534. It has been shown (444 _et seq._), that during the process of respiration more oxygen disappears than is accounted for by the carbonic acid that is generated; that this excess of oxygen is absorbed by the blood; and that in the lung the oxygen merely enters into a state of loose combination with the blood, the union being intimate and complete only in the system. The complete chemical combination of the oxygen with the carbon takes place, then, not in the lungs, but in the capillary arteries of the system; consequently it is only while flowing in capillary arteries that carbonic acid is formed; that is, it is only in these vessels that the arterial combustion takes place: of course, therefore, it is only in these vessels that heat is extricated, and only from them that it can be communicated to the adjacent parts. According to this view, wherever there is a capillary artery, the combustion of carbon incessantly goes on, and there caloric is as incessantly set free; but since there is not a point of any tissue, in which there are not capillary arteries, there is not a point from which caloric does not radiate. As soon as formed, carbonic acid passes from the capillary arteries into the capillary veins; by the veins it is transmitted to the lungs; and by the lungs it is expelled from the system. The real operations carried on in the lungs, then, are the transmission of oxygen and the extrication of carbonic acid; but this organ is not the seat of the essential and ultimate part of the function; it is merely the portal through which the elements employed in the process have their entrance and exit. Thus the question concerning the greater capacity of arterial blood for caloric is of no importance whatever: the phenomena may be equally accounted for, whatever be, in this respect, the constitution of the blood. 535. The result of the whole is, the complete establishment of the fact, that the production of heat in the animal body is a chemical operation, dependent on the combination of oxygen with carbon in the capillary arteries of the system; that is, it is the result of the burning of charcoal at every point of the body. 536. The agent which maintains and regulates this internal fire is the nervous system. There is, indeed, reason to suppose that the nervous system, in some mode or other, contributes to the actual production of animal heat. It is established by direct experiment, that the quantity of carbonic acid formed in the system is inadequate to the supply of the caloric expended by it; that in a given time more heat is abstracted from the body by the surrounding medium, than can be accounted for by the consumption of the amount of carbonic acid thrown off by the lungs during the same interval. There is evidence that the source of this additional heat is the nervous system.