When the motion is a compound one, involving the action of several muscles at the same time, a multitude of impressions are sent back and forth to and from the brain through the nerves. But the person acting thus is unconscious of all this delicate and wonderful mechanism. He wills the movement, and instantly the requisite nervous power is sent to the required cells and fibres, and they perform the motions required. Many of the muscles are moved by the sympathetic system, over which the mind has but little control. Among the muscles and nerves so intimately connected run the minute capillaries of the blood, which furnish nourishment to all. [Illustration: Fig. 52.] Fig. 52 represents an artery at _a_, which brings pure blood to a muscle from the heart. After meandering through the capillaries at _c_, to distribute oxygen and food from the stomach, the blood enters the vein, _b_, loaded with carbonic acid and water taken up in the capillaries, to be carried to the lungs or skin, and thrown out into the air. The manner in which the exercise of the muscles quickens the circulation of the blood will now be explained. The veins abound in every part of every muscle, and the large veins have _valves_ which prevent the blood from flowing backward. If the wrist is grasped tightly, the veins of the hand are immediately swollen. This is owing to the fact that the blood is prevented from flowing toward the heart by this pressure, and by the vein-valves from returning into the arteries; while the arteries themselves, being placed deeper down, are not so compressed, and continue to send the blood into the hand, and thus it accumulates. As soon as this pressure is removed, the blood springs onward from the restraint with accelerated motion. This same process takes place when any of the muscles are exercised. The contraction of any muscle presses some of the veins, so that the blood can not flow the natural way, while the valves in the veins prevent its flowing backward. Meantime the arteries continue to press the blood along until the veins become swollen. Then, as soon as the muscle ceases its contraction, the blood flows faster from the previous accumulation. If, then, we use a number of muscles, and use them strongly and quickly, there are so many veins affected in this way as to quicken the whole circulation. The heart receives blood faster, and sends it to the lungs faster. Then the lungs work quicker, to furnish the oxygen required by the greater amount of blood. The blood returns with greater speed to the heart, and the heart sends it out with quicker action through the arteries to the capillaries. In the capillaries, too, the decayed matter is carried off faster, and then the stomach calls for more food to furnish new and pure blood. Thus it is that exercise gives new life and nourishment to every part of the body.