Regulation of Temperature.—­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.