The sources and distribution of heat. Wherever metabolism of protoplasm is going on, heat is being generated. All over the body heat is being set free; more abundantly in the more active tissues, and most of all in those tissues the metabolism of which leads to little or no external work. The metabolism of the tissues (including the blood) and of the food within the alimentary canal is the source of the heat of the body. But heat, being continually produced, is as continually being lost, as we have seen, by the skin, urine, and feces. The blood passing from one part of the body to another, and carrying warmth from the tissues where heat is being actively generated, to the tissues or organs where heat is being lost by conduction or evaporation, tends to equalize the temperature of the various parts and thus maintain a constant bodily temperature. Taking the body as a whole, under normal conditions, the chief sources of the production of heat are the muscles, and the abdominal viscera, more especially the liver; and of these the liver deserves attention, inasmuch as it is always at work, whereas the heat produced by the muscles is at least largely dependent on their contracting, and they may remain at rest for a considerable period. The brain, too, may be regarded as a source of heat, since its temperature is higher than that of the arterial blood with which it is supplied. Heat is lost by the skin, respiration, feces, etc. The great regulator, however, is undoubtedly the skin. The more blood passes through the skin the greater will be the loss of heat by conduction, radiation, and evaporation. The working of this heat-regulating mechanism is well seen in the case of exercise. Since every muscular contraction gives rise to heat, exercise must increase for the time being the production of heat; yet the bodily temperature rarely rises as much as a degree C., if at all. By exercise the respiration is quickened and the loss of heat by the lungs increased. The circulation of blood is also quickened, and the cutaneous vascular areas becoming dilated, a large amount of blood passes through the skin. The expenditure of heat may be tabulated thus: By the skin, in conducting, radiating, and evaporating, 77.5 per cent. Warming expired air, 5.2  ,,   Evaporating the water of respiration, 14.5  ,,   In warming urine, etc., 2.6   ,,   THE CIRCULATION. The heart is a hollow muscular organ divided by a longitudinal septum into a right and a left half, each of which is again subdivided by a transverse constriction into two compartments communicating with each other, and named auricle and ventricle.