and loss of heat can be regulated. This is conclusively shown by the fact that both in summer and winter their mean temperature remains the same. But it was not until the intro duction of thermometers that any exact data on the temperature of animals could be obtained. It was then found that local dif ferences were present, since heat production and heat loss vary considerably in different parts of the body, although the circula tion of the blood tends to bring about a mean temperature of the internal parts. Hence it is important to determine the tempera ture of those parts which most nearly approaches to that of the internal organs. Also for such results to be comparable they must be made in the same situation. The rectum gives most accurately the temperature of internal parts, or in women and some animals the vagina, uterus or bladder. Occasionally that of the urine as it leaves the urethra may be of use. More usually the temperature is taken in the mouth, axilla or groin.

Warm and Cold Blooded Animals.

By numerous observa tions upon men and animals, John Hunter showed the essential difference between the so-called warm-blooded and cold-blooded animals to lie in the constancy of the temperature of the former, and the variability of the temperature of the latter. Those ani mals high in the scale of evolution, birds and mammals, have a high temperature almost constant and independent of that of the surrounding air, whereas among the lower animals there is much variation of body temperature, dependent entirely on their sur roundings. There are, however, certain mammals which are ex ceptions, being warm-blooded during the summer, but cold blooded during the winter when they hibernate. But it must be remembered there is no hard and fast line between the two groups. Also, from work recently done by J. O. Wakelin Barratt, it has been shown that under certain pathological conditions a warm-blooded (homoiothermic) animal may become for a time cold-blooded (poikilothermic). He has shown conclusively that this condition exists in rabbits suffering from rabies during the last period of their life, the rectal temperature being then within a few degrees of the room temperature and varying with it. He explains this condition by the assumption that the nervous mechanism of heat regulation has become paralysed. The respira tion and heart-rate being also retarded during this period, the resemblance to the condition of hibernation is considerable. Again, Sutherland Simpson has shown that during deep anaesthesia a warm-blooded animal tends to take the same temperature as that of its environment. He demonstrated that when a monkey is kept deeply anaesthetized with ether and is placed in a cold chamber, its temperature gradually falls, and that when it has reached a sufficiently low point (about 25°C. in the monkey), the employment of an anaesthetic is no longer necessary, the animal then being insensible to pain and incapable of being roused by any form of stimulus ; it is, in fact, narcotized by cold, and is in a state of what may be called "artificial hibernation." Once again this is explained by the fact that the heat-regulating mechanism has been interfered with. Similar results have been obtained from experiments on cats. In man, injury of the spinal cord, as in gun-shot wounds, may abolish the heat regulating portion of the lower part of the body, in which case the body temperature can only be maintained constant by artificial means. These facts— with many others—tend to show that the power of maintaining a constant temperature has been a gradual development, as Dar win's theory of evolution suggests, and that anything that inter feres with the due working of the higher nerve-centres puts the animal back again, for the time being, on to a lower plane of evolution.

Variations in the Temperature of Man and Some Other Animals.—As stated above, the temperature of warm-blooded animals is maintained with but slight variation. In health under normal conditions, the temperature of man varies between 36°C. and 38°C., or if the thermometer be placed in the axilla, between 36.25°C. and 37•5°C. In the mouth the reading would be from .25°C. to I.5 ° C. higher than this; and in the rectum some .g ° C. higher still. The temperature of infants and young children has a much greater range than this, and is susceptible of wide diver gencies from comparatively slight causes.

Of the lower warm-blooded animals, there are some that appear to be cold-blooded at birth. Kittens, rabbits and puppies, if re moved from their surroundings shortly after birth, lose their body heat until their temperature has fallen to within a few de grees of that of the surrounding air. But such animals are at birth blind, helpless and in some cases naked. Animals who are born when in a condition of greater development can maintain their temperature fairly constant. In strong, healthy infants a day or two old the temperature rises slightly, but in that of weakly, ill-developed children it either remains stationary or falls. The cause of the variable temperature in infants and young immature animals is the imperfect development of the nervous regulating mechanism.

