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CHAP. III.] perature is lowered; whereas in fevers, where the temperature is high, the pulse is always found beating rapidly, and the breathing is quickened.

We know that a fire will not burn unless the air can get to it—that is to say, unless oxygen can come to combine with the carbon of the coal; it is the same in the animal body: the  carbon which is derived from the food eaten is, as it were, the fuel which sets our machinery in motion; but that it is not wholly expended in work has been proved by experiment and observation. The muscles convert chemical force into mechanical work, but to quote the words of Dr. Mayer, "The maximum mechanical effect produced by  a  working  mammal hardly amounts to one-fifth of the force derivable from the total quantity of carbon consumed; the remaining four-fifths are devoted to the generation of heat." When a muscle contracts, heat is developed in it. This has been found so even in the muscles of dead frogs. And in the case of persons who die of that terrible disease tetanus, in which a general contraction of the muscles takes place, and death is caused either by starvation through lockjaw, or suffocation, owing to the prolonged contraction of the muscles employed in breathing, the temperature of the muscles is sometimes found to be nearly 11° Fahr. above the normal. The arterial blood charged with oxygen when passing through an un-contracted muscle is changed into venous blood, which then retains about 7½ per cent, of oxygen; but if the muscle is contracted, the arterial blood is almost wholly deprived of its oxygen, the quantity