In the lower limbs, the muscles are larger because they have heavier work to do, supporting and moving the whole weight of the body; but they are simpler in their arrangement since they have not such a variety of movements to carry out. The principal muscle in the thigh is the great muscle running down the front of the thigh, and fastening to the upper border of the _patella_, or knee cap. This muscle, when it shortens, straightens or extends the limb, or lifts the foot from the ground and swings it forward as in walking, or raises the knee up toward the body when we are sitting or lying down. You can easily tell how much it is used in walking by remembering how stiff and sore it gets when you have taken an unusually long tramp, particularly if there has been much hill-climbing in it. On the back of the thigh, runs another great group of muscles, which bend or flex the limb when they shorten. When the knee is bent, you can feel their tendons, or sinews, stand out as hard cords beneath the knee; hence, this group is called the _ham-string_ muscles.[23] How the Muscles are Fed. 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. This is the reason why, during exercise, we breathe faster and deeper than at other times, and why our skin begins first to glow and then to perspire. If these waste-materials form in the muscles faster than the blood can wash them out, they poison the muscle-cells and we begin to feel tired, or fatigued. This is why our muscle-cells are often so stiff and sore next morning after a long tramp, or a hard day's work, or a football game. A hot bath or a good rub-down takes the soreness out of the muscles by helping them to get these poisonous wastes out of their cells. Thus when we play or run or work, we are not only exercising our muscles and making them gain strength and skill, but we are stirring up, or stimulating, almost every part of our body to more vigorous and healthful action.