The average temperature falls slightly from infancy to puberty and again from puberty to middle age, but after that stage is passed the temperature begins to rise again, and by about the Both year is as high as in infancy. A diurnal variation has been observed dependent on the periods of rest and activity, the max imum ranging from io A.M. to 6 P.M., the minimum from II P.M. to 3 A.M. Sutherland Simpson and J. J. Galbraith have recently done much work on this subject. In their first experiments they showed that in a monkey there is a well marked and regular diurnal variation of the body temperature, and that by reversing the daily routine this diurnal variation is also reversed. The diurnal temperature curve follows the periods of rest and activity, and is not dependent on the incidence of day and night; in monkeys which are active during the night and resting during the day, the body temperature is highest at night and lowest through the day. They then made observations on the temperature of animals and birds of nocturnal habit, where the periods of rest and activity are naturally the reverse of the ordinary through habit and not from outside interference. They found that in nocturnal birds the temperature is highest during the natural period of activity (night) and lowest during the period of rest (day) but that the mean temperature is lower and the range less than in diurnal birds of the same size ; that the temperature curve of diurnal birds is essentially similar to that of man and other homoiothermal animals, except that the maximum occurs earlier in the afternoon and the minimum earlier in the morning. Also that the curves obtained from rabbit, guinea-pig and dog were quite similar to those from man. The mean temperature of the female was higher than that of the male in all the species ex amined whose sex had been determined.

Meals sometimes cause a slight elevation, sometimes a slight depression—alcohol seems always to produce a fall. Exercise and variations of external temperature within ordinary limits cause very slight change, as there are many compensating in fluences at work, which are discussed later. Even from very active exercise the temperature does not rise more than one degree, and if carried to exhaustion a fall is observed. In travelling from very cold to very hot regions a variation of less than one degree occurs, and the temperature of those living in the tropics is practically identical with those dwelling in the Arctic regions.

Limits Compatible with Life.

There are limits both of heat and cold that a warm-blooded animal can bear, and other far wider limits that a cold-blooded animal may endure and yet live. The effect of too extreme a cold is to lessen metabolism, and hence to lessen the production of heat. Both katabolic and anabolic changes share in the depression, and though less energy is used up, still less energy is generated. This diminished metab olism tells first on the central nervous system, especially the brain and those parts concerned in consciousness. Both heart beat and respiration-number become diminished, drowsiness supervenes, becoming steadily deeper until it passes into the sleep of death. Occasionally, however, convulsions may set in towards the end, and a death somewhat similar to that of asphyxia takes place. Cats are unable to survive when the rectal temperature is reduced below i6°C. At this low temperature respiration becomes increasingly feeble, the heart-impulse usually continues of ter respiration ceases, the beats becoming very irregular, apparently ceasing, then beginning again. Death appears to be mainly due to asphyxia.

The heart has great powers of surviving reductions of temper ature. It is said that in Siberia the hearts of animals which have been buried hours or days have, when excised and perfused, been made to beat.

Too high a temperature hurries on the metabolism of the various tissues at such a rate that their capital is soon exhausted. Blood that is too warm produces dyspnoea and soon exhausts the metabolic capital of the respiratory centre. The rate of the heart is quickened, the beats then become irregular and finally cease. The central nervous system is also profoundly affected, consciousness may be lost, and the patient falls into a comatose condition, or delirium and convulsions may set in. All these changes can be watched in any patient suffering from an acute fever. The lowest limit of temperature that man can endure depends on many things, but no one can survive a temperature of (113°F.) or above for very long. Mammalian muscle be comes rigid with heat rigor at about 50°C., and obviously should this temperature be reached the sudden rigidity of the whole body would render life impossible. H. M. Vernon has recently done work on the death temperature and paralysis temperature (temperature of heat rigor) of various animals. He found that animals of the same class of the animal kingdom showed very similar temperature values, those from the Amphibia examined being 38.5°C., Fishes 39°, Reptilia 45° and various Molluscs 46°. Also in the case of pelagic animals he showed a relation be tween death temperature and the quantity of solid constituents of the body, Cestus having lowest death temperature and least amount of solids in its body. But in the higher animals his ex periments tend to show that there is greater variation in both the chemical and physical characters of the protoplasm, and hence greater variation in the extreme temperature compatible with life.

Regulation of Temperature.

The heat of the body is gen erated by the chemical changes—those of oxidation—undergone not by any particular substance or in any one place, but by the tissues at large. The ultimate source of heat is the food which is oxidized in the body and the oxidation of which liberates the same quantity of energy as it would do if burned. This energy is dis sipated either in mechanical work or as heat—the latter forming by far the greater proportion. Not all foods have the same heat producing or caloric value. The ordinary heat production of a person is made up somewhat as follows, per day: Protein 120 grams = 480,000 calories.

Fat . . . . i oo „ _ 940,000 „ Carbohydrate . 33o „ = 1,385,000„ 2,805,000 calories.

Wherever destructive metabolism (katabolism) is going on, heat is being set free. When a muscle does work it also gives rise to heat, and if this is estimated it can be shown that the muscles alone during their contractions provide far more heat than the whole amount given out by the body. Also it must be remembered that the heart—itself a muscle—never resting, must give rise to no inconsiderable amount of heat. From this it is clear that the larger proportion of total heat of the body is supplied by the muscles. These are essentially the "thermogenic tissues." Next to the muscles as heat generators come the various secretory glands, especially the liver, which appears never to rest in this respect. Also a certain amount of heat is produced by the changes which the food undergoes in the alimentary canal before it really enters the body.

