into combination with hydrogen in the blood, producing some of the water which is given off from the body, and also with compounds of hydrogen, nitrogen, and carbon, thus further increasing the animal heat, for heat is evolved whenever chemical combination takes place. Oxygen is carried in the arterial blood all over the body, and wherever there are capillaries, there the oxygen combines with the carbon, and heat is evolved, as elsewhere—in a grate, for instance. But it must be remembered that something is necessary to set up the combination. In the grate, we apply it in the form of a lighted match, and in the same way a certain degree of temperature is required in the body. The combination of oxygen with carbon is going on all over the body wherever the two meet—not only in the lungs, as some have been led to suppose, because it is from the lungs that the chief amount of carbonic acid, the result of their union, is given off.
The origin of animal heat has been for centuries a debated question, and until of late no thoroughly satisfactory results have been obtained; but the theory here explained rests upon facts gained by numerous scientific experiments, and upon inductions therefrom. There is therefore every reason to believe that it is the true one, although animal heat may be in part caused by the friction of the blood in its rapid passage through the arteries and veins, as well as by the chemical action. The theory is borne out by the fact that when the circulation is slow, as in old people and during sleep, the tem-
