Our muscles are not only the largest, but the "livest" part of our bodies.

Their contractions and movements are caused by their tiny "explosions" (like the chugging of an automobile, except that we can't hear them); and in this way they burn up the largest part of the food-fuel which we eat--mostly in the form of sugar.

When they have burned up their surplus food-fuel, they call for more; and when this demand has been telegraphed to the brain, we say we are hungry, and that exercise has given us an appetite.

While the muscles are at work, they demand that large supplies of fresh fuel shall be brought to them through the blood vessels; and this makes the heart beat harder and faster, and improves the circulation.

As they burn up this fuel, they form smoke and ashes, or waste materials, which must be got rid of--the fluid part by perspiration from the surface of the skin, and through the kidneys, and the gas, or "smoke," through the lungs.