But heat is also continually being lost. And it is by the constant modification of these two factors (1) heat production and (2) heat loss, that the constant temperature of a warm blooded animal is maintained. Heat is lost to the body through the faeces and urine, respiration, conduction and radiation from the skin, and by evaporation of perspiration. The following are approximately the relative amounts of heat lost through these various channels (different authorities give somewhat different figures) : faeces and urine about 3, respiration about 20, skin (conduction, radiation and evaporation) about 77. Hence it is clear that the chief means of loss are the skin and the lungs. The more air that passes in and out of the lungs in a given time, the greater the loss of heat. And in such animals as the dog, which do not perspire easily by the skin, respiration becomes far more important. The dog when lying in the sun adopts a special type of very rapid shallow respiration, whereby the proportion of air passing into and out of the respiratory passages is great as compared with that which invades the deeper portions of the lung—hence he can evaporate large quantities of aqueous vapour without working out a corresponding quantity of aqueous vapour. It is claimed that by this means his blood may be reduced to a temperature lower than normal in which case the stimulus must be sought in the temperature of the skin, rather than that of the brain.

But for man, the great heat regulator is undoubtedly the skin, which regulates heat loss by its vasomotor mechanism, and also by the nervous mechanism of perspiration. Dilation of the cutaneous vascular areas leads to a larger flow of blood through the skin and so tends to cool the body and vice versa. Also the special nerves of perspiration can increase or lessen heat loss by promoting or diminishing the secretions of the skin. There are great difficulties in the exact determination of the amount of heat produced, but there are certain well-known facts in connec tion with it. A larger living body naturally produces more heat than a smaller one of the same nature, but the surface of the smaller, being greater in proportion to its bulk than that of the larger loses heat at a more rapid rate. Hence to maintain the same constant bodily temperature, the smaller animals must produce a relatively larger amount of heat.

Food temporarily increases the production of heat, the rate of production steadily rising after a meal until a maximum is reached from about the 6th to the 9th hour. If sugar be included in the meal the maximum is reached earlier ; if mainly fat, later. Muscular work very largely increases the production of heat, and hence the more active the body the greater the production of heat.

Under basal conditions, i.e., at rest in bed before breakfast, the quantity of heat given out is proportional to the surface of the body. The latter may be calculated approximately from a knowledge of the height and weight of the person according to Du Bois' "Height and Weight Chart" fig. r. A heat loss of 40 calories per square metre of surface per hour is regarded as the normal basal metabolic rate. In miners the heat loss may become on the average of 24 hours over zoo calories per square metre of body surface per day. Such persons have developed the power of losing water from the skin to a very high pitch. An average loss of water of over 3 lb. per hour on a five-hour shift has been registered in the case of coal miners.

But all the arrangements in the animal economy for the pro duction and loss of heat are themselves probably regulated by the central nervous system, there being a thermogenic centre— situated above the spinal cord, and according to some observers in the optic thalamus. (See also NUTRITION.) BIBLIOGRAPHY.-M. S. Pembrey "Animal Heat," in Schafer's TextBibliography.-M. S. Pembrey "Animal Heat," in Schafer's Text- book of Physiology (1898) . C. R. Richet, "Chaleur" in -Dictionnaire de physiologie (1898) ; Hale White, Croonian Lectures, Lancet (1897) ; Pembrey and Nicol, Journal of Physiology, vol. xxiii. (1898-99) ; H. M. Vernon, "Heat Rigor" Journal of Physiology xxiv. (1899) ; H. M. Vernon "Death Temperatures" Journal of Physiology xxv. (1899) ; F. C. Eve, "Temperature on Nerve Cells" Journal of Physiology xxvi. (1900) . G. Weiss, Comptes Rendus, Soc. de. Biol., lii. (1900) ; Swale Vincent and Thomas Lewis, "Heat Rigor of Muscle" Journal of Physi ology (19oi) ; Sutherland Simpson and Percy Herring "Cold and Re flex Action," Journal of Physiology (1905) ; Sutherland Simpson Pro ceedings of Physiological Soc. ( July 19, 1902) ; Sutherland Simpson and J. J. Galbraith, "Diurnal Variation of Body Temperature" Jour nal of Physiology (1905) ; Transactions Royal Society Edinburgh (1905) ; Proc. Physiological Society p. xx. (19o3) ; A. E. Boycott and J. S. Haldane, Effects of High Temperatures on Man; K. N. Moss, Proc. Roy. Soc. London B. xcv. (1924) ; and Du Bois, Arch. Int. Med. xvii. (1916) .