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40 remaining in some cases only amounting to 1 3/10 per cent. When the muscles are in activity, therefore, there is increased combustion, more fuel is burnt up. As a consequence of this an increased amount of carbonic acid is expired from the lungs, and during great exertion the quantity of this gas which is expired may be even five times as much as that breathed out during repose. A further consequence of the increased combustion, moreover, is that an increase of food is required, and we all know how hungry vigorous exercise makes us.

It is noticeable, too, that the colder the temperature of the air is, the quicker combustion goes on in the body to keep up its heat; breathing is then performed more rapidly; more oxygen, too, is contained in cold than in warm air, and more carbon is required to combine with it in the blood. Thus a larger quantity of food is rendered necessary. In the far North the natives consume an amount of food, and especially of fatty carbonaceous foods, which seems fabulous to those who do not understand why it is required. It is said that an Esquimaux boy can drink two quarts of train oil and eat twelve pounds of tallow candles in a day; or whale blubber corresponding to that amount.

The foregoing satisfactorily explains the reason why exercise of muscle or brain makes us hungry, for brain work uses up the blood to give nourishment, and evolves heat just as muscular work does, and it also shows why we feel that we want more food, and especially more carbonaceous—more fatty food in cold than in hot weather.