In the second place, food is the fuel of the body and is just as truly burned as is coal in a furnace. Moreover, the quantity of heat which a piece of meat or a slice of bread yields when burned in the body is just the same as if it had been burned in a stove. Complete combustion yields a definite amount of heat wherever and whatever may be the place and manner of burning. Any kind of food may serve as fuel for the body, but those which consist mainly of sugar, starch and fat, which contain no nitrogen and so cannot build up the body, are used chiefly as fuel. These fuel foods form the bulk of our daily ration, comparatively little being required for purposes of growth and repair. We are hearing a good deal recently about alcohol as a food. When it is remembered that alcohol contains no nitrogen it will be seen that it cannot serve the first function of food, namely, the purpose of growth and repair. It can, however, serve as fuel food, for when taken into the body in small quantities it is assimilated and burned up, producing the same amount of heat as if burned in a lamp. In sickness this may be beneficial, at times when the body cannot assimilate other foods. But the injurious effects of alcohol upon the digestive and nervous systems are so important and far-reaching that its value as a fuel food sinks into insignificance in comparison. The process of combustion or burning in the fire box of our locomotive consists, as has been said, in oxygen of the air uniting with the carbon and hydrogen of the coal, forming carbonic acid and water, and setting free a definite quantity of heat for every pound of fuel so burned. So, in exactly the same way, oxygen, which has been taken up by the blood from the air in the lungs, unites with carbon and hydrogen in the tissues of the body and forms carbonic acid and water, yielding precisely the same amount of heat as though the combustion had occurred in a furnace. This idea of food, that it is literally fuel, is a very suggestive one. And as fuels differ in the quantity of ash contained and the amount of heat produced, so food materials differ in the quantity of undigestible residue and in their heat-producing power. Remembering the analogy of the steam engine, let us now inquire what becomes of the energy supplied to the body in the fuel foods eaten, and which is turned into heat by this process of combustion constantly going on. 1. A large amount of heat is constantly being expended in keeping the body warm. Like the locomotive, the body is warmer than the surrounding air, and is constantly losing heat to the atmosphere. Unlike the locomotive, however, the body has a nearly uniform temperature throughout, namely, 98 degrees Fahr. The delicate regulation of temperature which is automatically maintained in the animal body is one of the wonders of physiology.