The heat of the body is generated by the chemical changes - those of oxidation - undergone not by any particular substance or in any one place, but by the tissues at large. 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 - also a muscle, - never resting, does in the 24 hours no inconsiderable amount of work, and hence 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. The brain also must be a source of heat, since its temperature is higher than that of the arterial blood with which it is supplied. 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 while continually being produced is also continually being lost by the skin, lungs, urine and faeces. 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 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, who do not perspire easily by the skin, respiration becomes far more important.
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. Dilatation 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 greater difficulties in the exact determination in the amount of heat produced, but there are certain well-known facts in connexion 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 animal must produce a relatively larger amount of heat. And in the struggle for existence this has become so.
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.
But all the arrangements in the animal economy for the production 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.
M. S. Pembrey, "Animal Heat," in Schafer's Textbook of Physiology (1898); C. R. Richet, "Chaleur," in Dictionnaire de physiologie (Paris, 1898); Hale White, Croonian Lectures, Lancet, London, 1897; Pembrey and Nicol, Journal of Physiology, vol. xxiii., 1898-1899; 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 Physiology, 1901; Sutherland Simpson and Percy Herring, "Cold and Reflex Action," Journal of Physiology, 1905; Sutherland Simpson, Proceedings of Physiological Soc., July 19, 1902; Sutherland Simpson and J. J. Galbraith, "Diurnal Variation of Body Temperature," Journal of Physiology, 1905; Transactions Royal Society Edinburgh, 1905; Proc. Physiological Society, p. xx., 1903; A. E. Boycott and J. S. Haldane, Effects of High Temperatures on Man.