Animal Heat. Living protoplasm is constantly in process of disintegration and oxidation, and these changes are accompanied by evolution of heat. The greater the activity of change, the higher does the temperature tend to become. Not only, therefore, are the so-called cold-blooded animals really warmer than the surrounding atmosphere, but even plants recognisably evolve heat, and the temperature of certain flowers, where protoplasmic activity is highest, may sometimes almost reach that of the human body. See ARUM.
Even the infusoria evolve heat, as is shown by the slowness with which the surrounding water freezes. John Hunter showed that worms and leeches, slugs and snails, were all one or two degrees warmer than the air. Fishes generally are only two or three degrees warmer than the water they inhabit; but in some of the more active, like the bonito and tunny, a temperature of 99° F. has been observed, while the surrounding water was at 80½°. So, too, the frog, which usually averages about 1° warmer than the air, is 2° or 3° warmer while breeding; while in certain lizards and snakes, a difference of as much as 15° to 20° F. has been recorded. Newport's researches on insects show that while the temperature of the larva may vary from 1½° to 4° above that of the atmosphere, that of the pupa is almost imperceptibly higher, and that of the perfect insect may rise enormously; a difference of from 2° or 3° at rest, to from 9° to 20° in excitement, having been observed in individual bees, and a much more marked elevation in the temperature of the whole hive, which has been observed to reach 102° F. Among the animals commonly termed warm-blooded, the temperature, although generally higher in birds than in mammals, varies from species to species, yet is very nearly constant during health in each. Thus, while the average temperature of the human body is about 98.4° F., that of the wolf is 3° or 4° lower, and that of the arctic fox 5° or 6° higher. In birds, the temperature varies from 100° in the gull and other aquatic birds, to nearly 112° in the swallow, while, on the other hand, a hibernating mammal like the lemming becomes temporarily cold-blooded, its temperature during the winter sleep being comparatively little above that of the atmosphere.
From the preceding details, it will be seen that while cold-blooded and warm-blooded animals thoroughly agree in evolving considerable amounts of heat, the difference between them lies in this, that in the former the means of loss of heat by the skin, &c. are great as compared with the normal production of heat, while in the latter the loss and production of heat are kept balanced.
Physiologically considered, the animal body is a machine for converting the potential energy supplied by food into the actual energy of heat and mechanical work. What Aristotle simply referred to the heart, and mechanical physiologists to the friction of the blood, and so on, is now simply regarded as one of the results of the disintegration of the complex protoplasm. Knowing the quantity and chemical composition of the food, it is easy to calculate the amount of energy furnished to the body. The average income of energy of the human body on normal diet is about 1,000,000 metre-kilogrammes, of which about 150,000 units can be expended in muscular work, the remaining 850,000 leaving the body in the form of heat. As to the channels by which heat leaves the body, Helmholtz has calculated that fully 2½ per cent. leaves the body with the fluid and solid egesta, about 5½ per cent. is spent in warming the expired air, about 14½ in evaporating the water expired by the lungs, and the balance, about 77½ per cent., by the skin, in conduction, radiation, and evaporation.
These general considerations once grasped, the apparent anomalies and variations in the temperature of different animals present no difficulty.
While heat is given off by the oxidation of the living matter of all the tissues, the greater part is the result of the activity of the muscular and glandular systems, and especially of the former.
For the lower or cold-blooded animals, the varying temperature is simply and directly associated with the varying amount of protoplasmic waste, and this again with the varying activities of the organism. The case of higher or warm-blooded animals (mammals and birds) presents, however, greater difficulty, since here the temperature remains practically constant throughout life (neglecting slight diurnal and seasonal variations, or the more serious perturbations due to the excitation and depression of the vital processes in various diseases). Some regulative mechanism must here be present, operating on the one hand to insure the regular maintenance of a minimum temperature, on the other to check its undue rise in periods of exceptional activity. This problem has been the subject of much physiological research, and is not yet fully exhausted; its essential solution is, however, due to Claude Bernard, whose Chaleur Animale (1876) may be taken as a centre round which the literature of the subject arranges itself.
The regulation of heat is on the one hand automatically effected by the variations in the quantity lost in warming the breathed air, in the flow of blood through the skin, and in perspiration. Thus if more air be passed in and out of the lungs in a given time, or if the vaso-motor nerves allow the skin blood-vessels to dilate and admit of a larger flow, or if perspiration increase, the body will become cooler, and vice versa. But on the other hand, there is every reason to believe from the researches of Hoppe-Seyler, Liebermeister, and others, that the production of heat through the activity of muscles and other organs is controlled by means of a special heat-regulating and nervous mechanism. Eulenberg and Landors have demonstrated a certain area in the brain, which, when stimulated, affects the temperature of the body. See PHYSIOLOGY, TEMPERATURE OF THE BODY.